wood shear walls in corridor only
wood shear walls in corridor only
(OP)
I'm designing a fairly large, 4 story wood apartment building on two-way, PT podium slab. The building is large enough that we are putting in an expansion joint to separate the main building from a small "leg" on the southwest corner. The main building is a large rectangle with a courtyard in the middle. The leg is rectangular(ish) shaped and is about 115 ft (E-W) by 82 ft (N-S) and has the corridor down the middle with units on both sides that runs E-W. The units are separated by party walls that support floor trusses, so there are plenty of load bearing walls to use as shear walls in the N-S direction. The problem is in the E-W direction. The corridor is really the only place I can get shear walls. The building plan and unit layout is so irregular that there are no exterior wall segments that I can use for shear walls. There's too many in-and-outs and large windows so no segments meet the aspect ratio requirements. I looked at converting some partition walls in the units into shear walls, but the plan is so irregular that none really line up and at 4 stories, I need a pretty decent segment length. Has anyone seen a layout like this before? I haven't done a wood building quite this complicated before, but I know I can't get by with shear walls in the corridor only; so does anyone have any suggestions? I will attach a sketch in the morning to better show the irregularity at the exterior walls. Thanks in advance.






RE: wood shear walls in corridor only
Mike McCann, PE, SE (WA)
RE: wood shear walls in corridor only
RE: wood shear walls in corridor only
RE: wood shear walls in corridor only
If you can't get comfortable with that approach, you could put in a steel moment frame at each exterior wall.
DaveAtkins
RE: wood shear walls in corridor only
RE: wood shear walls in corridor only
If you try to use the corridors alone, it won't work as a flexible diaphragm. You are limited to a 25' cantilever for a diaphragm by SDPWS. I have seen people use just the corridors before, but the deflection at the outside wall becomes pretty big.
Your best bet is to use a combination of the following:
-A rigid diaphragm analysis along with some plywood walls at the corridor. This will absorb more load at the corridor rather than a flexible assumption. A flexible assumption will put about 50% of the load at the corridors. A rigid assumption can put maybe 80% or so into the corridors. Note that plywood is about 3 times as stiff as gyp.
-Use Force Transfer around openings or Perforated shear wall analysis at the exterior walls. If you have a 2' segment on each side of a window, you can make a shear wall work.
-Add up all the little exterior shear wall segments that you can. It should be enough for wind, maybe not enough for seismic (depending on your region)
This is a big engineering problem in the industry and I am hoping for more guidance from AWC regarding a simplified force transfer procedure for shear walls. The math is extremely complicated and there really isn't a program out there that does everything right. I do most of my lateral design with some complicated spreadsheets.
RE: wood shear walls in corridor only
Are there not any interior demising walls you can use? Typically the suites have double 2"x_ walls between suites. These never have any openings and are a great option to add shear resistance. They typically have a gap between the walls to avoid sound transfer. In one case we had them sheet these walls with plywood.
RE: wood shear walls in corridor only
RE: wood shear walls in corridor only
Maine EIT, Civil/Structural.
RE: wood shear walls in corridor only
Simpson does make two shearwall options for these applications. They have a plate version, and prefabricated moment frames that are built to allow the contractor to easily add the wood furring for finishing. They also have a decent piece of software for them. The problem we have had with specifying these is the delivery time. Depending on where you are in the US they may be a good option.
Do you use woodworks? The shearwall program is quite helpful to go thru options. It will not help with the steel moment frames, but maybe you can persuade the arch to reduce some opening sizes to keep the project on budget. Adding the steel frames is a nuisance from the detailing end of things, and the type of contractor one typically gets for these type of projects are not always the most proficient with steel.
RE: wood shear walls in corridor only
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.
RE: wood shear walls in corridor only
I don't like it but what else can you do.
RE: wood shear walls in corridor only
RE: wood shear walls in corridor only
RE: wood shear walls in corridor only
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.
