Wood Diaphragm?
Wood Diaphragm?
(OP)
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.
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.






RE: Wood Diaphragm?
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.
RE: Wood Diaphragm?
RE: Wood Diaphragm?
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.
RE: Wood Diaphragm?
RE: Wood Diaphragm?
Also, thank you for all the extremely helpful posts, much appreciated!!
RE: Wood Diaphragm?
RE: Wood Diaphragm?
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,
RE: Wood Diaphragm?
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?
RE: Wood Diaphragm?
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