Environmental life cycle assessment of wire arc additively manufactured steel structural components

Wire arc additive manufacturing (WAAM) enables the production of structural components with topologically optimised geometries thus leading to significant self-weight reductions for a given load-carrying capacity. A common question arises regarding the environmental performance of WAAM structural components in comparison with conventional steel structural components. Thus, a comparative cradle-to-gate life cycle assessment has been conducted where the environmental impact of producing a topologically optimised WAAM steel beam is compared with that of producing a conventional hot-rolled steel I-beam. The beams are 2 m long, simply-supported and loaded vertically at midspan. The impact of using either carbon steel or stainless steel is investigated. The results demonstrate that the carbon steel and stainless steel WAAM beams have 7% and 24%, respectively, lower climate change impact than the corresponding I-beams. It is concluded that WAAM can lead to lower CO2-eq. emissions than conventional hot-rolling, provided that mass reductions of the order of 50% (which are readily attainable) can be achieved by employing WAAM in conjunction with, for instance, topology optimisation. Furthermore, it is shown that the shielding gas contributes greatly to the environmental impact of WAAM, and that, by using higher deposition rates or by utilising renewable energy sources, the impact of WAAM can be reduced by more than 30%

Prestressed cold-formed steel beams – parametric studies and design recommendations

Owing to their enhanced load-carrying and serviceability performances, prestressed coldformed steel beams can potentially open up new applications within the construction industry. In the proposed concept, an eccentric prestressing force is applied to cold-formed steel beams by means of a cable that is housed within a bottom hollow flange. During prestressing, tensile stresses are induced within the top region of the beam, thus delaying the occurrence of local instabilities under subsequent vertical loading. Consequently, the moment capacity of the beam is enhanced. Furthermore, owing to the prestressing, a pre-camber is also induced along the member, thus decreasing the overall vertical deflections significantly. Following discussion of the mechanical behaviour of the proposed beams, design recommendations are developed by employing interaction equations alongside the Direct Strength Method. Subsequently, finite element (FE) analysis is employed to investigate the effects of the prestress level and the section slenderness of the steel beam on the benefits obtained from the prestressing process. The parametric FE results are then utilised to assess the design recommendations

Prestressed cold-formed steel beams: concept and mechanical behaviour

An innovative concept, whereby the load-carrying capacity and serviceability performance of cold-formed steel beams are enhanced by utilising prestressing techniques, is presented. The prestressing force is applied by means of a high-strength steel cable, which is housed at a location eccentric to the strong geometric axis within the bottom hollow flange of the cold-formed steel beam, inducing initial stresses in the beam that are opposite in sign to those introduced during the subsequent loading stage. As a consequence, the development of local instabilities during loading is delayed and thus the capacity of the beam is enhanced. Furthermore, the pre-camber induced during prestressing, as well as the contribution of the cable to the bending stiffness of the system, decrease the overall vertical deflections of the beam. The conceptual development of prestressed cold-formed steel beams and a study investigating the potential benefits are presented. The mechanical behaviour of the proposed beams in both the prestressing and imposed loading stages is described in terms of analytical expressions, while failure criteria for the design of the cold-formed steel beam and the cable are also developed by employing interaction equations in conjunction with the Direct Strength Method. Geometrically and materially nonlinear finite element analysis with imperfections is employed to simulate the behaviour of the proposed beams. Sample numerical results are presented and compared with the developed analytical expressions and failure criteria, demonstrating the substantial enhancement in moment capacity and serviceability performance offered by these beams

Design of prestressed cold-formed steel beams

Structural design rules for prestressed cold-formed steel beams, considering both the prestressing and imposed vertical loading stages, are presented herein. In the proposed approach, the cold-formed steel member is designed as a beam-column using linear interaction equations in conjunction with the Direct Strength Method (DSM), while the prestressed cable is designed by ensuring that its tensile capacity is not violated during the two loading stages. In the present paper, the design approach and the failure criteria, which define the permissible design zone for the prestressed system, are first introduced. The suitability of the design recommendations is then assessed by comparing a set of parametric finite element (FE) results for several combinations of prestress levels, beam geometries and cable sizes, with the corresponding design predictions. Finally, following reliability analysis, the implementation of the design recommendations is illustrated through a practical worked example

