Composite Beam Plastic Neutral Axis 1

[Ganesh Persaud]

Hello Everyone,
I'm using aisc design manual, table 3-19 to do a composite beam design. My question is, how do you calculate the PNA of a composite section?

Thank you.

 

[Stenbrook]

If you look at page 3-12 in the 14th edition steel manual, it goes through the composite design theory that the tables are based off. On page 3-14 they have a diagram showing the fores in the composite beam and the PNA location. The PNA is located such that the total compression force is equal to the total tension force in the member. So, if your effective B is very large, your compression will be large, thus your PNA will be higher up in the beam section so that there is more steel in tension.

 

[271828]

The following are the basic steps.

Determine the maximum shear, V', that can be transferred between the steel beam and concrete slab. This is the minimum of the three values from Equation I3-1 from the 15th ed. AISC Specification.

The tensile resultant in the steel equals V'.

If V' < FyA for the steel beam, then the top part of the steel section is at Fy in compression and the remainder is at Fy in tension. Solve the following system of equations for the tensile and compressive resultants in the steel:

Ts - Cs = V'
Ts + Cs = FyA

The magnitudes of Ts and Cs will be true for only one elevation of the PNA. The PNA might be in the web or flange.

Edit: I added an example to show this process.

https://files.engineering.com/getfi...-d3459cb79f3c&file=Composite_Beam_Example.pdf

 

[Ganesh Persaud]

Thank you all for the helpful response, the example made it easier to see it