The discussion has been interesting. I agree that many of The discussion has been interesting. I agree that many of the shallow (and deep) foundation criteria seem a little ‘unbalanced'.
First, some history:
In the middle to late 1950's, expansive clays were becoming a significant problem in Colorado. The Denver area experimented with and moved toward drilled piers. My father, in the Colorado Springs area, experimented with and moved to shallow foundations. A lot of discussion and information sharing occurred at the 1960 ASCE conference in Boulder. In the early to middle 1960's, after a lot of trial and error, the basic criteria for deep (drilled piers) and shallow (narrow to no footings and soil improvement) foundation systems in Colorado had been fairly well determined. Unfortunately, between poor understanding and practice, coupled with some really dull legal decisions, the newer generations of geotechnicals and structurals are floundering.
I have a couple of related complaints about many in my own specialty profession. As sundale notes, too many geotechnical recommendations seem to be made with little thought as to the reasonableness and the actual consequences for design. In cases of expansive soils (excluding desicated, metastable soils), I am also distressed by Allowable Maximum criteria for specific structures which is indefensibly low.
I believe jhoulette has encountered some foundation design criteria similar to:
The foundation may be designed on the basis of an allowable bearing capacity of 3,000 psf maximum, and a minimum dead load of 1,000 psf must be maintained. Contact stresses beneath all continuous walls should be balanced to within + or - 200 psf at all points. Isolated interior column footings should be designed for contact stresses of about 200 psf more than the average used to balance continuous walls. The criteria use for balancing will depend somewhat upon the nature of the structure. Single-story, slab on grade structures and single-story crawlspace structures may be balance on the basis of dead load only. Multi-story structures may be balanced on the basis of Dead Load plus one half live load, for up to three stories.
The above criteria is complete, based upon the original criteria, which used to be typical for the greater Colorado Springs area. Unfortunately, the complete criteria is seldom given and is often poorly applied.
Please note the balancing criteria, DL for single story and DL + 1/2 LL for multistory. The Maximum and Minimum criteria is to be followed. The actual balancing number is to achieve a realistic, working foundation/soil load condition to improve overall foundation performance.
For many years, residential construction could be easily designed and properly balanced if the Allowable Maximum was at least 2.5 times the Minimum Dead Load. In the past 15 to 20 years, the size of residential structures in Colorado and elsewhere, has increase significantly. As a result, the ratio, Max : Min should be increased to 3 to 3.5 OR either Soil Improvement (structural fill) or a deep foundation system should be recommended. It can be surprising how a little Soil Improvement can significantly increase the Allowable Maximum Bearing for smaller structures.
To finish this long read, If the design criteria appears to be deficient for the structure loading, the geotechnical should be asked for clarification. If the issue persists, the geotech should provide real life recommendations appropriate for the structure and the proposed grading (which may have a tremendous effect).
The above criteria is complete, based upon the original criteria, which used to be typical for the greater Colorado Springs area. Unfortunately, the complete criteria is seldom given and is often poorly applied.
Please note the balancing criteria, DL for single story and DL + 1/2 LL for multistory. The Maximum and Minimum criteria is to be followed. The actual balancing number is to achieve a realistic, working foundation/soil load condition to improve overall foundation performance.
For many years, residential construction could be easily designed and properly balanced if the Allowable Maximum was at least 2.5 times the Minimum Dead Load. In the past 15 to 20 years, the size of residential structures in Colorado and elsewhere, has increase significantly. As a result, the ratio, Max : Min should be increased to 3 to 3.5 OR either Soil Improvement (structural fill) or a deep foundation system should be recommended. It can be surprising how a little Soil Improvement can significantly increase the Allowable Maximum Bearing for smaller structures.
To finish this long read, If the design criteria appears to be deficient for the structure loading, the geotechnical should be asked for clarification. If the issue persists, the geotech should provide real life recommendations appropriate for the structure and the proposed grading (which may have a tremendous effect).
DL+LL), as compared to the min/max pressures they give you, that are mathematically impossible to simultaneously satify. With a stemwall or basement wall, you can do an intermittant strip footing to maybe satify the prescribed min/max pressures (concrete weight is thus adjustable), but you can't really do anything with a isolated pad footing.
