topic Re: Doubly inclined columns in Robot Structural Analysis Forum

Doubly inclined columns

shannon.lloyd — Tue, 03 Sep 2019 16:06:15 GMT

Hi there,

I have a 'K' frame which has been designed so in theory the central strut restrains the incline in one direction (the main 45 degree incline), and the outer ties restrain the incline in the other direction (11.72 degrees from vertical) (see screenshot). The only thing is, the Robot model does not use the central strut, meaning the entire tie force from both inclines is being transferred through the two outer 'ties'.

This causes infinite displacements as the outer ties have not been designed to restrain that amount of load. Can anyone advise what can be done to model this accurately?

By the way, the axial load applied at the top of each column is 18MN and the vertical elements are SHS600x600x50 and horizontal elements are SHS500x500x36.

Hope someone can help,

Shannon

Re: Doubly inclined columns

Krzysztof_Wasik — Wed, 04 Sep 2019 09:42:50 GMT

Hi @shannon.lloyd

Could you share model for problem verification?

Re: Doubly inclined columns

shannon.lloyd — Wed, 04 Sep 2019 09:55:41 GMT

Hi,

Please see attached. Please note the model is rotated 11.5 degrees clockwise due to importing directly from AutoCAD. The 'Ux Uy fixed' supports are where the structure is supported by floor slabs.

Hope you can help!

Shannon

Re: Doubly inclined columns

Krzysztof_Wasik — Wed, 04 Sep 2019 09:57:57 GMT

Hi @shannon.lloyd

I cannot see attached file

Re: Doubly inclined columns

shannon.lloyd — Wed, 04 Sep 2019 09:58:55 GMT

Sorr, it didn't work - please see link below.

https://drive.google.com/file/d/1ETmYjNsycuejNVSCMCvM9HjR8Cn26yXw/view?usp=sharing

Many thanks,

Shannon

Re: Doubly inclined columns

shannon.lloyd — Wed, 04 Sep 2019 11:17:47 GMT

Sorry, it didn't work - please see link below.

https://drive.google.com/file/d/1ETmYjNsycuejNVSCMCvM9HjR8Cn26yXw/view?usp=sharing

Many thanks,

Shannon

Re: Doubly inclined columns

Krzysztof_Wasik — Wed, 04 Sep 2019 13:37:27 GMT

Hi @shannon.lloyd

Frame was statically instable

I have changed releases and support system to calculate it

See attached model

Let me know if you have any comments

Re: Doubly inclined columns

Krzysztof_Wasik — Wed, 04 Sep 2019 13:38:41 GMT

Hi @shannon.lloyd

Once again file, previous was incorrectly zipped

Re: Doubly inclined columns

shannon.lloyd — Wed, 04 Sep 2019 14:21:39 GMT

Hi,

Thank you so much for your help, I really appreciate it. Looking through the new releases and supports you've made, it makes sense now.

However the inner strut on each of the 3x K-frames is still taking barely any load when they should be taking 18MN * sin30 / sin(90-30) = 10.39MN (I meant to say previously that the columns are inclined at 60 degrees, not 45 degrees, on the main incline).

I am hoping that the outer ties will only need to support the tie force of 18MN * sin11.72 / sin(90-11.72) = = 3.73MN (I know this is the force pushing out from the centre of the whole structure, which I then resolve to find the axial tie force in the outer ties).

To get around this, is there a way I can set a limit on the axial load a bar can transfer? Sorry if this is confusing!

Thanks again for your help,

Shannon

Re: Doubly inclined columns

Krzysztof_Wasik — Wed, 04 Sep 2019 15:07:15 GMT

Hi @shannon.lloyd

Strut axial forces are about between 18 and 20 Mn

see screenshot

Horizontal takes between 8 and 10 MN (except of marked connectors)

It seems to be reasonable. Since you have not defined horizontal forces it makes sense.

You should go through steel dimension procedure to see elements capacity.

Steel design members process is presented in the following tutorials

Steel design

Re: Doubly inclined columns

shannon.lloyd — Fri, 06 Sep 2019 14:23:59 GMT

Hi,

Thanks so much for your help, it has been so useful. I am now investigating other forces in the structure if the inclined columns are removed (accidental loads) but can't get realistic values for forces/displacement and I think it is also due to the releases I am using. Do you know what I need to do to make this work?

Thanks again,

Shannon

https://drive.google.com/open?id=1cFAc8pzew4xGk3dMyptslMmUo-WzuCCl

Re: Doubly inclined columns

Krzysztof_Wasik — Fri, 06 Sep 2019 15:47:29 GMT

Hi @shannon.lloyd

Model is built with series of unstable systems

See below

additionally

bar 3 has releases in all directions on both ends (it is not allowed)

additionally columns

8to11 19 56

are unstable horizontally due to hinged connection in column center.

Releases and supports should be updated to remove static instability