Steel Beam Compression Effective Length 1

[epitome1170]

I am curious as to how other engineers judge the effective length of a hot rolled wide flanged compression member that is used as a beam and a axial member.

Personally, I will not call a beam braced in the Lbyy unless it is braced by another beam that is at least half the depth or braced by a bottom flange brace (i.e. angle).

However, I was recently told I was over conservative with this thought process and that if the beam was braced on the top flange (by the deck for instance) that the kL would actually be much smaller (i.e. 2-3 feet) since that is how often the deck is attached.

I have been unable to find any technical information in my cursory searches, so I wanted to get a feeling for the "common" engineering thought process.

 

[WillisV]

Metal deck generally does a good job for bracing drag struts against weak axis buckling, however it is generally insufficient to brace them for torsional buckling. If you run the numbers for typical spans and member sizes I have found that the design strength with torsional buckling as the controlling limit is only slightly higher than what you would calculate assuming that the deck does not brace the member in the weak axis at all. The exception is for a composite slab in which the restraint provided by the slab and studs can act as a torsional brace, thus negating the posibility of torsional buckling and reducing the controlling limit to strong axis buckling.

 

[BAretired]

I would consider the top flange of a simple span laterally braced by deck for bending, but the beam is not braced for axial load.

A continuous beam is braced by deck on the top flange but the bottom flange is not braced for negative moment.

BA

 

[epitome1170]

Obviously, I was not clear with what I was trying to ask. Sorry.

What I am asking is not the flexural effective length (which I agree is braced on the top flange by the deck), but is the compressive flexural length. For the design of compressive members (in this case beams), would you say that the deck is bracing the member?

 

[csd72]

I have been pressed to justify this sort of thing once and I tackled it as follows:

I calculated the stress in the top and bottom flanges separately.

The top flange took the full stress from the axial load plus the stress from bending and was checked against the reatraint from the deck.

The bottom flange was checked for the axial load minus the bending tension under the full length between braces.

the exact calculation of this depends on your code.

 

[patswfc]

Your approach is conservative based on the UK code for steel design, but i would generally assume the beam to not be restrained by the deck for weak axis strut buckling, as yo do.

Not sure what code you're designing to, but BS5950 (UK code for steel) has an annex that deals with the effective length of members that have one flange restrained with the other flange not restrained having compression along all or part of its length.

The method will reduce the slenderness of the member and hence increase the compression resistance, but not to the extent that your colleague suggests.

 

[epitome1170]

Thanks, patswfc. That is exactly what I was looking for (and not because it validates my point... although that is a nice plus).

I am currently using the AISC (American code for steel). Currently, I have not seen any criteria for increasing the compressive resistance by having one flange restrained, but that makes perfect sense and I would anticipate AISC to have something in their commentary about this.