Loads and use of BIS codes – Design of riveted and welded connections – Design of structural steel members in tension, compression and bending

Definition

Structural steel design is the engineering practice of calculating the required dimensions and configuration of steel members to safely support specific loads (dead, live, and environmental) by complying with Bureau of Indian Standards (BIS) codes, primarily IS 800:2007.

Main Content

1. Loads and BIS Codes

Dead Loads (IS 875 Part 1): Self-weight of structural elements, finishes, and permanent attachments.
Live Loads (IS 875 Part 2): Transient loads resulting from occupancy, furniture, and movable equipment.
IS 800:2007: The primary code for general construction in steel, providing the Limit State Design (LSD) methodology for safety and serviceability.

2. Riveted and Welded Connections

Riveted Connections: Mechanical fasteners used to join plates and sections. Though largely replaced by bolts, they are studied for historical restoration and heavy assembly.
Welded Connections: Fusing metal surfaces using heat. Common types include Fillet welds (triangular cross-section) and Butt welds (full penetration).

3. Structural Members

Tension Members: Structural elements subjected to axial tensile forces (e.g., bracing in trusses).
Compression Members: Elements carrying axial compressive loads (e.g., columns). They are susceptible to buckling.
Bending Members (Beams): Members subjected to transverse loads causing bending moments and shear forces.

       Tension Member           Compression Member
      <--- Force ---            --- Force --->
     [============]            [============]
      --- Force --->            <--- Force ---
                                 (Buckling risk)

Working / Process

1. Determination of Design Loads

Calculate characteristic loads based on the occupancy and usage of the building.
Apply partial safety factors (e.g., 1.5 for Dead Load + Live Load) as per IS 800 to determine the design load (Factored Load).

2. Connection Design

Determine the number of fasteners (rivets/bolts) or length of weld required based on the shear and tension capacity of the connection.
Ensure the connection satisfies the "block shear" criteria, where the member might tear out along a failure path.

3. Member Proportioning

Tension: Ensure the net effective area is sufficient to prevent yielding of the cross-section or rupture at the connection.
Compression: Calculate the effective length and radius of gyration to determine the slenderness ratio; ensure it stays within code limits to prevent buckling.
Bending: Design beams to resist maximum bending moment (governed by section modulus) and shear force (governed by web area).

Advantages / Applications

Strength-to-Weight Ratio: High strength per unit weight makes steel ideal for multi-story buildings and long-span bridges.
Uniformity: Factory-produced steel sections are highly predictable and consistent in mechanical properties.
Ductility: Steel can undergo significant plastic deformation before failure, providing a vital safety warning in overstressed conditions.

Summary

This unit covers the essential principles of structural steel design using the IS 800 code, focusing on load estimation, connection integrity through welding/riveting, and member stability under various stress states.

Limit State Design (LSD): A design philosophy ensuring safety and serviceability.
Buckling: The sudden failure of a compression member under high axial load.
Effective Length: The distance between points of zero moment in a member.
Yield Strength: The stress level beyond which steel undergoes permanent deformation.