Slab spatial composite effect in composite frame systems. I: Effective width for ultimate loading capacity

The slab spatial composite effect plays a significant role in the behavior of composite frame structural systems subjected to lateral forces. Since the ultimate loading capacity and stiffness are the two most basic characteristic behaviors in a design practice, this paper focuses on the influence of the slab spatial composite effect on the ultimate loading capacity, and the companion paper pays attention to the structural stiffness and internal force distribution. The extensive verifications of the proposed design methods for evaluating the ultimate loading capacity and stiffness considering the slab spatial composite effect based on various numerical examples and experimental programs are also carried out in the companion paper. In this paper, an elasto-plastic elaborate finite element model of the side joint is firstly developed in order to find the critical factors dominating the slab participation at the ultimate limit states. Furthermore, a parametric analysis is conducted to derive the simplified design formulas of the slab effective widths for calculating the ultimate positive and negative moments at the composite beam end. The predictions calculated using the proposed formulas correlate well with the numerical results. In addition, the proposed design methods are verified adapted for both side and middle joints. Finally, the proposed formulas are compared with some available formulas provided in the literature and current design codes. It is found that the proposed formulas can give the most satisfactory results for predicting the ultimate effective width and beam end moments.