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RE: PEMB Foundation Uplift
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- Subject: RE: PEMB Foundation Uplift
- From: "Donny Harris" <dharris(--nospam--at)kpff-la.com>
- Date: Fri, 3 Dec 2010 14:42:05 -0800
The load combination of 0.6D+W will always give a factor of safety of 1.5 or greater as long as you include the weight of the footing in the D and thus satisfy a "1.5 factor of safety" though I am not sure the 1.5 factor of safety is codified in the code I design too but maybe it is and I don't know it. I don't think it is the intent of the code to exclude the weight of the footing from the load combination and separating it without factoring it by 0.6 is just bad algebra. The expression for required weight of footing should be expressed as '5/3W-D' with D being the dead load less footing weight and thus the total load would reduce to 5/3W-D+D or 5/3W which will be greater than 3/2D mentioned. Using the expression 'W-0.6D' to determine required footing weight or even 150% of 'W-0.6D' will not satisfy the required load combinations... at least that is my interpretation. Donny Harris SE Los Angeles, CA Subject: RE: PEMB Foundation Uplift From: Long Chen <longfchen(--nospam--at)gmail.com> To: seaint(--nospam--at)seaint.org --000e0cd5d020df1427049675ae86 Content-Type: text/plain; charset=ISO-8859-1 Hello, Jeff, I think that the 1.5 factor is needed. Let's say that the wind load is W and the dead load beside the footing is D. To make it simple, let's say that both are given in positive values. In order to provide an uplifting safety factor of 1.5 or greater, we will need to have "D + footing weight" be greater than 1.5W. When you design the footing with the net uplift load 0.6D+W, you actually get a footing weight of W-0.6D since D and W counteract each other. Then the total dead load including the footing is D+(W-0.6D) = W+0.4D, which may or may not be larger than 1.5W. Then the safety factor achieved may be smaller than 1.5. But if you design the footing with 1.5 times the net uplift load, you will get a footing weight of 1.5 * (W-0.6D) = 1.5W-0.9D. Adding this back to D you get a total dead load including footing of D + 1.5W - 0.9D = 1.5W+0.1D. This value will always be greater than 1.5W since D can only be positive. And accordingly you will get a safety factor larger than 1.5 as required, regardless how small D is. Hope that can help. Regards, Long -- Long F. Chen, P.E. Civil/Structural Engineer Honolulu, Hawaii From: Jeff Hedman [mailto:jeff_h(--nospam--at)lrpope.com]=20 Sent: Wednesday, December 01, 2010 8:27 AM To: seaint(--nospam--at)seaint.org Subject: PEMB Foundation Uplift I designed a foundation for a PEMB. I want to know peoples methods for designing the footings for uplift. I have typically designed the footing to weigh as much as the net uplift load (0.6D+(W or 0.7E)). The calculated net uplift force has a factor of safety of 1/0.6 =3D 1.67 in = it from the reduction of the dead load. I received a plan check comment that states that with my calculations there is only a factor of safety of 1. But this is a factor of safety of 1 in regards to the net uplift force. Should we really be increasing the footing weight to maintain a factor of safety =3D 1.5 over the net uplift force? To me this is more like a factor of safety of 1.67*1.5 =3D 2.51. Not to mention we know = what the actual weight of the footing will be. This is a commercial building and the city inspectors are very good in requiring construction matches the plans, especially foundations. Am I wrong? Do others design PEMB footings that weigh 1.5 times the net uplift force? Thanks in advance for your replies Jeff Hedman 9675ae86-- ******* ****** ******* ******** ******* ******* ******* *** * 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 ******* ****** ****** ****** ******* ****** ****** ********
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