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# Civil Engineering/plate girder design

Question

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

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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