Steel Structures
Structural steel is strong, ductile and fast to build. Learn steel grades and sections, limit-state design (IS 800 / EC3), tension and compression members, beams with plastic moment and lateral-torsional buckling, bolted and welded connections, and fire and corrosion protection.
Skyscrapers, long-span roofs, industrial buildings, transmission towers and many bridges are built in steel. It offers high strength-to-weight, predictable ductile behaviour and prefabrication off-site.
Steel design checks each member and connection for yielding, buckling and fracture, with deflection and vibration limits for serviceability.
An analogy
A steel frame is like a meccano kit: standard rolled pieces bolted or welded together — but each piece must be checked for stretching, squashing, bending and buckling.
Sections and grades
Hot-rolled I-beams (ISMB/IPE/W), channels, angles, hollow sections (SHS/RHS/CHS), and built-up plate girders. Common grades: E250/S275/A36 and E350/S355/A572. Cold-formed light-gauge sections are used for purlins and light framing.
Members
Tension members: gross-section yielding, net-section rupture at bolt holes and block shear. Compression members: flexural buckling governed by slenderness and imperfections (buckling curves). Beams: plastic moment for compact sections, reduced by lateral-torsional buckling if the compression flange isn't restrained; web shear and bearing checks.
Connections
Bolted (bearing or slip-critical/friction grip with HSFG bolts) and welded (fillet, butt). Simple (pinned) connections transfer shear; moment connections transfer bending. Connection design often governs cost and erection speed.
Stability, fire and fatigue
Frames need bracing or moment connections for lateral stability; second-order effects matter for sway frames. Steel loses ~50 % strength near 550 °C — protected by intumescent paint, boards or concrete encasement. Bridges and crane girders are checked for fatigue at welded details. Corrosion protection by galvanising or paint systems.
Formulas
- Tension yielding:
T_dg = A_g f_y / γ_m0 - Beam (laterally supported):
M_d = β_b Z_p f_y / γ_m0 - Compression (buckling):
P_d = A_e f_cd, f_cd from buckling curves (a–d) - Bolt in shear:
V_dsb = (f_u/√3) · A_nb / γ_mb (γ_mb = 1.25) - Fillet weld:
strength = (f_u/√3) · 0.7s · L / γ_mw
Worked example: Laterally restrained beam
Given: I-section with Z_p = 650 × 10³ mm³; E250, γ_m0 = 1.10, β_b = 1
Find: Design moment capacity.
- M_d = 650 000 × 250/1.10 = 147.7 kN·m
Answer: ≈ 148 kN·m
Worked example: Bolt shear capacity
Given: M20 grade 4.6 bolt (f_u = 400 MPa); A_nb = 245 mm², single shear, threads in plane
Find: Design shear capacity.
- V = (400/√3) × 245/1.25 = 45.3 kN
Answer: ≈ 45 kN per shear plane
Common mistakes
- Wrong: Using full plastic moment for an unrestrained long beam. — Right: Check lateral-torsional buckling and reduce capacity.
- Wrong: Ignoring net area at bolt holes. — Right: Check net-section rupture and block shear for tension members.
Applications
- Industrial buildings: Portal frames, crane girders and trusses.
- High-rise: Steel or composite frames with braced cores.
- Towers & bridges: Lattice towers, truss and box-girder bridges.
Key takeaways
- Check yielding, buckling and fracture for every member.
- Beams: M_d = β_b Z_p f_y/γ_m0 (if restrained).
- Columns: slenderness and buckling curves govern.
- Connections, fire and corrosion protection are part of design.
Going further
- Composite construction: Shear studs make concrete slabs act with steel beams, raising stiffness and capacity.
- Plate girders: Deep welded girders with stiffeners carry long-span loads; tension-field action boosts shear capacity.
- Seismic steel: Special moment frames and eccentrically braced frames dissipate energy through controlled yielding.