Roof Strength Research Articles

Design optimisation of vehicle roof structures: benefits of using multiple surrogates

Rollover accidents have become more and more of an important issue for vehicle safety nowadays. On the basis of the final rule of Federal Motor Vehicle Safety Standard No. 216 (FMVSS 216), design optimisation of vehicle upper structures is performed to improve roof strength. Surrogate-based design optimisation is utilised to facilitate design space exploration and optimum search rapidly, in which the response of roof crush resistance force is approximated by surrogate models. Surrogates like kriging, radial basis function, support vector regression, etc. are used. Weighted average surrogates, including prediction -sum-of-squares-based (PRESS-based) weighted surrogate (PWS) and optimal weighted surrogate (OWS), constructed by above-mentioned individual surrogates, are also taken into consideration. The benefits of using the weighted average surrogate for function approximation and optimisation are investigated. The results demonstrate that OWS outperforms others in the context of prediction accuracy, and it also achieves the best improvement on crush resistance force by 41.7%, accompanied by 5.3% weight reduction compared with the original one. It seems an effective way to form an ensemble of surrogates to avoid a misleading optimum. The results also indicate that different surrogates would prefer different regions of design space. This indicates that simultaneous use of multiple surrogates could eliminate the surrogate dependency optimum, showing potential for surrogate-based design optimisation of non-linear problems.

Optimized Design Solutions for Roof Strength Using Advanced High Strength Steels

Optimized Design Solutions for Roof Strength Using Advanced High Strength Steels

Roof Strength and Injury Risk in Rollover Crashes of Passenger Cars

Objective: A 2009 study by the Insurance Institute for Highway Safety found that midsize SUVs with stronger roofs, as measured in quasi-static tests, had lower risk of ejection and lower risk of injury for nonejected drivers. The objective of the present study was to determine whether a similar association exists for other vehicle groups. Methods: Twelve small passenger cars were evaluated according to Federal Motor Vehicle Safety Standard 216 test conditions. Crash databases in 14 states provided more than 20,000 single-vehicle rollover crashes involving these vehicles. Logistic regression analyses were used to evaluate the effect of roof strength on the rate of driver injury while assessing and controlling for the effects of driver age, vehicle stability, state, and other factors where necessary. Results: Small cars with stronger roofs had lower overall rates of serious injury, lower rates of ejection, and lower rates of injury for nonejected drivers. Although the effect on ejection was somewhat smaller for cars than for SUVs, the overall pattern of injury results was consistent. For roof strength-to-weight ratio measured within 5 in. (SWR 5 ), a one-unit increase (e.g., from 2.0 to 3.0) was associated with a 22 percent reduction in risk of incapacitating or fatal driver injury in single-vehicle rollovers. This compares with a 24 percent reduction estimated for a similar change in roof strength among midsize SUVs. Conclusions: The association between vehicle roof strength and occupant injury risk in rollover crashes appears robust across different vehicle groups and across roof SWR5 values, varying from just more than 1.5 to just less than 4.0. If roofs were to increase in strength by one SWR5, a 20–25 percent reduction in risk of serious injury in rollovers would be expected. Still, even if all vehicle roofs were as strong as the strongest roof measured, many rollover injuries still would occur, indicating the need for additional research and countermeasures.

