API PUBL 937-A:2005 pdf download

API PUBL 937-A:2005 pdf download.Study to Establish Relations for the Relative Strength of API 650 Cone Roof Roof-to-Shell and Shell-to-Bottom Joints.
3.1.3 Roof-to-Shell Joint Failure Pressure
Using the SafeRoof program. the pressure for failure of the roof-to-shell joint was calculated to be 1.04 psi. The criterion for failure of the roof-to-shell is yielding at the roof-to-shell joint in compression. This yiclding then results in local buckling and kinking of the angic which causes the roof attachment weld to fail and to initiate detachment of the roof. Since the roof-to-shell failure pressure is greater than the balanced uplift pressure of 0.295 psi, signilicant uplift occurs before the roof-to-shell fails.
Both the roof-to-shell and the shell-to-bottom joints are in compression, as shown in the details of Figure 3-8 and Figure 3-9. This is due to the “doming” that has occurred in the roof(where it has lifted from the rafters) and the “bowling” of the tank bottom (which has resulted in a concave bottom). The deformations of both the roof and bottom result in inward radial displacements at the roof-to-shell and shell-to-bottom joints and a corresponding compressive circumferential stress.
At the bottom, the radius at which uplift starts is 98 inches (8.16 feet), so that the bottom has uplifted for a radial distance of 6.83 feet from the tank wall. The uplift displacement of the tank shell is 4.6 inches.
Iquivaknt stresses for the middle surface are plotted in Figure 3-10. These show that the top angle is at yielding (approximately 36,000 psi), while the stresses at the shell-to-bottom joint are large (approximately 26.000 psi), hut not yet at yielding. At this load, the circumferential stresses in the bottom near the shell are in compression. Figure 3-Il. The meridional stresses are in tension. with the largest (approximately 5.300 psi) values in the center of the bottom. However, in all cases, the meridional tension stresses in the bottom are much smaller than the circumferential stresses near the shell-to-bottom joini Therefore, they are not expected to cause failure.
3.2.3 Roof-to-Shell Joint Failure Pressure
The failure pressure of the roof-to-shell joint remains the same as the empty tank (1.04 psi), however the displacements at the shell-to-bottom joint are very diflrent than for the empty case. Figurc 3-20 and Figure 3-21. Although thcrc is some uplift, the radius of first uplift is 176 inches (14.6 feet). nearly equal to the tank radius of 15 feet. The magnitude of the bottom uplift is also much smaller, at 0.027 inch.
Because the uplift is less, the stresses at the shell-to-bottom joint for a full tank at the roof-to- shell joint failure pressure are also less than an empty tank. For the empty tank the stress was approximately 26,00() psi. while for the full tank the shell-to-bouom joint stress is approximately 13,000 psi. Thus, a tank lull oiproduct has the effect of actually reducing the stress at the shellto-bottom joint at the roof-to-shell joint failure pressure.
3.3 Empty Tank (with buckling)
As noted, buckling can reduce the strength of the joints. We will examine in detail the effect of buckling on the pressure at failure of the roof-to-shell joint. Buckling is approximated in the elastic, large deformation, static finite clement analyses by reducing the compressive strength of roof and floor when compressive stresses are detected.
3.3.1 Roof-to-Shell Joint Failure Pressure
Buckling is approximated in the SafeRoof analysis by reducing the circumferential stiffness of the elements in compression in the roof or Iloor. Based on beam flange buckling practice. buckling effacts are not included within a distance of 32 times the roof (or floor) thickness from the joint.
When buckling is included, the pressure for failure of the roof-to-shell joint was calculated to be 0.724 psi as compared to 1 .04 psi without buckling. These two values give a range at which the actual failure would be expected. Since both of these roof-to-shell failure pressures arc greater than the balanced uplift pressure of 0.295 psi, significant uplift occurs before the roof-to-shell fails, as shown in Figure 3-26.