Equivalent static loads are generated separately for each relevant eigenvalue and excitation direction. They are exported to static load cases to perform the linear static analysis in RFEM/RSTAB.
RF-/DYNAM Pro - Equivalent Loads | Calculation
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RF-/DYNAM Pro - Equivalent Loads allows you to determine the loads due to equivalent seismic loads according to the multi‑modal response spectrum method. In the example shown here, this was done for a multi‑mass oscillator.
The response spectrum analysis is one of the most frequently used design methods in the case of earthquakes. This method has many advantages. The most important is the simplification: It simplifies the complexity of an earthquake to such an extent that an analysis can be carried out with reasonable effort. The disadvantage of this method is that a lot of information is lost due to this simplification. One way to mitigate this disadvantage is to use the equivalent linear combination when combining the modal responses. This article explains this option by describing an example.
Both the determination of natural vibrations and the response spectrum analysis are always performed on a linear system. If nonlinearities exist in the system, they are linearized and thus not taken into account. Straight tension members are very often used in practice. This article will show how you can display them approximately correctly in a dynamic analysis.
In order to consider inaccuracies regarding the position of masses in a response spectrum analysis, standards for seismic design specify rules that have to be applied in both the simplified and multi-modal response spectrum analyses. These rules describe the following general procedure: The story mass must be shifted by a certain eccentricity, which results in a torsional moment.
![Reduction of Building to Cantilever Structure: The individual mass points represent the floors. The deflection due to the normal compression forces shown in (a) is (b) converted into equivalent moments of displacement or shear forces [2].](/en/webimage/009762/2420261/01-en-png-12-png.png?mw=350&hash=dd36dc43123116724231958668ad6cdcb13a0169)
- Response spectra in accordance with different standards
- The following standards are implemented:
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EN 1998-1:2010 + A1:2013 (European Union) -
DIN 4149:1981-04 (Germany) -
DIN 4149:2005-04 (Germany)
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IBC 2000 (USA) -
IBC 2009-ASCE/SEI 7-05 (USA)
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IBC 2012/15 - ASCE/SEI 7-10 (USA)
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IBC 2018 - ASCE/SEI 7-16 (USA)
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ÖNORM B 4015:2007-02 (Austria) -
NTC 2018 (Italy) -
NCSE-02 (Spain) -
SIA 261/1:2003 (Switzerland) -
SIA 261/1:2014 (Switzerland)
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SIA 261/1: 2020 (Switzerland)
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O.G. 23089 + OG 23390 (Turkey) -
SANS 10160-4 2010 (South Africa) -
SBC 301:2007 (Saudi Arabia) -
GB 50011 - 2001 (China) -
GB 50011 - 2010 (China)
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NBC 2015 (Canada) -
DTR BC 2-48 (Algeria) -
DTR RPA99 (Algeria)
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CFE Sismo 08 (Mexico) -
CIRSOC 103 (Argentina) -
NSR - 10 (Colombia) -
IS 1893:2002 (India) -
AS1170.4 (Australia) -
NCh 433 1996 (Chile)
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- The following National Annexes according to EN 1998‑1 are available:
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DIN EN 1998-1/NA:2011-01 (Germany)
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ÖNORM EN 1991-1-1:2011-09 (Austria)
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NBN - ENV 1998-1-1: 2002 NAD-E/N/F (Belgium) -
ČSN EN 1998-1/NA:2007 (Czech Republic) -
NF EN 1998-1-1/NA:2014-09 (France) -
UNI-EN 1991-1-1/NA:2007 (Italy)
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NP EN 1998-1/NA:2009 (Portugal) -
SR EN 1998-1/NA:2004 (Romania) -
STN EN 1998-1/NA:2008 (Slovakia) -
SIST EN 1998-1:2005/A101:2006 (Slovenia) -
CYS EN 1998-1/NA:2004 (Cyprus) -
NA to BS EN 1998-1:2004:2008 (United Kingdom) - NS-EN 1998-1:2004 + A1:2013/NA:2014 (Norway)
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- User-defined response spectra
- Direction-relative response spectrum approach
- Relevant mode shapes for the response spectrum can be selected manually or automatically (5% rule from EC 8 can be applied)
- Generated equivalent static loads are exported to load cases, separately for each modal contribution and separately for each direction
- Result combinations by modal superposition (SRSS and CQC rule) and direction superposition (SRSS or 100% / 30% rule)
- Signed results based on the dominant eigenmode can be displayed
Equivalent static loads are generated separately for each relevant eigenvalue and excitation direction. They are exported to static load cases to perform the linear static analysis in RFEM/RSTAB.
The equivalent load analysis calculation generates load cases and result combinations. The load cases include the generated equivalent loads, which are subsequently superimposed in result combinations. First, the modal contributions are superimposed with the SRSS or CQC rule. Signed results based on the dominant mode shape are possible.
Afterwards, the directional components of earthquake actions are combined with the SRSS or the 100% / 30% rule.
The input parameters relevant for the selected standards are suggested by the program in accordance with the rules. Furthermore, it is possible to enter response spectra manually. Dynamic load cases define a direction of response spectra effects and the structure eigenvalues that are relevant for the analysis.
I need many mode shapes to achieve a sum of 90% effective modal mass. Can I limit the number of mode shapes for generating equivalent loads?
RF‑/DYNAM Pro - Forced Vibrations and RF‑/DYNAM Pro - Equivalent Loads perform the multi-modal response spectrum analysis. What are the differences?
What method is applied in the RF‑/DYNAM Pro - Equivalent Loads add-on module?
I have used the RF‑/DYNAM Pro add-on module to generate the governing result combinations of seismic loads. What is the next procedure to perform the design of the individual structural components?
What should be considered when using a failure of columns under tension in the RF‑/DYNAM Pro – Equivalent Loads add-on module?
I would like to perform seismic design of an irregular structure. Does RFEM automatically carry out the modal analysis (consideration of multiple vibration modes) on the spatial system, or do I have to use special settings for this?
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