Redundancy or Alternate Load Paths

Redundancy or Alternate Load Paths:

In this approach, the structure is designed such that if any one component fails, alternate paths are available for the load in that component and a general collapse does not occur. This approach has the benefit of simplicity and directness. In its most common application, design for redundancy requires that a building structure be able to tolerate loss of any one column without collapse. This is an objective, easily-understood performance requirement. The problem with the redundancy approach, as typically practiced, is that it does not account for differences in vulnerability. Clearly, one-column redundancy when each column is a W8 x 35 does not provide the same level of safety as when each column is a 2000 il/ft built -up section. Indeed, an explosion that could take out the 2000 lb/ft column would likely destroy several of the W8 columns, making one-column redundancy inadequate to prevent collapse in that case. And yet, codes and standards that mandate redundancy do not distinguish between the two situations; they treat every column as equally likely to be destroyed. In fact, since it is generally much easier to design for redundancy of a small and lightly-loaded column, redundancy requirements may have the unfortunate consequence of encouraging designs with many small (and vulnerable) columns rather than fewer larger columns. For safety against deliberate attacks (as opposed to random accidents), this may be a step in the wrong direction.

Local Resistance:

In this approach, susceptibility to progressive/ disproportionate collapse is reduced by providing critical components that might be subject to attack with additional resistance to such attacks. This requires some knowledge of the nature of potential attaks. And it is very difficult to codify in a simple and objective way.

Interconnection or Continuity

This is, strictly speacking, not a third approach separate from redundancy and local resistance, but a means of improving either redundancy or local resistance (or both),m studies of many recent building collapses have shown that the failure could have been avoided or at leastreduced in scale, at farily small additional cost, if structural components had been interconnected more effectively. This is the basis of the 'Structural integrity' requirements in the ACI 318 specification (ACI, 2002).