LATERAL BRACING

[PT99]

I plan to double the span of a W12 steel roof beam from 25 ft to 50 ft. I plan to add a new W21 right below the existing beam and bolt the flanges to each other. The existing beam is laterally braced with steel beams at 6 ft o.c. supporting the roof .

My question is, in this arrangement, is the new combination of 2 WF beams still laterally supported or do I have to consider this new arrangement unbraced. And what bolt spacing with pairs of 3/4 A325 would you use.

 

[JAE]

If there's a sense that the W21 below can still somewhat freely rotate in a "torsional sense" - and I believe it can with only thin web members preventing that rotation, then it needs additional bracing of some kind.

The nature, spacing and stiffness of the bracing depends on your actual conditions. I'd be tempted to ignore the existing beam (and its bracing) and ensure that the W21 is properly dealt with.

 

[jhnblgr]

Only the top flange would be laterally braced

 

[PT99]

Thanks, I am going to consider this as entirely laterally unbraced
Bolt Spacing betwen the 2 beam flanges?

 

[XR250]

If the beams bracing the W12 are similar depth to the W12, I would consider the top flange braced.

 

[JAE]

Ok - now that XR250 mentioned this - I was perceiving a W12 braced only by decking or something on its top flange. My comments above would only be applicable if that were true.

If the upper W12 is braced by W12's or perhaps even a W8 sized beam then it may provide the necessary bracing required to prevent LTB in the W21.