Applied Loads and Stability of Steel and Steel-Concrete Composite Bridges - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 189

3.9 STABILITY OF STEEL AND STEEL-CONCRETE COMPOSITE
TRUSS BRIDGES
3.9.1 General
Truss bridges are generally used for spans over 40 m. For spans between 40
and 70 m, parallel chord trusses are used, while for spans greater than 70 m,
polygonal chord trusses are used. Trusses are, normally, designed to carry
axial forces in its members, which are either tension or compression or
reversible tension/compression depending on the worst cases of loading
and load combinations. Truss members are connected at joints using welds
or bolts. Joints are designed as pins and the forces in truss members are in full
equilibrium at the joints. In practice, gusset plates are used at the joints to
collect the forces in the members meeting at the joints, where equilibrium
takes place. Therefore, the size of the gusset plates should be as small as pos-
sible to simulate the behavior of pins. If the maximum force in a truss is less
than 3000 kN, single gusset plate trusses are used and truss members are
designed as angles. On the other hand, if the maximum force in truss mem-
bers is greater than 3000 kN, double gusset plate trusses are used and chord
members are designed as box sections, while diagonals and verticals are
designed as I-sections or box sections in case of long diagonals carrying com-
pressive forces. Cross girders are located at the joints of trusses to eliminate
bending moments on truss members. Figure 1.21 shows an example of a
highway truss bridge.

3.9.2 Design of Tension Members
The design of tension members is covered by EC3 [1.27,2.11]. The code
recommends that the design value of the tension force N Ed at each cross sec-
tion shall satisfy

N Ed

N t , Rd 1

: ð 3 :

85 Þ 0

For sections with holes, the design tension resistance N t,Rd should be
taken as the smaller of the following:
(a) The design plastic resistance of the gross cross section:

Af y
g M0

N pl , Rd ¼ ð 3 : 86 Þ

(b) The design ultimate resistance of the net cross section at holes for
fasteners: