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Department of Mechanical and Manufacturing Engineering Rollover Impact in Buses and Coaches Impact Mechanics- MECG-7780 Khashayar Pejhan 7718719

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This is a literature review on recent articles dealing with rollover analysis in buses and coaches. It includes experimental and FEM modeling samples

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Page 1: Rollover impact in buses

Department of Mechanical and Manufacturing Engineering

Rollover Impact in Buses and Coaches

Impact Mechanics- MECG-7780

Khashayar Pejhan

7718719

Page 2: Rollover impact in buses

2 Khashayar Pejhan - 7718719

Abstract

There exists a generalized interest in the improvement of safety issues in

buses due to the social alarm brought on by accidents in which buses are involved.

Accident statistics show that bus rollover accidents occur with relative low

frequency, taking into account all kinds of bus accidents. Nevertheless the risk of

mortality in the case of rollover is five times greater compared with any other

possible bus accident typology [1]. As a result the coach and bus producers have a

great responsibility in matter of safety issues regarding rollover. Although road

condition and driving skills are important influencing parameters that may lead to

or prevent rollover but producers must perform various test on their coaches to

ensure that the structure is as safe as possible in case of rollover. There are various

standards and regulations in this area that would be pointed out in this article, in

addition substitute computer simulation for these tests would be introduced and

some main research in this area would be explained to show their weak points and

strength. Although much have been done in area of rollover impact, but due to the

dynamic sense of regulation (becoming stricter each year) and incompleteness of

current tests and computer simulations, there are still necessary steps to be taken to

improve the bus structure for facing rollover impact. Some of these points are

mentioned for future studies in conclusion section.

Key words: Rollover-Impact- Bus Safety

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3 Rollover Impact in Buses and Coaches

Content

Title Page number

Introduction 4

Definitions, Regulations and Standards 7

Literature review and discussion 12

Jordan Rollover System 27

Conclusion 30

References 33

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1. Introduction

One of the main concerns of automotive manufacturers is safety. Due to the

significant amount of money and time that has been spent in this area in last decade

the fatality rate dropped to 1.44 fatalities per 100 million of vehicles traveling in

2004 [1].

Figure 1 shows the percentage of different types of crash in vehicles for a

time span of 1997 till 2001 [2] while Figure 2 shows the statistics of injuries and

fatalities in the same time limit. As it could be seen although rollover accidents

doesn’t seem to be an important type of accident to study due to their insignificant

part in whole crashes (only 8%) but according to Figure 2, more than 30% of

fatalities in accidents have happened in rollover crashes. As a result many

manufacturers and independent researchers have dedicated their time to analyse

this type of accidents.

Figure 1) Percentage of different types of crash in a 5 year time span [2]

8% 13%

16%

49%

14%

Percentage of different types of crash

Rollover

Rear

Slide

Front

Other

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5 Rollover Impact in Buses and Coaches

Figure 2) Total number of injuries (blue) and fatalities (red) in car crashes in 5 year time span due

to different type of impacts [2].

Prior to study about different researches that have focused on roll over, it is

necessary to present a definition for this type of impact. A rollover is a type

of vehicle accident in which a vehicle tips over onto its side or roof. The most

common cause of a rollover is traveling too fast while turning. Vehicles with a

high center of gravity are easily upset or "rolled." Figure 3 shows a SUV rolled

over in an accident.

Based on the 2001 Traffic Safety Facts published by NHTSA1, rollovers

account for 10.5% of the first harmful events in fatal crashes; but, 19.5% of

vehicles in fatal crashes had a rollover in the impact sequence. Based on an

analysis of the 1993-2001 NASS2 for non-ejected occupants, 10.5% of occupants

1 National Highway Traffic Safety Administration

2 National Automotive Sampling System

0

10000

20000

30000

40000

50000

60000

70000

Roll over Frontal Rear Slide Other

Injuries 30000 67143 2858 25714 17142

Fatalities 10138 12754 981 8176 655

Nu

mb

er

of

pe

op

le

Total number of injured and dead occupants in different crash types

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6 Khashayar Pejhan - 7718719

are exposed to rollovers, but these occupants experience a high proportion of AIS1

3-6 (3:serious, 6:Maximum) injury (16.1% for belted and 23.9% for unbelted

occupants). The head and thorax are the most seriously injured body regions in

rollovers. As a result, it is very important to consider certain standards for safety

regarding roll over in vehicles and many researchers have focused on topics related

to roll over of vehicles. Governing dynamic equations, protective structures, safety

test, injury analysis are some of the concepts that have got engineers attention in

recent years.

