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Private bridge open to the public

Private bridge open to the public

Private bridge open to the public

(OP)
So, I've got a little bit of a conundrum. We're designing a private bridge for heavy construction vehicles to access a wind farm construction site. The "bridge" is going to be short span precast concrete box culverts of 10 feet span or less. The loads we're designing for are H-20 loads, heavy articulated truck loads, and a 488 ton crane. Thus, the bridge will easily meet the load rating requirements of a public roadway. We're also designing them to the latest AASHTO LRFD 2012 specification. Now, the bridge is a private bridge but will be open to the public and is expected to be used by ATVs, pedestrians, light vehicles, and probably additional H20 construction equipment down the road.

Now, my question is the contractor and precast manufacturer want to save some cost by not designing it to the full song and dance of AASHTO. For example AASHTO prescribes 2" exterior cover on precast culverts, however they want to use 1.5" as they're not concerned about maintaining a 75 year bridge life and the structure will not be exposed to deicing salts. What are your thoughts on items like this? I can find no guidance from the local DOT website on issues such as this and I know this is going to be above the head of the local code official. As we're not changing the load rating would you say it's acceptable to use AASHTO modified as they request?

And, yes, the PE in charge has requested that unless we can find a good argument otherwise to just design for the full AASHTO. However, if that's the case I really need a good reason to tell the contractor why we had to use the full AASHTO specification beyond just "my boss told me so and it covers our butt".

Maine EIT, Civil/Structural.

RE: Private bridge open to the public

Have you spoken to the Owner? Unless a large portion of the savings are passed on the person / organization paying the bills, I would not consider this or any cost cutting measure. Even with that consideration, IMHO, it is not worth the risk. Surprising how much of infrastructure, especially bridges, are used long past their original design life.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Private bridge open to the public

(OP)
I'm three subcontractors deep so I don't even know who the owner is. Working with the contractor and getting information is like pulling teeth, I've had to ask 4 times just to see the contract drawings. However I suspect the person who would see the savings is the precaster who contracted us.

I do tend to agree, though. Doubly so as this is a private, remote area and I would be shocked if people were inspecting this structure regularly 50 years from now.

Maine EIT, Civil/Structural.

RE: Private bridge open to the public

The fact that you're three subcontractors deep and don't know even know who the owner is would make a good justification for sticking with accepted standards. "While ordinarily, we might be able to make adjustments to the design with the agreement of the owner, based on anticipated long-term usage of the structure, in this case, it seems unlikely that this could be coordinated in an expeditious manner" etc.

RE: Private bridge open to the public

Whats your soil cover on the structure? I think over 2' cover, you can go less than 2". Also if you use WWF, you can go down to an inch, or 3x diameter.



RE: Private bridge open to the public

Concrete is cheap. I can't imagine .5" of concrete is going to add much cost to your project.

RE: Private bridge open to the public

(OP)
Sounds good enough for me. I doubt they would get much saving anyway.

In these sorts of cases how do you guys usually determine what the applicable standard is? Do you just go strictest standard or play it on case-by-case basis.

Maine EIT, Civil/Structural.

RE: Private bridge open to the public

(OP)
Oh and we're right at 2 feet soil cover, I'm pushing for 2.5 feet so I can utilize that precise requirement ztengguy mentioned. I'm assuming they want to keep it rebar to decrease fabrication time (they have bars already in stock rather than waiting for the WWR company) but using WWR is a good idea.

I used concrete cover as an example. Some other things they wanted to push was use of ACI 318 vs AASHTO (not gonna happen), not using impact factors (maybe), ignoring the cutoff for box culvert design rules in AASHTO below 2 feet soil cover (not happening), and designing for reduced service life (probably not), etc. Most of them we can't do for obvious reasons but concrete cover was the one I was most on the fence about.

Maine EIT, Civil/Structural.

RE: Private bridge open to the public

Quote (In these sorts of cases how do you guys usually determine what the applicable standard is? Do you just go strictest standard or play it on case-by-case basis. )


From an Owner's perspective, a case-by-case-basis. Depends on the size (value) of the project, its complexity, magnitude of the savings, effects on schedule, long-term consequences of the change (sometime for the better), and other non-technical factors. In my experiences, never rule out a proposed change (well, almost never) but take a careful look at all aspects to see if the savings (to the Owner) are worth the change - many times they are.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Private bridge open to the public

I can only imagine that a 488 ton crane is causing shear issues, unless they are trucking it across in pieces.

RE: Private bridge open to the public

(OP)
Wide tracks that are twice as long as the span, only half the load will ever be on the bridge. But, yes, that's going to be an issue. It can be sent over in pieces if required.

Maine EIT, Civil/Structural.

RE: Private bridge open to the public

Stick with standards. When you deviate, you deviate from the standard of care, which increases your liability.

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