Optimising production costs of steel trusses

Abstract

The building industry is fragmented. In the case of steel structures, two parties are involved in the design. The engineering firm designs the structure, and at a later phase the steel contractor designs all the detailing. This fragmentation restrict the possibilities of creating a cost optimal design. Currently, engineers often optimise on total weight. The result is that at a later phase, the steel contractor will need to design more complex joints, in order to create sufficient resistance in the joints of the slender structure. This can lead to higher total cost of the steel structure, compared to when joint design has been considered at the early phase. This situation leads to the following research question: “How can engineers optimise the production costs of steel trusses with welded connections?”. In this research, I will focus on trusses with welded connections which comply with transport dimensions. To answer this question, a parametric optimisation model is developed. This model combines three software packages Grasshopper, Karamba3D and IDEA Statica Connection . Grasshopper creates the wireframe geometry that resembles the structure. Karamba3D performs the structural analysis and defines cross-sections through size-optimisation. Finally, IDEA Statica Connection will design all joints, check the joints plates for the strain percentage, and optimise the welds according to the directional method.
The model shows that selected beams from the size optimisation in Karamba3D often fail the joint analysis of IDEA. This is because the strain criterion is exceeded. This failure can be resolved in the model either by modifying the cross-sections, changing the geometry of the structure, or applying stiffeners in the joint. When the truss does not show failure in the plates of the joints, the weld volume can be optimised. The optimisation model will automatically optimise the welds of each joint in the structure by using the directional method in IDEA. Optimising the welding volume, creates a significant reduction in welding volume compared to the full strength method. However, in the current state, deformation capacity in the joint cannot be guaranteed. This can prevent a plastic hinge to form and may cause premature brittle failure of the structure. To be able to use the welding volume optimisation, it should be complemented with an extra check to define whether the yield strength of the connected parts is lower than the rupture strength of the welds. Further research is recommended, to complement the weld volume optimisation with the needed additional check. Additional research into the cost-optimisation of steel structures with bolted connections and moment-resisting connections is also encouraged.