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.
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