ABSTRACT
Live loads acting on bridges, including natural traffic during their common exploitation, can be utilised as an efficient and convenient tool of the structural condition evaluation. The bridge characteristics being searched and analysed in this way are very diverse and related to technical parameters, technical condition, load carrying capacity or prediction of future performance of the examined structure.
The described methodology is illustrated by case studies of experimental tests under real traffic executed on three over 100-year-old bridges over Odra River in Wrocław:
suspended Grunwaldzki Bridge (Figure 1) of the total length 172.6 m, Grunwaldzki Bridge – side view. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig135_1.jpg"/>
Osobowicki Bridge – multi-span masonry arch structure of the total length 223.10 m and
Pomorski Bridge – three-span masonry arch structure (18.20 m + 20.20 m + 18.20 m).
Within the process of bridge condition evaluation various methods and tools can be used. Live loads applied for this purpose can be treated as factors triggering structural response which always provides extensive information on the tested object. The structure’s reaction to the applied “impulse” is received in a form of measureable quantities by means of various measuring systems.
Bridge characteristics which undergo measurements and evaluation with application of live loads are:
dynamic characteristics like mode shapes and frequencies, dumping (Bień et al. 2011, Zwolski et al. 2013),
dynamic amplification factor – DAF (Bień et al. 2008, Bień et al. 2011),
modulus of elasticity (Helmerich et al. 2012),
structural stiffness (Machelski & Kamiński 2015),
influence lines (Machelski & Kamiński 2015),
hidden geometry like internal voids, walls or reinforcement (Brencich & Colla 2002),
boundary conditions (Costa et al. 2013, Kamiński & Bień 2015b),
load carrying capacity (Bień et al. 2012),
fatigue resistance (Bień et al. 2009),
technical condition based on presence of defects (Kamiński & Bień 2015a),
interaction with conduits supported by bridge (Bień et al. 2011)
Collection of bridge characteristics determined for the given bridges is listed in Table 1. Characteristics determined on bridges by live loads.
No.
Bridge characteristics
Intervention Criteria
Grunwaldzki
Osobowicki
Pomorski
1.
dynamic characteristics
+
+
+
2.
DAF
+
+
3.
modulus of elasticity
+
4.
structural stiffness
5.
influence lines
6.
hidden geometry
+
7.
boundary conditions
+
8.
load carrying capacity
+
+
+
9.
fatigue resistance
+
10.
technical condition
+
+
+
11.
interaction with conduits
+
Relationships between listed bridge characteristics being evaluated and features of the applied live loads (including their types, no. of vehicles and time of measurement against the time of loading) are studied. Relationships between the bridge characteristics and testing methods based on live loads (including controlled quantities and procedures of bridge characteristics determination) are also carefully considered.
One of the case studies – Grunwaldzki Bridge (see Figure 1) – is a famous in Poland over 100-year-old steel suspension structure. It represents a unique tram-way-road structure with span of 114.0 m. The bridge plays an important role in the transportation system of the city and is still intensively exploited.
316The main goal of investigations performed in 2008 was to estimate the remaining life time of the bridge in terms of fatigue resistance (Bień et al. 2009). In course of systematic inspections some defects of the bridge superstructure were noticed, however no fatigue cracks were detected.
Within carried out fatigue analysis the stress ranges and daily number of load cycles were experimentally determined. For this purpose accelerations and strains in selected bridge members were measured during one-day experimental vibration test under operational (natural) loads. The measuring gauges were placed only in a quarter of the span in the following elements: three crossbeams, two stringers, suspension strand, hanger and stiffening girder. Sixty measurements sessions (10 minutes each) were carried out during one day between 7.30 am and 21.25 pm. Numbers of cycles were extrapolated to intervals of time between the measurements as well as over the night hours.
During the tests the traffic density was very high, however it was possible to isolate single heavy vehicles which could potentially influence fatigue strength. Exemplary recorded strains for an examined stringer induced by various vehicles (tram, concrete mixer truck, lorry, traffic congestion and a single car) are presented in Figure 2. It shows also location of the analyzed element as well as the location of the measuring point in element’s cross section. Strains induced in stringer by various vehicles. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig135_2.jpg"/>
Extreme values of stresses induced by real traffic in the investigated structure members have been obtained on the basis of strain history, while stresses in the measuring points caused by dead load have been calculated by means of numerical analysis. Maximum values of stresses were found in a stringer. In general it was noted that only deck elements which suffer from the highest stress ranges are potentially exposed to fatigue phenomenon.
In fatigue analysis for the number of load cycles calculation the Rain-flow counting method was used while for the remaining life time estimation the sequential law of fatigue damage accumulation was applied. Finally it was concluded that the predicted remaining life time of the Grunwaldzki Bridge is more than one hundred years.
