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8/3/2019 Design Pilecap
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Seismic Pier Design
forSteel Pipe Pile Extensions
with Concrete Cap Beam
State of Alaska
Department of Transportation &Public Facilities
Bridge Section
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Overview
s The purpose of this document isto assist in the design anddetailing of Multiple Column /Pile Extension Piers
s The basis of this document is
founded on the Full-Scale Testof a Three Column / Pier CapBridge Substructure System
Under Simulated Seismic
Loading by Seible, et al.
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Typical Pier
Figure 1
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Step 1
s Collect the dead load forces in
the pier cap and columns due tostructure self weight, asphalt,
utilities, etc
s These forces should include:
P - axialVy and Vz - shearMy and Mz - moment
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Step 2s Collect the seismic forces in the
pier cap and columns frommultimodal computer analysis or
other methods
s These forces should include:
P - axialVy and Vz - shear
My and Mz - moment
s
Consider both Load CombinationI (100%L + 30%T) and LoadCombination II (100%T + 30%L)
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Step 3s Determine the combined axial,
shear and moment forces
s Note that the responsemodification factor, R, appliesonly to seismic moments inductile members (i.e. where
plastic hinges form)
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Step 4s Using the worst case load
combination, determine theamount of longitudinalreinforcement required in thecolumn, Asc
s Do not over reinforce the column
- this will lead to more cap beamand joint reinforcement
s Use factors
as defined inAASHTO
plastic strength
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Step 4s There are many computer
programs to aid in the design ofconcrete columns,
Recol_M Imbsen & Associates
ULTCOL Washington DOT
s Print out the P-M interactioninformation for later use in thecap beam design
s Note that AASHTO specifies acolumn reinforcement ratio
1% < < 4%
but 3% is a practical upper limitdue to joint reinforcementlimitations
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Step 5
s The ultimate applied shear, Vult,is the minimum of either thedesign EQ shear or the shearassociated with plastic hinging ofthe column, Vp
s Include the column overstrengthfactor for the concrete gap
portion of 1.3 and 1.25 for thesteel pipe when calculating Vp
s If the required moment capacity
of the column is close to thebalance moment, use thebalance moment in subsequentcalculations
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Step 5s The shear associated with plastic
hinging is calculated as shownbelow. It is good practice to use the
Vpfor design if practical
Vp = M1 + M2He
where:M1 = moment at top of column
= Mn * 1.3 - concrete columnM2 = moment at bottom of column
= 1.25 * Mp - steel pipeHe = effective height of column= H + lm (see figure 1)
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Step 6s Determine the size and pitch of
the spiral in the column
Vult < * Vn
where:
= 0.85 (16th) 0.9 (LRFD)
Vn = nominal shear capacitysee AASHTO code or
UCSD shear designequations
D = column / pile diameter
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Step 7
s Determine the amount of the capbeam steel required noting that:
s
the height of the cap beam, hb,must be greater than thedevelopment length of thecolumn longitudinal steel and
D < hb < D * 1.15
s the width of the cap beam, bj,must satisfy the following:
D + 12 in < bj < D + D/2
s Use the maximum overstrengthmoment of the column to loadthe cap beam (i.e., Mp at Pmax)
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Step 7
s The required development lengthof the longitudinal columnreinforcement is:
ld = 0.025*db*Fy/ (fc)
where:db = diameter of barFy = rebar yield strength (psi)
fc = concrete strength (psi)
s To use this length, welded hoop orspiral reinforcement must be usedin the joint (defined in step 9)
s Always extend longitudinal columnbars to the top of the cap beam
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Step 7
*Mn > MultWhere: = 0.9 for bending
Mn = nominal bending capacity
Mult = Mp + Mdl
Mp = plastic moment capacity of
column associated with PmaxCheck that the cap beam is not over orunder reinforced and that temperature andshrinkage steel requirements are satisfied
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Step 8
s Determine the size and spacingof shear stirrups required in thecap beam
Vult < * Vn
where: = 0.85 (16th ed.) 0.9 (LRFD)Vn = Vc + Vs
Vult = Vdl + Vp-capVp-cap = shear in cap beam
due to plastic hingingof column
= 1.5*Mp (approx.)
