Development of Brazilian highway live load model for unlimited fatigue life

Abstract

This work studies the fatigue live load model used in Brazil for highway bridges. Using the unlimited fatigue life approach, the current live load model is evaluated in relation to the actual traffic and a new fatigue live load model is proposed. Weigh-in-motion (WIM) stations data on two important Brazilian highways are used. The main structural analysis performed in this paper consider the bridges as box girders or multiple girders. The ratio between real traffic and the live load model load effect (bias factor) are determined for single and continuous spans in terms of bending moment and shear force. It is found that the bias factor of the current live load can vary a lot and may not ensure unlimited fatigue life. The proposed model, on the other hand, presents more uniform bias factors and is in accordance with the unlimited fatigue life approach for the WIM data.

Fatigue safety level provided by Brazilian design standards for a prestressed girder highway bridge

“…Complete prestressing presents higher safety levels than limited prestressing for both longitudinal reinforcements and stirrups. Although the Brazilian live load model is not in accordance with the unlimited fatigue life approach, as stated by Carneiro et al [4], the design criteria, which involve partial safety factors and stress limits for fatigue design, guaranteed satisfactory fatigue safety levels for the evaluated bridge. It is important to mention that according to EN 1992-2 [49], a fatigue design verification is generally not necessary for prestressing and longitudinal reinforcing steel, in regions where, under the frequent combination of actions (including prestressing) only compressive stresses occur at the extreme concrete fibres (like complete prestressing of NBR 6118 [15]).…”

“…In this work, the data from the BR-381 (Fernão Dias Highway, which connects the states of São Paulo and Minas Gerais) station is used (same data used in Carneiro et al [4]). The system is installed in two road lanes in the same direction (Minas Gerais to São Paulo), as indicated in Figure 3c.…”

“…In the static weighing process, overloaded vehicles can trace alternative routes, the multiple presence occurrences are unknown, and the data is not collected 24 hours a day, in days and months in a row. [4], [5]; b) weighting sensor before grout in BR-381 [4], [5], [33]; c) final appearance of the pavement in BR-381 [5], [33] (position sensors, which detect when vehicles change lanes, was not installed).…”

“…For fatigue verification in girders, it requires use of a coefficient ψ 1,fad = 0.5 for spans lengths up to 100 meters. Carneiro et al [4] evaluated the live load model as in NBR 7188 [1], from the unlimited fatigue life point of view, using traffic data from two HS-WIM stations in Brazil (BR-381 and BR-290) and proposed a new fatigue live load model for unlimited fatigue life. The authors indicated that the bias factor of the Brazilian model can vary a lot and may not correspond to the infinite fatigue life approach.…”

“…With that in mind, this work assesses the fatigue safety level provided by Brazilian design standards in relation to a concrete highway bridge, using weigh-in-motion (WIM) data of an important federal Brazilian highway, BR-381 (the same data used in Carneiro et al [4]). The fatigue service life and fatigue reliability indexes for a design life of 50 years are evaluated for prestressed concrete girders designed according to Brazilian codes, in terms of bending moment, shear force and torsion.…”