One of the components of a structural model is the member. Before discussing the properties of a structural member, we need to define its references.

## Longitudinal and Transverse Axes

Imagine a straight member of length \(L\) with a rectangle shape throughout its length.

Now, this member would have a *longitudinal axis* that runs from one end to the other. For a member with symmetrical sections, this axis is a straight line that runs through the *centroid of the member's shape*. Using a 3D-Cartesian grid reference, let's designate it as the \(x\)-axis. Perpendicular to it would be the \(y\)- and \(z\)-axes which are called the *transverse (or lateral) axes*.

There are several properties associated with its longitudinal axis.

## Cross Section

Imagine taking a knife and cutting a plane surface along the member. The exposed shape is what we call the cross-section. We can cut any member along any plane; however, when we refer to the cross-section, these are cuts made perpendicular to the member's longitudinal axis.

Likewise, there are cross-sectional properties we need to consider when designing structures.

## Summary

There are two critical axes: (1) longitudinal and (2) transverse.

The longitudinal axis runs from one member's end to the other.

The transverse axes are generally the reference axes perpendicular to the beam's longitudinal axis.

The cross-section of a structural member is the plane surface exposed after placing a cut section.

The cross-section is usually the perpendicular cut to the beam's longitudinal axis.