Monitoring of glulam structures by theodolite intersection
The use of glued laminated timber (glulam) in building and construction provides a designer with an attractive, environmentally friendly alternative to steel and concrete. However, along with other engineering materials, glulam is subject to creep and in order to use it efficiently without the risk of unacceptable long-term deflections occurring, a reliable method of estimating creep in glulam is desirable. With this mind, the Structural Timber Research Unit (STRU) at the University of Brighton embarked, in 1988, on an extensive glulam research programme. Since then, the deflection of prepared glulam specimens has been measured in the laboratory under controlled conditions and in 1992, the programme was broadened to include measurements to determine the behaviour of glulam in full-scale structures. A number of test sites have been established in a variety of buildings and the method chosen to monitor glulam beams in these is theodolite intersection using a Leica Electronic Coordinate Determination System (ECDS3). Borrowed from industrial and engineering surveying, the use of the theodolite intersection technique to monitor glulam is unique and it has, despite the practical difficulties and size of structure involved, enabled the movement of beams to be monitored with an accuracy of 0.1 mm. By processing three-dimensional intersection coordinates, vertical deflections and creep have been determined and results show that the creep response of the glulam beams monitored in full-scale structures can be represented by a seasonally modulated exponential function. This correlates well to results from the laboratory tests and demonstrates that measurements taken in the laboratory can predict the behaviour of glulam in-situ. The creep factors obtained also agree well with the values given for these in Eurocode 5: Design of Timber Structures.