I am attempting to design a small foundation for a light pole. The pole is 4" square and 25' tall and must with stand a 120mph wind. Need help please.
Usually, light poles are supported by small, drilled concrete piers of sufficient diameter to allow proper bolting of the pole to the pier.
The design involves considering the lateral forces on the pier and the desired amount of lateral deflection that can be tolerated. A geotechnical engineer can help with this in terms of allowable stresses on the soil and determining an appropriate soil modulus to measure its stiffness.
Your local municipality may also have some set standards for light poles, traffic poles, etc. that you could utilize.
See Gaylord & Gaylord, "Structural Engineering Handbook". Look under signs in the index. They have a chart for drilled piers for light poles.
Use the wind code to determine wind coefficients/drag forces that will impact on the pole - Usually a manufacturer can offer some data on this issue.
Design foundation for resultant shear and bending forces such that foundation contact area is always under positive (compression) pressure over entire area.
Bearing capacity of coastal soils which are usually sandy is best determined by the standard penetration test (SPT) method. Care must be taken with the proximity of the water table.
Also check out the AASHTO Publication "Standard Specifications for Structural Supports for Highways Signs, Luminaries and Traffic Signals". It has a lot of information that will help you out.
