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.

 

[lyonkster]

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.

In this case this carport is designed by a company that specializes in custom structures, mostly large garages and homes. It's not one of those online carport/garage companies that are very prevalent. In fact they designed our 1000 sq ft garage that we're just about to have them build for us. The engineers they are using are the same ones they use for other projects, so this will be a one-stop supplier - they do the design work and provide the materials.

you are not trained to design the foundation, which is why you need a competent engineer to prepare a Foundation Plan.

I'm not planning to design the foundation. However, I am an inquisitive person, and I like to make sure that what someone may recommend actually makes sense, especially when it doesn't feel right. I'm sure all engineers have been known to do that. Many years ago I had a foundation sized by a structural engineer who came up with something that was going to cost an arm and a leg, with numerous concrete pads and steel posts. I asked him, "can't we use a continuous foundation/stem wall and wood framing?" His answer was "yeah, you could do that too." Cost was less than half, and no need to be dealing with steel. It's not that he was incompetent, it's just that he pulled out whatever was the most convenient to him at the moment, not necessarily spending any time on thinking about alternatives. I like to know my options, whenever there are some.

 

[lyonkster]

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.

Just to be clear, this is a local company with local engineers, not a nationwide mail order place.

And yes, as I hope I've made clear already, I agree with all the comments here to just go with the recommended caissons. And hope that those purlins are sized correctly .

 

[BAretired]

In my neck of the woods, we have predominantly glacial clay till. We have frost penetration to a depth of six feet or more in open areas. A four foot deep caisson could heave with the frost; so deep piles are often used here to avoid that problem.

 

[Eng16080]

Concerning the purlins, they seem a little light for a high snow load area, but if the snow load is on the lower end, they’re perhaps ok. At least, that’s based on a quick calc. I just scribbled. Not necessarily questioning the engineering here. (It’s always easy to be a critic.). If this engineer is good, they can probably answer your questions.

 

[RPGs]

Just to be clear, this is a local company with local engineers, not a nationwide mail order place.

Understood--I don't think anyone here is trying to outright deride their design or your curiosity, of which the latter is prudent in any case (as you've experienced). Best wishes to you, of course.

 

[BridgeSmith]

I did not mean to imply anything negative about the company that's supplying the carport; just giving you a head-up that nobody likes being responsible for the results of a design that's not entirely theirs. The carport needs an adequate foundation to perform properly, and the performance of the foundation depends on the stability of the structure attached to it.

 

[lyonkster]

Thank you folks, I appreciate all the responses here! To address a couple of the points made:

- Our frost line here is 12" deep

- The snow load that was used for the garage is 25 lb/sq ft

As many of you suggested, I'll just leave the 24" dia caisson design; it probably won't cost that much more than a smaller diameter, so I'm happy to go with the proposed design.

But I will ask them to double check on the 4x6 purlins; I may even ask for one size larger, as I don't think that will cost much more than 4x6.

Thanks again!

 

[Eng16080]

Well, to be fair I didn’t realize snow loads got down that low! At that point, the roof live load almost controls. I was thinking of a light snow load as being more like 40 psf. You must be down south-ish like the Carolinas or Georgia.

With that said, if there’s any concern at all, there’s little downside to going a little bigger on the purlins for likely minimal extra cost.

Best wishes!

 

[lyonkster]

Well, to be fair I didn’t realize snow loads got down that low! At that point, the roof live load almost controls. I was thinking of a light snow load as being more like 40 psf. You must be down south-ish like the Carolinas or Georgia.

We're actually in Portland, but it's a mild marine climate (CZ 4C), so fairly mild temps and not a very high snow load (at least in our area; nearby mountains are a different animal of course).

 

[Eng16080]

Ok, that makes sense. Surprised frost depth is only a foot there.

 

[lyonkster]

Yup, this is from the county website:

Residential prescriptive design (non-engineered) criteria

Seismic: Design Category D
Wind speed: 97 mph
Roof snow load: 25 psf below 400 foot site elevation. All other elevations can be determined on Structural Engineers Association Organization (SEAO) website
Frost depth: 12 inches
Weather classification: Moderate

 

[lexpatrie]

You must be somewhere north of me, Eng16080 to consider a roof snow load of 40 psf to be light.

 

[Eng16080]

lexpatrie, I think so. Most of my work is in NH and Maine. NH has a wide range of snow loads. I regularly deal with 40 psf up to 100 psf. I tend to forget that 40 psf isn't a light snow load in most places.

 

[lexpatrie]

Here I thought 60 psf ground snow was heavy. I've often daydreamed about say, Whittier, California (right?) or someplace nice and Alaskan but it's never happened.

 

[Eng16080]

Yeah, I think some areas in Colorado and California are even a lot higher than what I run into.

60 psf ground snow load is still rather heavy to be fair. That's what the snow load is where I live. Actually, it seems that's more what it was when I was a kid. Now we don't seem to get snow like that most winters.

 

[RPGs]

I'm only mildly joking when I say we're spoiled here on the mid-US East Coast with our low seismic and snow, but at this point any ASCE hazard map search coming up with more than 35 psf ground snow load has me raising my eyebrows in mock alarm.