Seismic Design and Analysis of a Cold-Formed Steel Exoskeleton for the Retrofit of an RC Multi-Story Residential Building

Abstract

The awareness of the vulnerability of existing structures under both seismic and energy perspectives highlights the need for integrated retrofit solutions that combine structural and thermal enhancements. From this perspective, this study explored the efficacy of the Resisto 5.9 Tube system, which is a seismic retrofit solution for masonry and reinforced concrete (RC) structures that also improves the energy performance by integrating a thermal coat integrated within its basic steel framework. This research involved application to a RC building of a design procedure specifically developed for this system that was aimed at facilitating its adoption by designers involved in seismic retrofitting analysis. After designing the system components, nonlinear static analyses were performed using finite element software to compare the building’s seismic performance before and after the application of the Resisto 5.9 Tube. The results demonstrate a significant increase in the seismic safety coefficient ζE from 0.26 to 0.42, which proved the potential of this intervention to enhance the seismic safety of existing RC buildings.

Full text

Citation: Meglio, E.; Formisano, A.
Seismic Design and Analysis of a
Cold-Formed Steel Exoskeleton for
the Retrofit of an RC Multi-Story
Residential Building. Appl. Sci. 2024,
14, 8674. https://doi.org/10.3390/
app14198674
Academic Editor: Dario
De Domenico
Received: 9 August 2024
Revised: 18 September 2024
Accepted: 19 September 2024
Published: 26 September 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
applied
sciences
Article
Seismic Design and Analysis of a Cold-Formed Steel Exoskeleton
for the Retrofit of an RC Multi-Story Residential Building
Emilia Meglio * and Antonio Formisano
Department of Structures for Engineering and Architecture, School of Polytechnics and Basic Sciences,
University of Naples Federico II, 80125 Naples, Italy; antoform@unina.it
*Correspondence: emilia.meglio@unina.it
Featured Application: The system under study will provide a modification of the classic use of
exoskeletons installed outside buildings on their facades. The lightweight intervention technique
will act as an integrated retrofit system of existing RC and masonry buildings by improving
both the seismic performance and the energy efficiency with a negligible increase in the mass, a
satisfactory augmentation of the stiffness and a reasonable reduction in the thermal dispersion
through the structure envelope.
Abstract: The awareness of the vulnerability of existing structures under both seismic and energy
perspectives highlights the need for integrated retrofit solutions that combine structural and thermal
enhancements. From this perspective, this study explored the efficacy of the Resisto 5.9 Tube system,
which is a seismic retrofit solution for masonry and reinforced concrete (RC) structures that also
improves the energy performance by integrating a thermal coat integrated within its basic steel
framework. This research involved application to a RC building of a design procedure specifically
developed for this system that was aimed at facilitating its adoption by designers involved in
seismic retrofitting analysis. After designing the system components, nonlinear static analyses were
performed using finite element software to compare the building’s seismic performance before and
after the application of the Resisto 5.9 Tube. The results demonstrate a significant increase in the
seismic safety coefficient
ζE
from 0.26 to 0.42, which proved the potential of this intervention to
enhance the seismic safety of existing RC buildings.
Keywords: integrated retrofit; steel exoskeletons; existing buildings; reinforced concrete structures;
design method; pushover analyses
1. Introduction
In recent years, the need to enhance both seismic performance and energy efficiency in
existing buildings has gained increasing recognition, which has been driven by the impact
of recent seismic events, climate change and the vulnerabilities of older structures [
1
].
Reinforced concrete (RC) buildings, particularly those constructed in the post-war period
without seismic considerations, constitute a significant portion of the Italian building
stock. These buildings share common characteristics that make them highly vulnerable
to seismic activity [
2
,
3
]. Typically, RC buildings in Italy are composed of frames oriented
in only one direction, with hollow brick infill walls lacking thermal insulation. This
configuration is particularly susceptible to seismic forces and energy inefficiencies [
4
,
5
].
Given the prevalence of these structures in urban environments, the need for effective
seismic retrofitting solutions is urgent. Traditional methods, such as internal strengthening
or the confinement of beam–column joints [
6
–
8
], though effective, can be invasive, time-
consuming and disruptive to building occupants. A promising alternative is the use of
external retrofit solutions made of various materials, such as timber, steel, concrete or
aluminum alloys. For instance, external CLT timber panels were shown to increase the
Appl. Sci. 2024,14, 8674. https://doi.org/10.3390/app14198674 https://www.mdpi.com/journal/applsci

Appl. Sci. 2024,14, 8674 2 of 15
load bearing and displacement capacity and improve the global seismic response of RC
buildings [
9
], while steel buckling restrained braces (BRBs) have been used to increase the
strength and capacity for sustaining lateral deformations [10].
Steel exoskeletons offer an innovative solution for seismic retrofitting by providing
additional lateral strength and stiffness to existing buildings while allowing for intervening
only from the outside of the building, thus minimizing disruptions to the interior activities
of occupants. Studies demonstrated the potential of steel exoskeletons to improve the seis-
mic performance of RC buildings while also meeting structural, thermal and architectural
requirements [
11
]. RC structures retrofitted with steel exoskeletons, at times in combina-
tion with local interventions, showed significant improvements in the global structural
behavior, while also reducing the number of RC elements affected by brittle failure [
12
,
13
].
Cold-formed lightweight steel systems offer an effective structural solution, as they com-
bine the advantages of lightweight construction with strong performance during seismic
events. Some studies focused on the design of this type of system by conducting both
experimental tests and numerical analyses to assess the effect of earthquakes on lightweight
structures [
14
–
19
]. Furthermore, integrated solutions that combine structural and energy
retrofitting are nowadays very competitive in the building sector and, therefore, represent
an important issue in the research field. Traditional interventions, such as RC shear walls
combined with insulation panels, allow for improving the thermal and seismic behaviors
of existing RC buildings while also having a low environmental impact [
20
]. Innovative
solutions, such as the so-called seismic energy coats, integrate various technologies for
seismic retrofit and energy requalification using insulating panels, while also granting short
execution times and low invasiveness [21–24].
This research focused on a multi-story residential building located in northern Italy and
was constructed in the late 1970s. It is an RC structure with the typical configuration with
frames only in one direction. It was selected as a case study to evaluate the effectiveness
of the Resisto 5.9 Tube system developed by the company Progetto Sisma Srl (Fiorano
Modenese, MO, Italy). This recent solution for integrated retrofitting combines seismic
reinforcement with energy efficiency improvements. Resisto 5.9 Tube features a cold-
formed steel exoskeleton for seismic resistance combined with external thermal insulation
panels [
25
]. This seismic coat is a novel application of traditional exoskeletons because it
provides an increase in the existing structure stiffness while maintaining its mass as basically
unaltered. By allowing for installation from the building exterior, the system minimizes
disruption to occupants and preserves the interior activities during installation phases. This
system was investigated under the seismic perspective by means of finite element analysis
(FEA) using Pro_Sap software (version 23.6.1). Before implementing the reinforcement
in the calculation software, it was designed following a procedure specifically developed
for Resisto 5.9 Tube. The purpose of the design procedure was to define the thickness of
the bracing elements at each building level in order to optimize the performances of the
existing structure to be retrofitted [
26
]. Finally, non-linear static analyses were carried out
to compare the structural behavior before and after the retrofit by focusing on changes in
the seismic safety coefficient and seismic risk classification according to Italian regulations.
2. Materials and Methods
The case study was a multi-story RC residential building located in Rozzano, which
is in the province of Milan in Italy. The city of Rozzano is in the northern part of Italy in
a low-seismicity area, with a peak ground acceleration (PGA) of 0.075 g and in a climatic
zone E, as characterized by 2404 degree-days (Figure 1).
In this context, the enhancement of the building’s structural performance was the most
effective when coupled with improvements in thermal behavior. Resisto 5.9 Tube, which
was developed by the Italian company Progetto Sisma Srl, is a retrofit solution designed
to simultaneously enhance both the seismic resilience and energy efficiency in existing
masonry and reinforced concrete structures. This system can be customized to address the
specific needs of each structure by utilizing a design method tailored for this lightweight

