Emergency Bridge Deck Repair 1

[crossframe]

Performing a pothole repair on a 44 year-old bridge, a local municipality was using a small backhoe to remove the broken-up asphalt. They discovered they could keep scraping deeper and deeper into the concrete bridge deck, digging through the concrete as if it were gravel. They took four cores of the bridge deck themselves, producing two solid cores and two bags of aggregate. There is some question as to the drilling crews’ methods possibly breaking up the cores, or even losing a solid piece out of the bottom of the barrel, but this deck clearly needs help.

Buses, heating oil delivery trucks, and dump trucks were all observed crossing the bridge without any outward signs of distress. The underside of the deck was not accessible, but other than some staining, there was no visible evidence of this much deterioration. However, once we saw the bags of “cores” we instructed the municipality to restrict the traffic to passenger cars only.

The design of the replacement of this bridge will hopefully start soon, but the municipality wants us to come up with an emergency repair scheme so they may allow school buses and municipal buses (15 ton vehicle) to cross in the fall. The 1961 design load was HS-15.

One idea we’ve had is to place steel trench plates transverse to the beams in the areas of deteriorated deck. (Like a "Battleplate" deck or an orthotropic deck with no stiffeners.) The thought is that since these plates are readily available and they can span a couple feet over a utility trench, we may be able to use them to “help” the remaining deck span the 7’-9” beam spacing.

I’m having a severe mental block on how to analyze this. Using only the plate to support the wheel load gives a very thick and unrealistic plate thickness required. If the concrete deck truly is as bad as the cores, would a gravel-supported plate analysis be a reasonable compromise? Can the deck rebar be thought of as a “mesh” to carry some tensile forces?

We cannot possibly be the first ones to try this as a solution, but I can’t find any other examples of this type of temporary measure. I would appreciate any thoughts you may have.

Thanks.

 

[SlideRuleEra]

crossframe - Instead of steel plates, perhaps you could consider heavy duty metal bar grating. The large sizes are rated for highway traffic & and it is probably available on short notice. Should save adding excess dead load (compared to other solutions) to the span. Here is a link to the Metal Bar Grating Division of NAAMM

http://www.naamm.org/mbg/literature.php

You can download their two manuals (free .pfd) and take it from there.

http://www.SlideRuleEra.net

 

[Qshake]

Another idea would be to do some insitu concrete repair. Full depth and partial depth deck replacment are typical DOT approaches to a problem like this.

First, the deck would need to be sounded to produce an idea of what areas need repair. Based on this analysis plans can usually be drawn up that show the areas and based on the sounding results which ones need partial deck and full depth repair (might be hard not seeing the bottom) and then show retangular, saw cut, patches that are to be removed and concrete replaced.

Try several DOT bridge manuals for details.

I don't understand why the bottom of the slab is unaccessible unless it was too high above the existing ground to get a good look at.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[crossframe]

Thank you, SlideRule Era and Qshake, for your replies.

One obstacle to a "proper" resolution is that since this bridge is slated for replacement, the muncipality isn't interested in spending a whole lotta money on this. They want something to "just get us through." I certainly understand the Yankee frugality of not wanting to throw money at something that is going to be replaced.

In fact, when we brought up the full-depth and partial-depth deck repair options, we were told that if they were going to spend the money for a deck replacement, they would rather replace the whole bridge and fix some other issues as well (roadway & sidewalk width, alignment, higher rating, etc.). That's why this bridge is now scheduled for replacement.

The deteriorated area appears to be about a 40-ft section centered over the pier of a two-span bridge. It's about 30 feet over a river, so it's not truly "inaccessible" - a Snooper would certainly reach the underside of the deck. It's more a matter of ecomonics. Sounding this area would likely tell us what we already know from the cores - that the deck is shot.

I haven't had time to dig through SlideRuleEra's suggested grating manual, but I did notice spans up to 8 feet or so. Our hope is that we can find a solution that can be employed by municipal forces from the top side.

I think it's time to think more like a contractor and less like an engineer. Somebody, somewhere HAD to have solved this problem in this manner before.

