Hello,
This is Yuan, a bridge engineer with King County. I'm designing a new bridge that is 65' (clear span) simple span with 4% super elevation. The girders we proposed at this moment (30% design) are five WF39DG girders, 7.5' width, with UHPC closure between girders.
My question is that should I consider the UHPC width of 9" in my model? I feel that I don't need to and just model the flange with 7.5' wide, but wondering how it will be during fabrication. Is the flange of interior girder 4.5" shorter at each side to give the 9" UHPC closure spacing? And just one side shorter for the exterior girder?
By the way, I'm also wondering how to form the UHPC between the flanges with different depths. Please see the PGSuper file I attached for your information.
Thank you so much!
Yuan
| Attachment | Size |
|---|---|
 WFDG with UHPC closure | 113.63 KB |
There is two ways you can neglect the width of the joint but both have design consequences that you will have to compensate for.
1) Use the true top flange width of the girders and have out-to-out width of the bridge be smaller than actual. This will result in smaller spacing between stems and will have some influence on the live load distribution factors.
2) Make the top flange width wider than actual so the out-to-out width of the bridge is equal to the actual value. This will result in girder section properties and weight that are larger than actual influencing the stresses, prestress losses, camber, flexural strength, etc..
Modeling the joint width and the actual top flange width is easy in PGSuper. Both values can be entered directly. For all top widths being equal, top width = [(Out-to-out width) - (Joint Width)(Number Girders - 1)]/(Number Girders)
If you model the joint, you can accommodate it with the top flange by either making the exterior girders more narrow than the interior, as you've suggested. Alternatively, you can make the top width of all the girders equal.
I don't have experiencing with forming joints in this configuration. I can't imagine it being too much different than for constant depth joints. The forming system will have to be somehow suspended from the girder flange.
Rick,
Thank you so much for your reply. I prefer to model the joint and the actual top flange width as your mentioned. Just a couple of follow up questions, please see below:
1) As the bridge cross slope is achieved by varying top flange thickness so the girder top flange is not symmetric, is there any concern about keeping the prestressing force aligned with the section CG?
2) The UHPC closure is a trapezoid shape for my girder configuration (different flange thickness for adjacent girders), do you have any concern about force distribution at this location?
One more question, I didn't see the WF42DG in the PGSuper libraries, however, it is mentioned in BDM 5.6.7E, seems like it exists :) Do I need to create one by myself by copying other WFDG, then change the D7 dimension? Just in case I have to switch to this girder.
Thank you so much!
Yuan
1) For the span length and girder depth, the biaxial stress effects should be small. PGSuper has you covered on this front. Since the section is asymmetric PGSuper will do a general biaxial stress analysis that accounts for a non-zero Ixy until the girders are connected with the UHPC, then uniaxial bending is assumed. PGSuper also has the option to adjust the left and right side top flange overhangs. This will move the CG. You can pick your own values with the "Asymmetric" option or have the computed automatically such that the CG is aligned with the centerline of the web using the "Centered CG" option.
2) No concerns from a longitudinal bending perspective on the composite section. Though, let your engineering judgement by your guide.
Regarding the WF42DG in BDM 5.6.7E... this is a typo. It should be WF45DG.
There are the WF-TDG and WF-DG girders. The TDG are the "thin" deck girders. The "DG" are the full depth decked girders. The difference in height is 3" (the DG's have a 3" thicker top flange). If you look at the DG girder standard drawings, they reference the WF45DG.
Correction - BDM 5.6.7E the typo is WF42DG should be WF39DG for the beam that's 3" taller than the WF36TDG
Hi Rick,
Just a follow up question regarding the WF42DG in BDM 5.6.7E (ii). Do you mean the WF42DG should be WF39DG in the following paragraph in BDM:
"These girders shall be limited to simple span bridges with roadway with cross slopes of 0.04ft/ft or less. WF42DG, WF45DG, and WF53DG girders may be erected with the web plumb or perpendicular to the roadway surface. Erect all other girders with the web plumb."
If so, the WF39DG may be erected with the web perpendicular to the roadway surface, is it right?
Thank you so much!
Yuan
Yes, that is correct