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FW: Lateral restraint of floor beams

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I like to think of lateral-torsional beam buckling as column buckling of the
compression flange.  The flange would first want to buckle vertically (about
it's weak axis), but is restrained by the beam web.  With this restraint,
the flange would tend to buckle horizontaly with the beam web resisting this
movement and casuing section rotation about the stable tension flange.  The
connection described below is an attempt to resist this horizontal movement
by creating a rigid frame using the strut connection as moment resisting,
beam webs (weak axis) as columns and the tension flanges as the pinned
bases.  This may be adeqaute for very small magnitudes.

I prefer to use a more direct method for bracing the compression flange by
attaching to floor diaphragms, horizontal bracing or properly designed
flexural members.  Keep in mind that bracing for stability has a strength
and stiffness criteria in order to be effective.

Curt La Count, P.E.
Jacobs Engineering
Portland, OR
 ----------
From: Horning, Dick/CVO
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Lateral restraint of floor beams
Date: Wednesday, September 02, 1998 4:49PM

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