DIGITAL DOCUMENTATION , BRIDGE DECK LINEARITY DEFORMATION AND DECK THICKNESS MEASUREMENT USING TERRESTRIAL LASER SCANNER ( TLS ) AND CLOSE RANGE PHOTOGRAMMETRY ( CRP )

Bridges are one of the vital and valuable engineer structure from decades. As they play a major role in the road transportation sector. Few old bridges lacks its documents about the measurements of the structure. The study has been carried out on three different types of bridges like Truss, Beam and Cable bridges. Documenting these bridges can be utilised to reconstruct or renovate the bridge in case of any disaster or damage. 3D documentation is made from the point cloud Dataset acquired from Terrestrial Laser Scanner TLS (Riegl VZ 400) and Close Range photogrammetry – CRP (Nikon DSLR 5300). TLS and CRP point cloud are merged together to increase the density of points. Over the duration of time the bridge gets older and due to the load on the bridge deck, linearity in the deck effects and this linearity deformation measurement is important to know the present deformation in the deck. To know exactly at which part there is more linearity deformation, deflection is calculated at sample intervals between the present linearity conditions of the deck to the idle linearity conditions of the deck. The bridge deck thickness is also measured with the point cloud dataset. A slice is cut through the deck of point cloud dataset, the difference between the top and bottom layer of the deck gives us the thickness of the deck including the road. This thickness can be used to measure when a new deck layer is constructed or during filling up of any potholes. This study is mainly focused to help the construction and maintenance authority, bridge monitoring department and researchers. * Corresponding author


INTRODUCTION
Different engineering structures have different roles in the modern day to day life.Bridges play a major role in connectivity between two sides.In this study three different types of bridges are focused which are Truss Bridge, Beam Bridge and Cable Bridge.Digital documentation is very reliable method for storing the geometrical information for longer duration of time.Due to heavy traffic conditions and harsh environments bridge deterioration takes place which can be monitoring using non-contact technique like Terrestrial Laser Scanning (TLS) and Close Range Photogrammetry (CRP).Many researches has been carried out for generating photorealistic 3D models using TLS and CRP which includes documenting A'Famosa Fortress, Porta de Santiago, Malaysia, (Wei, Chin, Majid, & Setan, 2010).Joint use of TLS and Photogrammetry is used to enhance the position of the edges and linear surface features.Also generating a 3D virtual model for documenting of Al-Khasneh, Petra, Jordan (Alshawabkeh & Haala, 2006).An automatic approach for efficiently monitoring the status of buildings under construction like as-built dimension calculation and control can be done with the help of CAD models generated from lidar scans.The approach is made more efficient by increasing the registration quality by point matching method which uses iterative closest point (ICP) algorithm (Bosche, 2010).Bridge measurement documentation can be done with the help of TLS which is fast , safe and applicable to different weather conditions (Truong-Hong & Laefer, 2014).An automatic approach of workflows have been used to extract the minimum under clearance of Bridges using laser point cloud data (Tang & Akinci, 2012).A case study to find where the minimum under clearance and the geometry of a pre-stressed concrete beam is monitored with TLS and verified with total station for its accuracy in the paper (Riveiro, González-jorge, Varela, & Jauregui, 2013).TLS has also been used to estimate Stress on a beam structure when subjected to a concentrated load and the structural responses were monitored with finite element analysis (Lee & Park, 2011).The measurements obtained from the point cloud and laser distometer with their difference and the mean error obtained are tablulated in the table 3.

Table 3: comparison of measurements of Truss Bridge
The deflections in the deck linearity is measured at different sample intervals between both the ends of abutment.The Cable Bridge deck thickness obtained is 792.57mm.

CONCLUSIONS
The 3D digital documentation prepared can be stored for longer time period and can be used for reconstruction or renovation.The mean difference between the measurements obtained from the TLS and CRP point cloud dataset with the laser distometer for the Truss, Beam and Cable Bridges are 0.0552m, 0.0241m and 0.0333m respectively.The deck linearity deflection can be used to monitor it timely.The maximum bridge deck linearity deflection obtained for Truss Bridge is 112.23 mm at the centre of the deck, Beam Bridge is 53.52 mm at the right side of the centre pier and Cable Bridge is 103.91 mm 4 th left cable.The deck thickness measurement is an important factor to monitor the construction of deck or when a new layer of road is to be constructed.The bridge deck thickness obtained for Truss, Beam and Cable bridges are 370 mm, 622.46 mm and 792.57mm respectively.

Figure 1 :
Figure 1: Study Area Map Table1: Bridges with Point Cloud datasets and the number of scans and images acquired3.METHODOLOGY, INSTRUMENT'S AND SOFTWARE'S USEDThe workflow that was followed during the research has been shown in the figure 2. The data was acquired from different scan positions.All the scans were co-registered together to form a single point cloud.Many overlapping images were captured at the study sites.All the images were aligned and Tie points were generated.Dense point cloud was generated with the help of Tie points.TLS and CRP point clouds were merged together.Noise and Outliers were filtered out from the merged point cloud.Then the required outputs were extracted.The values were validated with the help of laser distometer.

Figure 2 :Figure 3 :
Figure 2: Methodology TLS Point cloud was acquired with the instrumetnent Riegl VZ 400 and RiSCAN PRO software further preprocessed in it.CRP point cloud was generated from the images in the Agisoft

Figure 7 :
Figure 7: Parts of Truss Bridge

Figure 8 :
Figure 8: Truss Bridge deck linearity deflections All the deflections obtained are shown in the graph format to easily identify the maximum deflection through out the deck.No support was present except the abutments at either ends of Truss Bridge deck.

Figure 9 :
Figure 9: Truss bridge deck linearity deflection graph The maximum Truss Bridge deck linearity deflection obtained is 112.23 mm shown in red.

Figure 10 :Figure 11 :
Figure 10: Bridge deck thickness of Truss Bridge The Truss Bridge deck thickness obtained is 370mm.

Figure 13 :
Figure 13: Parts of Beam Bridge

Figure 16 :
Figure 16: Bridge deck thickness of Beam BridgeThe Beam Bridge deck thickness obtained is 622.46 mm.4.3 Study area 3: Cable Bridge

Figure 19 :
Figure 19: Parts of Cable Bridge

Figure 20 :
Figure 20: Cable Bridge deck linearity deflections In the figure 21, the bold black values corresponds to the deflection at the left cables, centre foundation support and right cables of the Cable Bridge.The maximum Beam bridge deck linearity deflection obtained is 103.91 mm shown in red at the 4 th left cable joint with the deck.

Table 5 :
comparison of measurements of Cable Bridge