The automotive vehicle has an exhaust system, also known as the after treatment System. It consists of different components such as Diesel Oxidation catalysts (DOC), Diesel Particulate Filters (DPF), Selective Catalytic Reduction Systems (SCR), etc. These components are mounted in the chassis with the help of strap joints. The strap joints provide flexible and serviceable connections between ATS bodies and chassis. The strap joint assembly consists of a T bolt, trunnion, and nut with strap. As the nut tightened, the tension induced in the strap, which results in clamping the body. If the tension in the strap is not sufficient, it may cause the failure of the joint. The failure can be yielding of material, slipping, and separation of the body. The strap joint should be designed so that these kinds of failures can be avoided. This paper presents the work on the calculation of the design margin using analytical and FEA methods. The failure modes addressed in this paper are yielding of the strap joint due to applied preload on the T bolt, slipping, and separation of the ATS body from the strap due to the application of dynamic loads such as acceleration G loads in multiple directions. The acceleration load is calculated from the PSD profile. For calculating design margins from the FEA method, nonlinear analysis is performed, followed by PSD analysis to estimate the bracket reaction forces. These reaction forces are used to calculate slip and separation margins. The tests were performed under similar conditions on a shaker table, and the results from both analytical and FEA methods were correlated with test observations to validate the proposed method of calculation of strap join design margin.