Appl. Sci. 2024,14, 8674 3 of 15
exoskeleton made of cold-formed steel profiles. The current research aimed to evaluate the
effectiveness of this design approach for the seismic retrofitting of a real RC structure. The
seismic performance was assessed through nonlinear static analysis performed using finite
element software by comparing the structural behavior before and after the application of
the retrofit technique.
Appl. Sci. 2024, 14, x FOR PEER REVIEW 3 of 15
(a) (b) (c)
Figure 1. Localization of the case study in the Italian territory (a), seismic hazard map (b) and cli-
mate zones map (c).
In this context, the enhancement of the building’s structural performance was the
most effective when coupled with improvements in thermal behavior. Resisto 5.9 Tube,
which was developed by the Italian company Progeo Sisma Srl, is a retrofit solution de-
signed to simultaneously enhance both the seismic resilience and energy efficiency in ex-
isting masonry and reinforced concrete structures. This system can be customized to ad-
dress the specific needs of each structure by utilizing a design method tailored for this
lightweight exoskeleton made of cold-formed steel profiles. The current research aimed
to evaluate the effectiveness of this design approach for the seismic retrofiing of a real
RC structure. The seismic performance was assessed through nonlinear static analysis per-
formed using finite element software by comparing the structural behavior before and
after the application of the retrofit technique.
2.1. Existing RC Building
The case study was a multi-story RC structure constructed in the late 1970s. The
building had a commercial ground floor and three upper residential levels, with each one
characterized by an inter-story height of 3 m. The building’s rectangular plan measured
approximately 40 by 13 m, and its structure consisted of RC frames oriented solely in one
direction (longitudinal direction X), which were interconnected by the slabs, perimeter
beams, and two stair and elevator cores (see Figure 2).
(a) (b)
Figure 2. Architectural (a) and structural (b) plan layouts of the case study building (units: m).
The building’s façades featured large openings, especially in the longitudinal direc-
tion at the ground levels, and the infill walls were made of hollow bricks without thermal
insulation (Figure 3).
Figure 1. Localization of the case study in the Italian territory (a), seismic hazard map (b) and climate
zones map (c).
2.1. Existing RC Building
The case study was a multi-story RC structure constructed in the late 1970s. The
building had a commercial ground floor and three upper residential levels, with each one
characterized by an inter-story height of 3 m. The building’s rectangular plan measured
approximately 40 by 13 m, and its structure consisted of RC frames oriented solely in one
direction (longitudinal direction X), which were interconnected by the slabs, perimeter
beams, and two stair and elevator cores (see Figure 2).
Appl. Sci. 2024, 14, x FOR PEER REVIEW 3 of 15
(a) (b) (c)
Figure 1. Localization of the case study in the Italian territory (a), seismic hazard map (b) and cli-
mate zones map (c).
In this context, the enhancement of the building’s structural performance was the
most effective when coupled with improvements in thermal behavior. Resisto 5.9 Tube,
which was developed by the Italian company Progeo Sisma Srl, is a retrofit solution de-
signed to simultaneously enhance both the seismic resilience and energy efficiency in ex-
isting masonry and reinforced concrete structures. This system can be customized to ad-
dress the specific needs of each structure by utilizing a design method tailored for this
lightweight exoskeleton made of cold-formed steel profiles. The current research aimed
to evaluate the effectiveness of this design approach for the seismic retrofiing of a real
RC structure. The seismic performance was assessed through nonlinear static analysis per-
formed using finite element software by comparing the structural behavior before and
after the application of the retrofit technique.
2.1. Existing RC Building
The case study was a multi-story RC structure constructed in the late 1970s. The
building had a commercial ground floor and three upper residential levels, with each one
characterized by an inter-story height of 3 m. The building’s rectangular plan measured
approximately 40 by 13 m, and its structure consisted of RC frames oriented solely in one
direction (longitudinal direction X), which were interconnected by the slabs, perimeter
beams, and two stair and elevator cores (see Figure 2).
(a) (b)
Figure 2. Architectural (a) and structural (b) plan layouts of the case study building (units: m).
The building’s façades featured large openings, especially in the longitudinal direc-
tion at the ground levels, and the infill walls were made of hollow bricks without thermal
insulation (Figure 3).
Figure 2. Architectural (a) and structural (b) plan layouts of the case study building (units: m).
The building’s façades featured large openings, especially in the longitudinal direction
at the ground levels, and the infill walls were made of hollow bricks without thermal
insulation (Figure 3).
Appl. Sci. 2024, 14, x FOR PEER REVIEW 4 of 15
(a) (b)
Figure 3. Longitudinal (a) and transversal (b) façades of the case study.
The information about the building was gathered through an experimental campaign
conducted both on site and in laboratories. This campaign aimed to assess the geometrical
configuration, loading conditions and mechanical properties of the structural materials,
which were then compared with the existing documentation. According to Italian stand-
ards [27,28], a normal level of knowledge (KL2) was achieved, which corresponded to a
confidence factor (FC) of 1.2 for reducing resistance values.
The RC columns had rectangular cross-sections of varying dimensions (30 × 30 cm,
30 × 40 cm and 30 × 50 cm), with longitudinal bars and stirrups of φ8/30. The RC beams,
which were dropped, also had rectangular cross-sections, with dimensions of 30 × 50 cm
and 30 × 60 cm, and had stirrups of φ8/20. All structural elements were constructed with
C25/30 concrete and were reinforced with B450C steel, as detailed in Table 1 and Table 2,
respectively.
Table 1. Mechanical properties of concrete.
f
ck, cube 1
[MPa] E
1
[MPa]
γ
1
[kg/m
3
]
30 31,447.2 2500
1
Characteristic compressive cube strength (f
ck, cube
), elasticity modulus (E) and dead weight (γ).
Table 2. Mechanical properties of steel.
f
tk 1
[MPa] f
yk 1
[MPa] E
1
[MPa] γ
1
[kg/m
3
]
540 450 210
,
000 7850
1
Characteristic ultimate strength (f
tk
) and yielding strength (f
yk
) values, elasticity modulus (E) and
dead weight (γ).
The infill walls were made of hollow bricks (dead weight γ = 800 kg/m
3
) with a total
thickness of 24 cm.
2.2. Cold-Formed Exoskeleton
The system under investigation was the Resisto 5.9 Tube, which was developed by
Progeo Sisma Srl. This innovative solution, which is known as a seismic energy coat, was
designed for the integrated retrofiing of existing masonry and reinforced concrete build-
ings. The system’s seismic-resistant component is a cold-formed steel exoskeleton that
features a braced frame that is securely connected to the existing RC columns and beams
using chemical anchors. This exoskeleton is installed externally, parallel to the building’s
façades, which allows for normal activities within the building to be continued during the
assembly process.
The steel frame consists of horizontal and vertical members, with both having hollow
rectangular cross-sections with a thickness of 2 mm, though with varying dimensions. The
horizontal members (60 × 25 mm) are inserted into the vertical members (60 × 45 mm) via
modular holes (Figure 4a). The components of the steel frame are connected using gusset
plates and bolts, which also aach the diagonal bracings to the frame and secure the con-
nection to the RC structure (Figure 4b). The diagonal bracings are fastened to the gusset
Figure 3. Longitudinal (a) and transversal (b) façades of the case study.

