Need a book? Engineering books recommendations...
PEMB Foundation - Foundation Resistance to Uplift[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: PEMB Foundation - Foundation Resistance to Uplift
- From: "mail" <mail(--nospam--at)strand-engineering.com>
- Date: Wed, 31 Oct 2007 08:03:24 -0400
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? ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
- Prev by Subject: RE: paid for release of CAD files to subs...
- Next by Subject: PEMB foundation uplift
- Previous by thread: Re: FW: 11 x 17 copiers / scanners (was 12 x 18 printers)
- Next by thread: printers/ wood software