Staad Analysis of horizontally curved plate girder

[TS59401]

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

 

[canwesteng]

STAAD does not compute warping stresses for beam elements. You need to model the beam as a 3d element. I recommend modeling the web as plates and the flanges as beams.

 

[JoshPlumSE]

Be cautious with torsion in curved girders. I don't genuinely know what STAAD is doing, but the forces and deflections can be way off if they are not considering the stiffness effects of torsional warping. There was a good report published a couple of years ago NCHRP Report 725: Guidelines for Analysis Methods and Construction Engineering of Curved and Skewed Steel Girder Bridges.

It's a bit dry / theoretical. But, I think that's a good place to start.

 

[TS59401]

canwesteng - Thank you for that heads up, on the warping stress. At least I can stop combing through the results looking for warping stress incorporation.

JoshPlum - Thanks for the reference, I will take a look.

 

[WARose]

[blue](canwesteng)[/blue]

STAAD does not compute warping stresses for beam elements.

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 |

[blue](TS59401)[/blue]

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?

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.

 

[canwesteng]

Is that post processing in one of the design modules? I believe STAAD will design the member to some codes for torsion accounting for warping in determining stresses in the member, but the actual beam elements have no capacity to resist warping stresses.

 

[TS59401]

WARose - yes I can find the warping stress called out in the output file, but when I double check the "code check for combined stresses" it lists a reference of H1-1b and not something in H3.3. Additionally I can duplicate the staad utilization ratio pretty closely with using only the major and minor access bending moments.

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.

 

[WARose]

[blue](canwesteng)[/blue]

Is that post processing in one of the design modules?

No. It's one of the icons up on the toolbar.

[blue](canwesteng)[/blue]

I believe STAAD will design the member to some codes for torsion accounting for warping in determining stresses in the member, but the actual beam elements have no capacity to resist warping stresses.

Not sure I follow you there.

[blue](TS59401)[/blue]

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.

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.

 

[JoshPlumSE]

There are multiple aspects of warping to consider.
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.

 

[WARose]

Good points Josh. As I alluded to before, I think the fixity situation explains a lot of the differences. Some years back I did Example 5.1 (in DG9) in STAAD Pro. STAAD Pro predicted a (rotationally) more flexible beam than DG9 did. (In fact, I couldn't get rotation that small even when the ends were completely fixed.) This of course also effects warping stresses, etc.

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.

 

[canwesteng]

WARose - can you point to me where you are seeing the warping stresses in STAAD? I can't find these. As far as I know, the beam elements in STAAD (and most FEA) can't account for torsional warping, and only in the design modules does STAAD determine the stresses as described by Josh.

 

[WARose]

[blue](canwesteng)[/blue]

WARose - can you point to me where you are seeing the warping stresses in STAAD? I can't find these.

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.

 

[canwesteng]

Appears that is happening in the design module, not the FEA engine. Other codes won't output torsional stresses (at least CISC won't), and in the beam post processing tab, it won't be included in the beam force/beam stresses output. Not that there is a problem there necessarily, depending on what you are working on, but for indeterminate curved members I model them as described above.