why there is a difference between the modal mass and dynamic mass?
And one more problem
the Base shear coefficient that the robot 2011 is calculating is too big , about 2.5 in dynamic mass or sometimes 0.55 in modal mass but the real value is about 0.35
I have used ubc 97 for applying seismic load.
but v didn't calculate from multiply c.w from the value for c and w that the robot 2011 has calculated.
Please help me
thank you .
Solved! Go to Solution.
You can convert loads to masses in 3 ways.
1 Seecting Modal case
2 Selecting Dynamic mass - mass will be added to all dynamic cases
3 Selecting Global mass - converted masses will be applied to all dynamic cases and also can be used in inertia forces (of course if such forces are applied on model)
PLease check your loads to mass conversion as Romanich suggested. If it is not the case it could be good to upload the file for us to analyze
First of all I sugest to use only one self weight as a mass. There are two such masses in your model: one coming from Modal case definition ("Disregard Density" checkbox), another from Loads to Mass Conversion.
Converted masses are not used for base shear scaling unless you convert them using Global Mass or Dynamic Mass.
After these modifications base shear coeff ~=1.75. I will check the codebook
As far as I can see from the codebook (formula 30-4) you want either to see Cv coeff or C from V(base shear) = C*W(total weight\mass). These values are not presented in program.
Base shear coeff presented in report is in fact the ratio of base shear calculated with scaling to base shear without scaling.
See attached screen shot