Composite structural systems are widely used in the construction industry, effectively combining the properties of the constituent materials. Classical examples include here steel - concrete composite members, which combines the tensile strength and ductility of steel with the compressive strength and stiffness of concrete. Hybrid systems, on the other hand, combine elements of different materials, or of same materials but different characteristics, allowing for tailored deformation/strength/ductility demands or different interaction levels between different parts for optimized structural performance, improved construction cost, low environmental impact, and longer durability. Examples include concrete core systems combined with steel framing, dual steel frames made of High Strength Steel (HSS) and Mild Carbon Steel (MCS), or more recent developments, like steel and cross-laminated timber (CLT) hybrid constructions, or hybrid steel- Fiber Reinforced Polymer (FRP) composite systems. This paper summarizes a series of research projects and practical applications of hybrid systems, all with direct involvement of the authors. Particularly, the structural connections between the members made of different materials are examined. Some of the structural solutions detailed in the paper have been confirmed by full scale laboratory tests but also by advanced numerical simulations. The paper aimed also to demonstrate the efficiency of hybrid systems over classic homogeneous or composite ones, but also the inherent technical difficulties faced by designers, due to the lack of consistent design provisions. There are also emphasized some further developments based on the activities presented in the paper.

Due to their lightweight nature, cold-formed steel (CFS) profiles can be assembled into a wide variety of structural configurations, including trusses and corrugated web beams. In residential and commercial construction, integrating service installations often necessitate web openings. Because these openings introduce structural vulnerabilities, implementing effective strengthening solutions is essential to preserve the beam’s original load-bearing capacity. Continuing prior research that investigated built-up beams with lipped channel flanges and trapezoidal corrugated steel webs connected via resistance spot welding or MIG brazing, this paper evaluates two full-scale strengthening techniques, a reinforcing steel plate spot welded to the corrugated web, and a border-type frame using MIG brazing along the opening perimeter. The experimental results indicate that the reinforcement solution based on MIG brazing minimised the detrimental effects of the opening more effectively, reaching a superior ultimate load capacity and ductility.

Light steel based composite (sandwich) panels are largely used for the envelopes of different kinds of buildings. Owing to their non-combustible properties, new generations of sandwich panels are made with mineral wool insulation materials. Even less efficient than classic foam cores in terms of thermal conductivity, these panels are still efficient for energy consumption. Under extreme loading conditions, such as those generated by external blast events, sandwich wall panels may experience substantial damage under both positive and negative pressure regimes. This study presents experimental and numerical results obtained from sandwich panels with a mineral wool core subjected to transverse loading up to complete structural failure. Numerical models were developed and calibrated using the SAP2000 structural analysis and design software to observe and replicate behavior. Using previous studies on blast resistance, numerical models were further used to preliminary evaluate the response of the mineral wool sandwich panels against close in detonations.

Many buildings or other types of constructions remain unfinished at various stages, put on hold, or suffer serios time delays from the schedule. Before resuming the works, these constructions may require extensive structural assessment to determine the condition of the structure, identify the damages, and provide recommendations for intervention. This paper summarizes the intervention works for the retrofitting of an existing 11 story building made with a hybrid system of reinforced concrete walls, steel frames, and composite floor decking. The building is located in a marine environment and moderate seismicity area. The constant exposure to aggressive conditions caused different extents of damage, and most affected were the composite floor steel decking. Experimental tests were carried out to validate a strengthening solution for the composite floor slabs based on adding a new concrete layer and post-installed shear studs. Numerical models have been calibrated against test data. Additional requirements were considered, i.e., reducing the potential for progressive collapse and improving global behavior for seismic response (torsional stiffness, floor diaphragm effect).

Robustness of multi-story buildings, and more globally, of any construction, is an important matter when the safety of built environment is considered. However, as seen from past natural or human-made originated accidents, an improvement of the design process allowing us to ensure an appropriate structural robustness is still required. The consequences can be aggravated in case of cascading events, e.g., seismic aftershocks, explosion or localized fire after an earthquake. The study presented in the paper investigates the robustness of multi-story steel frames following the loss of a column. The case study structures were designed for the envelope of seismic and persistent design situation demands, considering low seismicity conditions of the site. For the perimeter locations, the structures had adequate resistance to prevent the progressive collapse, due to the large overstrength resulted in beams and joints from either non-seismic or seismic design situations. For the interior spans designed for gravity loads, the solution with partial strength joints also provided the capacity required to resist the loss of a column, with maximum load multipliers, λ, larger than unity. On the contrary, if the beams (and end connections) are designed to provide only the minimum tying resistance specified in the codes, the progressive collapse initiates.