It was fortunate that we heard that one single comment by the paving crew foreman to alert us to this condition. Otherwise, the bridge probably would've kept on happily carrying it's normal loads. I guess it goes to show it pays to listen.

 

[Qshake]

Back to that grating option then....if you happen to receive a copy of Structure Magazine, there is an article on fast-track bridge replacement.

In the article, a narrow bridge deck (conventional slab on girder) was removed and replaced by deck panels until the precast-match cast deck panels could be laid for permanent use.

I would think these temporary deck panels (grate or otherwise) would provide a possible solution here. And yes that idea came from a contractor (in the article)!

Good Luck.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[SlideRuleEra]

crossframe - What type of beams support the bridge? Steel? Prestressed? Concrete Deck Girders?

This can make a big difference in the temporary solution. For example if they are deck girders, they may be suffering the same deterioration. But if they are steel, there are a few more options.

P.S. In what seems like another lifetime, I was a bridge contractor. Give us some details, and we will get this thing worked out, with a method that the City can implement themselves.

http://www.SlideRuleEra.net

 

[crossframe]

SlideRuleEra,

I HAD remembered your former life as a bridge contractor (and was hoping for your comments). Even though it was a number of years ago, I spent enough time as a resident to know that more happens after the mylars are stamped than during design.

This two-lane bridge was built in 1961 using working stress design to the 1960 AASHO specifications for H15-S12-44 live load. The four steel beams are A36 33 WF 130's and are spaced at 7'-9". The 7-3/4" concrete deck is reinforced with transverse #5's at 6" intermeduate grade (40 ksi) rebar.

Based on the visual inspection the City asked us to do, no areas of deck deterioration other than asphalt potholes were observed. Had it not been for that off-hand remark by the foreman, we never would've suspected the deck is as bad as it is.

We were hoping that a trench plate (or maybe the grating as you suggested) placed normal to the beams would provide enough distibution. Using only the plate seemed too conservative, since I've watched the occasional truck cross the bridge with no noted ill-effects. My mental block comes with how to quantify the capacity of the bridge deck.

Treating it solely as gravel renders rather poor bending capacity. We considered treating it as a thin (3-4") slab centered around the bottom mat of rebar, with the remaining slab treated as gravel, but you don't get much lateral distribution with 4" of fill, so essentially we had a 4" slab or 3000 psi concrete trying to carry a H-15 wheel load. If that failed, we'd be back to the trench plate by itself anyway.

Other pencil-sharpening techniques under consideration include noting that the weight distribution in school buses seems to be different than the AASHTO design vehicles, so that a 15-ton bus appears to have about a 9 kip rear wheel load, rather than a 12-kip wheel load for the H-15 truck. However, I think I'd like to avoid geting down to specific bus models.

I hope that's enough food for thought. Thanks again for your help.

 

[SlideRuleEra]

crossframe - A few more questions:

1. Is the roadway flat (perpendicular to the centerline), crowned, or superelevated?

2. If it is crowned, is the crown along the bridge centerline?

3. If it is crowned, what is the slope?

4. If it is crowned, are the tops of the four 33 WF 130s at the same elevation (with the slab "thin" at the edges & "thick" at the crown)?
OR
Is the slab a constant (7 3/4") thickness with build ups on the caps to "raise" the interior beams?

5. Is there a concrete curb or sidewalk (on both sides)?

6. Assuming that there is a curb or sidewalk, what is the top of curb height above the roadway (maybe 6" or so)?

7. Assuming that there is a curb or sidewalk on both sides, what is the width of the roadway (face of curb to face of curb)?

http://www.SlideRuleEra.net

 

[crossframe]

The deck is 28'-2" out-to-out, with a 1'-9.5" curb on one side and a 4'-8.5" sidewalk on the other, leaving a 21'-8" curb-to-curb roadway width. The orginal 1.5" course of asphalt created 7.5" curb reveals.

The roadway is crowned 1/4" per foot from the roadway centerline, which works out to be 1'-5.5" left of the center of the cross-section. The asphalt wearing surface has been paved and patched so often, this may or may not be the finish grade cross-slope.

