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Engineered timber CLT/Glulam

New build commercial There has been extensive research and development into creating the most efficient solution for new build office construction in Borough High Street. Photo: Heyne Tillett Steel steel and metal deck. Lightweight steel beams span long distances with large web openings creating flexible open plan spaces and a zone for services to route through. The beam weights are further reduced by using the metal deck slab as the top flange of the beam via shear studs. The optimum structural and services zone is then provided, reducing building height in restricted planning areas, reducing cladding costs and even potentially squeezing in an extra floor for some instances. It is hard for a timber solution to compete with such a wellrounded, industry standard approach to new build office developments. Currently, timber only comes into its own when: • an even more lightweight solution is needed • the structural zones are not a key driver for the project • the client is keen to use it for its aesthetic and wellness characteristics • the installation programme is key.

Research Heyne Tillett Steel (HTS) has been researching current timber designs. Currently, glulam beams are designed to stand alone with CLT sitting on top, taking no benefit of the CLT top flange. Composite glued-in factory timber panels, brought to site as cassettes, have several disadvantages: • holding up factory time to create the cassettes • potentially creating air spaces in transportation • adding cost to the solution, which may outweigh the depth reduction gain. Searching for a more standard and buildable win, HTS looked at composite action from screwed on-site connections. The theory assumes the CLT spans between secondary beams using its full depth and the central layers only contribute to the composite T along the length of the secondary beam. As the top flange of the T is in compression, it is assumed that any friction between panels and the locked joint is adequate to transfer the forces.

Eurocode 5 (EC5) currently has a clause in Appendix B that partially deals with this situation, yet is mainly based on creating I-beams from softwood, rather than Ts from two different engineered materials. The equation appears conservative and does not account for the type, length or incline of the screws used, giving minor gains on a glulam-only design. HTS partnered up with City University of London to test a series of screwed connection arrangements along with two full-size tests with different screw spacings. The initial small-scale shear tests showed that screws at a 60 degree incline gave the highest screw capacities. The larger tests were then undertaken with 7mm-diameter 260mm-long screws at 60 degrees towards the centre of the span and at varying centres. Initial results show a clear improvement on a glulam-only design and the current EC5 method. Both tests gave a tensile/bending failure in the glulam beam, rather than a failure of the screws as expected. It appears that the mobilisation of the effective width of CLT through its layers is more of an issue than the partial composite action from the screws. HTS is currently working towards finding a solution to calculating the effective width of the CLT and therefore the impact of the partial composite action provided by the screws.

What’s next TS hopes that its work and research will encourage others H to get involved in developing solutions to make timber more efficient for commercial buildings. Further research is being made into other relevant aspects, such as: • plated web openings • composite hard and softwood beams • other composite floor systems. Timber as a material is easy to work with, quick to install, requires no finishes and naturally creates a feel-good environment; it seems a no-brainer to use this in the commercial sector, too. n

About the author

Kelly Harrison Associate Heyne Tillett Steel

Further information • Cross-laminated timber: Design and performance, ISBN 9781-909594-63-0, Exova BM TRADA, 2017 • WIS 2/3-61 Cross-laminated timber: introduction for specifiers, Exova BM TRADA, 2016 Timber 2019 Industry Yearbook

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Profile for BM TRADA

Timber 2019 Industry Yearbook  

The annual publication of the Timber Research and Development Association (TRADA) includes topical and technical features on all aspects of...

Timber 2019 Industry Yearbook  

The annual publication of the Timber Research and Development Association (TRADA) includes topical and technical features on all aspects of...

Profile for trada