The primary difference between the top chord and the bottom chord of a truss lies in their position within the structure, the loads they typically carry, and the types of stresses they are designed to resist. They are fundamental components of a single truss unit, not different types of floor trusses themselves.
Based on the provided reference:
- TOP CHORD: An inclined or horizontal member that establishes the upper edge of a truss. This member is subjected to compressive and bending stresses.
- BOTTOM CHORD: The horizontal (and inclined, ie. scissor trusses) member defining the lower edge of a truss, carrying ceiling loads where applicable.
Key Distinctions Summarized
Here's a breakdown of the main differences between the top chord and the bottom chord in a typical floor truss configuration:
| Feature | Top Chord | Bottom Chord |
|---|---|---|
| Position | Forms the upper edge of the truss. | Forms the lower edge of the truss. |
| Typical Loads | Supports the floor loads (people, furniture, walls) from above. | Supports ceiling loads (drywall, insulation, light fixtures) from below. |
| Primary Stress | Primarily experiences compression (pushing force) and bending. | Primarily experiences tension (pulling force). Can also carry bending. |
| Function | Carries loads down to the supports via web members; forms the top plane. | Resists the spreading forces from the top chord and web members; forms the lower plane. |
| Reference Note | Establishes the upper edge; subjected to compressive and bending stresses. | Defines the lower edge; carries ceiling loads where applicable. |
Understanding Their Roles
In a floor truss, both the top chord and bottom chord work in conjunction with the internal web members (vertical and diagonal supports) to efficiently transfer loads to the supports (like walls or beams).
The Top Chord
As the upper boundary, the top chord directly bears the weight placed on the floor above it. This load pushes down on the chord, putting it under significant compression. Because the load is often distributed along its length (e.g., from floor sheathing or joists), it also experiences bending stresses between the points where the web members connect.
- Practical Insight: The size and material strength of the top chord are crucial for supporting the floor load and ensuring structural stability.
The Bottom Chord
The bottom chord acts primarily in tension. As the top chord is compressed and attempts to shorten, the web members push or pull on the bottom chord, trying to pull it apart. The bottom chord resists this pulling force. In cases where the space below the floor is a finished ceiling, the bottom chord also carries the weight of the ceiling materials.
- Practical Insight: The bottom chord must be strong enough to resist the tensile forces generated by the load on the top chord. Any splices or connections in the bottom chord are critical as they must be able to transfer these tension forces effectively.
In essence, while both are essential structural members of a truss, their location and the primary forces they withstand are fundamentally different, enabling the truss system to efficiently span open spaces.
