Nodereaction and base shaer for SDOF system

[n_siahpolo]

Dear all
i need to find maximum base shear of SDOF via nonlinear time history analysis. IN one hand, I use this recorder
recorder Node -file reac1.out -node 1 -dof 1 reaction
In other hand, I read the relative acceleration on top node of SDOF by this recorder :
recorder Node -file acc-NTHASN1.out -node 2 -dof 1 accel
Then multiply mass of SDOF to maximum acceleration.
Now I have to question:
1- Is it true to say maximum base shear is equal to maximum acceleration multiplying by total mass (V=ma)?
2- What is the unit of acceleration, is the unit is equal to predefined unit in modeling.
For instant after using to above recorder the results are :
Maximum total base shear : 5387 KN
Maximum acceleration : 56 m/s2 : mass=600 ton --> base shear=56*600=33600 KN
Why the result differences is to much?
Also in my SDOF model the input acceleration is equal to node 2 acceleration. it means the structure is to stiff that response same as earth. i checked the T, but T=1.7 sec in my model. Hence the SDOF is not such a stiff.

[fmk]

1. NO .. you ae forgetting the inerta forces that are not included in the reactions .. look at the equation of motion:
the eqn for the linear system is: MA + CV + KU = -MAg, so obviously KU is only equal to MAg (.e. shear force equals Mag iff nodal accel is 0)!


2. same as the units you use to define your model properties in.,