Appl. Sci. 2024,14, 8674 4 of 15
The information about the building was gathered through an experimental campaign
conducted both on site and in laboratories. This campaign aimed to assess the geometrical
configuration, loading conditions and mechanical properties of the structural materials,
which were then compared with the existing documentation. According to Italian stan-
dards [
27
,
28
], a normal level of knowledge (KL2) was achieved, which corresponded to a
confidence factor (FC) of 1.2 for reducing resistance values.
The RC columns had rectangular cross-sections of varying dimensions (30
×
30 cm,
30 ×40 cm
and 30
×
50 cm), with longitudinal bars and stirrups of
φ
8/30. The RC
beams, which were dropped, also had rectangular cross-sections, with dimensions of
30
×
50 cm and 30
×
60 cm, and had stirrups of
φ
8/20. All structural elements were
constructed with C25/30 concrete and were reinforced with B450C steel, as detailed in
Tables 1and 2, respectively.
Table 1. Mechanical properties of concrete.
fck, cube 1[MPa] E 1[MPa] γ1[kg/m3]
30 31,447.2 2500
1Characteristic compressive cube strength (fck, cube), elasticity modulus (E) and dead weight (γ).
Table 2. Mechanical properties of steel.
ftk 1[MPa] fyk 1[MPa] E 1[MPa] γ1[kg/m3]
540 450 210,000 7850
1
Characteristic ultimate strength (f
tk
) and yielding strength (f
yk
) values, elasticity modulus (E) and dead
weight (γ).
The infill walls were made of hollow bricks (dead weight
γ
= 800 kg/m
3
) with a total
thickness of 24 cm.
2.2. Cold-Formed Exoskeleton
The system under investigation was the Resisto 5.9 Tube, which was developed by
Progetto Sisma Srl. This innovative solution, which is known as a seismic energy coat,
was designed for the integrated retrofitting of existing masonry and reinforced concrete
buildings. The system’s seismic-resistant component is a cold-formed steel exoskeleton that
features a braced frame that is securely connected to the existing RC columns and beams
using chemical anchors. This exoskeleton is installed externally, parallel to the building’s
façades, which allows for normal activities within the building to be continued during the
assembly process.
The steel frame consists of horizontal and vertical members, with both having hollow
rectangular cross-sections with a thickness of 2 mm, though with varying dimensions. The
horizontal members (60
×
25 mm) are inserted into the vertical members (60
×
45 mm)
via modular holes (Figure 4a). The components of the steel frame are connected using
gusset plates and bolts, which also attach the diagonal bracings to the frame and secure the
connection to the RC structure (Figure 4b). The diagonal bracings are fastened to the gusset
plates with steel rivets, and they have a width of 50 mm, while the thickness is determined
by the design specifications (Figure 4c).
The elements of the exoskeleton were designed with modular holes, which allow for a
standardized configuration. The standard module features a spacing of 1 m between hori-
zontal and vertical members, though different dimensions can be used to tailor the retrofit
solution to the specific requirements of the structure being reinforced. The reinforcement
process is completed with the addition of insulating panels and finishing layers. The final
configuration of the system’s seismic-resistant part is illustrated in Figure 5.

Appl. Sci. 2024,14, 8674 5 of 15
Appl. Sci. 2024, 14, x FOR PEER REVIEW 5 of 15
plates with steel rivets, and they have a width of 50 mm, while the thickness is determined
by the design specifications (Figure 4c).
(a) (b) (c)
Figure 4. Elements of Resisto 5.9 Tube: horizontal and vertical members (a), connections (b) and
diagonal bracings (c) [29].
The elements of the exoskeleton were designed with modular holes, which allow for
a standardized configuration. The standard module features a spacing of 1 m between
horizontal and vertical members, though different dimensions can be used to tailor the
retrofit solution to the specific requirements of the structure being reinforced. The rein-
forcement process is completed with the addition of insulating panels and finishing layers.
The final configuration of the system’s seismic-resistant part is illustrated in Figure 5.
(a) (b)
Figure 5. Standard configuration of Resisto 5.9 Tube: detail of the connections (a) and RC structure
equipped with the seismic-resistant braced frame (b) [29].
The elements of the Resisto 5.9 Tube system—including the horizontal and vertical
members, as well as the diagonal bracings—are constructed from pre-galvanized steel of
type S320GD + Z. The anchoring bolts used in the system have a diameter of 12 mm and
are of resistance class 8.8. The mechanical properties of the structural materials are sum-
marized in Table 3 for the steel elements and in Table 4 for the connectors.
Table 3. Mechanical properties of steel profiles.
f
tk 1
[MPa] f
yk 1
[MPa] E
1
[MPa] γ
1
[kg/m
3
]
390 320 210
,
000 7850
1
Characteristic ultimate strength (f
tk
) and yielding strength (f
yk
) values, elasticity modulus (E) and
specific weight (γ).
Figure 4. Elements of Resisto 5.9 Tube: horizontal and vertical members (a), connections (b) and
diagonal bracings (c) [29].
Appl. Sci. 2024, 14, x FOR PEER REVIEW 5 of 15
plates with steel rivets, and they have a width of 50 mm, while the thickness is determined
by the design specifications (Figure 4c).
(a) (b) (c)
Figure 4. Elements of Resisto 5.9 Tube: horizontal and vertical members (a), connections (b) and
diagonal bracings (c) [29].
The elements of the exoskeleton were designed with modular holes, which allow for
a standardized configuration. The standard module features a spacing of 1 m between
horizontal and vertical members, though different dimensions can be used to tailor the
retrofit solution to the specific requirements of the structure being reinforced. The rein-
forcement process is completed with the addition of insulating panels and finishing layers.
The final configuration of the system’s seismic-resistant part is illustrated in Figure 5.
(a) (b)
Figure 5. Standard configuration of Resisto 5.9 Tube: detail of the connections (a) and RC structure
equipped with the seismic-resistant braced frame (b) [29].
The elements of the Resisto 5.9 Tube system—including the horizontal and vertical
members, as well as the diagonal bracings—are constructed from pre-galvanized steel of
type S320GD + Z. The anchoring bolts used in the system have a diameter of 12 mm and
are of resistance class 8.8. The mechanical properties of the structural materials are sum-
marized in Table 3 for the steel elements and in Table 4 for the connectors.
Table 3. Mechanical properties of steel profiles.
f
tk 1
[MPa] f
yk 1
[MPa] E
1
[MPa] γ
1
[kg/m
3
]
390 320 210
,
000 7850
1
Characteristic ultimate strength (f
tk
) and yielding strength (f
yk
) values, elasticity modulus (E) and
specific weight (γ).
Figure 5. Standard configuration of Resisto 5.9 Tube: detail of the connections (a) and RC structure
equipped with the seismic-resistant braced frame (b) [29].
The elements of the Resisto 5.9 Tube system—including the horizontal and vertical
members, as well as the diagonal bracings—are constructed from pre-galvanized steel
of type S320GD + Z. The anchoring bolts used in the system have a diameter of 12 mm
and are of resistance class 8.8. The mechanical properties of the structural materials are
summarized in Table 3for the steel elements and in Table 4for the connectors.
Table 3. Mechanical properties of steel profiles.
ftk 1[MPa] fyk 1[MPa] E 1[MPa] γ1[kg/m3]
390 320 210,000 7850
1
Characteristic ultimate strength (f
tk
) and yielding strength (f
yk
) values, elasticity modulus (E) and specific
weight (γ).
Table 4. Mechanical properties of anchoring bolts.
ftk 1[MPa] fyk 1[MPa] E 1[MPa] γ1[kg/m3]
800 640 210,000 7850
1
Characteristic ultimate strength (f
tk
) and yielding strength (f
yk
) values, elasticity modulus (E) and specific
weight (γ).
Finally, the improvement in the building’s thermal performance is achieved using
expanded polystyrene insulating panels, which are applied at the intended thickness and
covered with various types of finishing materials.

