Foundation Design of a Light Pole 5

[Lina]

I would like to know of any good method to design a light pole 14' heigh over ground surface, 17" bottom diameter, and subject to a wind force of 392lb located at 11' over the ground surface. Do you know of any text book that contains good design examples of similar topics. Uplift design on foundation and overturning due to wind forces.
Thanks

 

[Ron]

Lina...there are several ways you can approach the foundation for this. It can be a square foundation with two considerations:

(1)Mass to resist overturning
(2)Reinforcement for bending

--or--

It can be a small drilled pier foundation designed for the same parameters.

Based on the base size, a 30 or 36 inch diameter pier would be necessary, with the length determined by the necessary fixity to resist the moment at the base and obtaining enough soil resistance (shear through dual projected planes as rotation would theoretically occur).

For the square foundation, not much dependence on soil type as long as you have reasonable bearing capacity (1500 psf or better). With the drilled pier, the resistance to overturning and thus the length of the pier is dependent upon soil conditions.

The vertical loads on either are essentially negligible, but don't forget to include the mass of the concrete. The pier can be designed as a column, but again, you have to determine the fixity of the system.

 

[Polecat]

Lina:

Most textbooks are painfully devoid of this subject, but there are several papers published concerning the overturning resistance of poles directly buried in sand or clay. Many of them are quite old, however, and may not be readily available. Here is a partial listing:

. A Highway Research Board (Washington DC) Bulletin 247 that appeared in 1959.
. ASCE Journal of Soil Mechanics, May, 1964 & 1965 by B.B. Broms.
. Matlock, Hudson & Reese: ASCE Journal of Soil Mechanics in October 1960.
. Timber Construction Manual 1994, American Institute of Timber Construction.
. An excellent ASCE paper by Naik & Peyrot, 1976 on "Analysis and Design of Laterally Loaded Piles."

There are others as well which I can share with you if you need them.

The Polecat

http://www.spiraleng.com

I invite people to visit my website if you wish to discuss these issues further.

 

[dik]

We use couple of methods for designing posts, typically used as sign standards. I've set them up as MathCAD files.

The first (Embedment of Poles, Sheeting and Anchor Piles, M.A. Drucker, Civil Engineering, Dec 1934)

Max passive pressure at base:
K = Kp * GammaS * W / 1000, where Kp - Coef of Passive Soil Pressure, GammaS - weight of soil(pcf), W - width of embedded post(ft)

Y = ((K - H^2) - 2 * F)^2 / (K - H^3 - 6 * F * L), where H - height of post, F - horiz. force at top of pole, L - total length of post

Max Permissible Soil Pressure at Base; Pmax = K * H

Max Soil pressure at Base; Pbase = Y - Pmax

Point of max soil pressure at top; Ht = (K * H^2 - 2 * F) / Y; Ht = if (Pbase <= 0,0,Pbase)

Max soil pressure at top Ptop = K * (H - Ht); Ptop = if (PBase <= 0,0,Ptop)



The second method if from Civil Engineering Handbook by Urquhart, 1940

Pmax = K * H; where K and H are as above

Pbase = (12 * F * H)/(H^2 * W) + (6 * F)/(H * W); Pbase = if(Pbase <= 0,0,Pbase), where F, H, and W are as above

b = (Pbase * W * H^2) / (3 * (2 * F + Pbase * W * H))

Point of max soil pressure at top; a = H - b, Ht = H - (a / 2)

Max soil pressure at top; (a^2 * Pbase)/(4 * b * H)

For more significant structures, I use a set of Reese and Matlock tables that are excellent, albeit, old...

 

[breaks]

Lina,

Another reference that you may want to check out is AASHTO's &quot;Standard Specifications for Structural Supports for Highway Signs, Luminaries and Traffic Signals&quot;. Section 8 deals briefly with foundations and contains a chart (Figure 1.8.2C(4)) titled &quot;Embedment of Posts with Overturning Loads&quot;.

Matt

 

[Qshake]

I'm with Matt (Breaks) on this one. Aside from the simple statics of the problem, the best reference I've seen on the subject is the reference noted by Breaks.

 

[haynewp]

The UBC has information on overturning with pier foundations.