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.

 

[EngDM]

Is the intent for the W21 to take the entire load, or are you designing this as composite with load sharing?

If the bolts are designed to transfer the shear flow such that this acts as a composite section, the compression in the W21 flange wouldn't be as high as the outer most flanges of the composite section, so you may be able to lower you load in the compressive W21 flange with the stress/strain distribtution through the cross section. The load might be low enough in this case to work as fully unbraced.

 

[PT99]

The W21 will take the entire load, as the existing W16 spans only 1/2 the distance. It also could be A36.

However my client now is considering changing this approach (because of increased total depth - he hadn't realized how deep the new beam needed to be) to just new W21's to replace the W16. This presents a new problem because all the purlin beams frame into the W16 at 6 ft o.c.. So, I am figuring on burning off a few inches of each of the lower purlin flanges to allow for fitting in the W21. Any thoughts on whether this is not a good idea. I am not figuring on welding the few inches of flange back.

 

[EngDM]

The W21 will take the entire load, as the existing W16 spans only 1/2 the distance. It also could be A36.

However my client now is considering changing this approach (because of increased total depth - he hadn't realized how deep the new beam needed to be) to just new W21's to replace the W16. This presents a new problem because all the purlin beams frame into the W16 at 6 ft o.c.. So, I am figuring on burning off a few inches of each of the lower purlin flanges to allow for fitting in the W21. Any thoughts on whether this is not a good idea. I am not figuring on welding the few inches of flange back.

You'd have to check the purlins in shear and check their existing connections for the reduced section depth as it could induce a tearout.

 

[KootK]

Any thoughts on whether this is not a good idea.

Depends. What did the original detail look like and what would it look like now? I'm having trouble visualizing it based on the information provided so far.