Bridges

Question 1

Bridge

Answer 1

A structure that enables a service, such as a road, a railway or a pipeline to pass over another service or obstacle without disrupting either.

Question 2

Types of bridges

Answer 2

1. Arch bridge
2. Suspension bridge
3. Cantilever bridge
4. Incremental launching bridge
5. Cable-stayed bridge
6. Draw bridge

Question 3

Arch bridge

Answer 3

Curved structure constructed in the vertical form and used to span an opening. They are used where there is a need to cross a steep valley or where it would be too costly to support the bridge at intervals along the span

Construction: 1. Arch can be built out from both sides of valley to meet in the centre, supported by cables over towers each end (abutment). When the cables meet they are removed and road is constructed.
2.Building each half of arch on each side of the valley then lowering the two sections with cables and cranes until they meet and joined

Question 4

Suspension bridge

Answer 4

They consist of of tall towers on each side of the span, with the two main steel cables spanning between them. The road is then suspended from the main steel cables using hangers.
They are used where very long spans are needed.

Golden Gate Bridge in California- Two main cables 920mm diameter, 2 332m length.

Question 5

Cantilever bridge

Answer 5

Constructed from both sides of the valley to meet at the centre, both sections cantilevering over the support

Question 6

Incremental launching bridge

Answer 6

The entire bridge deck is built in sections on one side of the span. A shutter is fixed as each section has a steel nose that span forward to rest on the piers (pillars) constructed to support the deck.
The next section is then cast in the shutter and joined to the first using reinforcing bars. As each section is completed and pushed out of the shutter, the bridge deck is extended to span over piers.

Question 7

Cable-stayed bridge

Answer 7

Uses cables that are directly attached to the tops of the towers or piers. The road is then suspended from these cables. Two or more towers each with its own set of cables support a section of roadway.

Question 8

Draw bridge

Answer 8

Used over a waterway where there is insufficient clearance between the water and the underside (soffit) of bridge.
The deck is hinged at the supports near the bank and made in two sections that can be raised and lowered.

Question 9

Bridge Design opponents

Answer 9

1. Bridge abutments
2. Piers
3. Superstructure
4. Bearings
5. Expansion joints

Question 10

Reinforced concrete vs structural steel

Answer 10

Many are not in favour of steel as it rusts, whilst those not in favour of concrete says it crumbles.
Choosing between concrete and steel: *Span
*Foundation conditions
*Loads
*Architectural considerations
*Economic considerations

The two materials produce the best results when combined as one material compensates for the disadvantage of the other.
When building over a canal, river or railway, precast construction must be considered.

Question 11

Prestressed concrete

Answer 11

In ordinary reinforced concrete, the cement and aggregate mixture is poured into molds around reinforcing rods and bars, as it hardens the formwork bears the loading.

When the forms are removed, the concrete and the reinforcing steel is under stress. This transfer of loading combined with shrinkage of the member, temp fluctuations and vibration by construction methods will result in cracks in tension zones.

One can cater for the self-weight of the structure and the stresses can then be accommodated during manufacturing and construction by prestressing the concrete using steel cables.

Question 12

Prestressed concrete beams

Answer 12

Deck slab can be constructed using prestressed beams, placed side by side on bridge abutments.
This type of construction is needed for longer spans.
The usual loading(prestressing) of wires is about 1000N/m²
Wires are tensioned prior to concreting by jacking systems fitted to beam mould, concrete is then poured.
When hardened, the prestressed beam is removed and tends to bend upward but deflects to horizontal position under load

Question 13

Prestressing methods

Answer 13

1. Pre-tensioning: Steel wires in mould are tensioned and concrete is poured into the same mould. Wires are held in tension while the concrete gains strength and hardens. Once suffucient strength is gained, the steel ands are released and concrete holds them in position, the wires in turn hold the concrete in compression.
2. Post-tensioning: Structural members can be cast without the steel but with holes or ducts running through it. Once concrete has cured, cables or bars are laid through the holes, stretched and sealed with cement.