Considerations in bridge design: functional design

Reading Time: 6 minutes

Bridge can be defined as any structure use to provide access from one point to another over obstructions such as valleys, rivers or to connect two points. It can also be defined as a structure providing passage over an obstacle without closing the way beneath. The passage can be for a road, a railway, pedestrians or canal (Gupta and Gupta, 2012). There are classes of bridge based on.

Material of construction, there are reinforced concrete (RC) bridge, stone masonry bridge, timber bridge, steel bridge, iron bridge, prestressed concrete bridge and composite bridge.

Purpose, there are railroad bridge, highway bridge, pedestrian bridge, green bridge (built for sustainability eg vegetation or bath foraging paths), aquaduct (built to convey fluids).

Configuration of superstructure, there are arch bridges, frame bridges, suspension bridges, beam/girder bridges.

Static systems, there are simply supported bridges, continuous bridges, cantilever bridges.

Nature of permanency, there are permanent and temporary bridges.

Relative position of floor, there are deck bridge, through bridge.

Requirements of an ideal bridge:

An ideal bridge should be:

1. Economical
2. Serve its intended purpose with safety and convenience
3. Have an aesthetic and elegant look

Factors affecting the selection of bridge type (Gupta and Gupta, 2012)

1. Cost and strategic considerations
2. Sub-soil conditions of the bed of river
3. Channel section, that is the nature of river
4. Hydraulic data
5. Physical features of the site
6. Climatic conditions
7. Navigation requirements
8. Volume and nature of traffic
9. Economic span length of the bridge
10. Foundation conditions
11. Appearance

What a design engineer should know before starting design of bridge (Gupta and Gupta, 2012)

1. Site of construction
2. Alignment
3. Clearance
4. Discharge to be passed under bridge
5. Number of spans
6. Economic span
7. Water way
8. Scour depth
9. Foundations
10. Afflux

Definition of some terms connected to bridge

1. Valley: this is the depression between two banks or sides with water inbetween them
2. Viaduct: This is a deep valley without perennial water.
3. Span: The centre-to-centre distance between two piers or supports is known as span. For simply supported bridge, it is the centre-to-centre distance between two abutments.
4. Abutments: This is the end supports of the superstructure of a bridge.
5. Piers: These are intermediate supports of the superstructure of a bridge.
6. Superstructure: It is that part of the bridge over which the traffic moves. It consists of the parapet, roadway, girder, arches, or trusses over which the road is supported.
7. Substructure: The substructure supports the superstructure and distribute the loads to the soil below. It consists of foundation, piers, abutments, wing walls and approaches.

Bridge Design

According to Tang (2017), the objective of a bridge design is to produce a safe bridge that is elegant and satisfies all functionality requirements, at a cost that is acceptable to the owner. A successful bridge design must be natural, simple, original, and harmonious with its surroundings. Figure 1 shows the flow chart of bridge design.

 

There are two aspects of bridge design. These are:

1. Functional design
2. Structural design

Functional design involves the definition of the sizes of the different components of the bridge based on environmental conditions while structural design involves the sizing of the structural members and provision of necessary reinforcements based on the loads coming on the bridge. In this post I am going to enumerate and explain with an example the different determinants of functional design of bridge.

1. Length of the bridge (Lineal profile): This is usually based on the width of the river to be bypassed by the bridge and other allowances.
2. Route location: For economic purposes, it is necessary that the bridge should cross the obstacle especially river at right angle.
3. Number of spans/openings of the bridge: This depends on the length of the bridge if the length of bridge is (˃ 12m) introduce piers. For reinforced concrete bridges, the openings or span should be 10 – 25 m (but not more than 25 m).
4. The width of the bridge: This is usually the same as the width of the highway joining to the bridge. That is roadway width = bridge roadway width. However, it is necessary to create footpath continuing from the kerbs where there is high pedestrian movement. Vehicle carriageway width for the bridge = Bridge roadway width – (2 x pedestrian way width).
5. Freeboard or Headroom of the bridge: This is the height of the bridge above highway (flyovers) or water. For highway bridge, it is a minimum of 4.5 m. For bridge crossing a river, if the river is navigational, it should be a minimum of 0.5 m but if the river is not navigational, there is no need to border about the headroom. To establish the headroom over water, it is necessary to know the highest possible water level the river can attain. This can be gotten from observation during the peak of rainy season or from interviewing necessary authorities.
6. Bridge construction height: This is the height from the highest point of the bridge to the bottom level of the longitudinal beam.
7. Bridge structural design: After establishing the functional design parameters above (1 -6), the next action is the structural design of the bridge. The structural design is done like slab design in buildings. First create panels of the bridge along transverse beams and longitudinal beams. For each panel choose the traffic load of the heaviest vehicle expected on the panel. Transfer the load to the slab, then to the beams, to the piers and finally the foundation.

Example

Assuming a bridge is to be constructed over a river with a lineal distance of 80 m. from the preliminary surveys, it was found that the highest water level of the rivers is say 70 m above the river bed while the crown level of the approaching road is 100 m from the river bed. If the approach road have a roadway width of 12.5 m and carriageway width of 10.5 m, determine the functional parameters of the bridge.

1. Sketch the profile of the river bed (Figure 2)

 

2. Determine the number of spans (L). Note that 10 ≤ L ≤ 25 m for reinforced concrete bridges. The choice here depends on the designer. We can assume 4 spans of 20 m each

3. Estimate the depth of the superstructure, hk

hk = [(L/8) – (L/10)](L/14) where L = 20 m

hk = [(20/8) – (20/10)] (20/14) = (2.5 – 2)1.42857 = 0.715 m = 715 mm

4. Check for the height of the freeboard

cl = hwl + hf + hk

Where cl is the crown level of the road

hwl is the highest water level of the river

hf is the height of freeboard and

hk is the construction height (depth) of the bridge superstructure

hf = cl –hwl – hk = 100 – 70 – 0.715 = 29.285 m ˃ 0.5 m Ok

 

5. For the deck design, an opening of the bridge is considered and paneled appropriately. In this case an opening of 20 m.

 

 

 

 

 

 

 

6. Having completed the functional design, the structural design can be commenced based on Figure 4 and other functional parameters determined above.

Thank you for visiting mycivillinks today. God bless you.