Appl. Sci. 2024,14, 8674 6 of 15
2.3. Retrofitted RC Building
Before implementing the design procedure and conducting a seismic analysis on the
reinforced structure, it was essential to define the geometric configuration of the reinforce-
ment system. The cold-formed exoskeleton was applied across the entire structure while
considering the architectural constraints, such as openings and balconies. A standard
module with 1 m spacing between the horizontal and vertical members was used wherever
possible. The system was anchored to the ground via a reinforced concrete (RC) foundation
beam that was securely attached to the steel vertical members. The new foundation was
connected to the existing one using grouted bars. The dimensions of the steel elements
match those specified in the standard solution, except for the diagonal bracing thickness,
which was calculated using the design procedure to optimize the reinforcement effective-
ness. The design procedure was based on pushover analysis, but any analysis method can
be employed. The schematic configuration of the Resisto 5.9 Tube system is illustrated in
Figure 6.
Appl. Sci. 2024, 14, x FOR PEER REVIEW 6 of 15
Table 4. Mechanical properties of anchoring bolts.
f
tk 1
[MPa] f
yk 1
[MPa] E
1
[MPa] γ
1
[kg/m
3
]
800 640 210,000 7850
1
Characteristic ultimate strength (f
tk
) and yielding strength (f
yk
) values, elasticity modulus (E) and
specific weight (γ).
Finally, the improvement in the building’s thermal performance is achieved using
expanded polystyrene insulating panels, which are applied at the intended thickness and
covered with various types of finishing materials.
2.3. Retrofied RC Building
Before implementing the design procedure and conducting a seismic analysis on the
reinforced structure, it was essential to define the geometric configuration of the reinforce-
ment system. The cold-formed exoskeleton was applied across the entire structure while
considering the architectural constraints, such as openings and balconies. A standard
module with 1 m spacing between the horizontal and vertical members was used wher-
ever possible. The system was anchored to the ground via a reinforced concrete (RC) foun-
dation beam that was securely aached to the steel vertical members. The new foundation
was connected to the existing one using grouted bars. The dimensions of the steel elements
match those specified in the standard solution, except for the diagonal bracing thickness,
which was calculated using the design procedure to optimize the reinforcement effective-
ness. The design procedure was based on pushover analysis, but any analysis method can
be employed. The schematic configuration of the Resisto 5.9 Tube system is illustrated in
Figure 6.
(a) (b)
Figure 6. Schematic configuration of the reinforcement: longitudinal (a) and transversal (b) façades
of the retrofied building.
2.4. Finite Element Modeling and Analysis
The seismic analysis was conducted using Pro_Sap, which is software developed by
the Italian company 2S.I. Software e Servizi per L’Ingegneria S.r.l. (Ferrara, Italy). The case
study building was modeled in the software both before and after the installation of the
Resisto 5.9 Tube system to assess its contribution to the seismic upgrading of the existing
RC structure. The structure was evaluated through nonlinear static analyses, and the seis-
mic safety factor was calculated in accordance with the Italian code. Pushover analysis is
the most common method used to investigate the seismic behavior of existing buildings,
considering that nonlinear dynamic analyses are more complex procedures that are un-
known to many designers. Only mechanical non-linearities were considered. RC members
were modeled as bidimensional beam elements with nonlinear properties using a concen-
trated plasticity model with assigned values for the plastic hinges’ capacities, while steel
X-bracings were modeled as non-linear trusses. The values of plastic moments to be con-
sidered in the non-linear analysis were automatically assessed by the Pro_Sap software
thanks to a linear calculation model called source model. The elastic–plastic behavior of
non-linear trusses for X-bracings was considered by assigning tension and compression
limits in terms of strength. Notably, the infill walls were not included in the structural
Figure 6. Schematic configuration of the reinforcement: longitudinal (a) and transversal (b) façades
of the retrofitted building.
2.4. Finite Element Modeling and Analysis
The seismic analysis was conducted using Pro_Sap, which is software developed by
the Italian company 2S.I. Software e Servizi per L’Ingegneria S.r.l. (Ferrara, Italy). The
case study building was modeled in the software both before and after the installation
of the Resisto 5.9 Tube system to assess its contribution to the seismic upgrading of the
existing RC structure. The structure was evaluated through nonlinear static analyses, and
the seismic safety factor was calculated in accordance with the Italian code. Pushover
analysis is the most common method used to investigate the seismic behavior of existing
buildings, considering that nonlinear dynamic analyses are more complex procedures that
are unknown to many designers. Only mechanical non-linearities were considered. RC
members were modeled as bidimensional beam elements with nonlinear properties using a
concentrated plasticity model with assigned values for the plastic hinges’ capacities, while
steel X-bracings were modeled as non-linear trusses. The values of plastic moments to be
considered in the non-linear analysis were automatically assessed by the Pro_Sap software
thanks to a linear calculation model called source model. The elastic–plastic behavior of
non-linear trusses for X-bracings was considered by assigning tension and compression
limits in terms of strength. Notably, the infill walls were not included in the structural
model; instead, they were represented as a uniform load distributed along the perimeter
beams. Vertical loads were applied to the floors at each level, as specified in Table 5. The
permanent loads on the top floor also included the weight of the roof, which was not
directly incorporated into the calculation software. The floors were modeled as rigid slabs
with a membrane thickness of 4 cm.

