Effects Of Variable Axial Load On Seismic Behavior Of Bridge Piers

 (First Phase)

See the report (2.19 MB)

OBJECTIVES:

As a joint project with other participating universities like UCB (University of California at Berkeley), UCSD (University of California at San Diego), and UCI (University of California at Irvine), funded by NSF through PEER, a comprehensive research program is currently being carried out at the University of Southern California emphasizing the fundamental tasks that:

The experimental results will be used to assess the adequacy of the standard analytical procedures. Among which, the plastic hinge method is particularly considered. The experimental results will also be used to evaluate an analytical model developed to simulate the moment curvature response of the section.

This is a summary of the results of the first phase of the study, in which two circular section columns, confined with spiral, have been tested under two different loading conditions. The loading cases for the two tested specimens here at USC Structural Lab., includes a fixed axial force, and a variable axial force proportional to the lateral force simulating the real loading cases, both with a quasi-static lateral force.
 
 

MODEL COLUMN:

The model column was a large-scale circular section column with a diameter of 16 inches, and a total height of 82 inches from top of the footing to the tip of the column. The effective length of the column was 72 inches, from the top of the footing to the point of application of the force. The footing was 34 inches wide, 48 inches long with a thickness of 18 inches. The longitudinal reinforcement consists of 12 #4 Grade 60 equally spaced around the section, and the confinement is provided by a W2.5 Grade 60 spiral with a diameter of 15 inches providing 1/2 inch cover, placed at equal distances of 1 1/4 inches. The details are shown in the Figure.

TEST SETUP

Test setup was different for the two columns. For the first test, a constant axial load equal to 30 % of the A_gf'_c was applied during the test. The test setup for the first column is shown in this Figure. The axial load is applied by a vertical actuator which is manually controlled, so that a constant axial force may be applied during the test. For the second column, the axial load was variable and proportional to the lateral load. The test setup for the second column is shown in this Figure in which an inclined lateral force was applied to produce an axial force proportional to the horizontal lateral force, simulating the actual case of lateral load with the overturning moment.

Experimental Results

Specimen One:

Different stages for test one is shown in the Figures. The Figures show the first specimen in a 1%, 2% and 4% drift ratio, and also expansion of the confined core concrete near the footing, rupture of the spiral and buckling of the rebar. This Figure shows the Horizontal Force-Displacement Hysteresis Curve on the testing monitor.

This Figure, shows the horizontal force drift ratio hysteresis curve resulted from test one. Compared with the test results on a similar specimen with zero axial load, the flexural strength, has increased as the axial force has increased, but with a severe decrease in ductility.
 
 

Specimen two:

Different stages for test two is shown in the Figures. The Figures show the second specimen in a 1%,4%,6% and 8% drift ratio, and also the buckling of the rebar, and rupture of the spiral and rebars.

This Figure, shows the same curve for test two. Since the axial force was proportional to the horizontal force and it's value has opposite signs in two opposite directions, the experimental capacities were different in the pull and push directions.

Here are some figures in which you may observe some experimental results.

Figures: A, B, C, D, E, F

 Conclusions: