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Artículo Científico / Scientific Paper |
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https://doi.org/10.17163/ings.n28.2022.06 |
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pISSN: 1390-650X / eISSN: 1390-860X |
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STRUCTURAL ANALYSIS OF A LONG-DISTANCE DOUBLE-DECKER
BUS DURING CRASHES |
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ANÁLISIS ESTRUCTURAL DE UN BUS DE DOS PISOS DE LARGA DISTANCIA DURANTE COLISIONES |
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Jimmy Brito Morocho1 |
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Recibido: 13-05-2022, Recibido tras revisión:
18-10-2021, Aceptado: 20-09-2021, Publicado: 01-01-2022 |
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Abstract |
Resumen |
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This
study presents an analysis of frontal impact and lateral overturn collisions
of a double-decker bus, carried out in accordance with Regulations 66 and 29
of the United Nations Economic Commission of Europe (UN/ECE), and the
Ecuadorian Standardization Service Institute (INEN) with its regulation
1323:2009. The INEN is on charge of regulating the buses for transportation
of Ecuadorian passengers. The continuous improvement of active and passive
safety of buses with respect to accidents, is currently a topic with great
social impact. In this context, the present paper applies the finite element
method (FEM) to analyze the behavior of a double-decker bus subject to
different collision scenarios, such as frontal impact and lateral overturn,
with the purpose of studying the effects of an accident of this type of
structure, considering that the existing regulations are not specific for
this kind of vehicles. The obtained results enable taking into account
different considerations when designing these elements. |
Este estudio presenta un análisis de colisiones de impacto frontal y volcamiento lateral de un autobús de dos pisos, conforme al Reglamento 66 y 29 de la Comisión Económica de las Naciones Unidas para Europa (UN/ECE), y el Servicio Ecuatoriano de Normalización (INEN) con su normativa 1323:2009, encargado de regular los autobuses para el transporte de pasajeros en el Ecuador. En la actualidad la mejora constante de la seguridad activa y pasiva de los autobuses con respecto a los accidentes es un tema de gran impacto social. En este contexto se analiza la colisión de un autobús de dos pisos aplicando el método de elementos finitos (MEF), el cual es sometido a diferentes escenarios de colisión como es de un impacto frontal y un volcamiento lateral, con la finalidad de estudiar los efectos de un accidente de este tipo de estructuras donde la normativa no es específica para esta clase de vehículos. Los resultados obtenidos permiten tener en cuenta consideraciones importantes al momento del diseño de estos elementos. |
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Keywords: Alamouti, D—BLAST, MIMO, SDR, USRP |
Palabras clave: colisiones, volcamiento, impacto frontal, reglamentación, energía, autobús |
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1,*Universidad Politécnica Salesiana, Cuenca-Ecuador. Corresponding
author ✉: mamaya@ups.edu.ec. Suggested citation: Brito Morocho, J.; Amaya Pinos, M.; López López, L. and Espinoza Molina, F. “Structural Analysis of a Long-distance Double-decker Bus During Crashes”. Ingenius, Revista de Ciencia y Tecnología. [Early Access]. doi: https://doi.org/10.17163/ings.n28.2022.06. |
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1. Introduction The bus is one of
the main transportation means in Ecuador, due to its efficiency, flexibility
in service routes and costs for the user; however; in 2018 this transportation
mean was responsible for 8 % of the road crashes, significantly contributing
to the accident rate and to the number of victims [1]; therefore, there is a
great interest in improving both active and passive safety of passengers
since the most frequent accidents are frontal impacts and overturns, which
are considered the most serious and generate a great social impact due to
both human and economic losses. In
2015, the States Members of the United Nations adopted the 2030 Agenda, which
states different sustainable objectives; it is intended to «make cities and
human settlements inclusive, safe, resilient and sustainable» [2], and with
the goal for 2030 of «providing access to transportation systems that are
safe, affordable, accessible and sustainable for all and improving road
safety, particularly through the expansion of public transportation, paying
special attention to the needs of vulnerable people, women, children, people
with disabilities and older people» [2]. International
initiatives and regulations developed by government organizations that are
sustained on the safety of human beings and which guarantee their integrity,
should be considered when generating new public transportation systems and
optimizing the existing ones. For example, double-decker buses have a large
mass and their center of gravity is located at a point very high with respect
to the floor, which significantly reduces its stability and resistance to a
