Disclosed herein is a middle or large-sized battery pack including a battery module assembly configured in a structure in which a plurality of battery modules, each of which comprises a plurality of battery cells or unit modules connected in series to each other while being mounted in a module case, is electrically connected to each other while being arranged in a lateral direction such that the battery modules are in contact with each other, a pair of side support members for covering sides of outermost battery modules of the battery module assembly, and at least one top connection member for connecting the side support members at a top of the battery module assembly, wherein at least one of the side support members has a region (weak portion) exhibiting low resistance to volume expansion, the weak portion being partially formed at the at least one of the side support members for inducing local deformation of the battery modules upon occurrence of swelling, thereby causing a short circuit.