Pedestrian bridge shear crack
Pedestrian bridge shear crack
(OP)
I am looking at options for the repair of a shear crack in a cast-in-place pedestrian bridge (early 70s). The condition was discovered during renovations and runs the full width of the bridge although the crack widths at the opposite end are acceptable.
The bridge is about 30 feet wide and spans about the same distance. I only have an architectural plan at this point so I haven't confirmed the longitudinal reinforcement, but seeing as it is basically a 12" thick flat plate I don't expect to find much effective shear reinforcement. The railings are integral cast-in-place, but as you can see they were discontinued near the support.
Thanks in advance.
The bridge is about 30 feet wide and spans about the same distance. I only have an architectural plan at this point so I haven't confirmed the longitudinal reinforcement, but seeing as it is basically a 12" thick flat plate I don't expect to find much effective shear reinforcement. The railings are integral cast-in-place, but as you can see they were discontinued near the support.
Thanks in advance.






RE: Pedestrian bridge shear crack
I would first try to understand or verify the original design to fully know why this happened in the first place before attempting a solution.
Perhaps not very aesthetic, but building a large haunch under the ends?
RE: Pedestrian bridge shear crack
RE: Pedestrian bridge shear crack
RE: Pedestrian bridge shear crack
OK, now regarding your post. I've got several possibilities:
1. Cut out last 4 feet and recast with stirrups and top steel.
2. Anchor steel beams to the bottom of slab and cantilever then from the slab to the support.
3. Use magical FRP (just kidding, but some engineers seem to treat this stuff that way)
RE: Pedestrian bridge shear crack
Thanks
RE: Pedestrian bridge shear crack
And you sid that "the crack widths at the opposite end are acceptable."
That makes me think that there were no expansion/seismic joints provided too as mentioned previously.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Pedestrian bridge shear crack
I have cropped an overview photo to maintain some confidentiality.
RE: Pedestrian bridge shear crack
Shore it first, of course.
RE: Pedestrian bridge shear crack
Is there a contact or connection with any rigid structure or mass at any place other than at two opposite ends?
Is this integrally cast with the building on the left, and if not, what is the support condition?
Is there a compatibility or restraint issue between supports and other structures making contact?
Is there vertical and/or horizontal displacement along the crack?
RE: Pedestrian bridge shear crack
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Pedestrian bridge shear crack
RE: Pedestrian bridge shear crack
With higher stiffness along the sidewalls, more shear would get deposited at the ends near the sides and less in the center of the 30 ft. width. Just a thought but it might be that the higher shears at the edges cracked the concrete and then the section "unzipped" across the width.
If the cracking is wider, or older, near the edges vs. in the center, this might be a cause to investigate.
RE: Pedestrian bridge shear crack
RE: Pedestrian bridge shear crack
Ground motion "up(?) w/r to the bridge walkway" is more likely the cause that "load causing the walkway to fail (down) due to load". So, the reduce that, as said above, support the short bridge from below with a complete arc-shaped filler (steel or concrete) but with a drainage path through for the runoff water.
Remove those ugly `1970's-era solid concrete rails and replace with "lighter appearing" steel or wood rails. Changes the bulky and heavy appearance, which eliminates the "dam look" of a solid fill underneath.
RE: Pedestrian bridge shear crack
Depending upon your soils, maybe the following would work and be expedient. It may be overkill, but would be a relatively quick, low liability solution: Drill piers on either side of the bridge. Dig a little under the bridge so you can form and place a concrete beam from pier to pier. If the slab will be resurfaced, you could punch through the slab and drill piers below, reducing the beam requirements and hiding the new work a bit.
(My post yesterday was late to the party - work kept pulling me away while I was typing here.)
RE: Pedestrian bridge shear crack
RE: Pedestrian bridge shear crack
My observation that the crack extended across the width of the bridge was based on seeing the same crack configuration (though only hairline) on the opposite side. The deck surface is rough with sand and grout where a layer of 4" thick pavers was removed. The underside of the bridge is hidden near the bearing by the precast panel hanging from the bridge by thru-bolts, so the true extent of the crack is not visible.
If the rails are attracting shear and dropping it off at the short length of slab between the support and the railing, then possibly the slab plate is only cracked at the edges. Either way I am leaning toward the idea of two piles and a beam below the deck at the end of the railing (about 24 inches from the support) due to the unknowns (crack and reinforcement) and not having an effective way to repair and reinforce the local shear capacity of the slab.
Thanks again for all the comments and advice.
RE: Pedestrian bridge shear crack
RE: Pedestrian bridge shear crack
The FRP repair is out of question, as this is only good for substituting for the bending reinforcement.
As temporary fix provide secondary support, as recommended already.
The best repair will be to recast end sections with shear reinforcement and expansion/contraction provisions. 1/4" elastomeric pad will be enough.
For the sizing of the shear reinforcement - use full force, and when the stresses in concrete are exceeded, carry all shear on steel only.
RE: Pedestrian bridge shear crack
With all that in mind, grasshopper would like to pose a few feasibility questions:
What would shear reinforcement look like in a 12"D by 30'W slab? Would you model it as (30) parallel 12"Wx12"D beams? Would the shear reinforcement be effective after cover requirements? In structural slabs, shear deficiency is normally made up with concrete drop panels or capitals I think.
RE: Pedestrian bridge shear crack