Scol
s Use shear at d from face ofcolumn
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Step 9
s Determine the size and spacingof welded hoops required in the
joint region of the cap beam
s This steel is needed to providedevelopment length andconfinement for the columnlongitudinal steel
s = 4*Ah < hbD*s 4
where:
s = welded hoop spacingAh = area of welded hoope.g. #5 hoop = 0.31 in2
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Step 9
s Continued
la = anchored length of Asc
Asc= Area of columnlongitudinal steel rebar
hb = height of cap beam
o = overstrength factor= 1.4
s = 0.3*o*Asc/ la2 > 3.5*(fc)/ FyD = core diameter of column
s Provide a cap beam height greater
than the anchorage length requiredfor the column longitudinal steel(see step7)
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Welded Hoop Steel
S < 4 * Ah < hb
D * s 4
Typically these bars will be field welded
after placement
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Step 10s Determine the average principal
tensile stress in the joint
pc,t = (fv+fh) + (fv-fh)2 + v2j
2 4
where:fv = Pc__
bj*(D+hb)
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Step 10s Continued
fh = Vc_bj
*hbvj = Mc _
hb*D*bjMc = moment in the columnVc = shear in the column
Pc = axial load in the columnD = column diameterhb = height of cap beam
bj = width of cap beam and
< 2 * D< D + hb
s Always check your signs (+/-)
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Step 10s Use Mc,Vc, and Pc which result in
maximum principal tension, pt
s If the principal tension (pt) isgreater than 3.5*(fc) thenadditional joint reinforcement isrequired - that is:
If pt < 3.5* (fc) then done
If pt > 3.5* (fc) then provide theadditional reinforcement definedin the following steps
If pt > 15* (fc) then joint will notwork - try different pier geometry
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Strut and Tie Models The model developed by UCSD
(shown below) was used togenerate the following jointdesign procedure
s Area of steel to resist tensileforces (Tes, Tbb and Tc) isdetermined from joint geometryand reinforcement pattern
AovjAivj
AB
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Step 11
s Determine the extra amount ofshear reinforcement (pairedhoops) required outside the jointregion, Aovj
s Space the stirrups evenly in aregion equal to the cap beamheight. Total area Aovj to each
side of the column
Aovj > 0.125 * o * Asc
where:Asc = area of column
longitudinal steel
o = overstrength factor = 1.4
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Shear Reinforcement Outside Joint
Avjo
> 0.125 * Asc * o
Put the paired hoops on each side of the joints
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Shear Reinforcement Inside Joint
Avji
> 0.095 * Asc * o
Space paired hoops evenly within joint region
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Step 13
s Additional top and bottomlongitudinal reinforcement isrequired to develop the jointstrut-and-tie mechanism
s Add the following amount of cap
beam longitudinal steel, Ab,inaddition to what is required to
resist bending alone, to both topand bottom
Ab > 0.17 * o * Asc
where:Asc = area of columnlongitudinal steel
o = 1.4
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Additional Longitudinal Beam
Reinforcement
Ab > 0.170 * Asc * o
Put additional longitudinal bars on top AND
bottom
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Step 14
s Provide seismic J bars within
the joint regions to prevent
buckling of the longitudinal steel
in the cap beam and to provideadditional confinement of the
joint region
s Two or three J bars perlongitudinal cap beam bar should
be adequate for most cases
s Space the bars evenly within thejoint so as to prevent buckling of
longitudinal cap beam bars
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Transverse Seismic J Bars
Place two or three three J barsper longitudinal cap beam bar
within joint region
Could use welded, headed bars if desired
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Detailing Notes
s Provide concrete core down pipepile below the depth of effectivefixity (point of maximum moment)
by at least 3 pile diameters or tothe point where the pile momentis about half the maximummoment
s Make sure that the longitudinalcap beam bars are fully
developed - may need to provide90o hooks
s Use headed reinforcement in
place of the J bars and on theends of the longitudinal capbeam bars if space is tight
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Detailing Notes
s Use paired shear stirrups (hoops)in pier cap beams. This providesbetter confinement of concreteand a more even distribution ofsteel within joint region to bettercarry the loads
s Generally, more smaller bars arebetter than few larger bars forserviceability. However, you muststill meet bar spacingrequirements for concreteplacement
s Although the earthquake load
case often governs the pierdesign, you must still examinethe other load combinations(strength and serviceability)
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Referencess Nilsson et al Reinforced Concrete Corners
and Joints Subjected to Bending Moment(1976)
s Park and Paulay Reinforced ConcreteStructures (1976)
s Schlaich, Schfer, and Jennewein Towards
a Consistent Design Methodology (1987)s Collins and Mitchell Prestressed Concrete
Structures (1991)
s James G. MacGregor Reinforced Concrete;Mechanics and Design (1992)
s Priestley, Seible, and Calvi Seismic Design
and Retrofit of Bridges (1996)
s AASHTO AASHTO LRFD Bridge Design
Specifications (1996)
s ASCE-ACI Committee 445 Recent
Approaches to Shear Design of Structural
Concrete (1998)s Silva, Sritharan, Seible, and Priestley Full-
Scale Test of the Alaska Cast-in-Place Steel
Shell Three Column Bridge Bent (1999)