Plastic Neutral Axis calculation of a three segment composite section 1

[ingdott]

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?

 

[BAretired]

Well, perhaps I am being extra dumb today, but you have indicated that dcarr and I have both interpreted the problem correctly, yet he is talking about a horizontal member while I was talking about a vertical member.

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

 

[rb1957]

this is a might confusing ...

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

 

[dcarr82775]

If I understand things correctly, the horizontal member of the frame is made up of the three individual pieces shown in the sketches.

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.