Special inspections for Deep Foundations - CBC 1705.3 1

[Jrswett]

From the special inspection form in my jurisdiction, California Building Code (CBC)
Table 1705.8 – Cast-In-Place Deep Foundation Elements
For concrete elements, perform tests and additional special inspections in accordance
with Section 1705.3. (requires verifying mix design, rebar placement, concrete slump, make concrete cylinders for strength tests, inspect form work - would all be required for a cast in place deep foundations)

1705.3 Exception: Special inspections and tests shall not be required for:
2) Continuous concrete footings supporting walls of buildings three stories or less above grade plane that are fully supported on earth or rock where:

2.1) The footings support walls of light-frame construction.
2.2) The footings are designed in accordance with Table 1809.7.
2.3) The structural design of the footing is based on a specified compressive strength, f 'c, not more than 2,500 pounds per square inch (psi) (17.2 MPa), regardless of the compressive strength specified in the approved construction documents or used in the footing construction.

1809.7 Prescriptive Footings for Light-Frame Construction i.e. shallow foundations.

Now I normally have not been required to do special inspections per 1705.3. The structure is light frame, the concrete is 2500 psi. However it is supported on grade beams, not prescriptive shallow foundations. I discussed this with the building official and he also said they do not normally require this. His logic was that the grade beams and cast in place drilled shafts far exceed prescriptive footings for light-frame construction per 1809.7. Can anyone confirm this is the correct interpretation? We are also assuming that the requirement of exception 2 are all AND clauses, that it must meet 2.1, 2.2, AND 2.3. Do all deep foundations require performing tests and special inspections in accordance with CBC 1705.3

 

[dold]

My interpretation of 1705.3.2 is that the footing merely needs to consist of a chunk of concrete. Just enough to transmit the load to the soil neglecting any contribution from reinforcing, i.e., plain concrete. It doesnt say that explicitly, nor has anyone ever told me that explicitly. But how often have you run across a footing that would apply to 2.1, 2.2, and 2.3 that would be insufficient if designed as plain concrete? I don't even know what the prescriptive widths and thicknesses are, but I'd hazard a guess that the thickness is roughly correlated to a 35-45 degree projection from the wall?

You must 'design' grade beams and footings considering more than simply bearing pressure. Light framed wall strip footings could literally be a pile of rubble and still mostly perform as intended as long as some sort of width:depth relationship is maintained.

Just my $0.02.

Furthermore, I suspect you would not be using piles and grade beams if your subgrade could otherwise meet the "fully supported by earth or rock" clause of 1705.3.2...?

To answer your question: I would require special inspections for grade beams and piles. No questions asked.

 

[Jrswett]

That's what I thought too. Yes the deep foundation neglects the upper soils layer and relies on cast in place / buried grade beams spanning between cast in place drilled shafts that transmit the forces to the bedrock layer.

It's that 2.2 exception that has me questioning this. I've done a ton of shallow foundations that exceed the requirements of 1809.7 as directed by the geotech. They almost always require 18" burial depth instead of the minimum 12" per 1809.7. Sometimes they will do a 3ft or 4ft burial depth if there is a shallow fill layer. And I've seen bearing capacities of 1000 psf, though typically the 12" wide 1 story and 15" wide 2 story footings per 1809.7 are acceptable for the distributed loads. This special inspection requirement never came up.

Wondering why I haven't seen this special inspection requirement before for deep foundations. I think inspecting the rebar and the forms is a good idea. Maintaining temperature and moisture is a breeze since it's all buried in the ground.

Having to do concrete strength tests, slump tests, air tests, temperature tests and verifying the mix design is a bit overkill for 2500 psi concrete. But whatever, do what you gotta do. Are these typically done on site or does the concrete supplier do some of these tasks? I'll probably call them and ask. Seems like they would want to make sure it's all good before they send over a half dozen concrete trucks.

 

[dold]

This is all stuff that any qualified special inspector deals with. It's all done on site. The only thing you have to do is state on your drawings that special inspections of concrete construction is required and your drawings should have tables of special inspections. It sounds like you might not deal with a lot of special inspections? I.e., mostly residential work?

 

[Jrswett]

Yup mostly residential work. I do special inspections for epoxy rebar post-installed in hardened concrete frequently. They usually want the engineer of record to do the special inspections. Inspecting the rebar work would be fine. I haven't done concrete cylinders and slump tests since college, but also within my capabilities. Would have to buy the equipment.

 

[dold]

Ah I see. Yea I would definitely not be doing any sort of concrete testing myself. The only engineering firms that would be EOR and also perform all of the special inspections would be firms who are fully equipped and certified to do so. I doubt the building department means they want EOR to do that themselves. More than likely they just mean you need to do some paperwork citing which inspections need to be done and the owner or contractor selects am inspection agency. The owner or contractor needs to hire a special inspector to perform these tasks. Sometimes building department will request that the engineer performs an "observation program" which just means EOR comes out to the site at specific milestones and confirms the construction meets the design intent. But not the materials testing, etc.