truss uplift anchor

[sklgleb]

Hi all,

I'm working on a house in the Florida Keys - we have a 180mph wind with exposure D. My roof truss uplift reaction (common truss) are ~5k! - can anyone suggest a proper anchor to a wood stud wall for this monster. I know there is now a question of going to a CMU wall, but we would like to avoid that.


Thanks in advance,

G.S.

 

[Teguci]

5/8" Diameter threaded rod from truss to foundation. Not a wood guy (probably obvious from my answer), but if its only a couple stories you probably don't need a take up device.

 

[KootK]

I was actually going to recommend a take-up shear wall hold down device semi-facetiously. Obviously, doing that at 24" o/c would be ridiculous. Another potential deal breaker is how you actually connect to the truss for 5000 lbs uplift. You'd almost need and upside down hanger of some sort to mobilize a bunch of the top chord fcp capacity.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JedClampett]

Can you space the trusses closer together?
Are you comparing LRFD uplift to allowable uplift hardware? 3000 lb will also be tough, but not as tough.
Can you suggest a heavier roof? Like clay tiles?
Are you sure Exposure D is appropriate? Coastal areas in Florida (I've never worked in the Keys, however) top out at exposure C. Don't ask me why.
Are you using partially enclosed analysis? Is that correct? That will really increase the wind loads.
Can you tell the architect to change the design? That will be fun.

 

[jayrod12]

I've never seen anything with that kind of capacity for a single ply truss. Simpson has capacities for multiply girder tie-downs. See the LGT series or the HGT series. They all require a 2 ply truss minimum though.

 

[sklgleb]

Everyone,

This issue has been resolved. I was always under the impression that truss anchors are per C&C... turns out you can use Main wind force...

 

[jayrod12]

Says who? I would be designing for C&C on a single truss. Even on a girder truss, I would just be designing using the appropriate factors for the trib area.

 

[KootK]

Yeah, I've heard the MWFRS thing for truss tie downs before, both on this forum and in this - admittedly industry biased - position statement: Link.

My feeling on the mater is similar to Jayrod's. Unless I grossly misunderstand the intent of the wind load provisions, I feel that it should be C&C based on tributary area.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

This was the recent discussion that I had in mind: Link. I also had a debate going with JAE that I never finished because I was in the Bahama's . Maybe he'll resurface for this...

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[DaveAtkins]

I would use a tributary area equal to the truss span squared, divided by three. Perhaps that will get you close to the MWFRS values. 5,000# uplift at each end of a common truss just doesn't seem realistic.

DaveAtkins

 

[boo1]

we see 5k girder loads, not single truss load
get the truss company to do the analysis, use their reactions. (watch the horizontal loads)

 

[dhengr]

Sklgleb:
When you are dealing with those kinds of uplifts on single roof trusses on a house, the next question is how the heck do you keep the roofing materials, whatever they be, and the roof sheathing on the trusses to actually apply the uplift? Then, the next question is...., don’t you need CMU exterior walls, as a min., just to keep flying 2x4's from entering you bldg., endwise like a projectile, during the storm?