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AD 329: Dealing with connection design in a 3D frame analysis

For a structure modelled in a 3D frame analysis, the ends of every beam element have six degrees of freedom and hence there are six possible forces or moments that can be transferred through each connection. The default assumption of a normal 3D general frame analysis package (as opposed to a design model package used
for whole building design) is that all degrees of freedom are constrained; then each connection transfers two shear forces (forces orthogonal to the member axis), two bending moments, a torsional moment (about the member axis), and an axial force. However, in many practical cases of building frames, the magnitudes of some of these forces and moments are very modest and can be neglected. Where this is the case, some of the constraints can be omitted and the following offers advice on modelling the connections in a 3D general frame analysis.

i)

Ensure that the end restraints to beam elements reflect the behaviour of the connections. For example, a normal moment connection between a beam and a column flange has significant moment capacity and stiffness for bending in the plane of the beam and column but is weak out of plane. The model should reflect  that behaviour and hence the ends of the beam element should be restrained about its major axis direction and pinned about its minor axis direction.

ii) If the presence of ‘pinned connections’ about the beam minor axis at the ends
of beam elements causes instabilities in the global analysis model, provide end restraints and then consider whether the out of plane moments are small enough to be ignored. ‘Small’ is a question of judgment, based on the magnitude of the forces and the scale of the connection.

iii) Provide torsional restraints at both ends of a beam member, to avoid instability in the analysis model. Unless there are torsional loads on a beam, the resulting torsional moments will normally be sufficiently small that they can be ignored.

iv) In all cases, the design assumptions must be compatible with the physical details
– and the practical connection details should be carefully considered before making assumptions in the design model.

If the frame analysis gives significant out of plane bending and torsional moments in a 3D analysis model the reasons should be carefully examined – it may be that an unstable part of the structure is being supported by twisting of a member and torsion on a connection, when the more appropriate approach would be to re-configure the structure.

Note that published connection design models only deal with major axis bending, shear and axial force. Configurations that give rise to significant minor axis effects or torsional effects will require more detailed design of the connections.

Further guidance is given in SCI publication
P148 (Modelling of steel structures for computer analysis, 1995) and chapter 3 (Connection design programmes) of BCSA publication No 41/05 (Steel details).

Original text provided by
Alan Rathbone of CSC (UK) Ltd

Contact: Abdul Malik
Tel: 01344 636525
Email: advisory@steel-sci.com 

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