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Metal additive manufacturing (AM), in particular wire-and-arc additive manufacturing (WAAM), has become over the last few years the breakthrough technology to reduce the environmental impact and increase the efficiency of steel structures. Although intense research effort has been paid toward the mechanical characterization of WAAM-produced thin walls, little attention has been devoted to the investigation of multi-layered thick parts. These latter would indeed expand the application of WAAM for large-scale constructions requiring thicker cross-sections to withstand high loading conditions. The present work provides a comprehensive experimental investigation of mild steel WAAM thick plates from the fabrication to the mechanical and microstructural characterization. First, the fabrication process is presented in full details. From that, microstructural and mechanical characterization is described and discussed, showing a homogeneous microstructure with little influence on the mechanical response along the wall plate thickness, also considering different specimen orientations with respect to the printing directions. The results confirm good mechanical properties of the printed outcomes, in line with those of structural mild steels manufactured with conventional technologies. Little influence on the response along the thickness is reported, thus proving the required quality of WAAM thick parts for applications in the construction sector. Graphical Abstract

Alloys fabricated by wire-and-arc additive manufacturing (WAAM) exhibit a peculiar anisotropy in their elastic response. As shown by recent numerical investigations concern- ing the optimal design of WAAM-produced structural components, the printing direction remarkably affects the stiffness of the optimal layouts, as well as their shape. So far, single-plate specimens have been investigated. In this contribution, the optimal design of WAAM-produced I-beams is addressed assuming that a web plate and two flat flanges are printed and subsequently welded to assemble the structural component. A formulation of displacement-constrained topology optimization is implemented to design minimum weight specimens resorting to a simplified two-dimensional model of the I-beam. Compa- risons are provided addressing solutions achieved by performing topology optimization with (i) conventional isotropic stainless steel and with (ii) WAAM-produced orthotropic stainless steel at prescribed printing orientations. Lightweight solutions arise whose spe- cific shape depends on the selected material and the adopted printing direction.

Current manufacturing techniques in the construction sector are slow, expensive and constrained in terms of architectural shapes. In other manufacturing sectors (such as automotive and aerospace) the use of automated construction systems significantly improved the safety, speed, quality and complexity of products. To realize real-scale structural elements for construction applications without ideally any geometrical constraints either in size or shape, the most suitable manufacturing solution for metallic elements is a directed energy deposition (DED) process referred to as wire-and-arc additive manufacturing (WAAM). The main advantage of WAAM relies on the possibility to create new shapes and forms following the breakthrough design tools for modern architecture as algorithm-aided design. At the same time, the printed part ensures high structural performances with reduced material use with respect to the conventional solution. The study presents a new approach called “blended” structural optimization, which blends topology optimization with basic principles of structural design and manufacturing constraints proper of WAAM technology, towards the realization of new efficient structural elements. The approach is applied to the case study of a I-type stainless steel beam on a multi-storey frame building. The approach could pave the way towards an efficient use of WAAM process to produce a new generation of structurally optimized elements for construction, with a more conscious use of the optimization tools and an efficient application of metal 3D printing.

Among various additive manufacturing (AM) technologies, wire-and-arc additive manufacturing (WAAM) is one of the most suitable for the production of large-scale metallic components, also suggesting possible applications in the construction field. Several research activities have been devoted to the WAAM of steels and titanium alloys and, recently, the application of WAAM to aluminum alloys has also been explored. This paper presents the microstructural and mechanical characterization of WAAM plates produced using a commercial ER 5183 aluminum welding wire. The aim is to evaluate the possible anisotropic behavior under tensile stress of planar elements, considering three different extraction directions in relation to the deposition layer: longitudinal (L), transversal (T) and diagonal (D). Compositional, morphological, microstructural and fractographic analyses were carried out to relate the specific microstructural features induced by WAAM to the tensile properties. An anisotropic behavior was found in regard to the specimen orientation, with the lowest strength and ductility found on T specimens. Reasoning to this was found in the presence of microstructural discontinuities unfavorably oriented with regard to the tensile direction. The results of tensile tests also highlighted an overall good mechanical behavior, comparable to that of conventional AA5083-O sheets, suggesting future use in the realization of very complex geometries and optimized shapes for lightweight structural applications.