Appl. Sci. 2024,14, 8674 7 of 15
Table 5. Vertical loads applied to the floors.
Level G11[kg/m2] G21[kg/m2] Q 1[kg/m2]
Intermediate floors 370 120 200
Top floor 500 - 120
1Self weight and defined dead loads (G1), undefined dead load (G2) and live loads (Q).
Seismic loads for the nonlinear static analyses were computed using the foundation
soil category (C) and topographic category (T1). A standard damping of 5% was applied,
while the soil–structure interactions were neglected, as the reinforced concrete and steel
vertical elements at ground level were assumed to be fixed at the base. The type of action
was calculated at the ultimate limit state (ULS) with different accidental eccentricities
(positive, negative and none) for each principal direction (longitudinal X and transverse Y).
Two force distributions were considered: a triangular distribution, similar to that used in
linear static analysis, and a mass-proportional distribution. The solid view of the existing
structure model, as implemented in the Pro_Sap software, is shown in Figure 7.
Appl. Sci. 2024, 14, x FOR PEER REVIEW 7 of 15
model; instead, they were represented as a uniform load distributed along the perimeter
beams. Vertical loads were applied to the floors at each level, as specified in Table 5. The
permanent loads on the top floor also included the weight of the roof, which was not di-
rectly incorporated into the calculation software. The floors were modeled as rigid slabs
with a membrane thickness of 4 cm.
Table 5. Vertical loads applied to the floors.
Level G
1 1
[kg/m
2
] G
2 1
[kg/m
2
] Q
1
[kg/m
2
]
Intermediate floors 370 120 200
Top floor 500 - 120
1
Self weight and defined dead loads (G
1
), undefined dead load (G
2
) and live loads (Q).
Seismic loads for the nonlinear static analyses were computed using the foundation
soil category (C) and topographic category (T1). A standard damping of 5% was applied,
while the soil–structure interactions were neglected, as the reinforced concrete and steel
vertical elements at ground level were assumed to be fixed at the base. The type of action
was calculated at the ultimate limit state (ULS) with different accidental eccentricities
(positive, negative and none) for each principal direction (longitudinal X and transverse
Y). Two force distributions were considered: a triangular distribution, similar to that used
in linear static analysis, and a mass-proportional distribution. The solid view of the exist-
ing structure model, as implemented in the Pro_Sap software, is shown in Figure 7.
(a) (b)
(c) (d)
(e) (f)
Figure 7. FEM model of the existing structure: 3D view (a), plan layout (b), longitudinal sections
(c,e) and transversal sections (d,f).
The same loading conditions were applied to the retrofied model, which was equiv-
alent to the unreinforced structure, but equipped with the cold-formed exoskeleton. The
steel elements were incorporated into the structural model as closely as possible to their
actual configuration, with some necessary simplifications. Horizontal and vertical profiles
Figure 7. FEM model of the existing structure: 3D view (a), plan layout (b), longitudinal sections (c,e)
and transversal sections (d,f).
The same loading conditions were applied to the retrofitted model, which was equiva-
lent to the unreinforced structure, but equipped with the cold-formed exoskeleton. The
steel elements were incorporated into the structural model as closely as possible to their
actual configuration, with some necessary simplifications. Horizontal and vertical profiles
were modeled as beam elements with moment releases, which accurately represented the
system’s constraint conditions. The diagonal bracings were modeled as nonlinear trusses,
which were capable of sustaining only tensile stresses. The connections between the hori-
zontal and vertical members and the RC structure were represented by rigid links, which
simulated the presence of the chemical anchors. The solid view of the retrofitted structure
model, as implemented in the Pro_Sap software, is shown in Figure 8.

Appl. Sci. 2024,14, 8674 8 of 15
Appl. Sci. 2024, 14, x FOR PEER REVIEW 8 of 15
were modeled as beam elements with moment releases, which accurately represented the
system’s constraint conditions. The diagonal bracings were modeled as nonlinear trusses,
which were capable of sustaining only tensile stresses. The connections between the hori-
zontal and vertical members and the RC structure were represented by rigid links, which
simulated the presence of the chemical anchors. The solid view of the retrofied structure
model, as implemented in the Pro_Sap software, is shown in Figure 8.
(a) (b)
(c) (d)
(e) (f)
Figure 8. FEM model of the retrofied structure: 3D view (a), plan layout (b), longitudinal sections
(c,e) and transversal sections (d,f).
3. Results and Discussion
The unreinforced and reinforced RC residential buildings were subjected to nonlin-
ear static analysis using the finite element software ProSap. The comparison of the results
was conducted by examining the capacity curves of the structures and the seismic safety
coefficients, as evaluated according to the Italian Technical Standard. The following sec-
tions provide a summary of the key results from the numerical analysis, along with the
outcomes of the design procedure.
3.1. Seismic Analysis on the Existing RC Building
The performance of the numerical analysis on the unreinforced building is crucial
not only for assessing the seismic safety and identifying vulnerabilities but also for de-
signing the cold-formed exoskeleton tailored to the building’s capacity and specific needs.
The RC building was subjected to pushover analysis in both primary directions, which
generated capacity curves for each direction. Figure 9 presents the pushover curves under
the most severe loading conditions for each thrust direction. The red square on the curves
marks the point at which the first structural damage occurred, which is a key parameter
for seismic design.
Figure 8. FEM model of the retrofitted structure: 3D view (a), plan layout (b), longitudinal sections
(c,e) and transversal sections (d,f).
3. Results and Discussion
The unreinforced and reinforced RC residential buildings were subjected to nonlinear
static analysis using the finite element software ProSap. The comparison of the results
was conducted by examining the capacity curves of the structures and the seismic safety
coefficients, as evaluated according to the Italian Technical Standard. The following sections
provide a summary of the key results from the numerical analysis, along with the outcomes
of the design procedure.
3.1. Seismic Analysis on the Existing RC Building
The performance of the numerical analysis on the unreinforced building is crucial not
only for assessing the seismic safety and identifying vulnerabilities but also for designing
the cold-formed exoskeleton tailored to the building’s capacity and specific needs. The RC
building was subjected to pushover analysis in both primary directions, which generated
capacity curves for each direction. Figure 9presents the pushover curves under the most
severe loading conditions for each thrust direction. The red square on the curves marks
the point at which the first structural damage occurred, which is a key parameter for
seismic design.
The main parameters of the analysis are summarized in Table 6, which highlights
that the structure exhibited greater ductility in the transverse direction (Y) and increased
stiffness and strength in the longitudinal direction (X). The initial damage in the structure
occurred in the longitudinal direction at a displacement of 0.61 cm, whereas in the trans-
verse direction, the first point of damage corresponded to a displacement of 1.75 cm. In
both cases it was represented by the formation of plastic hinges on the columns and beams.

