In this research, a detailed 3-D finite element model is developed by ABAQUS package for an existing six-story reinforced concrete framed structure in Saudi Arabia to study its response under blast loads. The building is office building designed for dead, live, wind and seismic loads. The design wind speed is 161 km/h while the mapped maximum considered earthquake spectral response acceleration at a short period and at a period of 1-second are 0.12 g and 0.034 g, respectively. Concrete damage plasticity model was used to model the concrete elements. The ‘brittle cracking’ concrete model was used to represent column removal in front of the explosion. The metal plasticity model was used to represent the steel. The model is exposed to blast loading with a 1.0 ton (2204.623 lb) of TNT at a distance of 5 m from the building. Another scenario was considered if the blast load is 10 m from the building to investigate the effect of the standoff distance. The structural vulnerability is assessed and compared. Alternative uncommon structural design of external columns is considered using composite cross section, which uses steel casing around concrete section. This uncommon design gave better results in terms of mitigating blast load effects on structures