L’innovazione consiste nella progettazione di un palo reticolare innovativo realizzato mediante tecnologia di stampa 3D. Il palo ha una sezione tubolare “atomizzata”, che forma una superficie esterna di tipo reticolare (“lattice structure”) costituita da elementi rettilinei continui. La continuità degli elementi rettilinei costituenti la struttura reticolare è garantita grazie alla tecnologia di realizzazione di tipo additivo per metalli di tipo Wire-and-Arc Additive Manufacturing, che supera la connessione di più elementi nella realizzazione di un unico elemento tubolare di tipo reticolare. L’innovazione si può estendere a una pluralità di forme (sezione di larghezza costante o variabile in altezza) e applicazioni a seconda della destinazione d’uso. Il palo reticolare garantisce prestazioni di resistenza confrontabili con il palo tradizionale, a fronte di un abbattimento del materiale impiegato (fino al 90% di risparmio di materiale), fornendo così anche un ridotto impatto ambientale.

The paper investigates the dynamic behavior of structural systems obtained by connecting a moment-resisting frame structure with a vertical rigid truss pinned at the base, known in literature as “strongback,” and equipped with added fluid-viscous dampers. The strongback, designed in order to remain in the elastic field under strong seismic ground motion, acts as a mast by imposing to the structure a linear lateral deformed shape. By regularizing the lateral drift profile of the structure, the strongback limits undesired effects such as weak-storey mechanisms, damage concentration and residual drifts. In addition, when supplemental dampers are inserted in the structure, a considerable amount of energy can be dissipated, thus reducing the peak seismic response. The aim of the work is twofold: i) to provide analytical formulations for the preliminary design of added dampers based on the Generalized Single Degree Of Freedom (GSDOF) concept, and ii) to evaluate the increase in energy dissipation capabilities for selected dampers configurations thanks to the presence of the strongback. The formulas are developed for different configurations of added viscous dampers: dampers inserted within the frame between all or selected consecutive storeys (inter-storey placement) and dampers located at the base of the strongback to realize a rigid “dissipative tower.” The effectiveness of the dampers configurations is evaluated through dynamic time-history analyses.

L’invenzione consiste in un dispositivo robotico mobile equipaggiato con una tecnologia di stampa 3D in grado di spostarsi sull’oggetto tridimensionale stesso, durante la sua realizzazione, attraverso un sistema di guida integrato nell’oggetto stesso. Il procedimento infatti prevede di stampare, contemporaneamente all’oggetto, uno o più elementi di guida in grado di interagire con il dispositivo mobile. Essendo un sistema mobile e flessibile, la tecnologia brevettata è adatta a stampare oggetti tridimensionali di qualsiasi geometria con dimensioni anche di molto superiori a quelle della stampante stessa. La tecnologia si presta a qualunque materiale adatto alla stampa 3D (polimeri, materiale cementizio, metalli).

Wire-and-Arc Additively Manufactured (WAAM) alloys are characterized by specific mechanical properties which can largely differ from the conventionally-manufactured alloys. In detail, the printing process results in a peculiar microstructure, characterized by preferential crystallographic orientation with reference to the printing direction, that leads to an anisotropic mechanical behavior of the printed part. Previous experimental tests on WAAM-produced stainless steel plates showed in particular a strong anisotropic elastic behavior. Based on the above, the present work formulates a specific anisotropic elastic model for a WAAM-processed austenitic stainless steel, considering an orthogonally anisotropic (or orthotropic) constitutive law and a procedure to calibrate the elastic parameters based on the experimental results. In detail, the procedure is applied to calibrate the numerical values of the elastic parameters of a specific WAAM 304L austenitic stainless steel. For this aim, specific investigations on both the mechanical and microstructural features were carried out. Experimental tensile tests were performed on specimens with different orientations with reference to the printing direction. In detail, Young’s modulus and Poisson’s ratios were evaluated for samples oriented along three different orientations with regard to the printing deposition layers: longitudinally (L), transversally (T) and diagonally (D) to them. Digital Image Correlation (DIC) optical measuring system was used to acquire the full strain fields during the test. Microstructural analysis was also carried out to study the inherent microstructure, characterized by a distinctive grain growth direction, and to assess the preferred crystallographic orientations of specimens extracted along the three considered directions. The experimental results are used to calibrate the orthotropic elastic model. From the calibrated model additional material properties in terms of Young’s and shear modulus for any printing direction are derived. The resulting values exhibit very large variations with the printing angle, with ratios between minimum to maximum values around 2 for the Young’s modulus and 3.5 for the shear modulus. This marked orthotropic behavior could open unexplored design possibilities based on deformability issues. Additionally, the calibrated orthotropic model can also be used for future experimental explorations of the mechanical properties of WAAM alloys and for stiffness-based structural design optimizations.

