design pilecap

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)

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