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]

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

Mike McCann, PE, SE (WA)

 

[XR250]

Punch the Architect?

 

[mike20793]

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.

 

[DaveAtkins]

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

 

[mike20793]

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.

 

[manstrom]

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.

 

[Brad805]

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.

 

[mike20793]

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.

 

[TehMightyEngineer]

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

Maine EIT, Civil/Structural.

 

[Brad805]

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.

 

[woodman88]

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.

 

[ofthesea]

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.

 

[mike20793]

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.

 

[mike20793]

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.

 

[woodman88]

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.

 

[msquared48]

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)

 

[ofthesea]

Mike,
Yes I did,I used all the transverse party walls between units as shearwalls to take the torsional loads.

 

[BAretired]

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

 

[mike20793]

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?