This paper presents the first main results of quasi-static cyclic tests performed on a half-scaled two-storey one-bay steel pendular frame equipped with Crescent Shaped Braces (CSBs). The CSB is a yielding steel bracing element characterized by a boomerang-like shape, designed to provide the structure with an optimal combination of lateral stiffness, strength, ductility capacity and hysteretic dissipation, thus capable of meeting multiple seismic performance objectives within the context of Performance Based Seismic Design. In previous research studies, the behaviour of single CSBs under cyclic loads was analytically, numerically, and experimentally investigated through tests conducted on 1:6 scaled specimens. These results verified the potential capabilities of the device. As a further step of the experimental validation, a set of tests have been designed to evaluate the performances of CSBs when inserted into realistic frame structures. A two-storey prototype structure has been designed to meet selected seismic performance objectives, which could not be achieved with traditional diagonal braces. From that, a single-bay two-storey frame, representative of one braced frame of the whole structure, has been designed, detailed, and manufactured (half-scaled) for the experimental tests. For a full assessment of the behaviour of the braced frame, two configurations have been designed and tested. This paper presents the results of the test performed on the first configuration with one CSB device placed at the first storey only. The attention has been mainly focused on the global force-displacement response and the contribution of the connection plates, energy dissipation capacities and local strains around the knee-point region of the CSB. The results show that the CSB behaves following the theoretical predictions, while the connection plates provide a non-negligible contribution in the whole response of the system that should be properly accounted for in the design phase.

Wire-and-arc additive manufacturing (WAAM) is an innovative technology that involves deposition of subsequent layers of molten materials. Due to the high deposition rates, this technology is suitable for the production of large-scale complex structures. Further enhancement in the productivity can be achieved by an interlayer cooling strategy that reduces idle time between depositions. However, the effect of the interlayer cooling on microstructure and mechanical properties has to be addressed. In this view, the present work compares microstructural features and mechanical properties of WAAM-produced plates of austenitic AISI 304 L, focusing on the effect of both active interlayer air cooling and possible anisotropy induced by the additive process. Microstructural and mechanical characterization was conducted on samples extracted along the longitudinal, transverse, and diagonal directions to the deposition layers of WAAM plates, processed with and without interlayer active cooling. Results showed no remarkable influence of cooling conditions on the microstructure and mechanical properties of WAAM plates, which are indeed affected by the anisotropy induced by the additive process. The observed anisotropy in the elastic modulus, independent from different cooling conditions, was related to the crystallographic texture consequent to the highly oriented microstructure typically induced by the process.

