wind uplift load path in wood walls

[bjb]

A very basic question about a very common situation.

The condidtion:
A wood building with an 8 foot floor to ceiling height and the distance from the top of foundation to the top of the double top plate is 8'-1 1/8". Wood roof trusses are spaced at 24", and wall studs are spaced at 16". Simpson ties are used at each end of each truss, so the trusses are anchored to the double top plate. The wood sheathing panels are 4' by 8'.

The question:
Are you relying on the wood sheathing to form the continuous load path for uplift, or do you have to use metal anchors connection the tops and bottoms of the wall studs to the top and bottom plates? This is what Simpson shows in their brochures, but of course they are in business to sell anchors.

I have a copy of the Windstorm Mitigation Manual and it says not to use the sheathing because if your sheathing is a 4'x8' panel, and the distance between the top and bot plates is 8'-1 1/8" then the top ply of the double top plate probably won't get nailed. Also, you probably have roof uplift acting concurrently with a horizontal wind shear so if you did try to use the sheathing you would need additional nails than those required just for shear wall action.

It seems like the best thing to do is to provide metal anchors between the studs and the plates, but I never see this done in my area, hence my question. At my former office the firm owner said that he always used the wall sheathing for the uplift, and he is a well respected and knowledgeable engineer. On the other hand, that was in the days before we used the IBC.

 

[RARWOOD]

I am not clear on what you are asking, but I am assuming that you have a gable roof and are developing the load path to resist uplift load on your entire roof system.

For example using an older code with a projected area method you calculate a required wind up lift on your system of 15 p.s.f. If the dead load of your roof is 5 p.s.f on a horizontal projection then your net roof uplift using the entire 5 p.s.f. dead load, would be 10 p.s.f.

Now depending on the size of your roof it might be posible to transfer the uplift through the sheathing into the roof trusses. Then by toe nailing and the use of blocking and the wall sheathing you might be able to transfer the load out of the trusses into the top plate into the sheathing and studs ect. down to the foundation.

The above approach was not all that uncommon in the past. What probably make this approach unuseable today are several factors. Wood buildings today are larger. The use of trusses has increased the roof spans and the amount of load that can be transferred into the structure. Also based on my experience the wind loads required by code today are larger then they were twenty years ago.

As always the key is to develop your load path. As long as you can justify the transfer of your forces through blocking and sheathing or other means you should be OK.

In the past, around 10 years ago I had a lot of experience with the design of 4 and 5 story platform framed multi-family buildings. Most of the ones I viewed during construction, in the midwest were braced against wind load using gypsum sheathing with no special hold downs or connection hardware.

I'm not sure that I designed any 4 story buildings, although I designed several 3 story wood buildings. On the ones that I designed and stamped I used hold downs and connection hardware, because I could not justify by calculations that not using the hold downs would work.

I know the buildings that did not use any special hold downs are still standing so that approach works, I just can't justify it with numbers. One note the buildings I observed were all in areas where sesmic loading was not an issue.

To learn more about the topic you should obtain a copy of Donald Breyer's book on "Design of Wood Structures". What I like about the book is that Breyer gives a number examples of resisting loads with toe nailing and blocking.

 

[bjb]

What I'm asking about is the connection between the wall studs and the double top plate. My trusses are anchored to the double top plate, but then I have to worry about the load path in tension between the double top plate and the wall studs. In the past, before using the IBC, in NY we had our own unique building code that was very light on the uplift and lateral requirements. It was common to just count on using the wall sheathing to make sure the double top plates were anchored to the wall studs. I am not aware of any failures from this approach, but then again these buildings have probably never seen a full design load. Now we're using the IBC, and what worked before doesn't always work now.

Now with the IBC the loads are higher than before, and the 0.6D+W load combination is more stringent than we had before. After sleeping on this matter, I am favoring using Simpson type connectors that attach maybe every other stud to the double top plates, and having similar conncetors at the bottom of studs. There is a booklet called the Windstorm Mitigation Manual that goes into the load path requirements for uplift. They state that using the wall sheathing to tie the wall studs to the double top plate is questionable because if you have an 8 foot floor to ceiling height, the distance from the bottom of you fndn sill plate to the top of your double top plate is about 8'-1 1/8". Therefore, with sheathing panels that are 4'x8', you won't get full coverage on one of the plates. A few years ago I had a project where the contractor didn't extend the sheathing to the top of the double top plate, so I had the contractor install connectors between the studs and the double top plate.

