> Step One
All of the Dynamic Analysis in Pro/MECHANICA use the method of Modal Superposition and require the modal analysis to be run first or as part of the Dynamic design study. When dynamic loads are applied the response is calculated based on the dynamic characteristics of the structure determined by the modal analysis. Also, dynamic stresses are calculated based on the element P-level derived during the modal analysis convergence process.
For a Dynamic Time analysis the loads are input as a function of time or time varying. The load may be Base Accelerations or Forces applied to the structure. Base Accelerations are applied to any Constraints or Springs-to-Ground. A time history response may be calculated at a particular location using a Measure or a snapshot of the full model response at a point in time may be made. It is also possible to animate the full model results over a time period. This type of analysis would be useful for short duration transient loads where calculating the maximum response or settling time was the goal.
For a Dynamic Frequency analysis the loads are input as a function of frequency. This would be similar to doing a sine sweep test on a shaker table using pure sinusoidal input. The load may be Base Accelerations or Forces applied to the structure. Base Accelerations are applied to any Constraints or Springs-to-Ground. If more than one load is applied the phase of the load may be defined. A frequency response of amplitude and phase may be calculated at a particular location using a Measure or a snapshot of the full model maximum response at a frequency may be made. This type of analysis would be useful for simulating any sine sweep type test or the response of a structure to a cyclic load such as vibration induced by the imbalance of rotating equipment. This may also be useful for calculating the frequency bandwidth of a structure for servo control problems.
For a Dynamic Random analysis the loads are input as a Power Spectral Density (PSD) over a frequency range. The load may be Base Accelerations (G^2/Hz) or Forces (Load^2/Hz) applied to the structure. Base Accelerations are applied to any Constraints or Springs-to-Ground. In a Dynamic Random analysis the loads are applied at all frequencies simultaneously. The result is more statistical rather than an absolute number. The PSD response or RMS value may be calculated at a particular location using a Measure. The RMS response of the full structure may also be calculated and displayed. This type of analysis is often used for simulation of transportation or acoustic environments where the loads are continuous, long duration and random.
The Dynamic Shock analysis technique was developed in the days of hand calculations to calculate or estimate the maximum response of a structure to a shock or transient input load such as an earthquake, explosion or transportation shock. The input to this type of analysis is a Response Spectral curve. The curve represents the response at different frequencies for a damped single degree-of-freedom (DOF) structure tuned to each frequency resulting from some applied transient load. In the days of hand calculations one would calculate the natural frequency of the structure with the assumption that it had a single DOF. You would the look at the amplitude of the curve at that frequency and use that as a static load on the structure to estimate the maximum response. Generally there are different curves provided for different damping levels. The big assumption is that your structure behaves as a single DOF and that the static response will look like the mode shape of the first mode. As computer solutions became available and the ability to calculate multiple modal frequencies became more common, this technique was extrapolated to estimate the combined effects of these multi-DOF systems. Some of these combination methods are included in the Pro/MECHANICA implementation of this Dynamic Shock technique. It is important to remember that this technique in its pure form is only valid for single DOF models, however, the multi DOF methods are still used extensively in the building, military and transportation industries. Many times load specifications are provided as Response Spectrum curves.
The first step before performing a Dynamic analysis is to determine if it is necessary. To do this you need some estimate of the first natural frequency of the structure either by performing a hand calculation or a Pro/MECHANICA modal analysis. The rule-of-thumb is that if the structure first natural frequency is 5 times the frequency of the dynamic load then the structure will behave as if the load were static and no dynamic analysis is necessary. As the structure frequency gets closer to the input load frequency you will get some dynamic amplification and a dynamic analysis may be necessary.
The following flow chart shows the step required for running a Dynamic Time, Frequency or Random analysis.
<TSD Engineering HOME>
Copyright 2003, TSD Engineering