Anchor bolts and washer plates in wood shear wall sill plate

[bnickeson]

I'm reading through requirements of shear wall fastening in the 2008 SDPWS. Section 4.3.6.4.3 deals with washer plates on anchor bolts in shear walls. Normally, they require washer plates within a half inch of the edge of the sill plate (which would be fairly large 4 1/2" square washer plates for a 2x6 wall) unless certain exceptions are met. I'm having trouble understanding the rationale of some of these exceptions.

We all know that the purpose of these washer plates are to prevent any cross grain bending in the sill plate due to shear wall uplift. So below are the exceptions that they give:

"Standard cut washers shall be permitted to be used where anchor bolts are designed to resist shear only and the following requirements are met:" So they are saying that the anchor rods must only take shear, as they typically do in shear walls, and the following requirements have to be met for the exceptions to be allowed. Easy enough.

"The shear wall is designed in accordance with provisions of 4.3.5.1 (standard wood shear walls) with required uplift anchorage at shear wall ends sized to resist overturning neglecting dead load stabilizing moment." OK, what? I'm trying to understand why they would require that hold downs are sized to ignore the counteracting dead load moments. If the washer plates are there to prevent any cross-grain bending from uplift, why would you need them if your shear wall has absolutely no uplift due to large dead loads applied to it? I'm designing a three-story structure in Florida that has some significant dead loads on the exterior shear walls and not even close to any uplift due to either shear wall overturning, roof uplift, or a combination of the two. So why should it be required to include the plate washers? I also have areas with perforated shear walls that have a large amount of dead load on them and no uplift at the foundation. By these code requirements I'd have to include plate washers as well despite not having any mechanism in which cross grain bending of the sill plate occurs. I just don't understand it and I'm sure I'll get push-back on the price tag of them as well.

"Shear wall aspect ratio, h:b, does not exceed 2:1" I suppose I can understand this exception, but again, if you have no net uplift due to dead loads, why should the washers be required?

"The nominal unit shear capacity of the shear wall does not exceed 980 plf for seismic or 1370 plf for wind." I'm not entirely sure what the capacity of the wall has to do with the actual load present on the wall or the uplift on the wall. If I only have 200 plf on my shear wall and sheathe it one side with 6" o.c. nailing I can get away with not having washers. But if I sheathe it both sides with 4" nail spacing then I have to include plate washers?

I can understand having a requirement to include plate washers on any bearing wall with net uplift or a shear wall without hold downs. But the way they list the exceptions in the code just makes little to no sense at all. Does anyone have any experience or justification to eliminate these requirements? Section 4.3.6.4 states that "Shear Wall Anchorage and Load Path Design of shear wall anchorage and load path shall conform to the requirements of this section, or shall be calculated using principles of mechanics." Could that be considered a way out of the washer plate requirements since mechanics states that a wall with significant dead load and no uplift will not have any cross grain bending?

 

[FSS]

Why not just apply the design loads and calculate what you need? No need for cookbooks if you perform engineering analysis.

 

[TXStructural]

How much time/money are you spending to save a few dollars on washers? And you started this off saying it was about fastening of shear walls, implying that there could be net uplift.

 

[bnickeson]

FSS - That's what I was getting at with my last paragraph. The code provision states that the plate washers shall be used, but the root code section above it also says "or shall be calculated using principles of mechanics." I'm thinking that statement will allow me to justify not using the washers where there is no uplift. I was just more confused than anything why they stated the provisions as they did, especially the part about overturning neglecting dead loads.

TX - This building is very large and has a lot of exterior and interior walls that behave as shear walls. We would need approximately 1000 additional plate washers if I chose to require them which is fairly significant. And I mention in the post that most of these walls have large dead loads on them and, thus, no uplift.

 

[JAE]

Can anyone else comment?

 

[CELinOttawa]

This question is code specific, and you should be seeking an interpretation from the authority who publishes your code.

 

[AELLC]

I think by specifying no washers at all, you are exposing yourself to liability. What about uplift during construction, when there is less dead load on the roof?

Even ordinary washers are required to keep the anchor bolt nut from digging into the wood.

Are you wanting to eliminate the washers on your own volition, to save the builder money?

 

[a2mfk]

What's a good washer go for these days, a buck? Nothing to sweat on a big 3 story building. I have never heard of any engineer getting questioned on the cost of nuts, bolts, washers, etc. But if you truly have net uplift on interior shear walls (even no uplift due to overturning, only using 0.6D to resist?), then in theory you don't need them.

There are a lot of sections of code that to me are prescriptive safety nets for amateurs. If you are sharpening your pencil and running all the numbers, you are more like the guy tightrope walking over the grand canyon.

 

[bnickeson]

Standard washers will be used everywhere unless plate washers are specified, that's a given. The question is whether we can get away with not using the large plate washers. In the grand scheme of things it's probably not a large chunk of money as a percentage of the entire construction project, but I'd prefer not to throw in $10,000 worth of plate washers if we can get away with not using them. Since I read that section more in-depth, now I'm just more curious than anything. a2mfk's mention of a prescriptive safety net could make sense. The commentary also references research on cyclic tests which would imply the provisions were included for seismic purposes originally.

 

[RWW0002]

It may be worth noting that IBC 2003 and IBC 2006 have similar provisions for shear walls in Seismic Category D, E, or F, but make no mention that I am aware of for low seismic areas. (See 2305.3.11) There has been some research done on "partially restrained shear walls" that may be applicable and worth looking into for further information on the use of dead load to stabilize overturning moment.

 

[bnickeson]

You're exactly right...it's been a few years since I've done a high seismic design by that code but I remember that provision now. It seems it's made its way into the code for any lateral forces now instead of just high seismic.

 

[a2mfk]

$10 plate washers? I need to get into the plate washer business

If it were my design and my numbers showed zero uplift I wouldn't waste money on the more expensive washers. Hell, in theory, you wouldn't need a nut for just shear? Codes are not perfect and they can't cover everything, that is where your judgment comes in as an engineer.

I do applaud your attention to detail.