4. Dynamic loading tests of bridges and monitoring

In situ dynamic testing of bridges gives very useful information for numerical modeling and assessment of real bridge dynamic parameters and service conditions. In many countries, the requirement of putting the bridges into operation is the execution of bridge static and dynamic loading tests, which aim is to prove and confirm the projected parameters (standards criteria, serviceability, safety limit states, etc.) of tested bridge structures according to technical standards, e.g., in Slovakia by standard STN—Slovak Technical Standards [28]. Results from static or dynamic test enable to calibrate a bridge analytical model and can be utilized as basic data for a bridge health monitoring program and for other sophisticated calculations of the bridge dynamic response (seismic, fatigue, etc.).

#### 4.1. Test procedures

In this section, bridges dynamic test procedure is shortly described. Bridges are tested according to the rules of the dynamic loading test (DLT) [28]. Excitation of highway bridges are commonly due to the passage of single, fully loaded, multi-axles lorries. The testing vehicles' gross weight usually lies near the legal limit which is defined by standards and regulations. In the case of railway bridges, locomotives are used. Also normal traffic flow is used for both highway and railway bridges.

δ calculated by designer and dynamic amplification factor (DAF—from DLT), greater than 1 (the amount by which the static effects are increased by bridge-vehicle interaction contribution).

Bridges Subjected to Dynamic Loading http://dx.doi.org/10.5772/intechopen.73193 127

Note 1 In relevant standards of many countries, it is defined how to obtain coefficient DLA (= δ) in a normalized way (e.g., Canada, France, Germany, India, Spain, Switzerland-UK, USA, and

Note 2 Nowadays for Slovak Republic (CEN member), from 1.5.2006, it is mandatory to applying new European Standard. For the chapter content, it is actual Eurocode 1—Action on

Note 3 In this EN, for road bridges, the dynamic amplification was included into the load models (fatigue accepted), although established for a medium pavement quality and pneumatic vehicle suspension, which depends on various parameters and on the action effect under consideration. Therefore, it cannot be represented by a unique factor. For example, in former Slovak Standard (till 2006)—STN 73 6203, Load actions on bridges for calculation of δ = DLA

Note 4 For railway bridges, the dynamic amplification was accepted and dynamic factor Φ (= DLA = δ) is possible to calculate according to the given algorithm (EN 1991–2, Section 6).

In this section, data acquisition and recording (DAR) processes during a bridge DLT are shortly described. More detailed DAR processes descriptions are in [23, 26, 29]. Dynamic deflections are measured by pick-ups at the characteristic points of the bridge, which is normally at the mid-span. The bridge structure dynamic response at these points, in the horizontal and vertical directions, is then recorded in the form of time histories signal. Deflections w(t) are also measured at additional points along the super-structure. Except for dynamic deflections, other relevant parameters are measured: speed of the loading vehicles, magnitude and time history

Instrumentation: During standard dynamic tests of bridges (Figure 10(a)), inductive displacement transducers—IDT are mounted at the bridge parapet or bottom of the bridge structure, which are used to monitor displacement amplitudes time histories. In these cases, recorded displacement amplitudes time histories contains both static and dynamic components of the bridge dynamic response. The measured baseline is given by an invar wire (max 30 m), strained between the measuring points of the structure and a fixed reference point under the bridge structure. The application IDT enables extracting the static component from displacement time histories w(t) by using filtering techniques. This procedure is applied for DAF calculation. When the measured structure cross section is situated over water (e.g., river, lake, bay, etc.), the IDT are usually replaced by accelerometers or velocity-type transducers, Figure 10(b), or strain gauges, Figure 10(c), (measuring of strain amplitudes time history contains both static and dynamic components of the bridge response due to moving load) with relevant hardware

of excitation forces, temperature of the structure and ambient air, wind velocity, etc.

former Czechoslovakia, etc.).

Structures, Part 2: Traffic loads on bridges (EN 1991–2).

was used a formula in unnumbered format

4.2. DLT data acquisition and recording

For the expected dynamic bridge response caused by well-defined individual testing vehicles, dynamic calculations are carried out before the bridge dynamic tests. The testing vehicle is driven with a constant speed (in each measurement travel) along the bridge and respectively in the same direction or in both directions. The tests begin with a vehicle speed of c = 5 km/h, which is increased after each passage in steps of 5 km/h, up to the maximum achievable speed [22, 26]. If a static test with the used testing vehicle is not performed before the dynamic tests, the bridge deflection caused by vehicle traveling at a speed c = 5 km/h can be considered to present the static deflection ws with sufficient accuracy (e.g., via filtering signals).

