Has anyone lengthed an existing bridge?

bridgebuster · Sep 14, 2019

Has anyone lengthened an existing steel bridge? Google wasn't any help. Here's my dilemma:

I'm working on a design-build tender. The project involves a number of bridge replacements and bridge retrofits. On the retrofit bridges, the owner wants to lengthen the existing W33 stringers and keep the deck. And they don't want any sag and they don't want a rigid frame.

In the image below, the as-built bridge is cambered 3/4" to level out. One problem is trying to predict the deflection. The two professors (yes they are) who have been studying it this week don't really understand the problem. At first they're telling me the new deflection is 1 1/2", but it turned out that value is for the W33 with a completely new deck; although the net deflection would be about 3/4" (1.5" - original camber). Then they came up with a stiffened girder to hold the deflection to 3/4". To me that probably doesn't work because the girder has leveled out.

In my mind, 1)the stringer extension has to be fully shored when placing the deck, 2)a strongback has to be installed above each stringer to hold it in its vertical position while the stringer is stiffened, 3)after the shoring below the extension is released, the strongbacks can be released but it's still going to deflect, even if the original end of the stringer is jacked.

I think the only way to eliminate the sag, short of bridge replacement, is post-tensioning, which may not go over well with the owner.

BridgeEI · Sep 14, 2019

What if you milled 2” off the deck, did your extension, then did a 2” concrete overlay to profile the deck so you didn’t have a sag? You could even get survey shots on the deck before profiling to see if it was even going to be an issue.

Im with you though. The only other work around I’d have is to add cover plates on the existing beams, shore the beams, do your extensions, then remove the shoring. It’s still going to deflect just like your proposal.

I’ve lengthened them in the past but deflection wasn’t a constraint for us.

bridgebuster · Sep 15, 2019

BridgeEI - that's a good idea to eliminate the deck sag. Yesterday I came up with a crazy scheme to minimize the girder sag. I add the 15' of lengthening at each end but leave it as a cantilever when the deck extension is cast. I calculated that produces an upward deflection of 1/2" (original DL camber is about 3/4"); after the bearings are installed and the temporary supports are released the deflection, assuming the composite section, is a little over 0.6", leaving a 1/8" sag. As a precaution I could stiffen the bridge with side plates at the bottom of the web and reduce the deflection to about 1/2". In theory - or more like in my mind - it balances out.

The bridge has vertical curve camber, (max. of 6")so no one will see the dead load sag but it was built with 14'-6" vertical clearance, which can't be reduced. I should be able to jack the superstructure an inch or two as long as I don't violate the vertical requirements for the bridge above.

BridgeEI · Sep 16, 2019

That’s a pretty good idea but how would the contractor you’re dealing with handle it? In my area, they for sure would put the bearings down before casting the deck, more so out of habit.

What are the grades on the bridge? Can you jack the bridge up 6”, or some distance, and do everything else normal? If the approaches tie in then it may be more feasible.

bridgebuster · Sep 16, 2019

The modified bridge will have integral abutments, no bearings. My thought was to set the end of the extended girder on blocks, after casting the deck remove the blocks to get the upward bounce; set the end of the girders then remove the temporary support.

There could be room to jack; there's a bridge above crossing perpendicular but I don't know the clearance, waiting for survey. The profile shows 15'-6" clearance, which is the minimum requirement. I'm not sure I have an inch or two to play around with. The approaches are being reconstructed so the grade won't be a problem; the plans show 6.4% grade.

wiktor · Sep 17, 2019

Bridgebuster,
I dd some studies for the widening of CBXE some years ago, but all the bridges were or frame, or simple supported.
The last solution seems feasible, but I will suggest the following changes:
1.jack up center bearings approximately 1.5" up
2. set extended girders on new integral abutment and connect to existing girders
3. remove the supports at integral abutment
4. cast the extended deck
5. set the extended girders on the bearings
6. remove supports at the old abutments
I did some very preliminary calculations using simplified model and the DL deflection is approximately 0.69". As I used non-composite section properties the real deflection will be max. 50% of it, so acceptable.
You were joking about the professors?

bridgebuster · Sep 21, 2019

Wiktor, thanks for your thoughts; I apologize for the late reply;. It's been a crazy week. We got clearance measurements so 1.5" of jacking could be done. We looked at a different scheme this week: Jack the girder at the existing abutment about 1"; splice it; cast the deck with the extension fully shored, then release the jacks. The deflections are acceptable. Meanwhile, the contractor has their own idea how to do things so we have to our scheme work with theirs.

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