الفهرس 
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Abstract
This thesis provides an approach for determining the behavior of the three axle semi-traillers that traverse uneven ground profiles (typical to the Egyptian environment )and for the design of the different structural elements under dynamic ”loading” rather than static one.
The chassis of the semi-trailer is descretized using the finite element method, where both the longitudinal and transverse beams are idealized using three dimentional beam elements. The leaf springs are idealized using linear spring elements.
The dexformations and stresses are calculated based on a static analysis of the chassis. Also natural frequencies and mode shapes are obtained.
A sinusoidal road profiles are assumed; and a dynamic response history analysis using mode superposition method is performed. The stresses and deformations obtained using the dynamic analysis are compared with those obtained from the static analysis. Then a parametric study is performed to estimate the effect of the springs stiffness coefficients on the design in an effort to get the optimum design values. Also the effect of different