×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

torsion analysis required?
6

torsion analysis required?

torsion analysis required?

(OP)
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.

RE: torsion analysis required?

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

RE: torsion analysis required?

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

 

RE: torsion analysis required?

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.

RE: torsion analysis required?

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.

RE: torsion analysis required?

(OP)
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.

 

RE: torsion analysis required?

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

 

RE: torsion analysis required?

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

RE: torsion analysis required?

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

RE: torsion analysis required?

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

RE: torsion analysis required?

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

RE: torsion analysis required?

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

RE: torsion analysis required?

2
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

RE: torsion analysis required?

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

RE: torsion analysis required?

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.

RE: torsion analysis required?

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

RE: torsion analysis required?

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.

RE: torsion analysis required?

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.

RE: torsion analysis required?

Hokie:

Thanks. Now I see differently.  

RE: torsion analysis required?

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.  

RE: torsion analysis required?

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.

RE: torsion analysis required?

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?    

RE: torsion analysis required?

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.  

RE: torsion analysis required?

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

RE: torsion analysis required?

BAretired,

Agree with you.

RE: torsion analysis required?

(OP)
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.

RE: torsion analysis required?

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.  

RE: torsion analysis required?

Quote:

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

RE: torsion analysis required?

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.

RE: torsion analysis required?

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

RE: torsion analysis required?

Dave:

Credit to you. Sorry to copied your idea inadvertently.

RE: torsion analysis required?

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?

RE: torsion analysis required?

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.

RE: torsion analysis required?

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

RE: torsion analysis required?

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?

RE: torsion analysis required?

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

RE: torsion analysis required?

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.

RE: torsion analysis required?

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

RE: torsion analysis required?

BA:

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

RE: torsion analysis required?

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.

RE: torsion analysis required?

BA nailed it long ago.  Why not use a W18 instead of a W24 and cantilever the W6s over the top?

RE: torsion analysis required?

Would it work as a composite beam with the W6 going across it? Just a thought.

RE: torsion analysis required?

Composite with what?

RE: torsion analysis required?

Lman: Is the project over yet? smile

RE: torsion analysis required?

hokie,

Just a brainstorming suggestion that adding concrete in this region may increase the options (even if unconventional)

Lman321,

I think the way to solve this is more financial and architectural than engineering. Give them a scheme that will work exactly how they have designed it no matter how expensive.

Then give them a scheme that is an architectural compromise such as diagonal braces, deeper canopy, or better still a canopy that is deeper toward the building but has the same edge.

Encourage them to get the options priced up.

You will then get to implement your 'compromise'.

I try to never say no to a client, instead I will say that the best I can do is this, this or this and let them make the decision.

RE: torsion analysis required?

csd,

Nothing against brainstorming, but I thought this was just a roof with metal deck, and hadn't thought of concrete as a practical part of the solution.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources