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wood shear walls in corridor only

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

I have seen this done before but do not like it. Gets into cantilevered three sided diaphragm issue.

Mike McCann, PE, SE (WA)


RE: wood shear walls in corridor only

Punch the Architect?

RE: wood shear walls in corridor only

(OP)
To make matters worse, this is the first time we've used this architect so we want to make it work. I attached a sketch to explain the layout better. I meant to mark it before I scanned it, but on the north side of the building, there is a segment that is 11 feet and in between the windows on the units on the southeast end that are about 12 ft combined. Under flexible diaphragm analysis, the segments aren't long enough and have a really large uplift force. Also, the floor trusses span between the party walls.

RE: wood shear walls in corridor only

I would try to use the walls which are perpendicular to the corridor to resist the moment. I know--this violates the assumption of a flexible diaphragm, but in the direction parallel to the corridor, the diaphragm is very deep.

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

(OP)
The original pricing layout we gave over a year ago had steel moment frames, but they got scrapped (probably due to cost vs wood). Since the diaphragm is so deep in that direction, will it act as a rigid diaphragm? Could I use the small 11 and 12 ft segments on the exterior and proportion the force based on their stiffness relative to the corridor walls? I can get comfortable with any approach as long as I can back it up with some calculations or a RISA model.

RE: wood shear walls in corridor only

Welcome to the world of multifamily wood. You wanted exterior walls? But we want windows! I deal with this all the time in 4 and 5 story wood on a podium. Some in medium seismic and wind zones. No way they will let you use steel frames.

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

Another vote for punching the architect, well, unless he is paying your bills. Then make it a friendly punch.

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

(OP)
Yes, the demising walls are double 2x party walls. The problem isn't in the N-S direction; there are plenty of load bearing walls to use as shear walls. The problem is the E-W direction where there are only corridor walls and exterior walls. I've no problem with the corridor walls; it's the exterior walls where I can't fit much because of the aspect ratio limits.

RE: wood shear walls in corridor only

I haven't read through everything but what about Simpson strong walls, a portal frame, or the like?

Maine EIT, Civil/Structural.

RE: wood shear walls in corridor only

I should have read closer. My error.

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

Design the corridor walls to resist 100% of the lateral load. Calculate the possible deflections at the exterior walls. Design the exterior shear walls to resist this deflection.

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 too have done these as rigid diaphragms and taken all the load through the corridor walls.
I don't like it but what else can you do.

RE: wood shear walls in corridor only

(OP)
Thanks for the suggestions. Woodman, so the diaphragm is still treated as flexible for your option? How should I calculate the deflection at the exterior walls? The SDPWS equation is for deflection at midspan of the diaphragm between supported shear walls.

RE: wood shear walls in corridor only

(OP)
ofthesea, how did you handle the inherent torsion when you analyzed it as a rigid diaphragm? Did you take it out in some small wall segments parallel to the corridor or did you use the N-S shear walls to resist the inherent torsion? Thanks again for all the help; the suggestions have been really helpful.

RE: wood shear walls in corridor only

The deflection would be similar for a reaction at each side vs a reaction at the center 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

Some Architects screw this up even further by breaking the diaphragm at the party walls for sound insulation.

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

Mike,
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

To me, the proportions don't look too bad for a cantilevered three sided diaphragm, but they could get much worse if the diaphragm is broken at party walls as Mike suggests.

BA

RE: wood shear walls in corridor only

(OP)
BA, that's what I originally thought too, but the diaphragm cantilvers 30+ ft in a few areas. We are going to specify that the diaphragm be continuous at the party walls. Any clue what that does to the sound transfer?

RE: wood shear walls in corridor only

Sound transfer SHOULD be secondary to structure, if it is going to be a safe, viable project structurally.

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

1. Is the diaphragm broken at the corridor? Or is it continuous across the width of the building?

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

#3 is interesting BA... There would have to be continuous blocking and strapping in line with the furthest inboard jog. I have done this before too, but not with a broken diaphragm. I will have to think about that one. ponder

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

(OP)
Fortunately, all the walls are stacked. There are some interior partition walls parallel to the corridor but none of the segments are long enough to work at 4 stories for flexible diaphragm. If I can get by with a rigid diaphragm analysis then the length won't matter as much since I can make the corridors long, perforated shear walls. Another engineer in my office suggested what woodman suggested: using flexible diaphragm and proportioning the corridor walls for 100% of the load and using the exterior walls to resist the cantilever deflection. As mentioned above, there are two segments on the south wall that are about 5.5 ft each and a segment on the north wall that's about 12 ft in length. They aren't long enough to take the tributary wind load, but they may be stiff enough to resist the deflection at the cantilever diaphragm. Any thoughts? I'm still having a hard time wrapping my head around how to calculate the deflection at the cantilever end since the SPDWS equation is for deflection at midspan. I will re-read previous threads where this topic was discussed. I recall seeing a few.

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

OK... Here's another wild idea:

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

(OP)
Is it typical for the diaphragm to be broken at the party walls for sound transmission? I need to get with the architect on that ASAP. I was under the assumption that the diaphragm would be continuous, but it does make sense to break it to limit the transmission of foot fall noise from the adjacent unit. They (architects) sure don't make it easy on us (engineers).

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

mike20793:

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

(OP)
I got worried it was typical for them to break it and my assumption that it was continuous was invalid. We're so early on that he doesn't have any details other than a "typical" floor section (nothing on this building is typical, though!) that I used to get the floor loads. I am going to get with the architect tomorrow and pick up the design once those issues are cleared up. Thanks a bunch Mike and BA. As a relatively new engineer, these are the things I didn't/don't think of before starting the design and obviously they matter quite a bit. Thanks again to everyone else that responded. This has been a great discussion. I'll report back when I learn more.

RE: wood shear walls in corridor only

See Manstrom's comments- They are right on.

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

Another possibility, although somewhat unconventional, is to frame the exterior walls as a series of vertical beams spanning from ground floor to roof. Since the roof diaphragm is unbroken at party walls, it can be designed as a rigid diaphragm or a horizontal truss spanning from end to end. In that way, the corridor walls can take 100% of the wind load but a large part of it reacts at roof level instead of at each floor. Torsional effects would be resisted by the N-S walls.

BA

RE: wood shear walls in corridor only

I have had architects try and break the floor diaphragm at the separation walls for acoustical reasons. It simply will not work. You would essentially create individual 4 story tall structures out of every unit. It is possible but the detailing would be insane. There is a reason they don't do that.

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.

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