Shallow farm bridge design
Shallow farm bridge design
(OP)
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). https://www.fema.gov/media-library-data/20130726-1...
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?
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). https://www.fema.gov/media-library-data/20130726-1...
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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RE: Shallow farm bridge design
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
RE: Shallow farm bridge design
RE: Shallow farm bridge design
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.
RE: Shallow farm bridge design
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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.
RE: Shallow farm bridge design
RE: Shallow farm bridge design
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.