A mainframe computer’s structure consists of a frame or rack, drawers with central processor units, IO equipment, memory and other electronic equipment. The focus of this structural mechanical analysis and design is on the frame, earthquake stiffening brackets and tie-down methods. The primary function of the frame is to protect critical electronic equipment in two modes. The first mode is during shipping shock and vibration, which provides excitation primarily in the vertical direction. The second mode of protection is protecting the equipment during seismic events where horizontal vibration can be significant. Frame stiffening brackets and tie-downs are features added to mainframe systems that must meet earthquake resistance requirements. Designing to withstand seismic events requires significant analysis and test efforts since the functional performance of the system must be maintained during and after seismic events. The frame stiffening brackets and anchorage system must have adequate strength and stiffness to counteract earthquake-induced forces, thereby preventing human injury and potential system damage. The frame’s stiffening bracket and tie-down combination must ensure continued system operation by limiting overall displacement of the structure to acceptable levels, while not inducing undue stress to the critical electronic components.
This paper discusses the process of finite element analysis and testing of a mainframe computer structure to develop a design that can withstand a severe earthquake test profile. Finite element analysis modeling tools such as ANSYS, a general-purpose finite element solver, was used to analyze the initial frame design CAD model. Both implicit and explicit finite element methods were used to analyze the mainframe subjected to uniaxial and triaxial earthquake test profiles.
The seismic simulation tests involve extensive uniaxial and triaxial earthquake testing in both raised floor and non-raised floor environments at a test facility. Prior to this extensive final test, in-house tests were conducted along with modal analysis of the prototype frame hardware. These tests are used to refine the dynamic characteristics of the finite element model and to design the frame stiffening bracket and tie-down system. The purpose of the modeling and in-house testing is to have a verified finite element model of the server frame and components, which will then lead to successful, seismic system tests. During experimental verification, the dynamic responses were recorded and analyzed in both the time and frequency domains.
The use of explicit finite element modeling, specifically LS-DYNA, extends the capability of implicit, linear modeling by allowing the incorporation of test data time history input and the experimentally derived damping ratio. When combined with the ability to model non-linear connections and material properties, this method provides better correlation to measured test results. In practice, the triaxial seismic time history was applied as input to the finite element model, which predicted regions of plastic strain and deformation. These results were used to iteratively simulate enhancements and successfully reduce structural failure in subsequent testing.
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