×
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

steel frame design question

steel frame design question

steel frame design question

(OP)
please see attached graphic file of steel frame i need to analyze. i had planned to anlayze and design this frame with all pin connections (lateral bracing will be provided elsewhere). i am concerned about torsion and/or lat. tors. buckling on the cantilevers. in a normal grillage frame i think torsion is most always negliable and/or ignored by engineers. even though the connections will be designed as pin connections there will be some moment transfer. would you be concerned with torsion here? how much moment transfer should i consider? in analyzing, AISC design guide 9 looks a little intimidating...what do you do about anlayzing torsional/ltb capacity in this case?

RE: steel frame design question

Is there a diaphragm?

 

RE: steel frame design question

(OP)
JAE - this will be a free standing structure inside a warehouse supporting wood joists spanned over/across the center steel beam with wood deck.

RE: steel frame design question

The joists can act as bracing if you detail it properly, provided that the joists and deck, in turn, can transfer the bracing force to the LFRS.

That's kind of a convoluted way of saying that all three beams can buckle simultaneously if there is no positive attachment of the deck to some sort of bracing.

What will the wood joists and deck be supporting?  

RE: steel frame design question

(OP)
frv-

(see attached revised graphic) as long as my beam strength for the full unbraced length is ok for the magenta and red beams I am not concerned about ltb if that is what your saying? my concern was the cantilever portions of the red beams that could be torqued by blue beam (basic bolted shear connection planned for red to blue) when loaded (storage 125 psf LL).

As far as lateral bracing of structure...there is no seismic and since inside no wind so no lateral load to design for (does ibc have min.? I have hear of 5 psf for walls?...but these aren't walls? could you apply...how?) In any case lateral bracing planned to be provided by chevron or x bracing in certain bays of the larger framework....I am not using the wood joists as a diaphragm to transfer load but how would you go about calculating/detailing lateral restraint of the green joists spanning on top the w shape red beam and across/over w shape magenta beam any ways?



thanks for the help

RE: steel frame design question

The 3rd edition and the green ASD steel design manual discuss the connection you speak of.  I don't know where it is in the new 13th edition.  Basically this is the case you have whenever you have top of beams and girders at the same height with the beams connecting to the side of a spandrel girder.  There are two approaches

1)  Design the beam for the torsion

2)  Design the connection for torsion.  

A one bolt connection would be purely pinned and the connection could not restrain the twisting red cantelevered beams.  The shear would transfer as an eccentric load.  However once you add multiple bolts (or even better a deck) you can check the connection itself to see if it can resist the torque caused by the eccentric load.  T = P * lever

Lever = the lever between top bolt and bottom bolt OR deck bearing and bottom bolt   

T = Shear load times distance from center of bolts to center of girder.  

Thus,

P = an axial load induced into the deck and bottom bolt of course in opposite directions.

An easier way to picture the statics is to move you connection V load to the middle of the girder.  You would be left with a V load at the center of the girder and a Moment (T).  Now try to restrain the moment with the beams that are connection to the girder.  For the case of an axial load being pushed into a deck (top restraint) and bottom bolt in shear connection (the bottom restaint) is that the axial load or force P is very small.

I check the bolts and shear plate simply by adding to the shear load the force P.  To minimize these restraint forces you could use the joist on top as one restraint (design them as beam columns in compression).  The steel beam is then only in tension which of course will be good.

Another way of looking at the physics is this.  You have a steel beam sitting on a stiffened seat on the side of a steel column.  The stiffened seat is composed on two plates.  Lets call them the flange and the web.  The flange is the plate that the beam actually bears on.  Normally these two plate are welded all around to the column.  Now lets suppose that someone forgets to weld the top of the stiffened seat to the steel column.  The only weld made is near the bottom of the bottom web plate.  The shear applied to the top of the stiffened seat is say 2 inches off the face of the column.  The stiffened plate because its not welded on top won't be able to resist the torsion causing the whole stiffened plate to rotate.  ONLY IT DOESN'T because the beam is welded to the top flange plate.  The only way for the stiffened plate to rotate and fail is if the beams bottom flange were to seriously compress.  Thus the BEAM itself restrains the rotation via a compression force induced into the beam.  The bottom web plate of the stifffened plate pushes on the column and the shear is transfered via the little bit of weld that was actually made.  Of course you column would now have a lateral load placed on it as well.  It probably won't move.  These loads are small provided the depth of the lever is long.  

Thus when making your beam to girder connection use as deep as a shear tab as you can so that your problem is minimized.  Sorry for the overkill but I stressed out about this same problem a few years ago.

John Southard, M.S., P.E.
http://www.pdhlibrary.com

RE: steel frame design question

One more confession.  I had a shelf in my closet where the connection to the drywall failed.  The connection rotated out.  I shove a VCR video box between the connector and one of the members of the shelving.  The shelf is holding up all kinds of stuff and has never failed because the shelf itself is stiff enough and strong enough to restrain the eccentricity of the connection.  The bottom bolt is the only thing in the drywall holding up the shear.  

So connections can be designed to restrain spandrel beam torsion.  Look in the steel manual in the section on beam to girder connections.  It is discussed though probably not as in depth as here.

John Southard, M.S., P.E.
http://www.pdhlibrary.com

RE: steel frame design question

If you have a pipe and a number of cantilevers span from it, if the pipe fails in torsion, there's no secondary device to hold the thing in place. The canopy drawing I remember from some ACI code. That is primary torsion. For any of these other cases where a failure of this kind can't happen, that's secondary torsion. Southard entry above shows how to deal with the problem quite well. Through some degree of restraint, here by merely the shear tab, you can ensure the secondary torsion behaviour and analyze for it. If you make rigid connections the behaviour will be there even if less easy to ascertain by hand. I have checked many times the secondary torsion for members as heavy as reinforced concrete foundation beams and rarely seen them attain significant values, I even came to some kind of conclusion of that for building works of the ordinary kind rarely one sees over 1 m·ton torsor moment (even factored) from secondary torsion, and only start to worry over such value ... for steel double tee design the values must be even lower, and also the level at which you must start to care for, since far less efficient in torsion.

RE: steel frame design question

Does the blue beam span between the red beams?
Or, is the blue beam connected to the end of red beams?  

RE: steel frame design question

(OP)
southard2 & ishvaaag - Thanks for your thoughtful response, I am still digesting and will have question for you soon...

Stillerz -
Red, Blue, & Cyan beams are all W shapes at same elevation. The beams would sit on top pipe columns (black) with angle bracing (black). Green are wood joists sitting on top of beams supporting wood deck (the wood joists and deck are actually existing but thats another story).

I was seeing the blue beam connecting to the side of the red cantilever but I suppose if the red beams instead tied into the side of the blue beam the torsion of concern would be reduced?  

RE: steel frame design question

If you are worried about the minor eccentricities in the connection causing the cantilevered beams to twist inward towards the center of the structure, then, yes, i would think running the spandrel beam out past the cantilevers would be the best approach.
 

RE: steel frame design question

If you are concerned about torsion of the cantilevered beams, then ensure that the joists limit torsional deformation of the beams.  The structure simply does not warrant torsional analysis.

BA

RE: steel frame design question

Make sure the joists are supported across the full width of the (red & magenta) beams and you will eliminate any significant torsion in those beams. The beam rotation will be limited by the joist.

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