In the case of highway bridges, the tests on the undisturbed bridges pavement are also repeated with a plank or standard obstacles placed across highway pavement, Figure 9(a). The cross section of the standard obstacle (length 5000 mm) is a cylindrical sector of height 60 mm and chord length 500 mm [28]. During the tests of highway bridges pay load, tires and tire pressure are kept the same; it means that the vehicle dynamic properties remain approximately constant.

Pulse forces produced by the ignition of pulse rocket engines (PRE) during DLT are also used mainly on large bridge structures. Harmonically variable forces produced by vibration exciters and the free vibrations of the bridge are also applied.

This type of the DLT so-called proof-loading test is performed for checking if the construction of the bridge has been constructed according to the design project. These tests (DLT) comprise the evaluation of the dynamic loading allowance (DLA) ! from standard = dynamic coefficients

Figure 9. Testing load: (a) lorries traveling over the obstacles; (b) the PRE applied mainly on large bridge structure.

δ calculated by designer and dynamic amplification factor (DAF—from DLT), greater than 1 (the amount by which the static effects are increased by bridge-vehicle interaction contribution).

Note 1 In relevant standards of many countries, it is defined how to obtain coefficient DLA (= δ) in a normalized way (e.g., Canada, France, Germany, India, Spain, Switzerland-UK, USA, and former Czechoslovakia, etc.).

Note 2 Nowadays for Slovak Republic (CEN member), from 1.5.2006, it is mandatory to applying new European Standard. For the chapter content, it is actual Eurocode 1—Action on Structures, Part 2: Traffic loads on bridges (EN 1991–2).

Note 3 In this EN, for road bridges, the dynamic amplification was included into the load models (fatigue accepted), although established for a medium pavement quality and pneumatic vehicle suspension, which depends on various parameters and on the action effect under consideration. Therefore, it cannot be represented by a unique factor. For example, in former Slovak Standard (till 2006)—STN 73 6203, Load actions on bridges for calculation of δ = DLA was used a formula in unnumbered format

$$s = \frac{1}{\mathbf{0.9S} - (1.4l)^{-0.6}}$$

Note 4 For railway bridges, the dynamic amplification was accepted and dynamic factor Φ (= DLA = δ) is possible to calculate according to the given algorithm (EN 1991–2, Section 6).

#### 4.2. DLT data acquisition and recording

4.1. Test procedures

126 Bridge Engineering

constant.

highway and railway bridges.

In this section, bridges dynamic test procedure is shortly described. Bridges are tested according to the rules of the dynamic loading test (DLT) [28]. Excitation of highway bridges are commonly due to the passage of single, fully loaded, multi-axles lorries. The testing vehicles' gross weight usually lies near the legal limit which is defined by standards and regulations. In the case of railway bridges, locomotives are used. Also normal traffic flow is used for both

For the expected dynamic bridge response caused by well-defined individual testing vehicles, dynamic calculations are carried out before the bridge dynamic tests. The testing vehicle is driven with a constant speed (in each measurement travel) along the bridge and respectively in the same direction or in both directions. The tests begin with a vehicle speed of c = 5 km/h, which is increased after each passage in steps of 5 km/h, up to the maximum achievable speed [22, 26]. If a static test with the used testing vehicle is not performed before the dynamic tests, the bridge deflection caused by vehicle traveling at a speed c = 5 km/h can be considered to present

In the case of highway bridges, the tests on the undisturbed bridges pavement are also repeated with a plank or standard obstacles placed across highway pavement, Figure 9(a). The cross section of the standard obstacle (length 5000 mm) is a cylindrical sector of height 60 mm and chord length 500 mm [28]. During the tests of highway bridges pay load, tires and tire pressure are kept the same; it means that the vehicle dynamic properties remain approximately

Pulse forces produced by the ignition of pulse rocket engines (PRE) during DLT are also used mainly on large bridge structures. Harmonically variable forces produced by vibration exciters and

This type of the DLT so-called proof-loading test is performed for checking if the construction of the bridge has been constructed according to the design project. These tests (DLT) comprise the evaluation of the dynamic loading allowance (DLA) ! from standard = dynamic coefficients

Figure 9. Testing load: (a) lorries traveling over the obstacles; (b) the PRE applied mainly on large bridge structure.

the static deflection ws with sufficient accuracy (e.g., via filtering signals).

the free vibrations of the bridge are also applied.

In this section, data acquisition and recording (DAR) processes during a bridge DLT are shortly described. More detailed DAR processes descriptions are in [23, 26, 29]. Dynamic deflections are measured by pick-ups at the characteristic points of the bridge, which is normally at the mid-span. The bridge structure dynamic response at these points, in the horizontal and vertical directions, is then recorded in the form of time histories signal. Deflections w(t) are also measured at additional points along the super-structure. Except for dynamic deflections, other relevant parameters are measured: speed of the loading vehicles, magnitude and time history of excitation forces, temperature of the structure and ambient air, wind velocity, etc.

