Beam design is a crucial component of structural engineering. A beam is a horizontal structural element that supports loads applied perpendicular to its length. Proper design ensures the safety, durability, and serviceability of buildings and structures.
What Is a Beam?
A beam is a structural member that primarily resists loads applied laterally to the beam’s axis. Its main function is to transfer loads from the slab to columns and ultimately to the foundation.
Types of Beams (Based on Support)
- Simply Supported Beam – Supported at both ends
- Cantilever Beam – Fixed at one end, free at the other
- Fixed Beam – Fixed at both ends
- Continuous Beam – Extends over more than two supports
Thumb Rules for Beam Design
Thumb rules provide a quick estimate before doing detailed analysis and design. These are especially useful during preliminary design stages.
Beam Depth (D)
- Depth = Span/12 to Span/15
Example: For a 4 m span → D = 4000/12 = 333 mm to 4000/15 = 267 mm
Use a minimum of 300 mm for residential buildings.
Beam Width (B)
- Width = Depth/1.5 to Depth/2
Example: If Depth = 300 mm → Width = 200 mm (approx.)
Clear Cover
- For beams: 25 mm
Minimum Reinforcement
- Main Reinforcement: Minimum 0.2% of cross-sectional area (for Fe 415 steel)
- Shear Reinforcement: Use 2-legged stirrups at 150 mm c/c near supports, 200–300 mm c/c in midspan
Design Loads Considered
- Dead Load (DL): Self-weight of slab, beam, wall, floor finish
- Live Load (LL): Furniture, occupants (as per IS 875)
- Load Factor: 1.5 (as per Limit State Design)
Steps in Beam Design (As per IS 456:2000)
Determine Loads
Total Load on Beam:
= (Dead Load from slab + Live Load + Wall Load) × Span
Example:
- Slab Load = 5 kN/m²
- Span = 4 m
- Width of influence = 1 m
- Wall Load = 13.8 kN/m (230 mm thick × 3 m high)
Total Load = (5 × 1 + 13.8) = 18.8 kN/m
Factored Load = 18.8 × 1.5 = 28.2 kN/m
Calculate Bending Moment
For a simply supported beam:
BM (Mu) = (w × l²)/8
= (28.2 × 4²)/8
= 56.4 kNm
Calculate Effective Depth (d)
Using the formula:
Mu = 0.138 × fck × b × d² → Rearranged to find d
Assume:
- fck = 25 MPa (M25 Concrete)
- fy = 415 MPa
- b = 230 mm
Solving for d:
d = √(Mu / (0.138 × fck × b))
= √(56.4 × 10⁶ / (0.138 × 25 × 230))
≈ 345 mm
Provide overall depth D = d + cover + ½ bar dia ≈ 375–400 mm
Check Shear
Nominal shear stress:
τv = Vu / (b × d)
Compare τv with design shear strength of concrete (τc from IS 456:2000 Table 19). If τv > τc, provide stirrups.
Design Reinforcement
Use:
Ast = (Mu) / (0.87 × fy × jd)
Where jd ≈ 0.9d
Once Ast is found, provide bars accordingly.
Minimum and Maximum Reinforcement (As per IS 456:2000)
Minimum Tension Reinforcement
Ast(min) = 0.85 × bd / fy
Maximum Reinforcement
Should not exceed 4% of gross cross-sectional area.
Common Reinforcement Bar Details
| Beam Size (mm) | Main Bar (Bottom) | Top Bar (Support) | Stirrups |
| 230×300 | 2–12 mm | 2–10 mm | 8 mm @ 150 mm c/c |
| 230×400 | 2–16 mm | 2–12 mm | 8 mm @ 150 mm c/c |
Detailing Guidelines
- Provide crank bars (top at supports, bottom at mid-span)
- Use 45° or 135° hooks in stirrups
- Minimum bar spacing: 25 mm clear
- Use lap length = 50 × diameter (for tension zone)
IS Codes for Reference
- IS 456:2000 – Plain and Reinforced Concrete – Code of Practice
- IS 13920:2016 – Ductile Detailing
- IS 875 (Part 1, 2) – Load Considerations
Beam Design Example Summary
Given:
- Span = 4 m, Width = 230 mm
- Load = 18.8 kN/m
Result:
- Depth required ≈ 375 mm
- Provide a 230 × 400 mm beam
- Use 2–16 mm bottom, 2–12 mm top bars
- Stirrups: 8 mm dia @ 150 mm c/c (near support)
Software Tools for Beam Design
- STAAD Pro
- ETABS
- Autocad (for detailing)
- Prokon / SAP2000
Common Mistakes to Avoid
- Ignoring the effective cover in-depth calculation
- Not checking shear
- Over-reinforcement (steel congestion)
- Not considering load from adjacent elements (walls, slab)
Conclusion
Beam design is both an art and a science. While thumb rules give a quick estimate, proper analysis using IS codes is essential for structural safety. Reinforcement detailing, load calculation, and understanding moment/shear behaviour are key to sound beam design.
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