Autodesk Robot Structural Analysis

[#1]

how can we take tortional effect into consideration when calculating a building ?.

its seemsthat there is two methods to define the excentricity in the modal analysis dialog box ( Excenticity definition ) ).

ive tried these two methods within the same modal ( Car park with to stories (braced only with columns ) )

the first one( Mass Matrix excentricity ), gives me results which i cannot accept ( more than 100 mode with less than 10% modal mass )

with the second one , ive got much more acceptable results ( at less than 10 mode , i ve got more than 90% modal mass ).

please confirm that the correct method is the second one in this case , and what is the limits (if its the case) of these two methods.

Thank in advance.

[#2 · Accepted solution]

The recommended method is the second one, i.e. "Add nodal masses".

It results is increasing some masses of specific story while decreasing others to obtain required eccentricity of the center of mass of each story. If stories are not defined the eccentricity is considered for whole structure.

The first method, i.e."Offset of the mass matrix" is based on shifting mass matrix. This method has not so clear physical description as the previous one and in some cases it may result in additional local torsional vibrations not observed in reality.

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