Section Modifiers

Normally the section properties are derived from the section geometry, but in some cases it is convenient to modify some of the parameters.

Definition

Bending Axes

The section can be considered to bend about either the local ($y$ & $z$) axes or the principal ($u$ & $v$) axes. So for an asymmetric section restrained by a slab it may be preferable to ignore the $I_{yz}$ term and just have bending about the y & z axes. The subscripts $1$ and $2$ refer to $y$ and $z$ for local axes and $u$ and $v$ for principal axes.

Analysis Reference Point

In defining a section the reference point may be adjusted, but this can be overridden for the analysis if required.

Modifiers

Modifiers can be applied to any of the section properties. The modification can be

by – values modified by a factor

to – values modified to a value

Of significance is the special case of modifying the $K_y$ & $K_z$ values to 0. In this case beam elements are considered as simple beams, rather than shear beams.

Stress Calculation

Applying modifiers to the section means that the stress calculation is not straightforward, so there are options:

• Don't calculate
• Use unmodified properties
• Use modified properties

Miscellaneous

The clear modifiers option returns the properties to standard properties derived from the section.

The simple beams option sets the shear factor to zero so that shear stiffness terms are not included in the calculation.

The multiple sections option is mainly intended for situations where there are several physical sections modelled as a single section property but where the properties are simply a multiple of the base properties, for example a hanger which may comprise two identical bars which act together but don’t interact with each other.

When defining the properties of a section for bending there are three second moments of area, $I_{yy}$, $I_{zz}$, and $I_{yz}$, and three shear area factors, $k_{yy}$, $k_{zz}$, and $k_{yz}$, (often the first two are written as $k_y$, $k_z$). The stiffness matrix is always assembled in principal directions, or local directions ignoring the cross terms.When the bending axes are local the modifiers apply to $I_{yy}$, $I_{zz}$, $k_{yy}$, $k_{zz}$ but when the bending axes are principal these apply to  $I_{uu}$, $I_{vv}$, $k_{uu}$ and $k_{vv}$. So transformation of properties to appropriate axis happens prior to applying the modifiers.