STAAD.Pro Help

TR. Response Spectrum Specification per IBC 2006

This command may be used to specify and apply the RESPONSE SPECTRUM loading as per the 2006 edition of the ICC specification International Building Code (IBC), for dynamic analysis. The graph of frequency – acceleration pairs are calculated based on the input requirements of the command and as defined in the code.

General Format

SPECTRUM comb-method IBC 2006 *{ X f1| Y f2Z f3} ACCELERATION 
Note: The data from SPECTRUM through ACC must be on the first line of the command.  The data shown on the second line above can be continued on the first line or one or more new lines with all but last ending with a hyphen (limit of four lines per spectrum).

The command is completed with the following data which must be started on a new line:

{ZIP f8LAT f9 LONG f13| SS f14S1 f15} SITCLASS (f16) (FA f17 FV f18) TL f19


Table 1. Parameters used for IBC 2006 response spectrum
Parameter Default Value Description
X f1, Y f2, Z f3 0.0 Factors for the input spectrum to be applied in X, Y, & Z directions. Any one or all directions can be input. Directions not provided will default to zero.
DAMP f5 0.05
The damping ratio. Specify a value of exactly 0.0000011 to ignore damping.

Optional parameter to use "Missing Mass" method.  The static effect of the masses not represented in the modes is included.  The spectral acceleration for this missing mass mode is the f6value entered in length/sec2 (this value is not multiplied by SCALE). 

If f6is zero, then the spectral acceleration at the ZPA f7frequency is used.  If f7is zero or not entered, the spectral acceleration at 33Hz (Zero Period Acceleration, ZPA) is used.  The results of this calculation are SRSSed with the modal combination results.

Note: If the MISSING parameter is entered on any spectrum case it will be used for all spectrum cases.
ZPA f7 33 [Hz] The zero period acceleration value for use with MISSING option only. Defaults to 33 Hz if not entered. The value is printed but not used if MISSING f6 is entered.
DOMINANT f10 1 (1st Mode) The dominant mode method. All results will have the same sign as mode number f10 alone would have if it were excited then the scaled results were used as a static displacements result. Defaults to mode 1 if no value entered. If a 0 value entered, then the mode with the greatest % participation in the excitation direction will be used (only one direction factor may be nonzero).
Note: Do not enter the SIGN parameter with this option. Ignored for the ABS method of combining spectral responses from each mode.
IMR f11 1 The number of individual modal responses (scaled modes) to be copied into load cases. Defaults to one. If greater than the actual number of modes extracted (NM), then it will be reset to NM. Modes one through f11 will be used. Missing Mass modes are not output.
STARTCASE f12 Highest Load Case No. + 1 The primary load case number of mode 1 in the IMR parameter. Defaults to the highest load case number used so far plus one. If f12 is not higher than all prior load case numbers, then the default will be used. For modes 2 through NM, the load case number is the prior case number plus one.
ZIP f8   The zip code of the site location to determine the latitude and longitude and consequently the Ss and S1 factors.  (IBC 2006, ASCE 7-02 Chapter 22)
LAT f9   The latitude of the site used with the longitude to determine the Ss and S1 factors.  (IBC 2006, ASCE 7-02 Chapter 22)
LONG f13    The longitude of the site used with the latitude to determine the Ss and S1 factors.  (IBC 2006, ASCE 7-02 Chapter 22)
SS f14   Mapped MCE for 0.2s spectral response acceleration.  (IBC 2006, ASCE 7-02 Chapter 22)
S1 f15   Mapped spectral acceleration for a 1-second period.  (IBC 2000, equation 16-17.  IBC 2003, ASCE 7-02 section  IBC 2006, ASCE 7-05 Section 11.4.1)
SITE CLASS f16   Enter A through F for the Site Class as defined in the IBC code.  (IBC 2000, Section 1615.1.1 page 350.  IBC 2003, Section 1615.1.1 page 322.  IBC 2006 ASCE 7-05 Section 20.3)
FA f17   Optional Short-Period site coefficient at 0.2s.  Value must be provided if SCLASS set to F (i.e., 6).  (IBC 2006, ASCE 7-05 Section 11.4.3)
FV f18   Optional Long-Period site coefficient at 1.0s. Value must be provided if SCLASS set to F (i.e., 6).  (IBC 2006, ASCE 7-05 Section 11.4.3)
TL f19   Long-Period transition period in seconds.  (IBC 2006, ASCE 7-02 Chapter 22)

IBC 2006 indicates that the spectrum should be calculated as defined in the IBC 2006 specification. 

comb-method = { SRSS | ABS | CQC | ASCE | TEN | CSM | GRP } are methods of combining the responses from each mode into a total response.

The CQC and ASCE4-98 methods require damping. ABS, SRSS, CRM, GRP, and TEN methods do not use damping unless spectra-period curves are made a function of damping (see File option below). CQC, ASCE, CRM, GRP, and TEN include the effect of response magnification due to closely spaced modal frequencies. ASCE includes more algebraic summation of higher modes. ASCE and CQC are more sophisticated and realistic methods and are recommended.

