Task leader: Gina E. Bertocci, PhD, PE
Co-investigators: Larry Schneider, PhD, Miriam Manary, MSE, Dong Ran Ha, MS (graduate student)
Other participants: James Swinehart (Metalcraft Industries, seating system manufacturer), Tom Novotny (AES, seating system manufacturer), National (ANSI /RESNA) and International (ISO) Standards Committees (including consumers, clinicians, researchers and manufacturers)
Duration/Staging of task: This 24 month research task will be conducted in months 1-24 of the 60 month RERC cycle
While performance of all wheelchair components is key to occupant crash protection, seat design and integrity are of particular concern since vehicle seat characteristics and failure have been linked directly to injury risk in motor vehicle crashes (Warner, 1991; Viano, 1992; Strother, 1987; Saczalski, 1993; NHTSA, 1997; Blaisdell, 1993; Adomeit, 1979; Aibe, 1982). Frontal impact sled tests (20g/48kph) of commercial wheelchairs have shown seating system failures to be relatively common (see figure below) (ANSI/RESNA, 1996). Seat attachment hardware, seat support surfaces and seat backs (on rebound) are among the most common components to fail under frontal impact conditions (Schneider, 2001). Such failures can greatly increase the risk of injury and particularly the risk of submarining.
Prior to developing test methods to evaluate the crashworthiness of wheelchair seating systems it is necessary to quantify seating loads during a crash. Previous studies which have attempted to elucidate wheelchair seat loading under crash conditions have consisted of both computer simulation studies and limited sled testing. Computer simulation studies have shown that frontal impact seat forces are dependent upon crash pulse, rear securement point location, seat characteristics and restraint configuration (Kang, 1998; Gu, 1996; Bertocci, 1996; Bertocci, 2000). A limited series of frontal impact sled tests conducted by Gu and Roy with disc-type load cells incorporated into the ISO surrogate wheelchair and using a Hybrid III, 50th percentile male test dummy measured seat loads (Gu, 1996). Shaw also estimated seat loading in frontal impact sled testing using pressure-sensitive film placed on the seat and load cells located beneath the front wheels of commercial manual wheelchairs with various types of seating systems (i.e. sling, rigid foam mounted on plywood) (ANSI/RESNA, 1996). Recent frontal impact testing conducted by Bertocci and Manary using the SAE surrogate wheelchair evaluated seat loads using disc-type load cells incorporated into the wheelchair seat and also evaluated the effects of rear securement point location (see figure below) (Bertocci, 2001b). This recent series of sled tests provided validation of seat loads measured through a previously conducted computer simulation study (Bertocci, 1996).
To provide the means for seating system testing independent of specific wheelchairs, the University of Pittsburgh has developed a static test method to evaluate seating system components (Ha, 2001b). Relying upon computer simulation studies and sled testing described above, test criteria for seat and back loading were established. Test loading criteria was based upon a wheelchair secured using 4-point tiedowns and a wheelchair-seated 50th percentile male test dummy subjected to a 20g/30mph frontal impact. Seat back loading criteria was based upon either the rebound phase of a 20g/30mph frontal impact or a 20g rear impact, depending upon which was greater. For the purposes of our proposed test protocol, a successful test, or "pass", required that the seating component under evaluation be capable of withstanding the test load. Failure was defined as component fracture or excessive deformation leading to an unstable support surface.
The design of the test setup was intended to isolate the performance of the tested seat component specimen from all other seating and wheelchair components. Accordingly, seat components (e.g., seat and back support surfaces, attachment hardware) were attached to a specially designed rigid test fixture that was mounted to a tension/compression loading device. The test fixture consisted of two solid rods spaced seating width apart simulating a wheelchair seat frame (see figure below). The test frame eliminated deformation that may be associated with the wheelchair frame so as to focus on seat component performance alone. A vertical downward load was then applied, transmitted to the test seat using the lower portion of the ISO 7176 anthropometric test device (ATD), which represents the buttocks and thigh of a 50th percentile male. When testing a seat back, the upper portion of the ISO 7176 ATD representing the torso was used to apply the load. Seat and back attachment hardware were tested using a rigid surrogate seat or back surface, to isolate attachment hardware performance.
To date, we have conducted nearly 80 static tests of 40 unique commercial wheelchair seating components using our developed test protocol. In general, many of the products failed to withstand test criteria, failing often at loads which are 50% or less than that expected in a frontal crash. Failure modes were, in many cases, similar to those seen in impact testing.
