The maximum allowable deflection of concrete beams and slabs shall be limited in accordance with the usage requirements specified in building codes. Limits were specified based on the span or the relationship between span and effective depth.
Limit exclusion values for slabs and beams are more similar. However, there is a slight deviation in the values. Furthermore, these exclusion limits may vary from draft standard to draft standard.
Deflections to BS 8110
BS 8110 is primarily concerned with limiting the relationship between span and effective depth to keep deflection within permitted limits. BS 8110 Part 1 contains a table of deflection limits. The following values can be considered as allowable span up to effective depth for beams and slabs.
| Support condition | Rectangular section |
| bang | 7 |
| Simply supported | 20 |
| Continuously | 26 |
If the span is greater than 10 m, the span must be multiplied by the effective depth by the modification factor “10/span”. This factor does not apply to cantilever arms and must be verified through calculations.
The allowable support depth ratio should be varied by the factors for tension and compression reinforcement.
The effective span/depth relationships above served to limit the total deflection to span/250. Furthermore, it ensures that the portion of deflection after construction of surfaces and partitions occurs in the span/500 or 20 mm, whichever is smaller.
Using both methods, the maximum allowable deflection of concrete beams must be limited. The article also states: Plate Deflection Provide additional information about a calculation related to deflection testing.
Permissible deflection according to Eurocode 2
Eurocode 2 also limits deflection to span/250 and the effective span-to-depth relationship is used to verify the limits. The calculation method is slightly different to BS 8110 Part 1.
Additionally, deflection caused by the construction of surfaces and partitions is also limited to Span/500.
Furthermore, Eurocode 2 does not provide a table like BS 8110 Part 1, but does provide equations and diagrams to check deflections.
Allowable deflections according to ACI code
ACI 318 specifies the minimum thickness to limit deflection of each element.
The following table can be used to limit the depth of unprestressed beams. This process also allows us to limit the maximum allowable deflection of the concrete beams.
By following the above guidelines and other requirements specified in ACI standards, deflection limitation and deflection calculation can be performed.
