Experimental & Analytical Studies of Drilled Shaft Bridge Columns Sandrine P. Lermitte, PhD Student Jonathan P. Stewart, Assistant Professor John W. Wallace, Associate Professor Civil Engineering, UCLA Kerop Janoyan, Assistant Professor Civil Engineering, Clarkson University 7NCEE July 21-24, 2002

Project Objectives Study of shaft – soil interaction –Pile specimens –Cyclic displacements –Large scale –Well instrumented Identify damage states –Plastic hinge Develop/Assess models –P-Y Curves

Specimen Information Specimen Design –6 ft Diameter column –36 - #14 Bars (2%) –#8 6” o.c. Instrumentation –200 channels: Load, strain, displacement, soil pressure cells, rotation, curvature

Test Overview - Caltrans 40 ft 48 ft 105/405 Interchange - LAX

Cyclic Loading Number of cycles Lateral Deflection at Top of Column (in) After 9 in displacement, 16 stops per cycles

Test Results

Load – Displacement Response

Shaft/Column Modeling Fiber (section) model Material models (concrete, rebar) Soil modeling –API p-y curves (2 ft pile tests) –Experimentally derived p-y curves Analysis –Pushover, Cyclic (no-gap, gap)

Fiber Model Unconfined concrete Confined concrete #14 steel bar

Strain (in/in) Stress (ksi) Mean f’ c Mean+/_1 STDEV Material Properties Steel reinforcement Reinforced concrete Fy = 71 ksi Fult =102 ksi Ec f’c = 6.1 ksi Ec = 3,500 ksi

Soil Model API relations, Stiff clay

Trilinear API p-y Curves y (in.) p (lb/in.) 3 ft Depth y (in.) p (lb/in.) 6 ft Depth y (in.) p (lb/in.) 10 ft Depth y (in.) p (lb/in.) 24 ft Depth

Experimental p-y curves.p(z): double differentiating the shaft bending moment distribution, y(z) : single integration of the slope, or a double integration of the curvature

6 in Displ. Level West Pull Soil Reaction, p (k/in) Depth, z (ft) 6 in Displ. Level Displacement, y (in) Depth, z (ft) Derivation of experimental p-y curves at different depths Soil Reaction p (k/in.) Displacement y (in.)

Experimental p-y curves x 10 4 y (in) p (kips) 3 ft below ground 6 ft below ground 10 ft below ground 18 ft below ground 24 ft below ground 30 ft below ground

Comparison API vs. Experimental p-y curves 3 ft Depth y (in) p (lb/in) API Stiff Clay Curve Fitted Test Data Curve

y (in) API Stiff Clay Curve Fitted Test Data Curve 18 ft Depth p (lb/in) Comparison API vs. Experimental p-y curves

Trilinear Exp. p-y Curves x 10 4 y (in.) p (lb/in.) 3 ft Depth x 10 4 y (in.) p (lb/in.) 6 ft Depth y (in.) p (lb/in.) 10 ft Depth y (in.) p (lb/in.) 30 ft Depth

 top (in) F (kips) F  TOP  GROUND F/2 Pushover Analysis

Shaft/Column top displacement (in) Lateral Force applied (kips) API p-y Experimental p-y test results Top Force vs Displacement

Shaft ground displacement (in) Lateral Force applied (kips) API p-y Experimental p-y test results Ground line Force vs Displacement

x Curvature (/in) Height (ft) API Exp Test Curvature Profiles 36 in. top displacement

Gap at end of testing

DRAG ELEMENT CLOSURE ELEMENT PLASTIC ELEMENT ELASTIC ELEMENT RADIATON DAMPING a) Model A: proposed by Boulanger et al.(1999) b) Model B: “Gap model” proposed DRAG ELEMENT CLOSURE ELEMENT PLASTIC ELEMENT ELASTIC ELEMENT RADIATON DAMPING

DRAG ELEMENT CLOSURE ELEMENT PLASTIC ELEMENT ELASTIC ELEMENT y/y max p/p u l t Gap Model y/y max p/p u

80% drag 2 ft below ground y(in.) p u l t (kips) P-y - 2 ft 20% drag force

Cyclic Response - 20% drag force Shaft top displacement (in) Lateral Force applied (kips) Gap model 20% drag test envelope

y(in.) p u l t (kips) P-y -2 ft 80% drag force

Shaft top displacement (in) Lateral Force applied (kips) Gap model 80% drag test envelope Cyclic Response - 80% drag force

Comparison 80% drag model and test results for 9 in. and 40 in Displacement (in) Height (ft) Gap model, 80% drag Actual test response

Comparison 80% drag model and test results for 9 in. and 40 in x Curvature(/in) Height (ft) analytical model with 80% drag force experimental results

Summary & Conclusions Test Results 6 ft diameter CIDH shaft/column test –Provide needed data (scale, cyclic loading, M/V, instrumentation) –Behavior and modeling (p-y) Plastic hinge located ~0.5D below ground line Test shaft/column behavior –~20% lateral drift prior to loss of lateral load capacity –Buckling of vertical bars, hoop fracture, vertical bar fracture

Summary & Conclusions Analytical Studies Fiber model of shaft/column & p-y curves –API p-y relations are too soft (monotonic) –Experimentally derived p-y curves (monotonic/cyclic) Lateral load capacity of system is over-estimated Yield displacement is under-estimated Peak inelastic curvature is over-estimated Cyclic material models & gap behavior –Nonlinear gap model with elastic/plastic/drag/gap –Drag participation was varied –Excellent agreement (global and local responses) between experimental results and analytical results for 70 to 80% drag participation