This help topic seems to indicate RAM Concept is calculating a complete time history velocity curve to perform vibration analysis. Is there a way to output the time histories RAM is calculating?https://docs.bentley.com/LiveContent/web/RAM%20Concept%20Help-v16/en/GUID-FE76210C-9E4F-4353-89B6-EAC59B1CEB58.html
Unfortunately, that data is not output and cannot be extracted from the program.
Two additional questions:
Does RAM apply input footfall forces at discrete spacing within the defined force input area, and if so, what is that spacing?
Can RAM generate output in terms of VC class, and if so, how does it handle the frequency-dependency of the VC criteria relative to the computed vibration response?
All finite element nodes in the excitation area are considered excitation nodes. Response are then calculated at other nodes as described in the manual excerpt below (Section 74.4.1 in the RAM Concept Manual). The spacing of the nodes is dependent on the Element Size specified when generating the mesh.
"RAM Concept considers any node associated with an excitation area as an excitation node. Excitation area polygons can be drawn on Layers – Vibrations – Vibration Analysis – Excitation Areas Plan. Only the nodes of elements intersected by or entirely within the drawn excitation area polygons are considered as excitation nodes. If no excitation areas are drawn, every node on the floor is considered as an excitation node.
Vibration response is always calculated at all finite element nodes. The Maximum distance from excitation node to calculate response option in the Criteria – Calc Options dialog can be used to intelligently select the excitation nodes to consider that will likely yield the maximum response. However, a complete response can always be calculated by setting the Maximum distance from excitation node to calculate response to “No Limit”. This will significantly increase the analysis time.
When using the Complete Harmonic Analysis (slower) option for the resonant response, the response associated with the lateral and angular degree of freedoms are excluded by default. Ignoring these components is often appropriate because they are rarely critical for floor vibrations and including increases calculation time. An option for users to include the lateral and angular degrees of freedom in the Complete Harmonic Analysis is provided in the Vibrations tab of the Criteria – Calc Options dialog."
The VC criteria is very similar to the Response Factor that is output by RAM Concept. The excerpt below (Section 74.2.4 in the RAM Concept Manual) explains how the Response Factor is calculated:
"The response factor is a multiplier on the level of vibration at the threshold of human perception. Thus, a response factor of 1 would represent a level of vibration that is just at the threshold of human perception, and a response factor of 2 would represent twice the perceivable level. People are more sensitive to vibration at some frequencies than at others. The base curves for human perceivability are taken from BS 6472. Since vibrations can contain a range of frequencies, the response factor in RAM Concept is calculated individually for each harmonic excitation frequency by taking a baseline acceleration (aRMS = 1) from the curve for that frequency, then combined using square root of sum of squares (SRSS). For resonant response, the response factor is always calculated using accelerations."
Thank you, Karl. Is there any insight available on how Concept computes the response factor for Impulsive Response?
See the manual excerpts below:
Thank you for bearing with me on this.
What input parameters are used for Impulsive Response? I cannot find any information on what force or load is used to generate the impulse. These options are greyed out when Resonant response is left unchecked.
You may find the recorded webinar, which covers the vibration analysis in RAM Concept, informative. The equations used to calculate the impulsive response are discussed at around the 25-minute mark.
The Concrete Centre publication titled "A Design Guide for Footfall Induced Vibration of Structures" will also explain further. The short excerpt from that document below may also be helpful. The equations referenced in that excerpt match those used in the webinar slide deck.
"The effective impulse can be thought of as the equivalent 'perfect impulse' of infinitesimally short duration that induces the samepeak structural response as the direct application of that footfall time history. It is found that this effective impulse is empirically related to the walking speed and the natural frequency of the structure. The mean effective impulse is given in Equation 4.9 below; the coefficient of variation is 0.4, and the serviceability 'design' impulse with a 25% chance of exceedence is given in Equation 4.10."