Experimental Study of the Controlled Rocking Response of Steel Braced Frames

Controlled rocking of steel braced frames has been proposed for the seismic retrofit of structures. The controlled rocking approach allows a frame to uplift from its support at the base-of-column to foundation interface while displacement-based steel yielding devices are implemented at the base location to control the response. Past studies have focused on analytical investigations of the response of such a system along with design applications for controlled rocking bridge piers. This paper discusses shake table testing of a 4-legged, steel braced pier representative of a 1/5 length scale highway bridge pier. The model is subjected to a series of seismic excitations about one of its primary orthogonal axes, using horizontal and vertical base motions, and rotated 45deg. to investigate pier behavior that would be expected from bi-directional horizontal base inputs. Ground motions from the 1940 El Centro and 1994 Northridge earthquakes are used along with a synthetically generated motion. The steel yielding devices used during testing are triangular plates that yield in flexure as the pier uplifts and rocks at its base. Results of the testing focus on the overall behavior of the controlled rocking pier including global hysteretic response, uplifting displacements, and pier forces.