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Civil structures
Sample Answers
Question 1
Using the following terms, classify each bridge as a mechanical system, by completing the following table:
| Mechanical system | Bridge number |
|---|---|
| Beam/girder | 1, 8 |
| Truss | 3,5 |
| Arch | 2,6,7 |
| Suspension | 4,9 |
Question 2
Which of the bridges shown use a combination of these mechanical systems?
| Bridge number | Combination |
|---|---|
| 4 | Suspension and timber truss |
| 6 | Arch and steel truss |
| 9 | Girder and suspension |
Question 3
Bridges 1 and 2 were made mostly from naturally occurring materials. What are the major materials innovations used for each of the other types of bridges listed in the table below?
| Bridge number | Materials innovation |
|---|---|
| 4 | High tensile steel cable |
| 5 | Fabricated steel girders |
| 6 | Rolled structural steel girders |
| 7 | Post-stressed concrete |
| 8 | Integrated steel/concrete |
| 9 | Integrated steel/concrete |
Question 4
The main loads that act upon bridges are those that come from the:
- mass of the components of the structure
- traffic mass
- reactions at piers and abutments
- wind pressure.
Complete the following table by placing a cross in the relevant cell/s.
| Source | Angle | Live | Dead | Point | UDL |
|---|---|---|---|---|---|
| Mass | vert. down | X | X | ||
| Traffic | vert. down | X | X | ||
| Reactions | Vert or oblique | X | X | ||
| Wind | oblique | X | X |
Question 5
Diagram 1
The diagram above shows how the loads on an arch bridge are transferred to the abutments for support. Using a simple sketch of your own, show how loads on a beam bridge and a suspension bridge are transferred to the abutments.
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| Beam bridge loads | Suspension bridge loads |
Question 6
- Why is it necessary on some bridges to use roller or sliding bearing supports?
To enable the movement caused by deformation and expansion or contraction due to heat to be taken up.
- What type of reactions occur at:
- roller bearings
The reaction acts only perpendicular to the roller surface - fixed bearings
The reaction can act at any angle - sliding bearings
The reaction acts only perpendicular tothe roller surface.
- roller bearings
- What is another type of bridge support?
Rubber pads.
Question 7
- The drawing below represents the frame of a simple pin jointed truss that could be used for a bridge design. Calculate the reactions at A and D if the total live and dead loads are equivalent to 200 kN acting on the centre joint.
The reaction at A will be 100 kN acting vertically upward.
The reaction at D will be 100 kN acting vertically upward.
- Use the method of sections to calculate the force developed in member BC in the truss above.
The load developed in member BC will be 115.5 kN in axial compression.
Was your answer correct?
Now, use your web browser to find the web site at: http://www.jhu.edu/virtlab/bridge/truss.htm 
The program on this web page allows you to design your own truss, and calculate the forces in the members. Copy the details for the truss in question 8, and see what the forces are in each of the other members of the truss.
Now design a truss of your own, work out some of the forces in members yourself, then check your work by using the bridge truss web site again.
Question 8
- What is meant by the term ‘Factor of Safety’?
A factor of safety is used in engineering design to help ensure that a structure is easily capable of carrying the loads imposed upon it during service without failure. This may include loads of an unexpected nature: loads caused by earthquake, accidental overloading or impact loads, etc.
This is achieved by reducing the “engineering stress” for the material being used in a design to a “working stress” or “maximum allowable stress” by applying a set factor. Calculations for the size of structural members are then based on the working stress.
The engineering stress used depends to a large degree on the nature of the material involved and the type of structure. Sometimes the engineering stress used may be the “ultimate tensile (compressive) stress” for the material, or in other cases the “proof stress”, or “yield stress”.
- Member BC in the truss is made from square section Grey Gum timber with a compressive (crushing) strength of 82 MPa. If a factor of safety of 1.5 is required in the truss design, calculate the minimum cross-sectional size for this member.
- Reduce the ultimate compressive stress to a maximum allowable stress.
Maximum allowable stress = engineering stress / 1.5
= 82/1.5
= 54.67 MPa.