RE: wood shear walls in corridor only
At some point here, you, as the structural engineer, will have to dictate the structure to make this work. After all, YOU, and not the Architect, are stamping the STRUCTURAL drawings for the project.
There may have been a definite reason why the Architect looked for another structural engineer, huh? At some point, he will run out of engineers...
Mike McCann, PE, SE (WA)
RE: wood shear walls in corridor only
Yes I did,I used all the transverse party walls between units as shearwalls to take the torsional loads.
RE: wood shear walls in corridor only
BA
RE: wood shear walls in corridor only
RE: wood shear walls in corridor only
If they want the diaphragm to be broken, they WILL need to change something in the Architecture and either foot the bill for exterior moment frames, one at each broken diaphragm, or exterior shear walls at each broken diaphragm. The choice is theirs. Tell them you have put a hold on the design until one or the other is decided upon, broken diaphragms or not. There are no other alternatives here...
Mike McCann, PE, SE (WA)
RE: wood shear walls in corridor only
2. Except at the expansion joint, party walls don't align across the corridor. Is the diaphragm broken at every party wall?
3. If the diaphragm is broken at party walls, is it possible to maintain a continuous chord member on the exterior (window) walls without messing up the sound rating too much?
BA
RE: wood shear walls in corridor only
Are there any interior demising walls parallel to the corridor walls that could be used as shear walls, stacked or un-stacked? With some crazy horizontal framing, even the un-stacked might be able to be weaseled if the walls are present. This would limit the diaphragm deflection at the exterior walls. At this point, I would even look at plumbing walls as shear walls masking them 2X8 if needed.
Mike McCann, PE, SE (WA)
RE: wood shear walls in corridor only
This project went out for pricing a year and half ago and then went on hold and was just brought back up. We have a pretty aggressive schedule, but we are still early on in the design. I need to check with the architect about the points BA brought up. We will insist on continuous diaphragms, but I haven't done a wood building quite this complicated, so I'm not sure if it's possible with the architect's scheme. Thanks for bringing that up, BA.
RE: wood shear walls in corridor only
We have all seen RC channels used at the ceiling to avoid sound transmission, but what if it was also used on the floor, over the structural plywood diaphragm, and on the ceiling. Put another layer of 3/4" ply over the RC channels.
Probably would still transmit sound though as the party shear/bearing walls would still be connected to the structural diaphragm.
Talked myself out of it... Oh well...
Mike McCann, PE, SE (WA)
RE: wood shear walls in corridor only
Back to the original problem: I ordered a wood design book that has a chapter on "Advanced Topics in Lateral Design" and includes a section on open front structures and rigid diaphragm analysis. I ordered it last week, so it should be here any day. I wonder if I can get anything out of there. I did read through a part of the chapter that mentioned the limits in the SDPWS for cantilever diaphragm were more prescriptive requirements to avoid large deflections at the diaphragm edge. I will read through it more carefully and see what else it says. I'm starting to lean towards using rigid diaphragm analysis and use the small segments on the exterior to help with torsion or take it out in the party walls.
RE: wood shear walls in corridor only
It is not "typical", but, unfortunately, I have seen it done. Just look at his architectural details for the floor to floor for the party walls. That detail will tell you what you are dealing with.
Mike McCann, PE, SE (WA)
RE: wood shear walls in corridor only
RE: wood shear walls in corridor only
Designed one of these in a high seismic area as a rigid diaphragm (with the city's blessing ahead of time). As expected, most of the load was sucked into the corridor walls. When seeing that, the city balked and reneged. We then left the corridor walls designed with the rigid analysis and re-designed the perimeter perforated shear walls as if we had a flexible diaphragm. Very conservative analysis, but it made the city happy.
RE: wood shear walls in corridor only
BA
RE: wood shear walls in corridor only
OP, I hope you get some good info from the book you ordered. Another good resource is Breyer's wood book. There is a good example on Force transfer around openings. I would bet that you can find a few areas where you can create a frame around a window and reinforce the inside edges with straps.