collision or to an overturn. If it is taken into account that these passenger
transportation units travel long distances, it is relevant to consider all
aspects related to the safety in the event of a collision [3]. Among
the different types of accidents in which buses may be directly involved,
frontal impacts and lateral overturns are deadliest. A study conducted by
Transport Canada shows that frontal impacts represent 70 % of all bus
accidents. In addition, it is considered one of the collisions that produce
more deaths and serious injuries than any other accident. In general, when
two vehicles that approximate at a high speed are involved in these impacts,
the front structure of the vehicle is involved [4]. A
study presented by Ramírez et al. [1] also indicate
that traffic accidents that occur in roads involving public passenger
transportation systems are mainly frontal collisions. Similarly, they state
that the difference between the masses and configurations of the vehicles
during the impact, generate critical material damages and serious injuries or
even loss of life of the occupants. The resistance to
collisions is the capability of the structure to absorb the kinetic energy of
the overturn |
or frontal impact,
which should provide an appropriate protection to the vehicle occupants
during the traffic accident. This criterion is especially important in
passenger transportation vehicles such as buses [5]. This is the reason why,
the purpose of the simulations performed in the bus superstructure is to
analyze the amount of energy absorbed during a frontal impact or lateral
overturn collision in a double-decker bus. Such structure should be deformed
as little as possible and should avoid any element to get into the bus
survival space [6], according to regulations 29 [7] and 66 of the UN/ECE [8]
and regulation NTE INEN 1323:2009 [9]. 2. Materials y
methods The study of
interest starts with a 3D modeling considering all the details and dimensions
of the structure of the double-decker bus. CAD tools were used with the
purpose of obtaining the final model for the simulation stages using the FEM;
Solidworks was used for the preprocessing stage,
while Ansys – LS DYNA [10] was used for the
processing and post-processing stages. The
overturn study of the bus structure using the FEM was based on the NTE INEN
1323:2009 regulation [9] and on regulations 29 and 66 of the UN/ECE [7, 8].
The latter is pioneer in increasing the safety of public transportation year
after year, implementing regulations that enable guaranteeing the safety of
the occupants when a bus experiences a collision and no invasion of the
structure to the passenger survival space occurs during an overturn. In
the application domain, regulation 66 of the UN/ECE states that it only
applies to single deck vehicles, rigid or articulated, belonging to
categories M2 or M3; according to regulations NTE INEN 1323 [9] and 2656
[11], double-decker buses belong to category M3; for these reasons, the
overturn test of a doubledecker bus may be carried
out according to regulation 66, which supports the application of the regulation
indicated in this study [8]. Once
the overturn analysis was carried out, it was studied a frontal collision of
the structure, which enabled visualizing the effect of this type of collision
on structure deformation and how it invades the survival space [12]. 2.1. Delimitation
of the survival space The survival space
shows the geometrical features stated in regulation 66 of the UN/ECE
considering the bus dimensions, and it should be located along its entire length
as observed in Figure 1. Passengers and operators are in this space; during a
collision, this space must not be invaded by the bodywork structure or any
accessory that may affect the physical integrity of the occupants [8]. |
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Figure
1.
3D modeling of the survival space within the bus passenger compartment 2.2. Conditions
of the bus superstructure Annex 4 of
regulation 66 of the UN/ECE presents the perspectives of the structural description
of the bus superstructure; the profiles and structural materials should
comply with national and international standards [8]. The
structural profiles used in the bus bodywork are shown in Table 1. Table 1. Bus structural profiles Paragraph
1.3 of annex 9 of regulation 66 of the UN/ECE [8] indicates that the data
necessary to carry out the test must be met, where the values of mass, center
of gravity and moments of inertia of the bus structure must be obtained in
advance. The values of mass, moments of inertia and center of gravity of the
bus structure are shown in Table 2, and were obtained during the modeling
process. Table 2. Data sheet of the bus
structure
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The
location of the center of gravity of the bus structure must be clearly
defined, as shown in Figure 2.
Figure 2. Location of the center of
gravity of the bus structure 2.3.
Analysis using finite elements The accuracy of
finite elements models depends on the number of nodes and elements, as
observed in Figure 3, which depends on the size and the types of components
of the mesh; hence, the smaller the size and the larger the number of
elements in a mesh, more precise will be the results of the analysis [13].
