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li' G. Curti" and A. W. Hendry, W. G. Curtin and Partners, Liverpool, England

Introduction

Our office, up to a few years ago, did not consider brickwork as a slruclural malerial. bul ralhe r as an obsolele malerial used by lhe early Viclorian engineers because Ihey had nolhing belter. Our firsl use of load-bearing brickwork happened more by accidenl Ihan designo We were appoinled consullanls to a large secondary sehool for which we designed a reinforced concrete frame and used 9" brick partition walls between the c1assrooms for acoustic and other reasoos. The c1assroom block was Ihree sloreys high, reclangular in plan with a central corrido r along the maio axis. Classrooms opencd off lhe corridor and were separaled by 9" brick cross walls. We checked lhe load-bearing capacity of lhe walls and found, lo Qur surprise, that we did not need the reinforced concrete frame. We were at first sceptical af our calculations since the slress in the waUs was aboul len limes lhe slress in lhe normal Iwo slorey house, which in England frequently have 9" brick walls. However, a detailed check of the calculation showed Ihal lhe walls were adequale, and we decided lo conslrucl lhe schoal wilhoul lhe frame, Ihus saving over 8 percenl of lhe cost af the school. During construction, we were surprised to find Ihal far from being a slow melhod of ereclion, lhe brick structure was actually a little fasteT than concrete con­slruclion. This happy experience led us to adopt brickwork as lhe slruclural malerial and lhe cross-wall as lhe slruclural syslem for severallarge schoals.

Mosl of Ihese schools were lhree sloreys high wilh cross-walls at 20 fl.-25 fI. cenlres wilh 1100r Iive loads of 60 Ib. per sq. ft. and own weighl of 80 - 90 Ib. per sq. fI.

In 1961, lhe archilecl for a women's residence hall ai lhe Universily College, Bangor, Norlh Wales, approached us

Ihrough lhe Clay ProducIs Technical Bureau lo discuss lhe feasibilily of conslrucling a six slorey building in 9" brick cross-walls. Delails of lhe building are shown in Figures 38-1 and 38-2 from which il will be seen Ihal lhe walls were ai 10 fI. cenlres. As lhe lolal 1100r tive and dead loading would be aboul 100 Ib. per sq. fi., we knew lha I lhis would be feasible and mighl reduce lhe lhickness of lhe upper slorey walls lu 4-1/2". Delailed calculalions based on lhe Brilish Slandard Code of Praclice C.P. 111 (l948), currenl ai lha I lime bul now superseded, showed Ihal lhe walls could be 4-1/2" from foundation to roof. Though the caJculations were relatively sim pie and within the provisions af the code , we had no experience af using such a slender wall. Furthermore , the research work carried out by the Building Research Station, 00

which lhe Co de was based , had been carried oul on 9" and 13-1/2" walls and piers. We had no cerlainly Ihal lhe results of lesls on 9" and 13-1 /2" walls would be applicable lo 4-1/2" walls, so il was decided lo confirm (or olherwise) lhe validily of lhe calculalions by carrying oul a series of lesls on 4-1 /2" walls. The results of these tests are summarized under Research Tesling.

These results confirmed Ihal lhe scheme was feasible and Ihal lhe load faclor under laboralory condilions was generally in lhe range 15 - 25. (The new code of praclice allows higher working slresses, and lhe load faclor is now Iherefore reduced). This faclor of safely would nol necessarily be obtained 00 site since workmanship and materiaIs could not be so c10sely supervised.