Appl. Sci. 2024,14, 8674 9 of 15
Appl. Sci. 2024, 14, x FOR PEER REVIEW 9 of 15
(a) (b)
Figure 9. Capacity of the existing structure in the most severe condition: pushover curves (a) and
location of the first point of damage (b).
The main parameters of the analysis are summarized in Table 6, which highlights
that the structure exhibited greater ductility in the transverse direction (Y) and increased
stiffness and strength in the longitudinal direction (X). The initial damage in the structure
occurred in the longitudinal direction at a displacement of 0.61 cm, whereas in the trans-
verse direction, the first point of damage corresponded to a displacement of 1.75 cm. In
both cases it was represented by the formation of plastic hinges on the columns and
beams.
Table 6. Results of the pushover analysis on the existing building.
Dir. d
dam 1
[cm] F
dam 1
[kN] d
cu 1
[cm] F
max 1
[kN] d*
y 1
[cm] F*
y 1
[kN] K*
1
[kN/cm]
X 0.61 1323.7 3.25 4096 1.5 3008 2002
Y 1.75 1100.7 17.22 3058 3.49 2102 602
1
Displacement (d
dam
) and shear (F
dam
) corresponding to the first damage, ultimate displacement (d
cu
)
and maximum shear (F
max
). Asterisked terms indicate the equivalent bilinear yielding displacement
(d*
y
), shear (F*
y
) and stiffness (K*).
3.2. Seismic Design of the Cold-Formed Exoskeleton
The first instance of damage served as the starting point for the design method, which
aimed to determine the thickness of the diagonal members to optimize the retrofiing
solution. This initial damage was observed in the longitudinal direction at a displacement
of 0.61 cm. The design method, which was based on literature approaches [30,31] and
adapted for this specific exoskeleton type [26], began with the assumption that the retro-
fied structure should exhibit ductility comparable to that of the unreinforced structure.
The process started by seing a target displacement slightly lower than the displace-
ment at the first point of damage, which was fixed at 0.55 cm. Using the equivalent bilinear
curve representation of the structure and the elastic spectrum in the acceleration displace-
ment response spectrum (ADRS) format, an iterative procedure defined the ultimate dis-
placement of the retrofied structure and, consequently, its stiffness. This iterative process
continued until the ductility of the retrofied structure matched that of the unreinforced
one (µ = 2.16). At the end of the procedure, an equivalent bilinear curve representative of
the cold-formed exoskeleton was derived by subtracting the design parameters of the ret-
rofied structure from those of the existing structure. According to the procedure, the de-
sign stiffness of the Resisto 5.9 Tube was determined to be 1.039.603 daN/cm. This value
represents the total stiffness of the exoskeleton in the longitudinal direction and needed
be redistributed across each level and module of the Resisto 5.9 Tube. The distribution of
stiffness along the building height was achieved using a proportionality factor r
k
, which
relates the overall stiffness of the existing structure to that of the Resisto 5.9 Tube. This
Figure 9. Capacity of the existing structure in the most severe condition: pushover curves (a) and
location of the first point of damage (b).
Table 6. Results of the pushover analysis on the existing building.
Dir. ddam 1[cm] Fdam 1[kN] dcu 1[cm] Fmax 1[kN] d*y1[cm] F*y1[kN] K* 1[kN/cm]
X 0.61 1323.7 3.25 4096 1.5 3008 2002
Y 1.75 1100.7 17.22 3058 3.49 2102 602
1
Displacement (d
dam
) and shear (F
dam
) corresponding to the first damage, ultimate displacement (d
cu
) and
maximum shear (F
max
). Asterisked terms indicate the equivalent bilinear yielding displacement (d*
y
), shear (F*
y
)
and stiffness (K*).
3.2. Seismic Design of the Cold-Formed Exoskeleton
The first instance of damage served as the starting point for the design method, which
aimed to determine the thickness of the diagonal members to optimize the retrofitting
solution. This initial damage was observed in the longitudinal direction at a displacement of
0.61 cm. The design method, which was based on literature approaches [
30
,
31
] and adapted
for this specific exoskeleton type [
26
], began with the assumption that the retrofitted
structure should exhibit ductility comparable to that of the unreinforced structure.
The process started by setting a target displacement slightly lower than the displace-
ment at the first point of damage, which was fixed at 0.55 cm. Using the equivalent bilinear
curve representation of the structure and the elastic spectrum in the acceleration displace-
ment response spectrum (ADRS) format, an iterative procedure defined the ultimate dis-
placement of the retrofitted structure and, consequently, its stiffness. This iterative process
continued until the ductility of the retrofitted structure matched that of the unreinforced
one (
µ
= 2.16). At the end of the procedure, an equivalent bilinear curve representative
of the cold-formed exoskeleton was derived by subtracting the design parameters of the
retrofitted structure from those of the existing structure. According to the procedure, the
design stiffness of the Resisto 5.9 Tube was determined to be
1.039.603 daN/cm
. This value
represents the total stiffness of the exoskeleton in the longitudinal direction and needed
be redistributed across each level and module of the Resisto 5.9 Tube. The distribution of
stiffness along the building height was achieved using a proportionality factor r
k
, which
relates the overall stiffness of the existing structure to that of the Resisto 5.9 Tube. This
factor was then applied to the floor stiffnesses of the existing structure to determine the
floor stiffnesses of the exoskeleton, as detailed in Table 7.

Appl. Sci. 2024,14, 8674 10 of 15
Table 7. Design stiffness of the Resisto 5.9 Tube system.
Level H1[m] d01[cm] Fi1[kN] Ksi 1[kN/cm] rk1KRi 1[kN/cm]
4 12 2.13 5074 2382.2
5.18
12,349.6
3 9 1.84 4428 2406.5 12,475.9
2 6 1.30 2952 2270.8 11,772.1
1 3 0.59 1478 2505.1 12,986.8
1
Level height from the ground (H), floor displacement (d
0
), shear (F
i
) and stiffness (K
si
) of the existing structure,
proportionality factor (rk) and design floor stiffness of the exoskeleton (KRi).
To evaluate the regularity in height of the existing structure and determine whether
the exoskeleton is needed to redistribute and homogenize the floor stiffnesses, reference to
the Italian Code was made. According to the code, for a structure to be considered regular
in height, the stiffness of successive floors should fall within the range in Equation (1):
−0.1 ≤Ks, i ≤0.3 (1)
The existing buildings met the requirements for regularity in height; therefore, it was
not necessary to use the exoskeleton for redistributing the stiffnesses.
To simplify the design procedure, the floor stiffness was divided by the number of
exoskeleton modules in each direction (nRi) to determine the stiffness of a single diagonal
(k
Ri,s
). The design area of the diagonal (A
i
) could then be computed using the following
Equation (2):
Ai =
kR,i,s ·Li
2·E·cos2αR
(2)
where Li is the average length of diagonal members, E is the elasticity modulus of the
material and
α
is the inclination angle of diagonal members. Knowing the width of the
diagonal members (b
i
), the design thickness (t
i
) for each level could be readily computed.
Table 8summarizes the results for one of the longitudinal façades, which represents the
most severe condition.
Table 8. Design thickness of diagonal members.
Level nRi KRi, s [N/mm] Li[mm] Ai[mm2]bi[mm] ti[mm]
4 93 6639.6 1291.4 50 50 0.99
3 93 6707.5 1291.4 50 50 1.00
2 93 6329.1 1291.4 47 50 0.94
1 53 12,251.7 1317.9 97 50 1.93
Ultimately, it was decided to use the same diagonal member thickness of 1 mm for all
levels, except for the first level, where a thicker member (2 mm) was required due to the
large number of openings.
3.3. Seismic Analysis on the Retrofitted RC Building
The numerical analysis of the reinforced building aimed to evaluate the effectiveness
of the designed retrofitting system in enhancing seismic safety and reducing structural
vulnerabilities according to Eurocodes 2, 3 and 8 [
32
–
34
]. After installing the Resisto 5.9
Tube, the retrofitted RC building was subjected to pushover analysis to derive the capacity
curves. Figure 10 illustrates these curves under the most severe loading conditions for
each direction. In these curves, the red square indicates the point where the first damage
occurred in the structure reinforced with the cold-formed exoskeleton.

