The influence of the beam flexural stiffness in the seismic response of inverted V concentric braced frames

Chevron concentric bracings (CCBs) are widely used in the design of seismic resistant multistorey steel building owing to both the high structural efficiency and the architectural functionality, thus allowing the placement of doorways, windows and plants. The seismic performance of CCBs is strongly related to the behaviour of the beam of the braced span [1-3]. It is well known that the buckling of the brace in compression results in a vertical force applied on the beam in the braced bay. Depending on the strength of the beam, two main collapse mechanisms can be achieved as follows: 1. weak beam mechanism: the beam experiences in plastic bending deformations. 2. strong beam mechanism: the beam is sufficiently strong to resist elastically the unbalance force due to brace buckling, thus allowing the yielding of the brace in tension. The first collapse mechanism should be avoided, because it leads to a poor dissipative behaviour with a significant deterioration of overall force-displacement curve. Therefore, this kind of collapse mechanism is characterized by ductility and energy dissipation demand larger than those experienced in case of strong beam mechanism. [4, 5]. The current seismic codes (e.g. EN1998-1 [6], AISC341 [7]) provide capacity design criteria to achieve strong beam mechanism. In particular, according to EN1998-1 [6] the beams of the braced spans should be designed to resist the following conditions: - all non-seismic actions without considering the intermediate support given by the diagonals; - the unbalanced vertical seismic action effect applied to the beam by the braces after buckling of the compression diagonal. This vertical force should be calculated using Npl,Rd for the brace in tension and γpbNpl,Rd for the brace in compression, being the factor γpb used for the estimation of the post buckling resistance of diagonals in compression (generally assumed equal to 0.3).