AS 3990 Clause 4.3, AS 4100 Clause 5.2.5 and non-uniform compression plates 1

[rather_be_riding]

Clause 4.3 doesn't specifically provide a slenderness limit for edge supported plates under non-uniform tension or compression. For example, a web with compression at one edge and tension at the other. Similarly, AS 4100 specifies slenderness limits for such a case but doesn't provide a value/formula for Ze for such a case (only an outstand or supported in uniform compression). What approach do you use?

 

[bugbus]

Check AS 5100.6 Clause 5.1.4. The approach is to ignore a certain depth of the web in the compression zone, depending on how slender it is.

The only time I've found this to be relevant is for plate girders in old bridges, where an accurate measurement of the web thickness couldn't be made on site (and no details on the original drawings), and so we ended up with some pretty conservative assumption of the plate thickness and a very slender web (> 115 slenderness). I believe most, if not all, rolled and welded sections would have compact webs.

I find the wording in AS 5100.6 very confusing compared to AS 4100. Whereas AS 4100 categorises sections as compact, non-compact and slender, AS 5100.6 only has two categories, compact and "not compact". I've never understood why the approach is different to AS 4100. It's not really clear how AS 5100.6 treats sections with plate elements that fall between lambda_p and lambda_y (a.k.a. non-compact sections in AS 4100). There is apparently no reduction to the effective section modulus as there is in AS 4100. Either way, I tend to use software that checks steel based on AS 4100 and I think it makes the most sense.

 

[rather_be_riding]

Thank you gusmurr; that was helpful. I agree that the 5100 approach seems to be missing something that 4100 gives WRT non-compact sections; however in this case it also provides something 4100 is missing.

I've chased up a copy of 5100 quick smart and read the section in question. I find it strange that the deducted section is the same regardless of the degree of element slenderness. i.e. - if you're just 0.001% above the slenderness limit, you lose the same area that you do if you're 10x beyond it. At both extremes the approach seems to be flawed. I wonder how applicable this approach is to a steel box girder?