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Re: UBC97-- Redundancy Factor

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Redundancy, when applied to structures, means that there are alternate load 
paths should any one structural member fail.

Years ago in Great Britain, a gas explosion blew out the walls of an upper 
level corner apartment in a high rise apartment building which caused the 
floors above it to collapse and take out the all the corner apartments 
below.  IIRC, the floor system was simply supported precast concrete planks 
and the walls were masonry bearing walls.  Once a bearing wall was gone, 
there was no support for the concrete planks which collapsed on the lower 
apartments, overloading the bearing walls and causing them to collapse.  As a 
result of this explosion, there was a great push for redundancy, particularly 
in Great Britain.  Once again, IIRC, the British codes required considering 
any member failing and providing alternate load paths with the forces in the 
remaining members being no greater than 150 percent of the allowable forces.  
In the U. S., the precast industry came up with providing negative moment 
reinforcing across supports to try to support the slab as a cantilever.

Calculating alternate load paths considering *any* member to fail (be 
removed from the structure) is very difficult and time consuming although 
with computers it is much more achievable that it was with hand 
calculations.  However, IMO, that is the only way to say that a structure has 

With regards to stability, codes do not permit using an identical member to 
provide lateral support.  Nor should we consider an identical member that may 
be subjected to the same loads as a failed member as providing redundancy.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

P.S.  To our British colleagues:  If I pulled wrong information from the deep 
recesses of my mind, please feel free to correct the information.