Sanity check on caissons for a carport

[lyonkster]

I had a steel carport designed by a company that will supply the materials, and I will then assemble the carport (picture attached). I will also be responsible for preparing the foundation. The company that designed the carport recommended using four 24" dia X 4' deep caissons under the posts of this carport.

I've similar looking carports at nearby apartment buildings, and they have caissons that are 12" diameter, so 24" seems like overkill to me.

Is it possible to run a sanity check on the caissons and see what's actually needed? It doesn't seem like it should be very difficult to do, but I don't know the first thing about running the calculation.

Thanks in advance.

 

[ALphaSpace]

Could you ask them for the calcs? and you could look through those yourself.

Or even ask them this question, I’m sure they’ll give you a reason.

 

[lexpatrie]

Pier diameter is probably not being controlled by the lateral resistance needed, or weight of the carport.

The 24" diameter suggests side cover and breakout of the anchor bolt group is involved, but if you're not mixing the concrete yourself and getting a truck to drop it off and pour it into the holes, is a smaller diameter hole going to result in a lower charge, or just more concrete sloshed out of the truck somewhere on your yard? Larger diameter would help control settlement as well as lateral resistance, versus whatever they were contemplating on that apartment building, particularly if it had a concrete slab "trapping" the piers so they are going to have difficulty moving at the top.

 

[lyonkster]

Thanks for the thoughts! I did ask them to look into smaller caissons, and they said they'll review and let me know. So that may be the easiest path forward, if they give me some calcs (or more likely they'll say "yup, that's the right size", without the calcs). We don't need a permit for this structure, so I was not planning to pay for stamped plans, but I guess that is always an option. Really I just want to know what is driving the large size (anchor edge distance, tippingm, etc). So far they said that it's this large because the carport "has no shear", but I'd like to get some more clarity than that.

I haven't priced the extra concrete, and perhaps the extra cost is lower than getting engineering calcs performed. There's also the cost of digging for the caissons, and I haven't priced 24" dia vs say 16" dia either. Again, maybe it's not a big extra expense. The flange being bolted to the caissons is 10" across, and the bolts appear to be on a roughly 10" dia bolt circle.

 

[hokie66]

It may be as simple as that size auger is typically mounted to boring machines in your area. But if it is indeed structural, the mass may be required to resist wind uplift. For a 10" bolt circle, a 12" pier is not enough. 24" sounds about right, allowing for some tolerance on location.

 

[lyonkster]

Hmm, interesting point - the apartment carport caissons had the posts embedded in the concrete, not flange mounted. Could the flange be driving the large diameter of mine? I'm not opposed to changing the design to embedded posts, if that's the only reason for the large caissons.

 

[Heaviside1925]

OP,
I would not consider how the neighbor's installation as a go by. I agree with hokie66 that this likely for uplift. Without knowing the soil type and if it has been disturbed, all the company may be able to use is mass to counter uplift to provide recommended foundations. The following link may assist you with a sanity check.

https://fortifiedhome.org/fortified-porch-carport-uplift-calculator/

 

[BAretired]

I've similar looking carports at nearby apartment buildings, and they have caissons that are 12" diameter, so 24" seems like overkill to me.

Based on what? You don't know how deep they are. Four feet is shallow.

Do you have any idea what the soil properties are in the upper four feet? Probably not, and neither does the steel supplier. Better dig a hole to find out. A 12" diam. pile does not provide 3" cover on the anchor bolts, so 18" diam. is minimum, but 24" does not seem unreasonable, particularly if the caissons are considered bearing piles. They can't be considered friction piles at a depth of only four feet.

 

[lyonkster]

Those are fair points, and make me feel better about the proposed 24" diameter. Indeed the apartment carport has concrete slab between the posts, which I won't have. And I have no idea how deep their caissons are. So thank you for the feedback, I will ask the carport company to tell me what they based the sizing on, but will likely plan to go with what they proposed.

to some extent I was going by this example for carport posts, but admittedly they don't specify how they chose their 16" dia X 2.5' deep caissons.

https://www.absoluterv.com/cantilever-carport-installation/

 

[lyonkster]

BTW, here are the caissons at the apartment complex. Mine would be twice as wide.

 

[gte447f]

We have no idea what the controlling design load case or criteria is, but my guess would be that the foundation design is probably controlled by wind uplift. 24" vs 12" provides 4 times as much uplift resistance just due to weight of concrete, neglecting any contribution from skin friction, etc. 24" sounds perfectly reasonable to me. Certainly not overkill. Also keep in mind, that the engineer for the carport kit probably knows nothing about the local site conditions and probably takes no responsibility for the foundation design, so the foundations noted on the drawings are probably just "suggestions".

 

[BAretired]

Is it possible to run a sanity check on the caissons and see what's actually needed? It doesn't seem like it should be very difficult to do, but I don't know the first thing about running the calculation

.