The use of Additive Manufacturing (AM) technologies in the construction industry is still at its pioneering stage. The first investigations indicate that, among actually available metal 3D-printing strategies, Wire-and-Arc Additive Manufacturing (WAAM) process appears to be the most suitable for realizing large-scale steel structures. Nonetheless, the limited knowledge of the mechanical response of WAAM-produced alloys requires further experimental work for a reliable evaluation of the structural behavior of WAAM structural members. One specific issue which still needs to be fully investigated is the peculiar geometrical irregularity resulting from WAAM process that could have non-negligible effects on the mechanical behavior, such as anisotropy and non-homogeneous stress-strain fields. The present work explores the influence of the inherent geometrical irregularities of planar 308LSi stainless steel WAAM specimens on the tensile response. For this purpose, detailed geometrical characterization of the external surface of WAAM specimens was carried out with 3D scanning techniques and random field theory. Different sets of specimens were subjected to tensile tests to evaluate the influence of surface finishing (machined vs. as-built), orientation (with respect to the printing layers) and cooling strategy on the key mechanical parameters. The whole strain field was then studied in detail through Digital Image Correlation (DIC) monitoring technique. Key findings are given in terms of geometrical characterization of surface roughness and thickness variability, as well as a quantitative assessment of the influence of geometrical irregularities on the main mechanical parameters. The material response reveals a significant anisotropy related to a marked crystallographic micro-texture. The geometrical irregularities negatively impact some of the mechanical parameters of the as-built material, suggesting further considerations for practical structural design applications.

The technical note investigates the tensile force-displacement response of the hysteretic steel yielding brace known as Crescent Shaped Brace and characterized by a boomerang-like geometrical shape. The force-displacement curve is governed by three key performance points which correspond to the transition points separating the initial elastic behaviour, the flexural plastic behaviour, the geometrical hardening behaviour and the final axial plastic behaviour. In particular, the influence of the main geometrical parameter of the device, the so-called “lever arm”, on the strongly non-linear force-displacement behavior is analyzed by means of a simplified kinematic model. Based on this, analytical estimations of the key performance points are derived and compared with numerically simulated force-displacement curves.

In the last decades Additive Manufacturing has gained fundamental importance in the development of digital fabrication for the automotive, aerospace, biomedical and only lately civil engineering field. In particular, the technology of Wire-and-Arc Additive Manufacturing, based on a welding process adopted on a robotic arm, is the most suitable to realize structural elements which usually requires large dimensions of the printed outcome, with still a good mechanical response of the printed metal material. The authors have been part of a pioneering work which provides the first insight into the material and geometrical properties relevant to characterize 308LSi stainless steel elements to realize the first 3D-printed steel footbridge to be held in Amsterdam by 2020 and manufactured by the Dutch company MX3D. In detail, the work presents the first results of an intense geometrical study to characterize the intrinsic irregularities of the printed outcome, by means of hand measurements and high-precision 3D scan acquisition of different element types.

Additive Manufacturing has recently gained great importance to produce metallic structural elements for civil engineering applications. While a lot of research effort has been focused on different technologies (such as Powder Bed Fusion), there is still quite limited knowledge concerning the structural response of Wire-and-Arc Additive Manufactured (WAAM) metallic elements, as very few experimental campaigns aimed at assessing their geometrical and mechanical properties have been carried out. The paper presents selected results of a wide experimental campaign focused on the assessment of the main geometrical and mechanical properties of Wire-and-Arc Additive Manufactured (WAAM) stainless steel material, carried out at the Topography and Structural Engineering Labs of University of Bologna. In detail, the focus is on the characterization of the surface irregularities by means of various measuring techniques and on the evaluation of the main material mechanical properties, including tensile and compressive strengths, Young's modulus and post elastic behavior. Tests results have been interpreted through statistical tools in order to derive mean values and gather information about the variability of both geometrical and mechanical parameters.

The manufacturing of large scale structures with Additive Manufacturing (AM) represents one of the main challenges facing the construction industry today. The first recent large-scale realizations with the Wire-And-Arc Additive Manufacturing (WAAM) technology have shown the potential of WAAM in changing the way steel structures are currently designed and manufactured. However, despite these pioneering applications, the full comprehension of synergies and possibilities of WAAM in terms of architectural shapes, structural behavior and material response are far from being completely exploited. In this article, the overarching design process from concept to fabrication of a half-scaled diagrid column 3D printed with WAAM technology is illustrated. The research methodology is based on a computational workflow integrating various aspects such as material properties, manufacturing features and global architectural design. The final designed WAAM-produced diagrid column has been exposed at “The Big 5 - International Building and Construction” show, held in Dubai from November 25th till November 28th 2018.

