# 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.