Deflection issues when beams are split for modelling purposes. Comparison of deflections between split and non-split beams

In this article we will take a look at deflection issues when beams are split for modelling purposes, by comparing the deflections between split and non-split beams.

Modelling Approach

Two beams modelled in different ways.

1. Top beam, a physical beam of 18m span - supports at the ends as expected.
2. Bottom beam, split into three beams (each beam joined with a fully fixed support to act as a moment connection between beams). Note that doing this also has implications for design since the individual beams are checked for LTB etc. and effective lengths and moment profiles will differ.

 

Solver View

By reviewing the Solver Scene view

Blue circles denote the support locations for the beams placed.

Other nodes are the midspan pdelta nodes at the centre of each beam (required for displaying mode shapes etc.)

 

Loading

Same UDL load applied to both

 

Global Deflection results.

The software in the 3D Results view will show the global deflections at node points where support/beams/braces/nodes meet. Hence, you see results at the blue highlighted locations noted above.

 

Local deflection results. (Right click beam > Open Load Analysis View)

Viewing local deflections of individual members show local results for individual members

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Support ends of beams are taken as having zero displacement as you are looking at local displacement between support points.

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This means the single 18m span beam will show results as expected.

  

However, the beams split into separate beams will not be as the Engineer expects.

1st beam (L>R)

  

2nd beam (L>R)

  

3rd beam (L>R)

  

Method of modelling is important

From the above it is clear that you must model beams as physical beams of the actual required full span length where possible.

If you do model as separate beams, then you must ignore local deflection checks undertaken by the program and check the local deflections manually outside of the program. You can take the global deflection values from the 3D Results view to do a simple check against a limit.

Local deflections can clearly be obtained from the Load analysis view from the single 18m span beam and results obtained anywhere along its length by using the red slider and reviewing the results in the properties window.

  

From the 3D Results view however, it is only possible to obtain results at node points as previously advised above.

If you wish to obtain 3D global results at "other" positions then a node needs to be created.

You could create a brace, analysis element or similar, to connect into the element to produce the node. You could also provide a nominal cantilever or column, something to produce a node.

The screenshot below shows an analysis element.

  

Global deflection results can then be viewed at the new node point.

 

Consideration for effective length factors for Lateral Torsional Buckling checks

Whilst not discussed in depth here, consideration will also need to be given to LTB restraint factors if the beam is subject to lateral torsional buckling checks, since the software will always assume a support point/member end is torsionally restrained. Hence, for the split beam it is considering the individual beam span, L=6m, with an effective length of 1.0L, using the moment profile for the individual beam. For the single beam, the span, L = 18m, is being used with an effective length of 1.0L, with the moment profile for the full beam span.