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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 +

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  )


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.


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Babak Esmailzadeh Hakimi


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


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.

Sinam Consulting Engineers Co.

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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