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Re: Live Load Distribution Factors

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The approach that Kipp Martin suggests is a conservative approach for 
designing bridge beams, but a non-conservative approach for the deck slab.

It should be recognized that the beams are not rigid supports for the deck 
slab, except at piers and abutments (which is why you cannot use 
distribution factors at those locations), and will distribute wheel loads to 
adjacent beams due to the beam under consideration deflecting.  In this 
respect, the suggested approach is conservative.  However, this same action 
that makes bridge beams conservative, makes the deck slab analysis 
non-conservative as the deflection in the beam under consideration will 
increase negative moments in the deck slab over the beams adjacent to that 
beam.  Therefore, an adjustment (increase) in the negative moment of the deck 
slab needs to be made.

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

Kipp Martin wrote:

>>You can always go back to basic statics.  I've designed several bridges 
where the distribution factors were computed assumeing the deck slab acted at 
a continous beam over the girders.  The girders were assumed to be simple
supports.  The wheel loads were moved to various locations in the lanes to
envelope the results.<<