Instrumentation: During standard dynamic tests of bridges (Figure 10(a)), inductive displacement transducers—IDT are mounted at the bridge parapet or bottom of the bridge structure, which are used to monitor displacement amplitudes time histories. In these cases, recorded displacement amplitudes time histories contains both static and dynamic components of the bridge dynamic response. The measured baseline is given by an invar wire (max 30 m), strained between the measuring points of the structure and a fixed reference point under the bridge structure. The application IDT enables extracting the static component from displacement time histories w(t) by using filtering techniques. This procedure is applied for DAF calculation. When the measured structure cross section is situated over water (e.g., river, lake, bay, etc.), the IDT are usually replaced by accelerometers or velocity-type transducers, Figure 10(b), or strain gauges, Figure 10(c), (measuring of strain amplitudes time history contains both static and dynamic components of the bridge response due to moving load) with relevant hardware

devices to the recording technique at the MS by special low noise cables were usually used. The application of the wireless sensor network (WSN) platform for DLT or distributed measurement application, e.g., bridge structural health monitoring, is possible to eliminate the need for costly and work-intensive wiring measuring technique. The WSN platform simplifies remote monitoring applications and delivers low-power, reliable measurement nodes that feature

Bridges Subjected to Dynamic Loading http://dx.doi.org/10.5772/intechopen.73193 129

The application of the quick-setup WSN enables to implement a stand-alone remote monitoring system or easily connect with measuring PC and control systems (e.g., NI WSN, BK PULSE WSN). Figure 12 shows examples of equipment set for experimental measurement data wire-

The final experimental analysis is usually carried out in the laboratory. The bridge vibrations induced by the lorries crossing the bridge during the DLT with different velocities are analyzed in order to quantify and compare the different dynamic effects on the bridge structure. The analysis of the time histories of vibration recorded, when lorries crossed the viaduct

• double integration of the accelerations to displacements and evaluation of their maxima and RMS values. The displacements maximum value is used for DAF calculations;

• digital filtering with a low-pass Butterworth filter with a cut frequency, e.g., of 150 Hz and

Also from the bridge dynamic response and free vibration measured time histories, can be

Figure 12. The experimental measurement data wireless transmissions by NI WSN modules with portable PC layout and

with a high-pass Butterworth filter with a cut frequency of 0.5 Hz; and

• frequencies of one or more vibration modes of the loaded and unloaded bridge;

• maximum and RMS values of acceleration amplitudes evaluation.

• the natural vibration damping parameters, dominant in free decay;

less transmissions by NI WSN modules with portable PC layout and scheme.

bridge, is processed with the following operations:

• offset and linear trends removal;

obtained:

scheme.

local control capabilities.

Figure 10. Examples of bridge DLT instrumentation: (a) inductive displacement transducers (IDT); (b) accelerometers set up; (c) strain sensor (d) charge amplifiers devices; (e) accelerometer installation process to bridge bottom for DLT.

components (amplifiers, cables, wireless technique, etc.), Figure 10(d). The view of accelerometers installation on bridge bottom for DLT is showed in Figure 10(e).

The signals from the used pick-ups are amplified and filtered by the signal amplifiers and lowband pass filters, and then recorded by portable notebook with relevant software and hardware facilities in test measuring station (MS). Scheme and view of the equipment in MS used in situ tests are plotted in Figure 11. During DLT, the signals transmission from measurement

Figure 11. View of the equipment in MS with its scheme mounting used during the bridge dynamic tests.

devices to the recording technique at the MS by special low noise cables were usually used. The application of the wireless sensor network (WSN) platform for DLT or distributed measurement application, e.g., bridge structural health monitoring, is possible to eliminate the need for costly and work-intensive wiring measuring technique. The WSN platform simplifies remote monitoring applications and delivers low-power, reliable measurement nodes that feature local control capabilities.

The application of the quick-setup WSN enables to implement a stand-alone remote monitoring system or easily connect with measuring PC and control systems (e.g., NI WSN, BK PULSE WSN). Figure 12 shows examples of equipment set for experimental measurement data wireless transmissions by NI WSN modules with portable PC layout and scheme.

The final experimental analysis is usually carried out in the laboratory. The bridge vibrations induced by the lorries crossing the bridge during the DLT with different velocities are analyzed in order to quantify and compare the different dynamic effects on the bridge structure. The analysis of the time histories of vibration recorded, when lorries crossed the viaduct bridge, is processed with the following operations:


components (amplifiers, cables, wireless technique, etc.), Figure 10(d). The view of accelerom-

Figure 10. Examples of bridge DLT instrumentation: (a) inductive displacement transducers (IDT); (b) accelerometers set up; (c) strain sensor (d) charge amplifiers devices; (e) accelerometer installation process to bridge bottom for DLT.

