Prediction of the Seismic Response of Steel Frames with Concentric Diagonal Bracings

Concentrically braced frames and eccentrically braced frames are efficient seismic resistant systems but they are generally prone to develop storey collapse mechanisms. As underlined in some previous papers with regard to eccen- trically braced frames, this tendency depends on the overstrength factors and damage distribution capacity factors result- ing from the use of common design procedures. In this paper a previously proposed procedure which predicts the height- wise distribution of the damage at collapse of eccentrically braced frames is applied to concentrically braced frames. To apply this procedure, proper definitions of the overstrength factors and damage distribution capacity factors are derived. The effectiveness of the proposed procedure is tested on frames with concentric diagonal bracings characterised by differ- ent storey numbers and designed by common design procedures. The target response is provided by nonlinear dynamic analyses. The seismic input is constituted by ten artificial accelerograms. The paper proves that the proposed procedure is able to predict accurately the nonlinear dynamic response of systems in which the damage is mainly restricted to a few storeys. In the other cases, some not negligible scattering between actual and expected values of damage can be found at some storeys.