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When to consider pinned base for Portal frames

 
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Dr. N. Subramanian
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PostPosted: Wed May 26, 2021 12:27 pm    Post subject: When to consider pinned base for Portal frames Reply with quote

Dear All,


We had several discussions on when to consider pinned base for Portal frames. I just saw an illustration in SCI P 399 regarding this  and I am giving it for the benefit of designers:

In UK a base plate with four bolts outside the profile of the column section, where the column is welded to the base plate, as shown in the enclosed figure is considered as nominally pinned, as per SCI P 399 of the The Steel Construction Institute.

Hope it will be useful!
Regards
Subramanian



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vikram.jeet
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PostPosted: Wed May 26, 2021 1:45 pm    Post subject: Reply with quote

Steel structures Industrial  Buildings , Pre Engineered Buildings

In all above structures industrial having portal frames due to gantry movement for plant activities , it is seen that structures with base plate are taken on hinged base condition. The stability is provided by top roof beams , open web  trusses or steel girders against wind forces which are critical mostly. End connections of roof beams are moment connection in girders and in trusses additional forces to resist .

However I think,
In direction of moment , anchor bolts of base plate on two sides of column , if tightened fully by nuts touching base plate  , would offer  restraint  against rotation , These bolts are designed to take Shear only taking hinge case. There fore I feel it is necessary to tighten the nuts leaving little gap  from.base plate   for  rotation So  that stresses in bolts do not exceed.

Taking fixed base condition  for steel structures  is  always  better as anchor bolts are designed for shear , tension and moment , Though base plate size would be more.  ALSO this arrangement provides  4 nodes resisting horizontal forces vis a viz two nodes in hinged case.

Nuts for industrial buildings are always double nuts.
Just a thought on hinge base / fixed base plate connection
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es_jayakumar
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PostPosted: Wed May 26, 2021 1:58 pm    Post subject: Reply with quote

Sir,
Suppose this is the base of a Mono-Pole structure used for Telecommunication / power transmission. If we assume the base as hinged for design purpose, the structure will be unstable, under horizontal loads. Have we to increase the number of anchor bolts then  ?

regards,
E S Jayakumar
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Dr. N. Subramanian
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PostPosted: Wed May 26, 2021 3:20 pm    Post subject: Reply with quote

Dear Er Jayakumar


More than 2 bolts in a row can be assumed as fixed connection. In fact according to me, the connection shown in the figure will have some fixity,  which will be advantageous for the structure.

Warm regards
Subramanian


es_jayakumar wrote:
Sir,
Suppose this is the base of a Mono-Pole structure used for Telecommunication / power transmission. If we assume the base as hinged for design purpose, the structure will be unstable, under horizontal loads. Have we to increase the number of anchor bolts then  ?

regards,
E S Jayakumar
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vikram.jeet
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PostPosted: Wed May 26, 2021 3:44 pm    Post subject: Reply with quote

Dear Er Jayakumar ji,
Respected Dr NS sir has very  nicely answered that more   than two bolts will have some fixity.

For Monopole structures the connection needs to be totally fixed only as it is case of vertical cantilever striving on fixed base .

In portal  frames , base plate can be hinged / fixed , but even hinged base assumption is not ideal one because as said by Dr NS sir, there will be some fixity .
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Dr. N. Subramanian
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PostPosted: Wed May 26, 2021 7:41 pm    Post subject: Reply with quote

Dear All,

When I read further, in section 11.3.1, I found the following figure for nominally pinned bases, which is better than the earlier figure, as the exact locations of anchors in base plate are shown!. It  is also showing the anchor bolts in the web region of the columns. I do not know why there are two figures for the same definition, one is confusing and the other is correct.

Regards
NS

Dr. N. Subramanian wrote:
Dear Er Jayakumar


More than 2 bolts in a row can be assumed as fixed connection. In fact according to me, the connection shown in the figure will have some fixity,  which will be advantageous for the structure.

Warm regards
Subramanian


es_jayakumar wrote:
Sir,
Suppose this is the base of a Mono-Pole structure used for Telecommunication / power transmission. If we assume the base as hinged for design purpose, the structure will be unstable, under horizontal loads. Have we to increase the number of anchor bolts then  ?

regards,
E S Jayakumar



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vikram.jeet
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PostPosted: Thu May 27, 2021 3:49 am    Post subject: Reply with quote

Sir



The plan view is very clear .earlier sent figure X  sec across web region   the bolts are more spaced than shown in plan/ or flange width is shown lesser, AND the sec sent with plan is L sec of base plate is ok.



Bolts in web region  predominantly transfer shear. Thus hinge connection.


Bolts (with increased plate size  )outside of flanges in addition to
Bolts in web region  , will  make base plate connection fixed.
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pvgraju
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PostPosted: Fri May 28, 2021 6:24 am    Post subject: When to consider pinned base for Portal frames Reply with quote

Dear Sir,

Euro code 5 gives column base rotational stifness value k. Diagram also given. Based on this i think we can decide when the base is fixed,pinned or semi rigid.

Regards,

P.V.Gavarraju
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sakumar79
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PostPosted: Fri May 28, 2021 11:13 am    Post subject: Reply with quote

Dear Sirs,
    Normally, it is practical to consider a pinned base for steel Portal frames unless stability will be an issue. This is because there will be a slight increase in steel cost but considerable saving in foundation cost
    As noted by Er Vikram Jeet, the following simplification can be considered to understand pinned and fixed bases - typically, the stanchion is I section (either rolled section or built up as parallel or tapered). Here, the moment can be considered as an equivalent couple on the flanges and the axial force and shear forces will be carried mostly by the web. Hence, a pinned base will have connections (bolts) at the web while a fixed base will be designed to have connections both at web as well as on either sides of the flange. Sometimes, fixed base may neglect the web portion bolts. Of course, even a pinned base will be usually capable of taking a small amount of fixity, which can be neglected
   In cases such as cantilever posts, fixity is essential for stability of structure. If we provide bolts at web only, it will be able to take only small amount of moment and while it may survive initial conditions, it is dangerous to leave it like that. We need to find the design moment and provide bolts at the flanges for economical design of the joint.

Yours sincerely

S Arunkumar
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vikram.jeet
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PostPosted: Fri May 28, 2021 12:04 pm    Post subject: Reply with quote

Er Arun Kumar has very nicely described.
Flanges carry moment and web carries shear ,predominantly in steel strs.



sakumar79 wrote:
Dear Sirs,
    Normally, it is practical to consider a pinned base for steel Portal frames unless stability will be an issue. This is because there will be a slight increase in steel cost but considerable saving in foundation cost
    As noted by Er Vikram Jeet, the following simplification can be considered to understand pinned and fixed bases - typically, the stanchion is I section (either rolled section or built up as parallel or tapered). Here, the moment can be considered as an equivalent couple on the flanges and the axial force and shear forces will be carried mostly by the web. Hence, a pinned base will have connections (bolts) at the web while a fixed base will be designed to have connections both at web as well as on either sides of the flange. Sometimes, fixed base may neglect the web portion bolts. Of course, even a pinned base will be usually capable of taking a small amount of fixity, which can be neglected
   In cases such as cantilever posts, fixity is essential for stability of structure. If we provide bolts at web only, it will be able to take only small amount of moment and while it may survive initial conditions, it is dangerous to leave it like that. We need to find the design moment and provide bolts at the flanges for economical design of the joint.

Yours sincerely

S Arunkumar
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