Wood Diaphragm? 1

[clevering]

I am currently designing a 4 story wood structure consisting of apartment units and that have dividing walls at 30' on center (with typical gypsum sheathing). In addition there are cmu walls (either stair shafts or fire walls) about 100'-150' apart. My question arises in how to take out the lateral load and how to examine the diaphragm. All examples discuss a pristine rectangular box. In my building the walls at the exterior are offset and constantly moving in and out. This makes it impossible to establish a diaphragm chord.

I have found mixed advice/info. I have spoken with people that say they do not look at the diaphram as a 'beam', as it is outlined in the books (no need to look at the chord etc.). They adise me to just load the masonry walls by tributary area. I am concerned that the lack of chord means I can not span this diaphragm so far and therefor have to take the load in each dividing wall. (flexible simple span between) Can I assume a flexible diaphragm spanning between the masonry walls without this chord element? Do I use the dividing walls to stiffen the diaphgragm (make it rigid) and then still take it all into the masonry?

I have seen designs in the past, on similar structures, where there are no apparent 'shear walls' at the dividing walls. The dividing walls in one case have one 16ga strap without any holddowns. The capacity of this strap, nailed to the studs, is much less than what is required to resist the wind loads. This has caused the question: Are these designers taking it all in the masonry walls? If so, how are they checking the diaphragm deflection without a continuous chord? What is the common approach? etc.

 

[JAE]

Individuals who say you don't need a chord don't know what they are talking about. A diaphragm IS a deep horizontal beam that resists shear and bending between supporting shearwalls or brace lines. If the diaphragm depth to span ratio gets high, then the shear behavior does start to control and the true bending is diminished...but based on the dimensions you describe, there would be consideration for bending.

If your floor footprint jogs in and out, you can still develop a chord line, it just has to occur at the minimum width and be carried through the length of the building in a continuous line. You can use interior floor framing members to achieve this along with straps or other ties across intersecting walls.

 

[rholder98]

Yeah, JAE is right. You've gotta have a chord. That point was driven home to me when I performed an investigation on a large, warehouse-type retail store. It was steel deck, with tilt-walls along the perimeter. I was called out because the panels were moving differentially, causing the joints between to distort and widen. It turns out that the deck angle, while properly connected to the panels, was not spliced at each panel joint like it should have been. So, instead of a continuous deck angle (chord), you had 30 intermittent deck angles, leaving the panels to move almost freely. The tension force would have to have been transferred into the deck and roof members. It's a wonder the deck hadn't ripped apart.

 

[bjb]

If you can't get the diaphragm to span between the masonry walls you have to use the interior wood walls. If the gypsum sheathing does not have adequate capacity, then you will need wood sheathing under the gyp. board. I am designing such a building now. You also might need closer shear wall spacing because of limits on the diaphragm aspect ratio. It is critical that any diaphragm have chord members, and that the chords be properly attached to the diaphragm. If your diaphragm only spans between the dividing walls, you still need to have chord members. Check out "Introduction to Lateral Design" by APA. They have a lot of excellent documents on their webiste that discusses wood shear walls and diaphragms, and you can download them for free. You can also email them with technical questions. A wood diaphragm usually does behave in a flexible manner with masonry shear walls, but if your shear walls are wood studs with wood or gyp sheathing the diaphragm may behave in a rigid manner, especially in the longitudinal direction.

One additional thought about the masonry shear walls in a wood frame building. As the wood studs shrink it may cause the wood floors to hang up on the masonry, causing distress.

 

[RARWOOD]

I just learned something interesting from a Lateral Design Seminar presented by APA-EWS. The speaker indicated that one might want to think about the flexiblity of the plywood diaphragm if it has light weight concete toping.

 

[clevering]

Is it common to use interior walls sheathed with 5/8" gypsum board each side to take shear? In addition to the unit dividing walls at 35' on center (26 feet long) there are also up to two interior party walls per unit that are up to 15' long. Seems like these provide additional stiffness and maybe I could consider them in my lateral design. Are there any published values for the shear capacity of these types of walls? any thoughts?

Also, thank you for all the extremely helpful posts, much appreciated!!

 

[bjb]

There are values for these types of shear walls in chapter 23 of the IBC. Personally, I'm not real comfortable using gyp board for a shear wall mostly due to quality concerns. However, since they are recognized by the IBC I would use them if I thought it was appropriate. I am designing a building now with some interior wood shear walls, and the contractor doesn't think that it's a big deal.

 

[RARWOOD]

In the 90"s it was very common, around Minneapolis, to resist all the wind load in a three to four story wood framed building with gyp board.

The problem that I encounter when I specified two layers of gyp board for lateral resistance, was that the walls would get built with gyp board on one side directly attached to a stud with gyp board on the other side of the wall attached to a sound channel.

To the best of my knowledge that if the gyp board is attached to a sound channel, it provides no resistance to lateral load. On the buildings I worked, on I would call for two layers of gyp board directly attached to the studs with an additional layer attached with a sound channel.

Currently with the code today you could put gyp one side, with plywood and gyp on a sound channel on the other side.

You then could sum the lateral resistance of the plywood and gyp which were both attached directly to the wall studs.
,if you if you are using a standard 2x4 or 2x6 stud wall,

 

[clevering]

RARSWC,
Thanks for the tip. I have the exact condition you are referring to on my building. I am wondering if there is any sort of study for this type of wall where the gyp board is furred out with hat channels. I would assume there is some lateral resistance. Maybe I have to assume there is zero resistance though. Any thoughts?

 

[bjb]

It's really the fastening that makes a shear wall work, and if your sheathing is attached to the wall via resilient channels I don't think you can count it in your shear wall (at least I don't).

Regarding the project I had mentioned, the developer recently switched construction managers when our design was about 75% done. This is a design-build project, and the new CM wants to use all gyp board shear walls. For one wall a single ply of the gwb didn't work, so we are detailing 2 plies on the face without the resilient channel.

If you're dealing with the IBC, make sure you carefully read 2305.3.8 about summing capacities.

good luck