- Calculate the minimum allowable cross-sectional area
Area = Maximum load / max. allowable stress
= 115.5/54.67
= 0.00213 m² (2130 mm²)
- Calculate the minimum allowable size of the member
width x thickness =
square section: width = thickness = = 
w = t = 46.15 mm²
As 46.15 mm² is not a standard size, the nearest practical size would be 50 mm² grey gum timber.
- Reduce the ultimate compressive stress to a maximum allowable stress.
Question 9
An RSJ made from structural steel might be suitable as a structural member in bridge 5.
- How would this type of beam be manufactured?
Continuous cast steel billet would be cut to suitable lengths, heated to rolling temperature and placed between the shaped rolls of a rolling mill. The billet would be progressively rolled to correct size.
- What is the composition of structural steel?
Source: IMMA (1997) Handbook of engineering materials. Institute of Metals and Materials Australasia: North Melbourne.Carbon % 0.20 – 0.25 Silicon % 0.40 – 0.50 Manganese % 1.50 – 1.60
- What is the microstructure of structural steel?
Microstructure of structural steel
The microstructure above shows white and grey areas. The white areas represent primary ferrite grains. The grey areas represent a mixture of two phases known as pearlite. If we examine the pearlite areas at higher magnification we will see each of the two phases from which it is made.
Microstructure of Pearlite
The two phases of pearlite are clearly visible in the micrograph above. These phases are ferrite and cementite. The ferrite appears white, and is laminated against the cementite which appears grey.
Question 10
Reinforced concrete is used to construct bridges 7, 8 and 9.
- What are the constituents that are used to make concrete?
- portland cement
- sand
- aggregate: crushed basalt, dolerite
- water
- What is the function of the steel reinforcement in concrete beams and slabs?
Concrete has a tensile strength that is about 10% of its compressive strength. However, uniform section beams and slabs need to resist roughly equal tensile and compressive stresses. In effect, the concrete is used to resist the compressive stresses while the steel resists the tensile stresses.
- Why is the position of the steel reinforcement in the concrete very important?
Steel has good tensile strength, and so it is embedded within the concrete in that part of the beam or slab that is required to resist tensile stresses.
- Why is corrosion a significant issue in reinforced concrete?
If the steel reinforcement within a concrete slab or beam is subject to corrosion it will ultimately cause failure of the structure. As the steel corrodes, the corrosion product (rust) has a greater volume than the steel it forms from. This “growth” places local areas of concrete under tensile stress. Because the concrete is poor in tension and brittle, pieces begin to crack and break away. If left, the steel will finally be exposed, corrode at an increasing rate and lead to structural failure. The effect is referred to as “concrete cancer”.
Question 11
Asphalt is frequently laid on concrete as a road surface.
- What properties make asphalt suitable as a road surface material?
It is wear resistant, does not deteriorate on exposure to the weather, is flexible and reasonably tough, it is easily laid as a continuous strip.
- Why is asphalt known as a composite material?
Because, like concrete, it is composed of a number of phases. A phase is an homogeneous, physically distinct and separable component within a composite structure. Phases may be elements, compounds, or solutions.
- What are the phases present in the structure of asphalt?
Source: Boral Asphalt, Greystanes.Asphalt a petroleum derivative adhesive which bonds other phases together. 5% 5mm aggregate crushed basalt or dolerite rock 15% 10mm aggregate crushed basalt or dolerite rock 20% Sand clean beach sand 57% Crushing fines fine dust from the rock crushing process 2%
Question 12
Describe the process of using post-tensioning in bridge construction.
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| Post-tensioning mechanism | Post-tensioning tendons |
In a structure that is to be post-tensioned a number of sections are pre-cast in concrete. These sections have holes cast in them to allow high tension steel cables to be passed through. Some structures have plastic tubes cast into the concrete where the steel cables can pass through.
The pre-cast sections are assembled on a temporary supporting structure.
Steel cables are drawn through the holes (tubes) and firmly secured in sockets at one end.
Large hydraulic jacks are fixed onto the cables and butted against the structure at the other end. The cables are then loaded in tension by the jacks. When the correct tension is achieved the cables are wedged into sockets in the end of the structure and the jack load released. As the jack load is released the tension in the cables tends to pull the various sections of the structure together. The structure, where the cables are located, is placed in compression by this process.
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