This International Standard specifies a method of determining the ring stiffness of bends and branches made from thermoplastic material and for use with plastics pipes having a circular cross-section. The method can be used to determine the stiffness of bends, equal branches and unequal branches, provided the fitting allows a diametric deflection of at least 4 %. NOTE 1 If a fitting has the same wall thickness, wall construction, material and diameter as a pipe tested according to ISO 9969, then, because of its geometry, its stiffness can be equal to or greater than that of the pipe. In this case, the fitting can be classified as having the same stiffness class as the pipe, without testing. NOTE 2 Any unequal branch can be expected to have at least the same stiffness as an equal branch, provided that it has the same main diameter, wall construction and material as the equal branch. NOTE 3 A reducer having the same wall thickness, wall construction and material in the transition zone as a tested bend or branch can be expected to have at least the same stiffness as the tested bend or branch with the largest diameter of that reducer. NOTE 4 The result of the test reflects the resistance the fitting has against deflection when installed. Advice on the significance of the test result is given in Annex A.