Considering this definition, buses are generally believed to be very safe due

to their low centers of gravity and their tendency to be driven slowly. However,

since a number of passengers are being carried on these large vehicles, any

accident involving a bus is often devastating. This is especially true for a rollover

accident.

In each rollover accident of buses, an average of 25 people loses their lives.

[3]. Also, a rollover can cause up to five rotations to occur on the road. A “serious

rollover” occurs when the vehicle flips more than two times. Lastly, a combined

rollover involves two different dangers; for example, a head-on collision that leads

to a rollover, or a rollover that ends with the bus in a lake. The most typical

collision configurations involving buses and coaches are side, rear, frontal and

rollover. In most parts of the world, especially in Europe, safety requirements are

continuously inspected to improve passenger safety in buses or coaches.

In this study a review over different researches about roll over impact,

standard roll over test and related subjects would be presented with a focus on

studies on buses or coaches.

1

The Abbreviated Injury Scale (AIS) is an anatomical-based coding system created by the Association for the

Advancement of Automotive Medicine to classify and describe the severity of specific individual injuries.

Page 7: Rollover impact in buses

7 Rollover Impact in Buses and Coaches

Figure 3) a rolled over SUV (safety test) [4]

2. Definitions, Regulations and Standards

Before going through details of different test, standards and studies about

rollover various types of rollover should be defined. Rollover test are divided in to

two categories; static and dynamic tests. Although there are different examples in

literature for both types of tests but there has been a tendency to static tests due to

difficulty in simulating a dynamic test that can predict any type of rollover;

because there are many reasons for a rollover to happen.

Considering the reason behind the occurrence of rollover two general

categories could be defined for rollovers:

a) Tripped rollover

b) Untripped rollover

A tripped rollover happens when a tire hits an obstacle on the road, or when

the tire rolls over soft soil, in these conditions the lateral motion of tire would stop.

This would lead to rolling of vehicle around the object. Unlike this type of rollover

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8 Khashayar Pejhan - 7718719

that usually happen when vehicle leaves road’s surface there is another type of

rollover that happens on the road surface, untripped rollover. In this type of

rollover severe steering maneuvers (like J-turns, fast lane changes and etc.) lead to

losing control of vehicle. Improving structure and control systems of cars could

only prevent untripped rollover, because it is independent of external parameters;

therefore, the main safety analysis is concentrated on this kind of rollover rather

than tripped one [5].

There are many rules and regulations in different countries trying to set a

minimum standard for safety issues of motor vehicles. In case of rollover and

impact, NHTSA has considered three major rules, namely FMVSS1 208, 216 and

220 [6]:

FMVSS 208 (Occupant Crash Protection): This standard originally

specified the required type of occupant restraints (i.e., seat belts). It was amended

to specify performance requirements for anthropomorphic test dummies seated in

the front outboard seats of passenger cars and of certain multipurpose passenger

vehicles, trucks, and buses, including the active and passive restraint systems. The

purpose of the standard is to reduce the number of fatalities and the number and

severity of injuries to occupants involved in frontal crashes.

FMVSS 216:Roof Crush Resistance - Passenger Cars (except convertibles)

(Effective 9-1-75) and Multipurpose Passenger Vehicles, Trucks and Buses (except

school buses) with a Gross Vehicle Weight Rating of 2722 kg (6,000 lbs.) or less

(Effective 9-1-94). This standard specifies requirements for roof crush resistance

over the passenger compartment.

