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RE: Lateral restraint of floor beams

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I agree with your approach, and your analysis of the failure state.  We
posed the question to AISC some decades ago, and their response was that it
would require at least a 3-row web angle connection to provide enough
stiffness to consider the beam flange braced.
> ----------
> From: 	Dave Meney[SMTP:yenem(--nospam--at)iinet.net.au]
> Sent: 	Tuesday, September 01, 1998 6:57 PM
> To: 	seaint(--nospam--at)seaint.org
> Subject: 	Lateral restraint of floor beams
> 
> Hi,
> 
> I am a structural engineer of 10 years standing, running my own
> consultancy
> and specialising in the design of industrial and mining structures.
> 
> An issue which seems to be a recurring debate amongst competent structural
> engineers is regarding the intermediate lateral restraint of floor beams
> part of a grillage supporting (usually) floor grating and uniformly
> distributed loads.  The floor grating is secured to the beams by either
> welding every 4th load bar or clipping at the manufacturer's recommended
> locations.
> 
> Typically the floor between columns comprises secondary beams or trimmers,
> spanning between main beams, which span between grid beams.
> 
> Additionally, there may or may not be lateral bracing members, set down
> -110
> mm from the top flange on horizontal bracing cleats.
> 
> Generally, beam to beam connections are bolted web side plate connections,
> using minimum. 10 mm full depth cleats and min. 2-M20-8.8/TB bolts.
> 
> Here's the question - if the lateral bracing doesn't exist, can the
> trimmers
> (min. size 150PFC) be treated as lateral (L) or lateral/torsional (F)
> restraints  to the main beams.  Similarly, can these main beams (min. size
> 200PFC) be treated as restraints to the grid beams?
> 
> My approach has been as follows, where there is no discrete lateral
> bracing
> system such as a horizontal floor truss:
> 
> Neglect the intermediate members as providing any restraint to the main
> members.
> 
> Consider that the floor grating has some ability to "stiffen" the floor
> system and take this into account by not applying the load height factor
> for
> top flange loading.
> 
> Many structural engineers simply assume that all intersecting members
> provide lateral restraint to the intersected member, despite the absence
> of
> a lateral restraining system such as a horizontal floor truss.
> 
> 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 seek opinion from other engineers on the above-mentioned queries.  Am I
> being too conservative in my approach, or are  other engineers which use a
> simplistic approach incorrect and unconservative.  Discussions with
> colleagues have generated a mixed response and demonstrate that there is
> clear doubt amongst us.
> 
> My current need to resolve this doubt regarding effective lateral
> restraint
> concerns an existing structure, where the original designer has clearly
> assumed full moment capacity for those beams which have trimmers but no
> floor bracing.  I need to be confident with my design method before
> specifying a number of modifications which would be based mainly on a
> different design philosophy.
> 
> Thanks in advance,
> 
> Dave Meney BE(Civil) MIEAust CPEng
> 
> Yenem Engineering Services
> "Providing structural engineering services to the mining, process and
> commercial industries"
> 
> 54 John Street
> Gooseberry Hill
> Western Australia 6076
> Phone:  +41 8 9257 2695
> Fax:    +41 8 9257 2264
> Mobile: +41 0417 949 374
> 
> 
> 
> Yenem Engineering Services
> "Providing structural engineering services to the mining, process and
> commercial industries"
> 
> 54 John Street
> Gooseberry Hill WA 6076
> Phone:   (08) 9257 2695
> Fax:     (08) 9257 2264
> Mobile:  0417 949 374
> 
> 
> 
>