Seismic strengthening through external exoskeleton

Highlights Abstract Within the framework of the European project Pro-GET-onE, two cases of reinforcement obtained by applying a steel exoskeleton connected to the existing structures will be presented. The first case refers to a reinforced concrete building where a three-dimensional steel addition is considered with the aim to provide extra-space with high energy performance envelopes and structural improvement to the existing structure. The second case concerns a very common type of residence in the Netherlands, the terraced house. In this case the intervention will focus on the realization of a planar frame leaning against the existing wall weakened by the openings. In both cases this strengthening strategy gives an added value to the existing building with the integration of different technologies to achieve a multi-benefit approach by a closer integration between different aspects such as social, safety and energy and that is the reason that leads to this choice of intervention instead of the traditional ones. The seismic hazard in Europe is one of the most critical issues of civil engineering. The necessity of improving existing buildings, in terms of energy and structure is always a new challenge for designers. The use of integrated improvement systems can be the solution to common obstacle from the project to the realization, such as the invasiveness, the cost and the duration of the construction phase. The current scenary is rich in different intervention techniques due to the heterogeneity of the buildings. The study focuses on two cases of seismic reinforcement through the use of steel exoskeletons in different contexts through different design solutions. Following the description of the issues related to the vulnerability of the two case studies, the procedures for evaluating the improvement are illustrated. Finally, the results deriving from the application of the strengthening structures are presented, showing ample margins for improvement in both cases up to the achievement of demand values.


INTRODUCTION
The EU research project Pro-GET-onE focuses on the identification of integrated technological solutions that allow improvements in the energy performance and seismic safety of existing residential buildings. The criteria to follow are those of the search for sustainable solutions and fast realization, which do not involve the interruption in the use of buildings and which also offer functional improvements to residential units, with the extension of living space, as intrudeced by A. Ferrante et al. [1]. The research study is based on the unprecedented integration of different technologies to achieve a multibenefit approach by a closer integration between different aspects: 1. energy requirements -by adding (or substituting the existing with) new prefab and plug and play high energy performing envelopes and HVAC (Heating, Ventilation, Air Conditioning) systems; 2. safety -using appropriate external structures to increase the overall structural capacity of the building, while supporting the new envelope consisting in timber-based components for opaque parts/surfaces, and aluminum, glass, PV Photovoltaic, solar panels; 3. social and economic sustainability -increasing the real estate value of the buildings and the desirability of retrofit options by providing tailored and customized solutions for users, owners and house managers, increasing safeness and minimize disturbance of inhabitants.
Information from SHARE Project [2] indicates Italy, Greece, Romania and the Mediterranean countries of the European Union as the areas with the highest probability of natural earthquakes. In these areas, recent seismic events have shown how relevant is the issue of seismic vulnerability for existing buildings of reinforced concrete since many of these were designed without any reference to anti-seismic criteria. A different case concerns the Dutch province of Groningen. In this area, as will be shown in the fourth paragraph, the seismic action is of an induced nature and is caused by gas extraction carried out in the last decades. The traditional masonry houses have never been designed for seismic actions, as natural earthquakes do not occur in this region.
In the design process of a seismic improvement intervention, after a careful assessment of the vulnerability of a building, it is necessary to proceed with the choice and the adoption of streghtening interventions able to allow the structure to support the horizontal action of the seismic zone. This choice depends on numerous factors, including invasiveness, cost, global behavior and critical aspects of the structure. The Pro-GET-onE strategy proposes a type of integrated seismic improvement intervention that excludes the displacement of the inhabitants and at the same time entails an energy improvement through a system of volumetric additions in one case or with a planar addiction in the other. These objectives can be achieved thanks to the positioning of the new reinforcing structures outside the existing building, through the use of steel exoskeletons; technique, to date, in experimentation. Projects in which this strategy was used are the requalification of the office and warehouse buildings of the Magneti Marelli factory in Crevalcore (Italy) made by Teleios Srl [3,4], and the seismic reinforcement of the complex of the Department of Engineering Rural and Topographic of AUTH, located in Thessaloniki, Greece [5]. However, in the cases described, the exoskeleton does not provide integrated solutions for energy improvement and the possible volumetric expansion, as in the presented case. In the case of Groningen many houses have already been strengthened, including upgrading to Zero Energy standards.