Figure 3. Nodes and elements of a mes Important aspects, such as the
quality and type, should be taken into account to generate the mesh of the
bus structure; these aspects are related with the density and type of the
mesh used, which for this case study is a 20 mm hexahedral mesh [14, 15]. 2.4. Computer
simulation of the overturn test of a vehicle as equivalent homologation
method The bus overturn
test is a quite fast and dynamic process with well differentiated stages;
this should be considered when planning the test. The bus will tilt without
balancing and without dynamic effects until it reaches an unstable
equilibrium and starts to overturn, as specified in Figure 4 [8]. |
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Figure
4.
Specification of the overturn test of a vehicle at initial position in the
platform Annex 9 of regulation 66 of the
UN/ECE, is applied through the finite element method for computer simulation
of the overturn test of a double-decker bus structure. The mathematical
values entered in the software to simulate the overturn correspond to the bus
turning speed with respect to an axis located in the tilting platform and the
gravity, in order to simulate the movement of the structure with respect to
the platform. Equation (1) gives the value of
angular speed to be applied.
Where:
m = mass (kg) g = gravitational constant (m⁄s 2) ∆h = height variation (m) I = rotational inertia (kg·m2) The contacts between the master
surface and a set of slave nodes are defined for the simulation. The master
surface is defined through the rigid elements used to establish the surface
on which the structure of the bus impacts, as seen in Figure 5.
Figure 5. Bus in position
of first contact with the rigid impact Surface |
2.5. Computer simulation of the frontal
collision Recent statistics of
traffic accidents demonstrate that almost two thirds of the collisions are
frontal, and half of them show a coverage between 30 and 50 % of the front
surface [16]. Computer simulation tests are conducted according to regulation
29 of the UN/ECE [8], to assess the effects of this type of collision. Computer
simulation tests of frontal impacts against stationary objects, enable
observing the behavior of the vehicle during a collision; it is also an inexpensive
method, compared to a real Crash Test [10], [17, 18]. The
frontal impact is analyzed at 64 km/h, considering that the bus has a frontal
impact against a centered stationary barrier (figure 6). This impact intends
to simulate the most frequent type of collisions in roads that result in
serious or deadly injuries, since most frontal crashes in double-decker buses
directly involve the operators” cabin [19].
Figure 6. Specifications for simulating
the bus frontal impact It
is important to indicate that the speed limit for this type of vehicles in
straight roads is 90 km/h, according to the Ecuadorian National
Transportation Agency, situation which is considered in other research works
that analyze the frontal impact of a single-deck bus [20]. 3. Results and
discussion 3.1. Bus overturn
The application of
the overturn test of the doubledecker bus structure
using the finite element method, works well until the structure reaches its
maximum deformation at time instant t = 0.621s after impacting the rigid
surface. 3.1.1. Energies The values of height
obtained for the centers of gravity during the overturn, shown in Table 3,
are used to find the difference between the heights (∆h) of the center
of gravity, equation 3, which is a variable required in equation 2 that gives
the total energy (ET) that will be absorbed by the bus superstructure in the
overturn test. |
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Table 3. Centers of gravity According
to the equations of regulation 66, the total energy absorbed by the bus is 4,
42 × 107 J, and the maximum value of total energy absorbed
obtained in the simulation (Figure 7) is 4, 56 × 107 J. When
comparing the total energy calculated and the one obtained in the simulation
there is a difference of 3.32 %, which is due to the fact that the center of
gravity is not exact because not all mechanical and finishing components,
such as glasses, seats, etc., are considered in the modeling process.
Although this difference exists between the values obtained in the
calculation and in the simulation, it may be considered that they are
coherent and acceptable.
Figure
7.
Energies obtained from the simulation of the bus overturn test The
maximum value of Hourglass energy during the overturn test is 0, 0966 × 107
J, which represents 2 % of the total energy. According to annex 9 of
regulation 66, this value should not exceed 5 % for the simulation to be
accepted; therefore, this requirement is fulfilled. 3.1.2. Survival
spaces Considering that regulation 66 states that the structure should never invade the survival space or vice versa during |
the overturn, when the bus structure reaches maximum
deformation, it does not fulfill such regulation.
Figure
8 shows the displacement of the structure with respect to the survival space.
