Signed Results Using the Dominant Mode
Tips & Tricks
In RF-/DYNAM Pro - Equivalent Loads, a signed result option in accordance to the dominant eigenmode is available since version X.06.3039. For the modal combination of results corresponding to the single eigenvalues a quadratic combination rule has to be used. In RFEM and RSTAB, the SRSS and the CQC rule are available. It is only allowed to combine results not loads directly. The reason are the mode shapes which are arbitrarly scaled and signed.
Following from the quadratic result combination is the loss of signs. Internal forces at different position in the model can only be provided as result envelope where the maxima are all positive and the minima are all negative. The result combination is not unique.
To produce a more realistic figure of internal forces the signed results combination option has been introduced. Equivalent loads corresponding to each eigenvalue and mode shape are exported to RFEM or RSTAB into Load Cases. The results in Load Cases are always unique.
If the structure that you consider has a dominant eigenvalue (large effective modal mass factor) the signs of the corresponding Load Case can be applied on the results in the Result Combination. Using this option the Result Combination gets unique, too.
RF-DYNAM Pro - Equivalent Loads can select the dominant mode automatically by means of the largest effective modal mass factor, but you can also select a mode manually. In the exported Result Combination the option ‘According to LC / CO’ is automatically selected and the correct Equivalent Load Case is set.
Independent of RF-/DYNAM Pro this option is available in RFEM/RSTAB for any Result Combination that uses the quadratic combination. The calculation parameters Maximum and Minimum are extended by the option ‘According to LC / CO’.
Further notes and an example are shown in the figure.
- Quadratic Combinations SRSS and CQC with "Keep Sign" Rule
- Result Combinations in Dynamics 2
- Dynamic and Seismic Analysis Software
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Models to Download
Knowledge Base Articles
The National Building Code of Canada (NBC) 2015 Article 188.8.131.52 provides a clear procedure for earthquake methods of analysis.
Product Features Articles
- Response spectra in accordance with different standards
- The following standards are implemented:
- EN 1998-1:2010 + A1:2013 (European Union)
- DIN 4149:1981-04 (Germany)
- DIN 4149:2005-04 (Germany)
- IBC 2000 (USA)
- IBC 2009-ASCE/SEI 7-05 (USA)
- IBC 2012/15 - ASCE/SEI 7-10 (USA)
- IBC 2018 - ASCE/SEI 7-16 (USA)
- ÖNORM B 4015:2007-02 (Austria)
- NTC 2018 (Italy)
- NCSE-02 (Spain)
- SIA 261/1:2003 (Switzerland)
- SIA 261/1:2014 (Switzerland)
- 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)
- NBC 2015 (Canada)
- DTR BC 2-48 (Algeria)
- DTR RPA99 (Algeria)
- CFE Sismo 08 (Mexico)
- CIRSOC 103 (Argentina)
- NSR - 10 (Colombia)
- IS 1893:2002 (India)
- AS1170.4 (Australia)
- NCh 433 1996 (Chile)
- The following National Annexes according to EN 1998-1 are available:
- DIN EN 1998-1/NA:2011-01 (Germany)
- ÖNORM EN 1991-1-1:2011-09 (Austria)
- 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)
- 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)
- Entering user-defined response spectra
- Use of directional response spectra
- 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 separated 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
Frequently Asked Questions (FAQ)
- How do you distinguish between the response spectra with equivalent loads and the response spectra without equivalent loads in RFEM and RSTAB?
- Why do I only obtain the seismic loads calculated in one direction in the case of design with RF‑/DYNAM Pro?
- During the check in RF‑/DYNAM Pro, I get the message "No natural vibration case or dynamic load case has been selected for the calculation." Why?
- How can I enter or read out a response spectrum via the COM interface in DYNAM Pro?
- Is it possible to also create a user-defined response spectrum in RF‑/DYNAM Pro?
- RF-/DYNAM Pro - Equivalent Loads includes the result tables "5.8/5.9/5.10 - Equivalent Loads." Which sum is displayed in the case of the "All mode shapes" option?
- What is the approach for seismic design in RFEM/RSTAB?
- Can I display the equivalent loads generated by the RF‑/DYNAM Pro - Equivalent Loads add-on module?
- In the RF-/DYNAM Pro - Equivalent Loads module, it is possible to take accidental torsional effects into account.. Which calculation approach is behind the calculation? Which values have to be entered for the eccentricities eX and eY?
- How is it possible to consider tension members in RF‑/DYNAM Pro?