Interpretation ID: nht78-1.7

DATE: 12/05/78

FROM: AUTHOR UNAVAILABLE; J. J. Levin, Jr.; NHTSA

TO: Blue Bird Body Company

TITLE: FMVSS INTERPRETATION

TEXT:

December 5, 1978 NOA-30

Mr. W. G. Milby Manager, Engineering Services Blue Bird Body Company P.O. BOX 937 Fort Valley, Georgia

Dear Mr. Milby:

This responds to your September 20, 1978, letter asking whether a particular bus body joint is subject to the requirements of Standard No. 221, School Bus Body Joint Strength. The joint in question is the connection of two body panels under which runs a continuous body member for the entire length of the bus body.

Standard No. 221 establishes strength requirements for body panel joints which are defined as "the area of contact or close proximity between the edges of a body panel and another body component..." Body panel is further defined to mean "a body component used on the exterior or interior surface to enclose the bus' occupant space." The exterior body panels to which you refer are involved in the enclosure of the bus' occupant space, and accordingly, their connection is a joint falling within the requirements of the standard. The fact that an underlying body member runs under the panels perpendicular to the joint in no way excepts the joint from the requirements of the standard.

Your analogy of these panels to rub rails whose joints are not tested according to the requirements of the standard is inappropriate. Rub rails are added on to the exterior of a bus over the body panels. All parts of the rub rails fall outside the exterior skin of a bus, and therefore, they serve no purpose in enclosing occupant space. The panels to which you refer, on the other hand, are the primary, sidewall components enclosing bus' occupant space.

You ask how the agency will test this joint since it has a body structure member that runs perpendicular to it. You suggest that the agency cut an appropriate size specimen of the panels' joint, and underlying body member and pull one panel and the body member against the other panel and the body member. The agency disagrees. This procedure would not test the strength of the joint, since the stresses imposed by the test would be carried by the continuous body member being pulled against itself.

The agency tests such joints by cutting a specimen of the panels that includes a portion of the underlying body member. The ends of the body member are then removed to allow the testing device to clamp the two body panels that are to be tested. However, rivets or other bonding materials that connect the panels and the body member at the joint remain intact. This is what is intended by the standard's requirement that the underlying body structure be included within the joint strength test. Leaving the underlying structure intact at the joint permits a test of the joint's strength that closely approximates the actual strength of the joint as it is installed in a completed bus.

Responding finally to your last comment that the agency by its testing technique is hindering the development of integrally constructed bodies, the NHTSA disagrees. The agency believes that the strength of the entire bus body is dependent upon the strength of its parts. Each joint must be examined independently to ensure that it is strong enough to withstand accident forces. Since those forces vary with the nature of any impact and can result in severe stress on one small section of a bus, it is appropriate to measure the strength of individual joints. However, the agency's testing technique as outlined above considers the effect of the underlying bus structure thus encouraging the development of integrally constructed bodies.

Sincerely,

Joseph J. Levin, Jr. Chief Counsel

September 20, 1978

Mr. Joseph J. Levin, Jr. Chief Counsel National Highway Traffic Safety Administration Washington, D.C. 20590

Reference: 1) Frank Berndt to W. G. Milby, April 26, 1976 N40-30

Dear Mr. Levin:

The purpose of this letter is to seek an interpretation on two questions:

1. Whether a particular area on our bus body is a joint subject to FMVSS 221 and,

2. What constitutes a joint specimen for testing to determine compliance with FMVSS 221.

These questions arose recently during tests at Mobility Systems and Equipment Company under contract to NHTSA No. DOT-HS-7-01670. These issues must be resolved for future compliance testing both by Blue Bird Body Company and NHTSA. The questions are dealt with independently below.

1. The area in question is circled on the enclosed photograph which shows the basic location on the body. The most basic issue is that this should not even be considered a joint. As shown in the attached sketch 41, the alledged joint is only skin over a continous, one piece, 16 gauge structural member which runs the full length of the body parallel to the direction of force application. Even if the skin panels were removed, the body would be fully enclosed beneath them by the solid one piece structural member. The skin in this case in analogous to the rub rails in reference 1 wherein it states that "...the rub rails... are not themselves considered to have a function in enclosing the occupant space and are therefore not considered body components for the purposes of the requirements." We therefore ask for confirmation that the area shown in the enclosed photograph, and represented by sketch #1 is not a joint.

2. Irrespective of this basic issue, for this particular joint however, there are other questions which must be resolved that apply to all joints. Therefore, even though we do not agree that the particular area in question is subject to the requirements of FMVSS 221, the remainder of this letter is written as though it were a joint so that the other issues can be resolved.

The question is what should be gripped and pulled, in order to test a joint? In other words, what constitutes a joint specimen? Our interpretation is that, as defined in the enclosed simplified sketch #2, parts 1 and 3 should be gripped on one end of the specimen and parts 2 and 3 should be gripped on the other end of the specimen.

We have arrived at this conclusion through the following analysis of FMVSS 221:

S 6.3.1 says "Grip the joint specimen on opposite sides of the joint..." Although "joint specimen" is not explicitly defined in S 4, Definitions, it is implicitly defined in S 6.1, Preparation of the test specimen. This says "...cut a test specimen that consists of any randomly selected 8-inch segment of the joint, together with a portion of the bus body whose dimensions to the extent permitted by the size of the joined parts, are those specified in Figure 1, ..."

The underlined portion above tells us we must, in cutting the specimen from the body, cut parts 1, 2 and 3 as defined in the enclosed sketch. Indeed, because of the integral nature of the way parts 1 and 2 are assembled to part 3, it would be impossible not to include part 3 in cutting the specimen from the body. Therefore, the assembly of parts 1, 2 and 3 constitute the joint specimen referred to in S 6.3.1 for the particular joint under consideration.

An explanation of how parts 1, 2 and 3 are assembled together is in order at this point. Parts 1 and 2 are fastened to part 3 not only at the "alleged body panel joint" but continuously along the length of the body by rivet row A, making a continuously integral assembly of part 3 to parts 1 and 2. Therefore, the strength of the body panel joint in question is meaningless without considering the strength of part 3. Part 3 is a continuous structural member running the full length of the body. Therefore, "failure" of the alleged body panel joint in question cannot occur without failure of rivet row A and part 3.

The issue involved here is more than the simple resolution of how to test one particular joint segment on one manufacturer's bus body or what constitutes a joint specimen. It involves the basic economic incentive or disincentive the NHTSA is creating for manufacturers to build stronger, integrally assembled body structures. NHTSA briefly touched on this fact in the preamble to FMVSS 221 in the January 27, 1976 issue of the Federal Register by acknowledging comments stating that manufacturers could take the approach to FMVSS 221 of weakening the overall body structure in order to lower the required joint strength. This approach is very possible since FMVSS 221 does not set an absolute strength requirement but rather a relative strength requirement; i.e. 60% of the weakest joined body panel.

If the NHTSA does not agree with the interpretations outlined above, it will be establishing a policy which will discourage manufacturers from designing strong, integrally constructed bodies, and encourage them toward the most cost effective means of meeting the letter of FMVSS 221 regardless of total body strength.

For these reasons then, we look forward to your early confirmation of:

1. The alleged joint in question is not for the purposes of FMVSS 221, and

2. What constitutes a joint specimen.

Thank you.

Very truly yours,

W. G. Milby Manager, Engineering Services