Technical Background

SAC Phase 1 Analytical Studies of Building Performance

Project Title:

Earthquake Response of Strengthened Steel Special Moment Resisting Frames

Sub-contractors:

Gary C. Hart, UCLA/Hart Consultant Group

Sampson C. Huang, Hart Consultant Group

Roy F. Lobo, Hart Consultant Group

Matthew Van Winkle, Hart Consultant Group

Anurag Jain, Hart Consultant Group

Project Summary:

The damage to steel beam column welded moment connections has left engineers scrambling for solutions to a problem that hitherto was not known to exist. A number of solutions to these damaged connections have been proposed and categorized under three global courses of action:

a) reweld only

b) strengthen Connection

c) introduce active or passive energy dissipating devices.

This report presents the response of steel moment resisting frames for the first two repair options. Two different non-linear dynamic analysis programs are used to study the effect of repairing and strengthening these weld failures, or predicting the expected response assuming the connection is not repaired. This paper presents the results of the analysis of the same structural systems modeled using DRAIN-2DX for the case where there is no moment capacity reduction during the earthquake motion of the frame, and NSTAR-ST for the case where there is a moment capacity reduction during the earthquake due to weld failures. For the second computer program, the effect of dropping the moment capacity of a beam to 50%, 20% and 5% of the original moment capacity after the completion reaches its first yield capacity is studied.

The frames selected for the study correspond to a real building damaged during the Northridge earthquake. They are subjected to an ensemble of artificial earthquake records that have an equal probability of occurring at the site for comparative studies.

Based on the research described in this study, the following conclusions were reached:

It is not acceptable to strengthen a connection with weld of material failure without a detailed nonlinear dynamic analysis to evaluate the impact of the strength increase on the load path and increased demand on other connections.

Strengthening connections with weld or material failure can be expected to increase the load and ductility demand on connections that did not suffer damage.

It is very important to quantify through testing and analysis the expected moment/rotational capacity relationship for a building's connections. The determination of the expected moment drop is a critical parameter for quantifying the life safety of the frame.

The results for analyses where the drop in moment capacity is to 20% of the original indicates that drift control and life safety may be able to be provided with selected strengthening of connections or minimal expense for high tech options.

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