The signals from the used pick-ups are amplified and filtered by the signal amplifiers and lowband pass filters, and then recorded by portable notebook with relevant software and hardware facilities in test measuring station (MS). Scheme and view of the equipment in MS used in situ tests are plotted in Figure 11. During DLT, the signals transmission from measurement

Figure 11. View of the equipment in MS with its scheme mounting used during the bridge dynamic tests.

eters installation on bridge bottom for DLT is showed in Figure 10(e).

128 Bridge Engineering


Also from the bridge dynamic response and free vibration measured time histories, can be obtained:


Figure 12. The experimental measurement data wireless transmissions by NI WSN modules with portable PC layout and scheme.

• percentage of vibration critical damping via the 3 dB band with method and curve fitting techniques;

5. Case study

The dynamic loading test and the following dynamic monitoring of the Lafranconi bridge over the Danube in Bratislava (Slovakia) are shortly described in this section [38–40]. The dynamic response behavior of a prestressed concrete, seven span highway bridge (761.0 m long) was examined via DLT according to standard [24] in 1990. Excitations of bridge structure were induced by the passage of two fully loaded, multi-axles lorries as well as by the rocket engines. Applied structural measurement technique was developed for in situ testing of the bridges. The DLT results enabled to identify bridge global dynamic characteristics of the bridge, e.g., maximum and RMS of displacements amplitude, natural frequencies f(j), mode shapes, DAF ! δOBS (δOBS = wmax/ws) and the structure amplitude damping parameter (ϑ). The obtained dynamic characteristics were compared with the numerical computed data [29] and standard prescriptions. For maximum and RMS displacements amplitude, and so on, see technical report [2].

Bridges Subjected to Dynamic Loading http://dx.doi.org/10.5772/intechopen.73193 131

The main bridge structure is composed of seven span continuous beams with one bridge frame pier (P3). The total length of the bridge was 761.0 m with spans 83.0 m + 174.0 m + 172.0 m + 4 � 83.0 m. The highway bridge consists of two independent bridges (left and right bridge) with three traffic lanes each (i.e., three in each bridge for one direction only) and sidewalks on both sides. The bridge's longitudinal section is shown in Figure 14. The bridge structure including multispan junctions, the test program, field measurements, and applied instrumentation are fully described in [29]. The vibration amplitudes were measured and recorded in 18 selected points. The measuring station for recording accelerometer signals (DSM-1) was situated on the top of the pier P3, Figure 14. The time history of vertical as well as horizontal vibration amplitudes have been registered by accelerometers in the second and the third span of the bridge. In the other bridge spans were applied inductive displacement transducer with working

5.1. Dynamic loading test of Lafranconi highway bridge over the Danube

Figure 14. Longitudinal section of the Lafranconi highway bridge over the Danube in Bratislava.


#### 4.3. Bridges' dynamic parameters monitoring

Long-term bridges observation is discussed in literature, e.g., [29–37]. Some dynamic methods used by other authors [33], were applied to correlate relative changes of material, frequencies, and damping with carrying capacity. It was found that used monitoring techniques gave an early indication of incipient deterioration. The main scope of monitoring tests was to evaluate mainly the relative change of well-defined natural frequencies or the corresponding damping and the RMS value of the displacements amplitude of the bridge vibration due to traffic loading. The monitoring technique based on measurement of the bridge vibration time history due to regular traffic is not focused to give detailed bridge information but for making decision if more detailed bridge assessment methods should be used. The sophisticated bridge monitoring was introduced e.g. on the Akhashi Kaikyo bridge in Japan, (Figure 13) completed 1998. At that time, it was the largest and longest suspension bridge in the world. Bridge is a 3-span 2 hinged bridge with steel-truss-stiffened girders located near Kobe City.

Bridge has a impressive 1991-m center span between two main towers that rise 300.0 m above the sea level. The Akashi Kaikyo bridge, being easily affected by natural conditions and traffic means, requires high level of disaster prevention and bridge structure functionality with projected structure parameters. Therefore, to provide centralized control, traffic control and bridge structure and facility monitoring have been integrated into the Traffic Control Center. There, information acquisition and processing are performed continuously 24 h a day, providing vital traffic and bridge structure information.

Figure 13. Akashi Kaikyo bridge: (a) look-out on the bridge; (b) 24 h a day monitoring center (source: Kobe—Awaji— Naruto Expressway. Honshu—Shikoku Bridge Authority 1998, advertising material).
