Steel Truss Lateral Bracing

[Youngin]

Hello. I am looking for information on using metal decking to provide lateral bracing for the top chord of a steel truss. Can the 2% rule be used here? If the deck is attached at say 12"o.c., is there any reduction for the 2% rule? Any recommended material is much appreciated! Thanks in advance.

 

[DaveAtkins]

If you are using steel deck attached to the truss top chord at 12" oc, it is braced. No need to crunch numbers, unless there is something unusual you are not telling us.

DaveAtkins

 

[Youngin]

DaveAtkins, thanks for your response. The truss is a long span (150') and the top chord has about 500 kips of compression. I didn't know if the magnitude of this force would change matters at all.

 

[Lutfi]

You will have to provide some sort of X-bracing between the trusses during erection for stability. The bracing will remain after the erection is complete. Also if you have an uplift situation where stress reversal is a possibility, you may need to brace the bottom chord as well.

I agree with Dave in that the top chord will be braced via the deck attachment. Pre-engineered wood trusses follow guidelines for lateral stability during erection and it can be found in several publications.

Regards,


Lutfi

http://www.cdeco.com

 

[sda97]

Bracing the top chord of a steel joist with metal deck is one thing but bracing the top chord of a major truss with just metal deck is another thing. The size of the top chord of a major truss would typically not require continuous lateral suport . I think you have to look at the overall stucture to determine lateral bracing requirements.

 

[Youngin]

Thanks for the response Lutfi. My main concern was the use of the roof deck as lateral bracing for the top chord. It will be attached via deck screws or puddle welds (haven't decided yet), and I was concerned that seemed a little weak to brace a top chord with such force (500 k).

 

[Lutfi]

You would be surprised to learn how little force it takes to provide lateral stability. The decking weld or screw will do that.

I was talking with a colleague few months back about the same issue. He stated to me that column was stabilized laterally, in California during an earthquake, by presence of a file cabinet!

I also heard (I do not recall the source) of researchers at Cornell University testing compressing flanges using regular tape and cardboard. Stability was achieved to a point.

Regards,


Lutfi

http://www.cdeco.com

 

[Youngin]

Does anyone know of any published work that would give some insight on how to calculate the necesary bracing force?

 

[WillisV]

The AISC LRFD Design Manual 3rd Ed. - Spec Section C3.3 for Columns also applies to the compression chord of trusses. This would be nodal bracing. Note that in the large majority of cases it is the bracing stiffness requirements and not the force requirements (which is only 1% Pu) that controls.