During vertical static equipment design, upon modeling platforms, engineers must carefully differentiate between live loads (such as personnel, equipment, and operational materials) and platform weight. These loads must be treated in specific ways depending on the analysis case being considered. This article will explore two main cases for handling live loads on vertical vessel platforms: Case 1, where live loads are considered, and Case 2, where live loads are not considered.
Table of Contents
Case 1: Live Loads Considered (But Not as Masses)
In this case, live loads are incorporated into the design, but it is crucial to emphasize that live loads should not be treated as additional masses for the purpose of affecting the vessel’s natural frequency. This ensures that the dynamic behavior of the system is correctly modeled without artificially altering the natural frequency.
Vertical Forces: Live loads are treated as vertical forces applied to the platform’s center mass. The live load per surface area (i.e kg/m²) is multiplied by the paltform area which provides force (Kg => Newton). The resulting force is on the conservative side when referring to a pressure vessel platform, as explained on the practical consideration below.
Global Load Effects: Although live loads do not affect the natural frequency of the vessel, they do contribute to the overall load distribution and generate additional bending moments around the vessel’s axis (estimated in their conservative approach as above). These effects should be considered in the global load analysis.
Local Load Effects: Like wise, platform brackets and local loads interaction to vessel shell should typically be evaluated during this case, taking into account the full effect of live loads + platform own weight.
Practical Consideration:
In real-world scenarios, vertical vessel platforms are hardly ever fully loaded with people or equipment. Typically, only one or two persons are on the platform during operations, not the full live load capacity. Therefore, when we account for live loads in this case, we tend to be overly conservative, assuming full live load occupancy across the entire platform surface. This ensures a worst-case scenario is considered, but it is not necessarily representative of actual operational conditions.
Case 2: Live Loads Not Considered (with Seismic Analysis)
In this case, live loads are not considered in the pressure vessel stability, particularly when focusing on seismic analysis. This is a common approach, as the primary concern here is the permanent mass of the vessel and platform, which directly influences the system’s behavior during dynamic events.
Seismic Analysis: The seismic forces are applied based on the platform’s weight and the permanent structural components. Live loads are excluded from the analysis, as it is unlikely that the platform would be fully loaded with people during a seismic event. The platform weight and vessel weight remain the key contributors to the system’s dynamic behavior.
Natural Frequency: By excluding live loads from the seismic analysis, the vessel’s natural frequency remains based on its permanent mass. Since live loads are generally temporary and transient, they do not significantly alter the system’s dynamic characteristics during seismic events.
Practical Consideration:
Since live loads are unlikely to be at their maximum during a seismic event, and it is not typical for a pressure vessel platform to be full of people, they should not be considered in the seismic case. Instead, the analysis focuses on the platform weight and other permanent loads that affect the global stability of the vessel during seismic events.
Differentiating Between Live Loads and Platform Weight
A crucial distinction to keep in mind is the difference between live loads and platform weight:
Platform Weight: This is the permanent load that contributes directly to the vessel’s mass and dynamic behavior, including its natural frequency and seismic response.
Live Loads: These are temporary loads that vary based on operational activities. They should not be considered as permanent masses for the purpose of affecting the vessel’s natural frequency. Instead, they are applied as vertical forces and only affect the overall load distribution and global response.
Understanding and differentiating these loads ensures an accurate dynamic model for stability evaluation.
Conclusion
When implementing live loads on vertical pressure vessel platforms, it is essential to correctly differentiate between live loads and platform weight. In seismic analysis, live loads are generally not considered, as it is highly unlikely for the platform to be fully loaded with people during a seismic event. On the other hand, when live loads are considered in static or global load analyses, they should be treated as vertical forces rather than masses to avoid impacting the system’s natural frequency.
For more accurate modeling of live loads in various stability cases, software like VCLAVIS.com can be used. This tool allows for the proper consideration of live loads while maintaining correct assumptions about how these loads affect vessel stability. It ensures that live loads are accounted for appropriately in different stability scenarios without altering natural frequency calculations unnecessarily.
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