Steels_ Metallurgy and Applications, Third Edition

Low-carbon structural steels 183

this discussion on the use of steel in bridges will be confined to that part of the
code dealing with avoidance of brittle fracture.
As a safeguard against brittle fracture, BS 5400:1982 specifies the maximum
thickness of steel that can be used in bridge tension members, with respect to
the various grades of steel listed in BS 4360 (Weldable structural steels) and
the minimum operating temperature of the bridge. The various stages in the
determination of this temperature are outlined below:

1. The first stage is to determine the minimum shade air temperature for the
   location of the bridge from isotherm maps, based on Meteorological Office
   data.


2. This initial value is then adjusted for height above sea level by subtracting
   0.5~ per 100 m. Additionally, there may be the need to take account of
   locations where the minimum temperatures diverge from published data, e.g.
   in frost pockets where the minimum may be substantially lower than the
   published value or in coastal and some urban areas where the minimum
   temperature may be higher.


3. The minimum effective bridge temperature (MEBT) is then derived from a
   table in which the minimum shade air temperature is adjusted to take account
   of the type of bridge construction, e.g. steel or concrete decking.


4. Finally, the U value for the bridge is determined by rounding down the MEBT
   to the next impact test temperature in BS 4360, i.e. a value of- 17~ would
   be rounded down to -20~

Each part of a bridge which is subjected to applied stress must be classified
according to the following criteria:

9 Type 1 - Any part subjected to an applied stress greater than 100 N/mm 2 and
which has either
(a) any weld connection
(b) weld repair not subsequently inspected
(c) punched holes not reamed.
9 Type 2- All parts subjected to applied stresses which are not of Type 1.

Stress calculations will have provided information on the combination of steel
thickness and yield strength that will satisfy the required design strength and
the appropriate sub-grade of steel in BS 4360 that provides the required level of
impact strength can then be derived in two ways:

1. From a table in BS 5400 which provides a correlation between the limiting
   thickness for various grades and the U value-the minimum effective bridge
   temperature- differentiating between Type 1 and Type 2 stress conditions.


2. Directly from BS 4360, having calculated the required impact strength require-
   ments in the following manner:

for Type 1 Cv > % t

• 355 2

for Type 2 C v > ay t

• 355 4