Frame structures in Finite Element Analysis

Strength check of beam members and beam connections.

Modeling each connection in detail can be a very time consuming job. Model your FE model with simple beam elements and check immediately the frame structure and its connections according to international standards. Use the FEMDS add-on and read the results into FEMAP or ANSYS. This is the preferred way.

But better: Dimension your FE model using the FEMDS.COM hand calculation sheets for member and joint check:

Generally no need for detailed FE models (= time safer).

In the past, lots of tests were performed to investigate the strength of closed hollow sections (CHS). Also for the strength of connections of CHS to CHS lots of literature is available. Safe valuable engineering time by modelling a simple beam model and check if the design complies with the standards? If no detailed model is required, why wasting time to do so?

For the connection there is some hassle though. This concerns the analysis how the load is transferred through the connection. A distinction is made between T/Y, K and/or cross connection joints. When branch members transmit part of their load as T/Y, K and or cross connections, the adequacy of the connection shall be determined by interpolation of the proportion of the available strength of each in total. This differs per loadcase and can't be adequately done by hand. The FEMDS software solves this for the user.

Be aware of P-delta effect.

It is necessary in all cases to consider P-delta effects in the evaluation of individual members subject to compression and flexure. This combination of loads reinforces each other which makes that only considering a first order analysis can be too optimistic. Deflection due to applied loads generate extra moments increasing the deflection which increases the moment again.  This loop of interactions can't be covered by a simple linear analysis. This requires a second order analysis.

AISC 360-10 chapter C gives guidelines when it is permitted to skip the second order analysis or to perform a approximate analysis (using only a first order analysis which needs to be slightly adjusted), see also AISC 360-10 appendix 8. Generally we recommend to check the most critical load cases to rerun with a non-linear solver and check the members and connections with the FEMDS software. This is the most easy solution to a thorough strength analysis.

Interested how to perform such checks, see also the more detailed explanation of how the software works.