VOLUME: 07, Special Issue 07, Paper id-IJIERM-VII-VII, October 2020 1
STUDY OF COMPOSITE BRIDGE CONSISTING OF PRECAST CONCRETE DECK ELEMENTS
Devesh Singh Raghubansi1, P.C. Diwan2
1Research Scholar, Dept of Civil Engineering, SVN University, Sagar, (M.P)
2Assistant Professor, Dept of Civil Engineering, SVN University, Sagar, (M.P)
Abstract:- In a composite bridge consisting of precast concrete deck elements the vertical forces will, when applied above a girder, make both girder and deck deflect simultaneously.
In other places the elements of the bridge will deflect somewhat. A hinging action is needed to transfer the vertical forces so that the two elements are able to deflect simultaneously.
This can be achieved either by making a joint of the type where transversal reinforcement and casting is conducted in-situ (wet joint) or by using shear keys fitted into the opposite element (dry joint). The focus of this report is on the latter, but a literature review is conducted on both. The results from those tests concluded that the shear keys used could handle a greater load than what the Swedish code stated. It was also noted that the steel reinforcement used in the shear keys were problematic to put into the right position.
Because of these notations it has become of interest to investigate if the shear keys can be designed in a different way in order to optimize both size and the reinforcementused.
1. BACKGROUND
There are a lot of bridges in different parts of the world that need to be replaced due to e.g. aging, increased volume of traffic or traffic load. Most of these are located on busy highways and congested areas which in turn makes it difficult to replace them. The direct and indirect costs due toe.g. Rerouting of the traffic, loss of use that an extended construction-time causes or construction- cost for building a temporary bypass-road could become significant. Therefore, new construction methods to shorten the disruption-time are required.
In other words, new bridge systems need to be developed that will allow components to be fabricated offsite and moved into place for quick assembly while maintaining traffic flow.One construction alternative that has a high degree of prefabrication is composite bridges with pre- fabricate steel girders and precast concrete deck. Transferring of the vertical forces from one element to another can be done in different ways as will be shown through this thesis where to focus will lie on overlapping male-female concrete shear keys.
2. RESEARCH PROJECT ELEM
This master’s thesis is part of a European research and development project called ELEM that was started in 2008. Four countries Sweden, Germany, Finland and Poland are represented in the research
group. The overall objective with the project is to make prefabricated bridges more competitive.
Some of the questions that will be looked into are:-
1. Opening of dry joints oversupports.
2. Assembling tolerances of dryjoints.
3. Design of concrete keys – how can the shear forces betransferred.
2.1 Advantages
Compared to concrete bridges less time is in most cases spent on the construction- site when a composite bridge with prefabricated deck elements is erected. A shorter construction time is beneficial not only for the contractor but also for the road user and the society as a whole, an aspect that can in some cases be neglected.
Advantages gained when constructing bridges of this type are:-
1. Less number of man-hours at the construction site, resulting in a reduced contact with traffic for the workers and less time away fromhome.
2. Improved working environment due to the fact that less work has to be donein-situ.
3. Lower road-user costs.
4. Higher quality can, however not for a certainty, be expected. This especially concerns the deck
VOLUME: 07, Special Issue 07, Paper id-IJIERM-VII-VII, October 2020 2 elements since they are cast
indoors in controlled environment.
5. Less transports of different materials such as formwork, concrete and steelreinforcement.
6. The use of precast elements has the advantage of eliminating the influence of thermal effects and the influence of early shrinkage.
The elements will shrink after casting but they will, for example, not induce any stresses in thegirders.
2.2 Disadvantages
One of the problems that are faced when constructing prefabricated element bridges is the lack of experience. All new technologies are however faced with this problem, where the keys to survive lays foremost in persistence and hard work.
Another problem that was noted when a pilot bridge was erected in 2000 over the river Rokån in Sweden, was the small margin of error that can be allowed, Stoltz (2001).
High demands are also put on the surface layer and insulation when it comes to movement and deformation capability. Since the elements are not homogenously connected to each other they have the possibility to separate. This gap will allow water to penetrate and could result in corroding reinforcement or frost burst of theconcrete.
3. LOAD OF SELFWEIGHT
Value for the self-weight is taken out of the Swedish bridge code, Bro 2004 Section 21.11, and is for normal reinforced concrete 25.0 kN/m3. The value stays the same even though the multinational standard Eurocode would be used. Values are there gathered from SS-EN 1991-1-1, A.1, and set to be 24.0 kN/m2 and increased with 1.0 kN/m2 because of the reinforcement.
4. APPLIED TESTLOAD
The applied load during the testing has the dimension of 400×400 mm2 in accordance with Load Model 1 (LM1) in Section 4.3.2 SS-EN 1991-2. Where it is also states that the load for a single axel should be counted as 600 kN.
Figure 4-1. FE-model for load transfer through concrete shear key.
4.1 Unreinforced
The test results for the unreinforced specimens vary from a breaking force of 105 kN to a force of 166 kN, see Figure 4- 2. These results should be compared with the theoretically calculated results 156 kN and 184 kN.
Figure 4-2. Test-results of unreinforced specimens.
5. CONCLUSION OF TESTRESULTS As it seems, it is hard to predict the strength using simple hand calculations and since the results vary in a wide interval it will never be possible. It will not be possible to do any type of optimization of the shear key as the results vary too much and the most important thing is to construct them to work safely. However, the test-results show that both the Ø12 and Ø8 bars could take the design load of 300 kN until breaking. So for Ultimate Limit State (ULS) the shear key seems sufficient.
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