Limitations of Seismic Design
The following limitations apply:
- Where seismic design and detailing is required this is only supported in Tekla Structural Designer for the ACI/AISC and Indian Head Codes.
- It is up to the user to assess whether framing is split horizontally or vertically, whether system specific requirements need to be assessed - like mixed system moment frames, whether diaphragms are rigid or flexible - in all instances, the user will need to make the necessary adjustments for the situation in hand. The software does not handle these situations automatically.
- Linear modal analysis with non-linear element properties - currently the modal analysis is limited to a linear model so all non-linear elements are set to be linear.
- ELF can be run as 1st or 2nd order analyses, however if the Fundamental Period is determined using modal analysis the modal analysis is always run as 1st order.
- The RSA analysis itself is a 1st order linear process. For the 2nd Order RSA Seismic analysis, the peak responses are enveloped around the static results for 2nd Order Linear Analysis. Thus when the analysis is set to 2nd order in Design Options, in real terms the results are actually RSA Seismic + 2nd Order.
- Non-linear models (i.e. those containing any non-linear elements) are beyond the scope of the RSA method, so only the ELF method can be used for these.
- We do not consider any of the standard methods for structurally accommodating seismic actions - e.g. base isolators, damping systems
- We do not consider more accurate methods of analysis like time history analysis. As a result there are some situations with very tall buildings and very irregular buildings that Tekla Structural Designer does not cater for.
- Diaphragms - rigid and semi-rigid diaphragms (meshed floors) are available and it is the user's responsibility to ensure they are modelled suitably. Rigid diaphragms are only allowed in limited circumstances and, so called, 'flexible diaphragms' can be modelled as semi-rigid diaphragms with extremely low stiffness. Force transfer into and out of the diaphragm is not checked.
- Collector elements - no checks included.
- Non-structural elements - no checks included.
Specific limitations of steel seismic design
- Coincident V & A braces giving X type are beyond scope
- Various other requirements not checked
- e.g. V & A braces are restrained at their intersection
- e.g. tension braces resist between 30% and 70% of total horizontal force
- e.g. forces in restraining members not checked
- Connections are not designed
- Potentially unconservative results can be obtained for non-linear models - which requires the use of non-linear analysis and the ELF Analysis Procedure. For further details see product bulletin PBTSD-2101-1
A specific limitation of RC seismic design
- While checks can be done in both directions they are direction specific where applicable - there are no biaxial checks.