Design for Overturning Moments of Wood Shear Walls of Modular Housing

[smokiibear]

I've designed a number of foundations for Modular Houses (different from Manufactured Housing in that a chassis is not built into the structure of the home). Generally, the factory modular plans contain a loading diagram with reaction values and dimensioned locations. Pretty strait forward.

On a recent plan, the factory provided a foundation plan with moment reactions in lieu of tension loads. Some of their walls are segmented and some appear to be perforated. But they refuse to provide the uplift values or their locations. They inisit I use the non-stamped factory build plans, plus the engineering calculations from the factory, in conjunction with their foundation plan to interpret where the reactions are and what are their values.

Question 1) Assuming that the listed shear walls are part of a continuous rim joist, is there a method for providing holdowns along at intermitant points along the length of the rim to the foundation using the moment? My apprehension is twofold: On one side of the pendulem, the outermost holdowns are not sufficient to take the full tension loads from the full length of the shear wall/rim combination, the outermost connections would tend to fail, increasing the demand on the next connection toward the compression side, and so forth. On the other side of the pendulum, I think "contiunous" holdown support of the rim would work...but not sure how to qualify the solution. Any comments or suggestions?

Question 2)Should the factory be required to provide stamped drawings listing the precise locations of their reactions and the value of their reactions? Seems if they are not willing to provide a stamped response, and qualify unstamped drawings and responses as "for information only," that I'm taking some high risk in assuming their interpration of their engineer's work is accurate. Any other comments?

Thanks for your assistance!

 

[msquared48]

1: You designate which walls You want to use, and design those specific walls for the overturning force and shear. See 2 below though...

2. There should be some indication somewhere in the plans or their calculations, which they should furnish to you, of how those reactions were generated. The values should be based on the specific location where the structure is to be placed. The structure will see different uplift values depending where it is placed, especially under current ASCE7 specs for wind considering terrain. I have seen too much of this wave the hand, good for all locations technique over the years and have no stomach for it anymore. Salesmen for these companies have misled too many clients into thinking that they have something that can be placed anywhere, and that is just not the case.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[smokiibear]

Mike...thanks for the response. I totally agree with you. Some of these factory/saleman teams are agrivating to no end.

One of my biggest concerns on this project, though, is that they insist that overturning can be resisted along the length of a wall by just adding more holdowns and/or straps along the wall until the resistance is greater than the capacity. Their terminology for this is triangular moment resistance. while I'm sure a huge amount of lab work could be produced to quaify an approach like this, that additional holdowns could add some resistance, obviously it would not provide full capacity since the interior holdowns would not be completely engaged.

On this end, do you or anyone else have any experience in wood framing providing this type of solution? Or would you consider this a very poor or inadequate approach?

 

[msquared48]

Personally, I have never heard of the approach, and would have to be convinced of the approach to use it.

If you are expected to stamp the plans, you have the right, irrespective of their demands, to use the approach you are comfortable with. Sounds like they are trying to save money here, and it probably will not work using standard methods. If their method has been INDEPENDENTLY lab tested by as certified testing agency, and documented as such in an EIR, then I would consider it. Notice I said consider...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[RFreund]

I have heard of this and contemplated it before, however I have not used it nor seen it used in a text or other technical publication. Essentially they are assuming a linear stress / strain distribution if you will.

The two arguments I could see for using this approach are - assuming that the wall is rigid and/or assuming that the over turning forces are being distributed linearly to the top of the shear wall. Meaning that the tension force is not completely applied at the end of the wall. However if you can lift up the end of the wall, then you can't over turn it. My mind continues to struggle with this argument/situation.

EIT

http://www.HowToEngineer.com

 

[msquared48]

"if you can lift up the end of the wall, then you can't over turn it."

You mean "Can't, right?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dhengr]

Why don’t you have a talk with the AHJ, the building permit people, and ask that they insist on proper foundation info. on a stamped set of plans, for a permit? It seems that you are trying to do a proper found. engineering job without info. about where the real loads are, and their values. If the AHJ allows them to build and install junk without sufficient engineering info. on the plans, then why do they insist on a stamped set of found. plans? They really can’t have it both ways, and might see that with a nice informative engineering discussion.

Undoubtedly, their triangular moment distribution just means max. tension at one end of a wall, changing linearly to max. compression at the other end. That’s pretty much what we’ve always assumed with typical framing and A.B’s at ‘x’ feet o/c down the wall. You said “until the resistance is greater than the capacity” and I suspect you meant resistance or capacity is greater than the demand, right?. Maybe you have to explain the code and a bit about shear walls with large openings to the AHJ, so they understand your dilemma. As you look at their plans, specifically at bldg. corners, ends of walls and jambs at large openings does this give you any clue to sizing the holddowns? Certainly, that tends to indicate the holddown locations, whether they have made sufficient shear connections and provided sufficient stud/jamb material is their problem. Pick a worst case holddown force and install that everyplace, screw the cost. They are trying to pin all the responsibility and liability on the one local guy who has a stamp, while your fee is $1000, and their profit on the bldg. is $50k. Tell them and the AHJ that you are not in the reinsurance business, you are trying to do a good engineering job for the community.

