wall design thickness 9th vs. 13th manual

huangyhg

wall design thickness 9th vs. 13th manual
i have just noticed difference in wall design thicknesses (as well as section properties) for pipes and tubes between 9th and 13th manual. anybody knows why? thanks
this is an excerpt from an article in modern steel construction:
designating hss
鈥淗ss鈥?is the correct designation for structural tubing, rather than 鈥淭s鈥?(tube shape). in 1990, the industry adopted the term hss to keep in step with producers around the world. in 1997, the steel tube institute and aisc officially implemented this change with the publication of the specification for the design of steel hollow structural sections and the corresponding aisc/sti/aisi hss connections manual.  hss are available in round, square, or rectangular shapes. round shapes are specified using decimal numbers and three decimal points. for example, hss 5.563 脳 0.258 indicates a round hss with an outside diameter of 5.563鈥?and a wall thickness of 0.258鈥? rectangular and square shapes are similarly specified but in terms of rational numbers. for example, hss 5 脳 4 脳 3/8 indicates a rectangular hss with a depth of 5鈥? a width of 4鈥? and a wall thickness of 3/8鈥?
design wall thickness
it鈥檚 important to note that the nominal wall thickness is not the design thickness for designing with hss. as originally noted in the 1997 hss specification (and now incorporated into aisc鈥檚 2005 specification), the design wall thickness should be taken as 0.93 times the nominal wall thickness. this 7% reduction in strength is due to the allowance given by astm a500, which allows for a minus 10% tolerance on wall thickness. since hss producers can procure flat-rolled steel at or near this tolerance, the resulting manufactured sections fall close to this lower bound tolerance on wall thickness. aisc incorporated this practice by reducing the design wall thickness to 0.93 of the nominal thickness for all design tables in the hss connections manual and the lrfd manual of steel construction, third edition.
and just for some additional info - the 13th ed. manual actually uses different design thicknesses to calculate certain properties in the manual such as:
1.  1.5tdes was used for b/t and h/t ratios.
2.  2.0tdes was used for most of the geometric (i, s, etc) properties.
3.  2.25tnominal was used for calculating the "effective flat"
where tdes = 0.93tnom as jae discussed above.  
jae - thanks a lot. btw does 2005 spec mention this?
i don't think the aisc spec mentions it - it may be somewhere in the manual.
thanks, willisv for the extra facts.
it is discussed on p1-5 of the 13th ed. manual.
the cisc cautions using a500 astm sections because of the minium tolerances and has reduced section information for use in canada...
dik
2005 spec:
16.1-223 item 12