Horizontal Vessels Wind and Earthquake Loads

In horizontal vessels, wind and earthquake loads primarily affect the design of saddles and supports. The design of the shell is normally controlled by other loads such as design pressures, operating weight, saddle reactions, etc. A horizontal cylindrical vessel supported by two saddles acts like a uniformly loaded, simply supported beam.

Vessels with hemispherical or elliptical dished heads may be treated as an equivalent cylinder having a beam length equal to L + 4H/3, where L is the tangent-totangent length of the vessel and H is the depth of the curvature of the heads. Figure 400-18 shows a horizontal vessel with two saddle supports.

Horizontal Vessel with Saddle Supports

Given the simple beam analysis of a horizontal vessel supported by saddles, the following stress conditions may be determined:
1. Longitudinal bending stress
2. Tangential shear stress
3. Circumferential stress

Depending on the location of the saddles with respect to overall length, the maximum longitudinal stress will occur at midspan or at each support. Generate a shear and bending moment diagram for the vessel and check the maximum longitudinal stress, at the position of maximum bending moment.

Tangential shear and circumferential shear stress will be maximum at each saddle support. The stresses in the vessel vary according to the angle included by the saddle. Circumferential bending is caused by the tendency of a cylinder to “oval” when supported on line supports, like a saddle. Section 447 below outlines procedures for saddle and stiffener design.

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