End plate bolted joints are frequently used in seismic resistant multi-story steel frame structures. Their flexural behavior can range from flexible and partial strength to stiff and full strength. For partial strength joints, the main source of ductility is the T-stub macro-component. While most studies and design provisions envisaged the response under single critical events, e.g., strong earthquakes, experience shown they may be vulnerable in a multi-hazard environment, e.g., earthquake aftershock, or column loss (due to fire or explosion) after earthquake. The study presents the results obtained on a set of T-stub elements subjected to monotonic and cyclic loading, single or in sequence monotonic after cyclic. The influence of several parameters including the loading protocol and plastic deformation demands, are also investigated. Numerical models are calibrated against test data.

Explosions produced in urban or industrial areas due to accidental or intentional detonation of explosives are low-probability but high-impact events. If standoff distance from buildings or other constructions is small, explosions can cause structural failures and serious injuries or deaths of occupants. Although heavy/stiff enclosure systems were long considered appropriate to protect buildings against explosive threats, lighter and flexible steel-based systems, which are increasingly used for modern buildings, may also provide satisfactory performance. The study presents the results of full-scale blast tests on liner tray walls attached to a steel frame building. Walls were tested against increasing blast charges until failure. Due to very high peak pressures and very short durations, the loading regime could be classified as impulsive. The ultimate strength is given by the failure of the end fasteners, after large bolt hole elongations coupled with the pull-through of the end fasteners. If the insulation and outer cladding are lost, the liner trays are directly exposed to blast and the seaming fasteners at overlapping sheets are prematurely lost. As a result, the wall panels work mostly independently, as one-way elements. On the contrary, if the outer cladding protects the seaming fasteners, the overlapping provides additional capacity due to membrane effect (two-way behavior). Numerical models were calibrated using test data and were further used to obtain more information about liner tray behavior under blast.

Steel plate shear walls (SPSW) are efficient lateral load resisting systems, with high ductility and initial stiffness properties. When coupled with moment resisting frames (MRF) in sharing the seismic induced lateral loads, they can provide more resilient systems, if the later provide the restoring forces to recenter the structure. If the damage is localized in easily replaceable panels, repairing is easier and costs less. In the paper, dual SPSW-MRF systems are investigated for evaluating their seismic performances (plastic mechanism, global ductility, residual drift, recentering). Numerical models employed nonlinear shell approach for SPSW and bar elements for MRF and were calibrated against experimental data. Several repair/replacement sequences of damaged panels were investigated. For low to moderate damages induced during a seismic event, the MRF provides the structure recentering capacity. For significant damages however, plastic deformations in MRF prevent the structure from returning to its initial positions, as it remains with residual story drifts. Such residual drifts do not, however, mean that the building will be demolished, as the structure is still reparable but probably at higher costs.

Constructions should be able to remain stable for their designed lifetime, from 50 to 100 years, even more. As climate change intensifies, extreme weather events such as temperature variation, humidity, heavy rainfall, floods, and windstorms become more frequent and more severe. These events pose a significant threat to conventional building designs and infrastructures. Consequently, there is a growing demand for climate-resilient constructions that can withstand extreme weather conditions. Protecting infrastructure and buildings to cope with these threats is a complex challenge. Building materials, design, and construction techniques need to be adapted to ensure the durability and safety of structures in the face of changing climatic conditions. The safety margins and robustness of constructions for undesired events in technical regulations and standards should therefore be continuously re-evaluated so that the designed level of reliability is maintained. To control by design, alternatively to traditional prescriptive design codes, where the building must conform to a set of given requirements that results in a hard-to-quantify performance, a Performance-Based Design (PBD) method might apply to explicitly define and achieve the desired structural performance. Based on the review and analysis of dedicated literature and research reports related to this complex problem, potential technical solutions are discussed. Also, two real study cases, one caused by extreme wind and the other by extreme drifted snow, are presented.

The growth of industrialization and the rapid expansion of densely populated urban areas have increased the risk caused by technological and man-made hazards. In particular, the accidental or intentional detonation of high explosives (e.g., improvised explosive devices IEDs), can cause damages to buildings and infrastructures and severe harm people. These risks can be minimized by a better planning, design, and construction, e.g., avoiding brittle construction materials, especially within the building envelope, and adopting more robust construction techniques. The study presented in the paper investigates the capacity of light steel-based wall panels to resist the effects of external explosions. The experimental results showed the ultimate capacity of the wall panels is strongly dependant on the initial design conditions and panel-to-structure fastening solution. A numerical model has been also calibrated using Etabs program.

