Richard M. Ecker

Modeling of a Mainframe Server Frame Subjected to Seismic Loadings

This paper describes the finite element modeling of a mainframe server frame. The frame consists of a server rack or frame with its add-on stiffening brackets. The frame is anchored directly to the floor with bolts at each of the four corners. The Telcordia Zone 4 earthquake test profile represents a severe dynamic load input and will be used herein to analyze the mainframe server frame. The main objective of this modeling is to validate the frame design prior to actual seismic testing, which ultimately ensures the structural integrity of a functional mainframe system during a seismic event. The server frame finite element (FE) model is derived from a three dimensional CAD model of a standard sheet metal frame weldment assembly which is then simplified and meshed with finite elements. This FE model represents the server frame, welded connections, and stiffening brackets, which are specifically designed to withstand seismic test profiles. To represent the components that populate the server frame, point masses are tied to the frame at the same attachment points that exist in the real assembly. The validation of the FE model involves the use of a horizontal shaker test to assess the server frame’s stiffness. The goal of this paper is to show a good correlation between FE model and test results using two separate FE solver technologies: implicit and explicit. For an implicit solver, linear material properties were used to obtain modal behavior that approximates the actual server frame’s behavior. Once these outputs were achieved, further response refinement was attempted by porting the model to an explicit dynamic solver. An explicit solver allowed non-linear material properties and body to body contact behavior to be included in the FE model while applying the seismic test profile to the server frame using a time domain input. The explicit dynamic model outputs used to correlate to actual test results were the modal dynamics, the displacement of the top of the server frame, and the maximum reaction force at the anchored corners. Finally, a functional system was subjected to the Telcordia Zone 4 seismic test profile. The system was functional during and after the seismic test with no significant structural damage having occurred.Copyright

SEISMIC TESTING AND ANALYSIS OF MAIN FRAME COMPUTER STRUCTURE

Designing a mainframe computer structure that can withstand seismic events requires significant testing and analysis. The computer frame and anchorage system must have adequate strength and stiffness to counteract earthquake-induced forces, thereby preventing human injury and potential system damage. However, this same frame and anchorage system must also meet the requirement of ensuring continued system operation by limiting overall displacement of the structure to accepted levels. Therefore, the test and analysis scope need to include the mainframe structure and its anchorage attachment to the building’s concrete floor via a raised floor structure. This paper discusses the numerical modeling and its verification to quantify the robustness of a high end computer server structure subjected to a severe seismic event. The frame of the computer is the structure where components are installed (e.g., the central processing unit, the input-output drawer, the power supply component, etc.). The dynamic response of this structure is highly related to the weight of the components, the assembly’s inherent natural frequency, and the location of the structure’s center of gravity. The natural frequency of various mainframe configurations were analyzed and measured by either changing the weights (i.e., adding or eliminating components) or by changing the structural stiffness (e.g., adding reinforcement brackets). The main objective of the modeling was to ensure structural integrity following a seismic test of a functional server system. Finite element analysis (FEA) was employed as part of the overall frame’s structural robustness design verification, whereby the simulated modal analysis results were compared to both the experimental modal data of the frame structure as well as measured swept sine data. This design study builds toward the objective of constructing a verified model of the server frame and components, which lead to a guideline for implementing optimized reinforcement. As part of the verification, the mainframe structures were subjected to horizontal table vibration tests simulating the loads and environmental conditions endured during seismic events. During experimental verification, the dynamic responses were recorded and analyzed in both the time and frequency domains.