Evaluation of Structural Behaviour of a Fire Damaged Highway Bridge in Lagos-Nigeria with BRIMOS® Structural Health Monitoring
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Life-Cycle and Sustainability of Civil Infrastructure Systems – Strauss, Frangopol & Bergmeister (Eds) © 2013 Taylor & Francis Group, London, ISBN 978-0-415-62126-7 Evaluation of structural behaviour of a fire damaged highway bridge in Lagos-Nigeria with BRIMOS® structural health monitoring P. Furtner & R. Veit-Egerer Vienna Consulting Engineers, Vienna, Austria J. Buchmann & M. Buschlinger BBV Systems GmbH, Bobenheim-Roxheim, Germany ABSTRACT: In 2008 a fire caused extensive damage to the underside of the superstructure and the piers of the Eko Bridge. In several bridge parts the concrete cover at the superstructure’s underside failed and the ex- posed reinforcement bars were partly buckled. Furthermore at several piers concrete has broken off up to a depth of the first reinforcement layers. In areas with the most excessive damage, additional temporary sup- ports were erected – surrounding certain piers. In the Site Inspection Report was stated that without immedi- ate investigations the safety and stability of the bridges could not be reviewed. In order to broaden the insight on structural integrity and the load bearing capacity a dynamic bridge monitoring campaign was undertaken. Particularly evaluation and judgement, to what extent the fire has caused serious damage and tailored recom- mendations regarding possible retrofit and maintenance interventions were needed to support the decision process of the bridge owner. 1 STRUCTURAL INFORMATION The Eko Main Bridge East is located in Ijora, Lagos The bridge structure’s relevant eigenfrequencies and connects Lagos Mainland with Lagos Island (Fig. and corresponding mode-shapes 1). The prestressed structure consists of two separate => Load bearing capacity and operability load-bearing structures – one for each driving direc- => Evaluation of the bridge bearings tion. The Main Bridge East which was part of the dy- => Distribution of the global and local structural namic investigation has a total length of 190 m and stiffness in the bridge’s lengthwise and transver- was opened to traffic in the early 1970ies. The cross- sal direction section of the superstructures is composed by a three- Sensitivity analysis to investigate the progression, cellular box girder and has a width of 13.8 m. Ac- the character, the stability and probable changes cording to the drawings, a six-span bridge was de- in the energy content of the relevant eigenfre- signed (Fig. 2), in fact only five-spans and a cantile- quencies ver of approx. 5m length in succession to the last => Load bearing capacity and operability bridge bearing were built. Energy dissipation path in the structure’s length- wise direction => Dissipation of the induced vibration energy, 2 SCOPE OF THE ASSESSMENT PROCEDURE localization of problematic sections Vibration intensity at the entire bridge deck The dynamic measurement at the Bridge object Eko => Localization of weak points with regard to fa- Bridge East with BRIMOS® 11.02 was carried out on tigue threat the 6th of August in 2010 in order to derive the fol- lowing key performance indicators: 727 Figure 1. Location of the investigated part of Eko Bridge, Lagos. Figure 2. Plan (top) and cross-section view (bottom) of the investigated part of Eko Bridge. Parallel to the dynamic measurement an accompa- PLC (2008). The expansion joints are in poor condi- nying rapid visual inspection was carried out. As ex- tion due to lack of maintenance. Parts of the railing pected, the underside and the piers at the area of span and almost all manhole covers are missing. 2, 3 and 4 show severe damage caused by the fire (Fig. 3). For further details see Julius Berger Nigeria Figure 3. Visual impression of fire damaged bridge sections. 728 During the measurements with the BRIMOS® 2.1 Description of the measurement wireless equipment three accelerometers were dis- The measurements were done under ambient condi- tributed stepwise all over the structure, while the ref- tions on the one hand (environmentally excited vibra- erence sensor remained at its fixed position. When tions) and under mostly unrestricted traffic on the using the BRIMOS® Recorder and an additional ex- other hand. The traffic was limited to two lanes in- ternal sensor all sensors were moved stepwise along cluding passages of heavy vehicles. the measurement grid. In-process referencing enables The so-called In-Depth-Monitoring was conducted a coherent analysis of the recorded data. using two BRIMOS® Recorders with two additional For each sensor array one measurement file with a external accelerometers on the one hand and two sets length of 11 minutes and a sampling rate of 500 Hz of BRIMOS® wireless units on the other hand. In to- (= 2 milliseconds) was recorded. tal a sensor grid of 112 different positions was con- Based on the applied dense sensor grid the aerial ducted (Fig. 5). On every bridge measurements were bridge characteristics regarding bending and torsional taken on the inner (center) and the outer (cap) canti- behaviour could be observed comprehensively. levers. Figure 4. Photo documentation of the BRIMOS® Measurement at the Eko Bridge. Figure 5. Sensor positions for the 4 applied measurement arrays at the Eko Bridge. 