AISC Steel Shape Densities, and M4x3.45 vs M4x3.2 1

[cal91]

This is purely out of curiosity.

I was checking the densities of all the steel shapes in the AISC database to see how close they are to 490 PCF by taking the Weight / Area * 144. Just for fun.

I noticed that closed shapes such as HSS and Pipe have densities around 525 pcf, while all other shapes have around 490 pcf.

Did anyone realize this? Is the density of A500 and A53 steel 525 pcf instead of 490?

Also things to note are:

Lighter shapes have more variance (likely rounding errors).

Pipe12XS has the highest density at 539 pcf.

M4x3.2 has the lowest density at 456 pcf.

This made me realize that the M4x3.2 is the exact same shape as the M4x3.45 EXCEPT it weighs 0.25 plf less. Every other single property is the same. Does anyone know why?

 

[SlideRuleEra]

Steel shapes, W, S, M, HP, etc., have been allowed + 2.5% variation on both area and weight. At 490 lb/ft³ that is a range of 478 lb/ft³ to 502 lb/ft³, which matches your graph for shapes very nicely.

ASTM A500 permits wall thickness to be as much as 10% less than the specified nominal thickness compared to a 2.5% variation permitted in wide-flange shapes. To account for this larger variation, AISC Specification Section B4.2 requires that the design wall thickness must be taken as 93% of the nominal wall thickness.

This makes steel "appear" to be 7% heavier than it is. Therefore 490 lb/ft³ x 1.07 = 524 lb/ft³
The 524 lb/ft³ value is near the median of the HSS cluster on your graph. I suppose the spread around that median is from expected variations (similar to the allowable steel shapes variations.)

ASTM A1085-13 is changing all this. See this Modern Steel Construction article: Link

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[wannabeSE]

I am guessing the discrepancy for HSS is because the area is calculated using 93% of the nominal wall thickness while the weight uses 100% of the wall thickness. 93% of 525pcf is 488pcf. See AISC 360 section B4.2 for the requirement to use 93% of the wall thickness for HSS design.

 

[cal91]

That makes sense. Thanks!

What about the M4x3.2 being the same shape as the M4x3.45, yet weighing 0.25 plf less?

Any ideas?