Question: Why are the BMD values are too discontinuous and spiked at the node points?
Answer:
In Time-History Load cases, Staad calculates the end forces of the beam only and the maximum forces within the time zone are reported. It is very unlikely that the maximum of all the forces at all the points occur at the same instant of time. The maximum of each force result for every point are more likely to be distributed within the time range.
Staad plots the BMD of a beam member by joining the plot of end moments by a straight line, which is why you are getting a strange BMD diagram.
The BMD plot for a beam member for a Time History analysis is totally meaningless.
Considering all the maximum forces for all the points will result in highly conservative design. Thus, it has been advised not to consider the force values that you have obtained for design.
Apart from BMD, the derived quantities like forces , displacements should not be used as the results are the individual maximum over the entire time period.