Advice on how to engineer my foundation according to code

[Semper Fidelis]

I'm tearing down a shed on my property and erecting a 24'x36' steel truss Garage manufactured by Worldwide Steel.

I've figured out I need to get a soil test done (county requirements) and the guy is coming by today to do that. I'll have the results in 2-3 weeks.

I'll also be getting my engineered and stamped drawings of the structure in a couple of weeks.

The county requires a map showing where it will be so I have a GIS picture depicting where the shed will be. I'll also need to submit to the county how my footers and slab will be poured to support the building I'm erecting.

This is where I'm stuck. I have an engineering degree so I can do some math. How do I determine the depth of the footers for each of the 8 trusses along with the slab to demonstrate that the foundation will bear the structure and meet the uplift requirements? I think I ought to be able to show that each truss will have a certain downward load and that the overall foundation needs to support the load of each leg and deal with any uplift so if I can demonstrate that based on footer depth and the weight of the slab atop it then I can create a diagram showing each of the 8 footers and the slab and why each footer is a certain depth.

How do I proceed to matching the soil test with the engineered drawings to engineer the foundation?

 

[kbs2244]

Show what you have to your concrete contractor and then work back from his proposal.

 

[Semper Fidelis]

Show what you have to your concrete contractor and then work back from his proposal.

Thanks. I'm meeting with him tomorrow about a sidewalk installation and I'll do that.

I found this site: https://www.thespruce.com/foundation-footings-code-basics-1822269

It appears I'll have to make sure the area is graded properly for water to slope away. It also appears that the soil test will give me a load bearing value of what it will support per square foot so I can figure out how many square feet of the footings will need to be based on the soil's LBV combined with the weight that each truss will put on each footer.

 

[01-7700]

So you have a degree but not an engineering license. If you had the license you would not be engineering something you have no experience doing. Do you really expect to get a geotechnical education off the internet in a few posts?

 

[01-7700]

Thanks. I'm meeting with him tomorrow about a sidewalk installation and I'll do that.

I found this site: https://www.thespruce.com/foundation-footings-code-basics-1822269

It appears I'll have to make sure the area is graded properly for water to slope away. It also appears that the soil test will give me a load bearing value of what it will support per square foot so I can figure out how many square feet of the footings will need to be based on the soil's LBV combined with the weight that each truss will put on each footer.

Soil test will tell you the agricultural designation of the soils in your area, not the load bearing capability.

 

[Walter_TA]

Where in northern va do live? Does your county require stamped prints? Metal buildings are not that heavy. Do you have a good compacted base? What does the county require for footers? What I did was put in a ground beam with piers. I dug deeper at the edge and got my dirt guy to use his 18 in auger to make 8 holes for the steal beams to sit on.

 

[lakelandcat]

I had to do the same thing but my contractor that pulled the permit for the foundation took care of the eng. permit and I also had to have a exterminator spray before they poured. Mine was a 16x20 and they poured it on 12" footers that were 12" deep, with a slab that was 4"-6" thick. I had to provide a blueprint and site map, foundation contractor took care of the rest. He also took care of meeting with the inspector. Cashed my check and we were both happy. exterminator cost $100, engineer permit cost $250, 2 years ago.

 

[Semper Fidelis]

So you have a degree but not an engineering license. If you had the license you would not be engineering something you have no experience doing. Do you really expect to get a geotechnical education off the internet in a few posts?

No, but I just spoke to my concrete guy and he's experienced in how these things work. The soil test will tell me how deep the footers go. The engineered drawings from the building ocmpany will tell me how much load will beon each pier. Based on the LBV per square foot of the soil then I'll be able to calculate the size of each pier. The county doesn't require that I have a geotechnical education in order to submit a site drawing and how the foundation will be built to bear the load based on the soil's LBV and other characteristics.

I'm not sure why you have to be churlish. I'm asking for advice.

 

[Semper Fidelis]

Soil test will tell you the agricultural designation of the soils in your area, not the load bearing capability.

