CHAPTER 2 FATIGUE DAMAGES AND MECHANISMS
2.4 M ECHANICAL BEHAVIOR OF ORTHOTROPIC STEEL DECK
Figure 2.8 Effective stress at the weld joint [10]
(2) Stress concentration (Geometric properties)
In bridge structures, fatigues are initiated from stress concentrated part of welding joints. Strongly dependent on geometry of welding details. Fatigue strengths are determined by considering the constructional detail together with its metallurgical and geometric notch effects. The shape of the structure will significantly affect the fatigue life; square holes or sharp corners will lead to elevated local stresses where fatigue cracks can initiate. Round holes and smooth transitions or fillets will therefore increase the fatigue strength of the structure. Therefore, the Local geometry at potential crack locus, such as welded toes, some post fabrication processing methods were usually used to prevent the crack initiation. The geometry or global geometry of the item were usually optimized by reduce the welding joint and improve the rationality of structure.
(3) Loading and environment
Traffic load is certainly the most important influence factor of fatigue problems of OSD. It would lead to the external fatigue cycle directly. Therefore, only analyze on traffic flow volume and loading magnitude is not enough. The time history of the external forces and loading mode with reference to the actual structural item are all important parameters. By the way, the environmental conditions of OSD during service could also effect on structural durability.
In addition, as a complex steel structure with various welded joints, the mechanical behaviors of orthotropic steel deck under wheel loading are always related to the fatigue cracks.
as well as the distribution area of the load [94].
The load transferred from the deck plate to the longitudinal ribs, and the local deformation of the deck plate from the wheel load results in transversal flexural stress in the deck and longitudinal rib. Transverse bending stress in deck plate, and local effects on ribs and rib to deck plate connection as shown in Figure 2.9. Mg is the moment in the deck plate in one rib, Md is the moment in the deck plate between two ribs.
As a result of transverse load distribution in the orthotropic deck, the ribs will experience local out-of-plane deformation causing traverse flexural stresses in the walls of the ribs.
(a) Welded joint A
(b) Welded joint B
Figure 2.9 Local effects on rib-to-deck connection from wheel loading
When the wheel is positioned direct above the rib wall the highest stress will be found in the deck, if the wheel is placed between the ribs the maximum stress will occur in the rib. At the location of the supports the ribs will be subjected to negative flexural moments from the traffic load generating compressive stresses in the base of the rib [95].
There are several reported fatigue failures in the connection between the longitudinal stiffener and deck plate [16]. For the stress state at root tip of rib-to-deck connection, the compressive stress would occurred when wheel load as shown in Figure 2.9(a). However, the transverse location of the wheel load move from (a) to (b) would lead to adverse stress condition at root tip. When the vehicle pass over in longitudinal direction, an adverse transverse distribution would lead to a larger stress range or tensile stress at details.
2.4.2 Mechanical behavior of the rib splice joint
Significant bending moments arise in the U-rib profile due to the passing vehicles and its axles resulting in considerable longitudinal bending stresses in the cross-section of ribs, as shown in Figure 2.10 [24]. The location of the splice joints in the trough profiles is normally to be the region with the lowest bending moment ranges.
However, there are still considerable stress ranges. These stresses in combination with the presence of a backing strip, lack of weld penetration and misalignment result in high stress concentrations. Fatigue cracks initiate at the locations of these stress concentrations. The crack growth rate is strongly dependent
A
Md Mg
Mg-Md Mg＞Md
B
Md Mg
Md-Mg Mg＜Md
on the type and the quality of the weld between two ribs [96].
Figure 2.10 Mechanical behavior of the rib splice joint
2.4.3 Mechanical behavior of other sub-systems
The interaction between the components in the OSD are complicated under traffic flows, the deck acts as a structural unit and has to accomplish numerous functions simultaneously. Besides the rib and deck plate connections, an OSD have numerous welded joints with complex structural behavior, geometry and load situation [97]. To understand the mechanical behavior of different parts and simplify the structural response, the deck could be divided into sub-systems, as shown in Table 2.5. Besides, the global deformation of OSD cannot be ignore, including the axial, flexural and shear stresses from deformation of supporting main girders.
A full interaction model could be established and analyzed with finite element methods, and the separated sub-system method is possible to use for the limit state calculations and then combined by linear superposition.
Table 2.5 Assembly and description of orthotropic deck systems and their behavior [4]
System illustration Action and result
Deck plate deformation:
Transverse deck stress from differential displacement of ribs.
Crossbeam in-plane bending:
Bending and shear in crossbeam acting as a beam spanning between the main girders.
Crossbeam distortion and rib rotation:
Out-of-plane bending of crossbeam web at rib due to rib rotation.
Rib distortion:
Local bending of rib wall at crossbeam cut-outs.
Crossbeam
U-rib
Cut-outs
Crossbeam