Roof Strength and Injury Risk in Rollover Crashes

Objective: Previous studies have reported that roof strength as measured in FMVSS 216 is unrelated to occupant injury risk in actual rollovers. However, those studies did not control for important confounding factors. The present study reevaluated roof strength and injury risk using a single class of vehicles with similar driver and vehicle usage characteristics in the expectation that some potential confounders will be controlled by this focused analysis. Alternative roof strength measures also were examined in case strength-to-weight ratio (SWR) at 5 inches, the measure in FMVSS 216, is not the best measure. Methods: Eleven midsize SUV roof designs were crushed according to FMVSS 216 protocol. Force was measured continuously, and the maximum force and the amount of energy used to achieve crush of 2, 5, and 10 inches were recorded; these six measures of roof strength were evaluated both directly and after normalization for vehicle weight. After controlling for other potentially confounding variables, logistic regression models estimated the effect of each of the twelve roof strength measures on risk of fatal or incapacitating injuries to drivers in single-vehicle rollover crashes. Crash data were obtained from fourteen states. Results: All roof strength measures correlated inversely, and similarly, with risk of fatal/incapacitating injury in single-vehicle rollovers. Across all states, 12 percent of drivers sustained fatal/incapacitating injuries, but individual state rates varied from 6 to 24 percent. Among available variables, only state substantially affected the estimated effect of roof strength; it was kept in the final models with roof strength, driver age, and static stability factor (SSF). Driver age was retained because it had a significant effect on injury outcome and SSF was retained for theoretical reasons. This model estimated that a one-unit increase in strength-to-weight ratio (SWR) within 5 inches of crush was associated with 24 percent lower risk of fatal/incapacitating injury (95% CI: 15–33). Ejection was lower by 41 percent (95% CI: 30–51), and injury risk among nonejected occupants was lower by 16 percent (95% CI: 3–29). Conclusions: The FMVSS 216 measure of roof strength related strongly to occupant protection in real-world rollover crashes in this study, which restricted analysis to vehicles with reasonably homogeneous driver and usage characteristics and accounted for state variation in injury rates. Further research should determine whether these results generalize to other vehicle types with different drivers and rollover risk. Research should also focus on factors other than roof strength, because even large increases in roof strength would leave many rollover crash deaths unaffected.

Analysis on stability of pillar and stiff roof system in the gob area

Based on the open stope method, the stability of the gob area was decided by pillars and stiff roof. Therefore, it was dispensable to leave pillars with long-term strength and enough size to support the stiff roof during mining activities. Based on the mining conditions of Baixiang wollastonite mine in Changxing County of Zhejiang, while considering pillars with different shape, irregular size, and distribution, the load imposed on the pillars was analyzed, and the safety coefficient was calculated in order to determine their support status. The strength of stiff roof was calculated by means of analytical solution—theory of rectangle thin plate rested on elastic foundation. The system stability of pillar and stiff roof was analyzed according to the proportion of the total cross section area of pillars to the stiff roof area above the mined area.

Vehicle and Occupant Responses in a Friction Trip Rollover Test

Objective: A friction rollover test was conducted as part of a rollover sensing project. This study evaluates vehicle and occupant responses in the test. Methods: A flat dolly carried a Saab 9-3 sedan laterally, passenger-side leading to a release point at 42 km/h (26 mph) onto a high-friction surface. The vehicle was equipped with roll, pitch and yaw gyros near the center of gravity. Accelerometers were placed at the vehicle center tunnel, A-pillar near the roof, B-pillar near the sill, suspension sub-frame and wheels. Five off-board and two on-board cameras recorded kinematics. Hybrid III dummies were instrumented for head and chest acceleration and upper neck force and moment. Belt loads were measured. Results: The vehicle release caused the tires and then wheel rims to skid on the high-friction surface. The trip involved roll angular velocities >300 deg/s at 0.5 s and a far-side impact on the driver’s side roof at 0.94 s. The driver was inverted in the far-side, ground impact. Head impact and torso deceleration resulted in 8.75 kN neck compression load with about ½” of roof deformation, 5 g vehicle deceleration. The vehicle came to rest on its roof after 1½ rolls. Conclusion: The friction rollover involved centrifugal accelerations moving the driver’s head to the roof at ground impact. The driver was inverted and the torso dove toward the ground causing high neck compression loads. This test confirms the conclusions of Malibu II and CRIS testing that belted occupants can experience neck loads before much roof deformation. This test involved a modern vehicle with a strong roof. Increasing roof strength would not reduce neck loads; reclining the head-neck-torso angle before the far-side roof impact seems a more reasonable approach to improving safety.

