Different Forms of Bridge Structures and their Pros and Cons

Following forms of structures are generally considered for Bridges:

Superstructure

• Steel/Concrete Composite Deck
• Pre-cast Pre-stressed Concrete Deck
• In-Situ Reinforced Concrete Deck

Substructure

• Reinforced Concrete Bank Seat Abutments (Open Aspect)
• Reinforced Concrete Cantilever Abutments (Close Aspect)
• Reinforced Concrete Cantilever Wing Walls
• Reinforced Earth, Concrete Panel Clad, Wing Walls

Foundations

Bridge foundations generally fall into two categories:

• Strip footings, one for each pier and abutment. However, it is sometimes convenient to split the deck into two halves longitudinally along the centre line, this is then continued to the footing
• Piled foundations

Superstructure

a) In-Situ Reinforced Concrete Deck

Advantages:

• Robust with regard to impact
• Alleviate the need for heavy lifting plant
• Lower construction costs than other options
• Lower maintenance costs than steel option.

Disadvantages:

• Longer construction period
• Major disruption to traffic
• Higher traffic management costs
• Numerous concrete deliveries to site
• Requires central support.

b) Steel/Reinforced Concrete Composite Deck

Advantages:

• Least disruption to traffic than other options
• Lower traffic management costs than other options
• Aesthetically pleasing
• Does not require central support

Disadvantages:

• Most susceptible to impact damage
• Higher maintenance costs than other options
• Requires heavy lifting plant but slightly less than the pre-cast concrete option
• Total road closure required during beam lift
• May require fabrication area adjacent to site

c) Pre-cast Pre-stressed Concrete Deck

Advantages:

• Reduced level of disruption to traffic when compared to the in-situ reinforced concrete option
• Lower maintenance costs than steel option
• Does not require a total road closure
• Robust with regard to impact
• Shorter procurement period than steel

Disadvantages:

• Requires central support, increasing disruption to traffic
• Require heavy lifting plant, slightly greater than the steel option.

Superstructure

a) Reinforced Concrete Bank Seat Abutments (Open Aspect)

Considered on the basis of aesthetics, this method of superstructure support with the geometrical constraint is only structurally viable for a two-span structure. Bank seat abutments could be positioned at the top of approximately 1 in 2 sloping revetments giving cost savings due to the reduced approach works, when compared with a ‘closed’ abutment structure. Without pre-loading of the new earth embankment structures unacceptable settlement of the bank seats, designed as spread footings, are almost certain to occur. This will therefore necessitate the use of piled foundations, that detract from any cost savings that the bank seats would have had when compared to vertical cantilever abutments. Another disadvantage is that due to the overall increase of the total deck span, the cost of superstructure would increase in the order of 50%.

b) Reinforced Concrete Vertical Cantilever Abutments (Closed Aspect)

This comprises reinforced concrete retaining walls to the approach embankments that give end support to the deck structure. Positioned around 3.5m from the edge of carriageway they provide a ‘closed’ aspect, keeping the superstructure spans to a minimum. The faces of these could be clad in various materials with the aim of aesthetically complementing the local environment. However, a far less expensive option, which is generally acceptable is to provide a profiled patterned finish to the surface of the concrete.

c) Reinforced Concrete Cantilever Wing Walls

Using the same type of construction as the vertical cantilever abutments, these structures retain the sloping sides of the earth embankments. Finishes to these walls would be as that applied to the abutment walls.

d) Reinforced Earth Concrete Panel Clad Wing Walls

This form of construction is only deemed appropriate for the wing walls and not the abutment structure due to reinforced earth not being suitable to accommodate significant vertical loading. They are therefore considered as an alternative to the reinforced concrete option to be used in conjunction with reinforced concrete abutments. It seems generally accepted that they are more cost-effective structure when compared to reinforced concrete retaining walls. However considering their relatively short lengths, the unknown ground conditions and the absence of detail design it is difficult to judge at the stage what the most cost-effective solution will be.

Foundations

Foundation types depend primarily on the depth and safe bearing pressures of the bearing stratum, also restrictions placed on differential settlement due to the type of bridge deck. Generally, in the case of simply supported bridge decks differential settlements of about 20 to 25 mm can be tolerated, whereas multi-span continuous decks 10 mm is usually considered as a maximum.

a) Strip Footings

The overall size of strip footings is determined by considering the effects of vertical and rotational loads. The combination of these two must neither exceed the safe bearing capacity of the stratum nor produce uplift. The thickness of the footings is generally about 0.8 to 1.0 m but must be capable of withstanding moments and shears produced by piers or abutments.

b) Piled Foundations

The type of piles generally used for bridge foundations are:

• Driven Piles: preformed piles of concrete or steel driven by blows of a power hammer or jacked into the ground.
• Preformed Driven Cast In-Situ Piles: formed by driving a hollow steel tube with a closed end and filling the tube with concrete.
• Driven Cast In-Situ Piles: formed by driving a hollow steel tube with a closed end and filling the tube with concrete, simultaneously withdrawing the tube.
• Bored and Cast In-Situ Piles: formed by boring a hole and filling it with concrete.

First three types of foundations are known as displacement piles, and the problems of calculating the load carrying capacity and settlement require a different approach to that for bored piles.

Driven type piles can, depending on the strata, be either end bearing or friction piles; sometimes a combination of both.

Bored piles are generally end-bearing and are often of large diameter. To increase their bearing capacity the bottom can be under-reamed to produce a greater bearing area. However, additional safety precautions are required with larger diameter piles. Choice of pile type depends largely on the strata which they pass through, none of them however give the most economic and satisfactory solution under all conditions. The art of selecting the right sort of pile lies in rejecting all those types which are obviously unsuited to the particular set of circumstances and then choosing from those which remain, the one which produces the most economical solution.

Concurrently with the choice of pile type must go the choice of the strata which will carry the main loads from the structure, because this very often influences the choice. In most all cases the rejection of conventional pad or strip foundations arises because the computed settlement is more than the structure can safely withstand and hence the main purpose of the piled foundation will be to reduce this settlement. It follows, therefore, that if more compressible strata exist within reasonable distance of the surface, it is very desirable that a high proportion of the foundation load should be carried by more stable strata; the ideal solution is where piles support the load wholly in end bearing on hard rock where the settlement will be negligible. It follows that piles wholly embedded in the same soil that would under-lie a conventional foundation has very little effect in reducing settlement. With soft normally consolidated alluvial clays, the remolding effect of driven piles may well increase the settlement of the soil under its own dead weight and thus increase the settlement of the foundation itself.

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