Instability of tie rod trusses of glulam

Abstract: Many buildings today primarily serve the purpose of being envelopes that separate theindoor climate from the outdoors, for a large open area. Examples of these buildings arehall buildings of different sorts. In climates where the snow load often exceeds the deadload, like in Sweden, it can be hard to construct a budget-friendly yet strong roof structurefor these halls, especially if a span above 40 m is required. One solution that has gained popularity is the tie rod truss. Oftentimes it is not a real truss,but rather a beam that is equipped with one or more tie rods and one or more compressionstruts. The struts, which connect to the tie rods that run from one end of the roof to theother, support the main beam. This way, the tie rods can decrease the moment load on thebeams by carrying a tensile force. However, this results in an increased compressive loadon the beams. The large compressive forces in conjunction with the often slender beamsections can lead to instability out of the truss plane, and ultimately collapse. This is whathappened to the sports hall Tarfalahallen in 2020 in Kiruna, Sweden. The tie rod truss is an important structure; it is relatively cheap, saves material and is strongin the truss plane. But the instability problems can deter its use. The aim of this study istherefore to explore the different factors that could influence the stability of the structureand to what extent, by studying critical loads and the utilization of all structural elements. The study was performed by analyzing a finite element model of a real tie rod truss thattoday exists in a sports hall in Sweden. The analyses were done for different values of many parameters. The results were then extracted and processed such that the critical load and the utilization were plotted against the tested parameter values. The analysis and results processing were performed in an automated process that the author created himself. The results show that the stability of the model is strongly influenced by (1) the rigidity ofthe joints between beams and struts, (2) the rigidity of the joints between beams and roofing material and (3) the vertical position of potential stabilization systems that actperpendicularly to the truss plane. Pretension of the tie rods had a large effect on themaximum utilization and how it varies along the beams. Any potential reinforcement measure must be cost-effective, and therefore should addressthe three influential properties above. One suggestion is to add sideway bracing as close tothe joints between the struts and beams as possible.