Last updated on August 18th, 2023 at 11:11 pm
What are Pier and Abutment Caps? Give the specification for Constructing Pier and abutment Caps.
The pier cap or abutment cap (also known as bed block or bridge seat) is the block resting over the top of the pier or the abutment. It provides the immediate bearing surface for the support of the superstructure at the pier or abutment location and disperses the strip loads from the bearings to the substructure more evenly.
The pier cap should cover the entire area of the top of the pier and should project 75 mm beyond the pier dimensions.
This offset prevents rainwater from dripping down the sides and ends of the pier and also improves the appearance of the pier. The top of the pier cap except at bearings should have a suitable slope towards the outside
The cap should be of M20 concrete with a minimum thickness of 225 mm up to a span of 25 m and 300 mm longer spans’. The thickness is reduced at the end over the cutwaters.
The cap is provided with nominal reinforcement of not less than 1 percent steel distributed equally at top and bottom and provided in two directions both at top and bottom.
The reinforcement along the shorter side is in the form of hoops, extending for the full width of the pier cap.
The reinforcement along the length of the pier should extend from end to end of the pier cap.
In addition, provision should be made for the local strengthening of the cap with two layers of mesh reinforcement one at 20 mm from the top and the other at 100 mm from the top of pedestal or pier cap each consisting of 6 mm bars at 75 mm centers in both directions placed directly under the bearings.
What are the Material used for Constructing Piers and Abutments? Give the specifications of Material used for its construction along with values of Permissible
Materials for Piers and Abutments
Piers and abutments may be constructed with masonry, mass concrete, or reinforced concrete.
Masonry piers or abutments may use stone masonry (Granite) in cement mortar, or composite construction with stone masonry facing and mass concrete hearting.
Concrete construction will be economical in situations where good stones suitable for masonry and skilled stonemasons are not available locally.
Stonemasonry used for the pier construction should be of coursed rubble masonry, first sort in cement mortar 1:4. In the past mass, concrete was adopted in some cases, using 1:3.6 mix by volume with 38 mm size aggregate.
It was then permissible to add plums’, i.e., stones of 100 to 150 mm size, up to a volume of about 20% of the mass concrete in order to save cost Such stones had to be placed by hand not closer than 300 mm centers.
However, in recent practice, concrete of grade M20 with nominal surface reinforcement is being adopted.
For reinforced concrete piers, especially single-column piers, the concrete grade used may correspond to M25 to M35. Typical values of permissible stresses for mass concrete and masonry are given in Table 3.1.
Why the use of Single column pier is increasing day by day?
Single column piers are increasingly used in urban elevated highway applications, and also for river crossings with a skew alignment. In an urban setting, single-column piers provide an open and free-flowing perception to the motorists using the road below: Such piers when Groused for a skew bridge across a river results
Explain the different types of Caisson foundation.
Types Caisson foundations are of two types
1. Open caissons (also known as well foundations in India) ; and
2. rheumatic assassins.
An open caisson is one that has no top or bottom cover during its sinking. It is more popularly known as well foundation.
A pneumatic caisson Is a caisson with a permanent temporary roof near the bottom so arranged that men can work in the compressed air trapped under it. Pneumatic caisson can be used for a depth of about 30 m below water level beyond which pile foundations would have to be resorted to.
What is a Single Column Pier? Give its utility also.
Single column piers are increasingly used in urban elevated highway applications, and also for river crossings with a skew alignment. In an urban setting, single-column piers provide an open and free-flowing perception to the motorists using the road below.
Such piers when used for a skew bridge across a river result in the least obstruction to the passage of flood below on the bridge.
Which type of force is difficult to calculate in the case of abutment?
Earth pressure force is the most difficult to compute correctly. The magnitude of earth pressure varies with the character of the material used for backfill and the moisture content The earth pressure may be computed as indicated in Section 3.16.
It is important in abutment construction to place the fill material carefully and to arrange for its proper drainage.
A good gusto drainage system may be secured by placing rock fill immediately behind the abutment.
Give the formula for calculating efficiency factor,F by Converse- Labarre formula.
The efficiency factor F, by which the bearing capacity of a single pile is to be multiplied to arrive at the capacity of a friction pile in pile group, is frequently obtained from Converse- Labarre formula, given by Equation
What is the function of Batter Piles?
Batter piles (also known as ranking piles) are more efficient to carry horizontal force occurring at abutments and piers.
However, batter piles are difficult to drive with accuracy on the slope. These also need special equipment and expertise.
The maximum rake permitted for precast-driven piles is 1 in 4. When using a combination of vertical and batter piles, the analysis should consider the total system of the pile group with the pile cap, as these act together.
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