The bridge plans show the bridge deck formed straight between the undersides of the top flanges, with level overhangs. The deck is 8.75" at the crown, 7.75" typ. at the (interior) beams, and 7.5" min at curb (left) edge of deck. Sidewalk deck edge is thicker due to location of sidewalk construction joint.

If curb is at left side (sidewalk at right), beam 1 (left exterior) beam seat el=0.00, beam seat 2 el=0.16, beam seat 3 el=0.21, and beam seat 4 (right exterior, below sidewalk) el=0.04. (All elevations relative to beam no. 1).

Our thought was to remove enough sidewalk to plate/grating will bear above girder, rather than face a large cantilever.

 

[crossframe]

That last sentence should've read: "Our thought was to remove enough sidewalk THAT THE plate/grating will bear above girder, rather than face a large cantilever." Sorry for the lax grammer and spelling. My ride was leaving and I was hurrying.

 

[SlideRuleEra]

Well as so often happens - the answer (at least from this former contractor's viewpoint) came in the middle of the night. A heavily reinforced, crowned, non-bonded, cast-in-place concrete overlay. Here is the concept:

Since two spans are involved (assuming 20 ft of deteriorated bridge deck on each), plan on an overlay slab for each span (say 25 ft long x 21' 8" - full width of roadway).

Patch (or at least fill up) all the holes in the existing deck to create a relatively smooth surface. Just before concrete placement put down a layer of polyethylene sheet.

Drill & grout (or chemical anchor) a line of rebar dowels into the deck about 24' from the end of the span (in solid concrete).
Also drill & grout (horizontal) rebar dowels into the face of the sidewalk. Cover the face of the curb on the other side of the bridge with thin (say 1/2") expansion material.

Plan on the two monolithic slabs (one on each span) having a constant thickness that is essentially equal to the existing deck (say 7.5"). Heavily reinforce the slabs (0.6% rebar, minimum) in both directions. Use one size rebar (say #6) for everything (dowels & slab reinforcement). Crown the slab to mirror the existing deck profile (slab thickness remains approximately constant).

Place each slab, one at a time (on different days), with longitudinal screeding, belting, and possibly raking for traction. Screeding a 25' slab, by hand (1950's style) with a truss supported 6" steel channel "on the flat" is not that difficult (relatively speaking) & can produce a finish that rivals modern bridge techniques.

Add approach ramps (doweled to the existing deck), wet curing along the way, new doweled-in curbs, any required guardrail mods & signs and it should be ready to go.

A "fringe benefit" is that this whole approach has some built-in traffic control - it is one giant speed bump that ought to convince drivers to slow down.

With 30 yards (or so) of concrete & a truckload of rebar, I believe the City can handle this one themselves. They might want to get some professional help on the days the slabs are placed (as you know, that has got to be done right the first time).

Even if you have reasons not to use this approach (I know that all the details cannot be adequately described over the internet), let me offer some (contactor's) advice:
Do NOT remove the sidewalk. I understand from my sketch, based on your description, the reason to do so. The problem is that the concrete may "break" at unintended locations. If the bridge deck failed, parallel to the beams, before the sidewalk came off, you would have a "bunch of trouble".

Keep us posted, I want to know how this comes out (maybe a link to a newspaper article, etc.)

http://www.SlideRuleEra.net

 

[crossframe]

SlideRuleEra,

THAT'S a pretty well thought-out solution. I want to take some time to digest it.

It appears this method suggests placing a "new" deck on top of the old one. (Hence your speed bump.) That brings a few concerns.

The existing steel beams were only designed to H-15 loading. Albeit over the pier, this will be a decent amount of additional dead load. The current inventory and operating ratings for the beams are 15 tons and 20 tons, respectively, so we don't have much additional capacity there. One of the challenges of this whole situation is trying to restore a bridge to nearly the original design strength with tie-wire and duct tape.

Secondly, raising the roadway surface over the "speed bump" will place the vehicles closer to the tops of the railings. I don't know if a PL-1 (Test Level 2) railing will still be a PL-1 railing if there's no raised curb and the vehicular impact is 7.5" higher on the railing.

Though you've greatly simplified the work efforts, this seems to be getting close to the partial-depth and full-depth deck repairs that the City resisted in the first place.

