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plate girder
plate girder  
Hello Mr.Hakimi

for plate girder design according to bs 5950-2000  as “flanges only” method:

I want to find the moment capacity of plate girder with " asymmetrical" flanges ( as shown in the attached picture )

code bs 5950 says ( as “flanges only” method  ) the moment is resisted by the flanges alone  , so  Mc= PyF . SyF   :
where SyF is the plastic modulus of flanges only  , about plastic natural axis (PNA) .

my question is : how do we find the location of PNA ?

I have explained two solutions  in the picture to find Sxf  ( i don't know which one is correct)  :

solution 1 : PNA is the  plastic natural axis of the /whole section / then SxF is for flanges only SxF = Ac.yc + At.yt

solution 2 : PNA is the  plastic natural axis of the flanges / after deleting the web /  then SxF is for flanges only
SxF = Ac.yc + At1.yt1 + At2.yt2     : where Ac= At1+At2

which solution is correct ? ( with little explaination  or reference please  )

thanks

Answer
Dear Hassan

The solution 1 is correct provided that you control the ultimate strain of the steel section if you have a composite section with concrete. The better way is to control with the maximum steel ratio. If your question is about a pure steel solution 1 is ok.

Regards
BEH

Civil Engineering

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Babak Esmailzadeh Hakimi

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I can answer questions in Structural Engineering, Earthquake Engineering, Project Management and Project Control topics!

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I am experienced in the area of earthquake engineering in electrical power industry, i.e. Lifeline Earthquake Engineering. My focus has been on structural seismic design of power installations. Recently I have got involved in project management in the category of constructional engineering.

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Bs degree in Civil Engineering from Ferdowsi University ,1988, Mashhad Iran, Ms degree in Earthquake Engineering, Sharif University of Technology, 1993, Tehran, Iran

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