I do have the Breyer book and agree that it is excellent. I understand the concepts of roof uplift and the load path requirements, but I am curious as to how other engineers are handling this.

 

[UcfSE]

We commonly use straps all the way to the sole plate. I'm also in Florida so I probably have a lot more uplift to deal with than in New York. If you want to use the sheathing, you'll have to find a way to calculate the interaction between shear and uplift on it and your nails. Basically you can make your own load path if you choose. Just remember that sheathing is not a "load sink", you cannot just throw load to it and not check what happens. With all of this, it is easier for us to use straps for uplift and not have to worry about overstressing the shearwall nails, plus truss uplifts here can be very high, requiring something more than the sheathing for uplift anyway.

If you look in the SSTD 10-99 (1999 SBCCI Standard for Hurricane Resistant Residential Construction), section 305.6 there is some information about using sheathing for uplift resistence.

 

[jheidt2543]

I attended one of Simpson's seminars on connectors a few months ago (they are excellent bye the way). What I understood the Simpson engineer giving the seminar to say is that the wall sheathing could be used to transfer uplift loads down to the foundation. HOWEVER, just as in your example, the wall is usually taller than 8' and the sheathing is only 8'. So, there is no continuous load path, hence the need for the uplift straps.

This is even more of a problem in a two or more store wood frame building.

 

[skier1578]

I know that where I am, we are required to design for 90mph wind, which may be low or high for where you are. We are required to align and strap rafter/truss to stud, stud to stud (in case of multi-story structure), and then stud to sill @ 48" o.c. Where I am, this is only done if the wind designed for is greater than 80mph, or if the structure is unique enought to produce high uplift forces.

 

[bjb]

skier1578, where are you located? We don't have requirements to line up the trusses with the studs in my area.

 

[boo1]

If resedential framing at 24" oc, we commonly align walls and trusses. We always place studs inlign with girders and heavly loads trusses.

 

[sundale]

With the IBC, I have recently started calling out coil straps wrapped over the wall top plate and the embedded Simpson straps at 8' o.c. and larger wall opening jamb columns at the sole plate. I have received nothing but grief from the contractors about this. I am starting to detest wood projects because anybody who has a circular saw, a pickup truck and a dog always knows more about structural engineering than I do.

0.6D+W with 90 mph, exp. C C&C IBC pressures yields anywhere from about 100 plf to 250 plf net tension at the wall top depending on truss spans, dead loads, roof hts, etc. This is not huge, but neither is it trivial.

APA Technical Note E510 has a very good discussion about using the OSB sheathing as a tension tie. It is a free PDF download. As I read it, horizontal panel joints are OK as long as you get the nailing into the studs above and below the joint.

With net tension in the sheathing on a wood wall, do any of you other engineers consider cross grain bending of the sole plate using a traditional round anchor bolt? I like either a Simpson strap or a big plate washer over the anchor bolt to inhibit cross grain bending.

 

[bjb]

I agree with you sundale. I have the same tech note from
APA. Where are you located? In my area, which is upstate NY, very few designs have anchors attaching the top plates to the stud walls, and even full depth blocking is almost never seen at truss bearings with raised heels. I don't see how designs without blocking or lots of metal ties can meet the IBC, but we have only been using the IBC since 2003. It has been our observation that many other designers around here seem to be very loosely following the IBC, and they typically don't get called on it by the local building departments.

 

[sundale]

I'm located in Durango, Colorado near the 4 Corners. Most structural engineers doing wood here and are not strapping the same way as I am, I think. This is based solely on the grief I receive when I do show strapping. I used to work in Denver and this is a true statement there too, but to a lesser extent. I don't know if I'm too conservative or others are too liberal, or some combination thereof.

