Wood Shearwall with Openings

[kennyb04]

My question is in regards to wood shearwalls with openings. For a typical shearwall I would use the Segmented Shearwall Method using only strips of wall the are not at the opening. I have attached an example calc. (first page).

That example is for a shorter rectangular shearwall. The problem I am running into is shearwalls on the other sides of our building the continue up to the pitched roof. If I use the segmented shearwall method, it would look like the second page attached. These are simplified examples of what we truly have and if I use the segmented shear wall method on a wall this high I start to get really high hold down values with our true loads. Also, our h/b aspect ratios start pushing the limit or beyond. It seems like there is a lot of wall above the opening that should be used.

The third attached sheet is how my boss has done it previously. It is not an actuall method I have found and I'm not completely sure it is legitimate. I guess it is a variation of the Segmented Wall Method. The first segment being down to the opening as shown attached and finding the Tension and Compression reaction of the extreme outside. Then these are added the the outsides of the individual walls below. As seen in the results, I'm not sure if the interior reaction are as high as they truly should be.

Does anyone have any suggestions. I could try to use the Force Transfer Around Openings method, but not looking for something that complex. Does anyone have a recommendation on how to handle a shearwall that looks like the second attached page? All the examples I have found are for shear walls that look like the first page and I can't find anything to support the calculations shown on the third page.

Also, has anyone tried modeling a wood shear wall as a plate with appropriate properties in a FEM program and then pulling the shears and reactions from that? Does anyone have any objections to doing it that way?

 

[AELLC]

I have always used the method on the third page (alternative method). It makes sense if the entire gable end is sheared the same as the 2 shearwalls flanking the door.

However the usual case is a flat ceiling with ordinary gable truss bottom chord bearing on the end wall (platform framing) - in that case, the the truss transfers the shear to the top of the 2 shearwalls at a height of only 16', and I specify the drag load of the truss on the drawings. (as in most wood buildings)

 

[kennyb04]

AELLC, Thanks for the quick response. Unfortunately, our gable end shearwalls are going all the way up to the roof and collecting the shear from the pitched roof diaphragm so I will be looking at the entire height of the shearwall, not just 16'-0".

You mentioned you have used the third page alternative method for a case like this. Is this an actual method? It is one way I was shown how to do it, but can not find any reference to doing it this way. It is not exactly the Segmented Wall method, but I think it is the closest to the three methods I know of. One of the other methods I know of is "Shearwalls designed for force transfer around openings". This method looks more complicated then what we are looking for and we don't want to get into the straps and blocking. The other method I know of "Perforated Shearwall" I understand to be more empirical and will have a lower allowable load.

If I continue to analyze the wall like the third sheet posted, is the any moment that is not being accounted for being transered from section 1 to sections 2 and 3. Can I just find an internal reactions at the ends like I have done between the segments and add that in to the true reactions for segments 2 and 3?

 

[AELLC]

If this a wood structure, are you saying that the studs go all the way to the roof?

Also, is the entire end wall sheathed with WSP, clear up to the roof?

What I meant to say is we were doing this before the Code even mentioned this topic (as far back as 1974), and it is not mentioned in the Code to date, just something that someone proposed at a seminar and we gave it a lot of study.

The Code allows us to do certain things - it's our engineering judgement. We can design everything with Simpson HDU14's but that would hurt our future business.

 

[kennyb04]

Yes, it is a wood structure with studs going all the way up to the roof. The roof sheathing will transfer the horizontal roof load component to the wall that is sheathed all the way to the roof. The horizontal component of the roof load is transfered to the full height shear walls, not transferred throught a truss. It is a church that we are working on with both prefab. trusses and exposed timber trusses.

The third sheet is the method my boss has done for years (he has owned the firm 30 yrs.) so maybe he came up with that the same way you did. I hadn't seen it done like that and questioned it, so he wanted me to track down if that is a legitimate way to do it or not and if the code allowed its use or not.

