1. System effects in stressed-skin elements - an experimental investigation Birgitte Dela Stang, Danish Building and Urban Research, Denmark Tord Isaksson, Division of Structural Engineering, Lund University, Sweden Martin Hansson, Division of Structural Engineering, Lund University, Sweden Catharina Kockmann, Germany, visitor at Lund University, Sweden

2. Prefabricated roof elements 5 beams Plywood sheathing on top Transverse head beam A unit:

3. Prefabricated roof element (turned up-side-down)

4. Before testing the elements: Bending stiffness of all beams (60) Bending strength of selected beams (15) Relation between stiffness and strength Sorting beams for elements (5 beams in each) Manufacturing of units

5. Distribution of bending stiffness

6. Relation between stiffness and strength

7. Unit 1 Unit 2 Unit 3 Unit 4 Unit 5 Unit 6 Unit 7 Unit 8 Unit 9 Sorting of beams in elements Weak Medium Strong

8. Sorting of beams

9. Manufacturing

10. Wood-wool light weight building board is removed

11. Sets of 3 units are placed on top of air cushion

12. Load cells are placed between beams and support

13. A saw cut is made through the transverse head beam to obtain “true” reaction forces

14. Load is applied by air-filled cushion until failure of system

15. Analysing test results: Predicting load carrying capacity from bending stiffness of single beams Determining load carrying capacity of units and sets of 3 units Evaluating failure modes and load sharing System effects?

16. Predictions No connectionRigid connection Lower boundUpper bound Predicted strength

17. Test results - Total load on 3 units

20. Test results - Load on single beams

21. System effects?

22. Conclusions Strength of beams are predicted from bending stiffness of individual beams Failure of one beam did not lead to failure of system Failure of two adjacent beams lead to failure of system No system effects were found Predefined strong units proved to be strong