1 Federal Motor Vehicle Safety Standards

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9 Rollover Impact in Buses and Coaches

FMVSS 220 (School Bus Rollover Protection): This standard establishes

performance requirements for school bus rollover protection. The purpose of this

standard is to reduce the number of deaths and the severity of injuries that result

from failure of the school bus body structure to withstand forces encountered in

rollover crashes. This standard requires a quasi-static application of a loading equal

to 1.5 times of the unloaded vehicle weight (UVW) to assess the static response of

the bus roof structure. The resistance of the roof structure is judged as satisfactory

when: the downward vertical movement at any point on the application plate does

not exceed 130 mm and each emergency exit of the vehicle can be opened during

full application of the force and after the release of the force [7].

These regulations are not always perfect and new concerns or new

technological improvements lead to modification of regulations. For example,

FMVSS 208 is a dynamic roof crush test standard and almost all vehicle industries

execute this test on their products. But in 2006, Mao et al. [8] showed that this test

lacks repeatability. Through their simulations they showed that 2 models with

completely similar roofs could react totally different to such test. This difference is

such significance, that even the number of rolls each model has in a test differs

from another similar model.

In the case of FMVSS 216, the vehicle’s roof should be loaded quasi-

statically up to a specific level, and its roof crush resistance then should be

checked. The first weak point that could be observed in this test is the fact that in

real rollovers, there are rollover forces and different velocities that are not

considered as mandatory conditions for performing this test. In addition, the force

application in this test is in a way that makes windscreen an important load

carrying part of body which limits roof deformation. The force applied to roof in

this test is not also in a same angle and magnitude as in actual accident.

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These problems led to some action in NHTSA and a new version of FMVSS

216 was proposed in 2005, in the new version the standard includes more vehicles

because of the increase in the magnitude of limit weight for vehicles (from 6000 lb.

to 10000 lb.). In addition the applied force was increased 2.5 times to simulate the

real rollover more precisely. Finally NHTSA revised the amount limit for intrusion

of roof, and made it mandatory for manufacturers to maintain enough head room

for a mid-sized adult male occupant.

ECE R66 - Another important regulation in this criterion is the ECE R66

regulation. If a vehicle passes ECER66 test, it is guaranteed that parts like pillars,

luggage racks and etc. would not intrude the residual space during a rollover. [9].

As shown in Figure 4, the envelope of the vehicle’s residual space is defined by

creating a vertical transverse plane within the vehicle which has the periphery

moving this plane through the length of the vehicle.

Figure 4) Residual space in ECE R66 test [7]

The ECE R66 test is actually a kind of lateral tilting test. As in Figure 5, in

the rollover test procedure, a vehicle resting on a tilting platform is quasi-statically

rotated onto its weaker side. Depending on the attachments of the staircase and the

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11 Rollover Impact in Buses and Coaches

door frame to the bus frame, it is usually the road side of the bus. When the center

of gravity reaches the highest (critical) point, the rotation of the table is stopped.

Further the gravity causes the bus to free-fall into a concrete ditch. The flooring in

the ditch is located 800 mm beneath the tilt table horizontal position [10].

Figure 5) ECE R66 test specifications [10]

Usually, the rollover test is carried out on that side of the vehicle considered

more dangerous with respect to the residual space. The decision is taken by a

competent Technical Service on the basis of the manufacturer’s proposal,

considering the following:

1) Lateral eccentricity of the center of gravity and its effect on potential

energy in the unstable starting position of the vehicle,

2) Asymmetry of the residual space,

3) The different, asymmetrical constructional features of the two sides of the

vehicle,

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12 Khashayar Pejhan - 7718719

4) Which side is stronger and better supported by partitions or inner boxes

(e.g., wardrobe, toilet and kitchenette.)

3. Literature review and discussion

One of the most important studies that have shown the importance of

research on rollover of coaches is the study by Martinez et al. [11] in 2003.

According to the data collected by them probability of different kinds of injury for

bus passengers is higher in roll overs than other types of accident (See Figure 6).

