Cauthep tc05-final

Nguyn Vn Nhm Nguyn Ngc Long Nguyn Mnh Ng Ngc Sn

1Chng I

M U

1.1 Nguyn l thit k Hin nay vic tnh ton thit k kt cu cng trnh c da trn cc trng thi gii hn. Trng thi gii hn l trng thi m nu vt qu n, cu hoc b phn ca cu s khng cn tha mn cc yu cu t ra khi thit k na. Trong mi trng thi gii hn (TTGH), mi cu kin hay lin kt phi tha mn iu kin = rniii RRQ (1-1) trong : i h s iu chnh ti trng, l h s lin quan n tnh do, tnh d v tm quan trng trong khai thc ca cu; i h s ti trng, l h s xt n s bin thin ca ti trng, s thiu chnh xc trong phn tch v xc sut xy ra cng mt lc ca cc ti trng khc nhau, nhng cng lin quan n thng k v sc khng trong qu trnh hiu chnh; Qi hiu ng ca ti trng: lc dc, lc ct , mmen un v.v mt b phn kt cu hay lin kt do ti trng sinh ra; Rn sc khng danh nh hay sc khng tiu chun ca mt cu kin hoc lin kt. Sc khng danh nh c xc nh theo kch thc, ng sut cho php, bin dng hoc cng ca vt liu; h s sc khng l h s ch yu xt n s bin thin cc tnh cht ca vt liu, kch thc ca kt cu v tay ngh ca cng nhn v s khng chc chn trong d on v sc khng, nhng cng lin quan n nhng thng k v ti trng thng qua trong qu trnh hiu chnh; Rr sc khng tnh ton.

1.2 Cc trng thi gii hn V tng qut c ba trng thi gii hn: - Trng thi gii hn v cng l trng thi gii hn c lin quan n cng v n nh. - Trng thi gii hn s dng l trng thi gii hn lin quan n ng sut, bin dng v vt nt di iu kin khai thc bnh thng. - Trng thi gii hn c bit l trng thi gii hn lin quan n cc s c nh ng t, va x ca tu b, xe c vo cng trnh, c th c trong iu kin xi l. Do trng thi gii hn v cng c chia lm nhiu trng hp khc nhau nn trong tnh ton cc cu kin hay lin kt phi tha mn iu kin (1-1) trong cc trng thi gii hn c th sau y:


2 - Trng thi gii hn cng I l trng thi gii hn tnh vi t hp ti trng

c bn khi trn cu c xe v khng c gi. - Trng thi gii hn cng II l trng thi gii hn tnh vi t hp ti trng khi trn cu khng c xe nhng c gi vi tc gi ln hn 25m/s. - Trng thi gii hn cng III l trng thi gii hn tnh vi t hp ti trng khi trn cu c xe v c gi vi vn tc 25m/s. - Trng thi gii hn mi l trng thi gii hn tnh vi t hp ti trng gy ra mi v t gy lin quan n tc dng lp i lp li v xung kch ca mt xe ti vi khong cch trc c nh (khong cch trc gia v trc sau l 9m). - Trng thi gii hn s dng l ti trng gii hn tnh vi t hp ti trng lin quan n khai thc bnh thng ca cu vi gi vn tc 25m/s v vi tt c cc ti trng ly theo gi tr danh nh (trong quy trnh c gi l ti trng tiu chun) dng kim tra vng, b rng vt nt trong kt cu b tng ct thp v b tng ct thp d ng lc, s chy do ca kt cu thp v s trt ca cc lin kt c nguy c trt do tc dng ca hot ti xe. - Trng thi gii hn c bit l trng thi gii hn tnh vi t hp ti trng c lin quan n ng t, lc va ca tu thuyn, xe c.

1.3 H s iu chnh ti trng H s i lin quan n tnh do, tnh d v tm quan trng ca cu trong khai thc theo quan h: 95.0IRDi = (1-2)

trong D , R v I ln lt l h s do, h s d v h s tm quan trng khai thc. Cc h s ny c ly theo hng dn di y.

1.3.1 H s do D Khi vt liu c tnh do nu mt b phn kt cu lm vic ra ngoi min n hi, bin dng s tng ln v c s phn b li ni lc sang cc b phn khc ca kt cu v nh vy kt cu hay lin kt c tnh do lm vic an ton hn kt cu v lin kt khng do. H kt cu cu phi c xc nh kch thc v cu to m bo cho s pht trin ca bin dng do trng thi gii hn cng v trng thi gii hn c bit. Cn phi xt n nh hng ca tnh do trong tnh ton, y h s do c t v tri ca biu thc (1-1), phn hiu ng ca ti trng nn cc cu kin v lin kt khng do c h s D ln hn. Quy trnh quy nh nh sau: - i vi trng thi gii hn cng : D 1,05 cho cu kin v lin kt khng do. D = 1,00 cho cc thit k thng thng.


3 D 0,95 cho cc cu kin hoc lin kt c bin php tng thm tnh do. - i vi cc trng thi gii hn khc: D = 1,00.

1.3.2 H s d Xt n tnh d l xt n bc siu tnh hay s lin kt tha so vi yu cu bt bin hnh ca kt cu. Tuy nhin khng phi b phn no ca kt cu siu tnh cng c xem l c tnh d, chng hn dn trn hnh 1-1 l dn siu tnh bc 1, cc thanh c nh du x l cc b phn c tnh d v h hng ca mt thanh no trong chng khng gy nn sp cu, tri li cc thanh khng nh du x l cc thanh khng c tnh d v h hng mt trong s cc thanh ny lm cho dn tr thnh kt cu bin hnh. Xt dm lin tc hai nhp nh trn hnh 1-2, dm c xem l c tnh d v khi xut hin mt khp do A hoc B dm vn l h bt bin hnh v khng b sp , dm ch c xem l mt kh nng lm vic khi ng thi xut hin c hai khp do A v B.

Hnh 1-1. Cu dn

Hnh 1-2. Cu dm

Qua hai th d trn c th thy ngay trong mt kt cu siu tnh cc b phn hay cu kin m h hng ca chng gy ra sp cu l cc b phn hay cu kin khng c tnh d, tri li cc b phn hay cu kin m s h hng ca chng khng gy ra sp cu l cc b phn hay cu kin c tnh d. Cng nh h s do, h s d R c xt n v tri ca biu thc (1-1), phn hiu ng ca ti trng nn b phn khng c tnh d, R c gi tr ln hn b phn c tnh d. Quy trnh quy nh nh sau: - i vi trng thi gii hn cng R : R 1,05 cho cc b phn khng c tnh d. R = 1,00 cho cc b phn c mc d thng thng. R 0,95 cho cc b phn c mc d c bit. - i vi cc trng thi gii hn khc: R = 1,00.


4 1.3.3 H s tm quan trng trong khai thc

Ty theo tnh cht quan trng trong khai thc ch u t c th quyt nh mt cu hoc mt b phn no ca cu l quan trng trong khai thc. Quy trnh quy nh ly h s tm quan trng trong khai thc I nh sau: - i vi trng thi gii hn cng : I 1,05 cho cc cu quan trng; I = 1,00 cho cc cu thng thng; I 0,95 cho cc cu t quan trng. - i vi cc trng thi gii hn khc : I = 1,00.

1.4 H s sc khng ca kt cu thp

1.4.1 i vi trng thi gii hn cng H s sc khng c ly theo cc ch dn trong bng 1-1.

Bng 1-1. H s sc khng

Hng mc Kt cu chu un Kt cu chu ct Kt cu thp hoc thp lin hp chu nn dc trc Kt cu chu ko, t trong mt ct thc Kt cu chu ko, chy trong mt ct nguyn p mt ta trn cc cht, cc l doa, khoan hoc bt bulng trn cc b mt cn Bulng p mt trn vt liu Neo chu ct Bulng A325M v A490M chu ct Ct khi Kim loi hn trong cc ng hn ngu hon ton: - Ct trn din tch hu hiu - Ko v nn trc giao vi din tch hu hiu - Ko hoc nn song song vi trc ng hn Kim loi hn trong cc ng hn ngu cc b: - Ct song song vi trc ng hn - Ko hoc nn song song vi trc ng hn - Nn trc giao vi din tch hu hiu - Ko trc giao vi din tch hu hiu Kim loi hn cc mi hn: - Ko hoc nn song song vi trc ng hn - Ct trong chiu cao tnh ton ca kim loi hn

1,00 1,00 0,90 0,80 0,95

1,00 0,80 0,85 0,80 0,80

0,85

Ly theo kim loi c hn Ly theo kim loi c hn

0,80

Ly theo kim loi c hn Ly theo kim loi c hn

0,80

Ly theo kim loi c hn 0,80


51.4.2 i vi cc trng thi gii hn c bit H s sc khng trong trng thi gii hn c bit, tr bulng, ly bng 1,00.

1.5 H s ti trng H s ti trng ph thuc vo loi ti trng: ti trng thng xuyn, ti trng tc thi hay ti trng thi cng. H s ti trng cn ph thuc vo t hp ti trng. Trong mi t hp ti trng cc h s ti trng phi chn sao cho gy ra tng ni lc tnh ton l cc tr (c gi tr m v dng), nu tc dng ca mt ti trng lm gim tc dng ca mt ti trng khc th phi ly gi tr nh nht ca ti trng lm gim tc dng ca ti trng kia bng cch ly h s ti trng nh nht. H s ti trng ca cc ti trng thng xuyn c ly theo bng 1-2, cn h s ti trng ca cc ti trng tc thi c ly theo bng 1-3. Khi cn kim tra cu vi xe c bit do ch u t quy nh hoc xe c giy php qua cu th h s ti trng ca hot ti (LL) trong t hp cng I c th gim xung cn 1,35. Cc cu c t l tnh ti trn hot ti rt cao (cu nhp ln) cn kim tra t hp khng c hot ti trn cu (t hp cng II) nhng vi h s ti trng bng 1,5 cho tt c cc ti trng thng xuyn (P = 1,5).

Bng 1-2. H s ti trng ca cc ti trng thng xuyn

H s ti trng Loi ti trng

Ln nht Nh nht DC: Cu kin v thit b ph 1,25 0,90 DD: Ko xung (ma st m) 1,80 0,45 DW: Lp ph mt cu v cc tin ch 1,50 0,65 EH: p lc ngang ca t Ch ng B ng

1,50 1,35

0,90 0,90

EL: Cc ng sut do lp rp 1,00 1,00 EV: p lc t thng ng n nh tng th Kt cu tng chn Kt cu cng b vi lp Khung cng Kt cu mm b vi lp v khng phi cng hp thp Cng hp mm bng thp

1,35 1,35 1,30 1,35 1,95 1,50

Khng p dng

1,00 0,90 0,90 0,90 0,90

ES: Ti trng t cht thm 1,50 0,75

Vi cu vt sng cc trng thi gii hn cng v trng thi gii hn s dng phi xt n xi mng m, tr do l thit k.


6 Vi cu vt sng khi kim tra cc hiu ng ti: ng t, lc va xe, lc va

tu trng thi gii hn c bit th ti trng nc v chiu su xi c th da trn l trung bnh hng nm, tuy nhin kt cu phi c kim tra vi cc hu qu do l nh kim tra xi trng thi gii hn c bit v ti trng nc tng ng nhng khng c cc ti trng ng t, va x ca xe, ca tu thuyn. Khi kim tra chiu rng vt nt trong kt cu b tng d ng lc trng thi gii hn s dng c th gim h s ti trng ca hot ti xung l 0,80. Khi kim tra kt cu thp ca trng thi gii hn s dng th h s ti trng ca hot ti phi tng ln l 1,30.

Bng 1-3. T hp ti trng v h s ti trng

TU

WA WS WL FR CR TG SE SH

Cng mt lc ch dng mt trong

cc ti trng

T hp ti trng

TTGH

DC DD DW EH EV ES

LL IM CE BR PL LS EL

EQ CT CV

Cng I P 1,75 1,00 - - 1,00 0,50/1,20 TG SE - - - Cng II P - 1,00 1,40 - 1,00 0,50/1,20 TG SE - - - Cng

III P 1,35 1,00 0,40 1,00 1,00 0,50/1,20 TG SE - - -

c bit P 0,5 1,00 - - 1,00 - - - 1,00 1,00 1,00S dng 1,00 1,00 1,00 0,30 1,00 1,00 0,50/1,20 TG SE - - -

Mi, ch c LL, IM v

CE

- 0,75 - - - - - - - - -

Ghi ch ca bng 1-3: BR Lc hm xe. CE Lc ly tm. CR T bin. CT Lc va xe. CV Lc va tu thuyn. EQ ng t. IM Tc dng xung kch ca xe. LL Hot ti xe. LS Hot ti cht thm. PL Ti trng ngi i. SE Ln. SH Co ngt. TG Chnh lch nhit khng u (gradien nhit ). TU Chnh lch nhit u. WA Ti trng nc v p lc dng chy.


7WL Ti trng gi ln hot ti. WS Ti trng gi ln cu. H s ti trng ca gradien nhit TG c ly bng: 0,00 trng thi gii hn cng v c bit; 1,00 trng thi gii hn s dng khi khng xt hot ti; 0,50 trng thi gii hn s dng khi c xt hot ti. i vi cu thi cng phn on, phi xem xt t hp sau y trng thi gii hn s dng: DC + DW + EH + EV + ES + WA + CR + SH + TG + EL

1.6 Ti trng v cc h s

1.6.1 Ti trng thng xuyn Ti trng thng xuyn l ti trng v lc tc ng khng i hoc c xem l khng i sau khi hon thnh vic xy dng cu. Ti trng thng xuyn ca cu ni chung bao gm tnh ti v ti trng t. i vi kt cu nhp th ti trng thng xuyn l tnh ti bao gm trng lng tt c cu kin ca kt cu, ph kin v tin ch cng cng km theo, trng lng mt cu, d phng ph b v m rng. Khi khng c s liu chnh xc c th ly khi lng ring nh trong bng 1-4 tnh tnh ti.

Bng 1-4. Khi lng ring ca vt liu

Vt liu Khi lng ring (kg/m3)

Hp kim nhm 2800 Lp ph nha ng 2250 X than 960 Ct cht, ph sa hay t st 1925

Nh 1775 Ct nh 1925

B tng

Thng 2400 Ct ri, ph sa, si 1600 t st mm 1600 Si, cui, dm nn hoc balat 2250 Thp 7850 xy 2725

Ngt 1000 Nc

Mn 1025


8 1.6.2 Hot ti v cc h s

1.6.2.1 Xe ti thit k Xe ti thit k (LL) l mt xe gm 3 trc c cc thng s nh sau (hnh 1-3): Ti trng trc trc 35kN. Ti trng mi trc gia v trc sau 145kN. Ti trng tng cng 325kN. Khong cch t trc trc n trc gia 4300mm. Khong cch t trc gia n trc sau (4300 9000)mm. Khong cch tim hai bnh theo chiu ngang 1800mm.

