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)

 

[EZBuilding]

Interesting project. Is the building "open" on three sides? If so I am not sure if partially enclosed would apply.

My initial gut feeling was to utilize a positive pressure equivalent to a wall pressure, i.e. wind hitting the sloped grade and being redirected towards the bottom of the structure. Similar methodology to a roof overhang. I can see open monoslope roof also applying here, especially in the direction perpendicular to the span you have drawn.

 

[TehMightyEngineer]

Is the building "open" on three sides?

Affirmative, but the owner may enclose 2 sides at some point (making it definitely partially enclosed) but I don't have to design around that condition if I feel it's not needed and can leave it for the next guy (probably me).

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

 

[msquared48]

What is taking the lateral load? Batter pile?

As for rhe uplift, would not worry about it unless greater than 25 psf.

Mike McCann, PE, SE (WA)

 

[TehMightyEngineer]

What is taking the lateral load? Batter pile?

Haven't gotten that far but it's whatever I recommend. My thoughts were either batter piles and/or cable cross-bracing between the screw piles, or a small concrete shear wall and foundation at the corners.

As for rhe uplift, would not worry about it unless greater than 25 psf.

Interesting; how come? This building isn't very heavy, uplift/overturning seems like a big deal for the screw piles.

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

 

[Ron]

You actually have two structures that need to be considered separately. The structure on piers/piles is a roof to an open structure. The structure on the piles is then a separately considered structure (partially open) with lateral loads and some uplift from the bottom opening to contribute to the uplift from the roof. I think your monoslope approach for the first consideration is correct. After that, it is a simple partially open structure. Superimpose the loads.

 

[TehMightyEngineer]

Interesting, Ron. I believe I follow you.

You're saying I compute my lower (exterior) pressure via monoslope roof approach. Then compute my interior building pressure from the partially enclosed equations. Utilize the combined interior/exterior pressure in my wind design load cases for + and - internal pressure. Correct?

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

 

[mronlinetutor]

I just got a warning from Facebook on house building on a slope. Due to the different level, the house might experience an eccentric torsion due to the fact that the earthquake base shears at the earth surface of the piles are at a different level.

disclaimer: all calculations and comments must be checked by senior engineers before they are taken to be acceptable.

 

[TehMightyEngineer]

Low seismic region, SDC B, SDS = 0.254, SD1 = 0.128. Wind almost certainly controls over seismic, but I'll make sure I'm not getting any torsional irregularities.

My involvement is mainly to help the owner use the screw piles which are cheaper and faster to install than a traditional foundation for this site and the owner likes them. If I get backed into a corner I can always recommend to the owner they keep all or part of the traditional foundation. Won't win much repeat work if I do that, though.

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

 

[Ron]

TME...yes

 

[TehMightyEngineer]

Thanks Ron.

I did some more playing around with other ideas but pretty much ended up reaching the same conclusion.

Make sense, you warm blooded engineers must have figured out wind loads on elevated buildings long ago.

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

 

[msquared48]

THE.

I got the 25 psf figure allowing 15 psf dead load for the roof structure and 10 psf for the floor atructure.

Mike McCann, PE, SE (WA)

 

[racookpe1978]

Low earthquake zone. Hurricane or snow zone? Or just simple winds?

 

[TehMightyEngineer]

Racook: Low earthquake, no hurricane (115 MPH per ASCE 7-10), heavy snow (80 PSF ground), fairly simple stuff.

Mike: Gotcha. I calculated 27.4 PSF on the front half of the "basement" (where the ground pinches the wind) and 11.4 PSF on the front half. See below:

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

 

[CANPRO]

Perhaps this is a rehash of what was already discussed, but as I see it, you have 2 conditions to check with the wind: (1) the wall to floor connection, with consideration of internal pressures as usual, no consideration for wind on the open underside, and (2) the connection between floor and pile/post and everything below with no consideration for internal pressures, with consideration of the additional wind on the open underside.

Have you talked to any screw pile contractors in your area? On a recent search for information I called quite a few who couldn't offer any technical info/support, but I eventually found a contractor who offered more of a delegated design/build kind of deal.

 

[TehMightyEngineer]

CANPRO: I agree.

Once I've run the numbers and figured out my demand on the screw piles I'll be reaching out to local assistance. The screw piles the client wishes to use they've used in the past and technical support is apparently available and the screw piles will be tested during their install.

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

 

[msquared48]

THE:

From your sketch above, I see a concrete retaining wall on the high side which means to me that you will need at least a stepped concrete retaining wall at the ends of the backside wall to retain the sloping earth too, and those walls will add to your uplift resistance if connected to.

I still really do not see much of an uplift problem here as the whole building will have to uplift to fail if properly reinforced...

Am I missing something here?

What is your total uplift force, and your total dead load?

Mike McCann, PE, SE (WA)

 

[JAE]

TME - my take on it:

I think you are mainly interested in MWFRS winds here correct? I'll assume so for now.

