# Document Information

Document Type:FAQ

Product(s):RAM Structural System

Version(s):Any

Original Author:Bentley Technical Support Group

# How does a dynamic analysis using response spectra for seismic loads differ from a static analysis using equilivalent lateral forces?

Dynamic load cases are not story forces in the same sense as the static load cases. When a dynamic load case with a code selected or user-generated response spectrum is analyzed, the program finds the Eigen solution and determines the modal response for each of the modes included in the analysis. The modal response is based on the response spectra assigned to the dynamic load case, which defines the acceleration vs. period spectrum curve. The modal responses are then combined in some fashion to determine the total response of the structure.

# How can I change the number of modes used in the dynamic analysis?

Create an Eigen solution dynamic load case and enter the number of modes to consider. The number of modes can be changed at any point by changing this load case in RAM Frame (Loads > Load cases).

# How many modes should I consider?

Generally, building codes require that the dynamic analysis include a sufficient number of modes to obtain a combined modal mass participation of 90% of the total building mass in each of the two orthogonal directions (see ASCE 7-05 12.9.1). The mass participation can be found in the Period & Modes Report under the section titled “Modal Effective Mass Factors” The “%Mass” values represent the mass participation for one particular mode; the “%SumM” values represent the cumulative mass participation. The X and Y modes with the highest mass participation represent the fundamental modes; the program will use the period and frequency associated with these modes in the calculation of the static seismic and wind loads when these values are chosen to be calculated by the program.

There is not a fixed number of modes that need to be specified in order to obtain the 90% mass participation. Rigid diaphragms have 3 degrees of freedom for each diaphragm (x-translation, y-translation, and z-rotation). If you need to include more modes than 3*number of diaphragms to obtain 90% mass participation, then you may need to increase the stiffness or look for instabilities in the model.

Note that when semirigid or pseudo- flexible diaphragms are used, there are 2 mass degrees of freedom for each node (x-translation and y-translation). As a result, default number of modes used by the program may be very large when no Eigen solution dynamic load case is created. In such cases, the number of modes can be changed as noted above.

# When I look at the Period & Modes Report, I see that the cumulative mass participation for rotation is always 0% no matter how many modes are included in the analysis?

If a diaphragm includes a two-way deck or it is defined as a semirigid diaphragm, it is meshed and represented with finite (shell) elements. Each node of the finite elements has a nodal (point) mass associated with it. In other words, the diaphragm mass is represented with a network of spatially distributed nodal masses. Note each node includes point mass defined in the global X and Y-directions but not include rotational mass moment of inertia. Because the array of masses does not include rotation, the mass participation for this degree of freedom will always be 0. For the same reason, the mass participation for the rotational degree of freedom will also be 0 when pseudo-flexible diaphragms are used.

It should be known that this type of modeling still suffices for capturing all essential dynamic properties. In other words, the proposed solution accurately captures any dynamic actions related to rotational inertias or any twisting modes due to having center of rigidity and mass center at different locations.

**What scale factor should I apply to the response spectrum?**

The response spectrum analysis should be factored by the quantity I/R for both forces and drifts. Some codes require the dynamic forces (but not drifts) to be scaled so that the dynamic base shear is at least a specified percentage of the base shear calculated from the equivalent lateral force procedure (see ASCE 7-05 12.9.4, for example). The total base shear is reported in the Building Story Shear Report (Report > Building Story Shear). When the ground level is set to a level other than the base of the model, it is best to use the value reported for the level immediately above the specified ground level.

# Should I use SRSS or CQC modal combination?

The CQC method is recommended as the SRSS method can give inaccurate results for 3D structures. When 2D structures are analyzed, the methods will produce similar results.

# Does the dynamic analysis consider accidental torsion?

Most building codes require the inclusion of accidental torsion for both the equivalent static load procedure and dynamic analysis. The code requirements for evaluating accidental torsion for dynamic load cases are implemented in RAM Frame when rigid diaphragms are used. To include eccentricity, set the eccentricity to “+ and –" for the x and y directions in the Response Spectra dialog when the dynamic load case is created. The % eccentricity is defined in the Mass dialog in RAM Frame (Loads > Masses). It should be noted that accidental torsion effects can be included in analysis for only rigid diaphragms. For pseudo-flexible diaphragm, it is assumed that a flexible diaphragm is not able transmit diaphragm torsional moments, and hence accidental torsion effect is ignored for flexible diaphragms. In RAM Structural System v14.0.4, accidental torsional effects are also not considered for semirigid diaphragms. These effects are included in v14.2.2.

# Why do the analysis results for all members in the model have positive values?

The modal response (nodal deflections, member forces, reactions) determined from a response spectra analysis have signs associated with them. However, once the analysis results are determined by combining the modal results using either the SRSS or CQC method, each response has only a positive value. Because the sign of the result is important for member design, there is an option in the RAM Frame (General > Criteria) to consider the sign in the results. When this option is selected, the sign of the analysis result will match the sign from the predominant modal result. You should not design continuous footings, gusset plate connections, or other elements based on the forces from multiple members using dynamic results unless the sign of the analysis result has been considered.

# Why don’t the values reported for the building story shear correspond to the values reported for the change in story shear?

The combination of modal results can affect interpretation of the results, especially building story shear and change in story shear. The program calculates a total story shear and a change in shear for each mode in the analysis. The results are each combined using either the SRSS or CQC combination. As a result, the reported change in shear is not linearly related to the reported building shear. For the same reason, the reported building story shears should not be expected to match the sum of the reported frame story shears. If one is trying to determine equivalent static forces to apply to the diaphragms to match the total base shear, the reported shear from one level should be manually subtracted from the shear reported in the adjacent level rather than using the change in shear values reported in the Building Story Shear Report.

# How do I generate load combinations with dynamic loads cases?

Dynamic load cases are not included in the load combination templates included with the program. However, you can create load combinations with dynamic load cases using the custom load combinations dialog. You may find it helpful to first generate load combinations with the templates using equivalent lateral force seismic load cases (E) and then modify the load combinations to use the dynamic load case (Dyn) by changing E to Dyn. You can also copy and paste load combinations from another load combination dialog or text document into the custom load combination dialog or create a custom load combination template to include the dynamic load cases.

# How can I account for 100/30 orthogonal load effects when using dynamic load cases?

Experts have recommended using a SRSS combination of two orthogonal response spectra analyses to determine the critical forces in a dynamic analysis (see reference below). This method is not implemented in the current version of RAM Structural System. You will need to use load combinations to account for this code requirement.

“A Clarification of the Orthogonal Effects in a Three-Dimensional Seismic Analysis,” E.L. Wilson, I. Suharwardy, and A. Habibullah, EERI Earthquake Spectra, Vol. 11, No. 4, Nov. 1995.

# See Also

Structural Product TechNotes And FAQs

STAAD.Pro Response Spectrum FAQ

# External Links

Bentley Technical Support KnowledgeBase

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