The lower deck of the bus is not affected by the structure deformation since
it is rigid enough to withstand an overturn collision; however, the survival
space of the upper deck is invaded by the structure in 48 mm when reaching
the maximum deformation during the overturn; thus, it does not fulfill the
requirement of regulation 66 of the UN/ECE.
Figure
8.
Displacement of the double-decker bus structure with respect to the structure 3.1.3. Speed The bus speed is
high until it impacts on the rigid surface, and after this instant the speed
decreases progressively (Figure 9); however, the most important time interval
is when the bus impacts on the surface, since our interest is the contact
between the two surfaces during the overturn test; the speed value does not
decrease to zero, because the overturn simulation is carried out until the
maximum deformation of the bus structure according to the regulation.
Figure 9. Behavior of the speed
during the bus overturn test |
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3.2. Frontal
impact The bus speed prior to
the collision is given by the speed change (∆V) that the vehicle
experiences and by the deceleration, which is a function of the mass and the
rigidity of the objects that collide. The impact zone of the bus with the
barrier covers the entire vehicle width; therefore, the bus frame absorbs
most of the kinetic energy during the collision, and moreover, the
mathematical model was adjusted to obtain the same conditions of a real
physical test. 3.2.1. Energies
Figure
10. Energies obtained from the simulation of the
bus frontal impact The
maximum total energy generated in the simulation of the bus frontal impact is
4, 56 × 108 J, which remains constant, i.e., is the same before and
after the collision; this indicates that the energy produced in the collision
is dissipated through the deformation, better known as internal energy
(Figure 10). During
the bus impact, the maximum value reached by the Hourglass energy is 0, 106 ×
108 J (Figure 11), which represents 2.3 % of the total energy;
thus, it fulfills the regulation which indicates that this value should not
exceed 5% of the total energy. 3.2.2.
Deformation The cabin of the
driver reaches a deformation of 250 mm, as can be observed in Figure 11, due
to the impact with the wall. The structure profiles with greater deformation
are those that impact directly with the surface; moreover, the chassis wings
behave as an underrun bar, which prevents the cabin of the driver from experiencing
an excessive deformation, but this does not prevent that structure debris may
damage the integrity of the controllers of the transportation unit. |
Figure
11. Bus deformation during the frontal impact on the stationary Surface 3.2.3. Speed The bus starts with
a speed of 17 800 mm/s (64 km⁄h), which then
decreases continuously due to the impact on the stationary barrier; thus, the
cabin of the driver is deformed in a short time interval until reaching a
standstill, and the most affected parts are the ones that impact directly on
the surface (Figure 12).
Figure
12. Behavior of the speed during the simulation of the bus frontal impact 4. Conclusions This study set
up two computer simulation processes, namely lateral overturn and frontal
impact, of a doubledecker bus, according to
regulations R66 and R29 of the UN/ECE [8] and regulation NTE INEN 1323:2009
[9]. This enabled to estimate the resistance of the vehicle superstructure
during a collision, and also to observe the behavior of the structure with
respect to the survival space and the deformation modes of the vehicle. The
collision analysis enables to evaluate the elastoplastic behavior of the
steel that makes up the structure of the double-decker buses used in
interprovincial transportation, through the computer simulation based on
explicit dynamics. |
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The
upper part of the structure was affected during the overturn, since there was
a deformation that generated an invasion of 48 mm of the survival space; this
can be observed in Figure 8. Since
the center of gravity is not exact, there is a percentage error of 3.32 %
between the value of total energy obtained in the overturn simulation and the
one calculated using the formulas suggested by regulation 66 of the UN/ECE
[8]. It is important to indicate that this numerical error is considered
small, and therefore the results obtained for the bus overturn are valid. The
forces generated by the frontal impact of the bus structure on a stationary
surface produce high deformation, especially in the cabin of the driver where
it reaches values of 250 mm in a very short period of time. This
is because the most critical parts are the profiles that receive the impact
directly. The bus superstructure is one of the main passive safety components
in these vehicles, and therefore design optimization is essential for
minimizing the damages that may be caused to passengers and operators of the
transportation unit. In
general, the frontal part of the bus structures does not have any protection
system to safeguard the life of the cabin occupants during a frontal impact.
These elements of the bodywork structure are not capable of totally
dissipating the kinetic energy, which should be considered in the design of
these elements. References
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