LRFD Foundation Design

[penpal97]

I am designing a bldg according to IBC 2006. I have designed everything up to the foundation using LRFD. Now I am at the foundation, and Section 1804 only allows ASD to check the soil capacity. Is this right? Any ideas how I can move forward besides doing a separate ASD analysis just for the foundation? If I get a soils investigation and it tells me allowable soil bearing capacity, where can I get factors for soil bearing, sliding, etc? (Please ignore the fact that I do not already have a subsurface investigation. I am working on it.)

 

[JAE]

We use LRFD, but then use ASD for the foundations. Always have.

 

[UcfSE]

If the building code says you have to use ASD, then you don't have an option. As JAE stated, we normally use ASD for foundations regardless of the design method used for the superstructure. I would go back and find your service loads for your foundations.

Just fyi, I've done retaining wall designs using LRFD using AASHTO LRFD and DOT specs. You need the code in order to have permission to use LRFD and also to get your resistance factors. You then need the geotechical engineer to provide you with nominal capacities instead of allowable. There's a lot that goes into determining appropriate LRFD factors for use with foundations.

 

[PEinc]

LRFD, what a PITA!

 

[penpal97]

Thank you all for your responses.

 

[dik]

I generally take the loads down separating the dead load and the live load... then it's just a matter of dividing by the respective load factor... else you can conservatively use a value of 1.3 or whatever. Generally, foundation costs are pretty insensitive to a tight or loose design...

Dik

 

[Mccoy]

syuen,
if you (are compelled to)design according to ASD, then I believe you should not worry about safety factors beyond the global factor which is customarily used (usually = 3).
Your allowable soil bearing capacity is treated as if it were the soil resistance according to LRFD.
I'll add that, ideally, the two will often converge, but LRFD design is governed by data variability and uncertainty as well, whereas ASD is not, being a more crude estimate, since soil is an inherently dishomogeneous system. In situations where you have a sufficiently numerous data set, and data are not too scattered nor too clustered, ASD and LRFD should not result in widely different results. LRFD just yields a value which should ensure that probability of failure is kept within reasonable (very low) limits.
Weird enough that IBC denies its application to foundation design, since one of the main targets of LRFD is full integration between upper structure and foundation design, structurals and geotechs.