Pedestrian bridge shear crack

[charliealphabravo]

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.

 

[JAE]

That looks very wicked.

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?

 

[MiketheEngineer]

Until you can figure out what has happened - I would close it

 

[JedClampett]

How long is the bridge? Were there expansion joints installed? Maybe the expansion/contraction started a crack and the shear forces expanded it?

 

[Teguci]

I second the closing aspect. Shear failure is sudden. Get some shores up uder the end before re-opening as a minimum.

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)

 

[ztengguy]

any other pics? maybe a wider shot to see the whole picture?

Thanks

 

[msquared48]

Well, from what I see, there must be some shear seel there, or it would not still be in place with that crack pattern.

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

 

[charliealphabravo]

Thanks for all your thoughts on the matter. The structure is closed. The span and width are 30'. Both ends look like they may be rigidly cast integral with a pile system and there are no apparent control or movement joints. The smallest section occurs at the building where the railing is discontinued and where the crack has appeared. This is all consistent with the JC's suggestion I think.

I have cropped an overview photo to maintain some confidentiality.

 

[hokie66]

Thanks for posting the second photo, which leads to the solution. As the bridge is close to the ground, just build a new wall under that end. Then cut out the part that is broken and recast, keeping it free of the building.

Shore it first, of course.

 

[TXStructural]

I'm not sold on the cause of the crack. It is hard to tell at what we are looking. Based on the pattern in the thinset, it also appears that the tile was installed before about 1" of vertical movement.

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?

 

[paddingtongreen]

@hokie, won't he have the same trouble? the span will be slightly reduced but there is even less likelihood of there being shear reinforcing.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

It has the appearance of a shear crack, but I think it would have started as a restraint shrinkage crack. It is restrained on both ends. Relieving the restraint should prevent that type of crack in the future. You wouldn't expect a 12" one way slab to have or require shear reinforcement.

 

[JAE]

I know the OP said that the shear crack was all the way across the width, but with those sidewalls, one thought that popped into my head was that there might have been a concentration of shear near the edges where the walls were...if indeed the walls are integral with the slab.

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.

 

[hokie66]

Good point, JAE. The slab spanned to the upstands, which just stopped near the support. Solution is the same...put the new support in line with the end of the upstands.

 

[racookpe1978]

As a pedestrian walkway "bridge" it appears that those "siderails" are a heavier load than anything else that has ever been on it.

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.

 

[TXStructural]

Without more info, I'd vote that the sides are acting as stiffeners for the slab, causing the crack to happen at the weak section (no sides). Without knowing the original design intent, I would suggest supporting from underneath near the broken end.

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.)

 

[asixth]

I reckon it's the sidewalls as well. A 30' slab will only want to shrink so much, so restraint may crack, but wouldn't generate 0.5" cracks. The shear force at the support line should be well within the shear capacity of the concrete alone. I would say that there is no reinforcement or that at some time it was subject to a large point load possibly from a vehicle. That is my forensics investigation. With the crack the size it is, can you see any reinforcement thru the crack?

 

[charliealphabravo]

No reinforcement was visible at the crack. The crack was exposed when a thin tile was removed from the side of the slab. The tile was not apparently cracked suggesting that the crack was covered over in the past.

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.

 

[Ron]

I agree with JAE...the mass of the sidewalls coupled with the end gap where the sidewalls don't tie created a stress concentration. Solution.....put a couple of pipes in, make the ends attractive, fill under the bridge.

 

[wiktor]

The crack is due to a combination of two factors - not present or not sufficient shear reinforcement and lack of expansion/contraction provisions.
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.