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Base Plate for Box column

 
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Bubalan
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Joined: 04 Aug 2011
Posts: 84
Location: Tokyo, Japan

PostPosted: Mon Jun 09, 2014 9:11 am    Post subject: Base Plate for Box column Reply with quote

Dear Sefians,

Iam designing a Base plate for a Box section column,

Support assumed in Staad model is pinned;

But, since the bolts are arranged outside in all four directions, should it not be like fixed condition. if it is fixed in condition, should i design the base plate for 2 moments (taking the reactions assuming the support as fixed in staad) or designing the BP for max moment is sufficient.

Professional advice required, since its a 6500Ton  boiler structure.

regards
Bubalan Sundaram
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umeshrao
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Joined: 23 Aug 2010
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Location: Bangalore, India

PostPosted: Mon Jun 09, 2014 12:09 pm    Post subject: Re: Base Plate for Box column Reply with quote

Bubalan wrote:
Dear Sefians,
Iam designing a Base plate for a Box section column,Support assumed in Staad model is pinned;But, since the bolts are arranged outside in all four directions, should it not be like fixed condition. if it is fixed in condition, should i design the base plate for 2 moments (taking the reactions assuming the support as fixed in staad) or designing the BP for max moment is sufficient.Professional advice required, since its a 6500Ton  boiler structure.regards Bubalan Sundaram


Dear Mr. Sundaram,
You may arrive at the area of base plate to distribute load on pedestal considering load on the box section. provide only two bolts to take shear only, presume reaction is only compression on pedestal and no tension. Project base plate in shorter direction just sufficient to weld box section to base plate, about 20 or 25 mm.
regards
Umesh Rao
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Dr. N. Subramanian
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PostPosted: Mon Jun 09, 2014 7:05 pm    Post subject: Re: Base Plate for Box column Reply with quote

Dear Er Bubalan,

If the anchor bolts are located outside the column section, the base plate has to be designed for AF and BM. Of course, you may provide this detail for axially loaded columns also. It is always better to provide 3 number anchor bolts per side of the base plate, when BM is acting. See Fig. 9.50 of my book on Design of Steel Structures, Oxford Univ. Press for different types of base plates. Section 9.17 and Section 13.12 of the book deal with the design of axially loaded base plates and eccentrically loaded base plates respectively.

Best wishes,
NS

Bubalan wrote:
Dear Sefians,

Iam designing a Base plate for a Box section column,

Support assumed in Staad model is pinned;

But, since the bolts are arranged outside in all four directions, should it not be like fixed condition. if it is fixed in condition, should i design the base plate for 2 moments (taking the reactions assuming the support as fixed in staad) or designing the BP for max moment is sufficient.

Professional advice required, since its a 6500Ton  boiler structure.

regards
Bubalan Sundaram
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Bubalan
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Joined: 04 Aug 2011
Posts: 84
Location: Tokyo, Japan

PostPosted: Tue Jun 10, 2014 2:04 am    Post subject: Base Plate for Box column Reply with quote

Thanks to Umeshrao sir and Subramaniam Sir for your advice,

I have read your book on Base plates, in that you have mentioned
" the moment in the direction of greater projection is reduced by the co-existence moment at right angles. Poisson`s ratio of 0.3 is used to allow for this effect"

In case of my structure, its a square box column of loads 732tons in axial direction and Mx is around 84.5Tm and Mz is 20.5Tm. does the coexistence criteria occurs for this scenario also?
So, is it enough to design for Mx moment alone with 8 to 12 bolt config with rib plates?

Regards
Bubalan Sundaram
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mtamil
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PostPosted: Tue Jun 10, 2014 4:54 am    Post subject: Reply with quote

Hi Bubalan,

I have come across a Chinese design of a boiler structure with heavy box sections, wherein the baseplate is completely integrated with Concrete Pedestal. In this method, A sole plate is welded with the Pedestal reinforcements and grouted first. Later, the column with small base plate is erected and completely welded with the sole plate. Some foundation bolts were also given for erection purpose. The condition here is the column is “fixed” to the supporting pedestal.

1. I think it is practical to assume the base is pinned, if we have alternate load path. For example frames completely triangulated (braced) shall not through high moments even if the supports assumed to be “fixed”. Therefore, irrespective of the bolt arrangement the support can be assumed as pinned with small error.
2. In case, the columns at the lower most level are not triangulated (braced) then, based on the arrangement of bolts, the support to be assumed as pinned or fixed and the baseplates shall be designed so.
3. It is also possible to emulate a pinned base with bolts on the peripheral bolts by raising  the bolting level. See the attached sketch.

