The Design of Modern Steel Bridges - TEDI

Loads on Bridges 69
minimum and maximum temperatures, respectively, for every 100 m height.
Local peculiarities like frost pockets, sheltered areas, urban or coastal sites
should also be taken into account. The minimum temperature in the bridge
structure is usually lower than the minimum shade air temperature by 2 Cto
4 C for bridges with steel orthotropic decks and higher by 1 Cto8C for
bridges with concrete decks; the maximum bridge temperature is higher than
the maximum shade air temperature by between 9 C and 20 C for bridges with
steel decks and by up to 11 C for bridges with concrete decks. The difference
between the bridge and the shade air temperatures depend upon the latter and
also on the type and depth of surfacing provided on the bridge deck; data for
these differences are tabulated in the British code[2]. Within this range of the
bridge temperatures, the variation with respect to the particular datum temperature
at which restraint was imposed on the bridge during its construction
determines the magnitudes of thermal stresses.
In the AASHTO code[5], a range of bridge temperatures of 18 C to
þ49 C is specified for a moderate climate and 34 Ctoþ49 C for a cold
climate.
The differential temperature pattern given in the British code[2] is based on
extensive measurements on bridges in the British Isles and deals with various
types of bridge decks and deck surfacings. For the common case of a steel plate
or box or truss girder construction with (1) a 230 mm thick concrete slab and
100 mm of deck surfacing and (2) a steel orthotropic deck with 40 mm of
surfacing, the temperature differential with the road surface in the hot and cold
conditions are as shown in Fig. 3.6.
The AASHTO code[5] does not specify any temperature differential, but the
proposals[8] stipulate the pattern shown in Fig. 3.7.
In the German code[6], the temperature at the time of construction is
assumed to be þ10 C and a variation of 35 C from the construction temperature
is to be considered; within this range, a differential temperature of
15 C, linearly varying between different parts of the bridges structure, is also
to be considered, for example between top and bottom flanges, between cables
and stiffening girders, between webs of box girders.
In composite structures, i.e. steel structures with concrete slabs, a temperature
increase or decrease in the top surface of the slab of 20 C and at the bottom edge
of the steel girder of 35 C, from the construction temperature of þ10 C, is
specified. In the German proposals[10] clarifications have been made that
differential temperature need only be considered in the vertical plane and the
magnitudes have been proposed to be reduced to 10 C with the deck hot, 5 C
with the deck cold for a steel deck bridge and 7 C with the deck cold for a
composite bridge with a concrete deck. With traffic load on the bridge, either
differential temperature or the traffic load may be reduced to 70%. A temperature
difference of 15 C between different members of a bridge that are generally
unconnected to each other should also be considered, for example between the
beam and arch, cables and deck structure, upper and lower chords of trusses.