FEA of Towers for Dome C

1
FEA of Towers for Dome C

• JL Dournaux / JPh Amans
• GEPI, Pôle Instrumental - Paris Observatory
• jean-laurent.dournaux_at_obspm.fr

2
FEA of Towers for Dome C

• Means to increase towers stability
• Mechanical behaviour of steels at low
  temperatures
• Description of the models used for simulation
• Results on nominal solutions
• Purposed solutions to improve stability of truss
  tower

3
How to increase towers stability (Hammerschlag
et al., 2006)

• Active methods consume power and can generate
  heat
• Some passive methods are
• Eigenfrequencies of towers gt 10 Hz
• Energy in the wind variations decreases quickly
  between 1 and 10 Hz
• Only plane motions of the platform parallel to
  the ground (i.e. no rotations x or y)
• Astronomical objects are far away
• But
• Changes in towers design have to keep the centre
  of the tower free to lift the telescope

4
Aim of the study

• Preliminary FEA of 2 concept towers designed by
  JP Amans (GEPI, Paris Observatory)
• Check the stability of these concept towers
• Improve performances of these 2 concept towers

Truss Tower Amans Tabouret Tower (ATT)
5
Mechanical Behaviour of Steels at low temperatures

• E increases slowly when T decreases
• Small differences in E values between summer
  (-30C) and winter (-80C)
• For a C steel 207 / 210 GPa
• For a stainless steel 168 / 174 GPa
• Ductility decreases strongly when T decreases

5 Stainless Steel / 6 C Steel (Thermeau, 2004)
6
Models Description

• Containers and platform are not modelled
• Boundary Conditions
• Towers feet are fixed
• Mass applied on the top of the tower 100 t
• No rotation between tubes (joints)
• No thermal gradients

7
Dynamic behaviour of truss tower

• Tube 300x5, C steel
• Small differences of dynamic behaviour in summer
  and in winter frequencies variations lt 0,1 Hz

8
Dynamic behaviour of truss tower

• Tube 300x5, stainless steel
• Small differences of dynamic behaviour in summer
  and in winter frequencies variations lt 0,1 Hz

9
Dynamic behaviour of truss tower

• The following results are for a Carbon steel _at_
  -80C
• The 1st modes _at_ 5,2 Hz harm because they create a
  strong rotation of platform
• Change towers design to increase these
  frequencies and reduce the rotation they produce

10
Dynamic behaviour of truss tower

• Different kind of add-ons are tested
• The most efficient is obtained by reinforcing the
  corners of the tower in all directions
• Reinforce only the 2 first floors allows to
  increase the frequency _at_ 5.3 Hz. The rate of
  rotation is almost not affected

11
Dynamic behaviour of truss tower

• Increase tube diameter of the 2 firsts floors
  allows to improve towers performances

12
Dynamic behaviour of Amans Tabouret(Stool) Tower

• Tubes 1016x4 291x3
• Modes 1/2 4/5 harm
• Eigenfrequencies are lower than for truss tower
  but the platform is more stable

13
Dynamic behaviour of ATT

• Towers performances seem to be less sensitive to
  tube diameter than Truss Tower

14
Conclusions

• Results
• Add appropriate add-ons and increase diameters of
  vertical tubes of first floors may allow to
  obtain 1st modes of truss tower beyond 10 Hz
• Platforms rotations are weaker for ATT but
  eigenfrequencies too
• Next steps
• Test other concept tower(s)
• Master student (Calculate transfer function to
  precisely determine wind effects by FEM,
  implement methods for vibration control, define a
  prototype) in collaboration with Pr André
  Preumont (ULB/INSA Lyon)