Increase Presumptive Soil Bearing UBC>IBC

[SteveGregory]

The UBC 97 code allowed you to increase the soil bearing values in the Table 18-1-A by 20% per foot up to 3 times the value in the table (footnote 2). The UBC values are slightly more conservative than the IBC 2000/2003 values in Table 1804.2.

The IBC 2000/2003 codes omit this provision. Is the concept used in the UBC code not valid? Or perhaps, the increase could be modified to use with the IBC values.

I am designing a flagpole nonconstrained footing (IBC 1805.7) for a pipe rack.

Any opinions?

 

[DonPhillips]

You can use UBC if that is the code in effect where you are designing, otherwise it the current IBC in effect. IBC requires you to provide data to the building official otherwise the presumptive values is what you are permitted. I say data because there may be a nearby soil report for a recent project that the building official may accept.

Don Phillips

http://worthingtonengineering.com

 

[SteveGregory]

The IBC 2003 code applies to this project. However, my experience and common sense tells me that the allowable soil bearing values should increase as the footings go deeper. This is what the UBC trys to account for in footnote 2 (from previous post) and the IBC seems to ignore.

This project has a short fuse. I would like to use a higher bearing value for my design with a deeper footing. Then, I was going to have the contractor hire a soils engineer to verify my design value.

Does this sound reasonable? Perhaps a 15% increase instead of a 20% increase per foot would be more like it.

 

[BigH]

Why don't you have a geotechnical engineer provide the infomration on the allowable bearing pressures rather than using presumptive values? In using presumptive values, one must be cognizant of the permissible settlements that they permit (is it 40mm or 25mm or 15 mm???); is the value based on shear (strength) issues or deformation? The presumptive values do not take into account soil layers (you might have the stress bulbs intersect two distinct and perhaps widely divergent soil types/behaviour); what combination of live and dead loads are inherent in the values given? Typically presumptive bearing pressures have to be ultra-conservative. For all projects, geotechnical engineers should be consulted for this information based on investigation and local experience/knowledge. Cheers.

 

[BigH]

Forgot to address one of Steve's points. It is not necessarily true that the net allowable bearing pressure will increase with depth of footing. Say you have a 5 m layer of dense sand over soft clay. If your footing is founded at, say 0.5m and is of a size less than 2m square, the whole of the pressure bulb is within the sand layer and the increase of stresses in the soft clay are so small that they will not cause consolidation settlement of the normally consolidated clay. However, suppose you found your 2m square footing at a depth of 2m. Then your pressure bulb will significantly stress the upper part of the soft clay layer and an increase of settlement will result (assuming that the increase of strength is greater than the preconsolidation pressure). In this case, your allowable bearing pressure will have to be lower so as not to exceed service level (permissible) settlements.

 

[SteveGregory]

As stated in my second post, our job schedule does not permit us the option of get a soils engineer during the design phase. Therefore, I must use the presumptive tables and I will require the contractor to hire a soils engineer to verify my assumptions.