1. ASCE 7 wind provisions have various building scenarios and yours "might" fall under an open building with obstructions below it...or a partially enclosed building - basically your initial assumptions.
2. ASCE 7 is limited to what's been studied, tested, etc. so we engineers are almost always limited to those applications
3. This requires us to take multiple stabs as deriving wind loads to our structure using those various close-but-not-excact provisions.
4. Once we take multiple stabs at it, we can then get a glimpse of the range of possible wind forces that our structure might fall under.
5. ...keeping in mind that wind forces from ASCE 7, like all codes, are minimum standards.
6. In addition, a monoslope roof with obstructions below (per ASCE 7) doesn't include a purely inclined obstruction (grade), which reminds me of a topographic effect - this might compel me to kick up my wind forces based on that...especially uplift forces on my "monoslope" roof (i.e. your floor underside).
7. The upper portion of the building I might just use standard horizontal and roof uplift forces, but use a larger overall building height (z) to further kick up my wind loads, using perhaps 2/3 of the maximum clear height below the main floor.
8. All this is based on the mantra: When in doubt make it stout.
9. I'd pay close attention to uplift hold-down connections.
10. I like your idea of X-braces below the floor and between main columns - but hope that there aren't tidal/coastline water issues as X-braces collect debris and make things worse.



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[TehMightyEngineer]

Mike: Sorry for the confusion. That was the original concept and that wall will not be there (unless I need it to be there). Screw posts for the whole foundation if possible per the owners request.

That section cut was drawn by the owner prior to switching to screw anchor foundations and I utilized it for my calculations to sketch out load paths.

Total dead load = ±130 kips (roughly 70 PSF)
Total wind uplift (factored) = ±87 kips (roughly 47 PSF)

JAE: Correct; C&C will be much simpler and unlikely to control anything underneath.

1. I've looked at the bottom as an open structure with obstructions below it. Seems reasonable enough.
2. Indeed. Not the first time I've had to get ASCE7 wind loads to fit odd cases.
3-5. Makes sense.
6. Yes, I was wondering if "obstructions" for an open structre truly accounted for the funneling effects my sloped grade will cause. Right now my uplift force on the narrow end of the bottom floor has net 27.4 PSF uplift on it. Pretty high pressure considering the roof only has 17.4 PSF net pressure. Seems like I'm probably close enough to conservative.
7. I've used a larger overall height for wind coming from the directions where it can pressurize the basement area. I agree this makes sense as the roof pressures will be higher.
8. Agreed completely, no skimping on the lateral loads for the screw anchors here.
9. Might need another eng-tips post when I get to that point but my understanding is these screw piles are weldable so I should be able to use those crazy simpson weldable brackets.
10. 1000 feet away from water and well above flood (see below). Good thought though.

It's also worth noting that the "open" sides of the "basement" are shielded by trees. I ran things as exposure C for all, but these could easily qualify as exposure B for those directions that pressurize the basement.

Fantastic advice as usual. Much appreciated to everyone. Now to finish modeling this and get some preliminary ideas of how feasible these screw piles are.

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

 

[KootK]

I'll venture a fair bit deeper than just the wind load question here. Take what is useful to you and leave the rest, as always.

1) In my experience, screw piles will be installed by a guy with a rig on the back of his F250 pulling said vehicle up the slope to do the install. Plan for your piles to be 6" misplaced and, probably, all a little battered.

2) In my opinion, because of #1, screw piles work best in combination with some kind of grade beam element that can be used to compensate for the pile install tolerances.

3) In my experience, it can be difficult to get lateral load resisting values from screw pile folks. The onsite testing will almost certainly be vertical load only. If there's lateral design values to be had, it will probably be based on assumed soil properties and testing done previously. Since you'll be relying on this type of data, spare yourself some pain and ask for it on Monday, before you invest too much time in trying to use the screw piles laterally. All that said, many a designer will choose to simply not worry about the lateral capacities of the piles for a very small scale building. If you go that route, you'll certainly not be the first.

4) Because of #2 and #3, I would seriously consider building your side walls in concrete rather using braced posts over screw piles. In addition to the tolerance and lateral capacity issues, you'd be able to rest easy in the knowledge that, worse come to worse, you can treat the substructure as a robust three sided building. Your contractor can surely still save a few pennies by leaving out that lengthy CIP back wall.

5) For the positive wind pressure in the giant crawl space, I'd assume that you did have the side walls in place, even if you won't. That way you'll get that "balooning" effect that we see in three sided buildings captured. This should be simple and conservative. And the rate at which incoming wind will exist the side wall is really one of the bits that makes this tricky.

6) Because of the tolerance issues, I'm also hesitant to use single screw piles under individual posts. You absolutely will have misplacement and pile eccentricity that you'll need to account for in terms of both the pile design and in creating a field adaptable connection detail to compensate. And that may well be a connection that will look a little crappy in real life. Not sure if anyone would be prone to caring about that. I'd be inclined to go with something like sonotube piers at the back wall and patio post foundations.

7) In my opinion, your owner will need a fairly robust building envelope on the underside of the entire main floor framing. That obviously costs money and I'd be inclined to bring that to the owners attention to sure that it's entered into the calculus associated with reducing cost by deleting the concrete walls around the sloping crawlspace. Moreover, I'd hope the benefits of having a pretty cheap and naturally occurring enclosed basement have entered into consideration as well.

I do realize that your construction team may well be set on the screw pile idea for various reasons. And sometimes, you just catch more flies with honey than vinegar, regardless of your personal opinions. Give the people what they want and hope they come back for more.