Technical Papers

This page contains information on several papers that were published in the 1990s by Prof. Ehsani and his associates. Following the collapse of freeway bridges in the 1989 Loma Prieta Earthquake, there was significant national interest in developing techniques to retrofit weak piers and columns in existing bridges and building in order to prevent similar failures in future. The concept of retrofit of bridge columns was originally developed by Prof. Ehsani and submitted to California Department of Transportation where as reported by the Engineering News Record the idea was inadvertently leaked to a competing company.

Our R&D on confinement and repair of columns with FRP jackets started over 20 years ago. We were the first team of researchers funded by the U.S. National Science Foundation to study repair and retrofit of columns and bridge piers with FRP wraps. Please keep in mind the first generation of FRP jackets we used in the 1990s to wrap those columns was far inferior to the newly-developed PileMedic™ system that we introduced in 2010. The earlier “FRP wraps” were, for example, very narrow and the uniaxial, meaning the fibers could only be positioned in the hoop direction to provide confinement for the column. In contrast, the new PileMedic™ has a biaxial construction so the laminate can not only provide confinement but it can also contribute to the flexural capacity of the column.

Referring to some of the earlier studies we published on tests of circular columns, this improvement is shown when you compare Figs. 9 and 10 with Fig. 8 or when you compare Fig. 12 with Fig. 11. In all cases, the enhancement caused by the confining wraps is substantial.

In another paper on rectangular columns, we studied the effects of keeping the same rectangular shape or enlarging the column cross section into an oval shape (Fig. 5). Another variable was whether the resin in the annual space was placed under gravity alone (passive confinement) or pressurized to place the jacket in tension from the start (active confinement). Fig. 10 is the response of the same column shown in Fig. 9 after it was wrapped. Likewise, Figs. 12 and 13 show the improvements on the behavior of unwrapped column (Fig. 11).

Finally, many of the unwrapped circular and rectangular columns that were damaged at the conclusion of the initial tests, were subsequently repaired by wrapping and re-tested. This portion of the study was aimed at showing the effectiveness of FRP wraps in repairing unretrofitted columns that are damaged in an earthquake. As seen in Figs. 12, 13 and 14 deteriorated columns could also benefit significantly from such repairs with FRP jackets.

For additional articles on piles and poles, we invite you to visit the Magazine Articles page on our website.