I successfully ran a modal vibration analysis with the minimum and maximum footstep frequency values set to 1.5 Hz and 2.5 Hz, respectively. I tried re-running the modal analysis with the minimum and maximum footstep frequency values changed to 0.83 Hz (50 steps) and 1.67 Hz (100 steps), respectively. I received the following error: An error has occurred. 0.83 is not a valid value for Minimum Footstep Frequency. I modified my footstep frequency values and changed the minimum and maximum footstep frequency values to 1.0 Hz and 1.7 Hz, respectively. However, RAM Concept will not perform the modal analysis. The modal analysis ends very quickly, and the Max Response Factor Plan, Max RMS Acceleration Plan, and Max RMS Velocity Plan all appear as the image shown below.
Why am I unable to set a Minimum Footstep Frequency below 1.0 Hz? Why will RAM Concept not perform a modal analysis with the new footstep frequencies?
Regarding the use of 0.83 Hz for the minimum footstep frequency:
A walking rate of 50 steps per second seems extremely slow and does not seem likely to induce a resonant response. It would also tend to excite a floor with very low fundamental natural frequency (< 4 Hz), which are seemingly rare in concrete floors. I do not think the critical response will typically be lost by using a minimum walking frequency of 1 Hz instead of 0.83 Hz. Increasing the minimum footfall frequency as you noted is what we would recommend to eliminate the first error that you noted.
Regarding the problems with using 1 Hz for the minimum footfall frequency:
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Thank you for the quick response. I have uploaded my model.
The Excitation Response Threshold Factor is set to high (see image below).
The information in this forum will explain how the factor is used and provide some guidance for determining a better value to use.
Thank you. The forum you referenced was very helpful.
How does the vibration analysis calculate stiffness? And how can I determine if the vibration analysis is being performed on cracked or uncracked concrete sections (specifically, a one-way concrete slab and post-tensioned beams)? I built a model in RAM Structural Systems then imported one elevated concrete floor into RAM Concept to run a vibration analysis.
The Criteria - Calc Options dialog has the options marked below for stiffness matrix options for the vibration analysis. The first option would use the gross section properties with any custom stiffness adjustments that might have been defined with the slab area or beam in RAM Concept or imported from the RAM Structural System model. The second image below shows the option that can be used to bring these factors into RAM Concept from RAM Structural System. The last three options are stiffness matrices from steps in the load history calculations and would include stiffness adjustments for cracking, creep, and shrinkage. A load history analysis would need to be run before these could be used in the vibration analysis. Since the load history calculations account for cracking, it is recommended to start with gross section properties with no stiffness adjustments (KMr = KMs =1) before running that analysis.