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Section 1
1-83
In addition, the dynamic results will not reflect the location
of a mass within a member (i.e. the masses are lumped at the
joints). This means that the motion, of a large mass in the
middle of a member relative to the ends of the member, is not
considered. This may affect the frequencies and mode shapes.
If this is important to the solution, split the member into two.
Another effect of moving the masses to the joints is that the
resulting shear/moment distribution is based as if the masses
were not within the member. Note also that if one end of a
member is a support, then half of the that member mass is
lumped at the support and will not move during the dynamic
response.
1.18.3.3 Damping Modeling
Damping may be specified by entering values for each mode, or
using a formula based on the first two frequencies, or by using
composite modal damping. Composite modal damping permits
computing the damping of a mode from the different damping
ratios for different materials (steel, concrete, soil). Modes that
deform mostly the steel would have steel damping ratio, whereas
modes that mostly deform the soil, would have the soil damping
ratio.
1.18.3.4 Response Spectrum Analysis
This capability allows the user to analyze the structure for seismic
loading. For any supplied response spectrum (either acceleration
vs. period or displacement vs. period), joint displacements,
member forces, and support reactions may be calculated. Modal
responses may be combined using one of the square root of the
sum of squares (SRSS), the complete quadratic combination
(CQC), the ASCE4-98 (ASCE), the Ten Percent (TEN) or the
absolute (ABS) methods to obtain the resultant responses. Results
of the response spectrum analysis may be combined with the
results of the static analysis to perform subsequent design. To
account for reversibility of seismic activity, load combinations can
See section
5.32.10