# Properties : Section

All beam type elements refer to a section defining the geometrical properties (e.g. area, second moment of area) referenced by the element or member through the property number. The set of sections in the model form the Section Library. Individual sections in the section library can have Section Analysis Properties that are modified for analysis, e.g. reducing the J value so that the element doesn't attract torsion. Sections are then grouped in Section Pools.

## Basic definition

The basic definition of a section includes:

**Name and Colour**

These are used to identify sections and show elements or members which refer to these.

**Material**

The material definition breaks into three parts. The first part is the material type. This is one of:

- Steel – allows a set of properties for various grades and gives option to assign to various steel design properties to the section
- Concrete – allows a set of properties for various grades
- FRP – allows a set of properties for various grades
- Glass – allows a set of properties for various grades
- Aluminium – allows a set of properties for various grades
- Timber – allows a set of properties for various grades
- <undefined> – not associated with any grade

The grade defines the actual material design properties. These depend on the material type. New grades can be initialized from the standard grades for the current design code. The final material attribute is the analysis material. This defines a set of elastic properties. If a grade is specified it is possible to refer to this for the analysis material (from Grade). In this case the required analysis properties are inferred from the grade.

**Profile, Reference Point and Point Voids**

The profile defines the shape and geometry of the section. These can be either catalogue (steel) sections, standard section shapes, or shapes defined by geometry (perimeter, or line segments). By default the reference point is at the centroid of the section. In this case an axial force applied at the centroid results in no bending. It some cases it can be convenient to change the reference point. The other change to the section geometry is to add point voids. This allows (small) ducts to be included in the section. This is also used in conjunction with bridge beams to identify locations for prestress tendons – in this case the voids are assigned a zero area.

**Rate**

This associates a unit cost against sections.

**Modified**

This is a flag to indicate that there are section modifiers associated with this section.

**Pool**

The section can be assigned to a pool. This groups sections for design.

## Section Properties

These are the actual section properties of the chosen section, so for example a 400×200mm rectangular section would have an area of 0.08m².

## Modifiers

Each base section property can be modified so that a different value is used in the analysis. So for example the user may want to increases the axial stiffness of the 400×200mm rectangular section defined above, so he can either factor the value or replace the value with a new value. So for example a factor of 2 on the area of the section would give an analysis value of 0.16m².

Modifiers on the $I$ and $k$ values apply to either the $xy$ or $uv$ values depending on the axes selected for the stiffness calculation.

If the section is modified then GSA will not calculate section stresses until it is stated whether the modified or unmodified properties are to be used.

## Analysis properties

The analysis properties are the base section properties modified as above. These are the values used in the stiffness calculation. Note that the modified area may or may not be used to determine the mass or gravity load, so in the above example the stiffness would be based on an area of 0.16m² but the mass and gravity loads would be based on either an area of 0.08m² or 0.16m².

**Bending axes**

The section is either assumed to bending in the local $y$ and $z$ directions independently or for non-symmetric sections the principal bending directions $u$ and $v$ can be used.

**Area**

The modified area of the beam cross-section for analysis.

**$I_{11}, I_{22}$**

The second moment of area of the beam about the $yy$ and $zz$ axes for the local axis option or $uu$ and $vv$ axes for the principal axes option.

**J**

The torsion constant for the section; note this is not the polar moment of inertia unless the section is circular.

**$k_{11}, k_{22}$**

The shear factor for shear about the yy and zz axes for the local axis option or uu and vv axes for the principal bending axes option. Note the principal bending axes is not the same as the principal shear axes. It is the proportion of the area of the beam which is effective in carrying shear (i.e. the [Shear Area]/[Total Area]). This can be defined as zero if the shear deformations are to be ignored.

## Use of section data in standard (GSS) analysis

Bending action is about the principal axes unless the user has chosen for the stiffness calculation in the “uv axes” of the section term. For all symmetric sections principal and local axes are coincident.

The shear centre is assumed to be at the section centroid.

## Use of section data in GsRelax analysis

The following points are only valid if the GsRelax solver is used.

If a section shape has been specified, the effect of plasticity on the interaction between axial force and biaxial bending is modelled. The net bending moment is limited to the plastic capacity of the section for the current axial force and direction of applied moment. The bending stiffness of a section is halved at first yield.

If explicitly defined section properties are specified, the interaction between axial and bending yield is ignored and the plastic section moduli used are approximations.

Bending action is in local element directions; the principal directions of the section are ignored.

Shear action is only modelled elastically and the shear centre is assumed to be at the section centroid.