This article is the summary of a dissertation for the degree of Master of Science Civil Engineering, IST.
Octávio Martins, Portugal
Press
HERE to download the virtual model of bridge construction by the incremental launching method.
The file (4mb) allows the user to move with total freedom in the place of construction.
It's necessary the instalation of the program Eon Viewer v.5.5 to open the file (freeware).
Abstract:
To facilitate understanding of the various phases involved in the incremental launching usage method, it was made an application of visual simulation through programming and three-dimensional modeling in virtual reality environment of the elements involved in construction.
The application of visual simulation was designed to allow direct access to any stage of the constructive process in which it is based and may be viewed from any point within the virtual scene, thus facilitating their understanding. The access to the application can be established through a web page placed on the Internet.
In this article, the main aspects of the constructive process by incremental launching are mentioned, the assumptions of the case study to use are defined, and with the support of tools for programming in the virtual environment the construction of the virtual deck is planned.
Contents
1. Introduction
2. Construction methods
3. Introduction to Virtual Reality
4. Case study
5. Geometric modeling and characterization of the elements
6. Programming of interactive model
7. Conclusions
References
1. Introduction
The construction method for the execution of bridges and viaducts must be taken into account from the earliest stages of the design [1]. Due to its importance, and because it involves several technical teams that depend on it, his detailed knowledge is very important. Sometimes this understanding is complicated by increasing innovation and upgrading of new construction methods of the main construction companies that compete with each other.
The construction of the bridge deck using the method of incremental launching exist from the 60s [2] but their implementation did not occur in the same way in different countries. As possible causes for the low use of the constructive method in the national territory, are the aesthetic constraints, the high investment in equipment that uses other constructive processes or the lack of projects that promote it. Accordingly, this article aims to provide a contribution to the dissemination of information available on the constructive method by incremental launching, through a recording of visual simulation of the phases that comprise the constructive process.
The current visual application is an object to the appropriate computer-based distance e-learning platforms. The application will be directed not only to professionals who directly or indirectly are related to the construction of this kind of works, but also to education, as learning tool. The technology of virtual reality (VR) has been applied as a complement to 3D modeling, leading to better communication between the workers, both in training and in work [3].
2. Construction methods
The processes of construction have a huge influence on the selection of the cross section of the deck and, consecutively, the structural solution [[2]]. This solution must verify the five objectives: functionality, security, durability, economics and aesthetics, inherent in any structure.
Perhaps in no other works in the field of engineering structures, the constructive process affects the design as the bridges [1]. This chapter refers to some of the main methods of construction (falsework supported on the ground, formwork carriage and cantilevering method made by formwork travelers) including the construction by incremental launching.
The falsework supported on the ground (scaffolding), exemplified in figures 1 and 2, is the most traditional method, due to its easy implementation and economy, and is the oldest construction type of bridges [4]. It is appropriate to viaducts with low height (up to 20m) [1], and always supported on a firm area. The falsework does not restrict the type of section, therefore the deck may be supported by ribs, beams or with box girder cross section.
Figura 2 - Ribeira of Seda viaduct, Portugal [Peri].
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Figura 1 - Falsework supported on the ground.
The formwork carriage, represented in figures 3 and 4, emerged with the need for extensive construction of viaducts that could run freely without any limitation by the type of terrain or the height of structure. The formwork can be used to support in-situ concrete, or just to elevate and suspend precast staves. It is suitable for the execution of long bridges, straight or with a small curvature [2]. The span lengths must be equivalent, with values of 30 to 60m and can reach 70m [1]. The deck is generally supported by beams or ribs. The basis of this operation method is to support the formwork on the final elements (pillars and abutments), may also use part of the built deck like support too. Thus, the displacement of the formwork is done from pier to pier.
Figura 4 - Cornellá viaduct, Spain, [ulmac].
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Figura 3 - Formwork carriage.
