What are the precautions to be observed by the prestressed concrete bridge engineer?

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What are the various forces considered in designing the road bridge?

While designing road bridges, the following forces should be considered:

1. Longitudinal forces are caused by the tractive effort of vehicles or by braking of vehicles.

2. Longitudinal forces due to frictional resistance of expansion bearings.

3. Centrifugal forces due to curvature.

4. Horizontal forces due to water currents.

5. Buoyancy

6. Earth pressure, including live load surcharge.

7. Forces and effects due to earthquake

What is the role of IRC in designing the road bridges?

The Indian Road Congress (IRC) has formulated Standard Specifications and Codes of Practice for road bridges with a view to establishing a common procedure for the design and construction of road bridges in India. The specifications are collectively known as the Bridge Code. Prior to the formulation of the IRC Bridge Code, There was no uniform code for the whole country. Currently, we would follow the IRC Bridge Code.

What are the different types of IRC Standards Live Load considered during the design of road bridge?

There are four types of Standard Loadings for which road bridges are designed:

1. IRC Class AA Loading
2. IRC Class 70 R Loading
3. IRC Class A Loading
4. IRC Class B Loading

 

What is the role of Admixture in Concrete?

Admixtures are available for increasing the workability of concrete (plasticizers) facilitating the reduction of water-cement ratio and for retardation of the setting of cement during hot weather concreting. Concrete properties such as durability, strength, and service life can be enhanced by the use of suitable mineral and chemical admixtures.

 

What is the role of tendons in pretensioning?

The tendon provides a pre-compression force to reduce cracking under service load and also serves as tension reinforcement under the ultimate load condition. The integrity of the grout duct and the surrounding concrete governs the corrosion protection of the high-strength, low ductility steel tendon. Grouting also helps to avoid fatigue failure in the steel at the anchorages.

What are the various conditions which meet the prestressing force?

The prestressing force must meet two conditions :

1. It must provide sufficient compressive stress to offset the tensile stresses which will be caused by the bending moments.

2. It must not induce either tensile or compressive stresses which are in excess of those permitted by the specifications.

What are the precautions to be observed by the prestressed concrete bridge engineer?

Precautions to be Observed by the prestressed concrete bridge engineer

The prestressed concrete bridge engineer should have a thorough understanding of the behavior of prestressed concrete structures and should be fully familiar with the technique of prestressing, besides the properties of the materials used. He would do well to observe the following precautions at the design stage and during construction in order to ensure a satisfactory

1.  The designer should familiarize himself with the details and sequence of the building construction procedure proposed to be adopted. He should take into account the erection stresses.

2. The design should provide for shortening of the structure in the direction of prestressing, as prestressing can compress the concrete only when shortening is possible.

3. Adequate provision should be made in the design to cater to radial forces due to change in cross-section along the length of the member. Draped cables and splay of cables in plan ear supports cause radial forces when the cables are tensioned. Change in the direction of the centroidal axis of the concrete member leads to unbalanced forces which act transversely to the member. The design calculations and structural detailing should take these into account.

4. The high permissible compressive stresses in concrete can be utilized only if the stiff concrete can be placed and vibrated properly to obtain the designed strength in the field. To ensure proper placement and compaction, special care should be devoted to the choice of the cross-sectional dimensions of the concrete and the detailing of the untensioned steel and the tendons.

5. Tensile stresses should be avoided under dead load. It is prudent in bridge design not to depend on the tensile strength of concrete.

6. Untensioned steel should be provided in the longitudinal direction to cater to ultimate load conditions, and transverse to the tendons, and especially in the anchorage zones to take care of the concentration of forces.

7. Prestressing steel should be handled carefully, positioned accurately, and held securely. Prestressing steel is highly sensitive to corrosion, notches, kinks, and heat. In thin webs, precise lateral positioning of cables is critical. Lack of precision in the positioning of cables may lead to serious problems due to friction during tensioning.

8. The design of the formwork and the technique of concreting should be planned with utmost care to ensure adequate vibration of the concrete and to avoid cracking of the young concrete due to deflection of the formwork during concreting. The formwork should be checked for leaks at joints to avoid honeycombing in concrete.

9. The alignment of ducts should be checked after the threading of cables. Excessive washing with a tape of sheathing at joints should be avoided, particularly in the vicinity of the anchorage.

10. The sheathing of cables should be leakproof, as otherwise the tensioning of cables will be difficult.

11. Before commencing the tensioning of cables, it should be checked to see that the structure can move in the direction of tensioning to permit shortening. For reasons of safety, the cable line extended on each side should be kept free of persons.

12. Prestressing of tendons in long members should be taken up in stages, The first stage should be aimed at providing moderate compression to prevent concrete cracks due to shrinkage and temperature, Full pre-stress should be applied only when the concrete has attained its designed strength. It is worth remembering that the highest stresses in concrete usually occur during tensioning of the cables.

13. While tensioning, the cable force should be ascertained from both jack pressure and the cable extension. Records of the tensioning operations should be preserved carefully.

What are the various steps to be involved in designing the simply supported decks?

The steps involved in the design generally include the following:

1. List the properties of the materials used such as grade of concrete, high tensile steel, and untensioned steel. Usually, concrete of grade M40 is used for post-tensioned girders.

2. Assume preliminary dimensions, based on experience. The overall depth is usually about 75 to 85 mm for every meter of span. The thickness of the deck slab is about 150 to 200 mm with transverse prestressing and about 200 to 250 mm in composite construction. The minimum thickness of the web of the precast girder is 150 mm plus the diameter of the cable duct. The bottom width of the precast beam may vary from 500 to 800 mm.

3. Computer section properties. It is permissible to compute these based on the full section without deducting for the cable ducts.

4. Compute dead load moments and stresses for girders.

5. Calculate live load moments and stresses for girders for the severest

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Anand Kamble
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