Maximum allowable deflection in concrete beams and slabs needs to be limited as per the serviceability requirements specified in the design standards. The limiting values have been given based on the span or the span and effective depth ratio.
The value of the limiting deletion for slabs and beams are more similar. However, there is a slight deviation in the values. Further, these limiting values of deletion could vary to design standard to standard.
Deflections as per BS 8110
BS 8110 mainly concern about limiting the span over effective depth ratio to maintain the deflection within the allowable limit. BS 8110 Part 1 provides a Table indicating the limiting values of the deflections. The following values could be considered for the beams and slabs as allowable span over effective depth ratios.
| Support Condition | Rectangular Section |
| Cantilever | 7 |
| Simply Supported | 20 |
| Continuous | 26 |
If the span is greater than 10m, span over effective depth shall be multiply by “10/Span” which a modification factor. This factor shall not be applied for cantilevers and it shall be checked by calculations.
The allowable span depth ratio shall be modified by the factors for tension and compression reinforcements.
The above span over effective depth ratios was for limiting the total deflection to Span/250. Further, this ensures the part of deflection occurs after constructing the finishes and partitions to Span/500 or 20mm, whichever is lesser.
Following either method, the maximum allowable deflection in concrete beams needs to be limited. Further, the article, deflection of slab provide more information on a calculation related to the deflection check.
Allowable Deflection as per Eurocode 2
Eurocode 2 also limits the deflection to Span/250 and span over effective depth ratio is used to check the limits. The method of calculation is somewhat different from the BS 8110 Part 1.
Further, it also limits the deflection that occurs due to the construction of finishes and partitions to Span/500.
Further, Eurocode 2 does not provide a table as BS 8110 Part 1, but it provides equations and charts to check the deflections.
Allowable Deflections as per ACI Code
ACI 318 specifies the minimum thickness to limit deflection for each element.
The following table could be referred for limiting depths of nonprestressed beams. By this process also we can limit the maximum allowable deflection in concrete beam.
Following the above guidelines and other requirements stipulated in the ACI standards, limiting deflection and calculation of the deflections can be done.
