Almost every time we hear the word ‘efficiency’ used to describe a timber building, it concerns the energetical aspect. One of the most important and interesting aspects of the timber engineering process, however, is the analysis of the material’s structural efficiency, that is the optimization of the strength/production costs ratio.
To this purpose, it is very useful to make use of valid finite elements modelling softwares, especially for composite or particularly complex sections. Using the structural analysis software Dlubal RFEM, we have recently analysed the optimization of a slab composed of glulam beams joined to a CLT plate by means of screws. This experience, which led to a final saving in costs and production timings of about 20%, was exemplary of the real advantages of a bigger investment during the design process.
Briefly recalling the main stages of the analysis of the structure pictured above, the first step was generating a correct model, both geometrically and mechanically; to this purpose, we:
Then the tensile forces perpendicular to the direction of the grain were calculated, in proximity to the circular hole, and the eventual interactions with the saw-cut were evaluated. As regards dimensioning and testing of the reinforcement screws, the software’s indications were graphically and numerically aligned to the regulations (the diagram shows how the stress peaks are concentrated in areas which are rotated circa 45 degrees from the beam’s axis). The optimisation was therefore focused on finding the maximum dimension of the hole and its position in relation to, respectively, the beam’s dimensions and the position of the cut.
The main work focused, then, on the optimisation of connections. They represent one of the elements that most influence this kind of structures, in terms of both mechanical response (they affect the grade of interaction among connected items) and costs (raw materials and construction timing).
Once the connector had been chosen, the target was to calculate the minimum number needed to guarantee the required strength and to follow the strain limits imposed by the regulations. Therefore, we focused on the number of segments to divide the beam in and on the center-to-center distance of the screws in each segment.
At this stage, the software proved itself to be a fundamental resource, for at least three reasons:
Finally, it is important to emphasize that the present paper aims to inform about the advantages of a correct and in-depth design process, and not to promote a blind faith in the modelling software. In fact, although it is a powerful support in design, a software could never take over the designer, that still bears the responsibility to control and critically examine the obtained results.