Light weight steel-based panels are largely used for the envelope (walls, roofs) of different kind of buildings. They are typically designed from the weather-related conditions of the site (climatic actions, thermal comfort). Despite their low weight, they have adequate load carrying capacities under transverse loads. However, under extreme loading conditions, like the high intensity pressures associated with external explosions, the panels can be heavily damaged or detached from the structure. The study presented in the paper describes the results obtained on liner tray walls tested for transverse loading until complete failure. The test program included also shear tests on end fastenings. The liner trays were arranged as single span systems supported on side beams and loaded at the mid-span. After a quasi-linear response, the maximum flexural strength is reached. Then, due to a local instability, there is a drop in capacity, followed by an increase in response capacity due to development of catenary forces. If the end fasteners have adequate resistance, the ultimate capacity in the catenary stage can be significantly higher than the peak flexural capacity. The ultimate capacity depends also on the in-plane stiffness of the support columns.

The difference between local and distortional buckling of thin-walled cold-formed steel sections with stiffened elements, of members in both compression and bending are taken into consideration applying relevant design formulas available in the codes. However, for unstiffened elements, or for those unstiffened enough, the difference between distortional and local buckling modes, in some cases, and particularly for U-, Z- and L-sections, might be unclear. When these sections are edge stiffened, in the case of lipped flanges, it is not always easy to identify which of the two are dominant, while in the case of interaction with an overall buckling mode, the situation might be even more difficult. Local buckling modes involve the buckling of cross-sectional walls, while distortional buckling, also known as “stiffener buckling” or “local-torsional buckling”, is characterised by the rotation of the flange at the flange/web junction in members with edge stiffened elements. Both types of buckling are also known as “sectional” buckling modes. For interactive local/distortional buckling with the overall one, the correct evaluation of the “short” member capacity might play a significant role when checking the design capacity of a slender thin-walled member prone to interactive sectional-overall buckling mode. In the case of a stiffened flange, the effectiveness of the stiffeners can make the difference between local and distortion buckling. A boundary between local and distortional bucking of a given open section can be established to be considered in the design. To correctly apply the Ayrton–Perry formula, in order to get the ultimate capacity of a thin-walled cold-formed steel slender member, prone to interactive local or distortional buckling with the overall one, the strength of the short member has to be identified. This paper summarises the study of this problem, including the main results and conclusions, emphasising that it is difficult to separate the local and distortional modes in the case of cold-formed steel open sections used in practice, and there is some interaction between these two modes.

The building subjected to intervention is composed of concrete walls and steel frames and is located in a moderate seismic area.The steel frames are made with partial strength joints and are designed to carry gravity loads only.The building has 11 above grade stories, with a lateral setback starting from the 8 th story.The main structure was completed in 2003, but without installing the enclosure walls and roofing.Since then, most code provisions employed in the initial design were updates several times.Also, due to the aggressive environmental conditions, characteristic to the sea climate (the building is located close to the sea cost), the corrosion affected the steel frame elements (beams, columns, connections) and the profiled steel sheeting of the composite floors.In addition to the retrofitting intervention, the new architecture asked for completing the setback till the last story.The study presented in the paper summarizes the structural assessment and proposed interventions for retrofitting the gravity load system and improving the progressive collapse resistance using alternate path method and nonlinear static procedure.Ongoing studies will also consider more advanced analyzes using non-linear dynamic analyzes.

In the first part of the paper, three examples of sustainable mixed building technologies are presented, which, for the main frame, combine cold-formed with hot-rolled steel sections, using timber for floor joists and cladding studs. In the second part of the paper, two examples of single-family houses made of cold-formed steel wall studs sheathed with OSB are detailed. The buildings are located in moderate seismic regions. The paper presents aspects related to design and detailing, including aspects of structural features. For structural design, a prescriptive method was applied, based on the test results on full-scale shear wall panels made of cold-formed steel profiles and OSB panels, under monotonic and cyclic loads. The results of life-cycle analyses of these constructions, compared with various traditional solutions, are also presented.

Light weight panels are largely used for the envelope (wall assembly, roofing system) of different kind of commercial or industrial buildings. They are typically designed from the weather/climate related conditions of the site (climatic actions, thermal comfort). Despite their low weight, they have adequate load carrying capacities under transverse loads. However, under extreme loading conditions, like very heavy winds or external explosions, such walls can be damaged or destroyed, under either positive or negative pressures. The study presented in the paper describes the results obtained on wall sandwich panels tested for transverse loading until complete failure. The panels are arranged as single span systems supported on side rails and loaded at the mid‐span. After a quasi‐linear response, the maximum flexural strength is reached. Then, due to a local dynamic instability, a sudden drop in capacity is recorded, followed by a second increase in capacity due to development of catenary forces. If the end fasteners have adequate resistance, the ultimate capacity in the catenary stage can be significantly higher than the peak flexural capacity. The ultimate capacity depends also on the stiffness of the support system (side rails and columns).

