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.

 

[kslee1000]

Lman:

What is the roofing material? And how is the W6 welded to the W24, with all flanges (W12,W24 & W6) in the same plane?

I am trying to capture the bigger problem by look into a typical cross section taking thru W24. There is 12" deep beam on one side, and 6" on the other. If you connect the W12 using simple shear connection, the W6 will bend the upper 6" of the W24 (not twist), unless you can tie the W24 top flange to the W12. But with that, you might need to provide a pair of bearing stiffeners (against bottom flange of W6) on the web of the W12, which has the shear tab/angles on the way. Then, as suggested, if you weld the W12 to W24, I afraid now you are going to have a torsion problem along the W24, unless the unbalanced moment is small. Is there any better way to eliminate/minimize the local bending?

Interesting problem.

 

[kslee1000]

Another thought:

Can you provide stronger cantilever beams along column grids ".6 & .9", and replace the C6 with deeper stronger shape. This way may free up the end moment requirement on W6.

Just talking loud. Anybody?

 

[ali07]

BAretired:
You provide a good solution. But how you will support the deck in that option. It would rather the same cost to provide additional members at top again to support deck. More craft work for moment connection on both sides of W24x55.

I think it is not a balancing condition. 8ft cantilver shall be balanced by 16 ft span or otherwise design for torsion. Why not the beams W16X31 orientation changes to rest of W24 beam and replace 24K7 joist with beam and have moment connection of W6 beam only.

 

[BAretired]

ali07,

I am not convinced that a good solution has been presented by anyone thus far or that a good solution is possible within the parameters set by the architect. I am not happy with the short tieback even though I have not performed any calculations to show that deflections are excessive. I am not happy with the depth of the cantilever. I am not happy with changing the direction of the steel deck in the middle of the roof.

There are many factors in this thread which could be improved from a structural point of view, but the essential issue is to satisfy the architect. That is (hang onto your hat) not an easy problem.

When structural engineers are forced into silly situations, they must decide whether or not they are capable of changing the rules of the game. If they decide they cannot, they must decide whether or not they wish to continue to participate in the project. To make an informed decision, they must carry out extensive calculations to determine how the structure will behave. If they are not prepared to accept the responsibility for the behavior of the structure, they should withdraw.

Unfortunately, that is our lot in life.



Best regards,

BA

 

[ali07]

BAretired,

Agree with you.

 

[Lman321]

Wow! Thanks for a very interesting thread. I did not anticipate this much response. I'd love to discuss architect-structural engineer relationship but I am under the gun on this project so here is what I came up with. (please see the attachment).

I have decided to put moment connections at columns and at midspan of W24. I thought this was my best solution at this time.

Thanks again. I hope to post another interesting problem soon.

 

[kslee1000]

Lman:

As the W12 will have a 1/4"-1/2" set-back from the web, make sure the bottom flange (in compression) will not create problem on the web of W24. That was one of my concerns embedded in my previous response.

 

[BAretired]

I have decided to put moment connections at columns and at midspan of W24. I thought this was my best solution at this time.

Only at columns and midspan? Don't you need it for every beam?

Like kslee, I would be concerned about the transfer of compression from the bottom flange of the W6 through the web of the W24 and into the web of the W12. Would advise checking this very carefully before going to tender.

Best regards,

BA

 

[kslee1000]

How about weld a narrow plate (say 3"x6") centered on the bottom flange to smooth out the compression force (note that you have a T - web + 3/8" stifferener PL).

By all means, I still prefer to see the moment connection can be eliminated. How this sound: let W6 be simply supported by the W24 and the edge beam, which is supported by the cantilevers along the column grids. The requirement on the edge beam is determined by strength, and the combined deflection from itself plus that of cantilever. If this scheme works, you are home free without much headaches. Good luck.

 

[DaveAtkins]

kslee1000,

That is what I suggested earlier in this thread. I assume the W6 cantilever can't take that much load. Is that correct, Lman321?

DaveAtkins

 

[kslee1000]

Dave:

Credit to you. Sorry to copied your idea inadvertently.

 

[kslee1000]

If I didn't make mistake, the W6 should be able to sustain 200 psf of load (assume A36 steel, Fb=0.6fy, V=0.4fy, L=8', S=6', selfweight not deducted from the derivation) - high side for a roof?

 

[csd72]

why not get rid of all the intermediate cantilever beams, beef up the cantilevers from the columns and then provide another 2 beams parallel to the W24.

No torsion and less connections.

 

[BAretired]

kslee,

200 psf? How did you get that?

csd72,

Good idea, but will not satisfy the architectural requirement of minimum 6" depth for cantilever.

Best regards,

BA

 

[kslee1000]

BA:

Sx=5.56 in^3
Fb=0.6(36)=21.6 ksi
M=Fb*Sx=21.6*5.56=120.6 "-K=10 '-k
M=WL^2/8=W(8^2)/8=8W=10 '-k
W=1.25 klf
w=W/tributary width =1.25/6=0.208 ksf=200 psf

Sounds right?

 

[BAretired]

kslee,

The beam is an eight foot cantilever, not a simple span. M = wL^2/2, not 8. As well, the laterally unbraced length of the compression flange is 8'-0, so stresses must be reduced.

Best regards,

BA

 

[kslee1000]

I was answering DaveAtkin's question, both of us want to make W6 simply supported, but not quite sure the capacity of it.

So, if not for any other concerns, Lman may elect to further down size the W6, sure, a quick check would be required.

 

[BAretired]

If the architect will accept a 16" (or so) deep edge beam then this would be the way to go. I was under the impression he wanted to keep the depth of overhang to 6".

Best regards,

BA

 

[kslee1000]

BA:

You could be right, the Architect holes the key. Since, I guess, the cantilever portion of the roof is exposed to view.

 

[hokie66]

Lman321,

They are all trying to change your framing layout, but I think you are stuck with it.

I think you definitely need the moment connection at each W6, not just some of them. Anyway, your original drawing doesn't show a W6 at midspan.

Be careful about developing the bottom flange, as others have flagged.

The splice plate over the top will not win you any friends with the deck layer.