Understanding Steel Beam Design
Steel beams are crucial structural elements designed to support loads in buildings and structures.
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W-Series Beams
Wide flange beams with parallel flanges. Depth ranges from 4" to 44". Most common in building construction. Excellent strength-to-weight ratio.
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Section Properties
Section modulus (S) determines bending strength. Moment of inertia (I) affects deflection. Higher values indicate greater strength and stiffness.
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Load Types
Dead loads: permanent weight (structure, fixtures). Live loads: temporary (people, furniture, snow). Wind and seismic loads may apply.
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Deflection Limits
Typically L/360 for floors, L/240 for roofs. L is span length. Stricter limits may apply for specific uses.
Technical Specifications
Steel Grades
- A36: Fy = 36 ksi (common)
- A572 Gr50: Fy = 50 ksi
- A992: Fy = 50 ksi (preferred)
- A913: Fy = 65 ksi (high-strength)
Load Combinations
- 1.4D (dead load only)
- 1.2D + 1.6L (typical)
- 1.2D + 1.0L + 1.0W (with wind)
- 0.9D + 1.0W (uplift)
Common Spans
Typical maximum spans:
- Floor joists: 20-30 ft
- Girders: 25-40 ft
- Headers: 4-12 ft
- Lintels: 3-8 ft
Connection Types
- Bolted: Field assembly
- Welded: Shop fabrication
- Shear tabs: Common
- Moment: Rigid frames
Design Formulas & Calculations
Bending Moment
M = wL²/8 (uniform load)
Where:
- M = Maximum moment (ft-lbs)
- w = Distributed load (lbs/ft)
- L = Span length (ft)
Required Section Modulus
S = M/(Fb × 12)
Where:
- S = Section modulus (in³)
- M = Moment (ft-lbs)
- Fb = Allowable bending stress (ksi)
Deflection
Δ = 5wL⁴/(384EI)
Where:
- Δ = Deflection (inches)
- E = Modulus of elasticity (29,000 ksi)
- I = Moment of inertia (in⁴)
Load Capacity
w = (8 × Fb × S)/(L² × 12)
- w = Allowable load (lbs/ft)
- S = Section modulus (in³)
- L = Span length (ft)