Concrete Solutions 09 Predicting the Deflection of Concrete

Concrete Solutions 09 Predicting the Deflection of Concrete Structures in Practice Doug Jenkins - Interactive Design Services

Introduction Everything should be made as simple as possible, . . . but not simpler. Albert Einstein

Introduction l Are the simplified provisions for the calculation of deflections in AS 3600 “too simple” l It depends

When are Deflections Important? l Second order effects l Client expectations l Contract conditions l Code compliance l Aesthetics l Clearances

When are Deflections Important?

Sources of Deflection l l Short term stress-strain and bond behaviour of the concrete and reinforcement. Time dependent behaviour of the concrete. Differential strain effects. Construction sequence and other load sequence effects.

Sources of Deflection l Short term stress-strain behaviour: – Concrete flexural tensile strength. – Concrete tension-stiffening effect. l Time dependent behaviour of the concrete – – Concrete creep Concrete shrinkage Loss of tension stiffening Loss of flexural tensile strength

Sources of Deflection l Differential strain effects. – Differential shrinkage – Differential temperature l Load sequence effects. – – – Handling, transport and erection Propping loads Change in stiffness after overload. Construction loads on buried structures. Timing of composite connections. Effect of varying axial load

Effect of Shrinkage Symmetrical Reinforcement – No Load

Effect of Shrinkage Concrete Shrinkage – de-bonded steel

Effect of Shrinkage Apply compression to steel

Effect of Shrinkage Re-bond steel and release compression

Effect of Shrinkage Apply bending below cracking moment

Effect of Shrinkage Apply bending greater than cracking moment

Calculation of Shrinkage Curvature Apply “negative” prestress to reinforcement

Effect of Shrinkage Moment-Curvature, without and with shrinkage

Effect of Shrinkage l l Shrinkage stresses in the concrete will significantly reduce the cracking moment Shrinkage will cause significant rotations in any asymmetrical section: – – Asymmetrical reinforcement Cracked section

Case Study l l Large span pre-cast concrete arch in the UK (approx. 20 m span) Short term crown deflections under self weight estimated to be about 30 mm Initial deflections consistent with predictions Deflections after 6 month delay to backfill increased to 150 mm

Case Study

Case Study 1. 2. 3. 4. 5. 6. 7. Short term stiffness, gross concrete section As 1, but age adjusted concrete modulus As 2, but using Branson equation As 3, but EC 2, β = 1 As 4, but with Mcr reduced due to effect of shrinkage and differential temperature. As 5, but with β = 0. 5 As 6, but with curvature due to shrinkage included.

Moment-Curvature (long term)

Crown Deflection, mm

Case Study - Conclusions 1. 2. Analysis including all relevant effects matched measured deflections Most significant effects: – – Reduction in cracking moment due to shrinkage and differential temperature Shrinkage curvature

General Conclusions l Critical cases: – l Will much larger than expected deflections have a significant effect on the design? If so: – – Use conservative estimate of concrete flexural tensile strength, reduced by shrinkage and tensile differential temperature stresses. Allow for section curvature due to shrinkage Consider possible differential shrinkage Allow for cumulative second order effects at ULS

General Conclusions l Structures requiring particular attention: – – – Where the maximum bending moment is approximately equal to the concrete cracking moment. Asymmetric beams (e. g. Super-T), especially those subject to hot dry conditions. Construction sequence effects.

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