By Ray Delaforce
PV Elite Software Engineer, Intergraph
What is a load case? Well, first we consider the word “load.” A load is any direct force, or bending moment that is applied to a structure. Where do these loads come from? Consider this tower, for example:
The lateral forces in Image 1 cause a bending moment to be experienced in the tower. But note – the pressure is a load and the weight of the vessel is also a load. In Image 2, there are three resultant internal longitudinal or axial stresses generated in the tower. These are:
- Stresses from the pressure acting inside the tower
- Stresses from the total vessel weight acting on section X-X
- Stresses from the resultant bending moment as the location X-X
So, in essence, there are only three sources of load:
Now let us consider the term “cases” used in the phrase “load cases.” Reflect for a moment that we have these cases:
- Operating weight
- Hydro test weight
- No weight
- Empty weight
- Operating pressure
- Hydrotest pressure
- No pressure
- Vacuum pressure
- Seismic moment operating
- Seismic moment at hydrotest
- No moment
- Wind moment operating
- Wind moment at hydrotest
We can have any combination of these loads. In the case cited above, there would be a combination of 80 cases. However, PV Elite is limited to 20 cases.
We have three loads and many cases, but how are the stresses derived? In the case of a tower, or vertical pressure vessel, it is simply treated as a beam subject to these load; a beam that is also subject to pressure loading.
Consider the tower illustrated in Image 1. Let us see the internal stress induced in the tower:
As can be seen, the stresses on the right hand side of section X-X are different from the stresses on the left-hand side because of the stresses from the bending moment.
We consider a particular pressure code, namely the European code EN 13445 Part 3 (Design). That code mandates certain load cases be considered. As displayed in PV Elite, they are as follows:
Although these load cases look complicated, we are still only dealing with:
- Moment – from wind or seismic loading
How can we unravel the mystery of load cases and how they are handled by PV Elite? Stay tuned for next month’s Insider Newsletter for the conclusion of this exciting article! If you are not receiving the Insider Newsletter, sign up today.