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RE: Lateral restraint of floor beams

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Dave,

In our office, we do not consider that grating provides an adaquate
diaphragm for bracing the compression flanges of steel beams.  We provide a
direct and positive horizontal brace (either flexural or axial) with the
appropriate stiffness.  Connecting to an adjacent beam with a strut
generally does not qualify as an adequate brace for both strength and
stiffness reasons.  If you have access to the text "Steel Structures" by
Salmon and Johnson (my favorites since they were my professors in college),
they have an excellant explanation of this situation.  Generally the easiest
solution is to provide a diaphragm or horizontal truss to span to vertical
bracing or to design the beams as unbraced (not normally economical).

Curt La Count, P.E.
Jacobs Engineering
Portland, OR
 ----------
From: Dave Meney
To: seaint(--nospam--at)seaint.org
Subject: Lateral restraint of floor beams
Date: Tuesday, September 01, 1998 6:57PM

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