Understand load combinations

In PRE-Stress the default option is to have dependencies between each stage in the life cycle of an element. With these dependencies the program remembers cracks, stresses and strains from the previous stages, as it is in real life. One other criteria is that the concrete materials can't be of a lower grade in a later stage of the dependency chain, as the concrete will mature over time. Each combination has its own purpose in order to most accurately model what might happen with an element. Another key feature is that a load combination can't be dependant on a ultimate or accidental limit stage, as the element will be considered to have failed at that stage.

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Load combinations

Release

This is the first stage, it is supposed to simulate the point in time when the prestressing strands are cut in the factory. The concrete grade should be set to reflect this moment. 

Time: Less than or equal to 1 day after casting.

Load combination, Serviceability Limit State (SLS), Characteristic (EN 1990 6.5.3 (2) a):

Ed = 1.0 ⋅ Gk (6.14)

Storage/Maturing

The second stage in the dependency chain. Now the element is moved out to the storage in or near the factory to mature or await transport to the construction site. Concrete grade should now have increased a few notches from the casting.

This is the first combination where the initial long-term parameters should be enterred, most dominant is the addition of the autogenuous shrinkage, as the drying shrinkage hasn't really started and only a minor part of the creep and relaxation has occured.

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Time: About 1 week after casting

Load combination, SLS, Quasi-permanent (EN 1990 6.5.3 (2) c):

Ed = 1.0 ⋅ Gk (6.16)

Transport

Transport from the factory to the construction site.

concrete grade may have improved a little bit from previous stage, but may not have reached its final value yet.

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Time: About 2 weeks after casting

Load combination, SLS, Characteristic (EN 1990 6.5.3 (2) a):

Ed = 1.0 ⋅ Gk (6.14)

Transport - ULS

Not a default load combination, but if you want to look at the transport-phase as a ultimate limit state, then add a branch off the main chain to simulate this.

Time: About 2 weeks after casting

Load combination, ULS, Lift/dynamic (CEN/TR 15728:2016):

Ed,dyn = ψdyn ⋅ Gk ⋅ γ (Eq. 5.2a and Table 5), note that national regulations may give different values.

  • Swedish default values in PRE-Stress: ψdyn = 1.4 (mobile crane) and γG = 1.35, giving a factor: 1.4 ⋅ Gk ⋅ 1.35  = 1.89 ⋅ Gk

Erection

Erection of the building on the construction site. If temporary supports are needed in the construction phase they can be added as unsupported joints.

If there is a topping on the element it should be added as wet concrete in this stage. Loads acting in this phase may be related to the construction, but also installations and loads of machines. The live load of the structure should not be applied in this phase as the structure has not been taken into service yet.

Time: About 3-4 weeks after casting, may vary from project to project.

Load combination, SLS, Characteristic (EN 1990 6.5.3 (2) a):

Ed = 1.0 ⋅ Gk + 1.0 ⋅ qk,construction (6.14)

Intermediate stage (Short term)

The purpose of this stage is to get a short term load before the long term load, in order to get the biggest service load before the long term loadcombination and to develop the cracks in the parts of the element that will be fully cracked in those regions in the final stages.

Time: About 1 year after casting.

Load combination, SLS, Frequent (EN 1990 6.5.3 (2) b):

Ed = 1.0 ⋅ Gk + ψ1 ⋅ qk,1 + ψ2 ⋅ qk,i (6.15)

Final Stage (Long term)

This is the final long term combination. At this stage the fully developed creep, shrinkage and relaxation should be considered. Crack widths may be checked to see that they do not exceed the allowed crack widths according to the code.

Time: 2 days before design working life L20, L50 or L100 (20 years, 50 years or 100 years)

Load combination, SLS, Quasi-permanent (EN 1990 6.5.3 (2) c):

Ed = 1.0 ⋅ Gk + ψ2 ⋅ qk (6.16)

Final Stage (Short term)

This stage is the proper final short-term combination, and it is in this stage the deflection should be considered.

The load combination of this stage should be the same as Intermediate stage (short term), so analysis forces should be the same (moment, shear force, torsion etc.). But due to the addition of the long term parameters the deflection will be different, as this stage will consider the deflection with fully matured concrete, fully developed creep, shrinkage and relaxation.

Time: One day before design working life.

Load combination, SLS, Frequent (EN 1990 6.5.3 (2) b):

Ed = 1.0 ⋅ Gk + ψ1 ⋅ qk,1 + ψ2 ⋅ qk,i (6.15)

6.10a (ULS)

The first of the design load combinations with emphesis on dead loads.

Time: Design working life (L20, L50 or L100)

Load combination, ULS, (EN 1990 6.4.3.2 (3)):

Ed = γG ⋅ Gk + γq ⋅ ψ0 ⋅ qk,i (6.10a)

  • Danish default values in PRE-Stress: γq = 0
  • Swedish default values in PRE-Stress: γq = 1.35 (BFS 2015:6, EKS 10)
  • Swedish values in PRE-Stress: γq = 0 (BFS 2019:1, EKS 11)

6.10b (ULS)

The second of the design load combinations with emphesis on live loads.

Time: Design working life (L20, L50 or L100)

Load combination, ULS, (EN 1990 6.4.3.2 (3)):

Ed = ξ ⋅ γG ⋅ Gk + γq ⋅ qk,1 + ψ0 ⋅ qk,i (6.10b)

Fire (ULS)

This may be a default load combination, depending on the national annex used. In the Wizard when creating the element there is an option to check to enable this load combination.

PRE-Stress will handle this combination a little bit separate from previous combinations as it will reduce the concrete cross section due to the temperature of the element. That means that a fire load combination may not have any dependencies similar to the previous load combinations, as the strain distribution can't be retained with equilibrium from previous stages.

Time: Design working life (L20, L50 or L100)

Load combination, Accidental Limit State (EN 1990 6.4.3.3 (1)):

Ed = 1.0 ⋅ Gk + ψ1 ⋅ qk,1 + ψ2 ⋅ qk,i (6.11)

References

J
Joakim is the author of this solution article.

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