Residential Foundation 6

[JAS34]

Hello Engineers,

I am a licensed engineer. I am looking to design a residential foundation for my parents new home and help them save money. The foundation will consist of perimeter footings with concrete masonry block foundation walls above. Within the foundation walls will be fill dirt on which is to be poured the slab.

House plans are from a licensed architect. Also, the bearing capacity of the soil is 1500 psf.

I have site inspected the ongoing foundation work with the development going on in my own neighborhood. Also, I have studied Chapter 4 Foundations of the 2009 International Residential Code and made note of the minimum design requirements.

My questions to everyone are, basically, are there actual design calculations or a methodolgy to size the width, depth, and reinforcement of the footers? Is there design equations or a methodolgy to determine the slab thickness and wire mesh reinforcement required? Or is residential foundation design more of an empirical approach?

I basically have my own approach which is to calculate dead, live, snow, and wind loads and load combinations per ASCE 7-05. From that I can have an idea of the loading on the perimeter foundation walls. Is sizing the footers a matter of checking concrete shear and soil bearing pressure? Do I treat the footers as beams and design for bending and shear? Is the slab just a concrete shear calculations? Or, is all of it from proven empirical methods, design tables, or traditional methods of success, etc?


Thanks in advance to all of you who may offer me guidance,

JAS34

 

[rowingengineer]

in Australia the design is more about the stiffness requirements to minimize cracking due to clay movements. With high clay content sites the stiffness of the footing and number can be fairly hefty.

http://www.nceng.com.au/

"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected” Petroski 1992

 

[Ron]

JAS34...don't mean to jump on you here, but if you are a licensed engineer, you should know what you don't know...

You are obviously not familiar with foundation and slab design, so unless you intend to make this a careen choice, don't do it.

...and they are "footings" not "footers".

 

[concretemasonry]

When you are into typical residential design, you enter the world of prescriptive design by code. If you bother to try to analyze a typical home with a basement, you quickly realize that if you punch the clock and break everything up into individual elements and look at the time and cost of what was designed, you realize that an engineer is not needed in many typical residential designs because of the practical effectiveness of the concept of prescriptive standards for common repetitive uses.

As an example, if you break a foundation wall (block or concrete) into separate units of a strip footing and a deep wall beam you immediately loose the effectiveness of, what motivators refer to as "synergy", where the individual contributions are far exceeded by the elements working together. Separating the footing and walls yields unrealistic values for the footing thickness and width and also unrealistic wall design parameters, when the two are commonly working in unison and the floor slab of a basement is constructed later (4" thick and unreinforced) and is a non-structual element, except some code required the slab to be cast on top of the footing and have 3 1/2" of contact to provide uncalibrated, but additional lateral resistance to the wall portion. Comparing this to a different type of floor/footing/wall combination and construction process, the analysis would be different and each of the elements would be different and be similar to most of the prescriptive requirements.You are always allowed to design each element individually and address the fixities and combinations of loadings.

Dick






Engineer and international traveler interested in construction techniques, problems and proper design.

 

[MiketheEngineer]

Go to IBC 2009 and use the prescriptive method as suggested.

Typically with an 8'' pour with 20-24'' footings. Good for up to about 8' tall. I like to put 2- #4 bars in the footing - maybe 3, then 2 #4 top bottom and middle. If I get real excited I will put 1 - #4 vertically about 4' oc. Also put #4's at 45 degrees around all openings.

If that doesn't solve it --- nothing will

Over 8' go up the prescriptive ladder.

Make sure you have the floor diaphragm on BEFORE you back fill.

And I DON'T like CMU's for anything!! Just my thoughts....

 

[FixedEarth]

Pay a local structural engineer 5% of the Home's worth and keep their calculations as your first design example.

 

[cvg]

before you do anything, investigate the soil conditions - both the existing ground and the "fill". note that bearing capacity has absolutely nothing to do with either "settlement" or "expansive soils" or "high water table" ...

 

[Grouser]

Pay a local geotechnical engineer his $1000 and get a professional report with boring or test pit logs. Simple situations allow simple designs and conversly... What if the site was a medical waste dump or hazardous material dumping ground? It is called a foundation for a reason.

Cheers