Minus Z force applied bur shell displacement points upward

Grasshopper Karamba3D
1

analysis,help.3
analysis,help.3639×566 152 KB
analysis,help4
analysis,help4602×594 137 KB

Hi all, i made some simple analysis, as seen in the picture first i applied minus Zvector point load(-0.05), but my shell doesn’t sink down, actually it points upward, like in the pic one.
So i changed point load to plus(to 0.05)then shell actually sinks down like in the pic2.
Someone help me, is this kind of a bug?
Thanks in advance

11.24normalframe.gh (74.5 KB)

2

Hello @ayca5855,
in your definition there is no pre-strain or temperature defined. The ‘Analyze Th II’ component starts with the NII given. In the second step NII gets updated to zero and the system becomes unstable (see warning of the ThII-component) since it consists of membrane elements.
To solve the issue (see attached definition: 11.24normalframe_cp.gh (78.2 KB) ) you can either

  • add real pre-strain loads,
  • limit the number of iterations of the ‘Analyze Th II’-component to ‘1’ to avoid the NII update or
  • use an ‘Analyze’-component. The latter takes account of the NII forces given but does not update them.

– Clemens

3

thanks you for replying.
i am new to karamba. Actually first, i applied initial force eps0 =-0.768 in X and Y direction,(i saw other discussing saying eps0 have to be minus to be tensile force) at this time max displacement was 0.1823 which is higher than the max displacement when no initial strain applied. Higher the eps0 in initial force , in my analyze max displacement increases, strange :nerd_face:

Hence, i thought my method of giving tension might have been wrong and changed it to that (previous post, which i am wrong too)
Plus when i check for the N2 in model view component, i see nothing. Does is mean no geometric stiffness effects due to tension in shell, right.

i saw this discussion
Defining Beam Element Types(end of the discussion) saying when structure is too flexible, no axial forces result from the pre-strain?

i tried to increase Youngs modulus, still n2 cannot be shown in model view.
Changing Max iteration=1 means, it will ignore Axial forces in beams and in-plane forces in shells influence the structural stiffness right? i dont want that. HElp me
11.24normalframe.gh (89.5 KB)

4

Hello @ayca5855,

the ‘Analyze’-component uses the given NII. If these are not zero then a geometric stiffness is added. The ‘Analyze ThII’-component updates the NII forces irteratively. If it arrives at NII values smaller than the initial guess, displacements come out larger than those returned by the ‘Analyze’-component.

Your definition is a bit hard to debug since you set some values directly at the components (see attached definition: 11.24normalframe_II_cp.gh (73.1 KB) ). At the ‘Analyze ThII’-component ‘No Tensional NII’ was set to true. This means that the beneficial effect of tensile normal forces is neglected. In case of membranes that leads to kinematic modes.

– Clemens

5

thank you so so much. Sorry for bothering you many times