Reaction Recorder: What's being included?

[kovacsma]

Hello,

I essentially have a multi storey elasticBeamColumn associated with a leaning column, on which I am conducting dynamic analyses. I have a reaction recorder and an acceleration recorder at each storey node; the acceleration recorder has the input timeSeries added to it, to get total acceleration; gravity and PT forces also act vertically on the system, from the top floor node down to the bottom, as it is a model of a controlled rocking wall. I can run the model successfully with or without damping.

The analysis runs; data is output; it all seems just about right. Except:

I plot the (reaction) recorder outputs alongside the (mass*acceleration + RayleighDamping) recorder outputs (I multiply the acceleration recorder outputs by storey mass, in Matlab). To be clear: I have only taken the x-component of all these records.
I expect the two resulting time histories to be the same, but they're not quite. When the inputted time history is most active (ground accelerations are larger), the two plots generally track each other, but the mass*acceleration plot is a fair bit more wavy. When the input ground motion has died off (free vibration stage) the two responses come back in line.
To troubleshoot, I removed damping, and reduced the timestep to 1/10000th of a second; now I'm plotting Reaction recorder output and mass*Acceleration recorder output at each storey node. I get the same effect, which can be seen in the dropbox link below (I have included a screenshot of the time histories of these two outputs, overlaid on each other: red is mass*acc, black is the reaction recorder). https://www.dropbox.com/s/albdor7bqw0zr ... y.png?dl=0

Does anyone have any insight as to why this effect might be occurring? To me, it would see logical that the reaction recorders (x-component) at the storey nodes, connected by elasticBeamColumns above and below, and laterally constrained to a leaning column, would read the same as mass*acceleration (x-component), since I am not imposing any other lateral force.

I can say that I have also explored the internal element forces with element recorders, and the global x-component forces are in fact associated with the reaction forces, rather than the mass*acceleration result (I added the reaction results at a single timestep, from the top node to the ground node, and these values corresponded -- mass*acceleration did not).

Some insight would be appreciated.

Thanks,

Mike

[fmk]

the reaction forces are the static forces only .. this is what people normall want .. if you want to include the damping forces try reactionIncInertia