3.1. Types of Trusses

Common Roof Trusses: Pratt, Howe, Fink, Warren, Fan, North-light—selected based on span, loading, and aesthetic requirements.

Common roof trusses are structural frameworks designed to support roofs over buildings. There are various types of trusses such as Pratt, Howe, Fink, Warren, Fan, and North-light. The choice of which truss to use depends on several factors:

• Span: The distance the truss needs to cover without support.
• Loading: The types of loads, such as dead loads (weight of the truss and roofing materials) and live loads (people, snow, etc.), that the truss needs to support.
• Aesthetic requirements: The appearance of the truss can be important depending on the architectural style of the building. Each truss type has unique geometric properties that affect these factors.

Imagine a bridge designed to carry cars. Different bridge types (like arches or beams) are chosen based on how far the bridge must stretch over the river (span), how many cars it needs to hold at once (loading), and how it looks to passersby (aesthetic). Similarly, choosing a roof truss involves considering span, load, and style.

Loading Cases Dead Load - DL - Self-weight of truss members, roofing material, purlins, and cladding. Imposed Load - LL - Service load (maintenance, snow, etc.), distributed as per roof pitch and use. Wind Load - WL - As per windward/leeward action; includes uplift, downforce, and drag.

These are the three primary types of loads that act on roof trusses:

• Dead Load (DL): This is the weight of the truss itself along with materials like the roofing (shingles, panels), purlins, and cladding. It's a constant load that the truss must support.
• Imposed Load (LL): This refers to temporary loads that can vary, such as people (during maintenance), equipment, snow, or even rain. These loads depend on the roof's pitch and specific use.
• Wind Load (WL): Wind impacts the roof differently on the windward side (the side facing the wind) and the leeward side (the side away from the wind). Wind can create uplift (lifting forces), downforces (pushing down), and drag (pulling across). Trusses must be designed to withstand these dynamic forces.

Think about a trampoline. The weight of the trampoline mat itself is like the dead load. If children jump on it, that's similar to an imposed load—they add weight when they use it. On a windy day, the wind pushing against the mat is akin to wind load. Just as a trampoline needs to be sturdy enough to handle these different forces, roof trusses must be designed to support all types of loads.

Load Transfer Purlins transfer roof loads to truss joints (panel points). Truss members transfer forces to supports.

Load transfer in roof systems is crucial for ensuring stability and safety:

• Purlins: These are horizontal beams that run along the roof and collect the roof loads, directing them down to the truss joints (the connections where truss members meet). They act as a bridge for the loads.
• Truss Members: The structural elements of the truss itself, such as the top chord, bottom chord, and web members, transfer the forces they receive from the purlins down to the building's supports, like columns or walls. This transfer ensures that loads are evenly distributed and do not cause structural failure.

Consider a chain of people passing buckets of water in a relay. Each person (purlin) carries the bucket and hands it off to the next person (truss member) until it reaches a designated spot (the support). Each link in the chain must work effectively to ensure the buckets of water (loads) are delivered successfully without spilling.

Structural Analysis Methods Method of Joints: Solves equilibrium at each pin joint to find axial forces in members. Method of Sections: Cuts through the truss to solve for forces in selected members using equilibrium equations. Software-based Analysis: Large or complex trusses often analyzed using structural analysis software for accurate force distribution.

Analyzing the forces within a truss is essential for ensuring its safety and performance. There are several methods for this:

• Method of Joints: This technique involves looking at each joint (connection) of the truss and applying the principles of equilibrium (where all forces balance out) to determine the force in each member connected to that joint. This allows engineers to find internal forces.
• Method of Sections: In this method, the truss is 'cut' at a certain section, and engineers then use equilibrium equations to solve for forces in the members that are included in this section. This is useful for finding forces in specific members without analyzing the entire truss.
• Software-based Analysis: With larger or more complex trusses, engineers often resort to software tools that can simulate and accurately calculate force distributions based on various load conditions and truss configurations.

Think of a team analyzing a sports play. In the method of joints, it's like examining each player's position and role during the play to see how they contribute. The method of sections is akin to focusing on just a few key players to understand their specific contributions to the play. Meanwhile, using software is like using video analysis tools to break down every detail, making it easier to see what's happening from various angles.

--