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Construction Of Special Moment Resisting Frame

Muhammad Hassan

By Muhammad Hassan,
in Seismic Design

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Muhammad Hassan Newbie

Muhammad Hassan

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Hello Guys. According to your opinion is it possible to BUILD SMRF(Special Moment Resisting Frames) in Pakistan satisfying all code requirements? Need comments of senior engineers.
There is practice among Pakistani Designers that they take value of "R" for IMRF intead of SMRF even if there building is in UBC zone 3 or 4.

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UmarMakhzumi Enthusiast

UmarMakhzumi

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as per my understanding if we provide some additional dowel bars that are embedded in both column and beam at joint then it can resist moment. correct me if i am wrong.

Its more than that. You have to see what seismic zone your building lies and have to select the appropriate level of ductility and over-strength. I would recommend skimming through ACI Chapter 21. I have also attached some documents pertinent to the discussion that will help you develop a better understanding of aforesaid. Go through read them and if you have any question post in the forums :)

Goodluck!

How do Beam-Column Joints in RC Buildings Resist Earthquakes.pdf

How do Columns in RC Buildings Resist Earthquakes.pdf

How to make building ductile for Good Seismic Performace.pdf

How do Earthquake Affect Reinforced Concrete Buildings.pdf

What is seismic design philosophy of Buildings.pdf

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WR1 Newbie

WR1

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SMRF has all its provisions listed in ACI chapter 21...this is very important to read all those provisions and its serious to deal with earthquake forces....you cannot ignore a factor as with other design procedures..You have to take the appropriate values from the code. You cannot use R factor of Ordinary or intermediate frame for special frame !

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Khalid Iqbal Newbie

Khalid Iqbal

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SMRF has all its provisions listed in ACI chapter 21...this is very important to read all those provisions and its serious to deal with earthquake forces....you cannot ignore a factor as with other design procedures..You have to take the appropriate values from the code. You cannot use R factor of Ordinary or intermediate frame for special frame !

Rana you are right, but i think question is related to ,"Do we have the capability to implement design requirments in the field"

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WR1 Newbie

WR1

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i didnt get it! what do you mean by the capability? ofcourse if you are going to build SMRF you have to build it in proper way by any means..other wise if you say you dont have the capability or resources to build it then obviously dont build it!

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Khalid Iqbal Newbie

Khalid Iqbal

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i didnt get it! what do you mean by the capability? ofcourse if you are going to build SMRF you have to build it in proper way by any means..other wise if you say you dont have the capability or resources to build it then obviously dont build it!

When you do commercial design in Pakistan, not every bldg has a super construction team. Considering local practise, the best thing is to keep you seismic parameters of the zone you are in and use a lower R Value. This would be little over design but safe.

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asadishaq Newbie

asadishaq

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we sue R=5.5 for IMRF and R=8.5 for smrf,why we reudce the value of base shear as R is inversely proportional to the base shear as per base shear equation.

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asadishaq Newbie

asadishaq

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i know the reason that increasing the R value increase the ductility of frame system..but we cant ignore the high value of base shear in zone 3 and 4..

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Yasir Saleem Newbie

Yasir Saleem

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asad R is structural participation factor for the base shear.we make our structures in two different ways one that whole structure bears the base shear force and still remains in the elastic range there is no plastic hinge formation or non linear range... and other way is that we design our structure such that some part of base shear is resisted by structure and remaining part is shared by the ductility of the structure and structure goes into non linear range and hinges are formed.

what we prefer that our structure behave the second way so it some part remains in the elastic range and other part in plastic.Plastic hinges are only preferred in the higher zones like you mentioned 3 or 4.In low zones we don't make plastic hinges as it makes structure uneconomical so we do more elastic and little bit ductile structure.. So the value of R is kept low as this value is divided by the base shear and the force which comes out is taken by the structure and remaining is taken by the ductile part of structure....In normal zones structures are low ductile and value of R is less.

Now if we go to the higher zones like 3 or 4 what we want is that our structure should be flexible and it resist all the seismic forces and should not fail at any stage. So SMRF structure is preferred and Strong column weak beam structure is designed. So when we take value of R like 8 or 9 it means that base shear divided by R the value we get is resisted by structure and the remaining force is very big goes to the ductile part of the structure so just to get that much ductility or flexibility in the structure plastic hinge phenomena is introduced in the structure. What happens in the plastic hinge that our structure gets all the deformation and it goes to the plastic hinge region but it don't fail or fall down but this is only in very severe cases or very high zones. In plastic hinge we keep very close reinforcement at the beam column joint what happen with this reinforcement that concrete is more confined as we know that in concrete structures failure only occurs due to the cracking of concrete and steel fails at very very late stage so with this extra shear reinforcement we make structure flexible such that large deformations occur but no failure of structure.

So i hope that you will get it now and Plastic hinges are made in SMRF structures as they are specially designed to resist severe seismic actions. and plastic hinges are formed that our structure should not fail at any cost it should go in non linear range ... I suggest you to read some thing about the push over analysis and non linear hinges in structures ...

I hope that now you are much clear what i mean but still if you have any doubts please donot hesitate to ask...

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