Design for Overturning Moments of Wood Shear Walls of Modular Housing
Design for Overturning Moments of Wood Shear Walls of Modular Housing
(OP)
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!
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!





RE: Design for Overturning Moments of Wood Shear Walls of Modular Housing
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
RE: Design for Overturning Moments of Wood Shear Walls of Modular Housing
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?
RE: Design for Overturning Moments of Wood Shear Walls of Modular Housing
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
RE: Design for Overturning Moments of Wood Shear Walls of Modular Housing
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
www.HowToEngineer.com
RE: Design for Overturning Moments of Wood Shear Walls of Modular Housing
You mean "Can't, right?
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Design for Overturning Moments of Wood Shear Walls of Modular Housing
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.
RE: Design for Overturning Moments of Wood Shear Walls of Modular Housing
RE: Design for Overturning Moments of Wood Shear Walls of Modular Housing
Are they really that large? Or is it 11.5 K total for the building?
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Design for Overturning Moments of Wood Shear Walls of Modular Housing
But yes, there are several uplifts at 11.5k
RE: Design for Overturning Moments of Wood Shear Walls of Modular Housing
RE: Design for Overturning Moments of Wood Shear Walls of Modular Housing
RE: Design for Overturning Moments of Wood Shear Walls of Modular Housing
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
www.HowToEngineer.com
RE: Design for Overturning Moments of Wood Shear Walls of Modular Housing
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.
RE: Design for Overturning Moments of Wood Shear Walls of Modular Housing
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.