Not to be disagreeable but the geo engineer I spoke to told me precisely the opposite - that the soil test will advise the LBV of the soil I'm getting tested. They will also tell me the depth requirement for the footers.

 

[01-7700]

Not to be disagreeable but the geo engineer I spoke to told me precisely the opposite - that the soil test will advise the LBV of the soil I'm getting tested. They will also tell me the depth requirement for the footers.

OK, so you're all set then. Soil test in my area usually means something different. Rock on.

 

[matt_i]

i think you can ballpark 2000 psf ground pressure in average soils when designing footing pads. My thinking is the depth has nothing to do with the square footage, its just to assure its conservatively below a ~500 year frost line for your area.

Then you get weights of the building + its snow load, you can probably roughly divide it into X weight distributed into Y columns, weight of the concrete itself, and add a safety factor (multiplier).

Then there should be considerations for wind load. Its a distributed load but then attempting to bend the columns as a moment.

After you do some calcs and initial design the concrete guy can probably say "wooo thats overdesigned" or "um, it seems a little light for these kinds of buildings we've done in the past".

 

[lakelandcat]

I think I understand where your going, different depth for something on sand as opposed to clay or 6"soil then bedrock. Right? Just trying to confirm.

 

[ard]

Not to be disagreeable but the geo engineer I spoke to told me precisely the opposite - that the soil test will advise the LBV of the soil I'm getting tested. They will also tell me the depth requirement for the footers.

What is the frost depth for your location? Usually the city/county will publish that on a website. THAT will tell you the depth.

The LBV gives you the width of the footings.

All generally speaking.

 

[ssdave]

The load bearing capacity of the soil is exactly that, the maximum capacity or safe capacity, depending on how it is written in your geotech report. What the engineer or designer that details the footing must take into account is the loads, the eccentricity of the loads, the factors of safety, and how the eccentricity or variances in the loading concentrate the loads, and transfer of load to the footing bearing area so that the concentration falls below the allowable capacity.

So, if your load is 8630 pounds and your bearing capacity is 2000 psf, your footing will theoretically be, at a minimum 4.32 sf. It will have to be much larger because of factors of safety, eccentricity, load concentration, etc.

Another factor that needs to be taken into account is that footings may flex, so the further the footing bearing surface is away from the point of loading, the less effective it is. To counteract this, the wider the footing is spread, the thicker it must be and the more dense the reinforcement steel.

If you're doing pier type footings, that's an entirely different design problem, as the sides of the footings contribute to the bearing capacity.

Often, the manufacturer will supply you with a table of recommended footings, for different soil bearing capacities. That would be the best place to start. A simple weight divided by bearing capacity is not a good way to design, as it will almost certainly be undersized.

What kind of footings are you using? Conventional spread footings and stemwalls? Conventional spread footings and cast in place column sections to receive concentrated loads from the structure? Or cast in place piers in drilled/dug holes? That will be the first design decision, and will determine how you design and size out the footing bearing area.

 

[ssdave]

Uplift requirements are a simpler calculation. You need to calculate the various wind uplift cases on the structure using the building codes section and factors for your area. Then, design the individual uplift points to have more weight than the maximum case uplift for that point. That's really just a matter of calculating the weight of the concrete in that section of foundation, and potentially, depending on the design, some of the soil that either is on the foundation, or surrounds it.

 

[Handyfarmer]

the old school rule of thumb would be below frost level, for depth, and then the footer should be 8" to 16" wider than the pier, and the pier twice the size of the post on it , so if the posts was 4 by 12, I would suggest a min of 8 wide, and 20 to 24 front to back and the footer, (depending on soil type, 16" by 32", and 8" inches thick, to 32" by 48" x 8" thick rebar reinforced,

