Understanding and Assessing Load-carrying Capacity for Beam Repair

Before embarking on any repair or rehabilitation work for under-designed or deteriorated beams, it’s crucial to calculate their load-carrying capacity accurately. This process involves evaluating the existing dimensions of the concrete member, estimating reinforcement area and concrete strength, and calculating the precise load acting on the structural element.

Step-by-Step Calculation Process

1. Measure Dimensions and Load Distribution

Measure the span of the slab supported by the beam.
Measure the span of the beam.
Estimate live loads on the slab based on building function.

2. Transfer Loads to the Beam

Calculate self-weight of the slab and additional dead loads.
Distribute loads from the slab to the beam based on its type (one-way or two-way).

3. Calculate Load on the Beam

Consider dead load, live load, and additional loads.
Use suitable load combinations per ACI 318-19.

4. Evaluate Beam Properties

Measure beam dimensions: width, depth, and height.
Determine number and size of embedded steel bars.

5. Compute Reinforcement Details

Calculate area of reinforcement.
Determine depth of rectangular stress block and neutral axis.

6. Assess Design Moment

Calculate design moment (Md) and applied moment (Mu).
Compare Md and Mu to determine if the beam needs rehabilitation.

Example: Compute Beam Capacity

Consider a beam with dimensions 250mm width, 380mm height, and 350mm effective depth. Follow these steps:

Loads on the RCC Slab:

Self-weight = 2.4 KN/m²
Live load = 2.4 KN/m² (Office use)
Finishing loads = 0.8 KN/m²
Total dead load on slab = 3.2 KN/m²

Loads on the Beam:

Self-weight = 1.68 KN/m
Dead load from slab = 12.8 KN/m
Live load from slab = 9.6 KN/m
Ultimate distributed load (Wu) = 30.816 KN/m

Compute Applied Moment:

Assume partial fixity of columns.
Applied moment (Mu) = 93.218 KN.m

Geometry of the Original Section:

Width (b) = 250mm
Height (h) = 380mm, effective depth (d) = 350mm
Used Bars: 4 No. 16

Compute Resistant Moment:

Reinforcement area (As) = 804.24mm²
Depth of stress block (a) = 62.33mm
Neutral axis (c) = 73.33mm
Design Moment (Md) = 64.61 KN.m

Conclusion

Since the resistant moment is less than the applied moment, the beam requires improvement to enhance its load-carrying capacity. This comprehensive assessment guides the selection of an appropriate repair method for effective rehabilitation.