In the following period, several types of joints with all kind of connectors were studied, for both short-term and longterm loading, and the results contributed nationally (NEN) and internationally (Eurocodes) to safe design rules for timber-structures. During this period there was, not only in the Netherlands, but also internationally an increasing interest in the application of new wood products such as: glulam, several types of boards, plywood, stress-skin panels etc. Building designers were experimenting with sophisticated types of structures: shells, hypars, domes, folded plates, etc.. The Stevin-research-group was directly or indirectly involved in several of these projects. The economics of the nineteen-eighties however unfortunately ended this from structural engineering point of view, interesting development.
More recently new ways of strengthening (reinforcing) the joints in timber-structures were investigated. Application of steel-plates, glasfiber, plywood and densified veneer was tested (Leijten e.o.). During this period a new type of fastener, an expanded tube in combination with one of abovementioned reinforcements, was successfully developed and tested. The influence of the number of fasteners in one row on the strength of joints was studied (Jorissen).
In the field of long-time-duration testing, the program on nailed, split-ring, and tooth-plate timberjoints was finalised (Van der Kuilen). Results show that some adjustment of design rules might be necessary.
In the field of "new type of structures or components" the activities mainly have been focussed on composite-beams of concrete and timber (Van der Linden). For successful, composite-action not only the long-term effect, but also the connectors between timberbeam and concrete-flange, play a key role and were widely studied.
In the Netherlands no big forestry industry is putting pressure on the building-branch to use as much wood as possible. We are importing for our demand. Historically for timber structures, quality has always prevailed over quantity. Although there is a demand for producing and using more inland wood, the quantity involved is minor. The nearby future for timber structures in the Netherlands will still emphasise on quality. Based on the natural properties of wood and woodproducts this quality of timber structure still will be governed by three main items: long-time-effects, (new) structural elements, and joints.
Our Stevin research-program for the nearby future on timber structures will therefore still focus on those three items. The way long-time-duration effects influence both the resistance and the solicitation side of the basic safety-analysis of timber-structures. It needs more study and research. Research on new structural elements will continue to focus on composite structures combining wood with other materials. Efforts will be made for the revival of the "sophisticated" structures of the seventies with modern materials and production techniques. The program on strengthening the joints will be continued. New combinations of strengtheningmedium and fasteners might be successful. Not a surprising new program. Wood is our oldest and finest building material. It took nature millions of years to develop it; it will take us centuries to understand it.
prof. ir. L.A.G. Wagemans
Timber Structures
Faculty of Civil Engineering and Geosciences
TU Delft