Eng-Tips does not provide free engineering services. I suggest that you retain a qualified engineer to design the foundation.

 

[lyonkster]

Perhaps my post was poorly worded, I did not mean to ask anyone here to do a calculation for me. I was mostly curious about what drives caisson sizing (weight, lateral, uplift), and was wondering if I could run some back of the envelope numbers as a sanity check. Seeing the responses that 24" is reasonable is very helpful to me.

 

[Eng16080]

I’m guessing you don’t get snow wherever this is located based on the purlin size and spacing. If that’s the case, it seems possible that a 12” diameter caisson might work based on gravity loads only and assuming the site soils are decent.

But, as others have said it’s possible that wind uplift might be controlling this size, meaning that that mass of concrete might be needed to “hold down” the structure or that it might be necessary due to edge distance required for the post to concrete anchorage.

Depending on the lateral force resisting system used here, the size may also be related to that.

Don’t take this as real engineering advice though. I don’t have enough info and am just speculating. Still, I hope this is helpful.

 

[Eng16080]

And one more thing I forgot to mention: If you were to hire a local engineer to justify using the smaller caissons, don’t be surprised if the cost of hiring them far exceeds any construction cost savings. Or at least take that into account. Hiring an engineer to save only a few thousand dollars is usually a losing proposition.

 

[BAretired]

Perhaps my post was poorly worded, I did not mean to ask anyone here to do a calculation for me. I was mostly curious about what drives caisson sizing (weight, lateral, uplift), and was wondering if I could run some back of the envelope numbers as a sanity check.

The snow load, the wind load and the soil properties all affect caisson sizing. We can't give you an adequate answer without doing calculations. As for a sanity check, you are not trained to design the foundation, which is why you need a competent engineer to prepare a Foundation Plan.

If this is a single carport, the size is overly generous; if it's a double, the lanes are too narrow. The minimum recommended lane width in North America is 8'-6".

 

[BridgeSmith]

Another thing to consider is if you don't take the manufacturer's 'suggestion', if the carport fails for any reason, the manufacturer will claim it's because the the foundation was inadequate. Even if you get an engineer to design the foundations, the result will be the same. The engineer won't take responsibility for the carport, and the carport manufacturer probably won't voluntarily warranty their structure if you deviate from their prescribed foundation size, even with a stamped engineer's design.

Anyway, an engineer's design will undoubtedly cost more than just using the larger foundations. The difference in the cost of the concrete is a few hundred dollars, and the difference in cost to drill the hole is probably about the same. For a project of your size, mobilization cost (getting the drill rig to the site) is the bulk of the cost of the drilled holes.

The only way you'd possibly save any money by getting a design is by drilling the holes with a hand operated auger. The biggest you can go with a 2- man auger is about 14", and those 2 men better be burly guys. Otherwise, you're looking at a skidsteer-mounted rig, which will be about the same for a, 12", 18" or 24" hole.

 

[lexpatrie]

Well, as we progress through this, there's the Design for Code Acceptance for decks, I think there's some designs for piers in there. I'd be surprised if the piers in that were intended to resist wind uplift or lateral load, for that matter. It would give you a sense of the relative size for "too much" (40 psf probably) downward load, which would allow you to diagnose if the pier size is based on something other than downward load, which I think we all kind of accept as implicit.

ETA: added a missing 't'. Guess where?

 

[lyonkster]

I’m guessing you don’t get snow wherever this is located based on the purlin size and spacing. If that’s the case, it seems possible that a 12” diameter caisson might work based on gravity loads only and assuming the site soils are decent.

We do have snow loads, and funny you should mention the purlins - I did in fact I question their 4"x6" purlins with the 15' span, seemed a bit undersized to me. I was told that it's "fine".

And one more thing I forgot to mention: If you were to hire a local engineer to justify using the smaller caissons, don’t be surprised if the cost of hiring them far exceeds any construction cost savings. Or at least take that into account. Hiring an engineer to save only a few thousand dollars is usually a losing proposition.

I'm sure that this is exactly right. I'm having trouble even finding a structural engineer to respond to me at this point, that's the situation around here these days. And yes, I'm sure their work will cost much more than just going with the recommended caissons.

If this is a single carport, the size is overly generous; if it's a double, the lanes are too narrow. The minimum recommended lane width in North America is 8'-6".

It's a single carport. I like to be able to open the car doors and trunk fully and still stay under the cover, we're in very wet Portland.

 

[RPGs]

Also keep in mind, that the engineer for the carport kit probably knows nothing about the local site conditions and probably takes no responsibility for the foundation design, so the foundations noted on the drawings are probably just "suggestions".

This was also my initial impression, having consulted on small structures intended to be constructed in multiple US states.

The folks in this thread have provided reasonable advice in terms of the relatively negligible construction savings you may gain by "sharpening the pencil" on the foundation in this case.