Hnh 1-3. Xe ti thit k

1.6.2.2 Xe hai trc thit k Xe hai trc thit k l mt xe gm hai trc c cc thng s nh sau: Ti trng mi trc 110kN. Ti trng tng cng 220kN. Khong cch t trc trc n trc sau 1200mm. Khong cch tim hai bnh theo chiu ngang 1800mm. Ghi ch: i vi xe ti v xe hai trc thit k, trn cc ng cp IV v thp hn, ti trng xe c th ly l ti trng trc nhn vi 0,5 hoc 0,65 cn khong cch trc xe v bnh xe khng thay i.

1.6.2.3 Ti trng ln thit k Ti trng ln thit k gm ti trng 9,30N/mm phn b u theo chiu dc cu. Theo chiu ngang cu ti trng ln c xem l phn b u trn chiu rng 3000mm. Khng tnh h s xung kch vi ti trng ln.

1.6.2.4 Ti trng ngi i Ti trng ngi i trn cu t khi l ngi i rng bng hoc hn 600mm c ly bng 3.10-3 MPa. i vi cu dnh ring cho ngi i b hoc i xe p ti trng ngi ly bng 4,1.10-3MPa. Khng tnh h s xung kch cho ti trng ngi.

4300 mm 4300 mm ti 9000 mm35 kN 145 kN 145 kN 1800 mm

Ln thit k 3500 mm600 mm ni chung

300 mm mt tha mt cu


91.6.2.5 Hot ti thit k HL - 93 Hot ti thit k HL 93 l mt t hp ca: - Xe ti v ti trng ln thit k. - Xe hai trc v ti trng ln thit k. Trong mi ln xe ti trng HL - 93 c xp chng gia xe ti hoc xe hai trc vi ti trng ln. Trn mi ln ch c mt xe ti hoc mt xe hai trc tr trng hp c quy nh ring (xem iu 3.6.1.3.1 quy trnh).

1.6.2.6 H s ln xe Khi trn cu ng thi c mt s ln xe cn phi nhn vi h s ln xt n xc sut xy ra hiu ng cc tr. a. S ln xe thit k: S ln xe thit k c xc nh bi phn s nguyn ca t s w/3500, y w l b rng khong trng ca lng ng gia hai va hoc hai ro chn, n v l mm. Cn xt n kh nng thay i chiu rng phm vi xe chy trong tng lai. Trong trng hp b rng ln xe nh hn 3500mm th s ln xe thit k ly bng s ln giao thng v b rng ln xe thit k phi ly bng b rng ln giao thng. Lng ng rng t 6000mm n 7200mm phi c 2 ln xe thit k, mi ln bng mt na b rng lng ng. b. H s ln: Ni lc cc tr ca hot ti c xc nh bng cch xt mi t hp c th ca s ln chu ti nhn vi h s ln xe nh trong bng 1-5.

Bng 1-5. H s ln xe

S ln cht ti

H s ln

1 1,20 2 1,00 3 0,85

> 3 0,65

Khng p dng h s ln cho trng thi gii hn mi v khi tnh mi ch dng mt xe ti thit k bt k s ln xe l bao nhiu. Khng p dng h s ln kt hp vi h s phn b ti trng tr khi dng quy tc n by hay khi c yu cu ring cho dm ngoi cng trong cu dm - bn.


101.6.2.7 H s xung kch

xt n tc dng ng ca ti trng, tc dng tnh ca xe ti hoc xe hai trc thit k (khng k lc ly tm v lc hm phanh) phi c nhn thm vi h s xung kch 1+IM/100, trong lc xung kch IM c tnh theo phn trm ca lc tc dng v c ly theo bng 1-6. Khng p dng h xung kch cho ti trng ln thit k v ti trng ngi i, tng chn khng chu lc thng ng t kt cu phn trn v phn mng nm hon ton di mt t.

Bng 1-6. Lc xung kch IM

Cu kin IM Mi ni bn mt cu: Tt c cc trng thi gii hn

75%

Tt c cc cu kin khc: Trng thi gii hn mi v t gy Cc trng thi gii hn khc

15% 25%

1.6.2.8 Lc ly tm Lc ly tm xut hin khi xe chy trn ng cong. Lc ly tm tc dng theo phng nm ngang pha trn v cch mt ng 1800mm, c ln bng tch s trng lng trc ca xe ti hay xe hai trc vi h s C:

gR3V4C

2

= (1-3)

trong : V vn tc thit k (m/s); g gia tc trng trng, ly bng 9,807 (m/s2); R bn knh cong ca ln xe (m). Khi tnh lc ly tm phi p dng h s ln.

1.6.2.9 Lc hm xe Lc hm xe c ly bng 25% trng lng cc trc ca xe ti hoc xe hai trc thit k t ln mi ln cho tt c cc ln c quy nh theo phn 1.6.2.6.a v c xe chy cng hng. Cc lc ny c xem l tc dng theo phng nm ngang cch pha trn mt ng 1800mm theo mt hng dc cu gy ra ni lc ln nht. i vi nhng cu c th tr thnh mt chiu trong tng lai th phi cht ti ng thi trn tt c cc ln thit k. Khi tnh lc hm phi p dng h s ln.

1.6.2.10 Lc va ca xe Khng cn tnh lc va ca xe c v tu ha nu cng trnh c bo v bi:


11- Nn p. - Ro chn c lp chn trong t, chu c va chm, cao 1370mm, t cch b phn cn bo v trong phm vi 3000mm. - Ro chn cao 1070m t cch b phn cn bo v hn 3000mm. a. Xe c, tu ha va vo kt cu: Nu khng c bo v th m, tr t trong phm vi cch lng ng b 9000mm hay trong phm vi 15000mm n tim ng st u phi tnh vi lc tnh tng ng t trong mt phng nm ngang, cch mt t 1200mm vi tr s bng 1800kN. b. Xe c va vo ro chn, lan can (hnh 1-4): Cc lc thit k lan can v cc tiu chun hnh hc phi nh quy nh trong bng 1-7 v c minh ha trong hnh 1-4. Cc mc ngn chn ca lan can c ly theo cc ch dn sau: L1 Mc cp mt, p dng cho cc khu vc cng trng vi tc xe c

thp v cc ng ph a phng c lu lng nh, tc thp. L2 Mc cp hai, p dng cho cc khu vc cng trng, hu ht cc ng

a phng v ng thu gom c iu kin tt ni c t xe nng v tc gim.

L3 Mc cp ba, p dng cho cc ng chnh c hn hp cc xe ti v cc xe nng.

L4 Mc cp bn, p dng cho ng cao tc vi tc cao, lu lng giao thng ln vi t l cao hn ca cc xe nng v cho ng b vi iu kin ti ch xu.

L5 Mc cp nm, p dng cho cc ng ging nh mc cp bn khi c iu kin ti ch chng minh cn mc ngn chn cao hn.

Bng 1-7. Lc thit k v cc thng s tc dng i vi lan can ng t

Mc ngn chn ca lan can Lc thit k v cc thng s tc dng

L1 L2 L3 L4 L5 Ft Ngang (kN) 60 120 240 516 550 FL Dc (kN) 20 40 80 173 183 FV Thng ng (kN) hng xung di 20 20 80 222 355 Lt v LL (mm) 1220 1220 1070 2440 2440 LV (mm) 5500 5500 5500 12200 12200 He (min) (mm) 460 510 810 1020 1070 Chiu cao lan can nh nht H (mm) 810 810 810 1020 1370


12

Hnh 1-4. Cc lc va vo ro chn, lan can

1.6.2.11 Ti trng gi a. Tc gi thit k: y ch xt tc gi nm ngang tc dng vo cng trnh cu thng thng, i vi nhng kt cu nhp ln hay kt cu nhy cm vi gi nh cu treo dy vng, cu dy vng, cn c nhng kho st, nghin cu ring v th nghim trong cc hm thi gi xc nh tc ng ca gi. Tc gi thit k c xc nh theo cng thc: SVV B= (1-4) trong : V tc gi thit k (m/s); VB tc gi git c bn trong 3 giy vi chu k xut hin 100 nm (p = 1%) ly theo bng 1-8; S h s hiu chnh theo a hnh v cao , ly theo bng 1-9.

Bng 1-8. Cc gi tr ca VB cho cc vng

Vng tnh gi theo TCVN 2737 1995

Tc gi git c bn VB (m/s)

I II III IV

38 45 53 59

Ghi ch ca bng 1-8: Khi tnh gi trong qu trnh lp rp c th nhn tc gi git c bn VB cho trong bng vi h s 0,85.

Lv

H

FvLt v LL

FL

R1

R2

_ R

_ Y

Ft


13Bng 1-9. H s hiu chnh tc gi theo a hnh v cao

cao mt cu trn mt t khu vc xung quanh hay trn mt nc (m)

Khu vc trng tri hay mt nc thong

Khu vc c rng hay c nh ca vi cy, nh cao ti a khong 10m

Khu vc c nh ca vi a s nh cao trn 10m

10 1,09 1,00 0,81 20 1,14 1,06 0,89 30 1,17 1,10 0,94 40 1,20 1,13 0,98 50 1,21 1,16 1,01

b. Ti trng gi tc dng ln cu: - Gi ngang Ti trng gi ngang PD c phng nm ngang, t trng tm din tch chn gi v c tr s (kN): tdt

2D A8,1CAV0006,0P = (1-5)

trong : V tc gi thit k (m/s); At din tch chn gi (m2); Cd h s chn gi ly theo th trn hnh 1-5.

Hnh 1- 5. H s cn Cd dng cho kt cu phn trn c mt hng gi c

Din tch chn gi At phi l din tch c chiu ln mt trc vung gc vi hng gi, trong trng thi khng c hot ti tc dng vi cc iu kin sau:

0,4 0,8 1,2

2,4 2,0 1,6

2,8

0,0 0,4 0,8 1,2 1,6 2,0 2 6 10 14 2218 14 26 30

H s ti thiu cho h mt cu t trn dm I hoc h c nhiu hn 4 dm loi khc hoc h dm hp H

s

cn

Cd

T s b/d


14+ i vi kt cu phn trn c lan can c, din tch At phi bao gm c din

tch ca lan can c hng gi (lan can u gi), khng cn xt nh hng ca lan can khng hng gi (lan can cui gi hay lan can pha sau). + i vi kt cu phn trn c lan can h, khi phi ly bng tng ti trng tc dng ln tng phn bao gm c lan can u gi v lan can cui gi khi xem nh lan can ny khng nh hng g n lan can kia. Khi s lan can ln hn hai ch xt hai lan can c din tch chn gi ln nht. + i vi cu dn ti trng gi c tnh ring cho tng dn, khng xt tc dng chn gi ca dn n i vi dn kia. + i vi tr khng xt n nh hng ca cc mt che chn. H s cn gi Cd ly theo hnh 1-5, trong trc honh l t s b/d vi b l chiu rng cu gia hai mt lan can v d l chiu cao kt cu phn trn bao gm c lan can c nu c vi cc ch sau: + Khi kt cu phn trn c mt chnh c, mp dc ng, khng c gc vut thot gi ng k th Cd ly nh trn hnh 1-5. + Trong cu dn, lan can, kt cu phn di tnh ring, mi b phn c h s Cd tng ng. + Mi kt cu phn trn khc, Cd c xc nh theo th nghim trong hm thi gi. + Gi tr Cd cho trong hnh 1-5 ng vi mt chn gi thng ng v gi tc dng nm ngang. + Nu mt chn gi xin vi mt phng thng ng, Cd c th gim 0,5% cho mi xin v gim ti a 30%. + Nu mt chn gi c phn ng v phn xin hoc hai phn xin vi gc nghing khc nhau th:

H s cn Cd tnh vi chiu cao ton b kt cu; i vi tng phn h s cn Cd gim theo ghi ch trn; Ti trng gi tng cng c tnh theo ti trng gi ln tng phn vi h s

cn gi Cd tng ng. + Nu kt cu phn trn c dc phi ly Cd tng thm 3% cho mi nghing so vi ng nm ngang nhng khng qu 25%. + Nu kt cu phn trn chu gi xin khng qu 5% so vi phng nm ngang phi tng Cd ln 15%, nu gc xin vt qu 5% phi c th nghim ring xc nh Cd. + Nu kt cu phn trn dc ng thi chu gi xin phi ly Cd theo kt qu kho st ring. - Gi dc: i vi m, tr, kt cu phn trn c b mt cn gi song song vi tim dc ca kt cu ln th phi tnh ti trng gi dc cu. Cch tnh ti trng gi dc


15tng t cch tnh ti trng gi ngang. i vi kt cu phn trn c mt trc c, ti trng gi dc ly bng 0,25 ti trng gi ngang tnh nh phn trn. Cc ti trng gi dc v ngang phi tnh ring r, trng hp cn thit cn kim ton theo hp lc th khng ly hai trng hp ring r trn m phi tnh theo hng thc ca gi. - Gi thng ng: Ti trng gi thng ng PV tc dng vo trng tm din tch chn gi thch hp c gi tr bng (kN): v

2V AV00045,0P = (1-6)

trong V l tc gi thit k c ly theo cng thc (1-4) trn, Av l din tch phng ca mt cu hay cu kin dng tnh ti trng gi thng ng (m2). Ti trng gi thng ng ch tnh trong cc trng thi gii hn khng lin quan n gi tc dng ln hot ti, ch tnh khi ly hng gi vung gc vi trc dc cu. Ti trng gi thng ng tc dng cng gi nm ngang. Cng thc (1-6) p dng vi iu kin gc nghing ca gi tc dng vo kt cu nh hn 50, nu ln hn cn xc nh bng th nghim. c. Ti trng gi tc dng ln hot ti WL: Trong trng thi gii hn cng III phi xt c ti trng gi tc dng ln hot ti. Ti trng gi ngang ln hot ti c ly l mt lc ri u hng ngang cu c tr s 1,5N/mm t ti cao 1800mm so vi mt ng xe chy. Ti trng gi dc ln hot ti cng l mt lc ri u nm ngang t ti cao 1800mm so vi mt ng xe chy, phn b dc cu v c cng 0,75N/mm. Ti trng gi ngang cu v dc cu c tnh ring r, nu cn kim ton theo hp lc th phi tnh theo hng thc ca gi.