The bridges built by the cantilever method, shown in figures 5 and 6, have its structural model strongly conditioned by the constructive process [5]. The structure advances from a short stub on top of a pier symmetrically in segments of about 3m to 5m length to the mid span or to an abutment. This system is the most used for building bridges with spans of 60m above, regardless of height [5]. It is also used with some frequency in the construction of arches of bridges and has a considerable field of application in the bridges built by precast staves. For extensions of less than 50m range, can be equated with the earlier solutions, depending on the number of piers and height.
Figura 6 - High-speed line, Taiwan, [VSL].
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Figura 5 - Cantilever method
The incremental launching method consists of casting 15m to 30m long segments of the bridge superstructure in a stationary formwork to push a completed segment forward with hydraulic jacks along the bridge axis, as shown in figure 7.
Figura 7 - Construction yard.
In 1961-63 the first bridge of reinforced concrete was built through incremental launching [2] in the city of Guyana, Venezuela. Andrä strengthened this constructive process, in 1965, with the discovery of the advantages of application of Teflon® on the support (figure 8), allowing greater slip of the deck during its incremental progress.
Figura 8 - a) Launching pad; b) Support temporary; c) Application of the lauching pad.
The incremental launching is designed to be applied in viaducts over valleys and mountains with spans of about 50m [6]. However, its use is common in flat areas where access at ground level may be conditioned, or on which it is inappropriate to impose disturbances, such as works on traffic, railways, etc.
The length of the optimal span is between 45 and 50m. The method can be applied also in bridges of reinforced concrete and in steel bridges and mixed. The bridge axis can be straight or curved circular. It is estimated that the minimum length of a bridge or viaduct to justify the cost of its implementation is 200m [7]. The application of this constructive process requires that the height of the cross section needs to be constant over its entire length, because each section will have different states of bending moment and thus different tensions. The appropriate type of section is the box girder, as they have a good relationship between the top and bottom flexural modulus.
The longitudinal pre-stressing of the deck is divided into 2 groups: the pre-stressing of constructive stage, installed and placed under tension before the deck launch, and the extra pre-stressing, placed under tension after the deck reach its final position.
As specific equipment for the proper use of the incremental launching method is: the launching equipment, responsible for the advancement of the deck (figure 9); the support bearings, which facilitates the sliding of the deck on the piers and the launching nose, located in front of the deck.
Through the application of visual simulation that was developed in this work, it is possible to observe the geometry and operation of equipment in the process.
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Figura 9 - a) b) c) Launching system (click on the pictures to enlarge).
3. Introduction to Virtual Reality
The term virtual reality (VR) refers to a variety of ideas and technologies [8], but can be defined as a computer generated environment involving real-time simulation and interactions through multiple channels of sensors. These sensors correspond to the five senses of human beings [9]. For the development of any application in the field of virtual reality, Burdea stresses the importance of three parameters, which are: immersion, interaction and imagination. By coincidence of words, he called the triangle as I3.
The connection of the user to the virtual world is not just through a computer screen. How Burdea notes, this connection is very important to define the user's immersion in the virtual environment, and can be achieved through simple and portable devices such as Head-Mounted Display (HMD), or more elaborate systems such as the Cave Automatic Virtual Environment (CAVE). Besides the movement of the user, he can also interact with objects within the virtual environment. From the simple handling of the keyboard, mouse or joystick, to more sophisticated sensory glove, the computer can connect the coordinates and movements of the glove with the coordinates of objects.
The VR is now present in various sciences such as medicine, fluid dynamics, space exploration, flight simulators, and many others [8]. The military industry in the area of aviation, was the first to use VR to counter excessive investment in flight simulators.
In engineering, architecture and construction, which are directly related to the objective of this work, the VR has been very useful. Designers can, through virtual environments, visually browse, inspect and submit 3D drawings in an immersive way with the appropriate size and scale [10]. As an example of a tool based on virtual reality, for the planning phase of a constructive, it is an application developed under a project of the IST / ICIST [11]. This application seeks to show, in stages, all the elements that constitute a double wall of brick, since the submission of armor needed for the creation of piers to the placing of doors and windows.
