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reconciling IBC and FEMA pile requirements

reconciling IBC and FEMA pile requirements

reconciling IBC and FEMA pile requirements

(OP)
Hi all,

Section 1810.2.2 of the IBC stipulates that piles must be braced for stability by providing grade beams at ~ orthogonal directions to the pile cap/gb intersections - i.e. free-standing piles are not allowed - exception 1 appears to require that the pile is embedded in soil over its entire length. Prior to building houses on elevated piles, this would have been my approach. I'm now working on single family residences in VE and coastal A zones. Going through the FEMA P55 coastal construction manual, the use of grade beams appears counter indicated as it induces greater scour depths. In my particular case, the piles are augercast (and CIP above grade). I'm at odds about how to proceed. Would the answer be to cast concrete beams at the top of the piles above grade to create a portal? This might be prohibitively expensive... Any and all input appreciated.

RE: reconciling IBC and FEMA pile requirements

For West Coast design and construction, piles are interconnected. Always. Either with a structural slab, or with grade beams, consistent with IBC 1810.3.12 and 1810.3.13.
While this is a seismic requirement, I do not recommend skipping the good practice of interconnecting (independent) piles below a light-framed structure - even in non-seismic zones. The horizontal tie forces in the floor members are undefined unless there are stiff and strong ties.

RE: reconciling IBC and FEMA pile requirements

(OP)
ATSE - yes, that has always been my interpretation - until I ran into coastal construction where both slab and grade beams are detrimental in a storm event, adding 2' to scour depth (maybe insignificant compared to the benefit of the grade beam?)

RE: reconciling IBC and FEMA pile requirements

Does 1810.1.3 allow you to skip the bracing provided it's designed as a column for all imposed loading?

Or 1810.1.4 might consider these a special type of deep foundation.

RE: reconciling IBC and FEMA pile requirements

I would normally design this type of pile system with a diaphragm or something similar at the elevated cutoff point. It seems to meet the same intent, as long as you make sure you design for the potential compression buckling in the extension area with a reasonable assumption for the fixed point in the embedded portion (i.e. not just at the surface).

This also seems to meet the intent of tying piles together to prevent spreading in earthquake scenarios.

Is the concrete to wood strength and stiffness differential making it so that you can't effectively consider your superstructure as an effective bracing/diaphragm element?

RE: reconciling IBC and FEMA pile requirements

I've done floodplain foundation design where it has to be open to water passage with velocities low enough to prevent scour. This might be similar to coastal. You are right, it is expensive construction. Check floodplain construction?
Are your augercast piles deep enough and reinforced for bending alone?

RE: reconciling IBC and FEMA pile requirements

Some good information here: Link

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