Though il was obvious Ihal lhe individual walls were safe, we had lo consider lhe problems of overaU slabilily under lhe action af wind forces. A series af cross-walls, acting alone, could suffer a 'house of cards' collapse. It was Ihoughl, correct1y as laler research showed, Ihal lhe provision of walls at right angles to the cross-walls, i.e. externai and corridor walls. would be more Ihan adequale lo preveni lhis. The effecl

329

330 Designing, Engineering, and Constructing with Masonry Products

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Figure 38-1. Typicalfloor plano University Co/lege, Bangor. Note: The load-bearing crosswalls are shown solid.

of wind forces acting parallel to the cross-walls was not quite so simple. If the two cross-walls connected by the corrido r slab were considered to act as a single structural entity similar to Vierendel girder, the wind stresses were insignificant. Since the corridor slab would form a very slender strut between the girder booms, the crass wal1s were then considered to act as separate cantilevers. Under this assumption the wind stresses, though now significant, were still tolerable. We believed that the actual wind stresses would be somewhere between the two calculated stresses. Tlus problem has since been developed extensively under tha heading of shear wall theory by many reasearch workers 1. The behaviour of interconnected shear walls has been considerably elucidated, but the applicability of elastic theory to brickwork structures still requires further study2

Research

Tests on 4-1/2"-thick storey-height brickwork panels were initiated in designing Bangor University College Hostel in 1961. These initial tests have beell described in deiai! elsewhere5, but a summary of the results and of those obtained from a second series of tests6 are given in Tables 38-1 and 38-2. These indica te lhe range of variables considered. Ali the tests were carried out in a specially designed loading frame in which the wall panels were loaded between sections of 4"-thick reinforced concrete slabs to give realistic end con­ditions and lo permit invesligation of the effect of bending moments introduced into the wall by loading on the floor slabs.

The conclusions fram these tests may be summarised as follows:

I. The typical wall failure mode was by transverse splitting although in eccentrically loaded walls ; spalling of the brick­work at the top of the wall was also observed .

2. Load factors on lhe British Slandard Code of Practice C.P. III were high , even on lhe revised (1964) version. There would appear to be scope for some increase of permissible slresses, especially for eccenlrically loaded walls.

3. The current limit for the heighl/thickness ratio of 18 could be safely increased , possibly to 24.

4. The effect of bending moments inlroduced inlO the wall panels by loading of the floor slabs on alternate sides of the wall appears to be insignificant.

5. Considerable increases in the strength ofbrickwork were achieved by placing light reinforcement in the horizontal joints, but further work is necessary to formulate design recommendations.

Tests on Seale-Model Stmctures

Lateral Resistance of Cross-wall. Following on tms work, it was considered desirable to study the lateral resistance af cross-wall structures. As this required studying the interaction between the various elemellts of the stmcture, a model technique was developed, using l/6th scale brickwork 7 lo preliminary experiments , tests on piers and walls , originaUy carried out at full-scale at the Building Research Station and elsewhere, were repeated at l/6th scale. These tesls confirmed that it was possible to reproduce on model scale the failure mo de of full-sized brickwork elements and to estimate their strength from the small-scale tests, taking into account brick strength, mortar strength, and other relevallt variables. Haviog thus established the validity of the model method, tests were conducted to study the strength of brickwork shear paoels

under varying amounts of precompression; the results of these tests are being reported in another chapter of this book.s

Strength and Rigidity of Multi-storey Cross-Wall Constructioo. The next stage was to study on model scale the strength and

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Design and Canstructian af Slellder Wall Brickwark Buildings 331

rigidity of a seetion of multi-storey, eross-wall construction. One of the test structufes after failure is shown in Figure 38-3. The strueture was subjected to lateral loading by hydraulic jacks at each storey height ; deflections were measured and eventually the structure was loaded to failure in the direetion of the longitudinal shear walls. The ultimate load behaviour was consistent with the previous work done 00 the shear panels , but it was found that existing shear wall theories did not give an accurate indication of deflections when applied to lhis structure. A simplified theory based on calculation of shear deflections from storey to storey , however, gives promising results and is being studied further.

Tests on Full-Scale Structures

The final phase of the work on slender cross-wall structures ~ to be a series of tests on full-scale structures. This will repeat some model work and is intended to give a final check on its validity. These tests will be carried out in a quarry so that laleralloads can be applied by jacking agajnst the rock face.