As you understand the problem, would you prefer this solution to the grating?

As I re-read the post, I think it may come across as a little ungrateful. I don't mean it that way. I'm just trying to flush out potential issues early, and I'm VERY appreciative of the time, thought and effort you've given this.

 

[SlideRuleEra]

crossframe - Grating rated for H-15 load at a 7 ft. clear span is going to weigh 40 to 50 psf and be 5" to 6" deep. And that does not address how to deal with the crown. Also looks like it would be best to stick with H-15 loading (see the "Specs" at this link)

http://www.blue-bird.com/products/school/all_american.php

Concrete at 7.5" thick is 94 psf. Thickness could likely be reduced (say 6" as a minimum), if all the facts are known. Cost is probably about 10% to 20% of the grating approach. Time required is probably about 3 weeks (for concrete) vs. 2 or 3 months (for grating- got to get it custom fabricated). I considered that the extra weight could be excessive, but I don't know the bridge span(s) length. What is it?

Even the steel plate approach has a weight penalty at 41 psf / inch of thickness.

Could the City consider making this a one-lane bridge (since the whole project is temporary anyway)? This would make things a lot easier.

If the roadway can't be raised at all, the cost-effective answer may be to abandon the project. I doubt if the City (or a Contractor for that matter) can selectively demolish the deck and restore it on a fixed budget. Once you start demolition an old saying takes effect, "If you look for trouble, you will find it". Costs will likely spiral though the roof as more problems are uncovered.

http://www.SlideRuleEra.net

 

[crossframe]

SlideRuleEra,

The bridge is comprised of two equal 58'-6" spans. The beams are continuous and the deck is not composite.

That Blue-Bird site is one of my "bus" sources. I agree we need to stay with a 15-ton vehicle, but the AASHTO trucks place 80% of their weight on the rear axle. School buses appear to have "only" about 65% on the rear. I was hoping I could use that 20% reduction, if it came down to that.

The City has already ruled-out a one-lane bridge.

The unintentionial "spread" of the concrete removal was an aspect I hadn't really considered. As I remember my last field visit though, I think I could've dug clear through with my miner's (rock) pick. I'm afraid we don't have far to look to find trouble, and the City's constraints don't help us much. Who knows? Maybe a spiraling cost or an over-excavated bridge deck would help to ease some of those restrictions.

The original thought, before we actually started throwing numbers at it, was to remove the asphalt over the pier (the 40'+/- section), place 1" (or so) plates directly on the "concrete" deck, and cover them with 1/2" to 1" of asphalt. We always knew there would be a weight penalty but we were trying to minimize it as much as possible, while employing a simple solution for the City.

Another option under consideration is to try to add bracing from below, though we preferred a more straight-forward topside approach. There are 12 WF 25 diaphragms spaced at 18'-6" (one at the pier, and spaced outward towards abutment places the end diaphragms at 3' from the bearings). They are arranged near the bottom flange, so there's about 18" between the diaphragm and the deck. I though maybe we could snake a beam through there and wedge it tight to the deck, reducing our effective span. We could brace to the beam web/bottom flange corner, if needed.

The City now appears to be more receptive to including this work as part of a bid package scheduled to be released. Having a contractor involved may allow us to consider more complicated and "bridge-like" alternatives.

My father built 13 houses over his lifetime, often with framing solutions coming to him in the middle of the night. And I was certain I corrected that "grammer" mis-spelling of 9:53 this morning!

 

[TTK]

After reading through this thread and all of the great ideas presented, it occurs to me that this problem is less of an engineering challenge and may have more to do with unrealistic owner expectations.

The owner wants a simple, low cost, temporary fix to be completed within the next few weeks which will safely support two lanes of legal load vehicles (presumably at mormal speeds). Heck, I would want all of those things if I were the owner, but is it really reasonable to expect a solution to meet ALL of these requirements?

The steel plate solution is the "quick fix" but if you try to apply simple beam theory without reduced safety factors the plate thickness will be significant as you have found....there will likely have to be shared engineering judgement used with the owner if this solution is selected.