1.7 Phn b ngang ca ti trng Cu l mt kt cu khng gian. C nhiu phng php tnh ra ni lc, chuyn v tng v tr ca kt cu, cc phng php ny c th hin trong cc phn mm tnh ton m khi thit k ngi k s c th dng tnh ton. Tuy nhin trong nhiu trng hp ngi ta c th a bi ton khng gian v bi ton phng thng qua h s phn b ngang ca ti trng. C rt nhiu phng php tnh h s phn b ngang c nghin cu, y ch xt phng php c chp nhn trong quy trnh 22TCN-272-05. So vi cc phng php khc, phng php tnh h s phn b ngang theo quy trnh 22TCN-272-05 c cc c im sau: - Tnh h s phn b ngang ring cho lc ct, ring cho mmen un. - C h s phn b ngang ring cho dm bin v cho dm trong. - C h s hiu chnh khi cu xin. - Cu dm v cu tit din hp c phng php tnh khc nhau.


161.7.1 Tnh h s phn b ngang cho cc cu dm - bn

Cu dm - bn l loi cu hay gp nht, trong dm ch c th l thp, g, b tng, mt cu cng c th l thp, g, b tng. Tuy nhin trong phm vi cu thp th dm ch bng thp cn mt cu bng g, thp hoc b tng. Nu mt cu khng lin hp vi dm ch th to thnh cu dm thp bn k. Nu mt cu bng b tng c lin kt cng vi dm thp th to thnh cu dm thp lin hp vi bn b tng ct thp. Nu mt cu bng thp v c cu to sao cho cng lm vic vi dm thp th to thnh cu dm c bn trc hng. Phng php tnh h s phn b ngang cho cu dm - bn (t y gi tt l cu dm) trong quy trnh 22TCN-272-05 ch p dng cho cu tha mn cc iu kin sau: - B rng mt cu khng i trn sut chiu di nhp. - S dm khng nh hn 4 tr khi c quy nh khc. - Cc dm song song vi nhau v c cng xp x nhau. - Phn hng ca ng xe chy khng vt qu 910mm tr khi c quy nh khc. - cong trong mt bng nh. - Mt ct ngang cu ph hp vi quy nh trong bng 1-10. Khi tha mn cc iu kin trn, ti trng thng xuyn ca bn mt cu v trn bn mt cu c xem l phn b u cho cc dm ch hoc phn b u cho dm ch v dm dc hoc phn b u cho dm dc nh trong cu dn. tnh h s phn b ngang ca hot ti cn thc hin theo trnh t: u tin tnh tham s cng dc, sau t tham s cng dc tra bng xc nh h s phn b ngang. H s phn b ngang ca hot ti y c th s dng cho cc loi xe m b rng ca chng tng ng vi b rng ca xe ti thit k.

1.7.1.1 Tnh tham s cng dc Tham s cng dc gK c tnh theo cng thc:

( )2gg AeInK += (1-7) vi

D

B

EEn = (1-8)

trong : EB m un n hi ca vt liu ch to dm (MPa); ED m un n hi ca vt liu bn (MPa); I mmen qun tnh ca dm (mm4); A din tch tit din dm ch hay dm dc ph (mm2); eg khong cch t trng tm dm n trng tm bn (mm).


17 Cc thng s I v A trong cng thc (1-7) phi c ly theo dm khng lin hp.

Bng 1-10. Cc loi mt ct ngang kt cu nhp

Cu kin Loi mt cu Mt ct in hnh

Dm thp B tng c ti ch, c sn, li thp mt co

(a)

Dm thp hp kn B tng c ti ch

(b)

Dm thp hp h B tng c ti ch, c sn

(c)

1.7.1.2 Xc nh cng thc tnh h s phn b ngang Cn c vo loi kt cu dm, mt ct thch hp (a hoc b hoc c trong bng 1-10), phm vi p dng, tra bng 1-11 tm cng thc tnh h s phn b ngang, sau thay cc gi tr tng ng vo tm gi tr ca h s phn b ngang. Vi cc cu ch c hai dm, h s phn b ngang c tnh theo phng php n by. H s phn b ngang c tnh theo cng thc bng trn c nhn vi h s ln xe m. Khi tnh theo phng php n by cn phi nhn thm vi h s ln xe m. Ghi ch ca bng 1-11: S khong cch gia cc dm (mm). L chiu di nhp (mm). ts chiu dy bn b tng (mm). tg chiu dy li thp hoc tm thp ln sng (mm). NL s ln xe thit k. Nb s dm, dm dc ph. de khong cch t tim dm ngoi n mp trong ca va hoc lan can. g h s phn b.


18 e h s hiu chnh.

d chiu cao dm ch hoc dm dc ph (mm). D chiu rng phn b trn mt ln (mm). gc cho (). Kg tham s cng dc (mm4).

Bng 1-11. H s phn b ti trng theo ln

Ni lc, dm

Loi kt cu H s phn b ti trng

H s iu chnh cho

Phm vi p dng

Mt ln chu ti 1,0

3s

g3,04,0

LtK

LS

4300S06,0

+

S ln chu ti 2

0,10,6 0,2g3s

KS S0,0752900 L Lt

+

1100S4900 110 ts 300 6000L73000 Nb 4

loi a, mt cu bng bn b tng

Dng gi tr nh hn: tnh theo cng thc trn vi Nb=3 hoc tnh theo nguyn tc n by.

5,11 )tg(C1

5,0

3s

g1 L

SLtK

25,0C

=

Nu < 300, C1=0,0 Nu > 600, s dng = 600

Nb = 3

loi a, mt cu bng li mt co

Mt ln chu ti S/2300 nu tg < 100 mm S/3050 nu tg 100 mm S ln chu ti 2 S/2400 nu tg < 100 mm S/3050 nu tg 100 mm

Khng p dng

S 1800mm S 3200mm

loi b v c, mt cu bng bn b tng

S ln chu ti bt k

Lb

L

N425,0

NN85,005,0 ++ Khng p dng

5,1NN5,0

b

L

M men, dm trong

loi a, mt cu bng tn ln sng

Mt ln chu ti S / 2800 S ln chu ti 2 S / 2700

Khng p dng

S 1700mm tg 50


19Bng 1-11. H s phn b ti trng theo ln (tip theo trang trc)

Ni lc, dm

Loi kt cu H s phn b ti trng

H s iu chnh cho

Phm vi p dng

Mt ln chu ti Tnh theo nguyn tc n by S ln chu ti 2 g = egdm trong

2800d77,0e e+=

-300 de 1700 Nb 4


Dng gi tr nh hn: tnh theo cng thc trn vi Nb=3 hoc tnh theo nguyn tc n by.

5,11 )tg(C1

5,0

3s

g1 L

SLtK

25,0C

=

Nu < 300, C1=0,0 Nu > 600, s dng = 600

Nb = 3

M men, dm bin

loi a, mt cu li mt co

S ln chu ti bt k u tnh theo nguyn tc n by. Khng p dng

p dng cho mi trng hp

Mt ln chu ti

7600

S36,0 +

S ln chu ti 2 2

10700S

7600S20,0

+

1100S4900 110 ts 300 6000L73000

4.109Kg3.1012

Nb 4


Nguyn tc n by.

+ tg

K

Lt20,000,1

3,0

g

3s

vi 00 600

Nb = 3

Lc ct, dm trong


S ln chu ti bt k u tnh theo nguyn tc n by.

Khng p dng


Mt ln chu ti Tnh theo nguyn tc n by.S ln chu ti 2 g = e. gdm trong

3000

d60,0e e+=

300 de 1700 Nb 4


Nguyn tc n by.

+ tg

K

Lt20,000,1

3,0

g

3s

vi 00 600

Nb = 3

Lc ct dm bin


S ln chu ti bt k u tnh theo nguyn tc n by.



201.7.2 Tnh h s phn b ngang cho cc cu dm hp

Vi cu dm hp k c hp n v hp c hai hoc nhiu ngn tt nht l tnh ni lc bng cc phng php khng gian nh phng php phn t hu hn, di hu hn trong cc phn mm tnh ton c sn. Vi cu nhiu hp (hnh b v c trong bng 1-8) bn c c th tnh theo cc cng thc cho trong quy trnh, y khng nu cng thc tnh v trng hp ny cho n nay cha gp Vit nam.


21Chng 2

THP V CC LIN KT

2.1 Cc loi thp kt cu Theo tiu chun thit k cu 22TCN-272-05, trong cu thp thng dng 4 loi thp sau: - Thp ccbon hay thp kt cu M 270M cp 250. - Thp hp kim thp cng cao M 270M cp 345 v 345W. - Thp hp kim thp ti v gia nhit M 270M cp 485W. -Thp hp kim thp ti v gia nhit vi cng chy do cao M270M cp 690 v 690W. Bng 2-1 cho cc c tnh c hc ti thiu ca thp, trong c cng chu ko nh nht (Fu) l cng nh nht khi t trong th nghim ko thp v cng chy nh nht (Fy) l cng ca vt liu gii hn chy trong th nghim ko thp.

Bng 2-1. Cc c tnh ti thiu ca thp kt cu theo hnh dng, cng v chiu dy

Thp kt cu

Thp hp kim thp cng cao

Thp hp kim thp ti v gia

nhit

Thp hp kim ti v gia nhit cng

chy cao K hiu AASHTO

M 270M cp 250

M 270M cp 345

M 270M cp 345W

M 270M cp 485W

M 270M cp 690/690W

K hiu ASTM tng ng

A 709M cp 250

A 709M cp 345

A 709M cp 345W

A 709M cp 485W

A 709M cc cp 690/690W

Chiu dy bn thp, mm

Ti 100 Ti 100 Ti 100 Ti 100 Ti 65 Trn 65 n 100

Thp hnh Tt c

cc nhm

Tt c cc

nhm

Tt c cc nhm

Khng p dng

Khng p dng

Khng p dng

Cng chu ko nh nht Fu

(MPa) 400 450 485 620 760 690

im chy nh nht hoc cng chy nh nht

Fy (MPa)

250

345

345

485

690

620

i vi tt c cc loi thp khi thit k u ly mun n hi E = 200000 MPa v h s gin n v nhit 11,7. 10-6 mm/mm/00C.


22 Trong cc loi thp bng 2-1 th M 270M l k hiu thp cn cp ca loi

thp l cng chy tnh bng MPa. Th d thp cp 345 th cng chy ca thp l 345MPa, cn ch W sau cp thp l ch thp chng g, th d thp cp 690W l thp chng g c cng chy 690MPa. Tt c cc loi thp cho trong bng 2-1 u l thp hn c. Chiu dy nh nht ca thp trong cu thp quy nh iu 6.7.3 nh sau: - Thp kt cu bao gm c lin kt ngang, lin kt dc v cc loi bn nt tr sn dm ca thp hnh, sn tng cng kn trong mt cu c bn trc hng (bn orthotrope), tm m v thp lan can u phi c chiu dy ti thiu l 8mm. - Chiu dy sn ca thp hnh, sn tng cng kn trong mt cu c bn trc hng phi c chiu dy ti thiu l 7mm. - Vi nhng kt cu hoc b phn kt cu chu nh hng n mn nghim trng th phi c bo v c bit chng n mn hoc phi quy nh chiu dy b n mn tng thm chiu dy thp khi thit k.

2.2 Lin kt bulng Bulng dng trong cu c th l bulng thng hoc bulng cng cao. Bulng thng c dng ch yu trong cc b phn ph nh lan can, ng thot nc v.v... Bulng cng cao c dng ph bin trong cu nht l cc mi ni thc hin ti cng trng. Lin kt bulng cng cao c th lm vic theo ma st hay theo p ta. Cc mi ni chu ng sut i du, ti trng va chm mnh, chn ng ln phi dng lin kt ma st, c th l: - Cc mi ghp chu ti trng mi. - Cc mi ni chu ct vi cc bulng lp vo l qu c. - Cc mi ni chu ct vi cc bulng lp vo l van ngn hoc di, lc tc dng ln mi ni khc vi phng thng gc vi trc di ca l van. - Cc mi ni chu ng sut i du. - Cc mi ni trong cc bulng cng tham gia truyn ti trng b mt c to nhm. - Cc mi ni ko dc trc hoc ko dc trc ng thi ct. - Cc mi ni chu nn dc trc vi cc l tiu chun hoc cc l van ch trong mt lp ca lin kt, phng ca ti trng thng gc vi phng ca trc di l van. Cc lin kt chu p ta c dng cho cc mi ni chu nn dc trc hoc cc mi ni trn h lin kt vi iu kin phi tha mn sc khng tnh ton trong trng thi gii hn cng .


232.2.1 Cu to bulng cng cao Bulng cng cao bao gm c bulng, ai c v vng m.

2.2.1.1 Bulng Bulng cng cao dng trong lin kt ca cc kt cu thp phi c cng chu ko ti thiu 830MPa cho cc bulng c ng knh t 16mm n 27mm v 725MPa cho cc bulng c ng knh t 30mm ti 36mm. ng knh bulng khng c nh hn 16mm. Khng c dng bulng 16mm trong cc cu kin ch yu, tr khi ti cc cnh thp gc 64mm v cc bn cnh ca cc mt ct c kch thc yu cu cc bulng lin kt ng knh 16mm. Bulng lin kt ng knh 16mm ch nn dng cho lan can, khng dng cho thp hnh. Cc thp gc m kch thc khng yu cu cn phi xc nh bng tnh ton th c th dng cc bulng nh sau: - Bulng ng knh 16mm cho cnh 50mm. - Bulng ng knh 20mm cho cnh 64mm. - Bulng ng knh 24mm cho cnh 75mm. - Bulng ng knh 27mm cho cnh 90mm. ng knh ca bulng trong cc thp gc ca cc thanh ch yu khng c vt qu mt phn t chiu rng cnh ca thanh c b tr bulng. Ty theo cng ngh thi cng bulng c cc cu to khc nhau: - Kiu 1 (hnh 2-1- a), bulng c cu to thng thng gm thn bulng, ai c v hai vng m. Khi xit dng mt cl hm u bulng, mt cl o lc xit n lc cng yu cu.

Hnh 2-1. Cc loi bulng

- Kiu 2 (hnh 2-1-b), bulng c u chm cu, ai c v hai vng m. Khi xit dng cl my xoay hai chiu, mt chiu hm u bulng ngoi ai c, mt chiu xit ai c n khi t u bulng.

(a) (b)

(c) (d) (e)


24- Kiu 3 (hnh 2-1-c,d), bulng c u chm cu, di chm cu c mt hoc

hai ngnh (cc ngnh ny c t trong l c rnh), ai c v mt vng m. Khi xit dng mt cl o lc xit n lc cng yu cu. - Kiu 4 (hnh 2-1-e), bulng c u loe hnh nn ct (chm) st phn u loe cu to on gai to ma st vi l, ai c v mt vng m. Khi xit dng mt cl o lc xit n lc cng yu cu.

