Effects of Superstructure Time-Dependent Deformations on Bridge Column Design Ebadollah Honarvar Gheitanbaf, Sri Sritharan, and Matt Rouse Civil, Construction & Environmental Engineering, Iowa State University 0.0 200.0 400.0 600.0 800.0 1000.0 0 500 1000 1500 2000 2500 3000 Microstrain Time (day) Superstructure Strain Rate B6 B7 B5 B4 B2 B1 P i /A=6.68 P i /A=6.81 MPa P i /A=6.77 MPa P i /A=6.18 MPa P i /A=5.87 MPa P i /A=4.73 MPa -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 0 500 1000 1500 2000 2500 F x (kN) Time (days) Base Shear B23 B24 B25 B26 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.000 0.050 0.100 0.150 Midas Displacement (m) Estimated Displacement (m) 0.000 0.050 0.100 0.150 0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 V/K eff (m) Estimated Displacement (m) Cracked Coulmns 0.000 0.010 0.020 0.030 0.040 0.000 0.020 0.040 0.060 0.080 V/K g (m) Estimated Displacement (m) Uncracked Columns Testing Testing Procedure Force control mode Displacement control mode Column Test Two specimen sizes to investigate any plausible size effect Two various loading scenarios: i. Instantaneous compressive axial displacement ii. Step Loading Beam Test Precracking loading Post-cracking loading: Failed the specimen to examine the possibility of any residual strains in the rebars -10000 0 10000 20000 30000 40000 -1000 0 1000 2000 3000 4000 0 20 40 60 80 100 120 140 160 Load (lb) Microstrain Time (min) Load/Tensile Strains L- L11H20 L- L9H20 Satec Load Problem Statement Analytical Investigation Results 1. Bridge superstructure shortens due to prestressing, and creep and shrinkage 2. Columns are pulled by the superstructure 3. Shear stresses are developed at the column base Methodology/Objective Concrete Relaxation Test Conclusions/Observations Concrete relaxation resulted in significant reduction in column base shear force Generally, the superstructure strain rate increased as the initial axial stress increased Top of column displacement was estimated fairly accurately using superstructure strain rate when it was compared to the analytical model results The maximum displacement and base shear force occurred for the most exterior columns, which cracked as well. The nearest columns to PNM experienced the minimum displacement and base shear The nearest columns to PNM experienced the minimum displacement and base shear force, thus remained uncraked. Concrete Relaxation Test-Results For the seven conduced test, strain remained constant, while force reduced over time 0 0.5 1 1.5 2 2.5 3 0 24 48 72 96 120 Stress (ksi) Time (hrs) Comparison of Compressive Stress Relaxation Small Column Specimen, loading age of 48 days Large Column Specimen, loading age of 67 days Small Column Specimen, loading age of 76 days Small Column Specimen Step Loading, loading age of 78 days Small Column Specimen Step Loading, loading age of 84 days Large RC Beam Precracking, loading age of 130 Large RC Beam Postcracking, loading age of 150 -550 -500 -450 -400 -350 -300 -250 -200 0 24 48 72 96 120 Microstrain Time (hr) Strains for Displacement-Control for 8 in. Diameter Column Location 1 Location 2 Location 3 Location 4 -0.02 0 0.02 0.04 0.06 0.08 0.1 0 20000 40000 60000 80000 100000 0 2 4 6 8 Time (Min) Actuator Displacement (in.) Force (lb) Loading under Force-Control Force Displacement 0.06 0.07 0.08 0.09 40000 50000 60000 70000 80000 90000 100000 0 50 100 150 Actuator Displacement (in.) Force (lb) Time (hr) Loading under Displacement-Control Force Displacement Analyzed six bridges with different lengths Two Short Bridge Frames; L f < 150 m (492 ft) Two Medium Bridge Frames 150 m (492 ft) < L f < 300 m (984 ft) Two Long Bridge Frames L f > 300 m (984 ft) Using the calculated strain rate, estimated the top of column displacement with respect to point of no movement (PNM) No residual strains in longitudinal rebars after the completion of the relaxation test Column relaxation led to appreciable reduction in column base shear Establish a realistic strain rate for the superstructure shortening Identify and quantify concrete relaxation phenomenon Analyze different bridge frame configurations using the Finite Element Analysis Propose a simplified design approach to calculate Propose a simplified design approach to calculate column moment/shear demands Strain Gages Strain Gages How to efficiently design bridge columns to accommodate stresses induced by superstructure time-dependent deformations in posttensioned concrete box-girder bridges