Soil Presumptive Load-Bearing Values per IBC 2018 table 1806.2

[Mike Mike]

I'm designing a sonotube footing for a large sign per IBC 1807.3: Embedded Posts and Poles, which references table 1806.2 for presumptive soil capacities. I understand everything in the IBC, but all of my software programs ask for a "Maximum Lateral Pressure Limit", defaulting to 1500psf. Does anyone know where this comes from? I would expect the only input to be "Allowable Lateral Passive Pressure", since nothing I'm reading in the IBC seems to put a ceiling on the allowable lateral pressure.

 

[Celt83]

the third column, "Lateral Bearing Pressure", in Table 1806.2 is the ceiling on lateral pressure.

Further for an embedded post, the S1 value is capped at the value corresponding to a 12ft embed depth.

As an example using the lowest presumptive value of 100 psf/ft for the lateral pressure and an embeddment of 16 ft:
S1 = min(100*16/3 , 100*12/3) = 400 psf

The basis of the IBC embeddment function is a modification of the method shown in the Patterson - Pole Building reference on SlideRuleEra's site: Link

EDIT:
Should add this also depends on what your software is doing, if it's doing a full LPile analysis then you should work with a Geotech on the soil values for use in the program. AASHTO also has another method for pile embeddment in Structural Supports for Highway Signs, Luminaires, and Traffic Signals, 1st Edition.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[Mike Mike]

Thanks Celt, I just did some digging and 1806.3.3 seems to cap it at 15ft. Did the old codes use to cap it at 12ft?

 

[Celt83]

equation 18-1 sets the 12 ft limit in the definition of "d"

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[Mike Mike]

Ah! So if presuming 100psf/ft, the cap is 400psf for unconstrained, 1200psf for constrained, and 1500psf for all other elements? Seems dumb of my software to default to 1500psf for a flagpole module.