SST Twist Strap at Every Roof Rafter?

[RareBugTX]

Hello all:

Would like to know your take on the need for roof rafter to wall straps at every rafter. I am just getting back to this after a few years of calling them out on the plans at every rafter. I believe that the load (20 psf uplift minus dead load) does not justify the need. Would like to know what you guys use. In this area wind controls. This is for residential frame construction and the roof slopes vary from 4:12 to 12:12. What triggered me the question is anchorage to foundation per code is @ 48" o.c. According to my numbers straps every third rafter would suffice.

Regards

Rarebug

 

[hokie66]

So if you only tie down every third rafter, what prevents uplift of the two between? It is not strictly the capacity of an anchor, but the load path which controls.

 

[asixth]

Depending of the rafter span and weight of the roofing material a strap might not be required and you can get away with just a multigrip. I live in a sub-tropical area where wind loads are high and light-gauge metal roof sheeting is used so it is common to see every rafter strapped to the wall frame and wall frame anchored to the foundations.

 

[XR250]

In our area, Raleigh-Durham, NC, they inspectors usually require them on every rafter - so it is irrelavent what I write on the plans.
Also, in our area, anchorage to the the foundation is only req'd at 6 ft. O.C. - even basements with 9 ft. of backfill.
I always specify the bolts closer - sometimes 16" O.C. at basements in order to properly react the backfill forces.
I have a question, and this is a pet peeve of mine - why do people call them "Roof Rafters"? Aren't they just "rafters". Is there another type of rafter?

 

[frv]

Hokie has got it. Regardless of whether the capacity of the anchor greatly exceeds the actual uplift on one (or two, or three) anchors, how will the "unanchored" rafters resist the uplift on them?

You have to trace your loads all the way down, regardless of "aggregate" capacity.

 

[XR250]

Roof sheathing and facia board spanning between anchored rafters?

 

[nac521]

IRC table R802.11 determines the prescriptive amount of required uplift connection strength at each rafter to plate connection. Although the use of toenails to achieve this is not prohibited, it is not a good design practice to rely on toe nails for this loading type. The installation angle and location of the toenails must be done properly to achieve the design strength and prevent splitting. Common practice in my area is to specify the Simpson H2.5A tie at each rafter.

 

[TXStructural]

In a hurricane, every part has to be attached sufficiently, otherwise you end up peeling the cladding and structure apart so the wind gets inside and fails the remaining structure. Not to say there aren't other ways to attach rafters, but why cut this corner to save so little.

You all might consider looking at the IBHS website at

http://disastersafety.org

for a bit more than minimum on load path connections. They have done quite a bit of research on the topic, and the free, downloadable FORTIFIED guides provide details which make buildings survive windstorms much better than minimum code.

 

[frv]

I don't know that I've ever considered fascia board anything other than architectural.

Will sheathing span 48"? Nail withdrawal between sheathing and "anchored" rafter? Can the anchored rafter support three times the uplift? I guess in theory, if you can get all these things to work, it would be OK to skip one or two straps. I doubt you'll get the entire load path to work without some odd detailing call-outs (e.g., and unusual number of nails at the anchored rafters); it just seems like poor detailing in any case.

 

[PrescottEngineer]

Simpson has a new "SWDC" screw on page 184 of their 2013 catalog. You can use one screw from the top plate to the truss or rafter. It is a lot faster than installing ten nails for an H2.5A so we have started specifying those on our projects.

 

[XR250]

@FRV.

My comment was more tongue and cheek about the fascia and sheathing. However, there is some redundancy there (I actually meant the sub-fascia)

 

[dhengr]

All you have to do is determine the appropriate loads and get them down to the foundation through a reasonable and justifiable load path. If properly structured, one large tie-down device at the center of the bldg. will probably suffice. That’s the intent of the bldg. code. But, some bldg. inspectors, in their reading of the cookbook, and the way they have been jerked around by some builders over the years, may like to see the same hold downs on every rafter, and it may not be worth many hours of fighting with them on the matter. As an informed and knowledgeable engineer, you should be able to explain and defend your design. Many builders do all kinds of things that are quite difficult to defend, under the rubric that they have always done it the wrong way, ‘that way,’ and by the grace of God, gotten away with it.

 

[a7x1984]

I believe that the load (20 psf uplift minus dead load) does not justify the need.

RareBugRA: Don't forget that the general load combination for uplift is .6D+.6W for ASD and .9D+1.0W for strength. Not allowed to rely on 100% of DL.

In Russia building design you!

 

[GlobeTrekker]

I have a question, and this is a pet peeve of mine - why do people call them "Roof Rafters"? Aren't they just "rafters". Is there another type of rafter?

Rafters are identified for the roofs, because they are (typically) installed on a
diagonal plane, whereas walls use "studs" installed in the vertical plane, and ceilings use "joists" installed in the horizontal plane.


.

 

[frv]

a7x1984. Be careful. Those load combinations are only valid in ASCE 7-10, which has not yet been widely adopted. It likely will become widely adopted in the next few years as IBC 2012 is adopted, but not yet. ASCE 7-10 changed the way wind loads were calculated and that directly impacts the validity of those load combos. Prior to ASCE 7-10, the load combos were 0.6D+1.0W (ASD) 0.9D+1.6W (LRFD).

globetrekker, I think you missed his point. His pet peeve is with the redundant word roof. In other words, you can't have a "floor" rafter. A rafter, by definition, is a roof member.

 

[dhengr]

ExcelEngineering:
Yes, there is other kinds... there is steel rafters, there is wood rafters, then there is those thar rafters what drift down them thar rivers. Sometimes called white water rafters, but not to be confused with them thar white wood rafters, at the bog box store. Then there are floor rafters when you intentionally want the floor to slope kinda like an almost flat ruuf. You certainly couldn’t call these floor joistes, since they slope. And, then you sometimes see carpenters who don’t understand how to use a tape measure or level installing window headers, which could as well be called window rafters given their slope. I think you should name your pet something other than ‘peeve.’ I’ll buy you a beer the next time we meet, to take your mind off your pet, Peeve.

 

[a7x1984]

My point was that full dead load can't be subtracted from wind load. I was using ASCE 7 10 as an example. That applies regardless of the new wind codes.

In Russia building design you!