EXPLANATION: STEEL SUPPORT STRUCTURE COLLAPSE ANNE FRANK CHESTNUT TREE
In Cobouw of 6 september an article was publiced about the support structure of the Anne Frank chestnut tree. On the internet I found this construction. I’m not sure if this is the support structure that was used. This steel structure should help the tree surviving a storm. The tree was already weakened due to a weak spot at the base of the tree. The support construction was connected with in the ground piled steel pipes by welding footplate on footplate. For really helping the tree the structure should not constrict the natural movement of the tree. A tree has to be abe to bend away when heavy winds arise.
There are made 2 fundamental mistakes in the design of this support structure.
1: The movement of the upper ring of the steel structure is lower than the lower ring. If we want to move the upper ring horizontally only compressive and tensional forces arise in the three legs of the support structure. When we want to move the lower ring horizontally also bending forces arise in the three supporting legs. The problem is that tensional and compressive forces give much lower displacements compared to bending forces. So with the same horizontal force the horizontal movement of the lower ring is higher compared to horizontal movement of the upper ring.This is just opposite to the natural movement of te tree. The natural surviving technic of the tree is to bend away at high windspeeds. The construction makes this impossible. That is why the tree broke at only 4 beaufort windspeed.
2: The support construction made with three horizontal footplates is welded on the plates that are welded on the piled pipes. Due to this design the welds that connect both footplates are under bending forces. Welds should only be loaded in shear and not on bending. This makes the connection between the three piled pipes and the three support pipes very weak. So footplates are wrong in this design. A simple way to improve the strength of this connection is welding 2 strips of steel that are directed more in piled and supportbeam direction. These steel strips can be hammered against the piled pipe and against one leg of the support structure. Now the welds only feel shear forces. This connection is much stronger compared to the used footplates.
A more detailed explanation can be read here.