torsion analysis required? 6

[Lman321]

Please refer to the attached roof framing plan.

W9x9 cantilevered 8'-0" from column line L. They are fully welded to W24x55. W12x14 are provided for lateral buckling.

I'm trying to avoid a complicated torsional analysis on the W24x55 along L. Is this possible?

Please advise.

 

[jike]

Why don't you provide the W12x14 with a moment connection to take the twist out of the W24x55?

 

[PEinVA]

If you detailed the connection of the backspan beam to brace against any rotation in the W24, I think you can avoid torsional analysis, but you still need to determine the loads from the W6.


Also make sure to include weak axis moment in your columns as well.



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[MikeE55]

Good advice to use moment connections - I would make the outrigger the same size as the backspan beam so that the load in the flange of the beams transfers straight through the web of the W24 rather than bending it.

 

[csd72]

It would really depend on th rotation of the W24 under these loads.

One connection that I have seen is to use standard single plat shear connectors either side to take the shear and then flange splice plates top and bottom with no connection to the W24. This theoretically avoids inducing the torsion into the W24.

 

[Lman321]

Thanks to all the responses.
The outrigger is part of an architectural feature so it has to be W6.
I have over 250 outriggers so the number of moment connections will have to be minimized to reduce cost.

 

[PEinVA]

Crazy architects!

I think if your W12 provides bracing at the top and bottom flange of the W24 you don't have to design for torsion. I guess it would be similar to designing a spandrel beam for brick facade on a steel framed building, etc.



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[DaveAtkins]

To minimize cost, try to make the exterior beam span column to column. Then you will only need a moment connection at each column. Of course, a W6 may not work for the additional load. If not, you should use a W6 on each side of the W24, and put a moment connection at each W6.

DaveAtkins

 

[slien]

Could you drop the W24 by 6" and run the W6 over the W24 tying into the adjacent bent?

 

[csd72]

Just make it a tube section and be done with it.

 

[hokie66]

All you have to do is follow jike's advice in the first reply above. Moment connection of W12 to W24.

 

[rfd23]

Jicke is right, provide moment connection W9 and w12 on both side of W24. make your life easier.

 

[BAretired]

You might want to check your deflections at the end of the cantilever too, particularly at midspan between columns on Gridline L. The combined deflection of cantilever, tieback and supporting beams could prove interesting.

Best regards,

BA

 

[BAretired]

Is the top of W6 x 9 flush with the top of the W24 x 55? If you can't drop the W24 by 6" as suggested by slien, could you substitute a W 18 x 65 (almost as strong) and run the W6 over the beam thus avoiding a costly weld each side of the W24?

Best regards,

BA

 

[kslee1000]

Per your sketch, the W6 cantilevers will experience is compability torsion (the outer edge tends to deflect more).
Like BA pointed out, make sure the tip deflection is small, then I wouldn't concern the torsion.

 

[apsix]

kslee1000
I think you mean the W24 is subject to compatibilty torsion (if the W12 is acting as a backspan to the W6 cantilevers).

 

[kslee1000]

apsix:

No. The w6 will have some torsion due to deflection of the cantilever floor area in direction of gridline L. However, the floor itself and the edge channel will provide restrain to some degree, that limits the free rotation of the w6 (again in direction of "L"), thus the torsional effect is small and negligible. Maybe the word "compatibility" is misleading.

 

[hokie66]

This is a roof, not a floor. The cantilevered W6 members are not subject to torsion. The OP was concerned about the W24 beam having to be designed for torsion. A W24 or any other W shape is not much good in torsion, so it is necessary to carry the cantilever moment into the backspan.

BA's suggestion to use a bit heavier W18 to avoid the moment connections is a good example of how structural engineers should think. Use simple solutions, don't worry if complicated ones really work.

 

[kslee1000]

Hokie:

Thanks. Now I see differently.

 

[kslee1000]

apsix:

It's too bad I didn't wake up at your call. You were right, I barked at the wrong tree. Thanks your trying to correct me.