Figure 6) Injury distribution in coach accidents, Spain 1995-1999 [11]

Therefore, it could be concluded that rollover is a menace to the safety of

bus passengers too. Rollover is a complex, chaotic and unpredictable event. This

complexity is due the fact that rollover would be influenced by different factors

like driver, road, vehicle and environmental factors. Whenever a car revolves at

least 90 degrees a rollover has happened (even if car rolls till it comes back on tires

at end). Some of major studies in the area of rollover of buses would be discussed

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

FatalitiesSeriouslyinjured

Minorinjured Not injured

9.60%

32.10%

55.26%

2.60%

2.50% 7.70%

43.30% 46.50%

Rollover Others

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13 Rollover Impact in Buses and Coaches

in details in this section and a quick over view on some other important articles in

literature is presented in Table 3.1 in a timely sequence.

Table 3.1) Some major researches about rollover

Author Research Description Year

A. Sances Jr. et

al. [12]

Biomechanical Analysis of Head and Neck Injury

with Rollover Glass Impact

(Glass consideration and injury)

2001

C. Parenteau et

al. [13]

Near and Far-Side Adult Front Passenger Kinematics

in a Vehicle Rollover

(Focus on passenger dynamics)

2001

D. Friedman et

al. [14]

Rollover Roof Test Results for a Production Vehicle

(Experimental value) 2004

D. Valladares,

R. Miralbes, L.

Castejon [15]

Development of a Numerical Technique for Bus

Rollover Test Simulation by the F.E.M. 2010

L. Yan et al.

[16]

Comparison of vehicle kinematics and occupant

responses between Jordan rollover system and an

over-the-road rollover 2012

As mentioned in previous section, standards and regulations are always an

interesting topic for researchers to work on. A good example is the study made by

Bojanowski et al. in 2011[7]. They considered a special case of Paratransit buses in

United States.

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Paratransit buses are a special type of buses in US. They are smaller than

ordinary buses and are being used for handicapped people. As a result they are

equipped with wheelchair lifts and some other special structures. Since in these

coaches, the passenger compartment is assembled on an adopted chassis of

ordinary buses, there is no specific regulation or crashworthiness standard for these

vehicles. Bojanowski et al. found that Paratransit bus manufacturers in US try to

comply with FMVSS 220, which is originally for school bus protection. So they

proposed a FEM model to check if they can predict whether paratransit buses can

pass other test too, or not.

According to their research [7] FMVSS 220 is a useful test but since in this

test a quasi-static loading is applied symmetrically to roof, it cannot be considered

as a trustworthy representative of dynamic loading in actual rollover in which

loads have varying value, direction and intensity [7]. This weakness influences the

repeatability of FMVSS 220 test in a bad way. On the other hand, since the number

of paratransit buses that are produced is not significant, performing expensive test

like ECE-R66 is not economically wise for small manufacturers.

As a result Bojanowski et al. proposed a FEM model, as shown in Figure 7,

to predict the paratransit bus behavior in both tests (FMVSS 220 and ECE-R66)

and compare the results. In order to validate their model, Bojanowski et al.

compared the results of their model with those of a full rollover test, performed at

Florida Department of Transportation (DOT) testing facility in 2010 [17].

The bus passes the rollover test if the residual space is not compromised

during the tests [18, 19]. The so called residual space was shown in Figure 4. As it

could be seen there are 6 hypothetical hinges in that figure pointed out as to .

In order to quantify a limit for passing the rollover test the rotation of these hinges

play an important role.

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15 Rollover Impact in Buses and Coaches

Figure 7) Paratransit bus in Rollover test and FE model [7]

The assumption is that during the rollover impact the deformation occurs

only at these points which are the weakest connections in bus. Then a Deformation

Index (DI) is defined as:

( )

( )

( ) (3-1)

The test would be evaluated on basis of DI as follows [20]:

Table 3-2) Evaluation of Rollover safety, regarding the Deformation Index

Acceptable Design

Deforming walls start to touch the residual space

Vehicle Failed the test

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Using a same concept Bojanowski et al. used their FE model to check the

condition of Paratransit buses under an ECE-R66 test. Figure 8 shows the results

they gained form FEM modeling using LS-Dyna software package. Obviously the

paratransit bus has not passed this test, and the residual space is intruded by

deformed structure.