4. Case study
The type of bridge that was used for this work resulted in a compilation of several cases reported in the literature and various photographic records available in some construction companies and engineering schools such as Federal Polytechnic School of Laudanize (http://is-beton.epfl.ch).
The cross section of the deck, abutments, geometry and size of the piers are based on the Itztalbrücke viaduct, built in 2007, near Rodental, Germany (www.abdnb.bayern.de, 2008). As the constructive process involved is due to a combination of cyclical actions, the case study consists only in one deck, supported by 6 piers, showing however the same central span with 58m (Figure 10), like the viaduct Itztalbrücke.
Figura 10 - Longitudinal view of the case study bridge.
Given its size, this virtual bridge is included in the group of long bridges and therefore the launching equipment could be the system Eberspächer [www.eberspaecher.org, 2008]. This equipment was referenced explicitly by Gohla [7] as owner of all the capabilities for this function. The bearing supports, used in construction phase, does not have any commercial reference because it was designed from the specifications and dimensions specified in [6].
The piers and abutments are fully built in visual model, because its implementation in work does not depend on the constructive process of the deck.
For the creation and modeling of the elements was used AutoCAD® 2008, from AutoDesk. For the virtual interaction was used the program Eon StudioTM, from EON Reality. This company, founded in 1999, referred to itself as a world leader in the management of three-dimensional (3D) visual content and the production of software for Virtual Reality. The software Eon StudioTM is a tool for creating interactive applications in 3D environment. Given the large cost of such software, it was important that the Instituto Superior Técnico (IST) have their licenses for their use, which are available for research in the laboratory of architecture of the IST (ISTAR).
To establish the communication between Eon StudioTM and AutoCad® was used 3ds Max ®, also from AutoDesk.
5. Geometric modeling and characterization of the elements
The unit of length used was the meter. The origin of the reference was defined at the base of the casting yard, represented in figure 11. Thus symmetry is obtained by the longitudinal vertical plane and the segments will be advanced from the vertical transverse plane (y = 0).
Figura 11 - Reference adopted for the modeling and programming of objects.
The form used for the 3D modeling was, in most cases, make the piece in two dimensions (2D) and then extend that section to create a solid. Later, using rotations, unions and subtractions between solids, inherent properties of AutoCad, obtained the desired element. The object that will move during the simulation were visually differentiated by assigning the "layers" or color in AutoCAD in order to be identified by the transition software (3ds Max ®). Taking as example the falsework used in the construction of segments of the deck, the figure 12 shows one of its elements. Through the operations described above, the metal parts were designed, then copied along the longitudinal direction of the deck and mirrored according to the longitudinal vertical plane (with x = 0). Finally, the wood beams were created, to make the connection between the score and shuttering, and, with the same procedures, were placed in their correct position, as shown in the third image of figure 12.
The remaining objects were executed in the same way, varying the geometry. It was necessary to carefully assess what level of detail required for modeling in view of its presentation in virtual reality, because the geometric level of detail will be set depending on the purpose with which the interactive model is created.
For the representation of reinforcement, shown in figure 13, one steel mesh was designed in two dimensions. In 3ds Max® these lines were setup as a way to go for a section in order to form a solid at each bar. It was not necessary to model all the reinforcement of the bridge, but only for a segment. The cross section used for steel bars was the rectangular instead the circle, to minimize the number of vertices and faces of all.
Figura 13 - Reinforcement steel mesh.
Figura 12 - Exterior formwork of the casting yard.
The launching nose was the most complex object produced in this work. His assembly in place is very repetitive, and will be showed in the application virtual, but the geometry of its elements is very uneven, as shown in figure 14. The application of materials in 3ds Max® is in itself a matter of high complexity. It is possible to reproduce material very close to reality due to the large number of tools and options that the software provides.
The materials created in 3ds Max®, in general, could have been made directly in the Eon Studio. The 3ds Max® was chosen because it has a library with some pre-defined materials, and has a working platform frankly better. It is possible to change the characteristics of a material, such as color, brightness, roughness, and other, and instantly see the effects.
For this work were essentially created two types of materials, with the use of image mapping as their difference. Through mapping coordinates and other properties, it is possible that the image is repeated in different directions.
