Deflection in beams

A Comparative Study of IS: 465, ACI- 318, AS:3600 and

Euro code 2 -Deflections in Beams

Samirsinh P ParmarAsst. Prof. DD University, Nadiad Gujarat, INDIAMail: [email protected]

Samirsinh P Parmar, DDU, Nadiad, Gujarat, India

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Limit State of ServiceabilityThe sections obtained by the Limit State of

Collapse are smaller compared to the Working Stress Method

Therefore, they must be checked for the width of cracks and the deflections

Important for the appearance, efficiency and durability of the structure


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Relevant Clauses in IS456Clause 35.1.1

It stipulates that the design should consider all relevant limit states to ensure an adequate degree of safety and serviceability.

Clause 35.3.1Deals with the Limit State of Serviceability in

DeflectionsClause 35.3.1

Deals with the Limit State of Serviceability in Cracks


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Short- and Long-term Deflections Short-term deflection refers to the immediate

deflection after casting and application of partial or full service loads

Long-term deflection occurs over a long period of time largely due to shrinkage and creep of the materials


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Factors Affecting Short Term deflections:(a) magnitude and distribution of live loads, (b) span and type of end supports, (c) cross-sectional area of the members, (d) amount of steel reinforcement and the

stress developed in the reinforcement, (e) characteristic strengths of concrete and

steel, and (f) amount and extent of cracking.


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Factors Affecting Long Term Deflections(a) humidity and temperature ranges during

curing, (b) age of concrete at the time of loading, and (c) type and size of aggregates, water-cement

ratio, amount of compression reinforcement, size of members etc., which influence the creep and shrinkage of concrete.


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IS 456- Calculation of Short Term Deflection[Clause C-2 Annex C]The code recommends the usual methods for elastic deflections using the short-term modulus of elasticity of concrete Ec

and effective moment of inertia Ieff given by the

following equation:


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IS 456- Deflections due to Shrinkage

k3 =0.5 for cantilevers, 0.125 for simply supported members, 0.086 for members

continuous at one end, and 0.063 for fully continuous members; ψcs is shrinkage curvature= k4εcs/D, εcs is the ultimate shrinkage strain of concrete

εcs= 0.0003 in the absence of test data [clause 6.2.4.1 of IS 456 ]


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IS 456- Deflections due to Creep


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IS 456- Control of DeflectionsClause 23.2 of IS 456 stipulates the limiting deflections

under two heads as given below: The maximum final deflection should not normally

exceed span/250 due to all loads including the effects of temperatures, creep and shrinkage and measured from the as-cast level of the supports of floors, roof and all other horizontal members.

The maximum deflection should not normally exceed the lesser of span/350 or 20 mm including the effects of temperature, creep and shrinkage occurring after erection of partitions and the application of finishes.

It is essential that both the requirements are to be fulfilled for every structure.


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ACI318-Calculation of Short Term Deflection[Clause 9.5.2.3]

The code recommends the usual methods for elastic deflections using the short-term modulus of elasticity of concrete Ec

and effective moment of inertia Ie given by the

following equation (Code provisions based on the “Branson Equation” ): <Ig


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ACI 318- Deflections due to Shrinkage and Creep[9.5.2.5]

The long term deflections due to the shrinkage and creep shall be determined by multiplying the immediate deflection caused by the sustained load, by the factor


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ACI 318- Deflections due to Shrinkage and Creep[9.5.2.5]

Multipliers for long term deflections


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ACI 318- Control of Deflections[Clause 9.5.2.6]


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AS 3600 and AS 5100- Code provisions for deflections


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Code provisions for deflectionsEurocode 2 and CEB-FIP 1990 (MC 90)

Members which are expected to crack should behave in a manner intermediate between the uncracked and fully cracked conditions and, for members subjected mainly to flexure, an adequate prediction of behaviour is given by Expression (7.18):


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Code provisions for deflectionsEurocode 2 and CEB-FIP 1990 (MC 90)


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Code provisions for deflectionsEurocode 2 and CEB-FIP 1990 (MC 90)Shrinkage curvatures may be assessed using

Expression (7.21):


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Thank You

Deflection in beams

Technology

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