Interrupted Footing Design 2

[jhoulette]

I'm curious if many of you have been given design criteria from geotechnical engnineers that specify a maximum and minimum soil pressure outside the state of Colorado?
If so, how do many of you design the footings when you can not meet the maximum or the minimum?
For example: you have a maximum of 3,000psf and minimum of 1,000psf soil bearing and you have to design a 12" deep square footing for a column load of 10,240lbLL and 2,560lbDL. Say you run with a 2'-0" square pad, which gives you a max load of 12,000lb and a min of 4,000lb. I have a total load of 13,400lb (600lb for footing) and a dead load of 3,160lb. I fail at the max and the min.
I know I can increase the size of the footing, maybe a 2'-3" square pad and then increase the depth to gain dead load. But this is for a residential single family house were cost of excavation and materials needs to be kept to a minimum. I welcome any advice.

 

[rgerk]

I have never seen a max and min for soil pressure. I understand your concern about residential construction costs, but don't let cost keep you from using what works. From a liability standpoint, I always make sure I use the recommendations from the geotech when I have them. That way, I can't be liable for soil issues for deviating.

A 2'-3' sq x 1' deep footing would work and 3" increase shouldn't increase cost very much.

Just my 2 cents.

 

[SERaymond]

Is important to consider the most critical case, I recommend to use the 1,000psf soil bearing capacity for the footing designs to guarantee security and prevent future settlements problems. In my opinion the use of most critical situation is a good guidance in combination with the codes recommendations.

 

[jhoulette]

Thank you rgerk for your comment, however a 2'-3" x 1' deep footing doesn't meet the minimum dead load. You would only have 650psf of dead load when you need 1,000psf pressure on the soil.
The min and max is very common here in Colorado. Typically, you can be given 2,500/800 or 3000/1000 or even 4000/2000 as your design criteria. The minimum is to help prevent movement from low swelling soils.

 

[eric1037]

So, if I understand it correctly, you would need the footing to be at least 2 feet thick to make up the extra load. Assuming unit weight of concrete to be 150 lb/ft^3.

I guess the only other way would be to use tie-downs like helical piers or drilled shaft foundation. Again, there would be extra cost for this.

 

[COEngineeer]

It sounds like the geotech should have recommended pier foundation and use mostly skin friction to support total load. There is no way you would meet the minimum DL if you have continous footing supporting exterior load that doesnt carry any floor or roof load. If you dont need that many piers sometime its more cost beneficial to just use helical piers.

 

[civilperson]

Design for equal dead load pressures and do not exceed the DL + LL maximums. Ignore the minimums or ask the geotech what basis this minimum is founded on.

 

[COEngineeer]

Probably it is because of expansive soil.

 

[jhoulette]

yes expansive soil. Thats why I feel that I should meet the minimum soil pressure. Also most of the houses that have problems are due to pads heaving up, not settlement. Last week I had to lower a steel beam supporting the main floor due to the footings heaving.
I don't what to use a drilled pier or helical due to the rest of the house being supported by spread footings. You gain enough deadload from the foundation walls and with adding voids in the footing to concentrate the load on the footings. It's just these isolated interior square pads that just don't have much dead load compared to the live load.
Some geotechs are putting into the reports that the footings sould be designed using full dead load and only half the live load, can they do that? It seems that would be against the code.

 

[COEngineeer]

Cant help you there jhoulette. I never design a house with spread footing when it's expansive. I feel that you are the engineer so you are the person to decide what to put. I would put drilled piers, or helical piers if you only have a few piers.

 

[jhoulette]

I should clarify, this is low expansive material. With many subdivisions the lot may even be over-excavated to get away from having to use drilled piers or helical piers.

So far it sounds like this is yet another goofy thing Colorado engineers do.

 

[sundale]

I have run across this a couple of times with Colorado geotech reports prescribing a min/max allowable bearing pressure on mildly expansive soil.

What these guys fail to realize is that, basically, the dead to live load ratio is SET for a wood framed house (or most any structural system). Floor DL = 10 psf, roof DL = 15 psf, floor LL = 40 psf, roof SL = 30 psf (Denver @ 5280'), etc...

There are certain ratios of DLDL+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.

I always call the geotech and ask them which provision, the min DL or the max TL, they would rather me violate. Writing a detailed phone log of the conversation or even getting something from them in writing amending their report is not a bad idea either.

 

[emmgjld]

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).

 

[rgerk]

jhoulette,

I guess I missed the part about meeting the minimum pressure using only dead load. That would mean making the footing thicker to meet the criteria. I agree with the others about getting clarification from the geotech. He bears a responsibility in giving you criteria that can be met in a specific design application.

Good luck.

 

[COEngineeer]

he will need to make it really thick to achieve minimum DL. Might as well make the pad smaller and go deep like it was a caisson and utilize the skin friction.

 

[emmgjld]

I am not sure how my post came to be repetitive and garbled. I apologize