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Overturning Resisted by Entire Windward Wall?

Overturning Resisted by Entire Windward Wall?

Overturning Resisted by Entire Windward Wall?

(OP)
Hi: Thanks in advance for any help. This site has been an excellent go-to source for many questions I've had over the years.

My concern is the recent hurricane in northeast and the engineering and design of newly elevated home building support structures in both A and V zones. FEMA's guide appears to indicate that all of the overturning force, not uplift, is resisted equally by the windward wall anchors. They show calc's to indicate that the overturning vertical couple component is equally divided amongst the number of anchors on that side of the house. For many of the 2-story, 17 ft wide by 40 to 60 foot long homes on the shoreline I don't see how this is possible along the long building wall line.

We've always taken lateral wind load into the floor and ceiling/roof diaphragms and collected it in the end shear walls. I typically don't allow any interior walls to act as shear collectors since virtually all do not run full width or height of the structure. While obviously the lower half of the first floor wall will impart overturning for the windward anchor below it, the remaining 75 to 80% of load is being collected by the first and second floor ceiling structures and roof diaphragms.

I'm sure FEMA has a rational reason...but I just don't see it at this time.

Again, thanks in advance to all for any and all help.

RE: Overturning Resisted by Entire Windward Wall?

(OP)
Brian: P85...Protecting Manufactured Homes. Frankly, I ignored this when I read it, but heard from another engineer designing these and he also stated that no, you equally divide overturning to each windward pile. I don't see it myself but I'm probably missing something.

RE: Overturning Resisted by Entire Windward Wall?

(OP)
Brian: See P85 Appendix F, Example Calcs, Page F-18, about halfway down page

RE: Overturning Resisted by Entire Windward Wall?

tpetsmith-

As best I can tell, this is due to the extremely low stiffness of the ground anchors (given as 1.2 k/in). If the structure overall is significantly stiffer than this, then as the anchors under the shearwalls begin to move the load will transfer along the windward wall to the other anchors.

I'm far from an expert however - can anyone with more experience with manufactured homes chime in?

Brian C Potter, PE
http://simplesupports.wordpress.com

RE: Overturning Resisted by Entire Windward Wall?

I assume you are talking about the load perpendicular to a face of the home causing the home to want to move laterally?

If so, I can see the screw anchors resisting up to 70% of the load since they are anchored to the frame of the mobile, and the floor diaphragm is anchored to the frame. However, I would have to think long and hard about the other 30%. Right now, I just don't see it.

Mike McCann
MMC Engineering

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