In one series of tests, the crashworthiness of five combination wheelchair back support surfaces and attachment hardware was evaluated using our static test procedure (Ha, 2000). Crashworthiness was tested by applying a simulated rearward load to each seat back system. None of the five tested wheelchair back supports withstood the simulated rearward-directed crash load of 2400 lb. All failures were associated with attachment hardware.
In another study by Bertocci et al., the crashworthiness of commercially available seat attachment hardware was evaluated using our low cost static test procedure intended to replicate seat loading conditions associated with a 20g/30mph frontal impact (Bertocci, 2000). Eleven unique sets of drop hook type hardware were tested and none of the hardware sets met the crashworthiness test criterion. All hardware failed at less than 50% of the load that seating hardware may be exposed to in frontal impact. The primary failure mode was excessive deformation, leading to an unstable surrogate seat support surface (see figure below). These results suggest that commercially available seating drop hooks may be unable to withstand.
Post-Test Drop Hook Test Specimen Failures (Bertocci, 2000) loading associated with a frontal crash and should not be recommended for use with transport wheelchairs. Bertocci et al. also evaluated the performance of various types of commercially available drop seats against the loading test criteria (Bertocci, 2001c). Five different types of drop seats (two specimens each) constructed of various materials (i.e., plastics, plywood, metal) were evaluated. Two types of drop seats (three of the total 10 specimens) met the 3750 lb frontal impact test criteria (see below). While additional validation of the test protocol is necessary, this study also suggests that some drop seat designs may not be capable of withstanding crash level loads.
Another study evaluating the crashworthiness of 2 specimens each of 3 unique sling seats and 3 unique sling backs using our static testing protocol was also conducted (Ha, 2001a). Two of six sling seats failed to pass the test and two of six sling backs failed to meet the test criterion. In general, sling-type support surfaces tended to have improved crash performance as compared to other types of seating surfaces.
In summary, our static testing protocol has provided a method for screening the crashworthiness of wheelchair seating components. Accordingly, this test method has been incorporated as an informative annex of the draft standard, ISO 16840-4 Seating Devices for Use in Motor Vehicles, to aid manufacturers in the design of transport-safe wheelchair seating. However, further validation of this test method to determine its dynamic similarity with sled impact testing is needed.
Recently the ISO and ANSI/RESNA wheelchair transportation standards committees have agreed that a dynamic test method to evaluate seating systems independent of a specific wheelchair frame is necessary. Such a test would promote safety across a broader range of available commercial seating systems. Accordingly, the University of Pittsburgh and University of Michigan have joined efforts towards developing a reusable surrogate wheelchair base that could be used to dynamically test seating systems. Commercial seating systems would be mounted to the surrogate wheelchair base (shown below) for sled impact testing. This reusable base has been designed to represent an average power wheelchair in terms of inertial characteristics (Bertocci, 1997). To-date, two pilot tests have been conducted verifying the strength and reusability of the surrogate base. Additional efforts are needed to verify that the surrogate base provides a crash response similar to that of commercial wheelchairs.
Objective 1: Further development and refinement of surrogate wheelchair base (SWCB). The surrogate wheelchair base described here is a first prototype. Additional confirmation and validation efforts are necessary.
Objective 2: Validation of surrogate wheelchair base dynamic crash response. As a part of this objective we will verify that the crash response of the SWCB is similar to that of commercial wheelchairs.
Objective 3: Comparison of static and dynamic test method results. Since dynamic testing can be more costly than static testing for seating manufacturers it is useful to determine if there is similarity between static and dynamic testing results. This objective will be accomplish by conducting frontal impact sled testing using the SWCB with seating systems that have been previously static tested by the University of Pittsburgh. We will conduct two sled tests each of the following seating components that have been previously static tested: i) drop hooks with rigid surrogate seat and back surfaces, ii) rigid plywood with molded foam seat and back, iii) contoured seat and back with manufacture provided attachment hardware, and iv) sling seat and back. In the case of failure, we will compare failure modes resulting from static and dynamic test methods.
Objective 4: Transfer of Recommendations and Findings to Standards Development Efforts
We expect that this task will result in test methods that can be used to evaluate the crash-worthiness of seating systems independent of a specific wheelchair frame. Such a method will benefit manufacturers of seating systems, eliminating the need to test all possible combinations of seating systems and wheelchair frames. The results of this task will likely provide rehabilitation technology suppliers and consumers a wider choice of transport-safe seating products. Since national and international standards groups have identified “after-market wheelchair seating systems” as a priority, the findings of this task will directly support the efforts of both the ISO and ANSI/RESNA Standards Committees. Transfer and implementation through standards provides an effective pathway for the results of this task to become integrated into industry practice and to be disseminated to manufacturers and suppliers. Additionally, one graduate student, Dong Ran Ha, MS, will be trained as a part of this task.
