huge differences in allowable bearing capacities? 1

[fixxxer0]

I am at a loss to why I am finding such huge differences between allowable soil bearing capacities in the IBC vs NYC building code.



See the International Building Code table 1804.2 in the below PDF:

http://www-scf.usc.edu/~mosheikh/ibc_chap18.pdf

Table 1804.2 clearly shows gravels as 3000 PSF.

vs.

NYC building code (based off of the UBC):

http://www.nyc.gov/html/dob/downloads/bldgs_code/bc27s11.pdf

Here Table 11-2 Note#4 shows gravels ranging from 12000 to 20000 PSF.

The NYC building code is about the same order of magnitude higher than IBC across the board for all soil/rock types. Footing sizes using IBC could be up to 7x as large!


Anyone know why, and which one should a shallow foundation be designed with?

 

[fixxxer0]

ignore that mention of the "(based off of the UBC)"

I can't seem to find how to edit my post.

 

[msquared48]

You finally found the reason why New Yorkers are such hard nuts to crack!

Sorry, but personally, I have no clue here why the difference of that magnitude...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Ron]

The IBC uses a lot of generalizations and cannot get nearly as specific as a local code that reflects local, known conditions. Same with wind loads and other provisions. In Florida, we have a wind map for each county, showing more closely the conditions that cannot be reflected in the ASCE 7 or building code maps.

 

[fixxxer0]

I figured it was something about being on the more conservative end of things... but the differences were so huge that I figured it must be something else.

NYC is the governing code for the project so obviously that is what I would use, I was just nervous about such large discrepancies.

 

[MiketheEngineer]

Test it - then use that

 

[fixxxer0]

the soil is going to be tested, i was just doing prelim stuff from the code when i noticed the ibv vs nyc thing.

obviously in the end, testing is the going to give the most realistic bearing capacity numbers.

 

[woodman88]

New York State land area equals 54,556.00 square miles.
Untied States land area equals 3,803,290.00 square miles.

Or it could just be that the New York code officials feel more comfortble with their numbers due to the land area (and the testing in this area of the soils done) it is being applied over than the IBC people.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[hokie66]

Agree that it is because of the relatively small area of New York City and the wealth of information which exists about the foundation conditions. woodman88, NYC itself is a lot smaller than that.

 

[dgillette]

Could it be that IBC includes consideration of settlement, in addition to bearing, but NYC is only for actual bearing capacity (with settlement requiring separate analysis)?

Try using Terzaghi or other bearing theory, assuming phi'=35, gamma=125 pcf, 3 ft wide strip footing, embedded 3 ft below grade. Does 3000 make sense w.r.t. bearing theory?

 

[BigH]

@dgillette - you've made the salient point. Based on my gray cells, I thought that the NYC code is allowable from a settlement point of view. Need to look at the code and the little notes at the bottom of the table to figure it out. Also, different jurisdictions might well have different permissible settlements. Usually, I/we have always used 25 mm (1 inch); yet in India, it is 40 mm. The OP should also know that different projects have different tolerances on settlements that are not addressed in building codes.

 

[hokie66]

Using presumptive bearing pressures is always risky unless there is good local knowledge of the founding materials. That being said, if the IBC says gravels are only good for 3000 PSF, I would like to know where they would find gravels that are that poor.

 

[FixedEarth]

In Glaciated states, 3 to 5 ksf bearing pressure is common. Sine NY soils are mostly overconsolidated, settlement would not be an issue at say 3 to 4 ksf bearing pressure, specially given a 3 to 4 ft embedment. We can not do this in CA soils.