9th Edition - Design Question for HSS tubes classified as "Slender"

[greauxpete]

Hello,



I was wondering if you someone could help with a design issue I am having with TU8x8x3/16.

The tube is classified as a “Slender” tube due to the high b/t ratio when compared to values in the AISC Manual 8th and 9th edition. I have figured it out in the 13th edition (its more straight forward but would like to compare side by side)



As a result, the AISC specification requires that the effective width (beff) be calculated and then
from the effective width calculate the effective section modulus (See attached spreadsheet).

The problem I am having is, the calculated effective width (beff), is larger than both the actual width, (bactual)

and the AISCM requires that the effective width (beff) be smaller than the actual width (bactual).

Am I comparing the effective width to the correct width? Should I be comparing the effective width

to the actual width (bactual) or the design width (bdesign = bactual – 3t)?



I have attached a spreadsheet with the comments/ question marked up in red.



I would appreciate any help/guidance you can provide.

 

[greauxpete]

Here is the Mathcad attachment.

 

[cap4000]

I have the out of print AISC 1997 HSS Connections Manual. It uses b/t < 35 with Fy = 46ksi. The b is called the flat width and the t design = 0.93t. The t is the nonmial thickness of the tube.

I calculate your b/t = 42.87 and not 39.667 as you have. You have a slender element but I think the 13edition Manual now uses 40 as the limit. I don't have the 13th edition.

Due to the tube thickness varying during fabrication AISC takes an average 7% reduction for the design t value. Thus the 0.93t value is the design t.

You are missing the 0.93 factor in both the denominator and numerator. Be very carefull !!!