Bridge Widening 3

[jike]

I am looking at a small town road bridge, 40 foot span. The current bridge is 21 feet wide. They want to add on 16 feet in width. The existing bridge has two 26" deep precast concrete girders with a 10" concrete deck. The girders are spaced 7'-4" on center, so the deck cantlevers 7'-4" also.

It seems logical to me to use the same construction as the existing bridge, however, someone else has proposed using a slab bridge for the addition.

If I use prestressed girders for the addition, I expect to get axial shortening in those girders. Should I have an expansion joint between the old and new deck to allow for separate movements?

If I use a slab bridge for the widening, it still seems that I should have an expansion joint between old and new.

The bridge has an asphalt overlay. How does a joint work with the asphalt overlay?

 

[SlideRuleEra]

Forty feet is a very long span for a flat slab bridge carrying highway traffic. Of course it can be done but other designs (prestessed concrete beams, steel beams, or even old fashioned deck girders) are probably more cost effective for spans over 30 feet. Your idea to use construction similar to the orignial sound reasonable.

In my experience (jobs of this size, in warm regions - South Carolina), there is no need for longitudinal expansion joints between the old and new deck. In fact they are often bonded together securely with rebar dowels. Both portions of the bridge will respond in similar fashion to temperature change, this applies even if you decide to go with a flat slab design for the addition.

 

[jike]

I just found out that existing bridge does have the old fashion concrete deck girders, not prestressed. It seems to me that the long term shortening of the prestress beams could cause cracking in the deck, or can this be taken up in the detail at each abutment?

I know in building construction, there are square dowels that you can use that have comp. material on the 2 vertical sides. This allows the vertical shear load to be transferred across the joint without the horizontal restraint. I suppose this can also be resisted by longitundinal rebar in the slab.

 

[CanyonRat]

40' is too long for a slab unless it's prestressed. I've been involved with a lot of widenings over the years and a few times have switched girder types with no problems. It does help to have the substructure on piles... Also worked on widening a few T-girder bridges.

Just match existing for ease of design. If the falsework/shoring is a problem then use prestressed girders or slabs.

Never have I used or seen a longitudinal joint at a widening. If you can pull off enough deck then splice onto the existing bars in the deck. If not epoxy and dowel new ones in. In both cases the existing concrete is coated with some type of bonding compound and the new concrete placed right up next to it.

This depends on whether you need to keep the bridge open to existing traffic, widening one or both sides, etc. Check the phasing of construction.

MikeD

 

[broekie]

You won't need a joint, but you definitely need to look at how construction is going to be staged. The existing 7'-4" sounds like an awfully long cantilever. I am not used to anything more than about 3'-0".

Does this bridge carry any truck traffic and/or is it load rated?

 

[jike]

I agree it is a long cantilever! It is rated for HS20-44. It has a 10" top slab and two 26" deep beams, all poured in place.

 

[yakpol]

Jike,
The new superstructure stiffness (per width of bridge) shall be close to the existing, if it's lower you might overload existing girders with the live load. Normally widening is constructed as a standing alone supported by permanent piers or shored, and then connected to the existing structure with the longitudinal closure pour strip. This staging does not add dead load on the existing bridge and allows to reduce differential creep and shrinkage.
In the case of large existing slab overhang, you will have to place new girder as close as possible to the existing, since the slab between girders will be now in positive flexure but reinforced mainly for negative moment. You might want to remove part of the overhang but to save rebar to couple with the new. Typical detail for prestressed girder bridge widening is to remove overhang all way to the girder, might not work in your case... Good luck!

 

[koodi]

Inspect the existing structure, make sure there is no structural deficiency. Determine if the existing structure meets the present design loads. If yes, widen per existing. This way the addition will have the same response and stiffness as the existing.