Cinder Block Foundation Capacity

[CNCLLC]

I am investigating the possiblity of installing a second floor to a residence supported by a cinder block foundation. Does anyone have any information on the allowable capacities of cinder block so I can determine if the existing foundation will be structurally adequate to support the addition of the second floor.
Thanks

 

[concretemasonry]

I assume the home was built according to a code. Even though you use the term "cinder block" it probably was code compliant. Through the years, residential codes have not changed much, so it may be similar to todays IRC. You should try to find out what model code was effect at the time of construction.

The older codes usually did not have a distinction between one and two story homes for basements although some permited greater heights of backfill or unbalanced soil loads for 2 story home. - The reason being that the vertical load capacity of a besement wall is more than enough for a two story and the extra vertical load reduces the problems with lateral loads.

To be prudent, you should makes several borings to determine the type of soil and capacity. Also verify the thickness and whether it is reinforced or unreinforced. If it has 12" thickwalls, it probably was unreinforced. You should also try to verify the footing width. The standard for most basements was 18" wide, although many 8" and all 12" had at least 20". With these narrow footings it was difficult to require or use lateral steel, but some used longitudinal steel, although many codes dio not require it.

For an addition under the current codes, you may have to reinforce the wall or at least show that the existing structure is equivalent. This will be up to the code representative. It may pay to make an inquiry.

Unless it is a very unusual situation (high walls, very long spans, etc., do not waste time trying to do a detailed engineering analysis, but refer to the 2006 IRC or possibly the Emprical Design section of ACI 530-02. In 40 years, I have seem many engineers to make calculations that agree with the accepted and time-proven IRC standards and I think several went crazy because they made too many "simplifying" assumptions. or did not have the computer time/capacity.

Without seeing the situation, your problem is probably more of showing adequacy under the current load, rather than a complete analysis or re-construction), which may be be feasible.

 

[civilperson]

Normal residential loading on 12" wide foundations under block walls will be fine for three stories or less. The soil capacity is the limiting case. If 2500 psf can be safely assumed on your foundation soil, then go ahead and add a second story.

 

[MiketheEngineer]

IBC says 1500 psf for soils unless a test is done. 12'' doesn't meet minimum of two stories for IBC which is 15'' I think.

Actaul loadings should be checked - this buildng could 300' wide!! - just kidding

 

[concretemasonry]

CNCLLC - This can be made into a big job if it is not just a second floor on a typical 1 story rambler.

MiketheEngineer - This is a residence.

Look at IRC 2006 or ACI 530.02 - Chapter 5, Table 5.6.3.1 for Empirical Design (unreinforced).

The seismic zone must be determined.

Dick