The fundamental difference between a line load and an area load lies in how they are applied and, crucially, how they are adjusted based on the supporting structure's spacing.
Understanding the Primary Distinction
According to standard engineering principles and as highlighted in the provided reference, the key difference is:
- Area Distributed Load: This type of load is applied over a surface area (e.g., pounds per square foot, psf). When applied to supporting members like joists or beams, this load is spacing-sensitive. It is converted or adjusted based on the spacing of the supporting members.
- Line Load: This type of load is applied along a linear element (e.g., pounds per linear foot, plf). It acts directly on the supporting member along its length and is not adjusted based on the spacing of supporting members or tributary width.
How Loads are Adjusted
Area loads are adjusted to determine the load effectively carried by each individual supporting member. This adjustment depends on the type of member:
- For members like joists, which are typically spaced at regular intervals (on-center spacing), the area load (psf) is multiplied by the on-center spacing to calculate the equivalent line load (plf) acting on each joist.
- For members like flush or drop beams, which support loads from a wider area, the area load (psf) is multiplied by the beam's tributary width (the width of the area the beam is responsible for supporting) to calculate the equivalent line load (plf) acting on the beam.
In contrast, line loads are already defined per unit length and are applied directly to the member. They represent a concentrated load along a line, such as the weight of a wall resting on a beam or the load from railings. Because they are already in a linear format, they do not require further adjustment based on spacing or tributary width of the member they are applied to.
Summary Table: Line Load vs. Area Load
To further clarify the distinction:
| Feature | Line Load | Area Load |
|---|---|---|
| Application | Applied along a line (plf) | Applied over a surface area (psf) |
| Spacing-Sensitive | No | Yes |
| Adjustment Method | No adjustment for spacing/tributary width | Adjusted by on-center spacing or tributary width to determine equivalent line load on supporting members |
| Represents | Load acting directly on a line | Load distributed over a surface, transferred to linear supports |
Understanding this difference is crucial in structural analysis to accurately determine the forces and stresses on individual building components.
