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Tower Base Plate

 
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A B Malla
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Joined: 26 Mar 2017
Posts: 9

PostPosted: Thu Feb 13, 2020 8:36 am    Post subject: Tower Base Plate Reply with quote

Dear all,

PFA, figure of base plate in one of the tower leg.

1)To my understanding, if we see it as base plate connected to one leg it behaves as fixed support. but, if we see it in over all tower, one leg may carry either axial tension or axial compression so, it behaves as pinned support.
     so, it should be assigned as pinned support in STAAD, and should be designed for bending stress due to axial loads in leg. how far my understanding is correct?


2)In Leg splicing how much gap is feasible from fabrication point of view.

3)In splicing, if two leg members (say 8mm thk & 6mm thk) are getting connected, there is a gap of 2mm, can we provide 2mm packing plate or there is any other option.



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es_jayakumar
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Joined: 24 Nov 2011
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PostPosted: Thu Feb 13, 2020 3:24 pm    Post subject: Reply with quote

Er. Malla,
As the tower base-plate is tightly secured to the foundation by a number of foundation bolts, the real behaviour of the supports will be fixed (even if the bolt holes are slotted). However, there will be little error in analysing it, taking supports as hinged. As the external loads are assumed to act at the joints (nodes), the members will not develop significant BM in either case. In brief, you can analyse the support as either hinged or fixed or partially fixed, without much difference in the values of axial forces in members.  The attached STAAD file is that of a tower design. You can check yourself, whether the axial forces in members  change significantly when the support condition is switched between hinged and fixed.
You can provide packing plates in the case mentioned by you.

E S Jayakumar



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A B Malla
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Joined: 26 Mar 2017
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PostPosted: Fri Feb 14, 2020 4:24 am    Post subject: Reply with quote

Dear sir,

Thank you for your valuable response.
Yes there is no significant change in axial forces by switching support conditions. i presume that is because of global forces rather than local forces. Also, if we consider support condition as fixed, should we design leg member connecting to base plate for Moments?

Regards
A B Malla
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mallesh_ng
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Joined: 27 Dec 2013
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PostPosted: Fri Feb 14, 2020 6:35 am    Post subject: TOWER DESIGN Reply with quote

Hi, In my point of view you have to consider structure as a whole, In that case two legs will be in tension & compression.

Since we are using angle members, which are not good in transferring moment , so its better to consider connection as pinned & to transfer only axial force to ground.

Base plate should be designed for bending produced by axial force, which is no where related to fixity of the column because all the bolts will be in tension in a particular leg.(Fixity matters when base plate with partial bolts are in tension)

Since most of the time columns will be in compression it advice to provide packer plate in the gap between two members.




Regards
Mallesh



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A B Malla
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PostPosted: Fri Feb 14, 2020 9:23 am    Post subject: Reply with quote

Thank you Er. Mallesh.


i was also thinking the same. your pictorial representation made it clear.

Regards ,
A B Malla
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