Thumb Rules for Beam Design and Steel Calculation for Multi-Storey Buildings

Designing beams for buildings of varying storeys involves understanding the load distribution, span length, and material strengths. Below are some thumb rules for beam design and steel calculation, including beam sizes based on span length.

Beam Design Thumb Rules

1. Beam Depth:

• Single Storey: Beam depth (D) = Span/20
• 2 to 3 Storeys: Beam depth (D) = Span/18
• 4 to 5 Storeys: Beam depth (D) = Span/16
• 6 to 10 Storeys: Beam depth (D) = Span/14

1. Beam Width:

• Typically, beam width (B) is 0.5 to 0.6 times the depth (D).
• Minimum width should not be less than 225 mm.

Beam Size Based on Span Length

• Span Length (L) = 3 m:
• Beam Depth (D) = 3000/20 = 150 mm
• Beam Width (B) = 0.6 * 150 mm = 90 mm (adjusted to minimum width of 225 mm)
• Recommended Beam Size = 225 mm x 300 mm
• Span Length (L) = 6 m:
• Beam Depth (D) = 6000/20 = 300 mm
• Beam Width (B) = 0.6 * 300 mm = 180 mm (adjusted to minimum width of 225 mm)
• Recommended Beam Size = 225 mm x 450 mm
• Span Length (L) = 9 m:
• Beam Depth (D) = 9000/18 = 500 mm
• Beam Width (B) = 0.5 * 500 mm = 250 mm
• Recommended Beam Size = 250 mm x 500 mm

Steel Reinforcement Thumb Rules

1. Top Reinforcement (Tension Zone at Supports):

• Single Storey: 2 bars of 12 mm diameter (Fe500 steel).
• 2 to 3 Storeys: 2 bars of 16 mm diameter.
• 4 to 5 Storeys: 2 bars of 20 mm diameter.
• 6 to 10 Storeys: 2 bars of 25 mm diameter.

1. Bottom Reinforcement (Tension Zone at Midspan):

• Single Storey: 2 bars of 12 mm diameter.
• 2 to 3 Storeys: 2 bars of 16 mm diameter.
• 4 to 5 Storeys: 2 bars of 20 mm diameter.
• 6 to 10 Storeys: 2 bars of 25 mm diameter.

1. Shear Reinforcement (Stirrups):

• Use 8 mm diameter stirrups at a spacing of 150 mm to 200 mm center to center.

Example Calculation

For a beam with a span of 6 meters in a 4-storey building:

• Depth (D): 6000/16 = 375 mm (rounded to 400 mm for practical considerations)
• Width (B): 0.6 * 400 mm = 240 mm (rounded to 250 mm for practical considerations)
• Recommended Beam Size: 250 mm x 400 mm

Steel Calculation:

• Top Reinforcement: 2 bars of 20 mm diameter.
• Bottom Reinforcement: 2 bars of 20 mm diameter.
• Stirrups: 8 mm diameter @ 150 mm c/c.

Important Considerations

1. Load Calculations:

• Dead Load: Weight of the beam and slab.
• Live Load: Occupancy load.
• Wind Load and Seismic Load: Based on the height and location of the building.

1. Concrete Grade:

• Use M20 grade concrete or higher.

1. Safety Factors:

• Apply safety factors as per local building codes.

1. Building Codes:

• Follow local building codes and standards (such as ACI, IS 456, etc.) for detailed design.

1. Deflection Control:

• Ensure deflection limits are within acceptable limits to avoid excessive sagging.

Final Note

These thumb rules provide a quick reference but detailed structural analysis and design should be performed by a qualified structural engineer using relevant design codes and standards, especially considering the specifics of load distribution, span length, and building use.