What are Structural References?

Datum References

The most common way to establish references is the Cartesian grid reference. When using this system, we generally consider the following:

• We place the origin $(0,0)$, usually at the bottom-left point of the structure. 
• The x-axis runs horizontally through the origin, while the y-axis runs through the origin perpendicular to the x-axis.

In this way, all values are in Quadrant 1 (all are positive).

Global and Local References

There are two types of references: global and local. The difference lies in how we see the structure as a whole.

To illustrate, let's consider a rigid plane frame $ABCD$.

When we analyze a structure globally, we interpret it as a whole. It typically means that the origin is at the bottom-left point of the structure. In this case, it would be at point $A$.

From origin $A$, we can properly identify the location of other joints $B$, $C$, and $D$ relative to point $A$. Usually, we use this type to solve for the reaction supports in the classical method (more on structural analysis).

When we analyze a structure using local references, we break it into its elements and establish the datum per component.

In our rigid frame example, let's break it into members $AB$, $BC$, $CD$, and joints $A$, $B$, $C$, $D$. After breaking it into its components, we can create our local reference for each. For example:

• In member $AB$, we let the origin be at $A$, and the x-axis goes from $A$ to $B$
• In member $BC$, we let the origin be at $B$, and the x-axis goes from $B$ to $C$
• In member $CD$, we let the origin be at $C$, and the x-axis goes from $C$ to $D$
• In joint $A$, we let the origin be at $A$, and the y-axis parallel with member $AB$
• In joint $B$, we let the origin be at $B$, and the x-axis parallel with member $BC$
• In joint $C$, we let the origin be at $C$, and the x-axis parallel with member $BC$
• In joint $D$, we let the origin be at $D$, and the y-axis parallel with member $CD$

These references are limited to the components of the structure; hence, they're named "local." These are beneficial when we want to analyze the forces, stresses, and deflection per member.

Summary

Let's summarize:

Placing a grid reference to identify critical points is necessary when solving structures.

The most common way to establish one is the Cartesian grid reference.

There are two types: global and local.

When we analyze globally, we interpret the structure as a whole.

When we analyze locally, we break a structure into its elements and establish the datum per component.