Transmission And Substation Foundations - Technical Design Manual (TD06088E)

The DL for an SLF application is so small that a foun- dation sized to resist V and M will typically be much more than adequate to resist DL. Therefore, DL will not control the SLF design and will not be consid- ered here. If DL is large enough to be of concern for an application where an SLF will be used, it may be evaluated based on bearing capacity equations ap- plied to the soil around the helical bearing plate and friction along the shaft. These evaluations are beyond the scope of this design example, which will only deal with SLF applications. Since SLF products are used as lighting foundations along public highways, it is appropriate to men- tion the American Association of State Highway and Transportation Officials (AASHTO) publication Standard Specifications for Structural Support for Highway Signs, Luminaires and Traffic Signals. This document is often taken as the controlling specifi- cation for jobs using SLF’s and will be referenced throughout this discussion. SLF Selection The SLF selection process is a trial and error proce- dure that may require more than one iteration. First, select an SLF diameter based on the applied bending moment (M) that must be resisted. That is, ensure that the applied moment is less that the allowable moment on the shaft. Determining the allowable mo- ment requires a structural analysis of the pipe shaft section capacities (often based on a reduced cross section through cable ways, bolt slots, base plate size, welds, etc). This effort should be familiar to en- gineers engaged in design work, so a sample of this process will not be given here. The design or selection of a foundation size to resist light pole loads in a given soil may be determined by various methods. Numerical methods using finite element and finite difference techniques may be used but have proven to be somewhat sophisticated for the rather simple SLF application. The Fourth Edition of the AASHTO specification lists a number of pre- liminary design methods that can be employed in the design process. Among those listed and discussed are the methods developed by Bengt B. Broms for embedment lengths in cohesive and cohesion- less soils and a graphical method dealing with the embedment of lightly loaded poles and posts. The Broms Method will be used for this design example as experience has shown these methods to both use- able and appropriate. Calculations are provided for both cohesive soil (clay) and cohesionless soil (sand).

wp =Wind Pressure EPAlf = Effective Projected Area of a Light Fixture EPAp = Effective Projected Area of a Light Pole Hlf = Moment Arm to EPAlf Centroid

SLF REACTIONS Vlf = [EPAlf x wp] Vp = [EPAp x wp] V =Vlf +Vp M = [Vlf x Hlf] + [Vp x Hp]

EPAlf

DESIGN EXAMPLES

Hlf

EPAp

DL

Hp

M

V

Pole Load Diagram Figure 7-30

Foundation in Cohesive Soil Figure 7-31

Page 7-47 | Hubbell Power Systems, Inc. | All Rights Reserved | Copyright © 2017