This is my understanding. Experts may pl. opine.

Regards
Tamilarasan



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Bubalan
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PostPosted: Tue Jun 10, 2014 5:35 am    Post subject: Base Plate for Box column Reply with quote

Dear Tamilarasan,

Thanks for the post. Our foundation arrangement is raft, and the steel column is 1 m inside the concrete which is reinforced.
The horizontal forces are to be taken care by the concrete raft as a whole, that is the assumption (since the horizontal loads are very large to the tune of 700 Tons , base shear is around 3500Tons )
As you have said, the axial loads play a major role in the design. (i have compared the reactions for both supports, only axial governs)

My worry is about the correct approach towards the actual structure.
if we put the structure inside the concrete will it not create moments?
if it creates moments, do we have to design for two sides moment or only one said by Subramaniam sir.

suggestions welcome
regards
Bubalan Sundaram
Tokyo
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bsc_rao
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PostPosted: Tue Jun 10, 2014 12:12 pm    Post subject: Reply with quote

Dear BuBalan,

If your baseplate is fixed to the concrete raft/pedestal by bolts on all four sides, then it is bi-directionally fixed and you will have to design it for P, Mx and Mz. Please then also make sure that the welds between the box section and the baseplate are capable of transferring the forces from the box column to the baseplate and that the baseplate itself is then capable of transferring those forces to the bolts, by having the suitable thickness.

Designing a pinned connection for a box section is possible - but only if you detail
it as a pin. The base plate itself can have bolts on all sides to fix it to the concrete, but the junction between the baseplate and the box column will need to be detailed as a proper hinged joint. A problem that arises here is that the structure will behave as pinned in let's say the X-direction, but what about the Z direction? That would not be pinned, and so you would have to design the base plate for the Mz moment.

I hope I have made myself clear. At least this would be my approach to your problem.

regards
BSC Rao
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bijay sarkar
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PostPosted: Tue Jun 10, 2014 3:45 pm    Post subject: Reply with quote

Dear Bubalan,

When whole base plate along with a portion of the steel column is inside the raft by about 1 Meter, then i think, question of type of base plate connection comes into play after RCC raft around & upside the base plate fails & gets spalled  out i.e. RCC raft fails. So raft failure should be taken care of.
However, the column is certainly fixed with the raft.

Furthermore, Boiler Structure as a whole is a kind of mechanical equipment with top hung four sided furnace pipe walls and for its better performance, control of deflection is utmost necessity. So normally, structures are braced in both the directions minimising the requirement of transfer of moments to pedestals. In Boiler Structure column, high Axial Load with negligible moment in a column base, theoretically, does not require any holding down bolt. In such cases, we provide four bolts at four corners of the base plate over RCC pedestals, as adivised by Umesh Rao Sir.

With Regards,

Bijay Sarkar
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Dr. N. Subramanian
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PostPosted: Thu Jun 12, 2014 6:41 pm    Post subject: Re: Base Plate for Box column Reply with quote

Dear Er Bubalan,

The statement " Eqn. 9.66 takes into account plate bending in two directions. The moment in the direction of greater projection is reduced by the co-existence moment at right angles. Poisson`s ratio of 0.3 is used to allow for this effect" has been made in pp. 756 with respect to Eqn. 9.66 of the book, which is thickness of base plate subjected to concentric load!

In case of AF with BM, the pressure diagram is not uniform and hence you have to refer to Section 13.12. Use eqn. 13.43 for the max. pressure calculation.  you have to select a size and use an iterative procedure to see that the permissible bearing strength of concrete below the base plate is not exceeded.

Best wishes,
NS
Bubalan wrote:
Thanks to Umeshrao sir and Subramaniam Sir for your advice,
I have read your book on Base plates, in that you have mentioned
" the moment in the direction of greater projection is reduced by the co-existence moment at right angles. Poisson`s ratio of 0.3 is used to allow for this effect"

In case of my structure, its a square box column of loads 732tons in axial direction and Mx is around 84.5Tm and Mz is 20.5Tm. does the coexistence criteria occurs for this scenario also?
So, is it enough to design for Mx moment alone with 8 to 12 bolt config with rib plates?

Regards
Bubalan Sundaram
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Bubalan
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Joined: 04 Aug 2011
Posts: 84
Location: Tokyo, Japan

PostPosted: Mon Jun 23, 2014 6:28 am    Post subject: Reply with quote

Thanks to all you who has actively participated in clearing practical problems to budding engineers.

Special thanks to Dr.Subramaniam sir, for his involvement to educate the engineers.


regards
Bubalan Sundaram
Tokyo
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