Wood Shear Wall Analysis 1

[cuels]

When looking at a regular gabled end of a building where the roof is constructed of manufactured trusses spaced at 24" o.c. I have a question about the dead load that is used to aid in the hold down of a segmented shear wall. Should I only include the gable end truss + overhang as the dead load holding down the wall in my overtuning moment analysis to determine the hold down requirements or is there anything that allows me to look at more of the roof acting as a dead load? If this is the case, the shear walls along the gable end of houses, buildings, etc. will always have a much higher hold down force requirement than the shear walls parallel to the ridge.

Also, along the same lines, has anyone ever analyzed the hold down force of the sheathing to sill plate and sill plate to embedded anchor bolt hold down force? If so, what allowable uplift force have you determined that it can resist? I have used the tables for allowable shear for structural panel sheathing in both the IBC and NDS for Wood Construction. Will that same shear value work for the sheathing to sill plate connection?

These are just a few thoughts that were running through my mind while I was working on a design for a detached garage with 14' high walls 40' x 40' square.

 

[JoshPlumSE]

To me it's a load path issue. I follow the gravity load path for that wall and determine what the total axial force is for that wall. That's what I use to reduce the hold down forces.

So, by that rationale those walls would have much lower reductions in their hold down force calculations than the walls that support the majority of the roof framing.

My 0.02$ on the sill plate anchorage topic:
No one that I know of accounts for this as part of their shear wall design. At least not unless they are using a true hold down at these locations.

The traditional assumption is that the only vertical support for the shear wall is the hold down in tension and the compression post on the other side. We know that it has to be a conservative assumption. But, that's what is typically done.

If you venture outside those assumptions, then you may be violating the "standard of practice". That's okay if your design is sound and works. But, the amount of extra effort (and the corresponding liability) it would take to demonstrate this is probably not worth it.

Josh

 

[JAE]

cuels,

If your gable-end shearwall extends all the way to an intersecting wall that actually supports the roof trusses (i.e the corner of the building) then there is some rationale to utilize the "holddown" weight from that intersecting wall.

Here's what you have to consider:

1. The intersecting wall has some level of in-plane rigidity due to its sheathing. As your gable-end shearwall tries to lift off the footing, it has to overcome both the holddown anchorage at the corner as well as lift the "connected" intersecting wall (along with its roof dead load) as well.

2. The question is - how much of that intersecting wall, and associated roof weight) goes in to holding down the shearwall end?

3. We have assumed that a 1:1 ratio might be a good starting place. So if your walls are 10 feet high, then 10 feet of tributary wall width can go into holding the end of the shearwall down. And thus, 10 feet of that supported roof weight can be included as well....BUT....

4. The tributary roof weight also might involve some roof wind uplift forces under the load combination. On the uplift side of the main gable-end wall you are dealing with windward forces perhaps on the intersecting wall so the uplift may not be as high as on the other side where you would have leeward roof uplift.

As you can see, it can get pretty convoluted so many engineers, I think, ignore the participation of intersecting walls to some extent.

 

[haynewp]

I have not counted on the wall sheathing to sill plate connection in the past. I have seen nails split the edges of the panels and the sill, and there is not much distance from the nail to the panel edge to prevent pull anyway. I am not sure the bottom of the panels always make it to the bottom of the sill either.

 

[JoshPlumSE]

JAE -

In your example, please explain the load path that transfers the vertical load from the studs of the intersecting wall into the Gable wall.

I'm having trouble understanding your rationale. Maybe the tributary load on the end post if that post is shared between the two intersecting walls....

 

[JAE]

JoshPlum,

The intersecting wall is a semi-rigid plate if you will, that is usually attached to a common end post at the corner with the shearwall (the sheathing of the intersecting wall is nailed with boundary/edge nailing along the end post.

If the shear wall tries to lift off the foundation it will have resisting it both the holddown (if present) and the weight of the intersecting wall. It will have to overcome its own weight and the weight of the intersecting wall before the holddown begins to "feel" the net tension.

