×
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

Analysis model

Analysis model

Analysis model

(OP)
I have an analysis model composed of bracings,beams and columns.

When I have verical loads only applied to my structure, I find the axial forces in bracing (in some of them) greater than the columns connected to it.

What could be the reason?

RE: Analysis model

Could be one of several reasons.  If you have an animation feature in your software, animate the model and see if it reacts as you expect.

Next look at nodal restraints.  You might have some restraint conditions that are not correct.

What type of software are you using?

RE: Analysis model

Is this a Chevron braced frame?  If so, then the natural load path will go through the braced frames rather than through beam flexure.  That's why the seismic codes tell you to analyze / design the beams as if the braced frames were not present.  

If this isn't a chevron, then is it possible that the axial stiffness of the brace (AE/L) is greater than the axial stiffness of the column?     

RE: Analysis model

(OP)
No axial stiffness of the brace is not greater than axial stiffness of the column.Both have same area but the brace is longer than the column.

 

RE: Analysis model

atee01,

we need more info, a sketch maybe?

RE: Analysis model

It sounds like you have a full building model, or at least a model with multiple frames and columns.

Pull out one braced frame with only its nodal loads.  You should be able to manually compute the brace and column loads or at least estimate them.  Also, at least you'll have an easier model to let you zero in on the problem.

This kind of exercise should be one of the *first* steps when the program seems to have done something weird.

RE: Analysis model

I know that when I use RAM Structural System, we design the gravity framing first, using gravity loads only.  Then enable the RAM Frame module.  What I'm trying to say is that by the time I'm on my lateral analysis, the gravity design is pretty close to complete, save for any iteration coming from RAM Frame.  But generally if you are just analyzing vertical loads, I'd not include braces for the intial sizing of your members.  Certain analysis softwares are quirky and things have to be done in a certain order or fashion to get expected results.  Depending what you're using I'd follow JoshPlum's advice.

Scott Shields
Ghafari Associates, LLC
 

RE: Analysis model

Check sshield's response, it make sense to me - take out the diagonals for gravity load analysis, and make sure your model is stable under such load.

RE: Analysis model

Again, yank out one of the braced frames into a model by itself and see what happens.

RE: Analysis model

(OP)
I removed the bracings and checked that the frame remains stable.

The compressive force in the column (the column which has less axial force in the presence of bracing) without bracing was 7.89 KN

When, I put all the bracings the compressive force in that columns was 3.5 KN and in the brace was 8.5 KN?

Any thoughts?

RE: Analysis model

It makes sense if you lateral loads on these braced frames are high compared to the vertical loads.

RE: Analysis model

Does the bldg drift in any direction due to gravity loads only?

RE: Analysis model

That was my first thought about drift too. But what is all of that you are representing in blue?   

RE: Analysis model

Ok, does the program allows "offset" commend?
If it does, "offset" the diagonal brace a certain distance from the upper brace-column joint.
If not, do offset manually.
The program couldn't recognize the secondary nature of the brace, it sees it as the stiffer main force resisting member in a truss, make sense? The offset should help to identify the main member - the column.    

RE: Analysis model

It appears from your graphic that the two outside bays that are normal to your end bay are not braced symetrically. In addition the end bay opposite your problem is not braced at all. If all that is true then you may be getting some global torsion about your center of stiffness. This would result in an axial force in the top horizontal member parrallel to your brace. This axial load would then be resisted by the brace which would be stiffer than the column.

Check your deflected shape for evidence of torsion and look for an axial force in the horizontal member framing into your brace. If this is the cause then your model is fine and you are getting realistic results.  

RE: Analysis model

I think Steve got it. The graph did shown two locations of excessive rotation for no obvious reasons, but caused by geometric arrangement of the diagonal braces. It is also evident that without the braces, the model is stable under gravity load alone.

RE: Analysis model

(OP)
I am also attaching the results of deformation for the selected beams above.

But,Steve as i mentioned , the brace is not stiffer than the column-both have same cross sectional area but length of brace is more than the column length

kslee2000, the graph attached was for translations-not rotations

RE: Analysis model

atee001:

Without numbering system on the graph, there is no way to interpretate the numerical results. However, I did notice there are excessive localized movements in direction of "X", and, albiet small, plane rotations about "Z" axis. Without attribute to irregularities in structural arrangement, I am puzzled as well. If there is no rotation (fiz) without the braces, try to rearrange the braces to a symmetrical manner, and see what results.

Or, have you tried to offset the braces from the common joint with columns?

RE: Analysis model

(OP)
Thanks a lot all.

however, I find that even on removing ALL the braces lateral drift exists.

Is this indicative of problem in defining the restraints?

RE: Analysis model

How about "fiz"?

RE: Analysis model

atee001:

I will print both out and have a look on Monday. Wish your project deadline last longer than that. :)

RE: Analysis model

Can you try a little experiment?  Can you remove all loads from the model then try applying a vertical load to the top node of each column and then check the reactions at the supports?  If the support reactions aren't all equal it might point us in the direction of a modelling error.  

What is the angle that the brace makes with the horizontal plane?  I'd like to see if the 7.89 KN, 3.5 KN and 8.5KN reactions make any sense.  The 3.5 + 8.5 cos (theta) should equal 7.89.

 

RE: Analysis model

atee001

If the load is horizontal at the top of your brace column loacation then the brace is much stiffer since it is being loaded axially while the column is beng loaded in bending.

 

RE: Analysis model

I'm not familiar with the software you are using.  Why are the support "icons" not showing up at all the column bases and why do the members have different releases at some of the column bases?

RE: Analysis model

atee001:

Your 2nd attachment turns out a blank page.
From the latest attachement, either the printing tool of the program was not accurate, or your support system is a mess. There are footings missing, also, the footing rows seem do not line up right.

Please have some one in your office check your model.

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