Analysis of Prestressed Concrete Box Girder Bridge

Analysis of Prestressed Concrete Box Girder Bridge: Abstract – Pre-stressed concrete bridge analysis is completely dependent on the standards and design criteria. Herein, the current study compares like a pre-stressed concrete bridge under the effect of two different loading standards and specifications. The two different loading standards considered herein are IRC 6: 2000 and AASHTO-LRFD standards.

Further, the pre- stressed box girder bridge is modelled and analysis in MIDAS CIVIL. On carrying out analysis, the primary structural analysis parameters which are important for the design of structure, are studied. These parameters are shear force, bending moment and torsion in the bridge elements along its length. It became observed that AASHTO standards are uneconomical than IRC standards, due to consideration of heavy weight vehicle load moving on the bridge span. Thus, it might be said that pre-stressed box girder bridge analysis and design should be carried out effectively and optimistically using IRC standards and specifications. Concrete Box Girder Bridge

Introduction and Research Summary

Heavy concrete structures like bridges, dams, silos, etc. are being built-up around the world, under different standards specified by respective countries. Many studies are being presented by earlier researchers on the comparative study of such heavy concrete structures. Mostly these structures are constructed using pre-stressed concrete, which have a large number of benefits over the normal reinforced concrete. While considering the design and analysis of bridges, many factors have their different influence depending on the moving loads condition as in addition with surrounding environmental factors. Thus, the analysis of such pre-stressed concrete (PSC) bridge also requires special attention by considering the standardizations available, like Indian Standard (IS) codes, Indian Road Congress (IRC) standard codes, and (AASHTO) standard codes.

Liu et al. [1] carried out research work on a bridges with moving loads and developed a new damage locator to locate multiple damages occurring in a span of bridge. Further, the name of that damage locator was given to be moving load damage-locating indicator. They observed that the indicator simulates damage level by a reducing the stiffness of member elements. Further, Adam et al. [2]carried out study on reliable dynamic loading analysis of unreliable composite bridge deck under moving traffic load condition. However, it was concluded that this analysis could be performed even without knowing the unsure material parameters and merely by considering worst case conditions of structural inheritance.

The present study aims to analyze the PSC box girder bridge under the influence of AASHTO-LRFD standard code and IRC and IS codes. Further, moving loads as per IRC 6: 2000 [6] and ASASHTO- LRFD [10] standards. In This work comparisons of the cross-section specifications given in both the standards and further analyze the bridge using MIDAS CIVIL software. Further, the analysis was

carried out for both the standards were compared and studied for primary analysis parameters like shear forces, torsion and bending moments on different positions of bridge along its length.

GEOMETRY AND MATERIAL SPECIFICATIONS

This section, geometry of box girder section used in this study is showcased in detail using Figure 1. The longitudinal span of bridge is 35 m, made up of 14 precast segments. This span is acting as a continuous span, having total width of 12.6 m. This 12.6 m flange width comprises carriage way of 9.6 m and remaining into footpaths on either side of way. Depth of box girder was kept as span by 25, which comes out to be 2.8 m.

1.      MODELLING OF THIS BOX BRIDGE GIRDER

Modelling of box bridge girder is done by using MIDAS CIVIL software package. Two similar models were prepared based upon abovementioned geometry and material specifications. The loading conditions were provided differently for both the models as per IRC 6: 2000 (Section II) and AASHTO-LRFD.

1.1  Loading details on Box Girder Bridge

Further, there were some common loading conditions in both the models, which are here below as:

a.      Dead load (DL)

The dead load sustained by the girder or component is made up of its own weight, as well as portions of the superstructure’s weight and any fixed loads sustained by the member. During designing, the dead load may be anticipated pretty well, and construction and service can be managed.

Moreover, weight of SIDL which includes earth-fill, wearing course, ballast, waterproofing, conduits, cables, pipes, etc. are installed on the structure. This superimposed dead load comprises total weight of approximately 20kN/m.

b.      Live Load (LL)

Vehicles passing over the bridge create live loads, which are temporary in nature. These loads are impossible to predict correctly, and once the bridge is exposed to traffic, the designer has very little control over them. For the evaluation of a two-lane box girder, the similar categories of loadings are used.

As per IRC – Vehicle Load: Class A and Class 70R; Dynamic Allowance: 33%

As per AASHTO – Vehicle Load: HL-93TDM, HL-93TRK; Dynamic Allowance: 33%

Modelling of Box Girder Bridge

Herein Fig. 2 below, model of the bridge girder is represented. This model has been prepared with the application of specifications given in section 2 above.

Results and Discussion

After carrying out analysis in MIDAS software, the results of box Girder Bridge were studied. Primary structural analysis parameters like, shear force, Torsion and bending moment were studied and compared in both the models, subjected to different loading conditions.

The comparison of results has been provided in below, which compares shear force, bending moment and torsion coming on to each segment of bridge along the length. Further, figure 3 shows the bending moment of bridge after the application of loadings.

