OPEN WOOD PORCH DRIFT 1

[RareBugTX]

Hello everyone:

Can anybody suggest references or give me any hints on how to determine a conservative method to calculate an acceptable limit for an open wood porch drift. A client is looking at not having a bad feeling when he is standing on an intermediate landing ( 12' foot above ground 6"x6" SPF treated lumber) of a newly constructed porch. In the City where I live there are no seismic concerns.

The landing dimensions in plan is about 6'x4'


Thanks and regards

RareBug

 

[RARWOOD]

This is an interesting question. In Minnesota I've talked to some people who believe that drift loads are not required for single family homes. Of course the fact something isn't required doesn't mean you should not design for it.

With the old Minnesota State Code a problem like this was easy to deal with, because the State Code limited total snow load with drift to three times the basic snow load. So if you were in doubt about the actual drift you could use 120 p.s.f. snow which was the maximum required by code ( 3 x 40 p.s.f.).

I am interested in what responses you will get because I've dealt with similar problems.

 

[tmacattack]

I am assuming your question is in regards to lateral drift, and not snow drift.

It has been my experience that most wood porches do not have a lateral force resisting system at all. In Chicago, where porches are scrutinized closely after last year's deadly collapse, I am sure to design x-bracing to resist the wind load on my wood framed porches. I am not aware of specific lateral drift requirements. However, I would make sure the numbers work for x-bracing for resisting wind load force, and that should stiffen up the porch to the point where lateral drift is not noticeable.

 

[RareBugTX]

Tmacattack:

Thank you for your feedback. Exactly, while I value also the snow consideration, my main concern was lateral displacement limits. The porch design in Chicago is becoming a science and Building Officials are very picky about the connection design and I strongly support this approach.
On the other hand I would like to know when designing x-bracing for such a structure, what drift figure would be considered a "stiff", or what numbers do you use. (h/500, h/600, etc)

Thanks

RareBug