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corne_mo
Posts: 19
Registered: ‎07-28-2011
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Bar deflection code check, cantilever check box usage?

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01-19-2012 10:18 PM

When setting up bar types for the code check for servicability limit state, we can choose to check deflection, nodal displacement or both. But in the are where we set the maximum allowable deflection, there is a checkbox for "cantilever".

 

I have run some tests using a one end fixed cantilever beam and a simply supported beam on two supports. For both I've used the same code check parameters (modified the original beam settings). If I don't check the cantilever checkbox the deflection is calculated correctly for the simply supported beam and not for the cantilever beam. If I check the box, deflection is calculated correctly for both.

 

In my opinion deflection is simply that, maximum displacement of the bar in correspondence to its original position. The difference between cantilever and non-cantilever is no code issue. So it must have something to do with how Robot calculates the deflections. I have searched the help but couldn't find any explanation of the use of this check box and the necessity of it.

 

To me it looks like this checkbox is there to cover a Robot inaccuracy during deflection calculations. Also, during my tests, checking the box gives correct results for the simple supported beam.

 

Can anybody tell me exactly why this checkbox is there and what its exact function is?

And how for example do I have to use it when I have a beam on two simple supports but with an overhanging (cantilever) part. I always model this beam as one beam (for example, node 1 = 0,0; node 2 = 2000,0; node 3 is 3000,0. Node 1 and 2 are supports and bar 1 is running from node 1 to node 3. How do I have to use the cantilever checkbox in this case? Or do I first have to examen if it is the part between nodes 1 and 2 that has the most deflection (no cantilever, so unchecked box), or the part between nodes 2 and 3 (cantilever, so checked box)?

 

Thanks in advance for your replies.

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Artur.Kosakowski
Posts: 4,586
Registered: ‎12-17-2010

Re: Bar deflection code check, cantilever check box usage?

01-19-2012 11:56 PM in reply to: corne_mo

 

Can anybody tell me exactly why this checkbox is there and what its exact function is?

 

In fact there's significant difference in the way the deflection is checked. If the check box is not marked then the deflection is checked as the distance between the line that connects the beam ends in the position they are after applying the load and the point along the beam that 'moves' the most whereas for cantilever the difference in displacement of the end nodes is checked as shown on the attached picture.

  

And how for example do I have to use it when I have a beam on two simple supports but with an overhanging (cantilever) part. I always model this beam as one beam (for example, node 1 = 0,0; node 2 = 2000,0; node 3 is 3000,0. Node 1 and 2 are supports and bar 1 is running from node 1 to node 3. How do I have to use the cantilever checkbox in this case? Or do I first have to examen if it is the part between nodes 1 and 2 that has the most deflection (no cantilever, so unchecked box), or the part between nodes 2 and 3 (cantilever, so checked box)?

 

The best is to model the part between the supports and the cantilever as separate bars and assign different bar types to them so that you can decide on the correct SLS parameters.

 



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Artur Kosakowski
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