Additive manufacturing (AM) has gained great importance in the recent development to produce metallic structural elements for civil engineering applications. However, research effort has been focused mainly on powder-based processes, while there is still limited knowledge concerning the structural response of wire-and-arc additive manufactured (WAAM) metallic elements, and very few experimental data concerning their mechanical properties. This paper presents the first results of a wide experimental campaign aimed at assessing the mechanical properties of WAAM plates produced using a commercial ER308LSi stainless steel welding wire. The aim is to evaluate the effect of the orientation in the tensile behavior of planar elements considering specimens extracted along three different directions with respect to the deposition layer: transversal direction (T), longitudinal direction (L), and diagonal direction (D). Compositional, microstructural, and fractographic analyses were carried out to relate the specific microstructural features induced by WAAM to the mechanical properties. The results show that the chemical composition of the plates meets the requirements of UNS-S-30403 for an AISI 304L austenitic stainless steel. The as-built samples were substantially defect-free and characterized by a very fine microstructure of γ and δ phases The fineness of the microstructure and the negligible defect content led to values of tensile strength and elongation to failure in line with the traditionally manufactured stainless steel elements. Anisotropy in the tensile properties between T, L, and D specimens was observed, and the highest elastic and plastic properties were measured in D specimens. This result is related to the crystallographic and mechanical fibering induced by the additive process, that led also, in case of D samples, to the highest density of cell boundaries, obstacles to the dislocation slip, located at 45° with respect to the loading direction, where plastic deformation preferentially occurs.

Early investigations suggest that the use of Additive Manufacturing (AM) technologies for construction has the potential to decrease labor costs, reduce material waste, and create customized complex geometries that are difficult to be manufactured using conventional construction techniques. Nevertheless, the full exploitation of AM technologies requires data on the material mechanical properties so that reliable and safety design requirements can be developed. Among different metal AM techniques, the so-called Wire-and-Arc Additive Manufacturing (WAAM) results to be potentially suitable to realize large-scale structural elements of any shape and size. However, the results of early experimental tests on WAAM-produced alloys suggest the need of ad-hoc considerations to properly interpret the geometrical and mechanical features of the printed outcomes. The present study analyzes the data obtained from the experimental results of tensile tests carried out on WAAM-produced 308LSi stainless steel elements with the purpose of calibrating design values and partial safety factors. In order to account for the anisotropic behavior proper of WAAM-produced elements, the design values of the main mechanical parameters have been calibrated for the three main orientations of the specimens with respect to the deposition layer. The calibrated design values and partial safety factors for the yielding and ultimate tensile strength are compared with recommended values for stainless steel structures as provided by EN1993:1-4 - Eurocode 3 (EC3). Additional considerations upon the Young's modulus values, highly influenced by the anisotropic behavior of WAAM-produced stainless steel, are presented as well.

The dynamic response of complex structures can be estimated by means of Generalized Single Degree of Freedom (G-SDOF) systems. The original concept of the G-SDOF system is revisited using an alternative viewpoint based on the equilibrium of the three resultant dynamic forces associated with the stiffness, mass and damping components, and applied to frame structures equipped with viscous dampers. In particular, it is shown that a generic structure with supplemental dampers can be reduced into two different G-SDOF idealizations, based on the global translational or rotational equilibrium. The mechanical analogies of the two G-SDOF systems provide physical insight into the dynamic behavior of structures with viscous dampers, since their energy dissipation capacities can be graphically assessed in terms of the resultant dynamic forces and corresponding lever arms. The approach is applied to shear-type structures with specific dampers configurations resulting in proportionally damped systems. Novel analytical estimations of their first circular frequencies and modal damping ratios are obtained, providing upper- and lower-bounds of the exact values. Limitations in the use of the approach for structures with different dampers configurations resulting in highly non-proportionally damped systems are also discussed.

