6 Creep effect on normal force

Task

During recent calculations we found out that the definition of creep factors of concrete slabs has a surprisingly high effect on normal forces in steel columns. The more statical undetermined a system is, the higher difference in normal forces can be expected.

In statical determined systems this effect does not exist.

Statical system

• 10 storey high building, floor height 3m

• Slab: C35/45 (Norway B35), XC1, thickness 200mm

• Geometry slab: 16 x 2m

• Steel columns designed for 80% utilization in FEM-Design nocreep

Loads

• selfweight

• permanent load: 1 kN/m2

• live load: 2 kN/m2

• loadcombinations

  • ULS: Sd = 1,35 gk + 1,5 qk

  • SLS: Sd = 1,0 gk + 1,0 qk

Results

Results are calculated with these programs.

• FEM-Design 19

  • no creep in slabs and walls

  • creep 2,5 / shrinkage 0,36

• Focus Konstruksjon 2021 (2D)

  • creep is not taken into account during ULS calculations, just for SLS

  • Calculation is done with normal creep definition and reduced creep

• OS-Prog K-bjelke

  • 1D calculation / creep is not taken into account during ULS calculations, just for SLS

  • Calculation is done with normal creep definition and reduced creep

The results are shown in the following 3 diagrams.

1. Normal forces of center steel column for the whole structure.

2. Reaction force of center support (= steel column) per floor.

3. Maximum deformation of slab for the whole structure.

Summary

There is a considerable effect of creep factor of the slab on the normal forces of a slab.

The reason for this behaviour is that in finite element calculations, forces always tend to move to the support with the highest stiffness. The more stiff a construction behaves, the higher this effect tends to be.

In structural systems where there is one part that is much stiffer than another - like concrete walls vs. steel columns, there is a high risk of underdimensioning of the weak part - the steel column.

In this calculation, the difference in normal forces in FEM-Design between the structure with and without creep definition in the slab is as high as 18%. Comparing the most extreme values of this example we get differences as high as 30%.