The Figure 15 shown the abutment of the bridge as one of the objects where the texture used has a concrete image. Besides the image applied in the concrete elements, was used an image of different texture to represent other material, wood, to be applied to the shuttering plates and the beams used for temporary support of the launching nose, represented in figure 16.
Figura 16 - Temporary support.
Figura 15 - Abutment.
Figura 14 - Launching Nose.
The remaining objects, mostly because they are the metallic type, can be set only with a material based on color, brightness, reflections, roughness, etc. Noting the hydraulic equipment, represented in that Figure 16, it is possible to check the differentiation between two materials, in addition to color. The outer cylinder, blue, has a uniform tone. The inner cylinder, light-colored, has a gloss and the reflection inherent of a sliding surface.
6. Programming of interactive model
Figura 17 - EON desktop.
Figura 18 - "Place" node.
In this item will not be exhaustively described all the procedures used to perform the programming and design of the application. Once all objects created and arranged in the correct position on the reference chosen, they were exported to the program EON StudioTM in order to start the programming of the virtual model.
The figure 17 displays the desktop of the EON StudioTM. In the middle window is available the hierarchical tree of the elements involved in the application to create. The hierarchy in this system facilitates the highly oriented programming with objects. During programming the nodes are added, which are essential for the design and implementation. By selecting any node in the middle window, its properties can be viewed and edited in the window on the right. The map of the simulation is in the left window. In this board, and quite intuitive, there is established the communication between the nodes, forming a network of connections, as detailed in figure 18.
With the purpose to give some immersive capacity to the application, were used two nodes from the EON system. The first node, of type "cgMaterial," served to create the river by the mixing of colors and the definition of other parameters. The second node, of "panorama" type, served to create the virtual environment of the scenario. This node allows the definition of an image for the sky, horizon and floor. The user will have freedom to move freely in the virtual environment, so the images set to the horizon and sky, must grant the projection to 360 degrees. To achieve the effect of movement of the river, the nodes of type "timesensor", "multiplication" and "cgMaterial", were dragged to the map of simulation, with the names "timesensor_Mar", "Multiplication" and "Water", respectively, as represented in Figure 19. The first node will be active during the application and is responsible for sending impulses to the node "cgMaterial (water)", after amplified by a multiplicative factor. These pulses provide a constant change of the parameter "time" of the node type "cgMaterial (water)", in other words, to change the amplitude of the waves in this material, thus creating the illusion of their movement.
Figura 19 - Nodes responsible for the movement of the river.
After defining the necessary considerations and details on the framework of virtual scenario, the planning of the sequence of events to create was established. In order to report an overview of the virtual environment, the application starts up in the opposite margin that where the casting yard is located, as shown in figure 20. The elements visible at this stage are the abutments, piers and beams of the foundation of the yard. The movement of the camera, from a margin to other, was done with the programming of "keyframe" node.
Figura 20 – Initial sequence. Approach the casting yard.
After the casting yard is achieved by the camera, begin the assembly of the elements which compose it. Some elements are placed in a lateral area of the yard, and will be moved back using the "keyframe" node, others are in the invisible state and became visible by turning on the "setrun" property of the node “frame”. After the beams and foundation support are positioned, is mounted the falsework exterior, composed of 26 elements equal. Only the assembly of one element will be represented in detail. The first image of Figure 21 illustrates the assembly of the element, and then the other elements appear in a synchronized way, duly assembled, as is the central image. The showing of the elements is done using the node of type "timesensor" to allow display in an incremental manner.
Figura 21 - Sequence assembly of the outer falsework.
During the animation, the positioning of the camera and its movement is very important, because the sizes of pieces to be displayed are different, and the action occurs in different places. This resulted in the existence of two sequences in parallel, dependent of each other.
