Staad Analysis of horizontally curved plate girder
Staad Analysis of horizontally curved plate girder
(OP)
I am working on a girder erection plan for a bridge with a relatively large radius curve (r=2200ft). I am trying to check the STAAD torsional analysis using AISC design guide 9. My STAAD model is a series or 10ft segments making up the curved spans of ~125ft, and these segments are loaded with a member load. STAAD is recognizing that there is a torsional moment, and even appears to calculate shear stresses and warping stresses (Torsion parameter set to 1.0) in the detailed output, but I cannot see that these stresses are being combined with the strong axis bending stresses.
My method for double checking with DG9 is to assume a concentrated torque created by the self-weight resultant acting through the centroid of the arc. The hand calculations produce much higher compression stresses than STAAD.
The torsional moment is not much (about 8 kip ft), but I cannot prove to myself that staad is actually considering it against the buckling stress for the compression flange.
I would appreciate any advice on reconciling these calculations. Is my hand approximation not relevant to the loading? Is should I be applying a different type of load in my model? Something else I am not seeing?
Thanks,
Tom
My method for double checking with DG9 is to assume a concentrated torque created by the self-weight resultant acting through the centroid of the arc. The hand calculations produce much higher compression stresses than STAAD.
The torsional moment is not much (about 8 kip ft), but I cannot prove to myself that staad is actually considering it against the buckling stress for the compression flange.
I would appreciate any advice on reconciling these calculations. Is my hand approximation not relevant to the loading? Is should I be applying a different type of load in my model? Something else I am not seeing?
Thanks,
Tom






RE: Staad Analysis of horizontally curved plate girder
RE: Staad Analysis of horizontally curved plate girder
It's a bit dry / theoretical. But, I think that's a good place to start.
RE: Staad Analysis of horizontally curved plate girder
JoshPlum - Thanks for the reference, I will take a look.
RE: Staad Analysis of horizontally curved plate girder
I've used STAAD for torsion all the time.....and it always gives me warping stresses in post-processing. A sample:
| MEMB 15 : Stresses caused by Torsion (MX = 9.360E-01) |
| START: FIXED END: FIXED |
| UNITS - KIP INCH |
| C.G. of Flange : |
| Shear Stress due to warping : 1.719E-01 |
| Shear Stress due to pure torsion : 0.000E+00 |
| Normal Stress (along axis) due to warping : 1.668E+00 |
| Junction of Flange & Web : |
| Shear Stress due to warping : 1.099E-01 |
| Shear Stress due to pure torsion : 0.000E+00 |
| Normal Stress (along axis) due to warping : 0.000E+00 |
| C.G. of Web : |
| Shear Stress due to warping : 9.745E-02 |
| Shear Stress due to pure torsion : 0.000E+00 |
| Normal Stress (along axis) due to warping : 2.777E+00 |
Are you sure you are combining stresses at the right location(s)? And by the way, it isn't unusual to see differences between DG9 and what STADD gives you. I specifically checked a problem worked in DG9 via STAAD.....and there were differences based (in part) on torsional fixity differences.
RE: Staad Analysis of horizontally curved plate girder
RE: Staad Analysis of horizontally curved plate girder
I suppose the warping stress could be in tension where the bending moments are both in compression, but DG9 seems to indicate that you add them together as if they are all in compression.
I also wonder if my torque at 8kip feet is too low and getting lost in rounding errors on my end. Any thoughts on that? These girders are 92in tall with 2ft wide, 1in thick flanges at the smallest section.
RE: Staad Analysis of horizontally curved plate girder
No. It's one of the icons up on the toolbar.
Not sure I follow you there.
Yeah, with a monster like that.....you'd probably be getting some pretty low stresses. But with a horizontally curved bridge member.....are you sure you shouldn't be getting more? With some of the dimensions you are calling out....I'd think it would be much more torque than 8 ft-k based on live load alone.
RE: Staad Analysis of horizontally curved plate girder
1) The stiffness of the model. Warping restraint adds a lot of torsional stiffness to the member. Therefore, if this isn't included in the stiffness formulation of the members, then the torsional moments you get from analysis will tend to be significantly too low.
2) Once you have a torsional moment, converting these moments into warping stresses.
It is very possible that a STAAD post processor does the 2nd option. However, I'm reasonably confident that they do not account for the first option. That's not a slight against them (or any other software). It's just one of the things you want to be aware of when torsion becomes a main criteria of design. I believe that report I mentioned goes into more detail about these issues.... of course.
RE: Staad Analysis of horizontally curved plate girder
I didn't see anything that made me think "I'll never use this again." But it definitely made me back check everything more closely from that point on.
RE: Staad Analysis of horizontally curved plate girder
RE: Staad Analysis of horizontally curved plate girder
First of all, be sure you have the correct commands for these to appear. In the edit file, state it this way:
<after load combinations in the edit file>
PERFORM ANALYSIS PRINT ALL
LOAD LIST 2
PARAMETER 1
CODE AISC
UNT 72 MEMB 1 TO 16
UNB 72 MEMB 1 TO 16
LY 72 MEMB 1 TO 16
LZ 216 MEMB 1 TO 16
*
TORSION 2 MEMB 1 TO 16
*
TRACK 2 MEMB 1 TO 16
*
CHECK CODE ALL
*
STEEL TAKE OFF ALL
*
FINISH
---------
The "Track 2" command will give you detailed info in the post-processing file on the warping stresses, code check, and so forth.
Then click on the "STAAD Output" icon on the tool bar and it should appear. (I still call it "post-processing" because that's what it was in a much earlier version.)
See the attachment (below) to locate these icons on the toolbar in STAAD.
RE: Staad Analysis of horizontally curved plate girder