Interpretation ID: nht76-2.12

DATE: 04/14/76

FROM: AUTHOR UNAVAILABLE; S. P. Wood; NHTSA

TO: Blue Bird Body Company

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Blue Bird Body Company's March 15, 1976, request for confirmation that calculation of the material tensile strength of body panels under S6.2(a) of Standard No. 221, School Bus Body Joint Strength, is based on the minimum thickness permitted by American Society for Testing and Materials (ASTM) Standard 525 for the thickness specified in ordering the material. This response also reflects the April 1, 1976, meeting held between Blue Bird representatives and National Highway Traffic Safety Administration (NHTSA) personnel at Department of Transportation headquarters.

Under ASTM standards, the thickness of listed materials is permitted to vary from the specified or "nominal" thickness by a small amount. If the thickness tolerance of a material is specified by the ASTM, the NHTSA bases its determination of thickness on the "minimum thickness" specified for that material in the 1973 edition of the Annual Book of ASTM Standards. If the thickness tolerance of a material is not specified by the ASTM, the NHTSA uses the minimum thickness permitted by the school bus manufacturer's material specification.

YOURS TRULY,

BLUE BIRD BODY COMPANY

March 15, 1976

Mr. Richard B. Dyson Assistant Chief Counsel National Highway Traffic Safety Administration

SUBJECT: FMVSS 221 - SCHOOL BUS BODY JOINT STRENGTH

In S6.2(a) the subject standard makes provision for the manufacturer to determine the material tensile strength as published by the ASTM. This information is required before a manufacturer can design body joints and tooling to manufacture those joints in compliance with the subject standard.

Material tensile strength as published by the ASTM has a tolerance and we need to know to which end of the tolerance we must design. The standard adequately addresses this problem in S6.2(a) by stating ". . . .the relative tensile strength for such material is the minimum tensile strength specified for that material in the 1973 Edition of the Annual Book of ASTM Standards."

This only addresses half of the tolerance problem. The ASTM standards show tolerances for metal thickness ranges. In the absence of specific guidelines of this problem and because we must commit for tooling immediately, we are using the minimum thickness based on our specified thickness and tolerance in ASTM A525-73. This approach seems to be justified in light of the tensile strength guidelines given in S6.2(a).

If this approach is not satisfactory, please contact us by telephone immediately. We will also appreciate a written reply to this letter at your earliest convenience.

W. G. Milby Staff Engineer

cc: Bob Williams; Jim Moorman; Jim Swift