🎥 📺MasterFrame Dynamics - Modal Analysis & Floor Vibration
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📺MasterFrame Dynamics - Modal Analysis & Floor Vibration


Posted 20 Mar 2014


The Dynamics module is an add-on to Master Frame, essential for determining the forces in a model due to dynamic forces like blast, impact, dynamic wind loading, moving loads, and vibrating machinery. The Seismic add-on is purchased separately and allows analysis using lateral force methods, response spectrum methods, or complete base acceleration time history analysis.

I. Modal Analysis (Natural Frequency and Mode Shapes)

Modal analysis is the foundation of all dynamics features in Master Series.

  1. Model Setup and Memory: Before analysis, model properties must be checked. Modal analysis uses matrices that require significantly more memory than equivalent static analysis. The memory needed is an indicator of analysis time (e.g., a model requiring 260 MB took 4 to 5 minutes). This check is crucial for models incorporating FE surfaces (meshes), a new feature, as degrees of freedom and memory requirements can become very large.
  2. Mass Contribution: Model mass is set up by using load groups defined in Master Frame and applying a factored amount of these loads into the Dynamics calculation.
  3. Analysis and Output: Users can choose to analyze all mode shapes, a subset (e.g., the first 'x' amount), or modes within a specific natural frequency range. The output includes mode shapes, natural frequencies, and mass participation in any of the given axes. Sorting modes by mass participation in the Y direction is particularly useful when analyzing vertically vibrating machines.

II. Time History Analysis

Time history analysis enables the study of the structure's response over time to specific dynamic loads.

1. Steady State Analysis

This feature models a harmonic force, such as a vibrating machine, applied to the structure.

  • Input and Settings: The user inputs a single or combination of sine waves that define the harmonic force (e.g., 8 Hz). Time history settings, including the time step (which controls accuracy) and the total time, are defined. For steady state analysis, the total time should account for at least one complete cycle to capture the cyclic, repetitive response and determine worst-case deflections.
  • Output and Design Integration: Outputs include maximum/minimum displacements, member forces at any specific time, and "node watches" which track the displacement, velocity, and acceleration of selected nodes over the analysis duration.
  • A snapshot of the nodal displacements and calculated member forces at critical times (e.g., when deflection is worst) can be sent back to Master Frame. This allows the creation of a load case incorporating dynamic effects, resulting in significantly higher bending moments (e.g., 7.4 kNm versus 4.61 kNm without dynamic loads).

2. Base Acceleration Analysis

This feature is used for importing seismic or other base acceleration data.

  • Import Features: Users can paste data from Excel or import data. Master Series ships with data for six historical earthquakes, or users can import their own formatted base acceleration sets.
  • Output: The structure's reaction to the seismic event is simulated (e.g., 32 seconds of an earthquake). Node watches provide graphs of displacement, velocity, and acceleration, along with root mean squared (RMS) values. Snapshots of member forces can be output to Master Frame for design purposes, allowing members to account for these dynamic seismic loads.

III. Vibration Design Tools

The vibration design package allows users to calculate RMS acceleration, response factors (RFs), and vibration dose values (VDVs) over a frequency range.

1. Setup and Approaches

The analysis begins by adding a vibration load case. Key choices include:

  • Code Approach: Users choose between the SEI p354 approach and the Concrete Society approach.
  • Analysis Type: Steady state or transient analysis is selected.
  • Fourier Series: If steady state is chosen, a Fourier Series must be selected (e.g., staircase, walking). SEI has five series options, while the Concrete Society approach primarily offers one "design" series. A user-defined Fourier Series can also be input.
  • Frequency Range: The range depends on the chosen Fourier Series (e.g., SEI walking between 1.8 and 2.2 Hz; Concrete Society uses 1 to 2.8 Hz, which is considered potentially more realistic for staircases than higher frequencies).
  • Mode Selection: The user can use all calculated modes or a selection (e.g., the first 10 modes with the most mass participation in the Y direction) to reduce analysis time.

2. Response Measurement (Single vs. Multiple Node)

The type of output depends on the node selection:

  • Multiple Nodes: Used initially to identify critical areas in the structure by determining the worst-case RFs and VDVs across a selection of nodes.
  • Single Node: Once critical areas are known, this option provides detailed results for a specific node, displaying the RF over the full testing frequency range to identify critical and subcritical resonant frequencies.

3. Loaded Node vs. Response Node

This setting determines where the vibration is applied relative to where the response is measured:

  • Response Node = Dynamically Loaded Node (Checked): The software applies a vibration at a node and measures the resulting response factor at that same node. This is a quick method to determine critical structural areas.
  • Separate Areas (Unchecked): This powerful feature allows users to measure the response factors in a specific response area (e.g., an operating theater) caused by vibration applied in a different vibration area (e.g., an adjacent corridor). This analysis takes longer, but can be used to achieve a reasonable reduction in response factors if vibrations are confined to non-critical areas.

4. Comparison of SEI vs. Concrete Society Approaches

Direct comparison often shows the SEI p354 approach yielding significantly higher, more conservative response factors than the Concrete Society approach.

  • Difference in Graphs: The Concrete Society approach tends to produce graphs with distinct peaks due to the use of more harmonics in its Fourier Series (e.g., four harmonics versus two for SEI staircase), allowing for greater interaction with the mode shapes.
  • Response Factor Magnitude: The sum of the Fourier coefficients in the SEI approach is often more than twice as large as in the Concrete Society approach, directly leading to larger response factors.