Complex sentence: The building, which was Victorian in origin and as a result had load bearing walls supporting upper floors on joists, was unable to withstand the force of an explosion and so collapsed adding to the devastation and injury count.
Split into simpler sentences: The building, which was built in Victorian times, had load-bearing walls supporting upper floors on joists. It was thus unable to withstand the blast and so collapsed, adding to the devastation and injury count.