Calculations

The design calculations for this contracl , as for most other brick contracts, were relatively simple. But , as with most brickwork calculations. there can be a bewildering choice of

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brick strengths and mortar mixes. The brick and mortar strengths can be reduced as the structure rises. but this 1eads to buying in small quantities and increases supervision difficul­ties. It is corrunon procedure in Qur practice and most others in England to have two ar three brick and mortar strengths. The bricks, selected were 7 ,500-lb.-per-sq.-in. crushing strength brick (a second class engineering brick) from foundation to third floor and 5,000-lb.-per-sq.-in . (a good 'common' brick) from there to lhe roof. The mortar mixes were I :0:3 for foundation to second floor , 1: 1:6 second to third floor and 1:2:9 from lhere to the roof. These strengths have been reduced on subsequent contracts following the introduction of the 1964 code.

As a check on the validity of the calculations, strain gauges were flXed to a ground flaor wall, and recordings were taken as the building rose and was loaded. The results of this investigation showed that at most points, the measured strains were proportional to the calculated stresses3 .

In addition to tests on design , we wanted to carry out some form of quality contrai as construction proceeded . At that time , the only test was for the crushing strength of bricks, and a mortar test cube was being discussed by a committee of the British Standards Institution. We were concerned with the interaction of the morta r and brick and the influence of workmanship. We proposed the use of 9" brick and mortar cubes to be made by lhe bricklayer from the bricks and mortar he was using for the walls.

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and Figure 38-2. Sectional elevatio n. University College, Bangar.

332 Designing, Engineering, and Constructing with Masonry Products

Table 38-1 Summary of Test ResuIts 00 4Y," Storey-Height Wall Panels (After S. Prasan and R.E. Bradshaw)

Width of panels: 36:' Height of panels: 8'- 4': First Series

Penn. Strcngth

Averagc Design Ratio

Wall Brick Mortar Ultimatc comprcssivc Strcss Load brickwork Mode of

No. Strcngth Strcngth Load stress (C.P. 111 1964) Factor brick Loading Failurc Rcmarks lb/in 2 Ib/in 2 tons Ib/in 2 Ib/in 2

5640 1166 60 9 15 141 6.5 0 .16 Axial transverse 5/8" join!s (1:1:6) splitting bad work-

manship 2 5640 945 134 2040 141 14.5 0.36

(i :1:6) 4 5640 80 44 660 141 4.7 0.12 buckling very weak

(I: 1:6) mortar 5 5640 1960 137 2080 199 10.4 0.37 transverse

(I: 1:6) splitting 7 7500 945 170 2580 180 18.3 0.34 "

(i: I :6) 8 7500 630 142 2150 180 11.9 0.29

(i: I :6)

3 5640 1201 145 2205 141 15 .6 0.35 Axial + vertical (i :1:6) 30lb/ ft 2 chase 3'6" x

super 3/4" x 1/2" deep.

6 5640 2041 128 1946 199 13.8 0.34 Axial + (1 :0:3) ave. 80 Ib/ft 2

super IOA 5640 1350 204 3100 199 0.55 Axial none reinforced

(i :0:3) I in 1 10B 5640 1350 204T 3100 145 max. 0.55 Sway* %" crushing at rcinforccd

(i :0:3) ave. 86 ave. reloaded top I in 1 11 4923 2240 141 2138 178 12.0 0.43 Axial rejnforced

(i :0: I) 1 in 4 12 4923 2380 192 2910 178 16.3 0.59 reinforced

(i :0:2) I in 3 13 4923 980 104 1580 178 8.9 0.32 " transverse reinforced

(i :0:3) splitting I in 5

14 5640 840 140 2128 249 max. 14.7 0.38 Ecc. %" (i :0:3)

'" 15 5640 945 116 1700 249 max. 18.2 0.31 Ecc. 1" crushing at (I :0:3) ave. 97 ave+ !op before

splitting

16 5640 945 100 1520 249'" 12.8 0.27 Sway* %" crushing at (I :0:3) 119 ave+ top

17 5640 975 124 1886 249 max.'" 14.1 0 .33 Sway* %" crushing at (I :0:3) ave. 134 ave+ top before

splitting

t These are maximum permissible edge stresses and include the 25 percent increasc for bending stresses allowed by C.P.II I, 1964.