2.2.1.2 ai c ai c phi c ch to ng tiu chun v c trng km. ai c phi c cng ti thiu 89HRB.

2.2.1.3 Vng m Vng m phi c ch to theo ng tiu chun v c trng km. Mt ngoi ca phn c bt bulng phi c dc so vi mt phng trc giao vi trc bulng ln hn 1:20. Trong bt c trng hp no cng phi c vng m di cu kin c xit cht. Cc vng m ti cng phi c trm qua cc l rng qu c hoc rnh kha tm ngoi. Chiu dy ca cc vng m tm kt cu hoc thanh vi cc l tiu chun khng c nh hn 8mm.

2.2.2 L bulng L bulng thng c cc loi sau: - L tiu chun l l c ng knh ln nht ln hn ng knh bulng t 2mm n 3mm. - L qu c l l c ng knh ln nht ln hn ng knh bulng t 4mm tr ln. L qu c c dng trong lin kt ma st bt k lp no ca lin kt v khng c dng trong lin kt kiu p ta. - L van ngn v l c x mt rnh l l c chiu di ln hn chiu rng nhiu nht l 7mm v c th dng cho bt k lp no ca lin kt ma st hay lin kt p ta. Cnh di ca l van khng ph thuc vo phng ca ti trng trong lin kt ma st, cn trong lin kt p ta chiu di phi c trc giao vi phng ca ti trng. - L van di v l c x hai rnh l l c chiu di ln hn nhiu so vi chiu rng v ch dng trong mt lp ca lin kt ma st hay lin kt p ta. Cng nh l van ngn trong lin kt ma st cnh di ca l van c th c v tr bt k so vi phng ca ti trng, cn trong lin kt p ta cnh di phi c trc giao vi phng ca ti trng. Kch thc ca cc loi l ph thuc v ng knh bulng v khng c vt qu gi tr cho trong bng 2-2.


25Bng 2-2. Cc kch thc l ln nht

ng knh bulng

Tiu chun Qu c van ngn van di

D ng knh ng knh Rng x di Rng x di 16 18 20- 18 x 22 18 x 40 20 22 24 22 x 26 22 x 50 22 24 28 24 x 30 24 x 55 24 26 30 26 x 33 26 x 60 27 30 35 30 x 37 30 x 67 30 33 38 33 x 40 33 x 75 36 39 44 39 x 46 39 x 90

2.2.3 Khong cch gia cc bulng

2.2.3.1 Khong trng v khong cch ti thiu Khong cch t tim n tim ca bulng vi l tiu chun khng c nh hn ba ln ng knh bulng. Khi dng bulng l qu c hoc l van (hay l x rnh) th khong trng (khong cch t mp l n mp l lin k theo phng ca lc hay vung gc vi phng ca lc) khng c nh hn hai ln ng knh bulng.

Hnh 2-2. Cc kch thc trong lin kt bulng

Ghi ch hnh 2.2: a - khong cch gia cc inh (bc bulng); b - khong cch t inh n u cu kin; c - khong trng gia cc bulng; d - khong trng t bulng n u cu kin; g - khong cch ngang; b1 - khong cch so le; a2 - khong cch ngang n mp cu kn.

2.2.3.2 Khong cch ti a bo m cho cc tm ghp p xt vo nhau v cch m khong cch gia cc bulng trn mt hng n lin k vi mp t do ca bn tp ngoi hay thp hnh phi tha mn iu kin: S (100 + 4t) 175 (2-1)

a b

c d g

b1

a2


26 Cng m bo cc iu kin trn nu c mt hng th hai b tr so le vi

hng lin k vi mp t do v c khong cch khng nh hn 38 + 4t th c ly so le S gia hai hng inh phi tha mn iu kin:

S 100 + 4t (4g3 ) 175 (2-2)

Khong cch so le ny khng c nh hn mt na khong cch cho mt hng n. Trong cc cng thc (2-1) v (2-2), t l chiu dy nh hn ca bn tp hay thp hnh (mm); g l khong cch ngang gia cc bulng (mm).

2.2.3.3 Bc ti a cho bulng ghp t hp Bulng ghp t hp khi mt ct ngang c hai hay ln hn hai tm bn hay thp hnh tip gip vi nhau. Khong cch gia cc bulng theo chiu dc khi ghp cc cu kin chu nn khng c vt qu 12t. Khong cch ngang (g) gia cc hng bulng lin k khng c qu 24t. Khi b tr so le khong cch so le ca hai bulng hai hng lin k phi tha mn:

S 15t- (8g3 ) 12 (2-3)

Khong cch dc gia cc bulng trong thanh chu ko khng c vt qu hai ln quy nh i vi thanh chu nn. Khong cch ngang gia hai hng bulng lin k khng c vt qu 24t. Khong cch dc ti a ca cc bulng trong cc cu kin c mt ct t hp khng c vt qu tr s nh hn gia hai yu cu chng m (xem phn 2.2.3.2) v ghp t hp (xem phn 2.2.3.3).

2.2.3.4 Khong cch dc ti a cho bulng ghp t hp u cu kin chu nn

Bc dc ca bulng lin kt cc b phn ca cu kin chu nn khng c vt qu bn ln ng knh bulng cho mt on di bng 1,5 chiu rng ln nht ca cu kin. Ngoi on ny (on u cu kin) bc dc c th c tng dn trn on chiu di bng 1,5 ln chiu rng ti a ca cu kin cho n khi no t bc dc ti a nh quy nh 2.2.3.3.

2.2.3.5 Khong cch bulng u mt cu kin Khong cch t tim l n u cu kin ca mi loi l khng c nh hn khong cch n mp quy nh trong bng 2-3. i vi l qu c hoc van khong t mp l n mp cu kin khng c nh hn ng knh bulng. Khong cch t tim bulng n u cu kin ln nht phi ly khng ln hn tm ln chiu dy ca bn ni dy nht v 125mm.


27Bng 2-3. Khong cch n mp ti thiu

ng knh bulng

Cc mp ct

Cc mp tm bn hay thp hnh c cn hoc cc mp c ct

bng kh t 16 28 22 20 34 26 22 38 28 24 42 30 27 48 34 30 52 38 36 64 46

2.2.3.6 Khong cch n mp bn cu kin Khong cch ti thiu t tim bulng n mp bn ca cu kin quy nh trong bng 2-3. Khong cch t tim bulng n mp cu kin khng c ln hn tm ln chiu dy ca bn ni dy nht v 125mm.

2.2.4 Sc khng ca bulng y cn xt sc khng trt dng cho lin kt ma st, sc khng ct v sc khng p mt dng cho lin kt p ta v cc sc khng khc.

2.2.4.1 Sc khng trt Sc khng trt danh nh ca bulng trong lin kt ma st c ly nh sau: Rn = Kh Ks Ns Pt (2-4) trong : Ns s lng mt ma st tnh cho mi bulng; Pt lc cng ti thiu yu cu ca bulng (N) ly theo bng 2-4; Kh h s kch thc l, ly theo bng 2-5;

Ks - h s iu kin b mt, ly theo bng 2-6, trong c mt loi A, loi B, loi C nh sau: + Loi A: Lm sch cc lp bn, khng sn, b mt c lm sch bng thi vi cc lp ph loi A. + Loi B: Lm sch b mt bng thi, khng sn, b mt c lm sch bng thi c cc lp ph loi B.

+ Loi C: B mt m km nng v lm nhm bng bn chi st sau khi m.

2.2.4.2 Sc khng ct Sc khng ct danh nh ca bulng cng cao hoc bulng ASTM A307 trng thi gii hn cng trong cc mi ni m khong cch gia cc


28bulng xa nht theo phng song song vi ng tc dng ca lc nh hn

1270mm ly nh sau: + Khi ng ren nm ngoi mt phng ct: Rn = 0,48 Ab Fub Ns (2-5)

Bng 2-4. Lc ko nh nht yu cu ca bulng

Lc ko yu cu Pt (kN) ng knh bulng mm M164 (A325M) M253 (A490M)

16 91 114 20 142 179 22 176 221 24 205 257 27 267 334 30 326 408 36 475 595

Bng 2-5. Cc tr s ca Kh

Cho cc l tiu chun 1,00 Cho cc l qu c v kha rnh ngn 0,85

Cho cc l kha rnh di vi rnh thng gc vi phng lc 0,70 Cho cc l kha rnh di vi rnh song song vi phng lc 0,60

Bng 2-6. Cc tr s ca Ks

Cho cc iu kin b mt loi A 0,33 Cho cc iu kin b mt loi B 0,50 Cho cc iu kin b mt loi C 0,33

+ Khi ng ren nm trong mt phng ct: Rn= 0,38 Ab Fub Ns (2-6) trong : Ab din tch mt ct ngang bulng theo ng knh danh nh (mm2); Fub cng ko nh nht ca bulng ( MPa ); Ns s mt ct cho mi bulng. Sc khng danh nh ca bulng trong cc mi ni m khong cch gia cc bulng xa nht ln hn 1270mm ly bng 0,8 ln sc khng danh nh tnh theo cng thc (2-5) hoc (2-6).


29 Vi bulng A307 nu sc khng danh nh xc nh theo (2-6) th khi chiu dy tp bn vt qu 5 ln ng knh bulng, sc khng phi gim 1% cho mi 1,5mm ln hn ca chiu dy tp bn vi 5 ln ng knh.

2.2.4.3 Sc khng p mt Din tch chu p mt hiu dng ca bulng ly bng ng knh ca n nhn vi chiu dy bn ni nh nht v mt pha. Khi l khot c ming loe (cho bulng u chm) th chiu dy phi tr i mt na chiu cao ca ming loe. i vi cc l tiu chun, l qu c, cc l van ngn c lc tc dng theo mi hng v tt c cc l van song song vi lc p mt th sc khng p mt danh nh ca cc l bulng pha trong v u cu kin trong trng thi gii hn cng , Rn c ly nh sau: - Vi cc bulng c khong trng (khong cch t mp l n mp l) khng nh hn 2d v khong trng u khng nh hn 2d: un Ftd4,2R = (2-7) - Nu khong trng gia cc l nh hn 2d hoc khong trng u nh hn 2d: Rn = 1,2 Lc t Fu (2-8) i vi cc l van c cnh di vung gc vi lc p ta, Rn c ly nh sau: - Khi khong trng gia cc l khng nh hn 2d v khong trng u khng nh hn 2d: Rn = 2 d t Fu (2-9) - Khi khong trng gia cc l nh hn 2d v khong trng u nh hn 2d: Rn = Lc t Fu (2-10) trong : d ng knh danh nh ca bulng (mm); t chiu dy tm bn (mm); Fu cng chu ko ca vt liu (MPa); Lc khong trng gia cc l hoc gia mp l vi u cu kin (mm).

2.2.4.4 Sc khng ko Cc bulng cng cao chu ko dc trc phi c cng n lc quy nh nh trong bng 2-4. Lc ko tc dng ln bulng bao gm ngoi lc tnh ton v lc nh do bin dng ca cc b phn lin kt. - Sc khng ko danh nh ca bulng (Tn) khng ph thuc vo lc cng ban u v ly nh sau: Tn = 0,76 Ab Fub (2-11) trong :


30 Ab din tch mt ct bulng tng ng vi ng knh danh nh (mm2);

Fub cng chu ko nh nht quy nh ca bulng ly theo phn 2.2.1.1 (MPa). - Tc dng nh ln: Lc ko do tc dng nh ln ly nh sau:

u2

u P328000t

a8b3Q

= (2-12)

trong : Qu lc ko nh ln trn mt bulng do cc ti trng tnh ton, ly bng 0 khi l m (N); Pu lc ko trc tip trn mt bulng do cc ti trng tnh ton (N); a khong cch t tim bulng n mp tm (mm); b khong cch t tim bulng n chn ng hn ca b phn lin kt (mm); t chiu dy nh nht ca b phn lin kt (mm). - Sc khng mi: Bulng cng cao chu ko dc trc b mi, ng sut trong bulng do hot ti mi thit k c xt xung kch cng vi lc nh ln do tc dng lp ca ti trng mi phi tha mn iu kin: (F) (F)n (2-13) trong : h s ti trng vi t hp ti trng mi, ly theo bng 1-2.

F ng sut trong bulng do hot ti mi sinh ra (MPa). Hot ti mi thit k l mt xe ti thit k c khong cch gia hai trc 145kN khng i l 9m.

(F)n sc khng mi danh nh (MPa), xc nh theo cng thc sau:

(F)n = 31

NA

21 (F)TH (2-14)

vi N = (365)(100) n (ADTT) (2-15) trong : A hng s (MPa3), ly theo bng (2-7); n s chu k ng sut cho mt ln chy ca hot ti, ly theo bng (2-8); ADTT s xe ti trong mt ngy theo mt chiu tnh trung bnh trong tui th thit k; (F)TH gii hn mi vi bin khng i, ly theo bng (2-9).


31Bng 2-7. Hng s loi chi tit, A

Loi chi tit Hng s A

( x 1011 MPa3) A 82,0 B 39,3 B 20,0 C 14.4 C 14,4 D 7,21 E 3,61 E 1,28

Bulng M164M (A325M) chu ko dc trc 5,61 Bulng M253M (A490M) chu ko dc trc 10,3

Bng 2-8. Cc chu k i vi mi lt xe ti chy qua, n

Chiu di nhp Cc cu kin dc

> 12000 mm 12000 mm Dm gin n 1,0 Dm lin tc

1. Gn gi ta pha trong 1,5 2,0 2. ni khc 1,0 2,0

Dm hng 5,0 Dn 1,0

Khong cch > 6000 mm 6000 mm Cc cu kin ngang

1,0 2,0

2.2.4.5 Sc khng ko v ct kt hp Sc khng ko danh nh ca bulng chu ko dc trc v ct kt hp (Tn) ly nh sau:

+ Khi 33,0RP

n

u th

ubbn FA76,0T = (2-16)


32Bng 2-9. Gii hn mi - bin khng i

Loi chi tit Gii hn (MPa)

A 165 B 110 B 82,7 C 69,0 C 82,7 D 48,3 E 31,0 E 17,9

Bulng M164M (A325M) chu ko dc trc 214 Bulng M253M (A490M) chu ko dc trc 262

+ Nu 33,0RP

n

u > th

2

ns

nubbn R

P1FA76,0T

= (2-17)

trong : Ab din tch mt ct bulng ng vi ng knh danh nh (mm2); Fub cng chu ko nh nht quy nh ca bulng, ly theo phn 2.2.1.1 (MPa); Pn lc ct tc ng ln bulng do ti trng tnh ton (N); Rn sc khng ct danh nh ca bulng (N), ly theo 2.2.4.2. Sc khng danh nh ca bulng trong cc lin kt ma st t hp ti trng trong trng thi gii hn s dng (bng 1-2) khi bulng chu ko dc trc v ct kt hp khng c vt qu sc khng trt danh nh tnh theo cng thc (2-4) nhn vi:

t

u

PT1 (2-18)

trong : Tu lc ko do ti trng tnh ton trong t hp ti trng s dng (N); Pt lc cng ti thiu yu cu ca bulng (N), ly theo bng (2-4).