The weight of that intersecting wall that participates in helping to hold down the end of the shear wall is subjective but I've looked at a 1:1 45 degree line to define the horizontal length of the intersecting wall as a conservative measure. If the intersecting wall has roof dead load on it, then that weight is also participating in adding dead weight to the end post of the shear wall through a sort of vertical diaphragm action of the wall.

But as I mentioned above, the wind uplift on that roof causes complications in this assumption.

 

[JoshPlumSE]

You'd be relying on the vertical shear between the post and the sheathing for the intersecting wall. That's the only load path that you can rely on to transfer that force, correct?

If so, then make sure that the nailing of the shear wall to the post can handle that shear. Most people do not design for this vertical shear, but it would likely control the shear design of that wall.

 

[JAE]

Correct.

 

[KBVT]

When I design holddowns for wood shear wall chords, I will only include the dead load of the intersecting wall at the end post based on the shear capacity of the nails connecting the two. I have a typical detail showing the nailing pattern in this condition so I know what the vertical shear capacity would be. Slightly more conservative then using a prescribed portion of the wall but it does allow the use of a smaller holddown in certain cases.

 

[3doorsdwn]

This is an interesting thread. Something I have never been too clear on: vertical shear in a run of the mill [no pun intended] wood shear wall. You see some design examples where the guy just goes into the IBC tables with horizontal shear only [per unit length]. But in some examples I have seen [in one reference I have] the problem will calculate both horizontal and vertical unit shears [at the boundaries] and check them against the appropriate tables. Never been to clear on what is appropriate. (To be conservative, I typically check both.) I think a lot of engineers assume that the maximum w/d ratios given in code cover us for that; but I think that is more for deflection control (than anything else).

 

[jhoulette]

You can reduce holddowns using OSB sheathing. I posted a blog at

http://evstudio.info/2009/07/31/reducing-holdowns-using-osb-exterior-sheathing

You can also learn more at

http://www.apawood.org/

APA has several design examples about lateral design.
When I'm designing shear walls on gable ends as you described I typically use additional load from the intersecting walls as JAE discussed. You just have to keep in mind, 'If this wall is going to lift up, what else is going along for the ride' Then put numbers to it. I typically get about 1,000lb of 'other' resistance in wood framing.

Jim Houlette

http://www.EVstudio.us

www.evstudio.info

 

[DaveAtkins]

3doorsdwn,

The horizontal and vertical shear in a shear wall are equal.

There may or may not be vertical load present to counteract the vertical shear, but the horizontal and vertical shears in the sheathing are still equal.

DaveAtkins

 

[JoshPlumSE]

DaveAtkins -

We were talking about the horizontal shear in the gable wall and the vertical shear in the wall perpendicular to it. So, they are aren't actually equal because they are different walls.

Generally speaking, the physical reality of what happens with wood shear walls will be considerably different from the normal design assumptions made by structural engineers. In this case, the standard of practice is to design shear walls for the average shear force seen at the base of the wall.

Unfortunately, IF you relied on the roof weight on the transverse wall to reduce the hold down forces in the gable wall then you would be required to consider the vertical shear in that wall (even though the average horizontal shear in the transverse wall is zero).

My main point in this thread is that you should NOT design the hold down for a reduced tension force based on the roof weight in the transverse wall.... To do so would be to treate the trasverse wall as if all of that roof weight is being trasferred via shear through that last panel.

 

[haynewp]

These may be of some assistance, there is some dicussion on pages 37 & 38 on using panels to resist uplift:

http://www.safehomeillinois.org/resources/building/windstorm_miti_lightframe.pdf

http://www.safehomeillinois.org/resources/building/comp_manual_lightframe.pdf

 

[jhoulette]

Haynewp - Thank you! Looks like a couple great resources. I look forward to reading them in full. Maybe a bit out dated, but great they went step by step when discussing the break down of the wall.

Jim Houlette

http://www.EVstudio.us

http://www.evstudio.info

 

[msquared48]

Holdowns are for uplift and anchor blolts for horizontal shear. Don't try to make one do the job of the other. They are too cheap and the failure too expensive.

As for the dead load, if there is a lot of roof area and wall length to include in the dead load calcs, I will. However, to err on the conservative side is good. (to a point)

Cheers.

Mike McCann
MMC Engineering