Section Property Errors

9 posts / 0 new
Last post
trj
Section Property Errors

In PGSplice you can only input U-girders following the WSDOT shape. When I input all parameters it gives erroneous section properties. When I double checked in PGSuper I get the same errors using the WSDOT shape, but if I use the TXDOT shape it gives me the properties I calculate outside of BridgeLink.

PGSplice properties: A = 1343in^2, Ix = 985,360in^4
Correct properties: A = 1782in^2, Ix = 1,175,174in^4
Obviously 400in^2 and 200,000in^4 makes a big difference....

Is there a way to override section properties manually in PGSplice?
Is there any thought in to putting a TXDOT U-girder option in the girder library in PGSuper?
Anyone have any thoughts on how I can use PGSplice and get accurate stress results?

Thanks.
-TRJ

Rick Brice
Section Properties

Which WSDOT PT U Beam are you quoting properties for? The WSDOT configuration for PGSplice has the U66PTG6 girder defined. The gross area is 1788.183 in2. During stress analysis transformed properties are used so you'll see different values depending on the number and size of ducts, pretensioned strands, longitudinal reinforcement and modular ratio of the materials (concrete strength changes with time).

You cannot override section properties, however you can create your own girders in the library.

TxDOT has not given any indication they will expand the girder shapes offered in PGSplice. WSDOT doesn't have any need to create this shape. Perhaps BridgeSight will offer it in the Professional version.

PGSplice gives accurate stress results. The output stresses are only as good as the input section properties.

Rick

trj
Section Properties

Thanks for the quick reply and sorry for the confusion. It is neither a WSDOT or TxDOT standard shape. It's a U-78 shape used in Arizona and is more similar to the TxDOT shape than the WSDOT shape. As I mentioned, in PGSuper when I create a new girder in the library using the TxDOT template I get accurate section properties but when I create a new girder in the library following the WSDOT template and the same basic dimensions (the girder shape graphically is shown almost identical) it is not computing properties the same/accurately. My guess is however the equations are set up doesn't work for a shape differing from the WSDOT basic shape.

My issue is that in PGSuper I cannot create my own girder shape using TxDOT templates and thus cannot get accurate section properties/stresses. It sounds like there is really no work around within PGSplice?

Rick Brice
Section properties

Please post the girder you are working with. Or a link to the AZDOT drawings if you know it. I may be able to figure something out if I know the actual girder shape.

trj
Section Properties

I've updated the post with 3 files. The girder's actual dimensions and screenshots of both section views from the girder library using the TxDOT and WSDOT templates.

Rick Brice
Can you add screenshots of

Can you add screenshots of the dimension parameters from PGSuper. The shapes look the same but clearly there is a difference. My guess is that difference is in the web thickness.

trj
section properties

Both have been uploaded

Rick Brice
The section properties are

The section properties are different because the girder shapes are not the same. The TxDOT shape has wider top flanges, their is a slight slope at the top, inside face of the beam, and both of these cause the webs to be wider than the WSDOT section.

Adjust the dimensions of your WSDOT section to get the properties you want.

Rick Brice
Section Properties

We don't recommend defining girders with dimensions that attempt to morph the section into something its not (http://www.wsdot.wa.gov/eesc/bridge/software/Documentation/PGSuper/4.1/u...).

Your case is attempting to morph the top flange into a thickened section of web. This is messing up the geometry of the web, and losing cross sectional area. I can work with the code to see if it can be improved, but that doesn't help you now.

You can use a WSDOT U-Beam to get close to what you want by changing the D7 dimension from 21.9" to 9.25". You'll loose a little area for the web thickening at the top of the girder, but the web will be the right thickness. If you want to add a little more area to the section, I suggest setting W4 and D6 to non-zero values. This will make the section look like it has an inner flange, but that should be ok. The only thing that should affect is the interface width for horizontal interface shear, which rarely governs.

Log in or register to post comments