Roof-to-ground contact force during a vehicle rollover

Current US Federal Motor Vehicle Safety Standards require that the roof strength of a vehicle must be such that it is able to withstand a quasi-static force equivalent to times the unloaded of the vehicle, with a maximum roof deformation of 5″ (127 mm). In the public forum, it is often argued that the 1.5 times the unloaded weight rule may not the most effective one for ensuring the safety of vehicle occupants. Therefore, the National Highway Safety Administration has proposed to increase the roof strength requirement to 2.5 times the unloaded of the vehicle for the available roof crush. Currently, no mechanics-based model is available in the literature dealing with the roof-to-ground contact forces when a vehicle is involved in rollover. Therefore, the current study was undertaken to develop two mechanics-based simple models, which can be used for estimating the required roof strength from the calculations of roof-to-ground contact force.

Numerical analysis on water-inrush process due to floor heave

To investigate the water-inrush process due to floor heave, simulation model was made and factors of mining height, immediate roof strength, water pressure, depth and strength of confiningbed, excavation step distance and bury depth were analyzed by using UDEC software. Five zones theory (containing two maximum main stress circles and three maximum main stress stratiform zones) along excavation direction was proposed, and two-ends-fixed beam mechanical model in maximum main stress circle was established. It is found that water inrush always occurs at the end of the beam where shear stress is higher, that is to say, it always occurs at near the starting cut and working face. Water pressure, height and strength of confiningbed are the determined factors, and mining height, immediate roof strength, excavation step distance and bury depth effect water inrush process due to floor heave.

Forecasting of destroyed height of overlying rock with the top coal caving based on ANN

Analyzed the rule of the Water Flowing Fractured (WFF) zone’s development during the fully mechanized top coal caving. Six influence factors of WFF’s height were selected, viz. mining thickness, base rock thickness, dip angle, uniaxial compressing strength of roof, mudstone proportion in overlying rock, and structure of overlying rock. The height-forecasting model of WFF was established based on the Artificial Neural Net-work techniques, and was applied in the first fully mechanized top coal caving face under sea in China.

Rollover Dynamics: An Exploration of the Fundamentals

This paper examines the commonly used constant deceleration method of rollover reconstruction in light of detailed analyses of actual rollover tests. By comparing methods used in the field to detailed experimental observations this paper will provide an understanding of the accuracy and limitations of those methods. The underlying data that provide the foundation for this article are derived from two high-speed, soft-surface dolly rollover tests of sport utility vehicles. Research focusing on automotive rollovers has garnered a great deal of attention in recent years. Substantial effort has been directed toward the evaluation of rollover resistance. Issues related to crashworthiness, such as roof strength and restraint performance, have also received a great deal of attention. Much less research effort has been directed toward a more detailed study of the rollover dynamics from point-of-trip to point-of-rest. The reconstruction of rollover crashes often requires a thorough examination of the events taking place between point-of-trip and point-of-rest. Increasing demands are placed on reconstructionists to provide greater levels of detail regarding the roll sequence. Examples include, but are not limited to, roll rates at the quarter-roll level, CG trajectory (horizontal and vertical), roll angle at impact, and ground contact velocity. Often the detail that can be provided in a rollover reconstruction is limited by a lack of physical evidence. However, there are many cases where the physical evidence - both on the vehicle and the ground - allows for detailed re-positioning of the vehicle during the roll sequence. Using such a high level of detail would lead one to believe that the reconstruction of the rollover dynamics will be an accurate representation of the actual event. But, just how accurate will it be?

Consideration of vehicle handling and stability with improved roof strength

The large number of serious injuries and fatalities occurring in the United States in rollover conditions indicates that substantial changes in the rollover protection system design approaches are needed. Effects of strengthening baseline roof structures on vehicle handling and stability are examined through use of finite element modeling, data studies, handling simulations, and handling tests. The results show that strengthening the roof does not adversely affect the vehicle handling and stability.

