Modeling and calculation of period for braced steel structures with semi-rigid joist-floor diaphragms

In the Current seismic codes, the earthquake load on the buildings depends to period of structure. More of these codes use the empirical relations to estimate the period of structures. These relations are suggested for the rigid floor structures. Then use of these relations for semi-rigid and flexible diaphragms is not accurate and cause error in determination of seismic load of structures. This topic is more important for the stiffened structures such as: structures with shear wall or bracing together joist-floors with lower rigidity in one direction and semi-rigid behavior. In this paper, the period of the braced steel building with block-joist floors for two different cases of real modeling without rigid diaphragm assumption and ideal modeling with rigid diaphragm assumption are compared and effect of flexibility of diaphragms on the period of structure is investigated. Also, for fast estimation of period of structures with flexible diaphragms, the simple equations are proposed.