Appl. Sci. 2024,14, 8674 11 of 15
Appl. Sci. 2024, 14, x FOR PEER REVIEW 11 of 15
(a) (b)
Figure 10. Capacity of the retrofied structure in the most severe condition: pushover curves (a) and
location of the first point of damage (b).
The main parameters of the analysis are summarized in Table 9, which highlights
that the overall behavior of the retrofied structure remained similar to that of the existing
one, with increased ductility in the transverse direction (Y) and enhanced stiffness and
strength in the longitudinal direction (X). Even after installing the exoskeleton, the first
point of damage in the structure still occurred in the longitudinal direction.
Table 9. Results of the pushover analysis on the retrofied building.
Dir. d
dam 1
[cm] F
dam 1
[kN] d
cu 1
[cm] F
max 1
[kN] d*
y 1
[cm] F*
y 1
[kN] K*
1
[kN/cm]
X 0.67 1579.8 3.63 4852 1.64 3562 2168
Y 1.82 1242.8 18.97 4100 4.52 2679 593
1
Displacement (d
dam
) and shear (F
dam
) corresponding to the first point of damage, ultimate displace-
ment (d
cu
) and maximum shear (F
max
). Asterisked terms indicate the equivalent bilinear yielding
displacement (d*
y
), shear (F*
y
) and stiffness (K*).
3.4. Comparison of the Results
The comparison of the capacity curves for the structure before and after the applica-
tion of the Resisto 5.9 Tube (Figure 11) demonstrates how the cold-formed exoskeleton
improved the stiffness, ductility and strength of the existing RC buildings. The percentage
variations in the key parameters of the seismic analysis, both before and after the retrofit,
are summarized in Table 10.
(a) (b)
Figure 11. Comparison of the capacity of the existing and retrofied structures in the most severe
condition: longitudinal direction X (a) and transverse direction Y (b).
Figure 10. Capacity of the retrofitted structure in the most severe condition: pushover curves (a) and
location of the first point of damage (b).
The main parameters of the analysis are summarized in Table 9, which highlights that
the overall behavior of the retrofitted structure remained similar to that of the existing one,
with increased ductility in the transverse direction (Y) and enhanced stiffness and strength
in the longitudinal direction (X). Even after installing the exoskeleton, the first point of
damage in the structure still occurred in the longitudinal direction.
Table 9. Results of the pushover analysis on the retrofitted building.
Dir. ddam 1[cm] Fdam 1[kN] dcu 1[cm] Fmax 1[kN] d*y1[cm] F*y1[kN] K* 1[kN/cm]
X 0.67 1579.8 3.63 4852 1.64 3562 2168
Y 1.82 1242.8 18.97 4100 4.52 2679 593
1
Displacement (d
dam
) and shear (F
dam
) corresponding to the first point of damage, ultimate displacement (d
cu
)
and maximum shear (F
max
). Asterisked terms indicate the equivalent bilinear yielding displacement (d*
y
), shear
(F*y) and stiffness (K*).
3.4. Comparison of the Results
The comparison of the capacity curves for the structure before and after the applica-
tion of the Resisto 5.9 Tube (Figure 11) demonstrates how the cold-formed exoskeleton
improved the stiffness, ductility and strength of the existing RC buildings. The percentage
variations in the key parameters of the seismic analysis, both before and after the retrofit,
are summarized in Table 10.
Appl. Sci. 2024, 14, x FOR PEER REVIEW 11 of 15
(a) (b)
Figure 10. Capacity of the retrofied structure in the most severe condition: pushover curves (a) and
location of the first point of damage (b).
The main parameters of the analysis are summarized in Table 9, which highlights
that the overall behavior of the retrofied structure remained similar to that of the existing
one, with increased ductility in the transverse direction (Y) and enhanced stiffness and
strength in the longitudinal direction (X). Even after installing the exoskeleton, the first
point of damage in the structure still occurred in the longitudinal direction.
Table 9. Results of the pushover analysis on the retrofied building.
Dir. d
dam 1
[cm] F
dam 1
[kN] d
cu 1
[cm] F
max 1
[kN] d*
y 1
[cm] F*
y 1
[kN] K*
1
[kN/cm]
X 0.67 1579.8 3.63 4852 1.64 3562 2168
Y 1.82 1242.8 18.97 4100 4.52 2679 593
1
Displacement (d
dam
) and shear (F
dam
) corresponding to the first point of damage, ultimate displace-
ment (d
cu
) and maximum shear (F
max
). Asterisked terms indicate the equivalent bilinear yielding
displacement (d*
y
), shear (F*
y
) and stiffness (K*).
3.4. Comparison of the Results
The comparison of the capacity curves for the structure before and after the applica-
tion of the Resisto 5.9 Tube (Figure 11) demonstrates how the cold-formed exoskeleton
improved the stiffness, ductility and strength of the existing RC buildings. The percentage
variations in the key parameters of the seismic analysis, both before and after the retrofit,
are summarized in Table 10.
(a) (b)
Figure 11. Comparison of the capacity of the existing and retrofied structures in the most severe
condition: longitudinal direction X (a) and transverse direction Y (b).
Figure 11. Comparison of the capacity of the existing and retrofitted structures in the most severe
condition: longitudinal direction X (a) and transverse direction Y (b).

Appl. Sci. 2024,14, 8674 12 of 15
Table 10. Comparison of the seismic analysis results: percentage variation in the capacity curve’s
parameters due to the application of the Resisto 5.9 Tube system.
Dir. ∆d*y1∆dcu 1∆Fmax 1∆K* 1
X +9.3% +11.7% +18.5% +8.3%
Y +29.5% +10.2% +27.5% −1.5%
1
Ultimate displacement (d
cu
) and maximum shear (F
max
). Asterisked terms indicate the equivalent bilinear
yielding displacement (d*y) and stiffness (K*).
The efficacy of the Resisto 5.9 Tube for the seismic upgrading of existing RC buildings
was assessed by comparing the seismic safety index
ζE
before and after the retrofit inter-
vention. The seismic safety index was evaluated according to the Italian Technical Standard
as a ratio between the capacity and the demand in terms of the peak ground acceleration,
as reported in Equation (3):
ζE=
PGAC
PGAD
(3)
This index represents the ratio between the seismic action that the building can sustain
and the one that is requested for a new structure. To classify a retrofit intervention as
seismic upgrading, the code requires that the seismic safety index must increase by at least
0.1. The cold-formed exoskeleton allowed for achieving an increase in
ζE
value of 0.16 and,
therefore, the seismic upgrade of the RC building was attained, as summarized in Table 11.
Table 11. Seismic safety index ζEbefore and after the retrofit intervention.
ζEExisting Building ζERetrofitted Building ζEIncrease
0.26 0.42 +0.16
Finally, a comparison was made in terms of the seismic risk classification according
to the Italian Guidelines [
35
]. The seismic risk class of a building was determined based
on two main parameters: the seismic safety index (IS-V), corresponding to the
ζE
index,
and the PAM value, which represents the expected annual loss. To each parameter a risk
class ranging from A+ to G, with increasing levels of risk, was assigned. The seismic risk of
the building was classified according to the lowest class between IS-V class and PAM one.
The worst condition was represented by the annual loss (PAM class), which was evaluated
as the area under the curve that depicted the direct economic losses as a function of the
average annual frequency of an exceedance of the seismic event that causes the structure to
reach the life safety limit state.
For the analyzed building, the retrofit intervention led to an improvement by one risk
class, from F to E, as shown in Table 12 and Figure 12.
Table 12. Seismic risk class of the RC building before and after the retrofit intervention: PAM and
IS-V parameters.
Status PAM [%] PAM Class IS-V [%] IS-V Class Assigned Class
Before
retrofit 5.644 F 26 E F
After retrofit 4.035 E 42 D E
In conclusion, based on the results achieved, it is evident that the Resisto 5.9 enhanced
the seismic performances of the existing building by improving the global seismic behavior
of the bare structure similarly to other exoskeleton solutions [13,36].

