Detailing for Uplift in High Wind Area 1

[elyengineering]

Have read through previous threads regarding C&C vs. MWFRS forces.

Am designing connections for residential trusses in a 150 mph wind zone. Two-story, wood-framed residence. I don't typically design in high wind area, so am less familiar with the typical detailing.

According to my calcs, with uplift of over 1,000# at the ends of trusses, this would necessitate strapping studs at all such locations continuously from roof to foundation.

Does this sound consistent with typical Florida design?

I'm in California, and in lesser wind ares, typically just carry uplift from shear wall/hold down forces continuously to the foundation.

This seems like a lot of studs to strap, but perhaps is the way it's done.

 

[DaveVikingPE]

1. Check the Florida codes. I don't have a link (used to, sorry...), but there's an insurance company that has a lot of graphics for wind detailing somewhere.

2. Simpson Strong-Tie has a couple of truss connections that make load path from the connection to the foundation; you could also use a threaded rod, etc.

 

[structuresguy]

Yes, 1000-1500 lb range per truss is very typical here in florida. And yes, the discussion of C&C vs MWFRS is also not settled. It is very easy to interpret the ASCE code both ways. In fact, the Wood Truss Council of America, which governs plate connected wood truss design requirements, has taken the position that the connection of a wood truss to the structure is designed for MWFRS loads. Only the top chord of the truss is designed for C&C. This matches my personal interpretation of the ASCE code.

Anyway, the loading you mention is still typical. And yes, a continuous load path to the foundation is required. And no, nails in pull-out condition (or toe-nailed) are not allowed in this load path.

So, you may find that you need to strap every stud all the way down to foundation. I have had to do this before. But if you look in the Simpson catalog, you will find many options on how to make the load path.

Typically, I try to make double top plates work to support the trusses if they land between studs. Then I strap each truss to the top plates, then the top plates to wall studs. Then at the floors use long straps to bypass the floor framing and connect wall studs above to studs below. Then straps studs to bottom plate, and connect bottom plate to concrete.

Also, make sure you have enough dead load to resist the uplift forces overall. In single story construction, many times the weight of the structure is inadequate, and quite large thickened slab edge foundations are required.

Hope this helps a little.

 

[elyengineering]

Thanks -- that was hugely helpful. Needed to make sure I wasn't a little crazy before I detailed those connections.

For the record, there's lots more hardware in a Florida house than a California house -- in 150 mph zones, anyway. Who knew!

 

[RareBugTX]

elyenengineering:

I do also would recommend you to request ALL the catalogs that Simpson Strong-Tie offers and...they are free!!!. I ordered mine through their web site and pretty soon I had the catalogs here at the office. The way they present the different options is very graphical and helpful.

Regards,


RareBug