332.3 Lin kt hn Lin kt hn c dng ph bin trong kt cu thp nht l cc mi ni trong cng xng v lin kt hn n gin v mt cu to, tit kim vt liu, tuy nhin trong cc mi hn ln cn quan tm c bit n bin dng v ng sut d. gim bin dng v ng sut d cn phi quan tm n cng ngh hn cng nh trnh t hn, th d khi hn mt dm ch I trnh t hn c thc hin theo th t 1, 2, 3 v 4 (hnh 2-3) v khi mi hn nhiu lp th lp sau c hn theo hng ngc li vi lp trc v.v

Hnh 2-3. Trnh t hn ghp

Khi hn kim loi c bn, kim loi hn phi tun theo cc yu cu ca quy chun. Phi s dng kim loi hn (kim loi ca que hn, dy hn) ph hp vi kim loi c bn (kim loi ca vt liu c hn) tr trng hp c quy nh ring.

2.3.1 Cc lin kt hn thng gp

2.3.1.1 Hn gc Mi hn gc c th dng khi hai tm c bn c t chng ln nhau (hnh 2-4 a,b) hoc khi hai tm c bn vung gc (cng c th xin gc) vi nhau (hnh 2-4c). Mi hn gc c th hn cng mi hn rnh (hnh 2-4c).

Hnh 2-4. Mi hn gc

Trong ng hn gc trn mt ct ng hn im giao gia hai tm c bn c gi l gc ca ng hn, mt t do ca ng hn l mt cong nhng khi tnh c th xem l mt phng, chiu di ng vung gc t gc ng hn n mt ng hn l chiu cao hay chiu dy ca ng hn (on AI trn hnh 2-5).

4 2

31

(a) (b) (c)


34

Hnh 2-5. Chi tit hn gc

Din tch hiu dng l tch ca chiu cao ng hn vi chiu di hiu dng ca ng hn. AB, AC c gi l kch thc ca ng hn gc. Kch thc ln nht ca ng hn gc ly nh sau: - Bng chiu dy ca tm c bn khi tm c bn c chiu dy nh hn 6mm. - Nh hn chiu dy ca tm c bn 2mm khi tm c bn c chiu dy ln hn hay bng 6mm. Kch thc nh nht cn ly theo bng 2-7, ng thi khng vt qu chiu dy ca tm c bn mng hn.

Bng 2-7. Kch thc nh nht ca cc ng hn gc

Chiu dy kim loi c bn ca b phn mng hn c ni ghp T (mm)

Kch thc nh nht ca ng hn gc (mm)

T 20 6 20 < T 8

Chiu di hiu dng nh nht ca ng hn gc phi ly bng bn ln kch thc ca n v khng nh hn 40mm. Sc khng tnh ton ca ng hn gc c ly nh sau: - Khi ng hn gc chu ko hoc nn song song vi trc ng hn, sc khng tnh ton c ly bng sc khng tnh ton ca kim loi c bn. - Khi ng hn gc chu ct, sc khng tnh ton c ly tr s nh hn sc khng tnh ton ca vt liu tm c bn v sc khng tnh ton ca kim loi hn:

=exx2C

yyr F60,0

F58,0minR (2-19)

trong : y h s sc khng i vi ct, ly theo phn 1-5 nu trn; Fy cng chy nh nht ca cu kin lin kt (MPa); Fexx cng phn loi ca kim loi hn (MPa);

C

B

I

Gc ng hn

Chiu cao

A


35 C2 h s sc khng i vi kim loi hn, ly theo phn 1-5 nu trn.

2.3.1.2 Hn c vt Mi hn c vt l mi hn trong rnh do mp ng hn c vt i. Mi hn ny thng c dng khi hn i u, tuy nhin cng c th dng hn gc, chng hn bn gc ca mt ct thanh hnh hp. Rnh c th l rnh vung khi chiu dy tm c bn nh, rnh ch V n, ch X, ch U (hnh 2-6).

Hnh 2-6. Mt s kiu vt mp khi hn

a. Sc khng tnh ton ca lin kt hn c vt ngu hon ton: - Sc khng ko v nn: Sc khng tnh ton ca cc lin kt hn c vt ngu hon ton chu ko hoc nn vung gc vi din tch hiu dng ca ng hn hoc song song vi trc ng hn c ly nh sc khng tnh ton ca kim loi c bn. - Sc khng ct: Sc khng ct tnh ton ca lin kt hn c vt ngu hon ton trn din tch hiu dng phi c ly theo tr s nh hn cho trong (2-20) hoc 60% sc khng tnh ton ca kim loi c bn chu ko. Rr = 0,60 C1Fexx (2-20) trong : Fexx cng phn loi ca kim loi hn (MPa); C1 h s sc khng i vi kim loi hn, ly theo phn 1-5 nu trn. b. Sc khng tnh ton ca lin kt hn c vt ngu khng hon ton: - Sc khng ko v nn: Sc khng ko, nn ca cc lin kt hn c vt ngu khng hon ton theo phng song song vi trc ng hn hoc sc khng nn

(a) Rnh vung

(c) Rnh ch X

(b) Rnh ch V

(a) Rnh ch U


36vung gc vi din tch hiu dng ly nh sc khng tnh ton ca kim loi

c bn. Sc khng ko ca cc lin kt hn c vt ngu khng hon ton chu ko trc giao vi din tch hiu dng ly theo gi tr nh hn ca (2-21) hoc sc khng tnh ton ca kim loi c bn. Rr = 0,60 C1 Fexx (2-21) trong C1 v Fexx nh nu trn. - Sc khng ct: Sc khng tnh ton v ct ca cc lin kt hn c vt ngu khng hon ton chu lc song song vi trc ng hn phi ly theo gi tr nh hn ca sc khng tnh ton ca vt liu c bn hoc sc khng tnh ton ca kim loi hn:

=exx2C

yyr F60,0

F58,0minR (2-22)

trong y , Fy , C2 , Fexx nh nu trn.

2.4 Sc khng ph hoi ct khi Trong cc lin kt cn phi xem xt tt c cc mt phng c th b ph hoi cc b phn v tm bn lin kt bao gm cc bn song song v vung gc vi lc tc dng. Cc mt song song vi lc tc dng phi c xt nh ch chu ct, cc mt vung gc vi lc tc dng c xem nh ch chu ko. Sc khng tnh ton ca t hp hai mt phng nu l sc ph hoi ct khi ly nh sau: - Nu Atn 0,58 Avn th Rr = bs( 0,58 Fy Avg + Fu Atn) (2-23) - Nu Atn < 0,58 Avn th Rr = bs ( 0,58 Fu Avn + Fy Atg) (2-24) trong : Avg din tch nguyn dc theo mt ct chu ct (mm2); Avn din tch thc dc theo mt ct chu ct (mm2); Atg din tch nguyn dc theo mt ct chu ko (mm2); Atn din tch thc dc theo mt ct chu ko (mm2); bs h s sc khng i vi ct khi, ly theo ly theo phn 1-5 nu trn; Fy cng chy nh nht quy nh ca vt liu lin kt (MPa); Fu cng ko nh nht quy nh ca vt liu lin kt (MPa). Din tch nguyn c ly bng chiu di mt phng nhn vi chiu dy ca chi tit. Din tch thc l din tch nguyn tr i tch ca chiu dy chi tit vi s l trong mt phng nhn vi kch thc ca l trong phng ca mt phng cng vi 2mm. Sc khng ph hoi ct khi phi c xt c lin kt bulng v lin kt hn.


372.5 Sc khng ca cc cu kin lin kt Sc khng ca cc cu kin lin kt bao gm sc khng ko v sc khng ct phi c xt n khi thit k cc cu kin lin kt nh: bn ni, bn nt

2.5.1 Sc khng ko Sc khng ko tnh ton phi ly gi tr nh nht v gii hn chy (2-25) hoc v gii hn t gy (2-26), hoc theo sc khng ph hy ct khi (2-23), (2-24). Rr = y Fy Ag (2-25) Rr = u Fu An U (2-26) trong : y h s sc khng i vi chy do ca b phn chu ko ly theo phn 1-5

nu trn. Fy cng chy (MPa). Ag din tch nguyn ca b phn (mm2). u h s sc khng i vi t gy ca b phn chu ko ly theo phn 1-5 nu trn. Fu cng chu ko (MPa). An din tch thc ca b phn (mm2). U h s trit gim; U = 1. Ch rng quy trnh cng quy nh din tch thc An trong cng thc (2-26) khng c ln hn 85% din tch nguyn ca b phn.

2.5.2 Sc khng ct i vi cc cu kin lin kt chu ct, sc khng ct danh nh (Rn) v sc khng ct tnh ton nh sau:

==

nyr

ygn

RRFA58,0R

(2-27)

trong : y h s sc khng i vi ct, ly theo phn 1-5 nu trn; cc k hiu khc nh cng thc (2-26).

2.6 Cc mi ni

2.6.1 Tng qut cc phn trn chng ta nghin cu lin kt bulng, trong c lin kt p ta (ct v p mt) v lin kt ma st cng nh lin kt ng hn. V nguyn tc c hai loi lin kt trn u c th dng cho mi ni cng xng hoc cng trng tuy nhin vi cc mi ni cng xng nn u tin dng lin


38kt hn, cn cc mi ni cng trng nn u tin dng lin kt bulng.

Cng vic thit k mt mi ni thng c cc bc nh sau: - Xc nh ti trng tnh ton tc dng ln mi ni. - Chn hnh thc lin kt. - B tr lin kt (bulng, ng hn) theo ng yu cu cu to. - Kim tra lin kt theo cc trng thi gi hn theo quy nh nh nu cc phn trn. - Hon chnh mi ni bo m cc yu cu v cu to cng nh v sc khng. Quy trnh quy nh: Cc lin kt v cc mi ni ca cu kin chnh phi c tnh ton trng thi gii hn cng khng nh hn s ln hn ca mt trong hai gi tr sau: - Tr s trung bnh ca mmen un, lc ct hay lc dc do ti trng tnh ton v tr ni v sc khng un, ct, hay dc trc tnh ton ca cu kin hoc: - 75% sc khng un, ct hoc dc trc tnh ton ca cu kin.

2.6.2 Mi ni bulng Khi thit k mi ni bulng phi tun theo mt s nguyn tc sau: - ni m ti mi ni thay i th mt ct bn nh hn phi c s dng tnh ton thit k. - Cc mi ni chu ko v chu un phi s dng lin kt ma st (bulng cng cao); - Cc mi ni thp gc, bn cnh gm hai thp gc, mi thp gc mt bn ca cu kin chu un - Bn tp c chiu dy khng nh hn 8mm.

2.6.2.1 Mi ni chu ko nn Mi ni ca cc cu kin chu ko phi tha mn yu cu sc khng ko ca cu kin lin kt (2-25) v (2-26). Mi ni ca cc cu kin chu nn c th c thit k cho khng nh hn 50% sc khng tnh ton thp hn ca cc mt ct ghp ni (hnh 2-7).

Hnh 2-7. Mi ni cu kin chu ko-nn


392.6.2.2 Mi ni ca cu kin chu un

Hnh 2-8. Mi ni cu kin chu un

Mi ni in hnh l mi ni dm ch I (hnh 2-8) bao gm mi ni sn dm v mi ni cnh dm. a. Mi ni cnh dm: Trong mi ni cnh dm ca dm ch I s c mt cnh dm chu ko v mt cnh dm chu nn. Khi tnh ton quan nim cnh dm chu ton b mmen xc nh theo phn 2.6.1 hoc c th tr i phn mmen do sn dm chu, nh vy mi cnh chu mt lc ko hoc nn dc trc c tr s bng mmen chia cho khong cch gia trng tm ca hai cnh. Vi cnh dm chu ko mi ni phi tha mn iu kin (2-25) v (2-26). Vi cnh dm chu nn mi ni phi tha mn iu kin: Pr = c Pn (2-28) trong : c h s sc khng i vi nn, ly theo 6.5.4.2 Quy trnh; Pn sc khng nn danh nh (s nghin cu phn kt cu chu nn). Quy trnh cng khuyn l trong kt cu chu un khng nn dng mi ni bulng cho bn cnh vi cc mi ni cng trng v bt k bn cnh no cng khng c qu mt mi ni trong mt mt ct ngang. b. Mi ni sn dm: Khi ghp ni bng bulng sn dm phi c ghp ni i xng vi hai bn ni (hay cn gi l bn tp) hai bn. Cc bn ni cho lc ct phi c chiu cao bng chiu cao sn dm do lc ct phn b trn ton b chiu cao sn v vi mt ct ch I th ng sut tip mp trn v di sn dm c gi tr ng k so vi ng sut tip ln nht. mi bn ca mi ni c khng c t hn hai hng bulng. Vi cc mi ni sn dm khi b dy chnh lch nh hn hoc bng 2mm th khng cn c bn m.


40 Quy trnh quy nh cc bn ni sn dm v cc lin kt (bulng) phi

c tnh trng thi gii hn cng cho: - Mt phn mmen tnh ton bng tch s gia mmen xc nh theo 2.6.1 nhn vi t s gia mmen qun tnh ca sn dm vi mmen qun tnh ca ton b din tch tit din. - Lc ct quy c (trong quy trnh gi l lc ct tng tng) ly bng lc ct do ti trng tnh ton nhn vi mt h s. H s ny bng t s gia mmen thit k xc nh theo phn 2.6.1 chia cho mmen gy ra bi cc ti trng tnh ton v mmen gy ra do lc ct (mmen ny ly bng tch ca lc ct vi khong cch t trng tm nhm bulng mt bn mi ni n lc ct). - Mmen do s lch tm ca lc ct quy c bng tch ca lc ct quy c vi khong cch t trng tm nhm bulng ca mi ni n lc ct ny, tc l n mt tip xc gia hai sn dm. trng thi gii hn cng ng sut un trong cc bn ni khng c vt qu cng chy nh nht quy nh ca bn ni.

