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One bridge or two?

One bridge or two?

One bridge or two?

(OP)
I have a bridge that is pretty wide. Out to out is 101', 4 lanes, with shoulders and shared use paths on both sides. At what point does this become two adjacent structures?

It's wide enough now that we need to consider expansion in the transverse direction with the bearing design, breaking up the deck pour with a longitudinal construction joint. I'm also having to deal with sloping the pier bent caps top deal with pedestal heights.

The bridge is 3 span with semi-integral abutments and no joints on the bridge so I'm hesitant too introduce a longitudinal joint. Its also in a coastal area with substructure units in salt water

The design is telling me I should, but maintenance wise it might be better to leave as a single bridge. What guidance is there to break this up into two bridges or leave it as one?

RE: One bridge or two?

4 lanes on each direction?

RE: One bridge or two?

Bearing capacity of the foundation says the geotechnical engineer. If you are doing piles to hard rock leave it as one, if you are doing a friction fit pile or rectangular footings it may be worth splitting it up into two.

RE: One bridge or two?

(OP)
@GeoEnvGuy - friction piles. I quickly ran both scenarios because we are right up against the limit of the pile capacity, currently getting CPT testing done to see if we can increase capacity. Pile capacities actually improve with a single bridge. Is the concern uplift of the far piles or something else?

@retired13 - 4 lanes total (2 each direction), but based on the width and future expansion (by removing the shared paths) the bridge has been designed for 8 lanes.

RE: One bridge or two?

I have split up piers before so I didn't end up with a 101' long pier cap.

At my previous employer I designed a 141' wide by 170' long 3 span steel beam bridge with conventional abutments. It ended up being one bridge because of phasing, the median section was needed to run traffic on. It is in service now and I'm not aware of any problems.

We also designed a 167' wide by 201' long 3 span PC beam bridge with conventional abutments. It was one bridge because of construction sequencing too. It's been in service for at least 5 years and I haven't heard anything bad about it either.

We didn't consider expansion in the transverse direction for the superstructure.

RE: One bridge or two?

Friction piles settle so it is a matter of the basic principle between foundation size and settlement. A larger foundation will settle more than a smaller foundation even if they have the same bearing pressure.

RE: One bridge or two?

Have you considered employing a single superstructure with multiple piers or bents at each line of support? We've done a couple of the those. The crossframes connecting the girders where the pier cap is discontinuous, may require a lot of scrutiny for seismic concerns. Either making them stouter or designing them as a structural fuse may be in order, depending on what ERS is overall.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: One bridge or two?

How about design a median to separate the bridge into 3 parts with flexible joints in between.

RE: One bridge or two?

The bridge could be designed as a single structure using guided pot bearings aligned according to the vectors of expansion - this actually shoul be done for any wide bridge, say above 4 lanes. The design done with assumption that the structure is expanding/contracting only in longitudinal direction is just plain stupid, and cracked webs in the external stringers, or tilted/sheared keys in guided bearings are quite common.

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