Channel dimensions used for tabulated properties

[JStephen]

I understand the tabulated cross-sectional properties and the tabulated dimensions for beams and channels don't necessarily correspond. Is there any resource that shows exactly what dimensions were used in calculating the cross-sectional properties? Looking at channels at the moment, but information for wide flange beams would also be useful.

A similar issue arises in that the "average thickness" of a channel could be interpreted several ways.

 

[SlideRuleEra]

JStephen - I don't have the current Manual of Steel Construction, but in the past (AISC 9th Edition, Page 1-9) calculations for rolled shapes (except angles) were made using the following conservative assumptions:

Published properties are based on the smallest theoretical size fillets produced.

Published dimensions for detailing are based on the largest theoretical size filets produced.

"The flange thickness given in the tables for S, M, C and MC shapes is the average flange thickness." I suppose this still leaves room for interpretation.

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[dhengr]

Jstephen:
To add to what SRE said, you have to look at the mill rolling tolerances for the various shapes and size groupings to pick the min. dimensions for that particular member, which will give you the min. (safe/conservative) section properties for design. But then, this may be gilding the lily a bit, since you probably don’t know your load to within an ounce per square mile either.

 

[JStephen]

I was thinking I had seen this come up before, but didn't find any answers with a quick search, either, so I asked again. I kind of figured somebody somewhere would have published this info after all these years!

Based on SRE's statement, which is what I remembered being the case, you can fairly easily work out a fillet radius that best matches the properties for wide-flange beams. I haven't played with this enough to know if you can set one consistent radius and match every property (complicated by roundoff on the tabulated results.)

For channels, you can interpret that "average thickness" two or three different ways, there is a fillet at the web, and appears to be a small fillet at the toe as well, so you've got several variables to work with, which makes it harder to back out the dimensions used.

Where this comes up, is in deducting a corrosion allowance from each surface of the beam, and trying to calculate the sectional properties of the corroded steel remaining- and it becomes more complicated if you don't have the full dimensions to begin with.

If I remember correctly, some of the torsional properties, which work their way into bending stresses in the newer AISC code, are based on the radius of the maximum circle that can be inscribed into that fillet area, so they're fairly sensitive to that fillet radius.

 

[SlideRuleEra]

I just remembered that for historic sections the precise calculated values for each producer are given in AISC's 1953 book "Iron and Steel Beams - 1873 To 1952". You can get that (public domain) book on my website.

AISC's current "Design Guide 15" is an update of the 1953 book, but I have not checked to see if it has the same level of detail. For the most part, Design Guide 15 "condensed" that type info.

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