Lateral Soil Bearing Pr, Unconstrained Soils and Foudations 1

[Engineerrrd]

Has anyone tried to use Equation 18-1 out the IBC 2006 for unconstrained soils and foundations to determine lateral bearing strength?

I tried using it and found it to be very conversative. I have an 8' cantilever post with a concentrated load of 375 lbs at the end. And according to the equations, it concludes that a 1' diameter footing 3' deep will not restrain the lateral load (I believe it suggest a ~12' deep footing). Did I miss something here?

Please let me know if you have used this in a design along with any comments or concerns you have about it.

Thanks.

 

[msquared48]

This equation is nothing in the codes (IBC, AITC, IBC) in this form and has been used for many years.

The 3 to 4 foot embedment with a 1 foot diameter footing seems reasonable to me. If you are getting a 12 foot embedment, I would look to your units to start. Otherwise, look to the lateral soil bearing you are using and the computation of the "S" value.

Mike McCann
MMC Engineering

 

[RWF7437]

MSquared48, Did you mean "nothing NEW in the codes" ?

Enginerrred,

Three references you may find useful are:

Pole Building Design by Donald Patterson, 1957. and

Archon Engineering's WinPost ( about $40 )available at Sales at archonengineering.com

and Chart for embedment of Posts, Outdoor Advertising Association of America ( circa 1960 ) which has been reprinted in many structural engineering handbooks.

good luck

 

[msquared48]

RWF7437:

Yep...thanks.

Mike McCann
MMC Engineering

 

[LobstaEata]

I get a 5 foot embedment considering allowable passive pressure of 250 psf. A 12 ft embedment results if allowable passive pressure is reduced to only 25-30 psf, about a factor of 10 difference.

 

[Engineerrrd]

Thanks guys for the advice.

LobstaEata,

Can you confirm that we are using the same variables? In my orginal calcs I used S1 = 100 psf (conservative due to non-testing), but in my calcs below I tried to match your results using S1 = 250 psf.

I have:
b = 1 ft
d = 3 ft (assumed)
h = 4 ft
P = 375 lbs
S1 = 250 psf

Resulting in:
A = 3.51 ft
d = 6.04 ft (required)

How did you get a 5 foot embedment? Can you let me know what we did differently?

 

[RWF7437]

http://www.slideruleera.net/miscellaneous.html

 

[LobstaEata]

Engineerrrd

One of the variables "h" was considered to be 8 ft per your original post, rather than 4 ft on the lastest post.

"I have an 8' cantilever post with a concentrated load of 375 lbs at the end."

Attached are output results for variables entered using our Enercalc module. It appears that if you are using 2 ft of inactive depth, which adds to the value taken from the Ruttledge Chart, then your embedment depth is pretty close. However, it seems that Enercalc may only consider 1 ft of inactive soil when considering engagement of passive pressure.

For the minimal additional cost, I'd use the 6 ft depth regardless and call it a day.

Hope this helps.

 

[miecz]

The IBC formula for poles is quite conservative. Note that 1805.7.2 allows "other methods approved by the building offical". If you have a reinforced concrete shaft, section 13 of AASHTO's Standard Specification for Structural Supports for Highway Signs, Luminaires and Traffic Signals has a method for "Drilled Shafts" that results in much lower embedment lengths, in your case, less than 4 feet. AASHTO 13.6 defines "Drilled Shaft" as "cast-in-place concrete and may include deformed steel reinforcement..."

The AASHTO method is identical to the method proposed in the Final Report for the Connecticut DOT "Foundation Design Methods for Poles and Towers" by Healy and Long, December 1973, School of Engineering, University of Connecticut.

 

[miecz]

Another thought. I'm reading IBC 2003 as I don't have 2006. The second paragraph of Article 1804.3.1 contains verbiage about doubling lateral bearing values for isolated poles. Although 1804.3.1 is titled "Increases in allowable sliding resistance", I think this second paragraph should have been separate from 1804.3.1, in which case it may apply to your case.