2.6.3 Mi ni hn Cc mi ni hn c thit k chu mmen, lc ct hoc lc dc thit k xc nh nh trong phn 2.6.1. Cc b phn chu ko v nn c th c ni ghp bng mi ni hn i u ngu hon ton, trnh s dng cc bn ni v vy trong mi ni dm I c th cnh dm dng mi hn cn sn dm ghp ni bng bulng. Cc mi hn cng trng cn hn ch vic hn t th nga mt. Khi hn i u cc tm c b rng khc nhau phi c s chuyn tip i xng nh trn hnh 2-9. Khi hn i u cc tm c b dy khc nhau, tm dy hn phi c mi hoc bo vi mt dc u khng ln hn 1: 2,5.


41 Bn knh 610 mm

u bn knh cong

u ni

(a) Chi tit chuyn i b rng

Chiu rng bn rng hn

u ni

Chiu rng bn hp hn

(b) Chuyn i vut thng

Bn knh 610mm

u ni

Chiu rng bn hp hn

(c) Chuyn i theo bn knh 610mm

19

8,7

2,4

Hnh 2-9. Chuyn tip tit din ti v tr hn


42Chng 3

CU DM

3.1 Khi nim chung Khi nghin cu v cu thp theo quy trnh Thit k cu cng theo cc trng thi gii hn xt ba loi cu dm: - Cu dm bn hay cu dm c l cc cu m gia dm thp v bn mt cu (c th bng b tng, g hay thp) ch c cc lin kt chng x dch tng i, khng c lin kt chng trt (neo). cu dm bn mt cu khng tham gia chu un cng vi dm ch. Hin nay cc cu ny ch dng lm cu tm phc v thi cng hoc cc cu c thi gian khai thc ngn. - Cu lin hp dm thp bn b tng ct thp (gi tt l cu lin hp) gm c bn mt cu bng b tng ct thp, dm ch bng thp, gia bn v dm c neo lin kt chc chn to thnh mt kt cu lin khi. Trong cu lin hp bn mt cu tham gia lm vic vi dm ch. Hin nay vi nhng cu c b rng khng qu ln (hn 20m) trong iu kin tnh khng thng thuyn cho php ngi ta hay p dng cu lin hp hai dm ch, khi khong cch gia hai dm ch c th t 3m n 14m, dm ngang vi khong cch khong 4m c y ln cao ngang vi mt cnh trn dm ch. Neo lin kt c b tr c dm ch v dm ngang, khi ty khong cch gia hai dm ch v khong cch dm ngang m bn mt cu c th lm vic nh bn k trn 4 cnh hoc trn hai cnh (hai dm ngang). Cu lin hp hai dm ch tit kim thp, gim thi gian thi cng v c gi thnh thp nn c dng kh ph bin cho c kt cu nhp gin n v lin tc. - Cu c bn trc hng l cu c mt bng thp c tng cng bng cc sn tng cng dc v ngang. Bn mt cu tham gia chu un vi dm ch. u im ca cu c bn trc hng l trng lng mt cu nh nn thng dng cho cc cu c khu ln. Cu dm thng c ba b phn chnh: mt cu, dm ch v h lin kt. Mt cu c nghin cu trong mn hc Tng lun cu, y ch xt cu to ca dm ch v h lin kt, trong nhng khi nim nghin cu phn trc s khng c nhc li.

3.2 Cu to ca dm ch

3.2.1 T l cu to chung Dm chu un phi c cu to theo t l sao cho:

9,0II

1,0y

yc (3-1)

trong :


43 Iyc mmen qun tnh ca bn cnh chu nn i vi trc thng ng y (hnh 3-1); Iy mmen qun tnh ca mt ct dm thp i vi trc thng ng y. Ngoi ra chiu dy ca cnh dm, sn dm, tr sn dm ca thp I cn khng c nh hn 8mm, cn chiu dy sn dm ca thp I hoc U cn khng nh hn 7mm.

Hnh 3-1. Mt ct ngang dm I

3.2.2 Chiu cao dm Vi cu dm bn hoc dm c: chiu cao ca dm thp khng nh hn 0,025L, trong L l chiu di nhp. Vi cu lin hp: chiu cao ca ring dm thp, nhp gin n h 0,033L; nhp lin tc h 0,027L. Chiu cao ton b ca dm lin hp (bao gm chiu cao dm thp v chiu dy bn b tng k c chiu cao vt), vi nhp gin n h 0,04L; vi nhp lin tc h 0,032L. Ch : Cc quy nh trn p dng cho dm c chiu cao khng i. Khi dm c chiu cao thay i phi hiu chnh cc gi tr c tnh n nhng thay i v cng tng i ca cc mt ct chu mmen dng v m hoc c th ly theo ch dn sau y: chiu cao tr gia h = (0,05 0,07)L, chiu cao gia nhp chnh v trn hai u h = (0,02 0,025)L.

3.2.3 mnh ca sn dm Sn dm phi c cu to sao cho: - Khi khng c sn tng cng dc (sn tng cng theo chiu dc dm):

cw

c

fE77,6

tD2

(3-2)

- Khi c sn tng cng dc:

cw

c

fE63,11

tD2

(3-3)

trong : tw b dy sn dm;

y

x


44 Dc chiu cao vng chu nn ca sn dm trong giai on n hi (mm);

fc ng sut trng tm cnh chu nn do ti trng tnh ton (MPa).

3.2.4 Chiu cao vng chu nn ca cc mt ct khng lin hp

3.2.4.1 Mmen chy v mmen do Mmen chy ca mt ct khng lin hp (My) l mmen cn thit gy ra chy u tin mt trong cc bn cnh khi khng xt bt k s chy no sn dm ca mt ct lai (dm lai hay dm kt hp l dm thp m sn dm c lm bng thp c cng chy do ti thiu quy nh thp hn cng trn ca mt hoc c hai cnh dm). Ni cch khc, mmen chy l mmen m ng vi n, im u tin trn mt ct c ng sut ln nht t ti cng chy. Mmen do (Mp) ca mt mt ct khng lin hp l mmen tng cng ca cc phn trn mt ct khi ton mt ct t n trng thi chy.

3.2.4.2 Chiu cao chu nn trong min n hi Trc trung ha ca mt ct khi vt liu lm vic trong giai on n hi l trc i qua trng tm ca mt ct. V tr ca trc trung ha c xc nh t iu kin tng mmen tnh ca cc phn din tch ca mt ct i vi trc trung ha bng khng. Khi xc nh c v tr trc trung ha d dng xc nh c chiu cao vng chu nn ca sn dm (Dc).

3.2.4.3 Chiu cao chu nn ca sn dm ng vi mmen do Nu Fyw Aw Fyc Ac - Fyt At th

)AFAFAF(FA2

DD cycwywtytyww

cp += (3-4)

Ngc li th Dcp = D. Trong : Dcp chiu cao chu nn ca sn dm ng vi mmen do; D chiu cao sn dm (mm); At din tch cnh chu ko (mm2); Ac din tch cnh chu nn (mm2); Aw din tch sn dm (mm2); Fyc cng chy nh nht quy nh ca cnh nn (MPa); Fyt cng chy nh nht quy nh ca cnh ko (MPa); Fyw cng chy nh nht quy nh ca sn dm (MPa).


453.2.5 Chiu cao vng chu nn ca cc mt ct lin hp Trong mt ct lin hp quy nh ti trng di hn trc khi b tng bn t c 75% fc (ng sut trong bn cnh chu nn do ti trng tnh ton) ch do tit din dm thp chu v c gi l tnh ti giai on I. Ti trng di hn (tnh ti giai on II) v hot ti do tit din lin hp chu.

3.2.5.1 Trnh t cht ti ng sut n hi mt v tr bt k trn mt ct lin hp do ti trng tc dng phi bng tng cc ng sut gy ra bi cc lc ring r tc dng vo: - Dm thp (tnh ti giai on I). - Mt ct lin hp ngn hn (cc ti trng tc dng ngn hn nn t bin cha kp pht sinh nh hot ti, nhit ). - Mt ct lin hp di hn (cc ti trng giai on II c thi gian tc dng lu di nh tnh ti giai on II, co ngt ca b tng, iu chnh ni lc phi xt n t bin trong b tng bn).

3.2.5.2 Mt ct c mmen un dng Khi tnh ng sut giai on II phi dng mt ct lin hp, c tit din dm thp, ct thp dc v mt ct tnh i ca bn b tng (trong phm vi b rng cnh tham gia lm vic vi dm ch gi l b rng hiu dng). - Vi mt ct lin hp ngn hn:

n

AA bt = (3-5)

- Vi mt ct lin hp di hn:

'n

A'A bt = , vi n = 3n (3-6)

Trong : At, At din tch tnh i ca b tng sang thp khi khng xt t bin v khi c xt t bin; Fb din tch tit din bn; n, n t s mun n hi ca thp v b tng khi khng xt t bin v c xt t bin. i vi b tng thng thng, n c th ly nh sau: 16 fc < 20, n = 10 20 fc < 25, n = 9 25 fc < 32, n = 8 32 fc < 41, n = 7 41 fc, n = 6 trong fc l ng sut trng tm cnh chu nn do ti trng tnh ton (MPa).


46 Qua nhng quy nh trn c th nhn thy tnh c t s mun n

hi cn phi thc hin php tnh lp, vi bc tnh u tin c th ly nh sau:

b

t

EEn =

vi Et = 200000 MPa, 'fy043,0E c5,1

cb = ,

trong : yc khi lng ring ca b tng, vi b tng thng thng c th ly yc = 2400 kg/m3; fc cng quy nh ca b tng tui 28 ngy, trong cu lin hp c th ly fc = 30 MPa (tt nhin ngi thit k c th quy nh mt fc no khc 30 MPa).

3.2.5.3 Mt ct c mmen un m tnh ng sut do mmen un gy ra c giai an II khng phn bit di hn hay ngn hn c xt t bin hay khng xt t bin mt ct lin hp bao gm mt ct dm thp v ct thp dc trong phm vi b rng hiu dng. Ct thp ti thiu trong vng mmen m: Trong cc min chu mmen m ca bt k nhp lin tc no, tng din tch mt ct ngang ca ct thp dc khng c nh hn 1% tng din tch mt ct ngang ca bn hay ni cch khc hm lng ct thp dc khng c nh hn 1%. Ct thp dc phi c cng chy do nh nht khng nh hn 420MPa v c c khng ln hn thanh N019 (l thanh c ng knh 18,8mm). Ct thp dc phi c b tr lm hai lp v phn b u trn sut b rng bn. Hai phn ba s lng phi t lp trn, khong cch gia cc ct thp trong mi lp khng c qu 150mm. C th b tr neo chng ct min chu un m. Khi khng c neo chng ct th thng thng ct thp dc phi c ko di n min un dng v vt qu neo chng ct mt on khng nh hn chiu di khai trin. Chiu di ny khng nh hn: - Chiu cao hu hiu ca mt ct dm; - 12 ln ng knh ct thp; - 0,0625 ln chiu di tnh ton ca nhp.

3.2.5.4 B rng hu hiu ca bn b tng ct thp B rng hu hiu l b rng bn b tng tham gia lm vic vi dm ch. a. i vi dm trong: B rng hu hiu c ly l gi tr nh nht ca: - 1/4 chiu di nhp hu hiu (vi dm gin n ly bng chiu di nhp thc t, vi dm lin tc bng khong cch gia cc im thay i mmen un tc l im un ca biu mmen un do ti trng thng xuyn).


47- 12 ln b dy trung bnh ca bn b tng cng vi s ln hn ca b dy sn dm hoc 1/2 b rng cnh trn dm thp. - Khong cch trung bnh ca cc dm k nhau. b. i vi dm bin: B rng hu hiu c ly bng 1/2 b rng hu hiu ca dm trong cng thm tr s nh nht ca: - 1/8 chiu di nhp hu hiu. - 6 ln chiu dy trung bnh ca bn cng vi s ln hn ca hai i lng: 1/2 chiu dy sn dm hoc 1/4 chiu rng cnh trn ca dm ch. - Chiu rng ca phn hng.

3.2.5.5 Mmen chy Mmen chy (My) mt ct lin hp c ly bng tng cc mmen tc dng vo dm thp, mt ct lin hp ngn hn v di hn gy ra trng thi chy u tin mt trong hai cnh dm thp (khng xt n chy sn dm ca mt ct lai). K hiu: My1 mmen un mt ct xt do tnh ti tnh ton giai on I; My2 mmen un mt ct xt do tnh ti tnh ton giai on II, co ngt...; My3 mmen un mt ct xt do ti trng ngn hn. Khi : 3y2y1yy MMMM ++= (1)

Khi b tr mt ct ta s tnh c My1, My2, cn cn phi xc nh My3. ng sut mp trn v di ca dm thp do My1, My2, v My3 nh sau: - Do My1:

dItt

1yd1t

tIt

t

1yt1t yI

M,y

IM

==

- Do My2:

dIItt

2yd2t

tIIt

t

2yt2t 'y'I

M,'y

'IM

==

- Do My3:

dIItt

3yd3t

tIIt

t

3yt3t yI

M,y

IM

==

T c: y

t3t

t2t

t1t F=++ (2)

yd3t

d2t

d1t F=++ (3)


48 Trong cc phng trnh (2) v (3) u c n s l My3. Gii h phng

trnh ny v ly gi tr nh hn ca My3 tnh ra mmen chy ca mt ct theo cng thc (1).