With the advent of a new arc-based additive manufacturing (AM) process, referred to as Wire-and-Arc Additive Manufacturing (WAAM), the scale of the metal printed parts increased up to several meters, thus becoming suitable for large-scale applications in marine, aerospace and construction sectors. However, specific considerations in terms of geometrical and mechanical properties ought to be made in order to effectively use the printed outcomes for structural engineering purposes. The introduction of the novel printing strategy referred to as “dot-by-dot”, consisting in successive drops of molten metal, enabled the use of WAAM for complex lattice structures, made by continuous grids of WAAM bars. Nevertheless, their proper design requires an accurate evaluation of the influence of the non-negligible inherent geometrical irregularities on the mechanical response of the bars. Hence, extensive experimental work is needed in order to evaluate the mechanical response of WAAM bars with geometrical imperfections. The present study is thus focused on the assessment of the mechanical response in tension of WAAM-produced 304L stainless steel small bars in terms of key effective mechanical parameters. As such, the mechanical characterization through tensile tests is supported by microstructural investigations and detailed studies on the geometrical features. Three batches of bars are studied, each one printed at different build angles representative of limit cases for practical applications. The microstructural analysis confirms the preferential grain orientation typical of WAAM process for all three build angles. The results of the geometrical and mechanical characterization clearly evidence the non-negligible influence of the inherent geometrical imperfections on the mechanical response in tension of the printed bars, with a detrimental effect of the build angle on the main key effective mechanical parameters. Overall, the results highlight the need of specific investigations on both geometrical and mechanical properties of WAAM bars for structural design purposes.

In the present paper, the coupled behavior of structural systems obtained by connecting a moment resisting frame structure with a vertical elastic truss, known in the literature as strongback, which acts as a mast by imposing to the structure a given lateral deformed shape, is investigated. The rigid behavior of the strongback, which has to be designed to remain in the elastic field under strong seismic ground motion, linearizes the lateral displacement profile of the adjacent frame through an exchange of mutual horizontal actions. The presence of the strongback should thus help in limiting undesired effects, such as soft-story and weak-story mechanisms. For the proper design of both the truss system forming the strongback and the frame members, the preliminary evaluation of the actions exchanged between the systems is of fundamental importance. The aim of the work is to develop analytical formulas for estimating mutual actions and internal actions in the frame members for the limiting cases of shear-type frames and pendulum-type frames. Finally, some numerical simulations of frame systems with strongback systems as subjected to earthquake ground motions are developed, including the cases of frames with flexible beams. It is found that the amplitudes and distributions of both mutual actions and internal actions in the frame are significantly affected by the beamto- column stiffness ratio. In the case of relatively stiff beams, the mutual actions tend to reduce the shear and bending moment at the lower stories, which are rather uniformly redistributed along the height. On the contrary, in the case of relatively flexible beams, large mutual actions and internal actions (shear and bending moments) are concentrated at the lower stories, with the upper stories remaining practically unloaded.

In the present paper different viscous dampers placements are compared through an analytical design procedure, developed in order to easily evaluate the performances in terms of cost and effectiveness. Once the performance objective is imposed through a target damping ratio and the corresponding reduction in seismic response of the structure, by assuming a specific deformed shape it is possible to evaluate the period of vibration of an Equivalent Single Degree of Freedom system, from which the structure peak response under earthquake levels can be estimated. From these, the total cost expressed in terms of maximum required damper forces and its effectiveness as the ratio of the total cost and the benefit in base shear reduction are evaluated for each dampers configuration chosen. The analytical steps of the procedure are specified for the case of linear along-the-height displacement profile for three specific dampers configurations: (i) dissipative tower DT; (ii) inter-storey placement, referred to as Stiffness-Proportional Damping (SPD); (iii) fixed-point placement, referred to as Mass-Proportional Damping MPD. The actual seismic behaviors of the design solutions studied are verified through non-linear time-history analyses.

This paper presents the basic concepts and the first engineering evaluations towards the development of the technology for the fully automated design and construction of 3D-printed grid-shell dome-like building structures inspired by the ideal visions of city domes. A suitable technique to realize such free-form shapes is the so-called Wire and Arc Additive Manufacturing (WAAM), whose process parameters and material properties are still not fully explored and known, as a comprehensive set of international standards has not yet been available. This work provides the first insight into the material and geometrical properties relevant to characterize 308LSi stainless steel elements. In detail, the results of material tensile tests and optical measurements are illustrated.

