It is important that you understand the techniques used by Section Maker to calculate section properties. Before using the program, you should read the remainder of this chapter to ensure you understand how to model sections to get accurate results.

If you create a section that is composed of a number of shapes you should be aware of how Section Maker treats these for calculation purposes.

First, don't partially overlap shapes, the properties won't be computed correctly. You may have a shape completely inside another or completely separate but they may not partially overlap.

Most section properties computed by Section Maker are exact and are based upon the theory of plane areas. However, some properties cannot easily be computed for arbitrary section shapes and so it is necessary for Section Maker to identify a shape as one of the standard shapes. The following properties will only be determined correctly when Section Maker has identified a section as a standard section shape.

§   Shear Areas

§   Torsion Constant and Torsion Modulus

§   Warping Constant

§   Shear Centre and derived properties

IMPORTANT: Non-standard shapes

When Section Maker does not identify the section drawn within the shape window as one of its standard section shapes some properties will not be computed correctly. When installing a section of unknown shape to the sections library the user should review the value of the following properties to ensure their accuracy.

Shear Areas – Approximated as gross area of section

Torsion constant – Approximated as sum of torsion constant of shapes

Warping Constant – Set to Zero

Shear Centre – Assumed to be coincident with centroid

H, r0 – LRFD properties based upon shear centre.

For a section composed of more than one material, Section Maker computes modulus weighted section properties as described in Pilkey. In determining the elastic properties of a section, the modulus weighted concept scales the properties of the individual shapes forming the section by the ratio of the elastic modulus of the shape and the elastic modulus of a reference material.

For example, a reinforced concrete section, Section Maker would default to using concrete as the reference material. In determining the overall properties of the section, the properties of the steel shapes would be factored by the modular ratio Es/Ec where Es is the Young's Modulus of the steel and Ec is the Young's Modulus of the concrete.

The calculation of the plastic properties, such as the plastic modulus, for a composite section uses a similar concept. Instead of scaling the properties of shapes by the ratio of the elastic moduli, the properties are factored by the ratio of the yield strength of the shape to the yield strength of the reference material.