Has anyone lengthed an existing bridge?
Has anyone lengthed an existing bridge?
(OP)
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.
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.
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RE: Has anyone lengthed an existing bridge?
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.
RE: Has anyone lengthed an existing bridge?
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.
RE: Has anyone lengthed an existing bridge?
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.
RE: Has anyone lengthed an existing bridge?
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.
RE: Has anyone lengthed an existing bridge?
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?
RE: Has anyone lengthed an existing bridge?