Figure 8) FE model of Paratransit bus after ECE-R66 test simulation (a: with skin, b: without skin)

[7]

Then, Bojanowski et al. calculated DI to check if it can also predict a same

result as the FE model. Figure 9 shows the variation of DI by time. As it can be

seen at 0.15 seconds after impact, paratransit bus fails the minimum requirements

for safety as DI passes 1.

Bojanowski reported the result in this way: “The bus is deformed in the

torsional mode with rear part being considerably less deformed. As an outcome of

the impact, the plastic deformations were developed at the front cap structure and

at the waistrail beam. The cantrail beam was also deformed locally at the

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17 Rollover Impact in Buses and Coaches

connections of the roof bows to the walls.” [7]. By this simulation Bojanowski et

al. showed that the connections between the body and the driver’s cabin are one of

the weakest points of the bus body structure.

Figure 9) variation of Deformation Index (DI) with time in ECE-R66 simulation of

Paratransit bus [7]

As shown in Figure 10 on the road side the only connection between the

body and driver’s cabin are two flat pieces of steel while on the curb side only two

welded spots connect the two parts.

In the next step they tried to use their model for simulate a FMVSS 220 test.

After considering proper loading condition the result they gained was to their

surprise in contrary with previous test. The paratransit bus passed FMVSS 220

conditions. Figure 11 shows the diagram they gained for force applied on roof

versus displacement of roof panel.

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18 Khashayar Pejhan - 7718719

Figure 10) Weak connections found by simulations on right and they shape in reality in left

picture [7].

According to FMVSS 220 a force equal to 1.5 times of unloaded vehicle

weight (UVW) should be applied to the roof, and the intrusion of roof structure to

residual space should not be more than 130.2 mm. As it could be seen in Figure 11,

at the time that applied force reaches 1.5 times of UVW the displacement is only

119 mm. So the paratransit buses pass FMVSS 220.

Figure 11) Force vs. displacement results of FE simulation of FMVSS 220 test [7].

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19 Rollover Impact in Buses and Coaches

This research proved that FMVSS 220 could not be trusted for paratransit

buses since it does not consider a fair simulation of actual loading condition in

rollovers.

In the final stage of their study, Bojanowski et al. tried to identify and rank

the most relevant components of the structure in the two tests. Not surprisingly

roof bows were the first choice of variables to be modified to bear more quasi-

static load. By improving them the roof resistance force was changed about 75%

but the important thing is that their influence on Deformation index was only 14%.

As Bojanowski reported in [7], these results lead to a conclusion that testing

paratransit buses according to the FMVSS 220 standard may lead in some cases to

erroneous conclusions regarding the bus strength and integrity of its structure.

One of the most recent researches in area of rollover of buses is the study on

rollover analysis of bus body structure by Jeyakumar and Devaradjane [9] in 2012.

They analyzed rollover of bus structure considering the conditions of ECE-R66

standard. A 36 seat ultra-deluxe bus was selected for this research and necessary

dimensions was gained by reverse engineering method. This study is one of the

most recent Finite Element Analysis (FEA) of bus body structure, which has used

LS-DYNA explicit solver. Their main goal as they mentioned in their article [9]

was: “to investigate the impact on passenger residual space along the entire vehicle

during a rollover by simulation method”.

Using their computer model (see Figure 12), Jeyakumar and Devaradjane [9]

simulated an ECE-R66 for bus and found that residual space is secure.

But there are some important factors that seem to be neglected in this

research. First of all, they used lump masses instead of real parts in bus without

checking the effect of this assumption on accuracy of results. The second important

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20 Khashayar Pejhan - 7718719

factor that was shown earlier is the importance of considering the special effect of

contacts between roof panel and structure. In this study they have considered same

contact elements for all contacts with same friction which is not precise enough,

regarding the importance of its influence. In addition, adaptive meshing seemed to

be necessary (more elements in contact areas and fewer in other parts which are

not important in rollover), but as their model shows, such consideration was not

made. So their model could become more precise and less time consuming by a

few modifications.

