Plastic Neutral Axis calculation of a three segment composite section
Plastic Neutral Axis calculation of a three segment composite section
(OP)
I want to calculate the plastic neutral axis of the section in the attachment (let's assume the materials are the same). I know that the areas above and below the PNA should be equal.
I want to write an excel sheet which could calculate the PNA with the dimensions entered by user. If it was sth like a T-beam, it would be easy, I think it would be enough to write the following :
"If PNA is within the flange, solve expression 1, If not, solve expression 2", and I would get the axis as a result of this If loop.
However for the case attached, I am not sure what I could do.. Should I write a more complicated if loop? Any ideas?
I want to write an excel sheet which could calculate the PNA with the dimensions entered by user. If it was sth like a T-beam, it would be easy, I think it would be enough to write the following :
"If PNA is within the flange, solve expression 1, If not, solve expression 2", and I would get the axis as a result of this If loop.
However for the case attached, I am not sure what I could do.. Should I write a more complicated if loop? Any ideas?






RE: Plastic Neutral Axis calculation of a three segment composite section
Pick one of the materials (I would go with the highest E) and "transform" the other pieces of the section to that material using n=Estiff/Epart. Now that you have the section as a single material finding the PNA should be relatively straightforward.
RE: Plastic Neutral Axis calculation of a three segment composite section
When I have certain dimensions, there is no problem, I can find the PNA with several trials by hand.. But I want to write a simple code with which the location of PNA can be calculated with several inputs of dimensions.. I am not sure about the right algorithm...
RE: Plastic Neutral Axis calculation of a three segment composite section
RE: Plastic Neutral Axis calculation of a three segment composite section
An expert is a man who has made all the mistakes which can be made in a very narrow field
RE: Plastic Neutral Axis calculation of a three segment composite section
If area top flange > area web + area bottom flange
...PNA is in top flange
Else If area bottom flange > area web + area top flange
...PNA is in bottom flange
Else
...PNA is between flanges
End If
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RE: Plastic Neutral Axis calculation of a three segment composite section
RE: Plastic Neutral Axis calculation of a three segment composite section
RE: Plastic Neutral Axis calculation of a three segment composite section
BA
RE: Plastic Neutral Axis calculation of a three segment composite section
my 2c anyway
RE: Plastic Neutral Axis calculation of a three segment composite section
If the pieces are not attached to each other, the compression flange will buckle at a very low stress. Then, the compression portion of the web will buckle at a very low stress.
The Plastic Neutral Axis calculation would have no meaning. My 2c.
BA
RE: Plastic Neutral Axis calculation of a three segment composite section
However, in the composite case, the plastic state would be like the second attachment.
Then, doesn't this show us that the buckling of the lower part would take place earlier when the section acts composite, since a bigger portion of the element is under compression?
RE: Plastic Neutral Axis calculation of a three segment composite section
RE: Plastic Neutral Axis calculation of a three segment composite section
BA
RE: Plastic Neutral Axis calculation of a three segment composite section
RE: Plastic Neutral Axis calculation of a three segment composite section
RE: Plastic Neutral Axis calculation of a three segment composite section
BA
RE: Plastic Neutral Axis calculation of a three segment composite section
RE: Plastic Neutral Axis calculation of a three segment composite section
RE: Plastic Neutral Axis calculation of a three segment composite section
RE: Plastic Neutral Axis calculation of a three segment composite section
If my interpretation is correct, the three members act separately, each taking load according to stiffness. The contribution of the two walls normal to the applied load can be neglected as they have negligible stiffness compared to the shearwall parallel to the applied load.
If beam ABCD is extremely stiff, then wall AE will be stressed in tension, DH in compression while BCFG deflects under both bending and shear. In any event, the concept of a Plastic Neutral Axis for the combined group has no validity. Don't waste your time trying to write a program to calculate it.
BA
RE: Plastic Neutral Axis calculation of a three segment composite section
i read the 2nd pic to show a I-beam in normal bending (the top flange in tension, the bottom one in compression)
the 1st pic looks like the OP, the three elements are independent, with the stresses showing normal bending (but these couldn't develop as the horizontal flanges are too flexible)
the 3rd pic does show very much ... the arrow could be the load, maybe being applied laterally to the upper flange so the flange is in in-plane bending and the composite section has a torque applied to it
RE: Plastic Neutral Axis calculation of a three segment composite section
If that is the case, the three pieces are not composite between the columns and shearwall. You have (3) individual members, not (1) member. Each will be loaded by their relative stiffness.