Load case combinations have been a part of the building codes and The American Society of Civil Engineers (ASCE) 7 for some time now. In the 2007 rewrite of ASME Section VIII, Division 2, we saw the introduction of load case combinations into the new version of that code. Word on the street is that UG-22 in ASME Section VIII, Division 1 will have load case combinations specified in a future edition, even as soon as the 2025 publication. Additionally, load case combinations are prevalent in third-party and industry standard specifications like JIP33, MODEC or those required by Engineering, Procurement and Construction companies (EPCs).
The following blog aims to further prepare those in the industry affected by these new code updates and provide guidance on how to successfully consider those combinations for wind and seismic loads.
Load case combinations take different types of loads (seismic, dead load, wind, and so on) in combination with each other, and then compare the combination to some sort of acceptance criteria. In these combinations, the different types of loads have some sort of multiplier on them. For example:
P is the design pressure (internal or external) and omegaP is 1 unless otherwise specified by the end user. Ps is the static head, D is the dead load (or transport load), W is wind and E is the seismic/earthquake value.
To simplify, the case would look at the combined effect of pressures, a 100% of the weight and either 60% of the wind load or 70% of the seismic load, whichever is worse.
D is the dead load, Ev is the vertical seismic and Eh is the horizontal or lateral seismic.
This combination looks at 90% of the weight in combination with the 100% of the vertical seismic load acting upward (Ev is defined as acting downward, so the – reverses that), and 100% of the horizontal or lateral seismic load. The load case does not explicitly specify what to do about pressure.
D is the dead load, L is the live load, W is the wind load and S is the snow load.
This combination looks at 100% of the weight, 100% of the live load, 60% of the wind load and 50% of the snow load. The load case does not explicitly specify what to do about pressure.
To answer that, a series of questions should be considered.
If the answer is yes and Part 4 is used for the design (Design By Rule), then the combinations in Table 4.1.2 must be satisfied.
If the answer is yes, then combinations from IBC, CBC or ASCE 7 may need to be satisfied.
If the answer is yes, then these additional combinations must be satisfied.
Additionally, other engineering decisions should be considered for load case combinations, like:
In considering all of these questions, engineers usually run a bunch of load case combinations to satisfy requirements. But, it has to be easier than that, right? It SHOULD be easier than that. First, let's look at why these calculations are run.
Let’s look at a couple of facts. Seismic loads are based on weight, and when the weight goes up, the magnitude of the seismic load goes up. Wind loads are based on projected area, and as the projected area goes up, the magnitude of the wind load goes up. With that understanding, let's consider the following load case combinations from ASME Section VIII, Division 2 as mentioned above:
Or, it could be that the highest tensile stress case on the bottom shell course is the sub-combination (internal pressure, empty, wind), with a static head set to zero because the vessel is empty:
Or it could be that the highest compressive stress case on the skirt is the sub-combination (external pressure, flooded, seismic horizontal and vertical downward action), with a static head set to zero to be conservative in combination with external pressure:
Basically, the equipment has multiple components that may fail under different loading combinations and each prescribed load case likely implies several permutations within itself.
Pressure Vessel software exists today that not only provides code assistance for design compliance but also helps guide engineers through updates like these that can halt design and overall production.
DesignCalcs, CEI’s leading pressure vessel design software, built to provide engineers with ASME Section VIII code assistance, supports the determination of lateral or vertical wind loads and lateral seismic loads for several building codes and ASCE 7.
The software defaults the multipliers on these loads based on the Allowable Stress Design (ASD) Basic Combinations (that include wind and seismic). In addition, it allows further permutations of these combinations by allowing these to be checked based on operating liquid levels, empty, pressurized (internal and external) and unpressurized factors. DesignCalcs, also considers the overstrength factor, but by default, and it only affects the resulting loads on the structural support elements in contact with the foundation.
Since this is an ASD basis by default, the allowable stresses are not increased during the application of the wind and seismic loads. These are just the default settings.
In order to accommodate different codes, multiple end user specifications, different interpretations of how to treat the overstrength factor, or even strength design combinations, DesignCalcs allows for the customizations of these different multipliers. If instead you prefer to calculate the resulting seismic multiplier on weight directly, or the wind pressure at a certain elevation, you can do that as well. The user can also override the default to increase the allowable for cases with wind and seismic.
So if you're worried about the load case combinations updates to the code or about third-party specs while designing your pressure equipment, considering pressure vessel software with code assistance can help you save time, money and headaches.
Schedule a Discovery Call Today to talk to a representative. And, if you're looking for online training to better understand code-assistance software from the experts who design it, consider using a CEI product in conjunction with the ThinkTank Academy.