Wire-and-Arc Additive Manufacturing (WAAM) has been recently adopted to create innovative structural forms and architectural shapes. As shown by few experimental investigations, the layer-by-layer deposition induces a remarkable anisotropy in the elastic response of the WAAM-produced alloys. A suitable topology optimization technique is implemented to account for this peculiar behavior of the material, which is generally disregarded in design tools that are conceived for traditional manufacturing. First, an orthotropic material model is derived from the data of an experimental investigation that was recently performed on alloys made with 308LSi stainless steel wire feed. Then, an optimization procedure is implemented that exploits, as design variables, not only the density field of an orthotropic material phase, but also the orientation of the symmetry axes of such material with respect to a reference frame (i.e. the printing direction used to build the whole structural element). Minimum weight problems with displacement constraints are solved to find optimal solutions that are compared to those achieved by performing topology optimization with (i) isotropic stainless steel or with (ii) the WAAM-produced alloy for prescribed orientations. Numerical simulations assess that the printing direction remarkably affects the stiffness of the optimal layouts, as well as their topology.

The proposed work presents the seismic performances of a structural system obtained by connecting a moment-resisting frame structure with a vertical elastic truss, known in the literature as strongback. The strongback system is able to limit the development of excessive inter-storey drifts thus reducing potential dangerous phenomena such as weak/soft stories. In addition, the strongback system can be equipped with viscous dampers enhancing energy dissipation properties of the whole structure. In the present paper, an applicative example is developed in order to compare the effectiveness of both the strongback system and different dampers placements.

A direct procedure for the seismic design of regular multi-storey frame structures with added viscous dampers is recalled in this paper and applied for the seismic improvement of a reinforced concrete frame structure. The application is carried out with reference to the real case-study of a five-storey hospital building located in Bologna. Three different solutions of seismic retrofit are presented and analyzed in terms of both performances and costs. They differ in terms of viscous dampers used, either Stiffness Proportional Dampers (SPD) or Mass Proportional Dampers (MPD), and number of external towers built, to provide an indirect implementation of the MPD system. The applied design procedure is based on the so-called “direct five-step procedure” which is intended to guide the structural engineer from the choice of a target reduction in the seismic response of the structural system to the identification of the corresponding damping ratio and the mechanical characteristics of the commercially available viscous dampers.

The paper presents the results of an experimental campaign aimed at assessing the structural performances of unreinforced masonry systems realized with a novel ductile mortar. The experimental campaign described in the paper is part of the research project “Zero Environmental Risks in Our buildings” (ZERO) whose broad objective is to develop a new class of construction materials and decorative products characterized by high environmental compatibility, i.e. Volatile Organic Compounds (VOC)-free, as well as superior performances in terms of both chemical and mechanical properties. Two mortar formulations, one for thin-joints and one for thick-joints, have been tested under flexural and compressive loads, and their performances compared with those of benchmark mortars available in the market. Masonry assemblies have been realized with different types of hollow clay blocks available in the market, including flat structural and non-structural blocks for thin joints and structural clay blocks for ordinary joints. The results of the experimental tests proved the effectiveness of the innovative ductile mortar in improving both shear strength and shear deformation capacities of unreinforced masonry assemblies.

The present work aims at providing the first considerations upon the application of innovative manufacturing technology for civil engineering purposes. In particular, among the 3D printing processes currently available, Weld-Based Additive Manufacturing (WAM) results to be the most suitable technique for the realization of innovative structural forms in metal material. The great potential of taking the printing head "out of the box" allows for the construction of innovative shapes by adding layer upon layer of welded steel. In particular, the study is focused on the realization of the first 3D-printed steel footbridge by a Dutch company held in Amsterdam, called MX3D, and its Additive manufacturing process, which results in specific constraints and limitations to be taken into account for design purposes. First, the design issues are described, by considering the printing parameters to be adopted for the realization of large-dimensions structures, and then the implications in terms of specific geometrical and mechanical characteristics are studied. These first engineering evaluations are intended to pave the way towards the development of a ground-breaking technology for the fully-automated design and construction of novel 3D-printed building structures through innovative robotic manufacturing processes whose parameters are still not fully known

