Pier foundations

[josh]

I have limited building experience, but will be building a 700-750 sq. ft structure in colorado this summer. The slope is not bad, but bad enough that grading will not be cost effective. I will be building a 24' by 32' shell of a house. I want to use piers. Local neighbors with limited building experience tell me the frost line is 5 ft, but there is viturally no heaving because the soil is so dry. I will also be using stud framing, not post and beam. So I have many questions:

1) Should I still place the footings below the frost line?

2) I am sure that is a resounding yes, so, will a pier more than 5 ft high create any issues?

3) In designing the pier layout, I understand all the calculations involved with footing / pier diameter, considering soil bearing capacity, etc. What I am more concerned with is what is the most effective pier layout when taking into consideration the cost of the floor (beams, girders, floor joists, etc.).

So I guess my question is, am I better going with larger and fewer OR more but smaller.

[Rich]

Where at in Colorado? 5' is pretty deep except for maybe Northern Minnesota - unless you are above 7000'. You mention that the ground is so dry so there won't be heaving - make sure you put waterproofing and perimeter drain all the way around your foundation to keep the water from getting underneath your walls.
I would probably opt for more and smaller.

[josh]

Yeah, it is just at 7000' ft. What would waterproofing and a perimeter drain entail?

Thanks for the reply.

[Rich]

I would put some type of spray on or roll on material with a drain board on the walls.. this would drain wall from the face of the wall down to a perforated pipe (4") that would slope around the foundation and eventually emptying away from the building 10-15'

[mjpliv]

A question for Rich -

I see the same issue cropping up all the time - size and spacing of concrete structures.

Fill me in, what are the requirements of most municipal building authorities in the United States as far as engineering support goes? I would assume that there is some sort of national standard as well.

In most cases published design tables can be used to design structures subject to uniform loadings under known conditions. Throw in a column load or vehicle load and you get a whole new kettle of fish.

I try to be very carefull not to "design" anything for anybody on this forum. I tend to stick to generalities in the concept of concrete design because -

1. I am not a qualified engineer.

2. Designing any structural component without detailed plan sets and details about it environment is just plain irresponsible.

3. I don't want to get sued and I don't want you to get sued (we have become a society of litigants)

So, at the risk of sounding like a broken record, in answer to the questions raised in this thread, consult with an engineer regarding the design of concrete piers that will eventually support your home, belongings and, most important, you and your family. Its fairly cheap peace of mind. Although the concept of a concrete pier sounds simple enough, most "off-the-shelf" designs would be for axial loads centered on the pier. This is not the normal load scenario. Most loads are concentric (off centered) and lateral loads must be calculated (unless the wind doesn't blow in your region)

[Rich]

I totally agree with you on the don't give design advice - that's why I have the small writing on the bottom of every post :)
As far as a national standard for engineering - I can't imagine there is one. For the house I'm building right now a pier and grade beam foundation was selected because of expansive soils. That ended me up with about 200 caissons and 2000 lf of grade beam with a spacing of 7'-10' for a house :shock:
Then we turn around and bid a school south of here and we get 24" caissons anywhere from 35'-40' spacing and no grade beams. I wish I would've seen the design before releasing mine for the caisson driller. Would've saved me approximately 3 weeks on the schedule and a whole lot of money too.
For high rise buildings - most of what I see is somewhere around 35' spacing for steel members and it just depends on other traffic loads in the garages for caissons/piers.
Not sure if that was what you were asking? :)

[mjpliv]

Actually I was curious about a national standard as it relates to a normal single family dwelling. I would expect that in the event the maximum loads or maximum spans are exceeded or a non-standard structural member or assembly is added there would be a requirement to provide documentation from a design professional (ie - engineer, architect, pre-engineered product technician).

Most span tables make several assumptions.

1. the member is fully loaded

2. the loads are uniform

3. the member is installed in the environment the table was designed for.

4. the loads are vertical.

Take a simple span beam with a uniform load and I will happlily accept the recommendations of a recognized span table. Throw in a concentrated load mid span and that table information is null and void.

Try a multi span beam of 12 feet and 8 feet for a total length of 20 feet. Again I would trust what the span tables tell me. Lets change that to a multi span of 12 feet and 4 feet. You would think that would also be fine with the same loads applied. I bet if an engineer designed the beam he would recommend it be installed as two separate beams.

Why? Because the engineer would calculate load cases - or play "what if". Lets say this beam in under your floor supporting joists. Lets say that the 12 span section is fully loaded because you are having a family reunion. This means that the beam is supposed to be (in a perfect world) at its maximum deflection value by code. Now suppose the hallway over the 4 foot span of the beam is empty. The only thing "pushing" down on the end of that end of the beam is 2 feet (half of the span) of dead load (about 15 pounds per square foot. If you look at the physics, when the 12 foot span deflects downward it will try to lift the 4foot span off the end bearing point (uplift) . All of a sudden, you have unexplained drywall cracks, or a window sticks, or a door no longer fits. If you cannot show enough deadload (floor, walls, etc.) to counteract the uplift then the beam must be installed in two pieces.

This is just one example to illustrate my point. At what point (in general) do your building authorities say, "Hey! I am not comfortable signing off on this (house, garage, addition, set of plans, concrete slab) because it falls outside of acceptable standards. Please provide me with an engineer's approval."?

[Rich]

oic.. well if we just look at a standard box for a house there is quite a bit that can be pulled, like you stated, right from span tables.
Specifically looking at concrete for a single family home - single level .. there isn't even a requirement for a footing. If I remember correctly this home only needs 6" of bearing soil (considering that the soils provide the minimum amount of bearing capacity). 2x4 walls @ 24" o.c. are fine also.
Like you said though - one oddity in the whole thing and it throws that recommendation out the window. Say we have a saltbox type roof - all the sudden we have eccentric loading on the walls, floor, and foundation. Standards get thrown out the window. Simple spans within the envelope of the building would pretty much stay the same but we might need to provide squash blocks at a certain point where we transfer loads through the floor.
I see a factor of safety of 3 all the time.. even worse is when an engineer takes over from another engineer.. all the sudden I have a factor of safety of 3 plus another of 2. Nobody trusting another and covering their butts. Of course, I don't blame them but that's one thing that continues to drive up the costs of these homes.
I suggest people to look through some of the publications by the NAHB, specifically their Optimum Value Engineering white papers. It gives you a good idea of questions to ask.. essentially they show the minimum design requirements that meet code with a "normal" factor of safety for odd wind loads etc. Some very interesting stuff. Oh and take a look at their shallow foundation designs too - very interesting.