Impact of Aspect Ratio on Two-Column Bent Seismic Performance

PI: David Sanders, UNR
Co-PI: M. Saiid Saiidi, UNR
Research Assistant: Khaled F. Moustafa, UNR

Sponsor: California Deaprtment of Transportation
Project Monitor: Dr. Saad El-Azazy

The design of structural bridge elements like columns, beam-column joints, and cap beams has changed tremendously over the past 30 years.

Many experimental tests have been done in order to determine the behavior of bridge bents under seismic loading. Most of those tests were performed under static-cyclic loading, whereas only a few of those tests were dynamic to simulate real-time earthquake effects.

Based on the literature review, most studies have focused on retrofitting older columns and/or single columns versus new design criteria on systems. Therefore, this study will be investigating two-column bridge bents under dynamic loading by testing them with real-time earthquakes on a shake table.

The main research objective is to investigate the seismic performance of two-column bridge bents with different aspect ratios using current Caltrans design criteria.

There are 3 bents to be tested. The three specimens are identical except for the aspect ratios, which are 2.5, 4.5, and 6.64. The first test will be of the bent with a column aspect ratio of 2.5. The bents will be tested by using increasing levels of the Sylmar earthquake motion until failure occurs.

Test Model

Based on the 0.3-scale model developed from a previous study, three specimens were designed using current Caltrans specifications and recommendations.

The current design included column confinement, ductility, column shear capacity, longitudinal and transverse reinforcement of cap beams, and the beam-column joints. 

Strain gages were attached to the longitudinal and transverse reinforcement in both columns and beams to determine when yielding occurs and strain profiles across the section. Novotechniks (displacement transducers) were installed in the expected plastic hinge zones and at the beam-column joints in order to record curvature and shear deformations, which will be used to obtain the concrete strains at critical sections. In addition, accelerometers and potentiometers were added to record frame displacement and acceleration during testing.

A total of 150 data acquisition channels will be used for the tests. Data will be sampled at 160 samples per second.

Loading System

One of the features of the present study is to load each specimen with mass (lead bricks) on the top and along side the cap beam to simulate the inertia mass and the axial compression force. The amount of lead weight was calculated based on the scale ratio between the model and prototype. To produce realistic stresses in the system, the weight causes an axial load in each column of 0.05 f'cAg. Ag is the gross area of each column.

Steel buckets were designed to contain the lead blocks.