This paper focuses on the fire behavior and collapse resistance of mid-rise steel buildings with composite floor systems. Two 10-story steel buildings with different structural configurations: (i) lateral load resisting system on the perimeter, and (ii) lateral load resisting system on the interior. They are designed (for structural design and fire protection) according to American design practices. Finite Element (FE)-based numerical techniques were used to model and simulate the behavior of these buildings in fire conditions. The fire conditions are simulated by assigning structural components the temperature values corresponding to the design fire event. The numerical technique involved macro level spring models for the connections. These connections were capable of modeling temperature dependent coupled multi-axial force-displacement response along with a coupled failure criterion. Beam elements and a combination of beam and shell elements are used for modeling columns, and composite slab systems, respectively. Explicit dynamic analysis technique is employed to simulate the structural response. According to the simulation results, gravity columns are the most important components for overall structural stability in fire conditions. If all the structural components were protected for equal FRR value, gravity columns were found to be the first to fail. If the columns were sufficiently protected, further heating causes failure of the connections at the discontinuous end of gravity beams in flexure where the gravity beam had reached its combined (positive moment capacity at the mid span plus negative moment capacity of the shear connections at the ends) flexural capacity. It was also observed that by using stronger connections, this type of failure could be delayed or avoided. If the connections survive the heating mode, there is a risk of connection failure in the cooling phase too. This risk is higher for interior connections. In the cooling phase, as the beams start to shrink, there is a tension demand on connections. It was analytically tested that by increasing the ductility of the connection, this type of failure can be delayed or avoided. © 2013 American Society of Civil Engineers.