A temporary full-depth deck replacement over the pier (no coincidence that it's the negative moment region that has deteriorated first) is the "best" structural solution but it obviously has cost and schedule implications for the owner.

The least costly temporary solution would probably consist of speed reduction with enforcement, heavy signage with lane restrictions and limiting to one truck at a time...along with some nominal structural improvement (say steel plates). This solution would likely not provide the level of service expected by the owner.

This reminds me of the old addage: "you can have it done well, quick, or cheap...just pick any two".

How about creating a decision matrix for the owner that presents the 3 or 4 best solutions with the pros and cons listed for each. The owner will need to decide what's most important to them: serviceability, cost, or schedule?

If it were my bridge I would likely choose the following depending on the length of the temporary fix before replacement:

O to 6 months: nominal steel plates over pier with reduced traffic speeds and safety factors, increased signage and enforcement.

6 to 12 months: heavy steel plates with bituminous tapers on approach to minimize vehicle impact. Shear strenthening of beams if required.

over 12 months: Full depth slab replacement over pier and partial depth as required over remainder of bridge. This solution would be even more appealing if winter plowing operations would need to occur during the temporary time frame.

Good luck and let us know what the owner ultimately selects for their "temporary" repair.

Regards,

TTK

 

[crossframe]

TTK,

I agree, the owner has some pretty ambitious desires for this structure, and I can't really blame them for asking. Heck, I've received some amazing "perks" over my life solely because I asked.

The discussion around the decision matrix you've suggested will take place. I just want to be sure that when we tell them that they can only have two of those three, it's true. I also think we owe them the most ingenious, creative solution we can come up with, and the most cost-effective solution they can live with. After all, we get paid to think.

I just want to make sure I didn't overlook a good, fast, and cheap solution (like the kid who suggested deflating the tires of the truck stuck under the bridge).

Thanks.

 

[Qshake]

I've got to say, in review of the threads, that something must be off for the steel plate option. Unless I just don't know the overall length and width of the area to be removed.

We use 1" steel plate for a host of applications and loadings. We have never found it in a state of distress. Have you used plate theory from Roark's Stress/Strain formulas or are you simple bending theory to size your plate? You may want to use the Roarks equations which are quite painless.

Another option might be to explore what can be done with plates and some supporting angles fashioned in a truss type arrangment to transfer load from the plates to a structural member (say W section) below that is affixed to the longitudinal beams.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[SlideRuleEra]

crossframe - Glad that you are hearing from TTK and Qshake, because I am "fresh out of ideas" for a top-side fix that meets the City's criteria.

If something changes, please post it.

http://www.SlideRuleEra.net

 

[crossframe]

SlideRuleEra, you've done more than your share. Thanks for everything.

A couple of posts were right on about simple beam vs plate analysis. I first did a quick beam analysis to try to get a feel for orders of magnitudes and an "upper bound" for plate thickness. Our STAAD model and (to be checked against our newly-acquired Roarks) is giving much more palatable results, even when ignoring the deck.

I think my main concern now is the 5'-5" of plate we'll have overhanging one interior beam if we heed SlideRuleEra's sound advice of leaving the sidewalk alone.

Thank you all for help and suggestions. I will post the final implemented solution.

 

[crossframe]

OK, since some have asked, here is the resolution:

For H-15 wheel loads, we found the plate stresses to be OK when we analyzed a 1" thick continuous plate over four supports (the stringers). We removed the wearing surface and exposed the bare concrete bridge deck. We also removed enough of the sidewalk to leave about 24'-3" to the opposite curb. Any localized deteriorated concrete was repaired.

A bituminous leveling course was placed on top of the concrete deck, and 8' x 24' x 1" steels plates were placed transversely (continuous over the stringers) over the questionable areas of the deck. Tabs were welded between adjacent plates, and holes were cut for the existing scuppers.

Finally, a 2+" bituminous wearing surface was placed over plates, and blended to the remaining pavement.

The additonal weight used up most of the reserve capacity of the original design, but we still got the rating to what was required.

Intermittent inspection, including removal of plates and inspection of the concrete bridge deck, will be performed during the remaining life of this bridge until its replacement is constructed.

Thank you all for your help and suggestions