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Limit State design - Steel Structures

 
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nazmul1990
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PostPosted: Sun Aug 28, 2016 5:32 pm    Post subject: Limit State design - Steel Structures Reply with quote

Hello All,
I want to design a steel building using Limit State method.
As we know the that Plastic moment  capacity  is found out to get the capacity of member and we use partial Safety factors. Now While I am designing a simply Supported beam,if I am loading it to the extent that plastic moment capacity is generated  and plastic hinge is formed,then "Will the member fail as Mechanism will be formed due to the plastic hinge created in the mid-span?"


Thanks in advance!

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ujjwal9717
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PostPosted: Tue Aug 30, 2016 2:37 pm    Post subject: Reply with quote

if the mechanism formed, then it is the state of failure of the structure.
it will not fulfill the limit state of serviceability.
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nazmul1990
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PostPosted: Tue Aug 30, 2016 3:36 pm    Post subject: Limit State design - Steel Structures Reply with quote

ujjwal9717 wrote:
if the mechanism formed, then it is the state of failure of the structure.
it will not fulfill the limit state of serviceability.




Hello Ujjwal,
My concern is regarding the formation of mechanism only.
In limit state design we are supposed to reach the plastic limit strength of material but a simply supported beam will fail if the plastic hinge is formed,then how/Why to use limit state method in that case?
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nazmul1990
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PostPosted: Tue Aug 30, 2016 3:38 pm    Post subject: Limit State design - Steel Structures Reply with quote

Engineers who have experience in limit state methodology of steel may also reply please Smile
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Abishek_Siingh
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PostPosted: Wed Aug 31, 2016 7:03 am    Post subject: Reply with quote

Just because plastic hinge and mechanism is formed does not mean there will be collapse. The member will undergo plastic strain which may be 10 times the elastic strain. After that the strain hardening stage is still left wherein the member uses it's final reserved capacity. That point which is characterized by its ultimate stress value, if breached, will lead to fracture.


So in simple words, the structure is designed to use its reserved strength above the elastic point but below the strain hardening point and its gives enough time to occupants to leave the building.

Best,

Abhishek
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nazmul1990
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PostPosted: Wed Aug 31, 2016 3:34 pm    Post subject: Reply with quote

Kumar_Abhishek_Singh wrote:
Just because plastic hinge and mechanism is formed does not mean there will be collapse. The member will undergo plastic strain which may be 10 times the elastic strain. After that the strain hardening stage is still left wherein the member uses it's final reserved capacity. That point which is characterized by its ultimate stress value, if breached, will lead to fracture.


So in simple words, the structure is designed to use its reserved strength above the elastic point but below the strain hardening point and its gives enough time to occupants to leave the building.

Best,

Abhishek





Thanks Abhishek  for the concepts...So to sum we can say that:<o></o>

1.Formation of internal hinge in the simply supported structure does not mean its collapse/failure.But it defines its maximum load carrying capacity.<o></o>

2.Secondly to find out the maximum load carryiing capacity of a frame or structural member group ,we need to load it until sufficient number of plastic internal hinges have been created to form mechanism(like in simply supported condition its 1").<o></o>

Having told the above points ,my next concern is how to analyze the structure for limit state/plastic design in Staad pro.Does it take into consideration the distribution of moments and formation of plastic hinges??<o></o>

Thanks,<o></o>
Nazmul<o></o>



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ujjwal9717
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PostPosted: Thu Sep 01, 2016 7:02 pm    Post subject: Reply with quote

if you want to study mechanism formation in detail for different supports and loading conditions , you can refer to any structural analysis book or Duggal S.K , limit state design of steel structure .
i am just referring you to the content.
thanks
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