Square Root of Summation of Squares method.
Absolute sum. This method is very conservative and represents a worst case combination.
Complete Quadratic Combination method (Default). This method is recommended for closely spaced modes instead of SRSS.
Resultants are calculated as:
ωnm ≤ 1.0
Note: The cross-modal coefficient array is symmetric and all terms are positive.
NRC Regulatory Guide Rev. 2 (2006) Gupta method for modal combinations and Rigid/Periodic parts of modes are used. The ASCE4-98 definitions are used where there is no conflict. ASCE4-98 Eq. 3.2-21 (modified Rosenblueth) is used for close mode interaction of the damped periodic portion of the modes.
Ten Percent Method of combining closely spaced modes. NRC Reg. Guide 1.92 (Rev. 1.2.2, 1976).
Closely Spaced Method as per IS:1893 (Part 1)-2002 procedures.
Closely Spaced Modes Grouping Method. NRC Reg. Guide 1.92 (Rev. 1.2.1, 1976).
Note: If SRSS is selected, the program will internally check whether there are any closely spaced modes or not. If it finds any such modes, it will switch over to the CSM method. In the CSM method, the program will check whether all modes are closely spaced or not. If all modes are closely spaced, it will switch over to the CQC method.
indicates that the Acceleration spectra will be entered.
Note: IBC / ASCE 7 does not have provisions for displacement response spectra.
select source of damping input:
  • DAMP indicates to use the f2 value for all modes
  • MDAMP indicates to use the damping entered or computed with the DEFINE DAMP command if entered, otherwise default value of 0.05 will be used
  • CDAMP indicates to use the composite damping of the structure calculated for each mode. You must specify damping for different materials under the CONSTANT specification
Select Linear or Logarithmic interpolation of the input Spectra versus Period curves for determining the spectra value for a mode given its period. Linear is the default. Since Spectra versus Period curves are often linear only on Log-Log scales, the logarithmic interpolation is recommended in such cases; especially if only a few points are entered in the spectra curve.

When FILE filename is entered, the interpolation along the damping axis will be linear.

Note: The last interpolation parameter entered on the last of all of the spectrum cases will be used for all spectrum cases.
This option results in the creation of signed values for all results. The sum of squares of positive values from the modes are compared to sum of squares of negative values from the modes. If the negative values are larger, the result is given a negative sign. This command is ignored for ABS option.
CAUTION: Do not enter DOMINANT parameter with this option.
This option results in the creation of a acceleration data file (with the model file name and an .acc file extension) containing the joint accelerations in g’s and radians/sec2. These files are plain text and may be opened and viewed with any text editor (e.g., Notepad).


The methodology for calculating the response spectra is defined in ASCE7-05, section 11.4.  The following is a quick summary:

  1. Input Ss and S1 (this could have been searched from database or entered explicitly)    
  2. Calculate

    Sms= Fa x Ss


    Sm1 = Fv x S1


    Fa and Fv are determined from the specified site classes A – E and using tables 11.4-1 and 11.4-2.  For site class F, the values must be supplied.  These are required to be provided by the user. You may also specify values for Fa and Fv in lieu of table values.

  3. Calculate

    Sds = (2/3) Sms


    Sd1 = (2/3) Sm1

The spectrum is generated as per section 11.4.5.

Individual Modal Response Case Generation

Individual modal response (IMR) cases are simply the mode shape scaled to the magnitude that the mode has in this spectrum analysis case before it is combined with other modes. If the IMR parameter is entered, then STAAD.Pro will create load cases for the first specified number of modes for this response spectrum case (i.e., if five is specified then five load cases are generated, one for each of the first five modes). Each case will be created in a form like any other primary load case.

The results from an IMR case can be viewed graphically or through the print facilities. Each mode can therefore be assessed as to its significance to the results in various portions of the structure. Perhaps one or two modes could be used to design one area/floor and others elsewhere.

You can use subsequent load cases with TR.32.11 Repeat Load Specification combinations of these scaled modes and the static live and dead loads to form results that are all with internally consistent signs (unlike the usual response spectrum solutions). The modal applied loads vector will be omega squared times mass times the scaled mode shape. Reactions will be applied loads minus stiffness matrix times the scaled mode shape.

With the Repeat Load capability, you can combine the modal applied loads vector with the static loadings and solve statically with P-Delta or tension only.

Note: When the IMR option is entered for a Spectrum case, then a TR.37 Analysis Specification & TR.38 Change Specification must be entered after each such Spectrum case.

See TR. Response Spectrum Specification - Custom for additional details on IMR load case generation.


8 FX 49.035
3 6 FX 98.07
ZIP 92887 SITE CLASS E FA 0.900 FV 2.400 TL 8.000