Overpressure for a fixed-roof tank dishes the roof, pulling the roof-to-shell junction radially inward and uplifts the bottom plate adjacent to the shell. It usually results in a failure of the fillet weld joining the roof plate to the shell when the stress in the junction reaches the yield point of the material before failure occurs anywhere else in the tank.
E. E. Morgenegg indicated the importance of frangible roof design and apprehended that roofs on tanks smaller than about 15 meters in diameter would not fail as intended although designed by API Standard 650 Appendix F.
So, the author developed the finite element code “CNCL-ND” and showed the nonlinear dynamic behavior of typical fixed-roof tank under impulsive internal pressure. Furthermore, in this paper, the author have developed a joint element for a continuous fillet weld joining the roof plate to the shell termed a “Fillet Weld Element” and define the fracture mechanics parameter Φ
w. For the shell-to-bottom junction, a simple method is proposed to estimate the stress intensity factor
KI* applying the Dugdale-Barenblatt model. By using these parameters, it can be evaluated the reliability of frangible roof design numerically.
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