Applied Loads and Stability of Steel and Steel-Concrete Composite Bridges - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 125

surface for the wheels of Load Models 1 and 2. For Load Model 3 (special
vehicles), where relevant, the models of special vehicles should be defined
and taken into account. The National Annex may define Load Model 3 and
its conditions of use. Finally, Load Model 4 (crowd loading), if relevant,
should be represented by a load model consisting of a uniformly distributed
load (which includes dynamic amplification) equal to 5 kN/m 2 . The appli-
cation of LM4 may be defined for the individual project. Load Model 4
should be applied on the relevant parts of the length and width of the road
bridge deck, the central reservation being included, where relevant. This
loading system, intended for general verifications, should be associated only
with a transient design situation.
In the United States, the AASHTO [1.23] specifies vehicular live loading
on the roadways of bridges based on designated HL-93 load model consist-
ing of a combination of the design truck or design tandem and design lane
load. Each design lane under consideration shall be occupied by either the
design truck or tandem, coincident with the lane load. The weights and
spacings of axles and wheels for the design truck specified in AASHTO
are shown in Figure 3.9 . A dynamic load allowance is considered for the
design truck load. The spacing between the two 145,000 N axles varies
between 4300 and 9000 mm to produce extreme force effects. It should
be noted that the total design force effect is also a function of load factor,
load modifier, load distribution, and dynamic load allowance. The design
tandem shall consist of a pair of 110,000 N axles spaced 1200 mm apart.
The transverse spacing of wheels shall be taken as 1800 mm. A dynamic load
allowance is also considered for the design tandem load. The design lane load
consists of a load of 9.3 N/mm uniformly distributed in the longitudinal
direction. Transversely, the design lane load is assumed to be distributed
over a 3000 mm width. The force effects from the design lane load are
not subject to a dynamic load allowance. The tire contact area of a wheel
consisting of one or two tires is assumed to be a single rectangle having a
width of 510 mm and a length of 250 mm. The tire pressure is assumed
to be uniformly distributed over the contact area.
The extreme force effect according to AASHTO [1.23] is taken as the
larger of the effect of the design tandem combined with the effect of the
design lane load or the effect of one design truck with the variable axle spac-
ing combined with the effect of the design lane load and for both negative
moment between points of contraflexure under a uniform load on all spans
and reaction at interior piers only; 90% of the effect of two design trucks
spaced a minimum P equal to the design wheel load (N). For the design