lateral support for steel beam

[struengineer]

I am designing a steel beam to support a floor system which is made of 18” deep timber joists and plywood floor. Joists are hung on the top flange of the steel beam with 12” space(30' span ). My question is:

Can I consider this steel beam as fully lateral supported? Does any one know a book or a website discussing about that. Thank you for help.

 

[DaveAtkins]

IF the beam is simply supported (no compression in the bottom flange), and IF there is enough floor system to prevent buckling, then these members should brace the beam.

DaveAtkins

 

[UcfSE]

If the connections are adequate and there is a load path to get the buckling force to the foundation, you should be able to assume the beam is braced. You really have to look at your connections though. Nails and hangers are not typically very stiff. Sometimes in this situation I might only assume the beam is braced only in the middle. Then you get some benefit and you'll have several connections in the middle to take the bracing force.

 

[struengineer]

Thanks guys!

The beam is continuous. My question is actually how to know “IF there is enough floor system to prevent buckling”

Simpson hangers are welded to steel beam on the top flange to support joist. I am not sure if it is good enough or not. In my case, considering lateral support and not will affect final cost a lot. However, I lack of any data to support my assumption.

 

[DaveAtkins]

It is kind of an intuitive thing, deciding how much floor system is needed to brace the beam. Is the floor diaphragm more or less square, or do these 18" joists on each side of the beam only span a couple of feet? The former would brace the beam, while the latter might not.

DaveAtkins

 

[UcfSE]

Have you tried the 3rd ed. AISC manual? There's a procedure there describing both strength and stiffness requirements for beam bracing. You need to figure out what you want to assume for bracing and then figure out your strength and stiffness requirements. From there you can see if your details meet those or if they need to be altered.

For instance, if your joists beared on the beam on a bearing plate and were toe-nailed, that probably isn't good enough to brace the beam. If on the other hand your floor sheathing was fastened to the beam flange directly with powder-driven fasteners and the floor sheathing attached to shear walls, then you're probably braced pretty well.

As Dave said, it can be very intuitive and is part of the art of engineering as opposed to the science. Given some rational engineering analysis and jugdment you can usually figure it out and be pretty safe. Besides the AISC manual, you can also look in Salmon and Johnson's Steel Strucutures. They have a rational method for designing beam bracing also. Keep in mind it is out dated, but probably works. For my part I assume a larger unbraced length than the joist spacing, say midspan bracing for instance, and things work out fine, but that's what makes me and my boss comfortable. You may be ok with something different. I'd go with the text book, the AISC and some gut feeling and see what you come up with.

 

[MeggieM]

There is a report called' lateral stability of steel beam' or similar names.

It gives example for this kind of condition, it does not suggest to hange the timber joists to the top flange, you should use timber block fixed on the bottom flange and sit the timber joist onto the block.

Further consideration please see that guide



Meggie