Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Lateral restraint of floor beams

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Dave Meney wrote:

. > My problem with this is best indicated by way of an example - two parallel
. > members as a walkway supporting grating, and connected together by 
. > trimmers at (say) 1200 mm centres. At the limit state for uniform 
. > loading, both beams have reached the same limit state.  How is it 
. > possible for each beam to then support each other's lateral restraint 
. > forces? Without a truss, these forces would be resisted by weak axis 
. > bending of the members, which couldn't be sustained if their stress limit 
. > state is already reached.
. > 
. > If the failure mechanism was such that both beams tended to twist towards
. > each other, then I see that the trimmers (and indeed the grating) would
. > stabilise the critical flange.  But couldn't each beam's critical  flange
. > twist in the same direction, and if so, the trimmers and grating would
. > simply be "carried" by the twisting beams and not provide any restraint.  
. > Of course, most grillages consist of more than two parallel members, but 
. > if each beam is designed economically there wouldn't be much reserve 
. > strength to resist horizontal bending moments.
. > 

I am not familiar with the Australian code, but I agree with your thinking.  
*Somewhere* in the U.S. Code(s) or Commentaries, I remember seeing the 
admonition that you can't use a similar structural member to provide lateral 
support to another structural member, however, I can't find the reference at 
this time, and, unless you have the U. S. Code, I don't think that a 
specific reference will do you much good.

What I have done in cases like your example and with Z-sections for metal 
buildings, is to install X-straps between the top flange of one member and 
the bottom flange of the other member, considering the metal deck or grating 
as being able to keep the top flanges a constant distance apart.  If 
necessary, a horizontal member connecting the bottom flanges could also be 
installed at the X-brace locations.

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