VULNERABILITY ASSESSMENT METHODS
In front of complex and articulated buildings, the tendency is to make the assessment explicit with few equivalent parameters of capacity, such as displacement, acceleration, return period, etc. However, as the number of components allows it, it is possible to accurately represent shortages, reinforcement interventions and improved results on the members subject to horizontal actions due to the earthquake.
European regulations define the procedures to be followed to evaluate existing structures in Eurocode 8 part 3 [6]. In addition to evaluations through modal and dynamic analysis with response spectrum, the complexity of the existing structure, the partial knowledge of their geometric-mechanical characteristics, together with the uncertainties on the seismic input, lead the choice towards analysis methods characterized by intermediate levels of complexity. This is the case of pushover analysis that, while reproducing the salient expectations of the non-linear response, it is based on the assumption of static actions applied to the structure.

STUDY
The structural improvement through the application of an external threedimensional exoskeleton in steel has been evaluated on the student house in Athens, pilot case of the project in the Mediterranean area. The study of the metallic structural system to be adopted in combination with the existing structure in reinforced concrete lead to two possible different strategies: the stiffness or the damping increase. The former provides, by the application In this section, the analyzes conducted aim to demonstrate the incidence due to the application of the external exoskeleton, using the stiffness increase strategy. The same results are shortly presented in A. Ferrante et al. [7] concerning the hypothesis on the Athens case study.

DEFINITION OF THE SEISMIC ACTION
Within the scope of EN 1998 [8] the earthquake motion at a given point on the surface is represented by an elastic ground acceleration response spectrum, henceforth called an "elastic response spectrum". The horizontal seismic action is described by two orthogonal components assumed as being independent and represented by the same response spectrum. The elastic response spectrum S e (T) is defined by the following expressions [8]: • LS DL -PVR = 63%; TR = 50 years; VR = 50 years -ag/g = 0,067; • LS SD -PVR = 10%; TR = 475 years; VR = 50 years -ag/g = 0,16; • LS NC -PVR = 5%; TR = 975 years; VR = 50 years -ag/g = 0,212.

ANALYSED BUILDINGS AND STRUCTURAL MODELING
The case study represents a part of the entire building of the student house, it is divided with a seismic joint from the rest and it is considered isolated (Figures 1 and 2).   The reinforcement bars have been defined and imputed by a simulated project using the permissible stress design. Sections, reinforcement and imposed loads are listed below in tables 1 and 2.

INITIAL STATE
In the evaluation of the vulnerability of the initial state, modal analysis, linear dynamic analysis with response spectrum and non-linear static analysis were performed, the latter in the two main directions of the building.
The modal analysis shows that the main vibrating mode is that in the transversal from the elastic response spectra in terms of acceleration as a function of the structure's own period. The results regarding the initial state are presented in Table 3.
It can be noted that the building already does not have a good seismic performance, showing greater vulnerability in the transverse direction where resistance is ensured for an earthquake equal to 41% of that expected in that area. The external steel exoskeleton is designed to achieve an increase in the stiffness of the structure and therefore in capacity to collapse. Table 3. PGA values derived from the pushover analysis and related to the considered limit state in both the main directions. The capacity/demand ratio are also presented.

PROJECT SOLUTION, 3D EXOSKELETON
The project structure consists of a steel frame for each floor, with bracing in  This connection consists of a steel profile (stiff in both the reference axes) connected to the exoskeleton by means of a flange and connected to the concrete joint with an UPN profiles fixed along the perimetral beams.
It represents the situation closest to the rigid joint simulated, even if it is difficult to realize.
The application of the structure on the entire perimeter of the existing floor has been studied in order to guarantee a reinforcement in both directions, as shown in Fig. 3.
The modal analysis following the application of the exoskeleton shows that the main vibrating mode in Y (U2) transverse direction, maintains an activated mass percentage around 75,7% with a period reduced to 0,488 s.
The maximum displacements in the DL limit state correspond to 1,5 cm in longitudinal directions (X) and 1,6 cm in the transversal ones (Y) showing a reduction of respectively 23% and about 40%. Finally, using the same settings used previously, the capacity curves of the structure were recalculated, and it was possible to derive the new shear values for the determination of the PGA.
In Table 4, the comparison is shown after the application of the external steel structure.   Fig. 4.
The improvement of the capacity of the structure must be followed by a rigorous verification of capacity/demand in terms of displacement, performed in this case with the method of the target displacement described both in the Eurocode 8 [8] and in the circular of the Italian technical standards [9]. As already mentioned above, the increase in stiffness leads to a reduction in the overall ductility of the structure which requires an accurate evaluation of the displacements. Table 5 below, shows the displacement values of demand and capacity for the X direction. It can be seen how the application of the external steel structure goes to increase the D/C values which consequently must be checked in parallel to the capacity increases in terms of acceleration.