3.2.5.6 Mmen do Mmen do (Mp) l mmen ng vi khi ton mt ct tr phn b tng chu ko t n cng chy. Khi tnh mmen do phi ly mmen i vi trc trung ha do. xc nh trc trung ha do c th tin hnh theo trnh t sau: - Gi nh v tr trc trung ha do, chng hn nm trn sn dm khi s c mt n s l khong cch t trc trung ha do n mp trn hoc mp di dm thp. - Tnh lc do ca cc phn: cnh dm thp, sn dm thp, ct thp dc (trong tnh ton c khi b qua ct thp dc), b tng vng nn. Lc do mi phn bng din tch ca phn nhn vi cng chy tng ng. Lc do trong phn b tng chu nn c tnh da trn c s: quan h t nhin gia ng sut trong b tng chu nn v ng bin c th coi nh mt khi ch nht tng ng c cnh bng 0,85 fc phn b trn mt vng gii hn bi mt ngoi cng chu nn ca mt ct v ng thng song song vi trc trung ha cch th chu nn ngoi cng mt khong cch a = 1 c. Khong cch c phi c tnh vung gc vi trc trung ha. H s 1 ly bng 0,85 i vi b tng c cng khng ln hn 28MPa. Vi b tng c cng ln hn 28MPa th 1 gim i theo t l 0,05 cho tng 7MPa vt qu 28MPa, nhng khng ly nh hn 0,65. Ch : c khong cch t th chu nn ngoi cng n trc trung ha (mm) nu trc trung ha ri vo bn b tng ct thp. Trng hp trc trung ha khng ri vo bn th c c th c ly l: + Chiu dy bn b tng nu bn khng c vt hoc xem nh b qua phn b tng chu nn vt; + Chiu dy bn b tng tng ng ly bng din tch bn v vt chia cho b rng cnh b tng ca dm. fc cng nn quy nh ca b tng tui 28 ngy (MPa). - Cn bng cc lc do cc phn ko v nn s xc nh c n s khi gi nh v tr trc trung ha. Kim tra li v tr trc trung ha theo n s tm c, nu ph hp th ngng tnh ton, nu khng phi gi nh li v tr trc trung ha v lp li qu trnh tnh trn. Cng c th dng cng thc phn (b) ca 3.2.5.7 tnh ra chiu cao chu nn ca sn dm (Dcp) t xc nh c v tr trc trung ha do.

3.2.5.7 Chiu cao sn dm chu nn C hai chiu cao chu nn ca sn dm: chiu cao ng vi mmen n hi (Dc) v chiu cao ng vi mmen do (Dcp).


49a. Vi mmen un n hi Vi mt ct c mmen un dng, v tr trc trung ha l trng tm ca tit din tnh i. Khi c trc trng ha n hi d dng xc nh c chiu cao chu nn ca sn dm Dc. Vi mt ct c mmen un m, xc nh v tr trc trung ha cho tit din gm dm thp v ct thp dc trong bn sau tnh ra Dc. b. Vi mmen do Chiu cao chu nn ca sn dm (Dcp) c th xc nh c nh trc trung ha do (xem phn 3.2.5.6), tuy nhin cng c th xc nh theo cc cng thc cho trong quy trnh. - i vi cc mt ct chu mmen un dng khi trc trung ha do i qua sn dm:

+

= 1

AFAFA'f85,0AFAF

2DD

wyw

ryrsccyctytcp (3-7)

trong : D chiu cao sn dm thp (mm); At, Ac din tch cnh chu ko, cnh chu nn (mm2); Aw din tch sn dm (mm2); Ar din tch ct thp dc (mm2); Fyt, Fyc cng chy nh nht quy nh ca cnh chu ko, cnh chu nn (MPa); Fyr - cng chy nh nht quy nh ca ct thp dc (MPa); Fyw - cng chy nh nht quy nh ca sn dm (MPa); fc - cng nn quy nh ca b tng tui 28 ngy (MPa); As din tch bn (mm2). - i vi tt c cc mt ct chu un dng khc Dcp phi ly bng khng v coi nh yu cu mnh trong cc mt ct c chc tha mn. - i vi cc mt ct chu mmen un m khi trc trung ha do i qua sn dm:

( )cycwywtytwyw

cp AFAFAFAF2DD += (3-8)

- i vi cc mt ct chu mmen un m loi khc, Dcp = D.

3.2.6 Cu to ca sn tng cng Sn tng cng gm cc tm thp ct t thp bn hoc thp gc, c hn hay lin kt bulng vo mt hoc c hai bn sn dm.


50 C hai loi sn tng cng dm ch: sn tng cng ng v sn tng

cng dc (trc y gi l sn tng cng ngang) (hnh 3-2). Do tnh cht lm vic khc nhau, sn tng cng ng c phn ra hai loi: sn tng cng gi v sn tng cng ng trung gian. m bo an ton khi vn chuyn v lao lp, quy trnh quy nh: Khi khng c sn tng cng dc, phi b tr sn tng cng ng nu:

150tD

w

> (3-9)

Hnh 3-2. Sn tng cng v lin kt ngang

v khong cch gia cc sn tng cng ng (d0) phi tha mn iu kin:

2

w0 t/D

260Dd

(3-10)

trong : tw chiu dy sn dm ch (mm); D chiu cao sn dm (mm); D/tw t s gia chiu cao v chiu dy sn dm, khi D/tw = 150 th d0 3D.

3.2.6.1 Sn tng cng ng trung gian Sn tng cng ng trung gian phi c lp kht cht vo cnh dm chu nn nhng khng cn phi p vo cnh dm chu ko. Cc sn tng cng ng lp lin kt ngang (vch ngang hoc khung ngang) phi c lin kt vo c hai cnh dm bng hn hoc bulng.

Sn tng cng gi Sn tng cng dc

Sn tng cng ng trung gian

Vch ngang Khung ngang


51 Khong cch gia u ca mi hn sn tng cng vo sn dm ch v mp gn ca ng hn bn cnh vo sn dm phi nm trong khong t 4tw n 6tw, trong tw l chiu dy sn dm. a. Chiu rng sn tng cng bt (hnh 3-3)

Hnh 3-3. Sn tng cng ng trung gian

Chiu rng (bt) ca sn tng cng ng trung gian phi tha mn iu kin:

+

ptf

yspt

t16bb25,0

FEt48,0b

30d50

(3-11)

trong : d chiu cao tit din thp (mm); tp chiu dy sn tng cng (mm); Fys cng chy nh nht quy nh ca sn tng cng (MPa); bf chiu rng ton b ca cnh rng hn ca dm thp trn mt mt ct (mm). b. Mmen qun tnh (iu 6.10.8.1.3) Mmen qun tnh ca bt k sn tng cng ng no u phi tha mn iu kin: It d0 3wt J (3-12) vi

0,52,0d

D2,5J

2

o

p

= (3-13)

trong : It mmen qun tnh ca sn tng cng ng quanh mp tip xc vi bn bng i vi cc sn n v quanh trc gia chiu dy ca bn bng i vi cc sn kp (mm4); tw chiu dy bn bng (mm); do khong cch gia cc sn tng cng ng (mm);

bf

bt

tw bt


52 Dp chiu cao bn bng i vi cc bn bng khng c cc sn tng

cng dc hoc chiu cao ln nht ca panen ph i vi cc bn bng c cc sn tng cng dc (mm). Khi trn dm c c sn tng cng ng v dc, mmen qun tnh ca sn tng cng ng trung gian cn phi tha mn iu kin:

10l

tt Id3

DbbI

(3-14)

trong : bl chiu rng ca sn tng cng dc (mm); Il mmen qun tnh ca din tch hiu dng ca sn tng cng dc ly i vi trc tip xc vi sn dm ch (mm4); D chiu cao bn bng (mm). c. Din tch mt ct ngang: (6.10.8.1.4) Sn tng cng ng trung gian cn phi tha mn iu kin v mt ct ngang:

ys

yw2w

r

uws F

Ft0,18

VV)C0,1(BDt15,0A (3-14a)

trong : Vr sc khng ct tnh ton, xc nh trong iu 6.10.2.1 Quy trnh; Vu lc ct do ti trng tnh ton trong trng thi gii hn cng (N); As din tch ca sn tng cng, c ly bng tng din tch cc sn trong trng hp sn kp (mm2); B h s ly bng 1 cho sn kp; 1,8 cho sn n bng thp gc; v 2,4 cho sn n bng thp bn; C t s gia ng sut un ct v cng chu ct quy nh trong iu 6.10.7.3.3a Quy trnh; Fyw cng chy nh nht quy nh ca sn dm; Fys cng chy nh nht quy nh ca sn tng cng.

3.2.6.2 Sn tng cng gi Sn tng cng gi chu phn lc gi v cc ti trng tp trung khc nn cn phi t sn tng cng gi v tr gi v ti cc v tr c ti trng tp trung m : Vu 0,75 bVn (3-15) trong : Vu lc ct do cc ti trng tnh ton (N); b h s sc khng i vi gi; Vn sc khng danh nh (N) ca sn dm (xem phn sau).


53 Sn tng cng gi cn c b tr c hai bn sn dm v cng kht vi c hai cnh dm ch cng tt. u sn tng cng phi c mi lp kht vo cnh dm hoc dng ng hn rnh ngu hon ton. a. Chiu rng sn tng cng gi: phi m bo iu kin

ys

p1 FEt48,0b (3-16)

trong : tp chiu dy sn dm (mm); Fys cng chy nh nht quy nh ca sn tng cng (MPa). b. Mt ct hiu dng ca sn tng cng gi: Sn tng cng gi chu nn v c xem nh l mt ct chu nn. Khi sn tng cng lin kt bng bulng vo sn dm, mt ct hiu dng ch bao gm cc cu kin ca sn tng cng. Khi sn tng cng hn vo sn dm ch, mt ct hiu dng bao gm tt c cc cu kin ca sn tng cng cng vi di nm trung tm sn dm ko di ra khng qu 9tw sang mi bn tnh t tim nu c mt i sn tng cng, tnh t mp nu c nhiu hn mt i sn tng cng (hnh 3-4).

Hnh 3-4. Mt ct hiu dng ca tit din chu nn sn tng cng gi lin kt hn

Din tch sn dm khng c tnh vo mt ct hiu dng chu nn ti cc tr trung gian ca cu lin tc trong cc b phn lai ca nhp khi:

7,0FF

yt

yw < (3-17)

trong : Fyw cng chy nh nht quy nh ca sn dm (MPa); Fyt s ln hn trong cc cng chy nh nht quy nh ca cnh dm (MPa). c. Khong cch gia cc sn tng cng ng u dm: Quy trnh quy nh khong cch gia cc sn tng cng ng u dm: - Khng vt qu 1,5 ln chiu cao sn dm khi khng c sn tng cng dc.

Sn tng cng

Sn dm ch

tw

Sn tng cng

tw

9tw 9tw 9tw 9tw

Sn dm ch


54- Khng vt qu 1,5 ln chiu cao mnh sn dm ln nht khi c sn tng

cng dc. Cc quy nh ny khng p dng cho mt ct lai.

3.2.6.3 Sn tng cng dc Khi sn tng cng ng khng m bo n nh cho sn dm ch th cn thit phi b tr thm sn tng cng dc. Sn tng cng dc c th l thp bn hn vo mt bn sn dm hoc thp gc lin kt bulng vo sn dm. Khong cch t mp trong ca cnh dm chu nn n sn tng cng dc l 2Dc/5 vi Dc l chiu cao phn chu nn ca sn dm mt ct c ng sut nn do un sinh ra c gi tr tuyt i ln nht. a. Chiu rng sn tng cng dc: phi tha mn iu kin

yc

s1 FEt48,0b (3-18)

trong : ts chiu dy ca sn tng cng dc (mm); Fyc cng chy nh nht quy nh ca cnh chu nn (MPa). b. Mmen qun tnh: Cc c trng mt ct ca sn tng cng tnh theo din tch hiu dng bao gm c sn tng cng dc v di trung tm ca sn dm khng vt qu 18tw. Mmen qun tnh ca sn tng cng dc phi tha mn iu kin:

13,0

Dd4,2tDI

203

ws (3-19)

Bn knh qun tnh ca din tch hiu dng phi m bo:

E

Fd234,0r yc0 (3-20)

trong : Is, r mmen qun tnh v bn knh qun tnh ca tit din hiu dng (mm4, mm); D chiu cao sn dm ch (mm); d0 khong cch gia cc sn tng cng ng (mm); tw chiu dy sn dm ch (mm); Fyc cng chy nh nht quy nh ca cnh nn (MPa).


553.3 Cu to ca h lin kt Trong cu dm thng c hai h lin kt: lin kt ngang bao gm vch ngang, khung ngang; v h lin kt dc. Nhim v ca h lin kt l: - Lin kt cc dm ch thnh mt h khng gian, bo m tnh bt bin hnh ca h, tng cng ngang cho kt cu nhp. - Chu cc ti trng ngang: lc gi ngang cu, lc ng t, lc ly tm khi cu nm trn ng cong - Truyn ti trng ngang xung gi.

3.3.1 Lin kt ngang Lin kt ngang c th l vch ngang hoc khung ngang (hnh 3-5).

Hnh 3-5. Lin kt ngang

Lin kt ngang c th t u kt cu nhp v cch qung theo nhp (lin kt ngang trung gian). S cn thit ca lin kt ngang trung gian phi c xem xt trong cc giai on vn chuyn, lao lp v trong giai on khai thc. Cn xem xt cc ni dung chnh sau: - S truyn ti trng ngang t y dm ti mt cu v t mt cu truyn ti gi. - S n nh ca cnh di dm khi chu nn di tc dng ca ti trng. - S n nh ca cnh trn dm ch khi cha hon thin bn mt cu. - S phn b ca tnh ti v hot ti thng ng cho cc dm v lin kt ngang c tc dng phn phi iu ha ti trng cho cc dm ch. Cc b phn ca lin kt ngang phi m bo iu kin mnh truyn c ti trng ngang, c th l:

- Vi cc b phn chu ko rL 240, trong L l chiu di khng ging (mm);

r l bn knh qun tnh nh nht (mm) (iu 6.8.4 Quy trnh).

- Vi cc b phn chu nn r

KL 140, trong L v r nh trn, cn K l h s

chiu di hiu dng. Vi lin kt bulng hoc hn c hai u, K = 0,75; vi lin kt cht c hai u, K = 0,875 (iu 6.9.3 Quy trnh). Cc bn lin kt ca lin kt ngang phi c hn hoc bt bulng vo c cnh chu nn v cnh chu ko ca dm ch khi:

(a)

Vch ngang

(b)

(a) Khung ngang (b) Vch ngang


56- Lin kt ngang c gn ni vo bn lin kt hoc sn tng cng ng

thc hin chc nng nh cc bn lin kt. - Cc dm mt cu c gn ni vo bn lin kt, hoc sn tng cng ng thc hin chc nng nh cc bn ni. Khi khng c thng tin, cn thit k lin kt hn hoc bulng chu c ti trng nm ngang 90 kN i vi cu thng.

3.3.1.1 Lin kt ngang trong cu dm thng mt ct ch I Lin kt ngang bng dm thp nh hnh (thp cn) phi cao t nht bng na chiu cao dm v cng cao cng tt (trong phm vi chiu cao dm). Lin kt ngang u dm phi vung gc vi trc dm m bo truyn ht lc ngang xung gi, nu lin kt ngang u cho th phi xem xt thnh phn dc do lin kt ngang truyn ti. Khi cc tr u cho hn 200 th cc lin kt ngang trung gian phi b tr trc giao vi cu kin chnh. Nu cc tr u cho th lin kt ngang khng cn song song vi ng qua tim cc gi.

