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MCEER Bulletin, Volume 21, Number 2, Summer 2007

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Volume 21, Number 2, Summer 2007

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Seismic Testing of a Self-Centering Post-Tensioned
Steel Frame

model with pted connection and close up of beam to column joint

PTED steel frame concept: 1/3-scale model on shake table (left); interior beam-to-column joint detail (right)

1/3-scale three-story steel frame model incorporating innovative Post-Tensioned Energy Dissipating (PTED) connections was seismically tested on a shake table for the first time in the Structural Engineering and Earthquake Simulation Laboratory (SEESL) at the University at Buffalo (UB) from March 19-23, 2007.

The PTED steel system, unlike traditional welded steel frames, incorporates high strength post-tensioned bars along with sacrificial yielding elements in each beam-to-column connection and is particularly appealing for hospital buildings from an initial investment standpoint. The tests, part of MCEER’s Hospital Project (Thrust Area 2), were conducted by Ph.D. student Dong Wang under the supervision of UB faculty member and SEESL Director Andre Filiatrault. The PTED concept is fully described in Christopoulos et al. (2002).

The photos show the PTED steel frame concept, where beams are post-tensioned to the columns instead of having the beams and columns welded together. PTED connections are designed to undergo large inelastic deformations without any damage in the beam or column and with minimal residual drift. The post-tensioning (PT) force is provided at each floor by high strength bars or tendons located at mid-depth of the beam.

Four symmetrically placed energy-dissipating (ED) bars are also included at each connection to provide energy dissipation under cyclic loading. These ED bars are threaded into couplers which are welded to the inside face of the beam flanges and of the continuity plates in the column for exterior connections, and to the inside face of adjacent beam flanges for interior connections. Holes are introduced in the column flanges to accommodate the PT and ED bars.

To prevent the ED bars from buckling in compression under cyclic inelastic loading, they are inserted into confining steel sleeves that are welded to the beam flanges for exterior connections, and to the column continuity plates for interior connections. The ED bars are initially stress-free since they are introduced into the connection after the application of the PT force.

The PTED connection relies on the PT force to maintain contact between the beams and columns. Horizontally slotted shear tabs may be welded to the column flanges and bolted to the beam web to provide stability during construction and to ensure an alternative vertical shear transfer mechanism from the beam to the column. The slots in the tabs need to allow the free opening and closing of the gap at the beam-to-column interface. Nonlinear elastic action is introduced by gap openings at each beam-to-column interface. Inelastic action takes place through yielding of the ED bars once the gap is opened.

The results of the test indicated that the PTED frame was able to exhibit similar seismic response as a fully welded frame but without any yielding and permanent deformations in the main structural elements.

Christopoulos, C., Filiatrault, A., Folz, B. and Uang, C-M., 2002. “Post-Tensioned Energy Dissipating Connections for Moment-Resisting Steel Frames,” ASCE Journal of Structural Engineering, 128 (9), 1111-1120.