 

[smokiibear]

You all are expressing great comments, exactly the ones I have been considering along the way. The plans they've presented are engineered and stamped. However, there plans have had multiple conflicts, showing details where shear walls are attached to the rim joist via straps, but nowhere on the plan showing how many straps and precisely at which locations. Several of their walls have openings in them, so are the shear walls designed as perforated, shear transfer around openings, or segmented. Since they only provided moment reactions on their walls and a horrid set of calcs, I couldn't backwards engineer to determine how the walls were designed. This is where I've put countless hours asking for their factory to clarify on the plans precise locations of uplift reactions and their values. If the loads were small, I wouldn't worry about it, and put hardware all over the house. Unfortunately, though, I'm working with uplift loads in the 11.5k range, which makes it necessary to use pre-installed all-thread. Even this, if it was one or two of them, not too big of a problem. Unfortunately, the house I'm working on is made up of 8 different modules, and has about 14 of these loads throughout. It's going to be a nightmare for the contractor to line the holdown posts up with the all-threads

 

[msquared48]

11.5 kips? Thats comparable to a Simpson HD12... That's big for a residence...

Are they really that large? Or is it 11.5 K total for the building?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[smokiibear]

Lol....that was my response when I first looked at their design. I've had two story homes in high seismic and snow where loads didn't exceed 3.5k of similar design.

But yes, there are several uplifts at 11.5k

 

[smokiibear]

One more tid bit. I asked the factory if they can provide posts at these high uplift locations. They refused. When I asked them what their stitching and glue schedule was for their multi-stud holdown posts, they also refused. What &(&*^&$^&&^&*^&&*^&*&^)(

 

[dhengr]

Show the deficiencies, errors, lack of framing and straps, etc. to the AHJ, and let them get after the modular building manuf’er. for compliance, or inspections and occupancy permit will be held up. Furthermore, when other manuf’ers. are providing what you legitimately need on their plans, this manuf’ers. plans will not be approved for permitting in the future. And, the AHJ should see this on sets of side by side plans. I think we Structural Engineers may have to kinda be the eyes, ears, and knowledgeable member of a team made up of the engineer, BO and local contractor in some instances. The BO understands the verbiage of the code, at times to the letter, but they don’t have the vaguest idea what it really means, and we may have an opportunity to befriend and educate, at the same time we are pressing them to really do their job. All in a nice way, of course. We become their go-to-guys when they have an engineering interpretation question, instead of an adversary. Work with the AJH and see if you can’t get them to apply some pressure, the contractor and manuf’er. do understand ‘failed inspections’ and no occupancy permit, and the AHJ holds those clubs. With the rate codes are changing and getting all mucked up in convoluted verbiage; and with the inexperience in many bldg. depts. there shouldn’t be anybody who understands the intent of the code and the need for that tiedown or code section any better than we do. Although, God knows, it is not hard to get lost in the meaningless verbiage inserted to keep dummies from doing dumb things.

 

[RFreund]

@M^2 - Little on this response but yes I meant *can't, thanks. So much for proof reading...

Interesting situation and I wish I knew more on the behavior of wood shearwalls / holddowns and how they fail. I just don't have an intuitive feel for whether or not the multiple holddowns will function just as effectively as one at the end. It seems as though multiple hold downs would ultimately keep the structure from overturning but how much damage / deformation would it take to engage the hold downs to result in equilibrium.

So are you responsible for the foundation design or the holddowns, anchorage and foundation?

EIT

http://www.HowToEngineer.com

 

[dhengr]

RFreund:
Perforated, or segmented shear walls do lead to fairly well defined jamb loads at the ends of each wall segment and/or at the edges of large openings. You design the holddowns for those vert. loads and assume that they all act at about the same rate. The wall top plate may be more than a double top plate, and in affect becomes a drag strut over the openings. Thus, each of the wall segments racks by the same amount laterally, but may pick up more or less load as a function of its stiffness. At high uplift forces you certainly can assume some sht.rk. cracking etc. in the area of the holddowns.

Smokiibear:
Just as a lark, and this shouldn’t take long, but it is in keeping with the spirit of the way they are operating, show a plan and a few notes, with 2" steel straps over the roof, at every other roof truss, down the sides of the bldg. and attached to auger anchors into the ground along side of the found. This is certainly in keeping with what they have given you. Show the end straps tensioned at x, the next one at .85x, next at .75x, etc., you know, std. linear moments. Then, add a not that if the evening new indicates strong winds or EQ’s tomorrow the homeowner should go out and crank up the tension a bit. Call Shmoo Modular Homes with any questions or problems. Give this package to the BO and the manuf’er. and see what reaction you get. You may want to color coordinate the steel straps if you’re in charge of the Arch. beauty on the project too.

 

[smokiibear]

Thanks for the input, fellows. Was really scratching my head as I walked through this....thinking all the same thoughts but second guessing myself.

RFreund: I am responsible for the foundation design and connections from the house/rim to the foundation. Usually the loads are so much smaller and easy to mitigate.

Dhengr: Thanks for the ideas. I will be noting on the plans various odd comments about the nature of the information provided, and making some disclaimers for sure.