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Shallow farm bridge design 1

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fenderbassman

fenderbassman

Structural
May 25, 2005
7
I am designing a farm bridge for access to a 16-acre field on the other side of a 12'-wide creek. It will be designed and posted for loads of 10000 pounds or less.
Delivery of concrete is difficult at this site, so concrete will be trucked-in by the bag and mixed by a portable mixer.

FEMA has posted a design on the internet for a farm bridge that utilizes a wood deck bearing on steel Wx stringers spanning the creek and supported at each end by a steel I-beam or concrete footing abutment.
The FEMA abutment is shown as shallow - not below local frost depth (4 feet).
IRC R403.1.4.1 Frost Protection excepts decks not connected to a structure from frost cover requirements - a bridge has a deck, therefore is it considered a deck??

Also - are simple bridge structures, esp. pedestrian or small farm bridges, ever designed with shallow foundations that allow movement with the freeze-thaw cycle? What would be the consequences?
 
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No, a bridge deck is not the same thing as a "deck" per the IRC, which is the residential code, covering items such as a porches and backyard decks.

As far as the consequences of not having the bridge abutment below frost line, it just means it would be subject to frost heaving. That could leave the abutment without support in some areas. Without knowing the assumptions that went into the design of the abutment, I wouldn't even guess at whether that would be ok. I suggest either extending the abutment to frost depth or subexcavating a couple feet below the abutments and backfilling with a well-graded granular material (crushed base).

Rod Smith, P.E., The artist formerly known as HotRod10
 
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OK guys, what is "frost depth". If this is a stream, the frost depth is thickness of ice in the winter. You might want to go to depth of scour instead.
 
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If you are able to set the abutment on the creek banks at a distance away from the creek, and way above the flood level, then FEMA Fig. 3.1-10 & 11 are here for you, with a foundation as suggested by BridgeSmith, but pay attention to the potential of soil wash away by the flow, so you might need some type of protection in front of the abutment that is similar to the galvanized steel plate in Fig. 3.1-11, and place it a few feet into the ground.

If the abutments are quite close to the creek, then you need to consider "scour" as call out by OG.

I don't know the context and intend of IRC on deck protect for frost. I think for your bridge directly over the water, freezing maybe is something to think about.
 
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Typically for a bridge we would always go below frost depth but it is possible to build a shallow foundation that can mitigate frost heave. If this is strictly a private crossing you probably have a lot of leeway. However, as OG points out - scour. Check out HEC-23 for guidance.

Link

Maybe you don't need to do a HEC-RAS analysis but it would be a good idea to find out if the site is prone to flooding, identify the high water mark, etc.
 
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One comment by this very old guy. Of all the bridges I've been involved with (hundreds) that cross a stream or pond never did I get involved with "frost depth". Plenty of water there in the soil to the side keeps "freezing depth" rather shallow (Wisconsin etc.) . It always involved other factors, the main one being depth of scour and of course support ability.
 
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Thanks for all the replies, especially the suggestion by BridgeSmith
to excavate down to frost, backfill with crushed base to an elevation with
the abutment structure bearing on the backfill. That will work, and save
a lot of money and trouble.

Again, thanks for the great advice.
 
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