Constructions, starting from the first man-made shelters, continuing with the first organized settlements and the means of transport between them, with fortresses and fairs, monuments and palaces, and ending with the construction works of modern times, they have all supported and marked the human civilization. First, it was the builder master, then the architect appeared, integrating both the knowledge needed for the construction's functionality and esthetics and the safety and durability. Then, when the scale and complexity of the constructions imposed the division of this integrated profession, construction engineering, responsible for safety and durability, but also for the establishment and application of building technologies, separated from architecture. The emergence and development of human settlements have always been influenced, favored, or restricted by the natural conditions and resources of the territory. Soil conditions, climate actions (e.g., wind, snow) and seismicity remained decisive factors in the development of the construction techniques. Constructions in Romania have developed largely in line with the general scheme described above. There was the Stone Age, then the Bronze Age, leaving traces of specific construction, then a Dacian civilization with remarkable achievements, followed by the Roman conquest and integration, with urban development and transport infrastructure at the level of the other provinces of the empire. After the withdrawal of the Roman administration from Dacia, a transition period followed, with numerous migration periods (invasions), during which it collapsed rather than developed. Then followed the early Middle Ages and the consolidation of medieval structures/states (the establishment of Romanian Voivodes), the Late Middle Ages, the Phanariot Rule and Ottoman vassalage in Moldavia and Wallachia, and Hungarian Rule in Transylvania. The modern Romania was marked primarily by the Unification of The Romanian Principalities and the introduction of the Austro-Hungarian administration in Transylvania, followed by the independence and the establishment of the Kingdom of Romania. The World War I followed, then the Greater Romania (after the union with Transylvania, Bessarabia, and Bukovina) and the interwar period, the World War II and the period of the communist regime, which started in December 1947. The collapse of the communist regime in December 1989 brought about major changes in all areas of economic and social life, and implicitly in the field of constructions. Constructions followed and characterized these historical periods, each with its particularities (aesthetics, technology, functionality). After the Roman period, starting from the early Middle Ages and ending in the first half of the 19th century, constructions in Romania were made on an empirical and intuitive basis rather than on professional and scientific education. There are two important moments here in the promotion and development of this profession in Romania. The first, on 15 November 1813, when a class of engineering and terrestrial measurements in Romanian language is being established at the Royal Academy in Iasi. It is actually the first high school for construction and geodesy in our country. The second moment is the establishment in Bucharest, in 1864, of the School of Bridges and Roads, Mines and Architecture, according to the French model. Three years later, in 1867, it turns into the School of Bridges, Roads and Mines, which becomes the National School of Bridges and Roads in 1888. This is indeed the first institution of higher education for construction engineers, which in the sense of that time was attributed to civil engineering. Following the Unification of the Romanian Principalities, Romanian cities are undergoing a vast process of modernization. The development and organization of urban infrastructure has also been envisaged through the development and modernization of public lighting, sewage system, and means of transport. The period between WWI and WWII has meant for Romania a period of remarkable economic, cultural, and spiritual progress, with significant achievements in the urban and transport infrastructure. Then, in the period 1948–1989, in Romania was built very much, perhaps not always justified and where it should have been. However, it should be said that, in the field of civil engineering and, in particular, in structural engineering, these structures tested the professional competence of the engineers who designed and executed them. The construction sector experienced a general decline in the early 1990s, then started to grow between the years 2005–2008, being one of the pillars of Romania's economic growth. After significant stagnation between 2009–2012, the construction sector started to grow again, with a very good year 2015, especially due to the residential construction sector. The non-residential construction sector saw lower growth rates. The achievements were however remarkable, as many of these constructions are comparable to similar constructions built in Europe and around the world. The history of construction is ultimately an image of the cultural, social, economic, and technological development of a society. This chapter tried to reflect this complexity and multidisciplinarity of the profession.

The study summarised in the present paper has investigated the post-earthquake robustness of multistorey steel framed structure prone to fire action. Building Frames, i.e., homogeneous system with moment resisting frames (MRFs) on one direction and centrically braced frames (CBFs) with inverted V braces on the other direction, of 4 and 8 stories, as case study structures, are numerically analysed. The structures were designed according to the relevant codes for persistent and seismic design situations in 2 locations with different seismicity. The structures were assessed and optimized for seismic loading using a push-over analysis. Afterwards, the robustness capacity was checked against thermal action, through a nonlinear dynamic analysis, using Extreme Loading for Structures (ELS) software [8].