Elevated building on posts, wind load on bottom

[TehMightyEngineer]

Just want to check on how the hive mind would handle this.

Designing a summer camp dormitory structure. The building will be elevated on screw piles and posts with the site sloping upward toward one side of the structure. See below:

My first thought was to design this as an open monoslope free roof with a roof angle of 0° per ASCE7-10 Fig 27.4-4; however, the sloping grade will clearly cause a different wind pressure than the open monoslope roof designs intend.

Next thought was I could design it as a partially enclosed structure but per the definitions at the start of chapter 26 with the 3 open sides I'm not sure whether I really fall under that category and not an open structure.

All things equal, partially enclosed seems like the more accurate and conservative method to me.

How would you consider wind on the bottom of this elevated building?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[TehMightyEngineer]

KootK: Great advice as always. Numbered replies below:

1 & 2) Agreed. Especially if they hit a rock and need to offset.

3) The screw pile company they want to use has an engineering department and a selection chart with capacities for downward, uplift, lateral load, and the flexural capacity of the pipe. I'll be reaching out to them once I've finished running the model to get a better idea of what went into those numbers.

4) Definitely considering this, even a partial foundation wall with a grade beam acting as a collector would be beneficial. What I don't want to introduce is differential deflection in the structure (stiff, lightly loaded foundation walls at the end; heavily loaded, less-stiff screw piles on the interior).

5) Agreed. I also might put in the structural drawing notes that if the "basement" is to ever be enclosed to provide vents to ensure we limit the ability for it to pressurize (required by code anyway for it to remain a dirt crawlspace IIRC).

6) The install procedure appears to be to drive them, cut off the excess, and weld a cap on them. Ideally I'd like to use this feature to continue the metal post up to the structure and use a Simpson CCO hanger field welded to the top. Should allow for some reasonable offset, but I agree that I'll need to have a plan to deal with extremely offset screw anchors. After talking with the screw anchor people I may go with sonotube piers or similar.

7) Well, keep in mind this is an un-insulated summer camp dormitory. Other than resisting rot I imagine there's actually some intent to make this feel more "rustic" than a normal building. (i.e. if the floor creaks it's a feature, not a defect. ) Point taken though, we are indeed adding another exposed face of the structure and it will cost more than a normal floor for sure. I believe the contractor quoted a price including this into the owners budget, so might be a non-issue. The original concept was to take advantage of using the basement as storage, but when the got the costs they decided that it's not worth the extra utility (it's a big camp complex, they have plenty of storage).

The owner was very specific that I win brownie points for making screw piles work. Something tells me as long as his savings > my design fee and it passes a permit process he'll be happy. His foundation cost for the original structure was $55,000 (yikes!) and apparently the smaller size screw piles install for $350/ea., so I believe the owner has the right idea with the budget/cost aspect.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[KootK]

What I don't want to introduce is differential deflection in the structure (stiff, lightly loaded foundation walls at the end

Meh. So long as you've got 15-20' of relatively forgiving wood framing between the walls and the screw piles, I suspect you'd be okay. Alternately, the CIP walls may be constructed to bear upon screw piles themselves. Although I don't imagine you'll be winning the popularity contest of life by suggesting CIP walls AND screw piles.