Reinforcement of vehicle roof structure system against rollover occupant injuries

With the increasing popularity of light trucks, sport utility vehicles and minivans in recent years, occupants' protection under rollover accidents require some urgent reassessment so as to further reduce the risk of severe and fatal injuries. European studies under the rollover project have shown that there is a relationship between head/neck injuries and the roof intrusion including its impulse. Typical roof damage accidents in Europe were investigated. Latest modification methodologies for the reinforcements to roof structures believed to significantly increase the crush resistance of the roof are suggested and discussed. The work also gives a comprehensive numerical analysis using LS-DYNA3D and experimental testing results of roofs under the crushing regulation FMVSS 216 as well as inverted drop tests. Simple design changes to the vehicle roof structure can produce a significant improvement in roof strength and energy absorption capability, although these changes will be accomplished with a weight penalty of 1.0% to 2.3% of test vehicle weight.

Rollover far side roof strength test and simulation

Rollovers are severe road accidents. The main injured parts of occupants' body in rollover are the head and neck that make contact with roof interior during this accident scenario. The more the roof structure intrudes during the rollover process, the higher is the chance of impact contact. Real-world rollover accidents roof damages and PC-CRASH rollover reconstruction of vehicle dynamic analysis show far side structure to experience more severe load in the rollover. To simulate this experimentally, roof crush test rig was designed and built on the basis of the rollover ground–roof interaction analysis. Full-size roof crush test was carried out and later numerically modelled using LS-DYNA 3D. Numerical simulation results show good agreement with test results. The methodology of testing, simulation and evaluation of vehicle structure provided a useful tool for safety car design aspects. The validated model provides a reliable platform for future structure optimisation.

Observations from repeatable dynamic rollover tests

In an attempt to understand the relationship between quasi-static and dynamic test results, repeatable, dynamic rollover tests were conducted on production vehicles to determine intrusion and intrusion velocities using the Jordan Rollover System. These tests included complete production vehicles and body bucks at reduced weight, to vary the roof strength-to-weight ratio. Data from these tests are compared with the results of quasi-static roof strength tests measured at greater roll and pitch angles than are used in FMVSS 216. Biomechanical data indicate that serious head, face, neck, or thoracic spine injuries are a consequence of rapid impacts with significant amplitude. The test data suggest a correlation between quasi-static roof strength and dynamic roof intrusion velocity. Localized failures (buckling and collapse of structural elements that often translate into the roof panel) are a more critical aspect of roof performance than its strength as measured in FMVSS 216.

Implementation of composite roof structures in transit buses to increase rollover roof strength and reduce the likelihood of rollover

A production transit bus with roof pillars of small section size and low-gauge steel, exhibits extensive lateral roof deformation (matchboxing) in rollover impacts. This matchboxing allows the rollover to continue well beyond a quarter roll. Finite element simulation demonstrates that a bus roof design incorporating inexpensive E-glass fiber epoxy composite roof pillars can reduce matchboxing and arrest the rollover of the bus at equivalent or reduced gross vehicle weight.

Assessment of vehicle roof crush test protocols using FE models: inverted drop tests versus updated FMVSS No. 216

Inverted drop test is a dynamic roof crush test which is simple, realistic, reliable and easily implemented. It can produce repeatable roof impacts that can be used to compare one test run with another and has long been used by the automotive industry and researchers to test roof integrity. However, practical full-scale vehicle roof crush tests are always very costly, especially for finding out the impact orientation for the worst structural behaviour. Computer simulation can provide an effective solution, since it is highly repeatable, and under different environmental conditions permits the variation of parameters to evaluate sensitivity. A full-scale Finite Element model of Ford Fiesta that was originally modelled in parts by Ford Motor Company using Radioss code was translated into LS-DYNA3D and refined through extensive employment of better elements and spotwelds formulation. The inverted drop-test simulation, therefore, was made using a combination of different pitch and roll angles in the search of the worst-case rollover condition. These parametric studies show the critical vehicle orientation at roof impact that causes the highest amount of roof crush to be 10° pitch angle and 12.5° roll angle. This is similar to the test conditions employed in the experimental test using a Renault Clio. The results obtained from simulating these two different vehicles show the existence of some design problems, i.e., a very weak roof that is supported by weak roof rails and weak reinforcing plates. Furthermore, the inverted drop-test simulation results depict clearly the differences between static roof crush tests based on FMVSS No. 216 that is loaded along a fixed orientation and only reflects the roof strength, rather than the pillar's strengths, response in that direction. The dynamic test in that sense is more realistic and closer in nature to a number of recorded real-world accidents.

