Work Item
ASTM WK70255

Revision of F2291-19e1 Standard Practice for Design of Amusement Rides and Devices

Keywords

None Provided

Rationale

Section 7.1.4.7 of F2291 allows for the use of expanded Gx (eyes front) limits in the case of a Prone restraint. Expanded limits were introduced in part because in the case of a ride in which the patron is essentially lying face down, the patron will be engaging the neck musculature in opposition to gravity (a Gx acceleration of 1 G magnitude in this orientation). Such a patron would thus be pre-conditioned to tolerate higher Gx acceleration than a patron who is essentially seated upright (approximately 0 G Gx acceleration). The current wording in Section 7.1.4.7(3) and the Gx note in Fig. 6 provide the following requirements related to the Prone Restraint Gx expanded limits zone: Specifications for entry into the Prone Restraint Gx expanded limits zone: oFrom Note 2: Over-The-Shoulder limits may be increased to Prone limits providing The onset is less than 15 g/s and The restraint is appropriately padded. The definition of a prone restraint: o7.1.4.7(3): A prone restraint is one in which the patron is oriented face down at a point or points during the ride cycle o7.1.4.7(3): A prone restraint is a restraint designed to allow the patron to accept higher acceleration in the Gx (eyes front) as compared to the base case and over-the-shoulder restraints oNote 3: Prone Restraint assumes body is supported by appropriately padded restraint In light of the reasoning behind the original Prone restraint classification, several issues are identified with the current language in 7.1.4.7 and the Note: 1.The current language uses only the term Prone, which by definition is lying flat. The prone orientation of a patron in fact exposes the patron to a Gx acceleration field (gravity) that pre-conditions the patrons neck musculature. However, any Gx acceleration of sufficient magnitude and duration would result in the patron engaging the neck musculature to keep the head from flexing forward, essentially counteracting the Gx acceleration field and preparing the neck musculature for additional Gx acceleration (pre-conditioning). The term Prone implies that the long axis of the torso must be perpendicular to gravity (i.e. 90 degrees from gravity) in order to use the expanded Gx limits. Therefore, by use of this term to describe the requirement, use of the expanded limits would be precluded if the patron was oriented at say 85 degrees - even though biodynamically there would be little difference between being oriented at 85 degrees or 90 degrees (i.e. it is likely that the patron would have pre-conditioned neck muscles if oriented at an angle of 85 degrees). It is thus proposed that the Prone orientation descriptor be changed to Pre-Conditioned. The prone orientation is only one case that satisfies pre-conditioning and the use of the increased tolerance, however, there are others. 2.The current language states that the expanded limits can be used if the patron is oriented face down at a point or points during the ride cycle. The standard thus currently allows for use of the expanded Gx limits for the entirety of any ride in which the patron was just momentarily oriented face-down for one brief point in time, which may be inappropriate. Correspondingly, there may be points in the ride where the patrons neck musculature is no longer required to prevent the head from flexing forward. In these cases, the pre-conditioned state of the neck will be lost. The G-force task group proposes based on biodynamic experience that once the sustained -Gx acceleration leaves the Prone Restraint area, the patron is no longer considered pre-conditioned. Additional entries into the Prone Restraint expanded limits area during the ride cycle must again satisfy the 15 g/s maximum onset requirement. 3.The current language states that the expanded limits can be used providing the onset is less than 15 G/s and the restraint is appropriately padded. (the Gx note 2 in Fig. 6) Onset is defined in section 7.1.4.8 at a specific time and the current guidance does not specify the time range over which this requirement holds. In addition, the onset is defined as a best fit linear least squares approximation. With this definition, the onset on entry into the prone expanded limit area would be negative and on exit would be positive. The current note is not clear given that the onset of the Gx acceleration can be both positive and negative. The G-force task group proposes to clarify the current requirements by requiring that the magnitude of the Gx acceleration onset be less than or equal to 15 G/s from the time of entry into the Gx area until leaving the prone extended limit area.

Details

Developed by Subcommittee: F24.24

Committee: F24

Staff Manager: Katerina Koperna

Work Item Status

Date Initiated: 10-18-2019

Technical Contact: Robert Cargill

Item: 003

Ballot: F24 (19-09)

Status: Will Reballot Item