Specimen design and testing piles extension connection to slab bridges

  • PI: David Sanders, University of Nevada, Reno
  • Research assistant: Mohamed Ayoub, University of Nevada, Reno
  • Sponsor: California Department of Transportation
  • Project monitors: Ahmed Ibrahim, Saad El-Azazy
  • Project date: April 2009

Project summary

The California Transportation Department (Caltrans) has a large number of highway bridges and many of them are slab bridges. Slab bridges are structures where the pile works as a pile under the soil level and as a column above the soil level. The pile is then directly connected to the superstructure.

Although slab bridges are common type of bridge, the current version of the Bridge Design Specification (BDS) and the Seismic Design Criteria (SDC) provide limited design guidance for pile extension connection details for slab bridges. Unlike the column to box-girder bridge connections which are under extensive discussion and have been extensively tested, slab-bridge connections have not been tested.

Larger trucks and seismic loads have made these connections even more critical. It is important that large scale tests be conducted on these connections.

Eight large scale column-slab bridge connections are the target of the research; three specimens have already been tested. The first two specimens were based on the existing Caltrans specification. The primary difference is that one specimen has a flat slab detail while the second specimen has a drop cap.

The current specimen is the second specimen of the second set. To overcome liquefaction problems, larger piles are used. This specimen has a 24-in diameter instead of the 16-in pile in the past specimens. The equations which used to calculate the shear are the same as those of the box girder. Constant axial load (76.5 kips) will be applied to the specimens while it is subjected to lateral load reversals with increasing drift levels.

Specimen selection and details

The embedded length of the column varies according to the soil type. In the plans submitted by Caltrans to the University of Nevada, Reno, the height of the column above the ground ranged from 9D to 20D. Therefore, an average height of the column above the soil surface equals 14.5D.

The point of the maximum moment for the individual column can be approximated at D to 2D under the surface of the soil. This increases the average total length to 15.5D to 16.5D.

The column inflection point will be approximately halfway between column top and maximum moment point in the soil. Columns with shear lengths greater than 6D are considered flexural members. Therefore, the height of the specimen was chosen to be 6D.

The reinforcement was chosen according to the Bridge Design Aid (BDA) of Caltrans.

Test setup

The transfer of axial loads to the specimen was accomplished through a steel I-beam that was placed across the column top. The test is conducted in an inverted position; so the lateral force will be applied to the top column and the slab placed at the bottom during the test.