Post- tensioned box girder problem 1

[stevey]

Hello all -

We're currently constructing a post-tensioned box girder bridge (3 span continuous - 31.1m - 39m - 31.1m) with 2 slanted external /5 vertical internal webwalls. The center webwall (405 mm wide) shifted out of horizontal alignment as much as 150 mm total at the top during the webwall pour(the shift is sinusoidal, with the curve amplitude split about evenly across the 125 mm displacement). Most of the shift is near the midspan of the interior span, where ducts are placed near the bottom of the box beam soffit. The paired 100mm rigid steel ducts are side by side in the location of maximum horizontal "wave". We can't tell whether the ducts have shifted as much as the top of the webwalls, although the general alignment of the webwall at the box soffit junction is fairly straight.

We are concerned that the webwall could potentially be damaged (blown out) locally upon tensioning the strands, and/or the bridge could have global serviceablity problems.

I can't find a lot on allowable horizontal tolerances for the stranding/duct layout, or potential consequences of letting the situation stand as is. We have contacted the post-tensioning subcontractor, and they are not terribly concerned by the situation after hearing the description of the problem and examining photos.

The bridge was designed by consultant, and they are reluctant (as are we) to accept the present condition without some numerical method of verifying the adequacy of the design. They have recommended that the contractor provide calculations verifying that the webwall condition will cause no problems.

We are considering having the contractor remove the misaligned webwall and repour. Both we and our consultant have some concern, though, that removing the webwall could make the situation worse. We think we could solve a local blowout problem by chipping the wall down beyond the inside face of the stemwall bars (in order to get adequate concrete around them for bond), and doweling and pouring a thickened webwall section, but have some concern that we could damage the ducts in the process.

Has anybody run up against this situation? Would chipping out the concrete webwall make conditions worse?

 

[dangnm]

Hi stevey,

I have designed numerous c.i.p box girders and I would not worry too much about the situation you describe as long as a few items are addressed.

First, prior to stressing the post-tensioning, are there cracks at the web junctions. If not, proceed to the next.

Second, if the maximum cell width exceeds 1.5 times its depth, then warping or distortion effects must be accounted for. If so, these effects should have been accounted for in the original design, then the designers should check if the existing design meets any of these additional effects.

Third, the designers should check for the slight longitudinal curvature in the web for the slightly increased friction where this new curve pinches the finally post-tensioned tendons.

If you have vertical web bars similar to the sloped external webs, you should have sufficient capacity in the web for post-tensioning.

All these costs for design checks should be the Contractors as the forms were probably not secured effectively. For the most part if there are no cracks, it is not a big deal, so the idea is to leave sound concrete in its place.

Life is not perfect.

Regards

VOD

 

[VoyageofDiscovery]

For some reason my Handle didn't appear, though I logged in.

VOD

 

[stevey]

Hi VoyageofDiscovery -

Thanks so much for the input. I didn't note any cracks at the box soffit / web junctions, but I'll have our inspector take a look. Our plans indicate that the aspect ratio of the box width / box height is about 1.1, so we appear to be OK here. I'll pass along your comments / advice to our consultant and request they check on the friction/curvature issues, but we will definitely have our Contractor and his Engineer formally respond with some numbers.

Thanks again for the timely response. Now, back to the perfect world ...


SteveY