729 relevant for further evaluation. Some of the corresponding mode shapes are shown in Figure 6 3 DYNAMIC SYSTEM IDENTIFICATION exemplarily. With regard to the nature of dynamic response the origin of resonance is specified The following table (Tab. 1) includes all identified respectively. eigenfrequencies which have been considered to be Table 1. Relevant eigenfrequencies for structural performance assessment. Driving Direction Lagos Driving Direction Lagos Eigenfrequencies ISLAND MAINLAND CAP CENTER Specification on CAP CENTER Specification on [Hz] origin of resonance origin of resonance 1. (1st bending mode) 2,92 2,89 span 1 2,91 2,91 span 3 & 2 2. (2nd bending mode) 3,22 3,33 span 4 3,27 3,32 span 1 3. (3rd bending mode) 3,69 3,77 span 1 & 2 3,74 3,76 span 3 4. (4th bending mode) 4,00 3,97 span 4 & 5 4,19 4,14 span 5 5. (5th bending mode) 4,50 - span 2-5 4,33 - span 4,3 & 5 6. (6th bending mode) 5,03 - span 4, 5 & 3 5,26 5,21 span 4,5 & 3 7. (1st torsional mode) 9,20 9,16 span 4, 5 & 3 9,27 8,95 span 4,5 & 3 8. (2nd torsional mode) - - span 1 & 5 12,43 12,47 span 1 & 5 9. (7th bending mode) - - span 4 & 1 12,55 - span 4 & 1 10. (8th bending mode) 15,11 14,81 span 1 & 2 14,99 14,25 span 1 & 2 Figure 6. System Identification – Resonance frequencies vs. type of structural response. 4 ASSESSMENT & EXPERTISE 4.1 Eigenfrequencies In the following chapter the key parameters - The frequency spectra in all three analysed dimen- extracted from structural mechanics (measured sions (mainly representing the effective dynamic vibrational response) are explained with regard to stiffness of the structure) of the measurements at the their relevance for civil engineering judgement of the driving direction Lagos Island and at the driving di- investigated structure. In the forefront – and referring rection Lagos Mainland only show a partly distinc- to bridge design – structural load bearing resistance tive dynamic characteristic. This indicates evident re- (bending resistance and torsional resistance) are strictions to the structural integrity of the static addressed. system. The relevant eigenfrequencies are primarily located in the range of 2.5 to 15.5 Hz. Most of those eigen- frequencies are global eigenfrequencies (bending and 730 torsion) linked with dynamic response of whole pretation of the whole measured frequency spectrum spans. Deviations between the eigenfrequencies of assumes greater significance. the different structures primarily arise from the dif- In order to distinguish individual frequency peaks, ferent geometrical properties (the ground view pro- a colour-scaling of these cards is required - represent- gression causes varying span lengths - see Fig. 2). ing the energy content of the oscillation and therefore the respective intensity. From this it follows, that damages are already visible in the frequency spectra 4.2 Evaluation of Structural Integrity in their beginning phase. The present investigation aims on an evaluation to So-called trend cards are an essential evaluation what extent the bridge’s operability and load-bearing instrument for full-scale measurements on bridge capacity was affected by the fire. For that purpose the structures. They represent the signal either in a fre- trend cards showing the structure’s relevant vertical quency-time or a frequency-bridge-length diagram. stiffness patterns in the lengthwise direction emerged Figure 7 & 8 demonstrate that the trend cards are ob- to be the most suitable instrument. For every meas- tained by evaluating frequency spectra taken from urement two trend-cards were derived. The first one several measurements, telescoping them together and shows the bridge’s relevant stiffness patterns in the viewing them from above. For reasons of descrip- range of 1.5 to 35 Hz (Fig. 7) the second one illus- tiveness a two-dimensional visualisation is chosen. trates the range of 1.5 to 10 Hz in detail (Fig. 8). It has to be mentioned that the basic frequencies with their long-wave vibration forms are insensitive to local damage. Therefore the assessment and inter- Figure 7. Distribution of stiffness in the lengthwise direction, 1.5-35 Hz (vert.) - Eko mainland Center. Figure 8. Distribution of stiffness in the lengthwise direction, 1.5-10 Hz (vert.) - Eko mainland Center. 731 Due to the bridge’s geometry (similar span length intensity is subdivided into 4 zones, ranging from and identical cross section) an equal distribution of low probability of damage (Zone I) up to very high the structural integrity along the entire bridge length probability of damage due to dynamic stress (Zone could be expected. In fact the damage led to a con- IV).