Wednesday, October 31, 2007

PEMB Foundation - Foundation Resistance to Uplift

I am designing a foundation for a PEMB. The load combinations used by the
PEMB manufacturer were ASD (governing 0.6D + W, we are in a high wind area
in SC). I have a couple of questions, which may have parts answered by
previous posts.

1. IBC 2003 Section 1609.1.3 Anchorage against overturning, uplift and
sliding.where a portion of the resistance to these forces is provided by the
dead load, the dead load, including the weight of soils and foundations,
shall be taken as the minimum dead load likely to be in place during a
design wind event. So, when designing a footing to resist the uplift of the
column rxn, it seems that you could take the weight of footing with a FS of
1.0 if you are pretty confident that the concrete will be there in the
presence of a design wind event. That is different from only using 2/3 of
the DL to resist the uplift or OTM when considering the entire building
which would have estimated DL values that may be conservative. Is there
something that I am missing about this passage, or does it fit into the
larger scope of the code in some way I do not see? I have seen on the list
discussions of a 1.5 FS for overturning. The only place I can find the
requirement of the 1.5 FS is under the retaining wall requirements in the
code (section 1806).

2. Also, it is common practice in this area to use monolithic slabs with
turndown footing foundations with enlarged footings under the columns. The
footing sizes end up being fairly large to provide enough weight to
counteract the uplift forces. I have seen designs where the engineer designs
the footing to be offset to the inside of the foundation which would cause a
tremendous vertical load eccentricity (this is easier to form for the
contractor though). We actually try to center the footing under the column
(which is not as easy to form) but avoids the eccentric vertical load. My
question is: is it necessary to design the footing for overturning due to
the eccentric vertical load from the uplift (assuming the horizontal rxn is
cancelled by the use of hairpin or slab compression - discussion for another
day)? If so, what factor of safety should be used? BTW, if you centered the
uplift load over the footing and did an overturning calc, the only RM force
is the weight of the footing. If you use a FS of 1.5, then you are getting a
reduced dead load of .67 (1/1.5), which takes me back to my first question -
do you have to use the FS of 1.5 for overturning on anything other than
retaining walls per the IBC?

3. A more general question: When do you do overturning, sliding, uplift
calcs? I understand checking the overall building for overturning, but if
the footing is assumed to crack at the slab to footing connection, when
would you need to design for overturning? Is an eccentric load reason enough
to check overturning on a footing? A retaining wall is designed for
overturning because of the significant lateral load exerted by the active
soil pressure, but if you did not have the lateral load, but just and
eccentric vertical load, would you still check overturning?

4. Do I need to change the reactions provided by the PEMB manufacturer in
any way to reflect service loads or can I simply use the loads provided?

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