Steel Design Guide Series: Staggered Truss Framing Systems

How to Apply Steel Design Guide Series: Staggered Truss Framing Systems

Mastering staggered truss framing systems can significantly enhance your capabilities as a structural engineer, particularly for mid-rise residential projects. This system offers a compelling alternative to conventional framing, often leading to shallower floor depths and efficient lateral load resistance. By understanding and applying the principles outlined in the AISC Steel Design Guide Series, you can confidently deliver innovative, cost-effective, and structurally sound designs, positioning yourself as a valuable asset in the competitive engineering landscape.

Before You Begin

Prerequisites:

• • Knowledge: Solid understanding of structural analysis principles, steel member design (tension, compression, bending), lateral load resisting systems, and basic building construction practices. Familiarity with AISC Steel Construction Manual is assumed.
• • Tools/Resources: Access to the "Steel Design Guide Series: Staggered Truss Framing Systems" publication, structural analysis software (e.g., ETABS, SAP2000, STAAD.Pro), steel design software or manual calculation tools, and relevant building codes (e.g., IBC, ASCE 7).
• • Time Required: 4-8 hours, depending on project complexity and familiarity with the system. This includes document review, initial analysis, and member sizing.

Step-by-Step Implementation

Step 1: Conceptualize and Layout the Staggered Truss System

Define the building geometry and identify locations for the staggered trusses. Typically, these trusses span between exterior columns and are oriented to create bays of habitable space. The trusses are usually one-story deep and often align with demising walls. Understand that Vierendeel panels will be required at corridor locations to provide continuity. A common mistake is to overlook the impact of the truss depth on ceiling heights; plan accordingly.

Step 2: Determine Gravity Load Paths and Initial Member Forces

Analyze the gravity loads from floors, walls, and the roof. Trace how these loads are transferred through the floor system (e.g., hollow core slabs acting as diaphragms) to the staggered trusses. The trusses then transmit these loads to the columns. For initial sizing, you can perform approximate hand calculations for the primary truss members (chords and web members) based on tributary areas. Avoid underestimating dead loads, especially for materials like concrete planks.

Step 3: Analyze Lateral Load Transfer and Diaphragm Action

Evaluate how lateral forces (wind and seismic) are distributed to the building's lateral load-resisting system. In a staggered truss system, the floor slabs, often hollow core units, are designed to act as diaphragms. These diaphragms collect lateral forces and transfer them to the staggered trusses, which act as primary lateral bracing elements. Ensure that the diaphragm chords, typically formed by the top and bottom chords of the trusses or structural elements connected to them, are adequately designed to resist in-plane bending forces.

Step 4: Model and Analyze the Staggered Truss Frame

Develop a structural model incorporating the columns, spandrel girders, and the staggered truss system. Accurately represent the truss geometry, member connections, and boundary conditions. Apply all applicable gravity and lateral load combinations as per the governing building codes. Pay close attention to the analysis of forces within the truss members, particularly the diagonals and verticals, which will experience significant axial loads.

Step 5: Design Truss Members and Connections

Based on the analysis results, design the individual members of the staggered trusses (chords, verticals, and diagonals) according to AISC specifications. This involves checking for tensile yielding, rupture, block shear, buckling (for compression members), and combined forces. Simultaneously, design the connections between web members and gusset plates, and between gusset plates and the truss chords. Ensure adequate bolt shear and bearing capacity, as well as weld strength.

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