AISC Design Guide 14: Staggered Truss Framing Systems

How to Apply AISC Design Guide 14: Staggered Truss Framing Systems

Mastering the design of staggered truss framing systems is a critical skill for structural engineers aiming to optimize multi-story steel structures. AISC Design Guide 14 provides the foundational knowledge and analytical tools necessary to implement these efficient framing solutions. By understanding and applying the principles outlined in this guide, you can deliver cost-effective, structurally sound, and innovative designs, enhancing your professional credibility and project success.

Before You Begin

Prerequisites:

• • Knowledge: A strong understanding of structural analysis principles, steel design according to AISC specifications (e.g., Steel Construction Manual), and building load calculations (gravity, wind, seismic) as per ASCE 7. Familiarity with diaphragm action and beam-column design is also essential.
• • Tools/Resources: Access to AISC Design Guide 14: Staggered Truss Framing Systems, AISC Steel Construction Manual, relevant structural analysis software, and applicable building codes.
• • Time Required: 4-8 hours, depending on the complexity of the project and the depth of analysis required.

Step-by-Step Implementation

Step 1: Conceptualize and Model the Staggered Truss System

Begin by sketching or modeling the basic layout of your staggered truss system. Determine the spacing and orientation of the trusses within the building plan. Consider how gravity loads will be transferred from floor slabs to the trusses and how lateral loads will be resisted. A common mistake is assuming a uniform load distribution without accounting for concentrated loads at truss support points. Ensure your structural model accurately represents the stiffness and load paths of the system, including the diaphragm action.

Step 2: Analyze Diaphragm Forces and Chord Design

Recognize that the floor system acts as a diaphragm, transferring lateral loads to the vertical bracing elements. Calculate the bending moment (M) in the diaphragm based on the lateral loads and building geometry. Determine the diaphragm depth (D) to compute the diaphragm chord forces (H) using the approximate formula $H = M/D$. Pay close attention to the connection details between the floor deck and the perimeter beams, as these must be capable of transferring the calculated shear flow and chord forces. Avoid neglecting the effect of eccentricities on diaphragm forces, as this can significantly alter chord stress.

Step 3: Determine Vertical and Diagonal Member Loads

Analyze the forces acting on the individual members of the staggered trusses. For vertical and diagonal members, consider all load combinations specified by ASCE 7, including gravity, wind, and seismic forces. Utilize load coefficients as presented in the guide to simplify the application of various load combinations. A typical approach is to apply a live load reduction where permitted. Ensure that the governing load case for each member is identified, as it will dictate the required member capacity.

Step 4: Design Vertical and Diagonal Truss Members

Based on the calculated axial forces (tension and compression) and any applicable bending moments, select appropriate steel sections for the vertical and diagonal members. Refer to the AISC Steel Construction Manual for section properties and design criteria. For diagonal members, the guide provides examples of member selection, such as using HSS sections, based on calculated forces. The design must comply with AISC requirements for member strength and stability.

Step 5: Design Truss Chords Under Combined Loads

Investigate the truss chords for combined axial forces and bending moments. Gravity loads typically induce uniformly distributed moments in the chords. Lateral loads, especially wind, can introduce significant bending moments in the Vierendeel panels of the truss. Carefully combine the forces from gravity and lateral load cases to determine the critical design forces. Ensure that the selected chord sections can resist both axial forces and the maximum bending moments in accordance with AISC Equation H1-1a.

Step 6: Verify Connections and Load Transfer