Thanks for the input. Also, we are having to use HDU14 or HDU11 hold downs already even using the method on the third sheet. Our transverse shear walls kept getting smaller on us due to additional openings on a long nave so we end up with a decent load on very short walls. The attached sketches were just simplified versions to get the idea across.

We have shearwalls at the ends of the building, but due to the length and transition between spaces, we have short transverse shearwalls in the interior of the space as well.

 

[kennyb04]

AELLC,

You mentioned "just something that someone proposed at a seminar and we gave it a lot of study"

I know that was a long time ago, but do you still have that seminar information or know where to find it. Or any other similar references. The best I can come up with is the regular segmented method. If we end up using that, like the second sheet I have posted, our tension and compression loads are going to be crazy high once we put our true higher loads on with a short 8' wide by 24-32 ft. high shear wall. Assuming the aspect ratio would even allow us to do that. Doing something like the third page I have shown helps that ratio significantly.

 

[AELLC]

Those notes were maybe about 1980-1985 and they are long gone.

There is so much we do that isn't in the Code, but is good engineering judgement. Especially with those king studs acting as tension ties and the entire wall sheeted with WSP.

No problem with those hold downs for a church. I would definitely use the 3rd method.

 

[jdgengineer]

I took a quick look at your sketch and nothing comes to mind abound a fatl flaw with your approach. I think it makes sense to me. I do think you will want to provide blocking and straps above the opening in order to define your wall piers as you have.

Also, one small thing I noticed with your calls. Your shear force to the wall piers should be distributed baer on stiffness (ie wall deflections) not just total length. Since your lengths are reasonably closest probably won't make a huge difference though. Shearwall deflection and therefore stiffness is based on a combination of flexiral stiffness, shear stiffness, nail slip, etc. your method is only taking into account the shear stiffness and neglecting the other contributing factors.

 

[jdgengineer]

One more thing, since you mentioned force transfer around openings. If you had a window I'd say it would be worth the effort to look into it as you would save yourself two hoedowns. However, with a door the method breaks down a little bit as you need wall segments above and below the opening wi the force transfer around openings method. Some people rationalize using the rim joist below as the wall segment but I think that is difficult to defend with the typical force transfer methods I have seen (SEAONC method or Breyers method)

 

[jdgengineer]

I thought about my reply this morning while I was at the gym and thought I would amend it slightly. The code allows three types of Shearwall analysis methods: pier analysis, perforated wall, and force transfer around openings. I agree with you that pier analysis is not the right approach and I don't think your wall piers would meet aspect ratios. Perforated wall is out because you have a sloped top plate. That leaves force transfer around openings. I would argue that you are calculating the forces around the openings (perhaps in a crude way). The code only requires this to be a rational approach and leaves the implementation to the user. While I have never done your method for force transfer I think it is acceptable. I do think you want a strap above the opening and across the wall (at least long enough to develop the strap). One way of calculating this is the drag-strut analogy. This is similar to the diaphragm transfer to several walls along a line of resistance. Essentially you would have a uniform shear dumping in load to your lower piers and a segmented shear at the piers removing it. You would then have a collector between the piers and you would size your strap for this force. That sounds reasonable to me.

 

[woodman88]

I would use the holdowns per the second sketch and pier ratios per the third sketch. Be sure to transfer the forces at the upper corners of the doorway.
The reason for this is that the third sketch area 1 will IMHO have four points of reactions, the ends and each side of the doorway. While I can justify the pier ratios per the third sketch, the calculating the reactions would take more time and effort than justified.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[sennafan]

The page 3 method is legitimate if you provide a shear transfer member along the wall across the head of the opening and continue iotacross the wall on either side. Blocking between studs and tension straps are one way to do this. The sheathing above and below the blocking line must be nailed to the blocking to transfer the uniform shear in the upper section to the lower sectinos. The tension straps need to be strong enough to transfer the difference in accumulated shear at the edges of the door. In your case - 2500#/20' = 125 plf above the door and 2500#/14' = 179 plf below the door head so the straps/blocking need to provide capacity of: (179 - 125 plf)*8' = 432#