(the building manufacture, should have the recommended pier dimensions,

if it is a really wide building they may have to have a collar running across under the floor to keep them from spreading, for 24 foot building not a consideration,

if building in clay soils, one may want go a few feet deeper and put in a rock and sand base, to help ease the clay heaving that can take place when it is wet,

~~~~~~~~~~~~~~~

your steel building is light guessing about a pound per square foot, for roof/side walls materials, 864 roof 1200 walls, more than likely the open web frame and read iron, nor much more than the wall and roof, so your building may not weigh much more than a heavy pick up truck, but one would need to figure in snow loads and such,

This simply deals with the dead weight of all of the steel making up the building. It is measured in pounds per square foot. Most of our buildings use 2.5 lb./sq.ft. as a standard dead load.

but most likely your looking at under 4000 pounds per pier at max snow load design of 30 pounds per square foot,

 

[brownbagg]

I'm not sure why you have to be churlish. I'm asking for advice.

no, he was right, I work for a geotech engineer, and if its not a pre-engineer bldg, you going need a visit from a geo eng

 

[landlord30]

in case anyone else is curious:

https://www.merriam-webster.com/dictionary/churlish

 

[barnee]

Soil testing should also be able to confirm depth to bottom of footing. I had plastic soils to four feet deep and had to have my footings extended to this depth even though my frost depth was only two feet. You also need deeper footings if they find that your soil is classified as fill.

You also need to design the rebar in the footings.

 

[_vin_]

Can you post what is to be included in the Geo Investigation? Another thing to keep in mind is that the geotech’s “recommendations” are more like requirements and have to be included in plans. Hopefully the site isn’t sitting on 10 ft of fill or something like that.

How many borings? 2 for 15 ft or auger refusal?

 

[wssix99]

No, but I just spoke to my concrete guy and he's experienced in how these things work.

That doesn't mean that he understands them and (for sure) he doesn't provide you any professional insurance in case your building falls down.


What county are you in? (This makes a difference in Northern Virginia.)

The soil test will tell me how deep the footers go. The engineered drawings from the building ocmpany will tell me how much load will beon each pier. Based on the LBV per square foot of the soil then I'll be able to calculate the size of each pier.

Your county will have a minimum depth that you have to go to get below your frost line. (This depth may be sufficient for you, structurally.) I recall that Fairfax was 24", years ago.

The posters above are correct. You cannot precisely figure out the actual bearing strength of your soil until you dig out the foundation and test it at that time. When you take a boring, it disturbs the soil in the sample and ruins it for this type of test. However; in your area - you may not need to do this...

Northern Virginia is historically non-urban and a lot of the land area is undisturbed. Knowing the local conditions, the engineer taking your boring can just check to make sure that you aren't building on top of fill. (and are on virgin soil) With that knowledge, they can give you a fairly accurate estimate of the strength of the soil - good enough to build with confidence. This method shouldn't be a big deal since you are putting up a very light building.

 

[ard]

Northern Virginia is historically non-urban and a lot of the land area is undisturbed. Knowing the local conditions, the engineer taking your boring can just check to make sure that you aren't building on top of fill. (and are on virgin soil) With that knowledge, they can give you a fairly accurate estimate of the strength of the soil - good enough to build with confidence. This method shouldn't be a big deal since you are putting up a very light building.

In my area, provided you are building in un disturbed soils (or excavate down to undisturbed soil) they have a standard soil bearing pressure they will allow without geotechnical testing.

For example, from their website:

West Slope Single Family Dwelling
General soil conditions default exclusive of the Tahoe Basin is 1000 #'s per square foot. The soil bearing pressure can be increased to 1500#'s per sq.ft. with a site visit and analysis by the project engineer. Any design soil bearing criteria >1500#'s per sq. ft. requires a soils report supporting the values used in the design.

 

[ishiboo]

With that knowledge, they can give you a fairly accurate estimate of the strength of the soil - good enough to build with confidence. This method shouldn't be a big deal since you are putting up a very light building.

I agree... with that size and (light)weight building, the foundation guys should be more than capable of telling you what you need... unless the excavation uncovers something unusual.

 

[davejo]

You might find that uplift will dictate how big or heavy your foundation needs to be rather than the weight of the building piled high with snow. That was my finding when using an online uplift calculator, they use a generous .6 safety factor