Appl. Sci. 2024,14, 8674 13 of 15
Appl. Sci. 2024, 14, x FOR PEER REVIEW 13 of 15
(a) (b)
Figure 12. Seismic risk class of the RC building: before (a) and after (b) the retrofit intervention.
4. Conclusions
The research focused on applying a cold-formed exoskeleton, known as Resisto 5.9
Tube, to a reinforced concrete (RC) structure for seismic-energy retrofiing. Resisto 5.9 is
made of steel-braced frames connected to the existing structure with chemical anchors
and integrated with insulation panels for the improvement of the thermal performance.
The investigation was addressed to the assessment of the structural behavior of a building
before and after the installation of the exoskeleton. The aim of the study was to show the
effectiveness of the proposed design procedure to obtain a solution that was able to im-
prove the global seismic behavior of a case study representing a common typology of RC
buildings erected in Italy in the 1970s.
The system was initially designed using an iterative procedure that determined the
required thickness of the seismic-resistant bracings based on the specific needs of the
structure. The design was set on a target displacement that was identified based on the
capacity curve of the existing structure before the first instance of damage (d*dam = 0.55
cm). Then, an iterative procedure allowed for determining the required stiffness for the
Resisto 5.9 Tube that aimed to obtain a similar ductility of the structure before and after
the application of the exoskeleton (µ = 2.16). Due to large openings on the ground floor
that limited the application of the exoskeleton, the design specified a bracing thickness of
2 mm for the first level and 1 mm for the upper levels. The effectiveness of the design was
evaluated through nonlinear static analysis of the structure both before and after the sys-
tem application. The results demonstrated that the exoskeleton significantly enhanced the
seismic performance of the existing RC buildings. From the comparison of the capacity
curves of the structure before and after the retrofit, it was observed that the ultimate and
yielding displacements increased in both the longitudinal and transverse directions. The
shear strength increased by 18.5% and 27.5% in the longitudinal and transverse directions,
respectively, while the initial stiffness increased only in the longitudinal direction with an
8.3% increase. The seismic safety index improved from 0.26 to 0.42, which surpassed the
minimum required increase of 0.1, as stipulated by the Italian Technical Standard for seis-
mic upgrading. Additionally, the application of the Resisto 5.9 Tube resulted in a reduc-
tion of seismic risk, which advanced the seismic risk class from F to E. In conclusion, the
Resisto 5.9 allowed for increasing the strength and the overall base shear of the existing
building, and thus, improved the global seismic behavior of the structure consistently
with other investigations on external exoskeletons found in literature. The ongoing re-
search aims to further assess the system’s efficacy, including its performance in the pres-
ence of infill walls, which were only represented as a uniform load on the perimeter beams
in this study. Despite the challenges posed by large openings, the positive impact of the
Resisto 5.9 Tube on the case study was confirmed, which demonstrated its effectiveness in
improving seismic resilience even when considering complex geometrical configurations.
Additional investigations will be performed to take into consideration not only the struc-
tural behavior but also the improvement of the energy performances of the building due
to the presence of the integrated retrofit system.
Author Contributions: Conceptualization, A.F.; methodology, A.F.; software, E.M.; validation, A.F.;
formal analysis, E.M.; investigation, E.M. and A.F.; resources, A.F.; data curation, E.M.; writing—
Figure 12. Seismic risk class of the RC building: before (a) and after (b) the retrofit intervention.
4. Conclusions
The research focused on applying a cold-formed exoskeleton, known as Resisto 5.9
Tube, to a reinforced concrete (RC) structure for seismic-energy retrofitting. Resisto 5.9
is made of steel-braced frames connected to the existing structure with chemical anchors
and integrated with insulation panels for the improvement of the thermal performance.
The investigation was addressed to the assessment of the structural behavior of a building
before and after the installation of the exoskeleton. The aim of the study was to show
the effectiveness of the proposed design procedure to obtain a solution that was able to
improve the global seismic behavior of a case study representing a common typology of
RC buildings erected in Italy in the 1970s.
The system was initially designed using an iterative procedure that determined the
required thickness of the seismic-resistant bracings based on the specific needs of the
structure. The design was set on a target displacement that was identified based on the
capacity curve of the existing structure before the first instance of damage (
d*dam = 0.55 cm
).
Then, an iterative procedure allowed for determining the required stiffness for the Resisto
5.9 Tube that aimed to obtain a similar ductility of the structure before and after the
application of the exoskeleton (
µ
= 2.16). Due to large openings on the ground floor that
limited the application of the exoskeleton, the design specified a bracing thickness of
2 mm
for the first level and 1 mm for the upper levels. The effectiveness of the design was
evaluated through nonlinear static analysis of the structure both before and after the system
application. The results demonstrated that the exoskeleton significantly enhanced the
seismic performance of the existing RC buildings. From the comparison of the capacity
curves of the structure before and after the retrofit, it was observed that the ultimate and
yielding displacements increased in both the longitudinal and transverse directions. The
shear strength increased by 18.5% and 27.5% in the longitudinal and transverse directions,
respectively, while the initial stiffness increased only in the longitudinal direction with
an 8.3% increase. The seismic safety index improved from 0.26 to 0.42, which surpassed
the minimum required increase of 0.1, as stipulated by the Italian Technical Standard for
seismic upgrading. Additionally, the application of the Resisto 5.9 Tube resulted in a
reduction of seismic risk, which advanced the seismic risk class from F to E. In conclusion,
the Resisto 5.9 allowed for increasing the strength and the overall base shear of the existing
building, and thus, improved the global seismic behavior of the structure consistently with
other investigations on external exoskeletons found in literature. The ongoing research
aims to further assess the system’s efficacy, including its performance in the presence of
infill walls, which were only represented as a uniform load on the perimeter beams in this
study. Despite the challenges posed by large openings, the positive impact of the Resisto 5.9
Tube on the case study was confirmed, which demonstrated its effectiveness in improving
seismic resilience even when considering complex geometrical configurations. Additional
investigations will be performed to take into consideration not only the structural behavior
but also the improvement of the energy performances of the building due to the presence
of the integrated retrofit system.

Appl. Sci. 2024,14, 8674 14 of 15
Author Contributions: Conceptualization, A.F.; methodology, A.F.; software, E.M.; validation, A.F.;
formal analysis, E.M.; investigation, E.M. and A.F.; resources, A.F.; data curation, E.M.; writing—
original draft preparation, E.M.; writing—review and editing, A.F.; visualization, A.F.; supervision,
A.F.; project administration, A.F.; funding acquisition, A.F. All authors have read and agreed to the
published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The raw data supporting the conclusions of this article will be made
available by the authors on request.
Acknowledgments: The authors acknowledge the Progetto Sisma Srl company, who patented
the Resisto 5.9 system and stipulated a research contract with the Department of Structures for
Engineering and Architecture of the University of Naples Federico II (scientist responsible: A.
Formisano) to evaluate the system efficacy for retrofitting RC buildings. Also, the DPC-ReLUIS
research project, where the current research activity was framed, is gratefully acknowledged. Finally,
sincere thanks are also due to 2S.I. Software e Servizi per L’Ingegneria S.r.l., which provided the
PRO_SAP software for conducting the numerical analyses.
Conflicts of Interest: The authors declare no conflicts of interest.
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