Figure 12) LS-DYNA model of bus in rollover test [9]

Another important factor that was not considered in the research, discussed

above, is the mass of passengers. First researchers to consider the effect of mass of

passenger weight on rollover crashworthiness were that of Guler et al. [21].

Likewise, Belingardi et al. [22] considered the passengers’ weight in their research

in which they used a FEM approach to study the structural behavior of a M3 bus in

rollover accident. Although this study was limited to a bay section of the bus, but

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21 Rollover Impact in Buses and Coaches

their analysis on passenger injury risk showed that even if a bus comply with ECE-

R66 regulations it may be inadequate, concerning passive safety.

Next case to be studied here is a comprehensive FEM analysis by Volvo in

2012 [23]. In their research, engineers of Volvo presented a more complete model

of the bus than the one in the research discussed above. The ultimate aim of this

study was to investigate the impact on passenger residual space. One of the most

interesting aspects of this study is the method of validation of results. In order to

have a reliable computer model, Volvo engineers executed some actual tests on

critical joints of bus’ body. Then they used their computer model to predict the

behavior of same joints. After getting acceptable results from the model they

modified model in way to be more conservative. So they were sure that if the

computer simulation shows that this bus passes the test, the actual case would be

the same.

As mentioned earlier, the advantage of this study comparing to [roll-bus] is

its precise modeling. In this case FE model of the full vehicle was comprised of

first order shell elements, spring elements, mass elements and rigid elements. So

the whole structure was not considered to be made of a same rigid element. Mass

balancing was also done in this study using balancing tool in HyperCrash V11

4021.

The test regulation that was set as an aim in Volvo is AIS 031, titled

“Resistance of the Superstructure of Oversized Vehicles for Passenger

Transportation” [18]. This regulation states that vehicle structure should be strong

enough that during a rollover it does not intrude inside the residual space.

Although it seems that AIS 031 and ECE-R66 are same, but a more detailed view

will show that there are some important differences between them.

1 HyperCrash is a CAE pre-processor tool developed to support the non-linear finite element solver, Alter RADIOSS

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Figure 13 and 14 shows the CAD model and FEA model of the coach

studied by Volvo in 2012.

Figure 13) CAD model of bus studied in Volvo’s research [23]

After preparing the CAD model, it is necessary to consider masses in the

system before performing meshing for FEM analysis. Engine, Power transmission

and suspensions were modeled by 1-D element. Mass elements were used for

modeling fixed masses. The whole vehicle had 1646623 shell elements, 7308

spring elements and 44 mass elements. In order to increase the accuracy the

elements on the roof were selected to be 10 mm long while in other parts where

there necessity for precision was not same as roof some elements were 30 mm

long. Considering the axle, it was set to be rigid and its mass was added at

appropriate locations, according to Volvo engineers [23]. The final FEM model of

bus is illustrated in Figure 14.

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23 Rollover Impact in Buses and Coaches

Figure 14) FEM model of bus studied in Volvo’s research [23]

The next step is the loading process. Figure 15 shows the model after rolling

over. Both sides of the bus were tested in a rollover situation. The colliding

surface was set as a rigid wall. A torque equal to 0.75 Mgh (Nm) was applied to

the structure by a rotational velocity to all parts of the vehicle. Table 3.3 shows the

formulas used to calculate proper loading condition.

Figure 15) Loading and rollover of model [23]

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Table 3.3) Loading condition and necessary formula

Loading condition Necessary formulas

Angular velocity and

gravity load applied on

whole body

( )

⁄ ( ( ) )

( )

(

)

Before presenting the results found by Volvo research team and analysing

them it is necessary to show how they have checked the accuracy of their model. In

order to check the accuracy of simulation results, Kumar et al. [roll-bus 4] checked

the total energy during simulation time period. Figure 16 shows the energy

distribution in buss rollover simulation. It could be noticed that while kinetic

energy transform to sliding energy and strain energy through time, total energy

remains same in whole simulation. As a result the results of simulation are accurate

and could be trusted.

