Timber-steel hybrid beams for multi-storey buildings

Abstract

The technical possibilities for building with timber experienced a big advance in the last years. The development of new timber and timber-based products together with growing environmental concerns are pushing the research and study on timber construction. One objective is the construction of multi-storey buildings in urban environment. Urban timber and timber-based building proposals are emerging, as many research projects and built examples illustrate. Many present day projects propose the combined use of timber elements with other materials for specific structural purposes difficult or costly to fulfil with an only-timber solution. In this work the potential of combining steel and timber in beams is identified and discussed (see chapter 2). A review of the studies carried out until now was compiled (see chapter 3). This material was summarized and classified depending on the timber-steel combination strategies used. The proposal of classification is based on the structural performance of the steel components (passive or active) and the steel geometrical arrangement. The structural purpose of combining the steel with the timber cross sections was identified for each case (see 3.1). Special attention was devoted to the identification of the design, construction, implementation or economical problems of the proposed solutions. The practical success of each timber-steel combination strategy is reflected in a list, when existing, of built examples. All this review and analysis was used for the identification of important design parameters to take into account when designing timber-steel beams solutions (see 3.5 and 3.6) Using the knowledge gained a design of timber-steel hybrid beams combining cold-formed steel profiles with timber is proposed. The hybrid solution developed is based on the combination of the elements without the transmission of horizontal shear forces between the steel and timber components. Both a symmetrical and a symmetrical cross-section arrangement are proposed (see 4.3). Conditions to take into account and objectives were stated and a cross-section design procedure developed for the case of single span beams (see 4.4 and 4.5). The particularities for the cross-section arrangement and application of these beams forming part of statically undetermined systems were also analysed and related design criteria given (see 4.8). Part of the theoretical work mentioned until now was proofed and tested. Short term loading tests of the timber-steel beams were carried out (see chapter 5) as well as a long-term loading test (see chapter 6) In a first testing series the hybrid symmetrical arrangement using cold-formed steel was tested and compared to other possible and statically equivalent beams. The steel design, assembly procedures and structural performance were tested and validated. A total of ten hybrid beams with different cross-section arrangements but always with a span of six metres were built and tested (see 5.3). In a second testing series only cold-formed steel elements were used for the hybrid asymmetrical arrangement. Hybrid beams with a structural depth of L/17 and L/20 were designed, calculated and tested. Special attention was paid to the study of the influence of different support conditions on the structural performance of the beams. A total of twelve hybrid beams with spans of six and eight metres were built and tested. A long-term loading test was carried out for the duration of one year. The performance of two hybrid beams was compared to two only-glulam beams. The behavioural trend was very clear and the advantages of hybrid systems when long-term deflections are relevant could be confirmed (see chapter 6). A summary of open future lines of research was made (see chapter 7). Part of the topics proposed were already partially studied or initiated, being the author of this doctoral thesis part of the researching teams. Timber-steel structural elements in general and beams in particular are therefore presented as an alternative for developing a holistic construction concept making the most of the properties from both materials starting at the structural element level. Its optimization for their application in multi-storey architectural solutions is the final objective and main application field.