3.3.1.2 Lin kt ngang trong cu dm thng mt ct hnh hp Phi b tr lin kt ngang trong cc tit din hnh hp ti m, tr chng li chuyn v v cong vnh. Lin kt ngang phi c thit k truyn mmen xon v cc lc ngang t hp ti gi cu. Nu lin kt ngang l thp bn th phi c lin kt vo sn v cnh ca mt ct hp. Trn lin kt ngang cn b tr khong h rng ngi chui qua. Tc ng ca cc ca ny n ng sut trong tm vch cn c xem xt v cn nhc c cn gia c vch khng. Trong cc hp n phi c cc lin kt ngang trung gian v phi xem xt khong cch gia cc lin kt ngang hn ch bin dng ca mt ct ngang hp.

3.3.2 Lin kt dc Lin kt dc trc y c gi l ging gi cn quy trnh mi gi l h ging ngang. Lin kt dc phi c xt n c trong giai on thi cng v trong giai on khai thc. C nhng lin kt dc ch cn trong giai on thi cng v khng cn trong giai on khai thc th sau khi thi cng c th tho b v gi l lin kt dc tm. v tr cn thit nn b tr lin kt dc trong hoc gn mt phng cnh dm. Lin kt dc phi c thit k : - Truyn ti trng gi ngang xung gi. - Truyn cc ti trng ngang theo iu 4.6.2.8 Quy trnh. - Bo m tnh bt bin hnh ca kt cu nhp trong qu trnh ch to v lao lp.


57 Cc b phn ca lin kt dc phi m bo yu cu v mnh nh h lin kt ngang (xem 3.3.1). Cc bn nt (bn lin kt) ca h lin kt dc phi c lin kt vo sn dm ch. Khi sn dm ch c sn tng cng, khong cch thng ng t bn nt n cnh dm gn phi m bo: - Khng nh hn mt na chiu rng bn cnh gn. - Khng nh hn 150mm. u cc thanh ca h lin kt dc trn bn nt phi c khong cch ti thiu 100mm n sn dm v n bt k sn tng cng no. v tr c sn tng cng th bn nt ca h lin kt dc phi c nh tm trn sn tng cng d bn nt cng hoc khc bn vi sn tng cng. Nu bn nt cng bn vi sn tng cng th bn nt phi c lin kt vi sn tng cng. Khi c thit k cho lin kt dc chu ti trng ng t phi tham kho cc quy nh iu 4.6.2.8 ca quy trnh.

3.4 Neo chng ct Trong cu lin hp neo lin kt dm thp vi bn b tng ct thp thnh mt kt cu lin khi, do bn mt cu cng tham gia chu un vi dm ch. phn trc nghin cu cu to neo gm cc loi neo mm (ch to t thp trn), neo cng v neo inh (goujon). Trong quy trnh 22TCN-272-05 ph bin l neo ch [ v neo inh, cc neo ny va chng c trt ca bn b tng trn dm thp va chng c bc b tng khi dm thp. Vic b tr cc neo ph thuc vo nhp gin n hay lin tc v nu l nhp lin tc th ph thuc vo vng mmen m hay mmen dng. Xut pht t quy trnh c nhng ch dn nh sau: - Trn nhp gin n phi b tr neo chng ct trn sut chiu di nhp. - Trn cc nhp lin tc cng nn b tr neo chng ct trn sut chiu di nhp. Trong vng mmen m phi b tr neo chng ct ni m ct thp dc trong bn c xem l mt phn ca mt ct lin hp. Tuy nhin trong vng mmen un m m mt ct khng lin hp th khng cn b tr neo nhng cn b tr neo b sung cc vng biu mmen un do tnh ti c im un trnh hin tng chuyn t ngt t tit din lin hp sang tit din khng lin hp. - Trong vng mmen un m ca dm lin hp lin tc c b tr neo chng ct, ct thp dc trong bn phi c ko di vo vng mmen un dng v vt ra ngoi neo mt khong cch khng nh hn: + Chiu cao dm. + 12 ln ng knh danh nh ca ct thp; + 0,0625 ln chiu di nhp tnh theo khong cch tim gi.


583.4.1 Cu to neo

3.4.1.1 Neo ch [ Neo ch [ c t trn cnh dm nh trn hnh 3-6 v c hn vo cnh dm bng ng hn c chiu cao khng nh hn 5mm.

Hnh 3-6. Neo ch [

Chiu cao ln nht ca neo phi m bo neo c chn su t nht 50mm trong b tng v chiu dy lp b tng trn mt neo khng nh hn 50mm, c ngha chiu cao neo ch [ phi tha mn iu kin: 50 h hb -50 (3-21) trong : h chiu cao neo (mm); hb chiu dy bn b tng ti v tr neo (mm). Vi neo ch [ khi b tng cn m sao cho b tng tip xc trn ton b b mt neo tr phn t trn dm thp.

3.4.1.2 Neo inh Neo inh (hnh 3-7) c lm t cc loi thp trn ko ngui vi cc cp 1015; 1018 hoc 1020 kh mt phn hoc hon ton xy, tun theo tiu chun k thut i vi cc thanh thp ccbon gia cng ngui v c gii hn chy nh nht l 345MPa, cng chu ko l 400MPa. Neo inh gm thn inh v m inh, c hai u c mt ct l hnh trn. Nh c m inh m neo inh chng c c lc trt v lc bc.

Hnh 3-7. Neo inh

h 50

hb

h


59 Quy trnh quy nh t l gia chiu cao ton b v ng knh thn ca neo inh khng c nh hn 4. Theo chiu dc dm, bc ca neo inh (khong cch t tim n tim inh) khng c vt qu 600mm v khng nh hn 6 ln ng knh thn inh. Theo phng ngang, khong cch t tim n tim ca neo inh khng c nh hn 4 ln ng knh thn inh, ng thi khong cch t mp thn inh n mp cnh dm ch khng c nh hn 25mm. Chiu cao ca neo inh cng phi tha mn iu kin (3-21).

3.5 Tng quan v thit k cu dm chng I nghin cu nguyn l thit k theo h s ti trng v h s sc khng (LRFD) l: i i Qi Rn = Rr trong cc h s iu chnh ti trng i, h s ti trng i v h s sc khng c xt chng I, y cn phi xt ni lc hay ng sut Qi v sc khng danh nh ca b phn kt cu hay ca vt liu Rn. thit k mt cu dm, trc ht cn la chn cc kch thc chung s b sao cho tha mn cc yu cu v cu to, t xc nh c ni lc hay ng sut (Qi) do ti trng thng xuyn, do hot ti v cc nguyn nhn khc sinh ra, ng thi cng xc nh c sc khng danh nh hay tnh ton ca cc b phn kt cu kim tra theo iu kin trn. Cu dm thp c th l cu dm I, dm hp thng hoc cong, chng ny ch gii hn cho cu dm c mt ct ch I bng thp cn hay t hp i xng vi trc thng ng v trc dm thng hoc gn nh thng. Tha mn iu kin trn kt cu chu un c thit k theo cc ni dung sau: - Sc khng un theo trng thi gii hn cng . - Kim tra vng di hn trong trng thi gii hn s dng. - Kim tra trng thi gii hn mi v t gy ca cc cu kin v yu cu mi i vi sn dm. - Sc khng ct theo trng thi gii hn cng . - Kim tra tnh kh thi ca kt cu. Trc khi i vo nghin cu cc ni dung c th cn nghin cu thm mt s khi nim. - Dm lai: l dm thp c thit k vi thp sn dm c cng chy ti thiu quy nh thp hn ca mt hoc c hai bn cnh. - Tit din c chc: Tit din hay mt ct c chc l tit din c kh nng pht trin s phn b ng sut do hon ton khi chu un trc khi xy ra mt n nh. T y v sau s gi l mt ct c.


60- Tit din khng c chc: tit din hay mt ct khng c chc l tit din

c th pht trin cng chy do trong cc phn chu nn trc khi xy ra mt n nh un dc cc b nhng khng th chng li mt n nh cc b phi n hi khi yu cu c s phn b ng sut do hon ton trn c tit din. T y v sau s gi l mt ct khng c. Qua cc nh ngha trn c th nhn thy tit din c khi ton tit din t n mmen do trc khi mt n nh, tri li tit din khng c l tit din xy ra mt n nh trc khi t n mmen do. Yu cu v tnh do: Mt ct lin hp mun tin dn n mmen do Mp th b tng bn phi c bo v khng b nt v nh khng c thi cng tnh ti giai on hai khi b tng cha t cng theo yu cu. Quy nh di y nhm m bo yu cu trn: - i vi mt ct lin hp c chu mmen un dng do tc dng ca ti trng tnh ton gy ra ng sut trong mi bn cnh vt qu cng chy ca n nhn vi h s lai Rh th mt ct phi tha mn:

++=

5,7ttd'D

5'D

D

hs

p

(3-22)

Trong : = 0,9 i vi thp c Fy = 250 MPa = 0,7 i vi thp c Fy = 345 MPa th, ts = chiu cao vt b tng v chiu dy bn b tng Mt ct thc ca tit din chu un: trong cc cu kin chu un c th b qua cc l bt bulng cng cao hoc cc l h c ng knh khng qu 32mm min l din tch b khuyt i khng vt qu 15% din tch ca bn cnh . i vi mi din tch b tiu hao vt qu 15% din tch bn cnh th khi tnh ton phi tr phn din tch b tiu hao. i vi cu lin hp nu khng b tr lin kt dc di th khi tnh ton mt ct thc tnh mi sc khng, chiu rng cnh di phi c tr i hai ln bw l b rng chu lc gi ngang, bw c ly theo iu 6.10.3.5.1 ca Quy trnh.

3.6 Sc khng un theo trng thi gii hn cng Theo cc trng thi gii hn cng sc khng un tnh ton i vi mmen un v ng sut c tnh nh sau:

==

nfr

nfr

FFMM

(3-23)

trong :


61 f h s sc khng khi un ly theo phn 1-5; Mn sc khng un danh nh (Nmm); Fn sc khng un danh nh mi bn cnh (MPa). Ch : - Khng p dng cc quy nh v phn phi li mmen un cho nhp gin n. - Vi kt cu nhp lin tc c th dng cc quy nh n hi hoc khng n hi tnh ton trong trng thi gii hn cng . Ch c cc cu kin lin hp hoc khng lin hp vi mt ct ch I chiu cao khng i, mt ct c v c cng chy do nh nht khng vt qu 345MPa mi c php p dng cc phn tch khng n hi.

3.6.1 Phn loi sc khng un Khi dm c mt ct ch I tha mn cc iu kin cu to nh nu trn, nu cng chy do nh nht quy nh ca thp khng vt qu 345MPa v chiu cao mt ct khng i th phi kim tra mnh ca sn dm c mt ct c theo cng thc phn 3.6.1.1. Nu cng chy do nh nht quy nh ca thp ln hn 345MPa v chiu cao mt ct thay i phi kim tra mnh ca bn cnh chu nn mt ct khng c theo cng thc phn 3.6.1.3.

3.6.1.1 mnh ca sn dm c mt ct c Nu iu kin sau c tha mn:

ycw

cp

FE76,3

tD2

(3-24)

th sn dm c xem l c tc l ton b mt ct t n cng chy m cha xy ra mt n nh v tun theo cc ch dn sau: - i vi mt ct lin hp chu mmen un dng, sc khng un phi c xc nh theo phn 3.6.2.2 v sc khng un dng ca mt ct lin hp c. - i vi cc mt ct khc vic tnh ton phi thc hin theo phn 3.6.1.2 v mnh ca bn cnh chu nn ca mt ct c. Nu iu kin (3-24) khng c tha mn th sn dm khng c coi l c v nu khng s dng cng thc Q (Q l mmen tnh ca din tch bn tnh i ngn hn i vi trc trung ha ca mt ct lin hp ngn hn trong cc vng un dng hoc l mmen tnh ca din tch ct thp dc i vi trc trung ha ca mt ct lin hp trong cc vng mmen un m) th: - i vi cc mt ct lin hp chu mmen un dng phi xc nh sc khng un ca mi bn cnh theo mt ct khng c nh phn 3.6.2.4. - i vi cc mt ct khc phi tnh ton mnh ca cnh chu nn c mt ct khng c nh phn 3.6.1.3. Nu iu kin (3-24) khng c tha mn v c s dng cng thc Q th phi tun theo cc iu kin s dng cng thc Q nh phn 3.6.1.4.


623.6.1.2 mnh ca cnh chu nn mt ct c

Xt quan h sau:

ycf

f

FE382,0

t2b

(3-25)

trong : bf chiu rng bn cnh chu nn (mm); tf - chiu dy bn cnh chu nn (mm). Nu (3-25) tha mn th phi tnh s tc ng qua li ca sn dm mt ct c v bn cnh chu nn nh trong phn 3.6.1.5. Nu (3-25) khng tha mn th bn cnh chu nn khng c coi l c v: - Khi khng xt cng thc Q phi xt mnh ca bn cnh chu nn c mt ct khng c theo phn 3.6.1.3. - Khi c s dng cng thc Q th phi tnh ton theo phn 3.6.1.4 v iu kin s dng cng thc ny.

3.6.1.3 mnh ca cnh chu nn c mt ct khng c Xt quan h sau:

cf

f

w

cc

f

f

fE408,0

t2b

tD2f

E38,1t2

b

(3-26)

trong : fc ng sut trong bn cnh chu nn do tc dng ca ti trng tnh ton (MPa); tf, bf, Dc, tw nh nu trn. Nu (3-26) tha mn th phi tnh ton theo cc quy nh nh phn 3.6.1.8 v vic ging bn cnh chu nn c mt ct khng c. Ngc li th mt ct khng m bo iu kin n nh, cn chn mt ct khc v tnh li.

3.6.1.4 iu kin s dng cng thc Q Nu mt ct ang xt chu mmen un m v khng c sn tng cng dc th phi xt theo cc quy nh phn 3.6.1.7 v mnh ca sn dm theo cng thc Q theo cch ty chn ca bn cnh chu nn. Nu khng tha mn nh trn th phi tnh theo phn 3.6.1.3 v mnh ca cnh nn c mt ct khng c.

Khi khng c sn tng cng dc: Khi c sn tng cng dc:


633.6.1.5 Tng tc gia sn dm v cnh nn ca mt ct c Xt quan h sau:

ycf

f

ycw

cp

FE382,0

t2b

FE76,3

tD2

(3-27)

Nu (3-27) tha mn th phi tun theo quy nh ca phn 3.6.1.6 v vic ging bn cnh. Ngc li th phi tnh ton tc ng qua li gia sn dm v cnh nn ca mt ct c

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