Steels_ Metallurgy and Applications, Third Edition

Low-carbon structural steels 185

Corrosion

protecUon 10.1%

Fire 28.9%

protection

Supply and 7.1%

erection

Fabrication 23.3%

Steel 36.8%

Detail 3%

109.4%

r~ 100%

30% I

~-.-.._,~~_, 87.8% :

27%

,"4o/=

,, L

1976 1981

]

1987/88

Estimated cost at s 543/tonne s 985/tonne s

survey date

I! 19.7%

7.5%

28.6%

25.6%

2.6%

Real cost 87/88 s 1455/tonne s 1330/tonne s

(Using GDP deflator)

Figure 2.22 Cost breakdown of multi-storey building frames (at 1987/88 prices)

Building code requirements

In the UK, steel-framed buildings are specified in BS 5950:1985 Structural use

of steelwork in buildings. This refers to the use of structural steels specified in

BS 4360 (Weldable structural steels) and the selection of a minimum level of

toughness in relation to the yield strength of the steel, its thickness and service

conditions.

The design strength Py may be taken as 1.0 x Ys but cannot be greater than

0.84 x Us where Ys and Us are the minimum yield strengths and tensile strengths

specified in BS 4360. The main types of steel used in building construction are

Grades 43, 50 and 55 and, for convenience, BS 5950 tabulates the permissible

design strengths for these grades, according to material thickness. This table is

reproduced as Table 2.16.

Brittle fracture needs to be considered in locations which are subjected to

tensile stresses in service. In this respect, the first step is the determination of

the appropriate k factor, according to the level of tensile stress and location of

material. This is determined from Table 2.17.

Having determined the thickness of material required from a consideration of

design strength (Table 2.16), the required grade of steel can then be selected

from a table in BS 5950, reproduced here as Table 2.18. This table includes

the weathering grades (WR50 A, B and C) and differentiates between situations

with k factors of 1 and 2, the limiting thickness/grade requirement being more

severe when k = 1. This table also differentiates between the internal and external

situations in buildings and applies providing the temperatures concerned do not

fall below -5~ and -15~ respectively. When the steel is subjected to lower

temperatures or where the steel grade or thickness is not covered in this table,