S and Z AISC steel design

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S is the section modulus for the member while Z is the plastic section modulus. See your steel manual or a steel design book for their differences.

 
Z is the current books (the 13th edition) way of calculating bending stress. So f_b = M/Z now instead of M/S

Also, Z is usually at least 10% greater than S was. Just gives us a little more to work with (for once).

Now calculating Z is ridiculously complex compared to S so most likely they won't ask you to calculate it (whereupon S = I/c which is pretty straightforward).

 
Last edited by a moderator:
dastuff said:
Now calculating Z is ridiculously complex compared to S so most likely they won't ask you to calculate it (whereupon S = I/c which is pretty straightforward).
Click to expand...
Actually, calculating Z is way simpler than calculating S.

Calculating S - steps

1. Identify location of centroid as a weighted average of component areas

2. Calculate Moment of Inertia about this centroidal axis - Use Parallel Axis Theorem

3. S = I/c

Calculating Z - steps

1. Identify the plastic neutral axis as the line that splits the AREA into two eual parts

2. Z = sum of the first moments of all component areas about the PNA

 
civilized_naah said:
dastuff said:
Now calculating Z is ridiculously complex compared to S so most likely they won't ask you to calculate it (whereupon S = I/c which is pretty straightforward).
Click to expand...
Actually, calculating Z is way simpler than calculating S.

Calculating S - steps

1. Identify location of centroid as a weighted average of component areas

2. Calculate Moment of Inertia about this centroidal axis - Use Parallel Axis Theorem

3. S = I/c

Calculating Z - steps

1. Identify the plastic neutral axis as the line that splits the AREA into two eual parts

2. Z = sum of the first moments of all component areas about the PNA
Click to expand...
Interesting... I haven't had to calculate it often (usually RISA does the leg-work for me) and I guess i went the long route since from what i remember calculating it for an I-beam was no fun. It was probably the case of the book complicating a subject in an attempt to make it simple :)

Thanks for the note, i'll have to look into it more.

And to answer PEin2010, No, I believe Z is used for both ASD and LRFD in the 13th edition (See F2-1 in the specs)

 
Last edited by a moderator:
Z = Plastic Section Modulus (Steel is in plastic range - this means that the whole section is at yield stress)

S = Elastic Section Modulus (Steel is all in elastic range - that means it hasn't passed the yield point yet)

 
Andrewstructure said:
Z = Plastic Section Modulus (Steel is in plastic range - this means that the whole section is at yield stress)
S = Elastic Section Modulus (Steel is all in elastic range - that means it hasn't passed the yield point yet)
Click to expand...
So what are we suppose to use in the NCEES exam.

 

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