STUDY
The Ducth case studies are buildings part of blocks of typical Martini K terraced houses which were built in the 1960s when no seismic requirements were considered in the Groningen area. Therefore, the houses are designed to mainly sustain vertical loads with only marginal horizontal loading from wind. The applied strengthening method consists in the application of a steel portal in the longitudinal direction with wood skeleton walls and insulation.
The calculation of the steel portal frames assumes that the entire stabilizing function in X direction is taken over by the new steel exoskeleton. The influence of masonry walls is neglected. However, it will have to be demonstrated that the load-bearing walls can continue to fulfil their function in the deformations occurring in the NC-limit state.

INDUCED SEISMICITY
The on-going process of accurately assessing the ground conditions and the peculiarities of the Groningen area that influence the design and retrofitting of buildings is very dynamic and assumes very frequent modifications and new releases of the national guidelines, NPR 9998 [10]. The peculiarities of the seismic area of Groningen are related mainly to the nature of the seismic events occurring here. Unlike the regular tectonic earthquakes, the ones in Groningen are of induced nature, caused by the gas extractions from the ground in the area. This fact influences both the depth at which the hypocenter is located and hence at which energy is being released as well as the spectrum of accelerations that the ground surface experiences.
One of the most important features is the nature of the seismic event as it directly influences the depth of the hypocenter. It is commonon knowledge that generally tectonic earthquakes have considerably deeper hypocenters than induced earthquakes (see Fig. 5). This fact influences the area on which the waves are spread at the surface of the ground. In fact, the energy is released over a much smaller area with serious consequences as higher values of peak ground accelerations are generated.
The intensity of an earthquake is measured in the well-known Richter scale.
The magnitude relates to the energy that is released by the earthquake.
Because of the shallow depth of the hypocentre it can be observed that for the same magnitude the accelerations and therefore the damage of an induced earthquake will be higher than that caused by a deeper tectonic earthquake.
Another main factor to be considered is the duration of the event in relation to the acceleration values.
In case of an induced earthquake, for example Huizinge earthquake in 2012 (B.Dost, D.Kraaijpoel, [11]), the shaking motion is of shorter duration (typically 1-2 seconds) and with higher frequency than in case of natural Dutch earthquakes, such as the earthquake in Roermond from 1992 (T. Camelbeeck et al. [12]) , with a recorded time signal of 5-10 seconds. These result in less damage for this type of induced earthquake in Groningen, compared to a tectonic earthquake with the same peak ground acceleration.
After considering all these aspects, the procedure for determining the seismic action ends with the determination of the response spectra, specific for the Groningen earthquakes.  Below two pictures of the terraced houses (Fig. 6).

ANALYSED
Concerning existing construction, which must be statically tested based on the definitions in NEN-8700, this static evaluation is not in the scope of the project. The seismic test criteria follow from NPR 9998 [10] and NEN- EN-1990 [13, 14 and 15]. For the determination of dynamic behaviour, the building is modelled in two main directions into a mass spring system, each mass representing a building floor level. The masses are connected by elements with bending stiffness, which represent the stability system between the floors. The mass spring system is supported by a rotational spring, whose stiffness is determined according to the foundation conditions, as can be seen in Fig. 7.

