Evaluating Existing Spread Footings

[ToadJones]

I am evaluating some existing spread footings for possible increased capacity.
The original drawings show a few locations where the bearing pressure is 9000 psf.
This seems extremely high.

When I check the existing footings for increased loads I am getting numbers in the 10,000 psf area. The footings would have to be on bedrock to satisfy these conditions and I am quite certain the building is not on bedrock which makes the original value of 9000 psf on the drawings seem absurd as well.

What were engineers using for allowable bearing values in the 1940's?
Is there any chance a soils test was done as this time?

Without a soils test now, am I ok to use the presumptive values of IBC Table 1806.2?

Does a soils test even help in evaluation of existing footings?

 

[msquared48]

I guess I would have to ask, is there any distress in the building area around these 9000 psf footings?

If not, then, assuming these footings were loaded as designed, perhaps the original data is good.

If there is damage, then the deduction is obvious.

I assume that the footings are in a confined location?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[JAE]

The 9000 psf does sound high - but can you get a new boring/analysis done near an existing footing to see what is really there? Over the last 70 years there probably has been a lot of long term consolidation (especially if it is clay) - and an increase in bearing capacity.

 

[ToadJones]

Mike-
The short answer is, yes, the building shows signs of distress that I am assuming are the result of differential settlements.

This leads to another question I guess...

A considerable amount of the loading on the columns is from overhead cranes. In some cases, the crane loading can double the foundation load. Should I be considering this crane loading as transient when combined with D & L loading?

 

[msquared48]

There are load combinations to use in conjunction with partial live loads, snow, and seismic loadings, if applicable. Usually they include full crane loading with partial other, and vice versa. Check ASCE7.

I also agree with JAE, but I am also concerned about the distress, not necessarily caused from overloading, but perhaps from vibration.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ToadJones]

I guess what I was hinting at was maybe the original design did not incorporate the crane loads the way I would do now.

I get very reasonable bearing pressures without the crane loading.

Following the combinations I typically check with cranes now, they arent even close.
One combiantion in particular is simply DL + LL + Live Roof + MULTIPLE CRANES.

This combination is a killer, but is also not very likely to occur very often.

 

[msquared48]

Are you suggesting that the crane was added later?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[SteelPE]

IBC 2009 section 1605.3 exception #1: Crane hook loads need not be combined with roof live load or with more than three-fourths of the snow load or one-half of the wind load.

Are these the original cranes placed in the building. Maybe someone upgraded the cranes and never bothered to check the footings?

 

[ToadJones]

With these buildings, cranes are added, upgraded, removed etc all the time.

I am 100% certain that the foundations were never upgraded.

SteelPE-
Thank you for the note.
I thought I had seen this before, but I am often forced to use load combinations by AIST also.

 

[ToadJones]

actually, this exception will only help a little as my snow loads are almost as high as the live roof loads.

 

[msquared48]

Sounds like you may have to add additional columns and footings along the crane line - the owner will love this suggestion.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ToadJones]

Yea, they'll be so excited to have a nice new runway for their cranes!

Owners of these buildings more often than not just proceed to beat the hell out of the building as long as product is going out the door.

They also love to call when something is just about to collapse and ask "how could this happen!?".
To which I usually reply "Well, when an inspection or analysis shows items that are grossly deficient and there was a recommendation to fix or replace the item.....

 

[SteelPE]

What kind of distress is the current structure showing? You mentioned previously about differential settlement. Is this all?

If you are running into differential settlement on the crane lines I imagine the would be putting unnecessary wear on their cranes..... which I imagine are not easy or cheap to fix.

If they are having problems with premature wear..... maybe you could approach the client and say that this will help them "get more product out the door" because they should not have to fix their cranes as much. Down time now means less down time in the future.

Also, how would adding more columns help with the footing problem? Unless the spans are large adding more columns might not have the desired effect.

 

[ToadJones]

Steel-
Its just like many old early 1900's industrial buildings. They are beat up. They seen their useful life IMO. There is many more problems than the settlements which themselves are rather apparent by simply looking down the runway girders and seeing the up and down.
The cranes do see unnecessary wear from the structure being out-of-tolerance, if you will. Fixing these structures is not cheap or easy. There is often machines, foundations, piping and utilities all over the place, etc. Even simple structural steel reinforcement can be a real challenge at times.

The line about new columns and footings was more or less a joke as it is simply not feasible.

 

[Splitrings]

I have seen where a geotechnical engineer increased the allowable bearing pressure by 20% for each foot of depth, up to a maximum of 3 times the list value in the IBC table. I have never seen in the IBC or anywhere else where this is stated. I should ask him. If this is acceptable, than a class #3 material with a listed value of 3000 psf would have a maximum allowable bearing pressure of 9000 psf at a depth of slightly greater than 6'.

 

[ToadJones]

20% of depth based on what? consolidation?

 

[Splitrings]

I would guess consolidation and lateral confinement. I will ask him.

 

[ToadJones]

ill be curious to hear what he says

 

[Splitrings]

It may be a day or two before he gets back to me. The IBC does state that the listed values are for "near the surface".

 

[azcats]

The 20% increase was also footnote #2 in the 1997 UBC table 18-I-A. I don't believe it's in the IBC codes but the presumptive values are increased somewhat in the IBC table.