Steel Designers Manual - TheBestFriend.org

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Steel Designers' Manual - 6th Edition (2003)
Cross-sectional considerations 407
The second method leads to a significantly higher result, being only some 1%
below the fully effective figure. The main reason for this is that the first method
applies the same reduction in strength, based on the proportions of the most slender
plate element, to the whole of the cross section, whereas the second method makes
a reduction only for the plate elements which are slender.
Both BS 5400: Part 3 and BS 5950: Part 5 employ a method for slender sections
in compression that uses the concept of effective area. Its implementation has been
illustrated in general terms by the second method of the above example. The bridge
code prohibits the use of slender outstands by specifying the absolute upper limits
on b/t given in Table 15.2 with no procedure to cover arrangements outside these,
and determining the effective area of the cross section by summing the full areas of
any outstands and compact webs and the effective areas, KcAc, of the slender webs.
Values of Kc are obtained as a function of b/t from a graph.
It is worth emphasizing that for design using hot-rolled sections the majority of
situations may be treated simply by ensuring that the proportions of the cross
section lie within the semi-compact limits. Reference 1, in addition to listing flange
b/T and web d/t values for all standard sections, also identifies those sections that
are slender according to BS 5950: Part 1 when used in either S275 or S355 steel.
Table 15.3 lists these.
Table 15.3 Sections classified as ‘slender’ when used as struts
Section S275 S355
All except All except
Universal 1016 ¥ 305 ¥ 487 1016 ¥ 305 ¥ 487
beam 1 1016 ¥ 305 ¥ 437 1016 ¥ 305 ¥ 437
1016 ¥ 305 ¥ 393 1016 ¥ 305 ¥ 393
914 ¥ 419 ¥ 388
610 ¥ 305 ¥ 238 610 ¥ 305 ¥ 238
610 ¥ 305 ¥ 179
533 ¥ 210 ¥ 122
457 ¥ 191 ¥ 98
457 ¥ 191 ¥ 89
457 ¥ 152 ¥ 82
406 ¥ 178 ¥ 74
356 ¥ 171 ¥ 67 356 ¥ 171 ¥ 67
356 ¥ 171 ¥ 57
305 ¥ 165 ¥ 54 305 ¥ 165 ¥ 54
305 ¥ 165 ¥ 46
305 ¥ 127 ¥ 48 305 ¥ 127 ¥ 48
305 ¥ 127 ¥ 42 305 ¥ 127 ¥ 42
305 ¥ 127 ¥ 37
254 ¥ 146 ¥ 43 254 ¥ 146 ¥ 43
254 ¥ 146 ¥ 37 254 ¥ 146 ¥ 37
254 ¥ 146 ¥ 31
254 ¥ 102 ¥ 28
254 ¥ 102 ¥ 25
254 ¥ 102 ¥ 22
203 ¥ 133 ¥ 30 203 ¥ 133 ¥ 30
203 ¥ 133 ¥ 25 203 ¥ 133 ¥ 25
203 ¥ 102 ¥ 23 203 ¥ 102 ¥ 23
178 ¥ 102 ¥ 19 178 ¥ 102 ¥ 19