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Steel Designers' Manual - 6th Edition (2003)
434 Beams
In building design when using hot-rolled sections, for which relevant properties
are tabulated, 2 it will normally be sufficient to ascertain a section’s classification and
moment capacity simply by referring to the appropriate table. An example illustrates
the point.
Example
Using BS 5950: Part 1 1 determine the section classification and moment capacity for
a 533 ¥ 210 UB82 when used as a beam in (1) S275 steel and (2) S355 steel.
(1) From Reference 2, p. 129, section is ‘plastic’ and Mcx = 566 kN m
Alternatively, from Reference 2, p. 26, b/T = 7.91, d/t = 49.6
Since T = 13.2 mm from Reference 1, Table 9, py = 275 N/mm 2 and e = (275/py) 1 – 2
= 1.0
From Reference 1, Table 11, limits for plastic section are b/t � 9e, d/t � 80e;
actual b/t and d/t are within these limits and section is plastic.
From Reference 1, clause 4.2.5, Mc = py S
= 275 ¥ 2060 ¥ 10 -3
= 566.5 kNm
(2) From Reference 2, p. 303, section is compact and Mcx = 731 kN m
Alternatively, from Reference 2, p. 26, b/T = 7.91, d/t = 49.6
Since T = 13.2 mm from Reference 1, Table 9, py = 355 N/mm 2 and e = (275/py) 1 – 2
= 0.88
From Reference 1, Table 11, limits for plastic section are b/T � 9e, d/t = 80e,
which give b/T � 7.9, d/t � 72; actual b/T of 7.91 exceeds 7.9 so check compact
limits of b/T � 10e, which gives b/T � 9; actual b/T of 7.91 meets this and section
is compact
From Reference 1, clause 4.2.5, Mc = py S
= 355 ¥ 2060 ¥ 10 -3
= 731.3 kN m
Thus whichever grade of steel is being used, the moment capacity, Mc, will be the
maximum attainable value corresponding to the section’s full plastic moment capacity,
Mp. Provided that S275 material is used, the beam is capable of redistributing
moments, since the plastic cross section behaves as illustrated in Fig. 13.4(a) and
so could be used in a plastically designed frame. Use of the higher strength S355
material, while not affecting the beam’s ability to attain Mp, precludes the use of
plastic design since redistribution of moments cannot occur due to the lack of rotation
capacity implied by the behaviour illustrated in Fig. 13.4(b). For a continuous
structure designed on the basis of elastic analysis to determine the distribution of
internal forces and moments or for simple construction, the question of rotation
capacity is not relevant and in both cases moment capacity should be taken as Mp.
The fact that the section is plastic for S275 material is then of no particular relevance;
a more appropriate classification would be to regard the section as compact or better.