Spida™ Web SoftwareAbout the Analysis
SpidaCalc is a comprehensive utility pole analysis tool. The analysis technique that is used in the program is an inverse stiffness matrix finite element analysis. This technique breaks down the structure into pieces that can be easily modeled with known geometric and material properties. This model amounts to a large and complex system of springs, which are combined into something called a stiffness matrix or [K]. Having a spring representation of the structure can be quite useful. If the spring constant of a spring is known and the force acting on that spring is also known, then the movement of the spring can be calculated by using simple algebra. The springs in the stiffness matrix are called elements and they are joined at places called nodes. Once a model of the structure is constructed, the loading is summed at each of the nodal points in the structure. These forces are arranged into a force vector or {F}. Once the [K] and {F} are known the displacements, or {x} can be solved for. In this algebraic process the inverse of [K] or [K]-1 must be calculated, giving this technique its name. Once {x} is found, stress and loading values can easily be backed out using classic strength of materials techniques.
The model in SpidaCalc includes all of the structural components of a utility pole: including the pole, guys, anchors and push braces. Guys are included in the stiffness matrix only if they are placed in tension, because of their nonlinear behavior. The pole taper and changing wind area are also easily accounted for in the analysis.
The loading vector is determined from physical properties of the structure and its attachments and specified load conditions. A couple examples of loading due to physical properties would be the tensions from wires and moment additions from equipment. The physical data for these inputs come from a customizable database. Loading from specified conditions would include wind forces on the pole, wires and equipment as well as possible ice loading. The specific values for the load conditions come from the NESC (National Electric Safety Code) and vary depending on location in the country.
An additional feature of SpidaCalc is the manner in which wire tension values are determined. The tension of a strung wire is a well understood property; it is based on gravity and the distance the wire has sagged. Using these two properties the horizontal tension can be calculated. However the sag and weight of a wire can change due to temperature change and ice loading, this will in turn change the horizontal tension in the wire. An iterative solver is used to determine the actual tension at every load condition and for all conductors in the analysis. This step eliminates the use of look up tables and more accurately represents the tension of each wire under load.
SpidaCalc uses the above procedure for calculating the stress in the structure
and it does it for every possible wind direction. Meaning, the model is
updated, the forces are generated and summed and the stresses are calculated
and stored at each of the 360 possible wind loading directions. This step
allows us to determine the true worst case scenario for every structural
component, which is a valuable tool for diagnosing the true health of the
structure. In addition to the stress in the structural components, the shear
and moment forces in the pole are also calculated and stored to assist in
design. All of these features allow for a useful and robust analysis tool.