Lightweight segmented timber shell for the Bundesgartenschau 2019 in Heilbronn. Form and Force: Proceedings of the International Association for Spactial Structures Annual Symposia 2019 Barcelona, October:1--8, 2019. [PUMA: groenevolt alvarez sonntag wagner Lightweight aldinger segmented Bundesgartenschau knippers engineering shell 2019 BUGA Heilbronn itke krieg timber menges bechert]