Numerical analysis of a small European vehicle under rollover condition

Quasi-static and dynamic roof crush simulations have been carried out successfully on a small European car that was loaded at different pitch/roll angles. The results obtained were validated against the MIRA experimental dynamic roof crush outputs and against the Bolton experiments performed under FMVSS 216. Overall, the outcomes show that bonded windscreens contribute to nearly 30 per cent of the roof strength, thus confirming similar results obtained by a number of other researchers. Furthermore, these results show that roof strength is a function of roll and pitch angles which also greatly influence the overall intrusion rate. The new findings illustrate that, having a fixed loading pitch angle, the roof strength decreases when the roll angles are between 15 and 45°. If the pitch angle is increased, the same phenomenon of reduced strength is observed. However, this does not signify that the roof becomes weaker with increasing pitch angle, because there are limits within which this can occur, and hence the worst-case loading that yields the smallest strength is found at pitch 10° and roll 45. Therefore, it is recommended in this paper that an update be made to the FMVSS 216, where a roll angle of 45° (not 25°) and a pitch angle of 10° (not 5°) be used as they constitute the worst-case loading condition. As a result, a more realistic test configuration with angles that replicate real-world accident data can be represented.

Buckling Load of Elliptic Paraboloidal Single Layer Reticulated Roofs with Simple Supports under Uniform Load

The present paper discusses buckling strength of elliptic paraboloidal reticular roofs under uniform load with a rectangular plan. The roofs are assumed as three way single layers with a simple support at all peripheries of a rectangular plan. The buckling analyses are performed for linear buckling, elastic buckling, and elastic-plastic buckling. The results of buckling loads are formulated based on two procedures for practical design use. One is an explicit formula for elastic buckling loads using a knock down factor and the other one is an implicit expression for buckling loads interpreted as a column buckling strength in terms of generalized slenderness ratio.

Buckling Load of Elliptic Paraboloidal Single Layer Reticulated Roofs under Uniform Load

The present paper discusses buckling strength of elliptic paraboloidal reticular roofs under a uniform load on a rectangular plan. The roofs are assumed as three way single layers with a pin-support at all peripheries on a rectangular plan. The buckling analyses are performed for linear buckling, elastic buckling and elastic-plastic buckling. The results of buckling loads are formulated based on two procedures for practical design use. One is an explicit but approximate formula for elastic buckling loads using a knock down factor and the other one is an implicit expression for buckling loads interpreted into a column buckling strength in terms of generalized slenderness ratio.

Static and dynamic roof crush simulation using LS-DYNA3D

Quasi-static roof crush simulations based on FMVSS 216 have been carried out successfully on a small European car, and different loading conditions have been tested. Results show that bonded windscreens contribute at least 30% of roof strength based on FMVSS 216. This confirms experimental test results carried out by other researchers. Furthermore, results show that the roll and pitch angles are a function of roof strength and greatly influence the overall behaviour. For instance, with the same pitch angle, the roof strength greatly drops as the roll angle goes up from 15° to 45°; but the magnitude of roof strength drop is minimal as the pitch angle increases. While for the same roll angle, the roof strength is seen to decrease as the pitch angle increases from 0° to 10°; but it does not mean that the bigger the pitch angle, the weaker the roof strength becomes. In fact in the case of a 10° pitch angle, the roof strength exhibits worst results. The recommendations on the updated FMVSS 216 are a roll angle of 45° (not 25°) and a pitch angle of 10° (not 5°). These constitute more realistic test angles that replicate real world accident data. In addition to quasi-static virtual testing, some dynamic simulations are also carried out based on two dynamic test methodologies, namely the inverted car drop in a cylindrical ditch and on a flat ground. Results show severe roof damage to occur in the former test scenario despite both having same initial conditions. The paper has also defined corridors for the best design of a car roof.