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LTB for I Sections

LTB for I Sections

LTB for I Sections

(OP)
Hi,

To avoid any issues with with lateral torsional buckling for beams supporting walls in a domestic situation which may not be considered to be laterally restrained by floors/roof/friction of blockwork on compression flange do any Engineers out there use RHS sections in lieu of UB's?

Also for the effective length do you also have to apply table 13 of BS 5950.

I know this topic always gets a lot if discussion amongst Engineers with opinions varying.

John.

RE: LTB for I Sections

1) Sure, RHS is a valid approach.

2) For CMU above, I've had deformed bar anchors welded to the top flanges of wide flange beams such that the supported wall itself provided rotational LTB bracing.

3) Sometimes it's hard to find a sufficiently stiff RHS. On occasion, I've used wide flange beams with a pair of channels welded to both sides of the beam webs to form an HSS. The best of both worlds mechanically but more expensive to fabricate.

How is it that there's no floor or roof framing near by to provide LTB bracing? That's a rare condition. If you provide more detail, we might be able to generate additional options.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: LTB for I Sections

(OP)
Hi kootk,

There is a timber joist floor located just above the UB with 12mm floor boards but a lot of Engineers in the uk don't seem to consider this as adequate to provide lateral restraint.

John.

RE: LTB for I Sections

Generally speaking yes, the
use of an RHS section effectively renders LTB considerations a moot point. The reason for this is best illustrated by examining eqn 3 from this link http://eurocodes.jrc.ec.europa.eu/doc/WS2008/SN003.... Although the equation is only applicable to singly symmetric cross-sections, the equation highlights the importance of two very important parameters in the calculation of Mcr.

You can see from the equation that Mcr is a function of the torsion constant, It (J depending on your notation) and the warping constant, Iw. For open sections like I-beams, the torsion constant will be very small and thus Mcr is predominantly a function of a beams warping stiffness.

Now, for an RHS section which is a nice closed, torsionally stiff section, J will be typically an order or magnitude or greater than the warping constant Iw.

Thus, for boxes which do not have an excessive depth to width ratio (we're talking much greater than 2 or 3 here), LTB is no longer a consideration. Of course, you now have an additional web that doesnt particularly do very much from a cost of steelwork point of view, and it might be awkward having to have one specific connection for a couple of RHSs where the rest are I beams.

Hope this helps, Pete

RE: LTB for I Sections

Also I agree, I dont think the timber joist flooring would provide sufficient lateral restraint. Also would definitely avoid the use of 5950 in eurocode times, if you go to TATA steel blue book theyll have tabulated values of Mcr for you for various spans although these will be somewhat conservatively derived as they are for general purposes only.

Pete

RE: LTB for I Sections

(OP)
Ps: There is also a roof load at higher level loading the wall to be supported.

John.

RE: LTB for I Sections

I say use that timber floor deck unless there's some code prohibition against doing so. Chances are, that same timber diaphragm is what's stabilizing the columns supporting the beam anyhow.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: LTB for I Sections

(OP)
Pete

In the tata blue book the tables give a c1 value of 1 to 2.5 for loading type/ end conditions, where is this explained?

John.

RE: LTB for I Sections

The document which i referenced in my previous post explains the c1 factors in sufficient detail. I dont know if theres any UK national annex provisions, but itll probably be explained in SCI P363 on which the blue book is based.

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