Why are intermediate restraints not picked up for cantilever beams?
Not version-specific
Tekla Structural Designer
Environment
British Standard
Eurocode
Question
When I define intermediate restraints on my steel cantilever beams, I notice that the effective lengths used for the relevant checks are ignoring the intermediate restraints I have defined. Why is this?Answer
The Development team made the decision that in reality suitable restraint cannot be provided for cantilevers.The basis for this restriction is that for BS 5950 design. Clause 4.3.5.5 and Table 14 of BS 5950 allow intermediate restraints to be taken into account only when the end condition at the tip is type 4.
With this being relatively unusual and difficult to achieve, we have implemented the safe approach and assumed that a type 4 tip will not be achieved.
In Eurocodes the NCCI SN009 is more lax, but it is unclear whether this is a mistake or whether further consideration of the rules in BS 5950 allowed a more optimistic approach.
As it stands in the software, intermediate restraints will be ignored for cantilevers.
Two work arounds exist:
- Alter the 1st sub beam flange factor for the span (it will not always be the top one if it’s a continuous beam with multiple spans) to suit the worst case effective length.
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- Create the cantilever as separate elements between the points of restraint setting the end releases of each element as appropriate.