On further discussion for shear force and bending moment values by comparing the results of two models based on different loading standards, the analysis results were found to much greater in AASHTO loading conditions than in IRC loading conditions. From figure 9 it can be observed that for element of box girder bridge is having 20% more Torsion in AASHTO loading conditions as compared to IRC loading conditions. Similarly, the values of shear force also higher for AASHTO loadings than IRC loading conditions. This was due to the heavy weight vehicles considered in American standards. As the supports are simply supported, the bending moment in end supports is zero in both loading conditions.

Figure 4 and 10 showcases the bending moment and shear force of bridge under both the loading conditions to provide a visual representation of analysis results for each element of the bridge.

Moreover, from Figure 4, 9 and 10 the values of bending moments, Torsion and shear force were higher only in case of mid elements and not for remaining elements. For all elements, other than end elements, the shear force, bending moment and torsion values were almost different in both the loading conditions.

CONCLUSIONS

This research compares of two different standards for the analysis of box girder bridge. The two standards considered herein are IRC and AASHTO standards, which differs with each other in loading conditions of vehicles, and many other factors. The equivalent study has been carried out by doing analysis in MIDAS CIVIL software. The results of primary structural analysis parameters were compared under loading conditions of two different standards.

1. It has been found that the maximum values were higher for AASHTO loading conditions than IRC loading conditions.
2. The consideration of heavy weight vehicles under AASHTO standard causes the structure to yield higher shear force, torsion and bending moment values in all the elements of box Girder Bridge.
3. This concludes that IRC code of standards can be used in analysis of like box girder bridge sections to design an economical structure.

References: Concrete Box Girder Bridge

• Liu F, Li H, Yu G 2007 New Damage Location for Bridges Subjected to a Moving Load Journal of Ocean University of China, 6(2), 199-202.
• Adam C, Heuer R, Ziegler F 2012 Reliable Dynamic Analysis of an Uncertain Compound Bridge under Traffic Loads. Springer-Verlag, Acta Mechanica, 223, 1561-1581.
• Reddy GS V, Kumar PC, 2014 Response of Box Girder Bridge Spans – Influence Based Moving Load Analysis International Journal of Bridge Engineering, 2(2), 21-30.
• Patil YS, Shinde SB, 2013. Comparative Analysis of Box Girder Bridge with Two Different Codes International Journal of Civil Engineering and Technology, 4(3), 111-120.
• Freyssinet Pre-stressing (pre-tensioning and post-tensioning) manual.
• IRC: 6-2000, Standard specification and code of practice for road bridges, section-II.
• IRC: 18-2000, Design criteria for pre-stressed concrete road bridges.
• IRC: 21-2000, Standard specification and code of practice for road bridges, section-III.
• IS: 1343-1980, Code of practice for pre-stressed concrete.
• AASHTO (2007). “AASHTO-LRFD Bridge Design Specifications”.

The analysis of prestressed concrete box girder bridges is a complex process that involves considering the effects of the prestressing, the dead load, the live load, and the environmental loads. The analysis can be performed using a variety of methods, including:

• Hand calculations: This is the simplest method, but it is only accurate for simple bridges.
• Computer programs: There are a number of computer programs available that can be used to analyze prestressed concrete box girder bridges. These programs are more accurate than hand calculations, but they can be more time-consuming to use.
• Finite element analysis: This is the most accurate method, but it is also the most complex. Finite element analysis uses a computer to divide the bridge into a number of small elements, and then analyzes the behavior of each element.

The analysis of a prestressed concrete box girder bridge typically includes the following steps:

1. Determine the geometry of the bridge. This includes the dimensions of the box girder, the spacing of the prestressing strands, and the location of the supports.
2. Calculate the dead load of the bridge. This includes the weight of the concrete, the steel, and any other materials used in the bridge.
3. Calculate the live load of the bridge. This includes the weight of vehicles, pedestrians, and any other loads that the bridge may be subjected to.
4. Calculate the environmental loads. This includes the effects of wind, earthquakes, and temperature changes.
5. Determine the prestressing force in the strands. This is done by considering the dead load, the live load, and the environmental loads.
6. Analyze the stress and strain distribution in the bridge. This is done to ensure that the bridge is safe and will not fail under the applied loads.

The analysis of prestressed concrete box girder bridges is a critical step in the design of these bridges. By carefully analyzing the bridge, engineers can ensure that it is safe and will perform as expected.

Here are some of the advantages of using prestressed concrete box girder bridges:

• They are very strong and can support large loads.
• They are relatively lightweight, which can save on construction costs.
• They are durable and can withstand a wide range of environmental conditions.
• They are easy to construct and can be built quickly.

Here are some of the disadvantages of using prestressed concrete box girder bridges:

• They are more expensive than other types of bridges.
• They can be difficult to repair if they are damaged.
• They are not as flexible as other types of bridges, which can make them more susceptible to damage from earthquakes.

Overall, prestressed concrete box girder bridges are a versatile and reliable type of bridge that can be used in a wide range of applications. They are strong, durable, and easy to construct, making them a good choice for many bridge projects.