e-mosty December 2018 e-mosty December 2018 | Page 67

Rib section types
The cross section shall be determined according to the
arch span, width of the bridge and vehicle load level.
The arch rib may be a single tube (generally used for a
bridge with a span smaller than 80m), dumbbell
section (a span smaller than 150m) or truss types (with
a large span, from 120m to the longest one with a
span of 500m), see Figure 7.
Figure 7: a) Single tube section b) Dumbbell Section c) Three-chord truss section d) Four-chord truss section e) Horizontal dumbbell truss
Legend: 1 – Chord 2 – Web Plate 3 – Web Truss 4 – Horizontal Connection Bar 5 – Horizontal Connection Plate
Rib Material
Main materials used for arch ribs are Q235 steel and
Q345 steel, but in recent years, Q235 steel has
gradually replaced Q345 steel and high strength steel
Q390 has also be used. Strength of infilled concrete
generally exceeds C40, only five bridges built before
2005 used C30.
To take full advantage of the mechanical
characteristics of concrete filled steel tube and
guarantee the stability of the steel tube wall, it is
necessary to limit the ratio of the tube diameter to
wall thickness. In 97% of bridges the ratio ranges from
35 to 100, with the majority between 35 and 70. The
wall thickness shall not be less than 8mm.
Design Calculation
The Code adopts the limit state design concept based
on the probability theory. Partial safety factors are
used in design calculation. The bridges shall be
designed based to two limit states:
a) Ultimate Limit State: The state where the bridge or
any of its components reaches the maximum load
capacity or the deformation / displacement which
makes it no longer able to resist peak loading.
b) Serviceability Limit State: The state where the
bridge or any of its components reaches certain
functionality or durability limit.
The Code gives guidance on three alternative methods
of analysis based on:
a) Resistance of a single CFST member in axial
compression, which is the basic formula of the
calculation of the ultimate load-carrying capacity
of CFST arch.
b) Resistance of long CFST columns with defined
eccentricity and slenderness parameters in
compression.
c) Ultimate load-carrying capacity and stability of
CFST arch based on a beam-column method with
an equivalent effective length.
4/2018