Frost Protected Foundations / Clay till / Winter is coming

[Slowzuki]

Hello,

Being a mechanical engineer with a civil technician for a father lead me to designing my own workshop.

It is a 32'x64' post and beam, single storey building designed to use the same bent (think large scale truss) every 8 ft in the long direction. The truss is suported on footings in a 6" slab on grade that runs the long direction of the building including one in the centre, such that the truss only spans 16 ft.

I live in a area where the design snow loading is 2.74kPa or about 40 psf. Our frost depth is 5 feet.

I used to ASCE design guide for frost protected foundations to design the correct insulation for the building. Basically ended up with R=10.5 which is about 3" of XPS on the derated ratings in the standard, and extending out 5 ft from the edge of the slab. The slab is 24" on the thickened perimeter and the central footing.

I approached a structure engineer to seek help on the selection of reinforcing and preparation of the pad. I had envisioned removing topsoil then leveling the site then adding 6-12" of non-frost suceptible fill compacted in 3 or 6" lifts. The soil is a firm clay till on top of a better draining gravel below.

After meeting with the engineer I'm told he has the reputation as the most conserevative local structure engineer. He has recommended so far doubling our fill and using a large Bomag type roller to compact it. He has also recommended 200 feet of french drain / interceptors around the perimeter.

He then interviewed me regarding the loads in the building, the largest being a 9000 lb farm tractor on quite large tires. He said he would be using this information to see if a thinner slab could be used.

Most garages in the area are built on 6" of 3/4" river stone / pea gravel with a 4" slab and nothing else. This meets the local building code.

I'm concerned there is maybe 3000$ of extra engineering going into saving 500$ on a truckload of cement. I'm also concerning I'm getting an expensive over designed slab to protect the engineer from liability. I told him cracking was acceptable for the end use, he replied it wasn't to him.

If I abandon plans of constructing post and beam, an engineer is not longer required by law to be involved with stamping drawings for the foundation. This is sort of silly as the outside footing loads are higher on a stick built shop with trusses spaning the 32'.

We also have available to us within 5 minute drive a sandpit with moderately coarse well draining sand with some fist sized rocks in it (say 2 per cubic ft). We have been trying to accomodate this in our design as it is nearly free for us and reduces trucking costs. The engineer says it is not suitable for anything we are doing.

Anyone have any comments for me, critism of my expectations welcome! Oh and he wants a soils test done. Am I out of line for being concerned with costs? To me this is a small slab.

 

[DRC1]

I agree that some of this does sound like overkill. Unfortunately, many engineers specify the same standard details over and over unil they no longer check to see if they are appropriate. I would go with the 3/4" stone base. Unlike soil, it does not need rigorous compaction. If you are sing gravel, you would need to wet & roll it. With the stone, you would only have to tamp it wiht a walk behind plate compactor (available at your rental center). I would shy away from the free sand. It may be difficult to get good compaction and if you dont tke out the rocks, you could get crack causing point loads on the slab.
The design of the slab should only take a couple of hours at the most, but as you point out your money may be better spent on the thicker slab.
As for the french drains, you do want to keep water away from the structure, but you may be able to accomadte what you need with surface ditching.
I dont know about the soil test, but I would want a test pit to see how thick the clay is and if it may be possible to drain the stone into the gravel layer.
Good Luck

 

[eric1037]

This is all about risk management. If you know the risks and the possible performance of the structure, then you can make an informed decision.

However, based on the fact that you hired an engineer, it seems like you are not 100% comfortable with the risks involved.

If you pay for recommendations from the engineer, that's just what they are: recommendations. However, don't go back to the engineer when things fail. I'm not saying they will, but you are taking the risk.

As far as I'm concerned, it's your building. If you don't mind cracks, then do what you want as long as it meets the local building code.

However, if you want to reduce your risk, then take the advice of the engineer. You may want to discuss with him each of the high cost items and figure out what risk is associated with each. Then you can make an informed decision based on your risk tolerance and your expectations for the performance of the structure.

As a side note, there are many engineers that are very conservative and give recommendations based on the least amount of risk. This both protects them and their clients. However, cost may quickly become a big issue. The really good engineers help their clients manage their risk.

 

[Slowzuki]

Thanks Eric, I actually hired him because I have no choice but to present stamped drawing to the building code officials if I go post and beam. I work in the fire protection field so I understand.

Unfortunately the National Building doesn't include foundations which don't have their footings below the frost line despite them being commonplace.

I will try to discuss costs with him. As a note, I'm already at about 5 hours of his time without him having even started a calc or design on the slab. He won't give a quote but I know our company would be billing about 500$ already for that time. I'm scared to think how much time he'll spend on the whole design.

 

[Slowzuki]

Sorry, National Building Code of Canada was what I meant to say.

 

[eric1037]

I just reviewed your original post and I have a better understanding of what you are asking.

It seems to me that the main issue is that your foundation will not extend below frost depth. Therefore, you need a stamped drawing.

I would weigh the costs of engineering plus the cost of construction vs. using traditional foundations.

If it becomes apparent that there will be a cost savings by going with the engineer's recommendations, you will have to follow them to the letter.

As far as his recommendations being conservative, I would say that considering the soil type and the frost depth, it seems reasonable.

The reason that cracking isn't acceptable in this case is that the slab is part of the foundation system. There may be differential settlements that affect the entire structure.

 

[Slowzuki]

Thanks Eric, I appreciate the honest response.

 

[Slowzuki]

An update, we have sort of resolved the problem. The structural engineer never phoned back with a quote. He said he would the day after I demanded a quote, it has been 8 days and counting.

The soil is a stiff clay / till, with allowable bearing pressures of 150-200 kPa by the building code.

To get us below the frost line to avoid trouble with the inspector, we are trenching 18" wide, lining with filter fabric, then filling with 3/4 - 1.5" clean crush in 6" lifts. There is drain tile laid a couple of inches off the bottom and it and the trench is inclined at 1" in 8 ft. This drain comes out above grade.

On top of this there is a 2 ft high by 10" wide grade beam insulated like a frost protected foundation.

Following this with all the loads calc'd, we have 60.1 kPa at the bottom of the trench.

Ken