INITIAL STATE
The initial structure is analysed by means of a spectral modal calculation.
The most important output of the spectral modal analysis is the highest horizontal force. The building-specific assumptions required for determining the boundary conditions for the calculation of existing structures are named in   It needs to be noted that this is the NPR 9998 of 2015. Knowledge of the seismic risk has advanced considerably since then.

This means that it is now known that the actual seismic risk is much lower than
what is shown here. Furthermore, the Dutch government has taken action in reducing the gasproduction, which has led to a foreseen considerable further reduction of the seismic risk.
The difference between the horizontal load in each direction is caused by the difference in stiffness per direction. The horizontal seismic loads, shown in table 7 (included in the point masses of the floors), must be transferred to the foundation through the stability system.
The calculations show that the seismic load is relatively low in x direction and high in y direction. The shear capacity of the masonry is exceeded, and the stability elements do not verify the requirement in the x-direction (see table   8). Table 7. Horizontal seismic force applied in the initial state condition. This low stiffness causes the largest percentage of the mass of the building to be brought into a relatively low-frequency vibration, with relatively low horizontal seismic loads occurring. This vibration is associated with relatively large horizontal displacements.
The bearing walls out of plane loads are also checked for calculating the bending moment due to the earthquake load. A behaviour factor is calculated according to Table 9.2 of the NPR [10]. The masonry is considered according to NEN-EN 1998-1 [13]. Wall calculations investigate the requirements of slenderness and strength. The 100 mm end walls are not verified, these walls need to be strengthened. The gable walls are not considered in the calculation, however, in the current situation these are only supported by the rafters of the roof, because they are not properly anchored to the wall. As a result, the top wall will come loose.
An alternative is the use of Non-Linear Pushover Analyses to more accurately The steel portal in X direction is designed such that it can plastically deform to withstand the spectral deformation induced by the earthquake. The steelframes are calculated with a non-linear pushover analysis. The structural model consists of columns to which steel beams are rigidly connected. For each house two portals at the front and two portals at the back are applied.
The portals are stacked and hinged with each other. Each of these takes into

PROJECT SOLUTION, 2D EXOSKELETON
The aim of the intervention is to seismically strengthen a large number of rowhouses and surroundings against the influence of earthquakes. For this purpose, the increase in safety is governed by the will to minimize the influence on inhabitants, the duration of the approach keeping the costs low and improving the sustainability of the houses. The constructive behaviour of the building after these reinforcements complies with the seismic requirements of NPR 9998 [10], NEN-EN 1998 [13, 14 and 15]. The reinforcement includes the following aspects: • Steel frame in X direction, the planar exoskeleton (An S235JR steel was The existing structure takes care of static loads like permanent loads (structural element own weight, permanent structural and no-structural weight) and variable loads (live, snow and wind). The new steel structure deals with horizontal actions due to the earthquake. The building-specific assumptions required for determining the relevant parameters for the calculation of strengthening measures are listed below in table 9.
The assumptions are according to NPR [10]. Table 9. Reference data for determing the design value of acceleration to derive the seismic action for the state of project.
The seismic analysis of the existing structure is used as a starting point for the seismic strengthening. The main improvement lies in the fact that the structure is now capable of taking horizontal seismic forces in X direction by means of the new steel exoskeleton. At the same time, the prevention of the out-of-plane movement of the end walls was achieved by ensuring the roof system is able to transmit the horizontal loads towards the foundations.
Concluding, the proposed specific interventions lead to the achievement of the threshold imposed by the verifications and therefore to resist the seismic action considered.
The main interventions have been used to also improve the energetic quality of the buildings. The steel frames were pre-manufactured with a complete insulation skin. The same was done with the roof panels, which also were fitted with solar panels. This way the houses were converted to zero-energy buildings.

CONCLUSIONS
The work presents the first results obtained within the ongoing research activity

REFERENCES
structure. In the two cases presented, there is the possibility of significantly improving seismic performance. In the first, going to increase the capacity in terms of collapse acceleration, evaluated on the overall behavior of the reinforced concrete building. While in the second through interventions and analyzes aimed at satisfying the verification of all the members.
The interventions were not aimed to improve the energy performance of the buildings, but with these big intrusions to the current buildings these measures could be added to the programme with relatively limited costs (but big energetic gains).