After placing the shuttering plates that end the construction of the falsework, the assembly of the launching nose is initiated. The nose assembly will begin by placing the longitudinal beams, as shown in Figure 22. This operation is done in 3 movements for each of the 3 pairs of elements of beam. Each set has an upward displacement, followed by a position in extension of the falsework, and finally is released on the temporary support, to be attached to the first segment. The continued assembly of the launching nose is similar to the falsework assembly. The camera is moved with the equipment to display, and shows the assembly of an element in detail, and finally, the other components of the equipment are incrementally displayed.
Figura 22 - Assembly of the launching nose.
To perform the sequence of assembly of the hydraulic unit on the front of the launching nose, the camera was rotated from its previous position, to a lateral position, as shown in figure 23. The elements, again, are displayed by their order of assembly on yard. First the base of the equipment properly reinforced is screwed, then is placed the hydraulic equipment. After the completion of the assembly, is given an order to display the hydraulic equipment on the opposite side. Finally, both contract and expand, in synchronized way, to provide understanding about the functioning of the piece.
The contraction and expansion of equipment are achieved through the handling of the 3 distinct elements that constitute the piece. In figure 24 is showed three images, illustrating for each one, the axis of rotation of each of these elements. The definition of the axes was done in 3dsMax® program, which are then recognized by the EON system, as part of each element.
Figura 24 - Axes of rotation of the equipment.
Figura 23 - Placement of hydraulic equipment.
After completion of construction of the initial phase of the yard, is triggered a constructive sequence of the segment. For the start of the displacement of the deck, the temporary support of the nose is removed and the segment is separated from the shuttering. To represent the advance of the deck, the launching equipment appears in detail. This equipment consists of two twin parts located in the base of each web of the deck, and through their four movements, illustrated in figure 25, provides the launching of the segments. For this operation were defined the axes of rotation for each element, and through carefully related displacements and rotations based on time it was the cycle of the four movements. This cycle repeats itself, neatly until they check the progress of the segment with 31m long, together with equipment to launch, the launching nose, the interior falsework, and the segment.
Figura 25 - The four stages of operation of the equipment launch: starting position, lift, push, drop and collect.
Before the execution of the second segment, some movements are performed with the visualization camera to facilitate the perception of that portion of the deck was moved and the remaining portion. The arrival of the nose to the first pier is during the advance of second segment. To simulate the deformation of the launching nose by bending moment, it was made a rotation of the element, having as the rotation axis the lowest point of its connection to the deck, represented in Figure 26 with the "O".
Figura 26 - Deformation of the launching nose at the moment of arrival at the first pier.
In Figure 27 it is possible to see the transposition of the pier by the nose. The small brown parallelepiped, which is shown in the figure are the launch pads and are placed manually by workers between the launching nose and the temporary support that is on the pier. This phase of the animation is programmed to run various actions at the same time, properly synchronized.
Figura 27 - Sequence of arrival at the first pier.
The construction of the remaining segments is performed in fast mode, because their construction is identical to the previous segments. The display returns to normal speed to be applied upon arrival to the abutment. Already in the final phase of construction, the yard is removed, giving space to the appearance of the landfill. The execution of virtual bridge could not end without provide the finishing which allow the sense of construction completed. For this operation, the camera moves along the bridge, while the guards and the finishing elements are positioned as shown in the right image of the Figure 28.
Figura 28 - Arrival to the abutment and implementation of landfill.
The application resulted in a file with 4mb. 113 nodes were programmed manually resulting more than 2400 lines of code, 212 nodes of the type “keyframe”, 122 nodes of the type “place”, 15 nodes of the type “timesensor” among others.
7. Conclusions
Compared with other visual representations, the application that resulted from this work, is: greater complexity of material and space in an attempt to provide more immersive capacity for its view; the introduction of a menu of events and displays, which allows the selection by the user of rapid events that he may want to view; concern with the user, trying to predict the possible sources of misunderstanding and giving them the prominence that it was considered necessary; showing the movements of the camera in a consistent way, to present all sequences of events not only in teaching but also in appealing to provide interest and attention from the user.
It is supposed that this work has the aim of providing not only the direct, fast and consistent understanding of the incremental launching method, but also show the capacity of representation in virtual environment in support of innovative techniques for construction.
References