Seismic response and energy dissipation in partially restrained and fully restrained steel frames: An analytical study

Abstract

The damage suffered by steel structures during the Northridge (1994) and Kobe (1995) earthquakesrnindicates that the fully restrained (FR) connections in steel frames did not behave as expected. Consequently,rnresearchers began studying other possibilities, including making the connections more flexible, to reduce thernrisk of damage from seismic loading. Recent experimental and analytical investigations pointed out that thernseismic response of steel frames with partially restrained (PR) connections might be superior to that of similarrnframes with FR connections since the energy dissipation at PR connections could be significant. Thisrnbeneficial effect has not yet been fully quantified analytically. Thus, the dissipation of energy at PRrnconnections needs to be considered in analytical evaluations, in addition to the dissipation of energy due tornviscous damping and at plastic hinges (if they form). An algorithm is developed and verified by the authors tornestimate the nonlinear time-domain dynamic response of steel frames with PR connections. The verifiedrnalgorithm is then used to quantify the major sources of energy dissipation and their effect on the overallrnstructural response in terms of the maximum base shear and the maximum top displacement. The resultsrnindicate that the dissipation of energy at PR connections is comparable to that dissipated by viscous dampingrnand at plastic hinges. In general, the maximum total base shear significantly increases with an increase in thernconnection stiffness. On the other hand, the maximum top lateral displacement Umax does not always increasernas the connection stiffness decreases. Energy dissipation is considerably influenced by the stiffness of arnconnection, defined in terms of the T ratio, i.e., the ratio of the moment the connection would have to carryrnaccording to beam line theory (Disque 1964) and the fixed end moment of the girder. A connection with a Trnratio of at least 0.9 is considered to be fully restrained. The energy dissipation behavior may be quite differentrnfor a frame with FR connections with a T ratio of 1.0 compared to when the T ratio is 0.9. Thus, for nonlinearrnseismic analysis, a T ratio of at least 0.9 should not be considered to be an FR connection. The studyrnquantitatively confirms the general observations made in experimental results for frames with PR connections.rnProper consideration of the PR connection stiffness and other dynamic properties are essential to predictrndynamic behavior, no matter how difficult the analysis procedure becomes. Any simplified approach mayrnneed to be calibrated using this type of detailed analytical study.