49 CFR § 571.221 - Standard No. 221; School bus body joint strength.
S2. Purpose. The purpose of this standard is to reduce deaths and injuries resulting from the structural collapse of school bus bodies during crashes.
S3. Application. This standard applies to school buses.
Body panel joint means the area of contact or close proximity between the edges of a body panel and another body component, including but not limited to floor panels, and body panels made of composite materials such as plastic or plywood, excluding trim and decorative parts which do not contribute to the strength of the bus body, members such as rub rails which are entirely outside of body panels, ventilation panels, components provided for functional purposes, and engine access covers.
Maintenance access panel means a body panel which must be moved or removed to provide access to one or more serviceable component(s).
Passenger compartment means space within the school bus interior that is between a vertical transverse plane located 762 mm in front of the forwardmost passenger seating reference point and including a vertical transverse plane tangent to the rear interior wall of the bus at the vehicle centerline.
Serviceable component means any part of the bus, of either a mechanical or electrical nature, which is explicitly identified by the bus chassis and/or body manufacturer in the owner's manual or factory service manual as requiring routine maintenance actions at intervals of one year or less. Tubing, wires and harnesses are considered to be serviceable components only at their attachments.
S5.1 Except as provided in S5.2, each body panel joint, including small, curved, and complex joints, when tested in accordance with the procedure of S6, shall hold the body panel to the member to which it is joined when subjected to a force of 60 percent of the tensile strength of the weakest joined body panel determined pursuant to S6.2.
S5.1.1 Body panels attached to each other shall have no unattached segment at the joint longer than 203 mm.
S5.2.1 The requirements of S5.1 do not apply to -
(c) Trim and decorative parts which do not contribute to the strength of the joint, support members such as rub rails which are entirely outside of body panels, doors and windows, ventilation panels, and engine access covers.
S6.1 Preparation of the test specimen.
S6.1.1 If a body panel joint is 203 mm or longer, cut a test specimen that consists of any 203 mm segment of the joint, together with a portion of the bus body whose dimensions are those specified in Figure 1, so that the specimen's centerline is perpendicular to the joint at the midpoint of the joint segment. Where the body panel joint is not fastened continuously, select the segment so that it does not bisect a spot weld or a discrete fastener. Support members which contribute to the strength of a body panel joint, such as rub rails on the outside of body panels or underlying structure attached to joint members, shall remain attached to the test specimen, except that material may be removed from the support members as necessary to clear the gripping areas of the joint members being tested.
S6.1.2 If a joint is less than 203 mm long, cut a test specimen with enough of the adjacent material to permit it to be held in the tension testing machine specified in S6.3.
S6.1.3 Prepare the test specimen in accordance with the preparation procedures specified in ASTM E8-89 (incorporated by reference, see § 571.5).
S6.2 Determination of minimum allowable strength. For purposes of determining the minimum allowable joint strength, determine the tensile strengths of the joined body components as follows:
(a) If the mechanical properties of a joint component material are specified in ASTM E8-89 (incorporated by reference, see § 571.5), the lowest value of that material's thickness and tensile strength per unit of area shown in that source shall be used.
(b) If the mechanical properties of a material are not specified in ASTM E8-89 (incorporated by reference, see § 571.5), determine its tensile strength by cutting a sheet specimen from outside the joint region of the bus body in accordance with Figure 1 of ASTM E8-89, and by testing it in accordance with S6.3.
(c) The cross sectional area of material removed to facilitate the installation of fasteners shall be subtracted from the cross-sectional area of the panel in the determination of the tensile strength of the weakest joined body panel.
S6.3 Strength Test.
S6.3.1 The joint specimen is gripped on opposite sides of the joint in a tension testing machine in accordance with ASTM E8-89 (incorporated by reference, see § 571.5).
S6.3.2 Adjust the testing machine grips so that the applied force on the joint is at 90 degrees plus or minus 3 degrees from the joint centerline, as shown in Figure 1.
S6.3.3 A tensile force is applied to the specimen by separating the heads of the testing machine at any uniform rate not less than 3 mm and not more than 10 mm per minute until the specimen separates.