Combination Method

The various methods available in STAAD.Pro for
combining the contribution from the individual modes is listed under
this heading. The details of these methods are explained in TR.32.10.1 応答スペクトル解析.
Note: Not all
methods are available for all code options.

SRSS is the square root of summation of
squares method.

ABS is the absolute sum method. This
method is very conservative and represents a worst case combination.

CQC is the complete quadratic
combination method. This method is recommended for closely spaced modes instead
of SRSS.

ASCE is the NRC Regulatory Guide Rev. 2
(2006) Gupta method for modal combinations and Rigid/Periodic parts of modes
are used. The ASCE498 definitions are used where there is no conflict.
ASCE498 Eq. 3.221 (modified Rosenblueth) is used for close mode interaction
of the damped periodic portion of the modes. This method should only be used
for a general response spectrum.

TEN is the Ten Percent Method of
combining closely spaced modes as per the NRC guideline 1.92 (1976).

CSM is the closely spaced modes method.
The peak response quantities for closely spaced modes (considered to be within
10 percent of each other) are combined by Absolute method. This peak response
quantity for closely spaced modes is then combined with those of widely spaced
modes by SRSS method.

GRP is the closely spaced modes grouping
method where the peak response quantities for closely spaced modes (considered
to be within 10 percent of each other) are combined using the absolute method.
This peak response quantity for closely spaced modes is then combined with
those of widely spaced modes by SRSS method. NRC Reg. Guide 1.92 (Rev. 1.2.1,
1976).

Save

Select this option to generate a file (with
.acc. extension) containing the joint
accelerations in g’s and radians/sec_{2}

Period vs. Acceleration table

Provide the values of period (seconds) and
corresponding acceleration (current length units/sec^{2}) or
displacement (current length unit). Spectrum pairs should be provided in
ascending value of period. As we provide the curve points, the program
displays the curve at the bottom of the dialog box.

Spectrum Type

Choose whether the response spectrum curve will be
input as Period vs. Acceleration or Period vs. Displacement (Custom and IS 1394
only).

Interpolation Type

From the spectrum data that are provided under the
Define Spectrum Pairs tab of the dialog box shown above, STAAD fetches
the spectral value for the actual modes of the structure using one of two
interpolation methods –
Linear and
Logarithmic. Linear interpolation is the
default method.
Since Spectra versus Period curves are often linear
only on LogLog scales, the logarithmic interpolation is recommended in such
cases; especially if only a few points are entered in the spectra curve.

Damping Type


Damping  This is to be used for
specifying a single modal damping ratio which will be applied to all modes. The
default value is 0.05.

CDAMP  Select this option to use
Composite Modal Damping. This evaluates the damping from that defined in the
material or constant definitions. A Damping ratio is defined by the material
definition or in the
Material Constants  Damping
Ratios dialog. If there is no damping information entered in the
material or constant definitions, the behavior is the same as MDAMP.

MDAMP  Selection this option to use
Modal Damping, which is used for individual damping ratios for each mode.
Individual mode damping ratios are defined using the
Modal Damping dialog.

Scale

Linear scale factor by which the spectra data will
be multiplied. Usually to factor g’s to length/sec^{2} units. This
input is the appropriate value of acceleration due to gravity in the current
unit system
You may choose to provide the spectral acceleration or displacement data as a set
of unnormalized values or as a set of normalized values. For normalized
values, the normalization factor is specified through the means of the
Scale factor. For example, if the curve is input in terms of g  the
acceleration due to gravity  and the current length unit is feet, the
Scale would be 32.2. For unnormalized values, the scale
factor is provided as 1.0, which also happens to be the default. The
spectra data will be multiplied by the scale factor during the
analysis.

Missing Mass

Select this option to apply the Missing mass correction. The static effect of
the masses not represented in the modes is included. If this
option is selected on any spectrum case it will be used for all spectrum
cases. 
ZPA

Zero Period Acceleration: It is used only with the
missing mass option. If no value is entered or a zero value is entered, the
default considered by the program is 33 Hz. If an acceleration is entered
corresponding to the Missing mass mode, then the ZPA value is ignored. If no
acceleration value is entered for the missing mass mode, then spectral
acceleration corresponding to the ZPA frequency is used.

Direction

Specify the global direction(s) in which the spectrum
is to be applied. The response spectrum may be applied in one or more
directions simultaneously. Directions not provided will default to zero.

Use Torsion

(IS 1893, IBC 2012, IBC 2018 and NRC only) Setting  Description 

Dynamic Eccentricity (DEC) 
Factor to be multiplied with static eccentricity
(i.e., eccentricity between center of mass and center of
rigidity). If not specified (or zero), a value of 1.0 is
assumed. 
Accidental Eccentricity (ECC) 
Factor for accidental eccentricity. Positive
values indicate clockwise torsion and negative values
indicate counterclockwise torsion. If not specified (or
zero), a value of 0.05 (5% of floor plan dimension
perpendicular to force at a given level) is used.


Signed Response Spectrum Results Options

Two method are available for added mathematical
signs to the spectrum response output:

Dominant Mode No.  Select this option
and (optionally) specify a mode number to define as a dominant mode. The sign
(sense) of this mode will be applied to other modes.

Signed  Select this option to create
signed values for all results by comparing the sum of the squares values for
positive and negative values to determine the governing sign.

Individual Modal Response Load Case Generation

Select this option to have the program
automatically generate primary load cases from the mode shape scaled to the
magnitude that the mode has in this spectrum analysis case before it is
combined with other modes. A load case is generated for each of first number of
modes specified, starting with the specified load case number.
Note: The Individual
Modal Response case generation is not available for SNiP II code response
spectra.