Finite-element modeling of prestressed cold-formed steel beams

The concept and structural benefits of prestressing cold-formed steel beams are explored in the present paper. In the proposed system, prestressing is applied by means of a high-strength steel cable located within the cross section of the beam at an eccentric location with respect to the strong geometric axis. The internal forces generated by the prestressing are opposite in sign to those induced under subsequent vertical loading. Hence, the development of detrimental compressive stresses within the top region of the cold-formed steel beam is delayed and thus the load-carrying capacity of the beam is enhanced. Owing to the precamber that is induced along the member during the prestressing stage, the overall deflections of the beam are also reduced significantly. In the present paper, finite-element (FE) modeling was employed to simulate the mechanical behavior of prestressed cold-formed steel beams during the prestressing and vertical loading stages. Following the validation of the FE modeling approach, a set of parametric studies was conducted, where the influence of the key controlling parameters on the structural benefits obtained from the prestressing process was investigated. The parametric results were utilized to determine how the benefits obtained from the addition of the prestressed cable can be maximized, demonstrating the significant enhancements in the performance of the cold-formed steel beam that can be achieved

Description of anisotropic material response of wire and arc additively manufactured thin-walled stainless steel elements

In contrast to conventionally-produced structural steel and stainless steel elements, wire and arc additively manufactured (WAAM) elements can exhibit a strongly anisotropic material response. To investigate this behaviour, data obtained from tensile tests on machined and as-built coupons extracted from WAAM stainless steel sheets are analysed. The observed mechanical response in the elastic range is described accurately using an orthotropic plane stress material model requiring the definition of two Young’s moduli, the Poisson’s ratio and the shear modulus. In the inelastic range, the anisotropy is captured through the Hill yield criterion, utilising the 0.2% proof stresses in the three different loading directions relative to the deposition direction; plastic Poisson’s ratios are also reported. The presented findings and constitutive description highlight significant variation in the properties of the studied stainless steel with direction, which opens up opportunities to enhance the mechanical performance of WAAM structures by optimising both the location and orientation of the printed material

The evolving basis for the design of light gauge steel systems

The importance of allowing for the many different types of structural interaction that have an effect on the performance of light gauge members when used in practical situations is emphasised. A distinction is drawn between internal interactions involving the various plate elements of the steel profiles and external interactions involving the other components in the system. Although full-scale testing of representative systems can capture this behaviour, the costs involved make this an impractical general basis for design; codified methods generally consider only isolated plates within members and isolated members within systems, thereby neglecting the potentially beneficial effects of both forms of interaction. Properly used, modern methods of numerical analysis offer the potential to systematically allow for both forms of interaction – provided the numerical models used have been adequately validated against suitable tests. The use of such an approach is explained and illustrated for three commonly used structural systems: roof purlins, floor beams and columns in stud walls. In each case it is shown that, provided sufficient care is taken, the numerical approach can yield accurate predictions of the observed test behaviour. The subsequently generated large portfolio of numerical results can then provide clear insights into the exact nature of the various interactions and, thus, form the basis for more realistic design approaches that are both more accurate in their predictions and which lead to more economic designs. Building on this, modifying existing arrangements so as to yield superior performance through specific modifications is now possible. Two such examples, one in which improved interconnection between the components in a system is investigated and a second in which prestressing is shown to provide substantial enhancement for relatively small and simple changes, are presented

Stability of steel struts with externally anchored prestressed cables

Externally anchored prestressed cables can be employed to enhance the stability of steeltruss compression elements significantly. To demonstrate this concept, a system comprisinga tubular strut subjected to an external compressive load and a prestressed cable anchoredindependently of the strut is studied. Energy methods are utilized to define the elasticstability of the perfect and imperfect systems, after which the first yield and rigid–plasticresponses are explored. The influence of the key controlling parameters, including thelength of the strut, the axial stiffness of the cable and the initial prestressing force, on theelastic stability, the inelastic response and the ultimate strength of the system is demon-strated using analytical and finite element (FE) models. To illustrate the application of thestudied structural concept, FE modelling is employed to simulate the structural response ofa prestressed hangar roof truss. A nearly two-fold enhancement in the load-carrying capac-ity of the truss structure is shown to be achieved owing to the addition of the prestressedcabl