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Section 5
5-185
Steps to calculate base shear are as follows:
1.
Time Period of the structure is calculated based on clause
1630.2.2.1 (Method A) and 1630.2.2.2 (Method B).
2.
The user may override the period that the program calculates
using Method B by specifying a value for PX or PZ (Items f9
and f10) depending on the direction of the UBC load. The
specified value will be used in place of the one calculated
using Method B.
3.
The governing Time Period of the structure is then chosen
between the above-mentioned two periods on the basis of the
guidance provided in clause 1630.2.2.2.
4.
From Table 16-Q and 16-R, Ca and Cv coefficients are
calculated.
5.
The Design Base Shear is calculated based on clause 1630.2.1
and distributed at each floor using the rules of clause 1630.5.
6.
If the ACCIDENTAL option is specified, the program
calculates the additional torsional moment. The lever arm for
calculating the torsional moment is obtained as 5% of the
building dimension at each floor level perpendicular to the
direction of the UBC load (clause 1630.6). At each joint where
a weight is located, the lateral seismic force acting at that joint
is multiplied by this lever arm to obtain the torsional moment
at that joint.
7.
If the value of C
t
is not specified, the program scans the
Modulus of Elasticity (E) values of all members and plates to
determine if the structure is made of steel, concrete or any
other material. If the average E is smaller than 2000 ksi, Ct is
set to 0.02. If the average E is between 2000 & 10000 ksi, Ct
is set to 0.03. If the average E is greater than 10000 ksi, Ct is
set to 0.035. If the building material cannot be determined, Ct
is set to 0.035. C
t
is in units of seconds/feet
¾
or in units of
seconds/meter
¾
. C
t
< 0.42 if the units are in feet, and C
t
> 0.42
if the units are in meter.
8.
Due to the abstractness of the expression "Height above
foundation", in STAAD, height "h" is measured above
supports. If supports are staggered all over the vertical
elevations of the structure, it is not possible to calculate "h" if
one doesn't have a clear elevation level from where to measure