Knowing the fact that simulation is accurate, Kumar [23] tried to check if

the residual space would be intruded after rollover or not. Figure 17 shows the

summary of the simulation results. As it could be seen, the driver side seems to

face less danger. The minimum distance between superstructure & residual space

was found to be 38.0 mm on the driver side.

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25 Rollover Impact in Buses and Coaches

Figure 16) Energy balance graph- LHS rollover [23]

Volvo engineers have also ignored some important issues. First and probably

the most important fact is that AIS 031does not include the driver and co-driver. So

although a bus may look safe to passengers by passing AIS 031, but drivers cannot

be sure if they are also safe in case of a rollover. In addition according to AIS

031[18] structural parts, members and panels and all projecting rigid parts such as

luggage racks, on-the roof ventilation equipment, should not intrude the residual

space.

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26 Khashayar Pejhan - 7718719

Figure 17) Summary of the results for both sides of bus [23]

But regarding the safety of passengers there many other hazards in a rollover

that could be considered as a menace to safety, Including:

Bulkheads

Partitions

Rings or other members reinforcing the structure of vehicle

Bars, kitchenettes or toilets (fixed appliances)

Unfortunately, none of the above is included in ECE R66/ AIS 031 and it would be

wise to consider them as possible research subjects, because there might be

mandatory regulations in future that consider them, and more importantly, they can

be dangerous for passengers in rollovers.

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27 Rollover Impact in Buses and Coaches

4. Jordan Rollover System- an ignored method for

testing buses:

Since the earliest dynamic rollover test was introduced in 1934 [16], many

different rollover test devices have been designed to obtain the realistic dynamic

rollover test results. One of these devices is the Jordan rollover system (JRS). JRS

is a versatile and repeatable rollover test system developed to evaluate the

performance of roof structure and occupant restraint system during rollover. It can

also be used to study the kinematics of the occupant.

Many physical tests [24] and computational simulations [25] have been performed

and showed the effectiveness of this method. But this method is not popular in

coach producers. The reason would be obvious by a glance at the methodology of

Jordan Rollover System (JRS).

Methodology of JRS

As shown in Figure 18, a vehicle is mounted on an axis through its centre of

gravity (CG), which permits it to be rotated about its longitudinal axis. A roadbed

is released to move under the suspended vehicle when the vehicle starts to rotate

and drop. The vehicle contacts the moving roadbed in a controlled impact

configuration. This configuration includes roadbed speed, vehicle roll rate, drop

height, impact yaw angle, impact pitch angle and impact roll angle.

The standard values of these parameters are listed in Table 3.4 [26]. Only the

lateral translation and yaw rotation are continuously restrained once the vehicle is

released.

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Figure 18) Jordan Rollover System [27]

After both the near side and the far side of the roof have contacted the

roadbed, the vehicle is halted in vertical direction to avoid further contact with the

tracks along which the roadbed moves [26].

Table 3-4) Initial impact configuration of JRS [26]

Initial condition Value

Roll rate

Drop height

Roadbed speed ⁄

Roll angle

Pitch angle

Yaw angle

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29 Rollover Impact in Buses and Coaches

A dummy can be seated in the vehicle to investigate the occupant responses

and to evaluate the protection performance of the occupant restraint system during

rollover. Obviously executing such a test for heavy vehicles like buses or coaches

is very difficult, and expensive. This means that a versatile and repeatable rollover

test for buses is either impossible or very difficult and restricted to rich companies.

Computer simulation is a possible useful approach to fulfill the need of such

test for heavy vehicles. In 2008, Friedman and Hutchinson [27] performed a

computer simulation of JRS experiment on a vehicle to compare the results

between JRS, as a repeatable experiment, with CRIS (Controlled Rollover Impact

System). Figure 19 shows some sample results gained from computer simulation in

[27].

