wood shear walls in corridor only 1

[mike20793]

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.

 

[msquared48]

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)

 

[BAretired]

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

 

[msquared48]

#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.

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)

 

[mike20793]

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.

 

[msquared48]

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)

 

[mike20793]

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.

 

[msquared48]

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)

 

[mike20793]

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.

 

[hawkaz]

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.

 

[BAretired]

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

 

[manstrom]

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.