+ The average values have been ca1culated by dividing lhe total safe load by lhe wall area. The lotai safe load has becn de· termined by assuming that the maximum edge stress is the va lue marked <D and that the wall has no tensile resistancc.

* The calculations for permissible stresses for walls subjected to sway are based 00 an eccentric load equal to the sway.

Table 38-2 Summary of Test Results on 4V," Storey-Height Wan Panels (After R. Bradshaw)

Width of panels: 36" . Height of panels: 8 '4" . Second Series

Pcnnissible Strength Avcragc design fatio

Wall 8rick Mortar Ultimate compressive strcss Load Duration brickwork No. Strength Strcngth load strcss (C.P. 111 1964) factar of Loading brick Loading Age Remarks

Ib/in 2 Ib/in 2 tons lb/in 2 Ib/ in 2 mmutcs days

4825 660 53.6 835 132 6.3 90(9) 0.17 Axial 38 1:1:6 (t/22) mortar/cement/

Iime/sand 2 6235 2015 120 1630 217 7.5 90 to 92 0.26 Axial 46

(2/ 1) tons (12) (t/l7) 90 to 120 tons(18)

3 6235 1585 114.5 1790 217 8.2 105 to 114.5 0.29 Axial 18 Joint thick-(1 / I) tons (17) (1 /55) ness: 3/16 in.

4 6235 1540 135 2110 217 9.7 90 to 135 0.34 Axial 18 (3/1) tons (23) (t/40)

15to135

5 6235 2450 157 2450 21 7 11.3 30(82) 0.39 Axial 78 (3/1) (t/62)

6 6235 865 150 2330 217 10.7 30t090 0.37 Axial 2 (3/1/w) tons (48) ( l/67)

8 to 150 tons

7 6235 2335 172 2670 217 12.3 20 to 22.5 0.43 Axial 18 (3/1 /w) tons(17) (t/24)

60 to 172 tons (44)

8 6235 700 149.5 2320 217 10.7 60(39) 0.37 Axial 7 Reinforced (3/ I) (t/29) every second

course 9 6855 1400 20U 3100 235 14.3 30to ISO 0.45 Axial 148 Reinforced

3070 tons (39) every second 30 to 188 course tons (97) 20 to 200 tons (I 55)

10 6235 1100 132 2060 120 17.2 30(69) 0.33 eec. %" 6 (3/1/w) (t/ 5.5-

l/7.8)

11 6235 630 117 1825 120 15.2 30(61) 0.29 eec. %" 5 (t/5.5-t/3.6)

12 6235 765 120 1630 120 13.6 35 (47) 0.26 ~" 7 (3/2) offplumb

13 6855 695 131 2035 129 15.7 35 (58) 0.30 %" 8 = (3/2) • offplumb

14 3710 80S 81.5 1265 77 16.4 30(42) 0.34 %" 19 (3/3/w) offplumb

15 3710 770 70 1085 140 7.7 25 (43) 0.29 Axial 7 (3/2)

16 3710 940 67 1040 140 7.4 30 (35) 0.28 Axial 13 Mortar at (3/2/w) 17 days

'"-=

334 Designing, Engineering, and Constructing with Masonry Products

The results af the crushing tests on site cubes were compared with those obtained from tests on similar cubes made under laboratory conditions, but as this was lhe first time 9" brickwork cubes had been used , we had no yardstick against which to compare our results and Talher arbitrary acceptance cri teria had to be specified. This situation will soon be changed, however, since lhe British Ccramic Research Association , Structural Clay Products Ltd. and others in Bfilian are carrying oul cube testing programmes to establish lhe correlation betwecn lhe cube strength and lhe wall strength. It is possible that this test may become standard practice in Britain in lhe neaT future4.

