# Section properties - serviceability limit state (SLS) (Composite beams: EC4 Eurocode)

A value of the short term elastic (secant)
modulus, E_{cm} is defaulted in Tekla Structural Designer for the selected grade of concrete. The long term elastic modulus is determined
by dividing the short term value by a user defined factor - default 3.0. The elastic
section properties of the composite section are then calculated using these values
as appropriate (see the table below).

This approach is used as a substitute
for the approach given in EC4 Equation 5.6 in which a knowledge of the creep
coefficient, ɸ_{t}, and the creep multiplier, Ψ_{L} is required. It
is envisaged that you will make use of EN 1992-1-1 (Ref. 6) when
establishing the appropriate value for the factor.

EN 1994-1-1, clause 7.3.1.(8) states that the effect on deflection due to curvature imposed by restrained drying shrinkage may be neglected when the ratio of the span to the overall beam depth is not greater than 20. This relates to normal weight concrete. Tekla Structural Designer makes no specific allowance for shrinkage curvature but does provide you with a Warning when the span to overall depth exceeds 20 irrespective of whether the concrete is normal weight or lightweight. Where you consider allowance should be made, it is suggested that you include this as part of the 'factor' described above.

Tekla Structural Designer calculates the deflection for the beam based on the following properties:

Loadcase type | Properties used |
---|---|

self-weight | bare beam |

Slab dry | bare beam |

Dead | composite properties calculated using the long term elastic modulus |

Live | composite properties calculated using the effective elastic modulus appropriate to the long term load percentage for each load. The deflections for all loads in the loadcase are calculated using the principle of superposition. |

Wind | composite properties calculated using the short term elastic modulus |

Total loads | these are calculated from the individual loadcase loads as detailed above again using the principle of superposition |