The present work explores the possibilities of 3D printing applied to structural engineering field to create innovative design and optimized shapes. By means of Wire-and-Arc Additive Manufacturing, structural members are manufactured by placing layer upon layer of welded steel material in an automated process. Additive Manufacturing, thanks to the theoretical freedom in the geometrical shapes that can be obtained, open completely new possibilities for designers. On the other hand, specific aspects related to material properties and geometrical irregularities characteristics of such innovative manufacturing processes have to be properly considered in the design phase. Along with digital design tools recently developed and applied in architecture and construction for the realization of new shapes and forms through parametric design, the work presents a new structural shape for diagrid columns to obtain structurally optimized forms adapted to be efficiently realized by means of Wire-and-Arc Additive Manufacturing process taking into account the specific features of the printing process. The outcome of the study is the final realization of the column in a 1:2 scaled dimension. These first engineering evaluations are intended to pave the way towards the design of a new family of optimized structural elements to be efficiently 3D-printed, towards the fully-automated design and construction of novel 3D-printed building structures.

In the present work, a Performance-Based Seismic Design procedure applied to multi-storey frame structures with innovative hysteretic diagonal steel devices (called Crescent Shaped Braces or CSB) is introduced. CSBs are steel elements of peculiar geometrical shapes that can be adopted in frame buildings as enhanced hysteretic diagonal braces. Based on their "boomerang" configuration and placement inside the frame structure, they are characterized by a lateral stiffness uncoupled from the yield strength and, if properly inserted, by an overall symmetric hysteretic behavior with hardening response at large drifts, thus preventing from global structural instability due to second-order effects. The procedure here presented is intended to guide the structural engineer through all the steps of the design process, from the selection of the performance objectives to the preliminary sizing of the CSB devices, up to the final design configuration. The steps are described in detail through the development of an applicative example.

The proposed work presents some selected results on a wide experimental campaign devoted to evaluate the mechanical response of unreinforced masonry (URM) systems realized with an innovative mortar, characterized by a ductile behavior. The improved response of the mortar induces higher resistance of masonry panels subjected to shear actions, thus improving their earthquake-resistant performances. The experimental campaign described in the paper is part of the research project called “Zero Environmental Risks in Our buildings” (ZERO), funded by the PORFESR 2014-2020 call of Emilia- Romagna region (North Italy). The final objective of the project is to introduce in the market a new class of construction and decorative materials, characterized by a high environmental-compatible profile (VOC-free) and improved chemical and mechanical properties. One of the objectives of ZERO has been the improvement of the ductile behavior of URM systems. To this aim, a wide experimental campaign has been carried out at the CIRI Building & Constructions at University of Bologna. The experimental tests have been devoted to characterize the material properties through specimens of the innovative mortar, as well as the coupled structural response of masonry systems realized by means of the innovative mortar and different clay blocks. In the present work, the preliminary results of the characterization of mortar specimens is described. In detail, three-point bending tests and centered compression tests have been performed to derive the flexural and compressive strength of the innovative mortar, as well as quantify the ductile response of the material. From the results, the innovative mortar is found to have improved resistance towards tensile action, and superior ductility values with respect to the specimens realized with traditional mortar commonly available in the market.

The behavior of “ductile seismic frames”, which is strongly related upon construction details and specific design rules, during strong earthquake appears to be often not in agreement with average expectations of modern societies which consider not admissible human lives losses and building collapses. The issue of safe constructions against natural hazards appears even more important for those essential facilities, such as hospitals and schools whose collapse could provoke dramatic consequences. In the present paper, a brief history of construction from the appearance of first framed structures to wall-based and tubular structures for tall buildings and to the more recent construction technologies for low-rise buildings using insulating concrete forms and sandwich panels, is provided within Earthquake Engineering perspectives. It is shown as the use of “tubular” structures for low-rise buildings (such as residential buildings) realized with modern construction technologies would ensure the achievement of quite superior structural performances, especially with respect to seismic induced actions.