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

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

Prefabricated roof element (turned up-side-down)

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

Distribution of bending stiffness

Relation between stiffness and strength

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

Sorting of beams

Manufacturing

Wood-wool light weight building board is removed

Sets of 3 units are placed on top of air cushion

Load cells are placed between beams and support

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

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

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?

Predictions No connectionRigid connection Lower boundUpper bound Predicted strength

Test results - Total load on 3 units

Test results - Load on single beams

System effects?

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