Figure 19) Jordan Rollover system initial impact configuration [27]

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CRIS is another dynamic test for simulating rollover impact which involves

a semi-tractor and trailer at the back of which is mounted a fixture holding the

vehicle. The fixture can be adjusted to cause the vehicle to drop from a given

height giving it a defined vertical velocity. However, there is no direct

measurement of the horizontal or vertical speed of the vehicle at impact. The

traveling speed of the semi-tractor/trailer controls the horizontal speed of the

vehicle at impact1 . This computer modeling and similar researches has shown the

capabilities of FEM analysis in simulating the actual JRS experiment. As a result it

seems very useful to model a JRS test for a bus structure and check the reliability

of structure in such impact.

5. Conclusion:

During a bus or coach rollover, the occupant will have a larger distance from

the center of rotation as compared to that of a car occupant. So it is necessary to

investigate more about this type of accident in buses to ensure that passengers are

safe enough. NHTSA has considered this issue and introduced some obligatory

standards that all bus manufacturers in North America should pass. One of the

most important and famous standards to mention is FMVSS 220. But as shown in

this research there are some weak points in this regulation.

Bus manufacturers frequently strengthen up the roof structure to pass the

FMVSS 220 testing procedures. The symmetric loading applied to the roof in the

FMVSS 220, actually examines primarily the strength of the roof bows, without

applying excessive loading on the rest of the structure. This approach need to be

1 The results of [27] showed that JRS is more conservative, more versatile and more repeatable than CRIS

and as a result in this review CRIS is not covered in details.

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31 Rollover Impact in Buses and Coaches

revised. As Bojanowski et al. [7] showed passing FMVSS 220 does not guaranty

the safety of residual space. So it would be vise to alter the focus from

strengthening the roof structure to considering more secure and strong connections

between the wall and the roof and the front cap.

In addition another important conclusion that could be made from this

review is the necessity of more focus on computer simulations. As shown on the

case of Paratransit buses, many manufacturers try to evade high costs of more

conservative test like ECE-R66 by some simpler test like FMVSS 220. As a result

they may produce vehicles that are not safe enough. In addition even if tests like

ECE-R66 motivate NHTSA to update its regulations, it will be a very expensive

test for many local manufacturers in North America. So by expanding the

computer simulations and trying to make them as similar as possible to actual case

engineers might find the weak points of the products without high costs of

producing expensive prototypes that might fail in very preliminary tests.

Another conclusion that could be made here is the importance of passenger

weight on accuracy of results. As was shown in section 3, by comparing two

studies ([9 and 22]) it was proved that considering passenger weight may put a

previously believed safe bus, in category of unsafe ones. The results gained [roll-

bus] in 2012 shows the necessity of more studies in future on FEM simulation of

bus rollover test with more accurate and detailed models that consider all neglected

influencing parameters like: more precise contact element selection, adaptive

meshing, full scale modeling, considering detailed geometry in order to simulate a

more accurate residual space and considering passenger weight.

It seems that industries have done comprehensive research on rollover, in

order to produce buses and coaches that comply with regulations, (as the example

presented from Volvo in part 3 [23]), but there are 2 reasons for continuing studies

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in this area. First, the current standards and regulation would be updated in a few

years, and they will be for sure stricter. Second, currently many parts of buses are

capable of intruding the residual space (like: bulkheads, partitions, rings or other

members reinforcing the structure of vehicle, bars, kitchenettes or toilets (fixed

appliances)), that have not been considered in current studies due to their

complication. So it would be appropriate to improve current progresses in safety

criterion and research on more complete computer models of buses and coaches.

The last conclusion to be made is about repeatable vehicle rollover dynamic

physical testing methods. Because of their heavy weight and large structures,

motor coaches and buses have not been tested in experiments like Jordan Rollover

System (JRS) or Controlled Rollover Impact System (CRIS). But since there are

many validated FE models for such test for light vehicles, it is possible to model a

JRS test for a bus in computer and check the reliability of its structure by checking

different parameters that this versatile test provides. Performing such test may lead

to major revisions in design of structures; because these test represents the actual

behavior of vehicle much more realistic than current standard, obligatory tests (as

FMVSS 220).

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33 Rollover Impact in Buses and Coaches

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