At Bangor, cube testing had a useful side effect on the site since it had a psychological effect on the men. We experienced a !ittle difficulty with the bricklayers at the beginning of the contract since their work, though acceptable by normal brick building standards, was not acceptable by engineering stand­ards. After some discussion with lhe men (when they wcre shown photographs of the research work etc.) , work improved considerably and the men worked well and enthusiastically. As a resul! of this experience, we were satisfied that these measures together with regular supervision ensured a high standard of workmanship.

Construction

In addition to the normal problems of design and construc­tion, we were faced with severa] detailing problems. Provision for expansion was made by isolating each block and also providing an expansion joint across the centre of each block. We stiJ1 have no data on expansion joint requirements but would on most jobs provi de expansion joints at about 75 fI. centres.

Close liason was necessary with the services engineers, and they had to accept the discipline of detailed planning of services before construction began because we would not permit the bashing of holes through the slender walls. Provision for services was made by forming vertical ducts at the junction of cross and corridor walls. Pre-formed holes were left in the walls , and reinforcement was embedded in the brickwork around the holes to distribute the stresses. No chasing of the brickwork was permitted, and , because this prevented using normal electricaI conduits, plastic coated cables were stuck to the walls and plastered over.

The architecl , fortunately , realised there was a planning discipline in cross-wall construction in that the walls should remain in the same plane from foundation to roof. We have not always been so fortunale and where architects have not accepted such discipline there has been a resultant increase in structural cost and construction time.

As engineers, we are nalurally interested in the architectural expression of the structural formo In the pasl. this has been achieved by projecting the waUs and slab beyond the face of a curtain wall - trus tends to be expensive and on trus contract

Figure 38-3. Test on a l/6th scale model crosswall structure.

the face wall was an 11" cavity brick wall. This forms an economical cladding to the structure which is virtually maintenanee free and gives a good internaI environment Where the cross-wall has been projected beyond the curta in wall , there is liable to be damp penelration lhrough the wall The provision of a vertical damp proof eoucse is unlikely to be satisfaetory since the brickwork projecting externally beyond the damp proof course is liable to peel off. This problem has been solved on another conlract by building lhe wall pro­jection in engincering briek which is practically impervious.

Qur experience on lhe Bangor contraet confirmed our view that slender brick wall construction is economical . simple, and quick to construct and gives adequa te performance for residential buildings. We have since used this form of construc· tion (4-1/2" cross-waUs) for student hostels at Aberystwyth and Oxford Universities, at Leeds, Chester, and Rugby teacher training colleges and at Wrexham Technical College. We have also continued to use the 9" cross-waU 011 l1umerous school buildings where larger 1100r spans are involved.

References

I C II A d S 'th E St f C d "TaU Buildings'"· . ou, ., an ml , . a ,ar , Pergamon Press. 1967.

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Design and Construction of Slender Wall Brickwork Buildings 335

2. Sinha, B. P., "Model Studies Related to Load-Bearing Brickwork". Ph.D. Thesis, University of Edinburgh, 1967.

3. Stockbridge, J. G., "A Study of High-Rise Load-Bearing Brickwork in Britian", M. Arch. Thesis, University of Edinburgh, 1967.

4. "Model Specifieation for Load-Bearing Briekwork". The British Ceramie Researeh Assoeiation, Spec. Publ. 56, 1967.

5. Prasan, S., Hendry, A. W., and Bradshaw, R. E., "Crushing Tests on Storey-Height Walls 4-1/2" Thiek", Proc. British CeramicSoc., No. 4.,1967. pp. 67-80.

6. Bradshaw, R. E. and Hendry, A. W., "CTUshing Tests on Storey-Height Walls 4-1/2" Thiek: Further Results", Proc. British CeramicSociety. In Press, 1967.

7. Murthy, C_ K., and Hendry, A. W_, "Model Experiments in Load-Bearing Briekwork". Building Seienee, VaI. 1. No. 4., 1966 pp. 289-299.

8. Sinha, B. P. and Hendry, A. W., "Raeking Tests on Storey-Height Shear-Wall StTUetures"., ehapter 23, Design­ing, Engineering, and Constructing with Masollry Products, GulfPublishing Co., Houston, Texas, 1969.