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Aibe, T., Watanabe, K., Okamoto, T. and Nakamori, T., Influence of Occupant Seating Posture and Size on Head and Chest Injuries in Frontal Collision. SAE Paper No. 826032, 1982.
American National Standards Institute (ANSI)/Rehabilitation Engineering Society of North America (RESNA), Seating Insert Eval Sled Tests, Greg Shaw - University of Virginia Auto Safety Lab, 1996.
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Bertocci, G.E., Digges, K. and Hobson, D., Development of Transportable Wheelchair Design Criteria using Computer Crash Simulation. IEEE Transactions of Rehabilitation Engineering, 1996. 4(3): p. 171-181.
Bertocci, G., Karg, P., Hobson, D., Wheeled Mobility Device Database for Transportation Safety Research and Standards. Assistive Technology, 1997. 9.2: p.102-115.
Bertocci, G., Szobota, S., Ha, D. and Roosmalen, L., Development of Frontal Impact Crashworthy Wheelchair Seating Design Criteria Using Computer Simulation. Journal of Rehab Research and Development, 2000. 37(5): p. 565-572.
Bertocci, G., Ha, D., Deemer, E. and Karg, P., Evaluation of Wheelchair Seating System Crashworthiness: Drop Hook Type Seat Attachment Hardware. Archives of Physical Medicine and Rehabilitation, 2001a. 82(April): p.534-540.
Bertocci, G., Manary, M. and Ha, D., Wheelchairs as Seats in Motor Vehicles: Frontal Impact Seat Loading. submitted to Medical Engr & Physics, 2001b.
Bertocci, G., Ha, D., van Roosmalen, L., Karg, P. and Deemer, E., Evaluation of Wheelchair Drop Seat Crashworthiness. To appear in Medical Engineering & Physics, 2001c.
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Ha, D., Bertocci, G., Karg, P., Evaluation of Wheelchair Sling Seat and Sling Back Crashworthiness. Submitted to the Journal of Rehabilitation Research and Development, 2001a.
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To-date no problems have been encountered that have greatly impeded the work progress or altered the originally proposed schedule. However, a change to the original plans regarding the SWCB has occurred. Original plans were to modify the 1st prototype to accommodate varied seat widths. Upon gathering input from the ISO standards group, it was decided that a 2nd prototype having a structure different from the 1st prototype was necessary. This 2nd prototype was developed at UMTRI with only slight impact to the originally proposed project schedule. This change has resulted in a 2-3 month delay which will not impact the final completion date of the project.
An investigation of wheelchair crash response and seat failure modes associated with previously conducted frontal impact sled tests was conducted and was used in the development of the SWCB prototype. The following trends were identified in a sample that represents most ANSI WC19 wheelchairs: 1) Seating failures are relatively rare and usually result from attachment hardware failures. 2) Much of the attachment hardware that fails is not positively locked to the frame, suggesting that simple design changes could improve seat system crashworthiness. 3) Manual WCs experience little rotation in a crash but account for the majority of WC seat pan failures, suggesting that shear loading is the worst-case scenario.
Progress toward planned outputs
Key findings to-date
Future plans through end of grant cycle
This project has achieved the originally proposed objectives described above. Over the past 4 years, SP4a investigators have submitted 3 conference abstracts and 2 peer-reviewed journal papers describing the findings of this project. These publications are expected to aid manufacturers in the design and development of crashworthy seating systems. Moreover, national and international industry standards have been developed as a part of this project that will allow manufacturers to evaluate the performance of their seating systems under frontal impact conditions. These standards define test methods that will now allow seating systems to be evaluated independent of a specific wheelchair frame. (Previously existing standards required that a complete wheelchair – frame and seating system – be tested together as one unit. This testing scheme did not address service delivery needs of providing a seating system from one manufacturer and wheelchair frame from another manufacturer.) Ultimately this will lead to an increase in availability of crash-safe seating systems, improving safety for those using their wheelchair as a motor vehicle seat.
Last updated: August 30, 2006
Past Research > Archived Tasks: November 2000 to October 2005 > SP-4: Investigate and Compare Methods Testing the Crashworthiness of Wheelchair Seating Systems and Peripheral Devices >