Creep and dependencies

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StruSoft Fredrik L
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Creep and dependencies

Postby StruSoft Fredrik L » 18 Nov 2017, 15:00

The different values of concrete creep that we will look at are:
  • Final creep-value
  • Effective creep-value

The final creep-value will be used in serviceability limit state-combinations (SLS) in Frame Analysis. While the effective creep-value will be used in ultimate limit state-calculations (ULS) for structures using second order theory (EN1992-1-1 5.8.4). Effective creep-value are therefor not of interrest for SLS calculations, only for ULS design calculations.

But the calculation of effective creep-value can be done in two different ways:
  1. Manually enterring the creep value. Can only be done if the ULS-load combination has no dependencies
  2. Automatic calculation of effective creep. Needs dependencies from a long term SLS load combination, and the final creep value.

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1. Manually enterred effective creep value
Define an ULS-load combination and set the dependencies to "-", this will open up an option to manually enter the effective creep-value under Input geometry > Input > Concrete input... > Long term parameters. Calculate the value using EN1992-1-1 5.8.4, formula (5.19) then enter the value under concrete long term parameters.

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2. Automatic calculation of effective creep value
If a chain of dependencies has been defined with at least one long term combination and ending in at least one ULS combination, then automatic calculation of effective creep value will be performed for the structure. Enter the creep-value under Input geometry > Input > Concrete input... > Long term parameters, calculated by either EN1992-1-1 3.1.4, EN1992-1-1 Annex B or by using the Longterm concrete calculation-app on StruSoft Portal

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In ULS this will be of utmost importance if second order analysis has been checked in the calculation. If a first order analysis are being performed a fictive second order moment according to the stiffness method or curvature method are being calculated for each member, so it is not irrelevant for that analysis either.

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ULS combinations should not be dependant of eachother as this will give wrong values, and they should only be in the end of a chain of SLS-combinations. It is even possible to have several SLS-combinations dependant of eachother, as shown in the picture below (taken from PRE-Stress software). Note that the ULS-combinations end each possible chain and that no SLS or ULS-combinations are dependant of any ULS-combination

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So in general should the SLS combinations be before the ULS combinations. the preparations in the program to calculate the effective creep value by comparing stresses in the current combination compared to previous combinations. If, for example, there is a chain with two long term combinations. In the first long term combination the creep value (φ_1) for that time and loading is calculated and entered in the first row of the long term parameters. When the calculation reaches the second long term combination, the increment for the creep from first long term combination to the second one are being calculated (Δcreep) should be entered. The current creep value for the second combination will now be φ_2=φ_1+Δcreep, so it is important to not enter the calculated creep value in each step as φ_1+φ_2 etc will be wrong. This creep-value will be the basis for the effective creep value, calculated by the program. At the moment (Frame Analysis 6.04.026) it is not possible to see this effective value in the print-out for each element, but it is considered to present this in the result-printout.
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Fredrik Lagerström
Technical Support and Developement
StruSoft AB

Telephone: +46 (0)40 530 100
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