AISC clarification Table 3-10

[engineerED930]

Can someone please explain what they mean by unbraced length in table 3-10 of AISC? Is it lateral bracing? (e.g. the roof rafters sitting over the steel beam I am specifying are 2-foot OC, would that mean that the unbraced length is 2'?)


Thank you in advance for your help.

 

[KootK]

It is lateral bracing of the compression flange. Yes to your beam being, effectively, continuously braced. The rafters themselves need to be prevented from translating but, in the majority of cases, the roof sheathing will do a good job of that.

 

[phamENG]

Generally, yes. But only if the connections and bracing members have sufficient strength and stiffness. Take a look at appendix 6 and the associated commentary.

 

[BAretired]

The unbraced length of a steel beam is the span beyond which the maximum factored moment starts to reduce due to lateral-torsional buckling.

A beam supporting rafters at 24" centers would be considered braced at 24" centers if the compression flange is braced at 24" centers.

 

[roykent]

Not really related, and I can't tell if that's a newer version of the AISC Manual or not, but Table 6-2 in the 15th Edition organizes this same information (Moment strength as a function of braced length) in a much easier to digest chart.

 

[engineerED930]

Not really related, and I can't tell if that's a newer version of the AISC Manual or not, but Table 6-2 in the 15th Edition organizes this same information (Moment strength as a function of braced length) in a much easier to digest chart.

It is the 13th edition.

 

[Eng16080]

the roof rafters sitting over the steel beam I am specifying are 2-foot OC, would that mean that the unbraced length is 2'

Yes, assuming positive bending. If there's negative bending, though, then the bottom flange would instead be in compression and I would not consider the rafters as providing lateral bracing.

 

[engineerED930]

If I braced the web, would it suffice the requirements or would I still have to brace the compression flange?

 

[canwesteng]

If you brace the section against rotation you could connect to the web. This is likely a more costly option than a fly brace down to the bottom flange though