My moral dilemma, in general, with wood design is the Code prescribed forces versus the prescriptive Chapter 23 "Conventional Construction" provisions and the IRC provisions. This stuff is more of a "recipe book" and not really code forces and capacities. Moreover, I don't think alot of this recipe stuff works, numerically, if you actually run the numbers on what the code says. Long span roof trusses, 10'+ plate heights, and architectural "glass boxes" with more window than wall are a far cry from the 2x8 rafters and 8' plate heights of the olden days. Is this "traditional" or exotic?

What are your thoughts on the APA E510 document in regards to edge/end distances at the top and bottom of the wall? In a single sole plate, your base nails have 3/4" edge distance. The geometry adjustment for D<1/4" is 1.0, i.e. ignore edge distances. If one chose not ignore end distance and treat it like a bolt, then 7 diameters for an 8d nail is 0.917", which is bigger than 3/4".

My other concern with using traditional round AB's for tension delivered via the sheathing is the torque and cross grain bending in the sole plate. I used to put a big plate washer, but got grief over that... The embedded Simpson straps are good and are like $13 each here in the 4 Corners.

It would be nice to use the sheathing as a tension tie and I think I'm going generate both strapped wall and sheathing tension wall details as contractor options. Unfortunately, the more wood details you show, the more bitching you receive from the wood butchers.

 

[skier1578]

bjb - I am in Southern California. At wind speeds of over 90mph, exp. C, this is required per the California Building Code. I don't have mine with me know so I can't give a specific sections. If my memory is any good, which is debatable, I want to say it is in section 23-1. This may hold true for the UBC and IBC as well, I'm not sure.

 

[UcfSE]

sundale, when you use the straps at, say 8-foot spacings, Are you designing the top plate to act as a beam between the straps?

There is a modification to the IBC 2308.12.8 requiring 3x3x1/4 washers at base plates. It is not in affect yet to my knowledge. At least then you will have something backing you concerning the use of large washers. I totally agree about their use. Cross-grain bending and twisting of the sole plate must be addressed. Unfortunately, you run into the "been doing it that way for forty years" crowd.

 

[sundale]

With 90 mph, Exposure C, for bearing wall conditions, I currently do a CS18 wrapped strap at 4' o.c. over the top plate. Implied with this is that the top plate has about a 4' span capacity, which seems reasonable. The strap tension is T/2 each face of the stud and I use 10dx1-1/2" nails so as not to split the stud's wood. Per the '06 Simpson catalog, you do not need a reduction for this for flat straps anymore (formerly a 0.77 adjustment w/10d to 10dx1-1/2")

At the bottom of the wall, I have been putting STHD8 (standard or rim joist version depending on whether platform framing or not) at a single jamb col for for 4' or less rough openings, both jamb col's for any bigger openings, and at 8' o.c. along the wall in general. This is definitely overkill as far a demand vs capacity goes but it is the only anchor that grabs the stud directly, which I do like. My reasoning for the 8' o.c. spacing is that you will get some corbelling action out of the sheathing and so I justify aligning a tension anchorage at the bottom of the wall with every other wall top anchorage.

After getting (too) much grief and reviewing APA E510, I think I could live with using the sheathing to anchor the top plate because your tensile end distance in the sheathing is bigger than 1". Specifically, you can do your panel edge nailing at the top 2x6 and your tension nailing at the bottom 2x6 of the wall's top plate. The field nailing is no problem.

At the wall base, however, I have my doubts about the 3/4" end distance in the sheathing. In reviewing APA E510, they say 1" end distance typical, which smells alot like a 7 diameter requirement.

 

[bjb]

If you're using the sheathing across to form the tension ties across the floors, you need to have a continuos piece going across. One of the concerns that I have with using the sheathing is that it matters what the sheathing layout is. Wherever you're using the sheathing, it only can work if there are no horizontal panel joints where the extra nailing occurs. I have seen contractors sometimes using "slivers" of sheathing, and not getting both plies of the double top plate covered with sheathing.

However, I do like the idea of using the sheathing where possible. I would indicate that no horizontal panel joints are acceptable in the uplift nailing zones, and make the nailing and the panel continuity part of the special inspection program.

 

[boo1]

Try Simpson SP4 top and bottom of the studs, instead of the CS18. Have them installed on the walls when they are being framed (up flat on the slab), leave one side un-nailed untill the roof is installed. This allows the stap to lay flat, as the roof loads compress the wall sections