Design of Steel Structures (IS 800)

Steel design in India follows IS 800:2007 (limit state method). GATE asks about member capacities, plastic analysis and connections.

Partial safety factors

γ_m0 = 1.10 (yielding), γ_m1 = 1.25 (ultimate fracture), γ_mb = 1.25 (bolts), γ_mw = 1.25 (shop welds).

Tension members

The design strength is the least of:

• Gross section yielding: T_dg = A_g f_y/γ_m0.
• Net section rupture: T_dn = 0.9 A_n f_u/γ_m1 (for plates).
• Block shear at the end connection.

Compression members

• Slenderness ratio λ = KL/r. Limits: 180 for members carrying dead and live load, 250 for members carrying only wind/earthquake reversal, 350 for tension members that may reverse.
• Euler stress f_cc = π²E/λ².
• IS 800 uses column buckling curves (a, b, c, d by section type) to give the design compressive stress f_cd, and P_d = A_e f_cd.
• Effective length factor K: 0.65 (both ends fixed), 0.8 (fixed–pinned), 1.0 (pinned–pinned), 2.0 (fixed–free).

Beams

• Laterally supported beam: M_d = β_b Z_p f_y/γ_m0 (plastic or compact sections, β_b = 1); limited to 1.2 Z_e f_y/γ_m0 for simply supported beams.
• Shear: V_d = A_v f_y/(√3 γ_m0), with A_v = web area (h t_w).
• Unsupported beams are checked for lateral torsional buckling.

Plastic analysis

• Plastic moment M_p = Z_p f_y. Shape factor S = Z_p/Z_e:

• Collapse loads (from mechanism method):
• Simply supported beam, central load: W_c = 4M_p/L.
• Fixed beam, central load: W_c = 8M_p/L.
• Fixed beam, UDL: w_c = 16M_p/L².
• Propped cantilever, UDL: w_c = 11.66 M_p/L².
• Load factor = collapse load/working load.

Connections

• Bolts in shear: V_dsb = f_u A_nb/(√3 γ_mb) per shear plane; bearing V_dpb = 2.5 k_b d t f_u/γ_mb.
• Fillet welds: throat t = 0.7 s; design strength f_wd = f_u/(√3 γ_mw).

Worked example

A rectangular section 100 × 200 mm (bending about the strong axis), f_y = 250 MPa.

• Z_e = 100 × 200²/6 = 666,667 mm³; Z_p = 100 × 200²/4 = 1,000,000 mm³.
• Shape factor = 1.5; M_p = 250 kN·m.

Practice