Residential Foundation Design 3

[redbridge]

I am designing a foundation for a single story brick home and I am needing help with the loads and geotech information. First of all what should the dead load be for a single story brick home? Also, a geotech guy came out to do an inspection and all he did was stick a rod in the ground and looked around and came up with a soil bearing pressure of 1,500 psi. This didn't seem very scientific, is that standard practice for determining soil bearing pressure.

 

[dik]

1500 psf?... if he's experienced and familiar with local soils, 1500 psf would be ok for a medium clay or whatever. He would know this from experience as determined by the resistance on the rod...

As far as loads go, dead load for roof might be 15 psf depending on the construction, the weight of the exterior wall with brick veneer might be 55 psf, the weight of the floor might be 12 psf, depending on finishes... you seem somewhat inexperienced and you might want to contact a local engineer...

Dik

 

[SlideRuleEra]

Lburg - Download the free .pdf "Residential Structural Design Guide: 2000 Edition", and see Chapter 3 for loads (a lot of other good info there, too). Here is the link

http://www.pathnet.org/sp.asp?id=1442

http://www.SlideRuleEra.net

 

[Gimbli]

I am a geotech and bearing capacity, with no offense to structurals, has always been a matter of discussion confusion to designers as per my experience. Many times I have been asked How much is it? (bearing capacity) Designers shall remember that bearing capacity is a function of soil properties (density, angle of internal friction and cohesion) and of foundation geometry (depth of foundation and footing size). Therefore, I consider unappropiate to assign bearing capacities without knowing column loads at least, in order to have an idea of possible based width. By the other side, 1500 psf is a very low bearing capacity and might be enough for the relatively light structure under question. My impression is that the geotech is just being conservative in lack of SPT data for instance.

 

[concretemasonry]

Lburg -

You have probably been given a very simple project that is very easy to over-engineer. Your geo-tech gave you a soil strength level that will work for any standard foundation design, although the soil may be shown to be substantially better.

Take a look at the IRC 2006 - Chapter 4 - Foundations.

If the home you are designing for is a typical structure there is really no need for detailed calculations to comply with the codes and requirements. It has been done many times before you. If you choose to do so, you can do a detailed analysis, go partially mad trying to make assumptions, run up the cost and be unduely criticized. Take a good hard look at the section of the code and try to understand it, keeping in mind you have to think in 3 dimensions with continuity and not just a narrow 2 dimension snapshot.

You will find some things that are approved, legal, logical, proven and scarry!!

1. Minimum footing footing width for a basement on a one story brick veneered wood frame home or a concrete block home with 1500 psf soil is 12"!! Not really surprising since 40 years ago (and today) wood basement panels were permitted without concrete footings by using 8" of compacted gravel.

2. The minimum compressive concrete strength is 2500 psi, despite most contractors minimums of 3000 or 43500 psi.

3. Thanks to a knee-jerk reaction (possibly erroneous) from the requirements for weaker, sectional discontinuous panel wall systems, the 2006 IRC has adopted the anchor bolt spacing between 8" and 12" as opposed to the previous requiremnts of 6' and 2' from the end of a sill. Some jurisdiction have over-ruled this point and have reverted to the traditional spacing for continuous walls (masonry and poured concrete).

4. When referring to Table R404.1.1(1), for PLAIN (unreinforced) masonry walls, the thickness should be 8", 10" or 12" depending on the wall height and the height of retained backfill. For REINFORCED 8" masonry walls the reinforcement will be #5 or #6 @ 48" o.c. For 12" REINFORCED walls, the reinforce would be #4 or #5 @ 72" o.c.

5. There are also tables for reinforced concrete walls, and ICF walls.

To put things in perspective, read the code carefully and try to understand the rationale. I am sure you can increase the code minimums to something realistic and comparible to standard construction that has been proven.

In addition to the above mentioned code, you can make your own detailed design and campare to what has be proven to be acceptable. You will find the results and differences very revealing when you step back and look at the big picture.

You will have to determine if the home fits into the applicable requirements of the code. The true features of a successful basement/foundation design is the correct provisions and specifications for subsurface drainage, backfill and surface water control.

If have spent about 40 years trying to calculate why a masonry basement works so well and still am challenged. To me, it is much more difficult to get precise on a small basement than it is to design a 20 story loadbearing masonry structure.

Dick

 

[concretemasonry]

Please forgive my error - The "If" in the last paragraph sould have been "I".

Maybe this could have directed to the "Structural engineering other topics" also, but your charge was to design the foundation.

Good luck!!!

Dick

 

[Touma59]

I am a geotech engineer with few grey hairs. While I very much like to simplify things and remain on the practical side and very much appreciate local experience, there are some basic considerations that should be accounted for which the "stick" does not account for. There should be a minimum amount of testing, lab & field, at least to safeguard against surprises in the soil and be able to answer questions later if a problem does occur.

Tsoft;

http://www.tsoftonline.com

 

[Mickney]

Also a licensed geotechnical engineer with some grey hairs. I am surprised that borings were not performed to investigate the subsurface soils. At an urban site, I have encountered a crust of decent fill underlain by an apparent stump burial pit. Only subsurface borings would have revealed this condition. As a licensed engineer, I would never issue bearing pressure recommendations soley from the use of a probe rod, particularly for a residential structure in this litigation happy society of today. I wonder how his insurance carrier feels about his approach. Playing devil's advocate here.

 

[redbridge]

Thanks for all your input, it has been very helpful. The contractor has hit solid ground at 4'. So we are thinking of doing a 24" wide footer with 3 runs of #4 bars on bottom and 2 runs of #4 bars on top spaced evenly (designed base on Residential Structural Design Guide). Also the geotech recommended excavating 2.5' below the footer and filling with gravel due to water. Does this design sound ok? Thanks for your help.

 

[concretemasonry]

What you are proosing will be totally adequate for an engineered foundation of most any one story home. I initially assumed the home had a basement. If there is no basement or unbalanced fill you design is far more than adequate. You may run into minor technical problems with the excessively wide footing relative to the tickness depending on the applicable code. If you have a thick footing, you may still comply - that would keep everything in shear and not force reinforcement for flexure that does not have to occur.

If there is a basement and the possiblity of water was noted on the geo-tech report, make sure drain tile is used. You may run into minor technical problems with the excessively wide footing relative to the tickness depending on the applicable code. If you have a thick footing, you may still comply - that would keep everything in shear and not force reinforcement for fexure.

You are in a realm with reasonably good soil conditions where engineering assumptions/calculations far exceed or confict with proven prescriptive/minimum standards.

I assume the home you are designing the foundation for is somewhat out of the norm for residential one story construction.