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 Response Spectrum Analysis.

• 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 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. 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).

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 Log-Log scales, the logarithmic interpolation is recommended in such cases; especially if only a few points are entered in the spectra curve.

• 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.

Linear scale factor by which the spectra data will be multiplied. Usually to factor g’s to length/sec² 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 un-normalized 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 un-normalized 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.

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.

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.

If the minimum base shear value exceed the calculated response spectrum base shear, then all analysis results of the response spectrum load case are scaled by the ratio of the minimum base shear to the calculated response spectrum base shear. This multiplying factor will be noted in the output.

Used to define the subsoil conditions based on which the response spectra will be generated. Based on the subsoil conditions the soil types may be of three kinds

• Type A: for Rock or stiff deposits of sand
• Type B: for deep deposits of medium dense sand,gravel or medium stiff clays.
• Type C: Loose cohesionless soil deposits or deposits with soft to medium stiff cohesive soil.

Please refer section 3.2 of Eurocode8 for detailed guidelines regarding the choice of soil type.

Select either a Elastic or Design response spectrum for the loading type.

Two types of response spectra curve can be generated based on either RS1 (for response spectra type 1 curve) or RS2 (for response spectra type 2 curve) f.

Used to define the subsoil conditions based on which the response spectra will be generated. Based on the subsoil conditions the soil types may be of three kinds

• Type A : rock or other rock-like geographical formation.
• Type B: very dense sand, gravel or very stiff clay.
• Type C: Deep deposits of dense or medium dense sand, gravel or stiff clay.
• Type D: Deposits of loose-to-medium cohesionless soil or of predominantly soft to firm cohesive soil.
• Type E: Surface alluvium layer

Please refer section 3.2 of Eurocode 8 for detailed guidelines regarding the choice of soil type.

Defines the subsoil conditions on which the response spectrum will be generated.

1. Non-weathered rock and rock-like geological formation or permafrost subsoil.
2. Weathered rock or deep deposits of medium dense sand, gravel or medium stiff clays.
3. Loose cohesion less soil deposits or deposits with soft to medium stiff cohesive soil.

Defines the subsoil conditions on which the response spectrum will be generated.

1. Non-weathered rock and rock-like geological formation or permafrost subsoil.
2. Weathered rock or deep deposits of medium dense sand, gravel or medium stiff clays.
3. Loose cohesion less soil deposits or deposits with soft to medium stiff cohesive soil.

• Frequent
• Fortified
• Rare