Factors Affecting Geometric Design 4b27q

Factors affecting geometric design A number of factors affect the geometric design and they are discussed in detail in the following sections.

Design speed Design speed is the single most important factor that affects the geometric design. It directly affects the sight distance, horizontal curve, and the length of vertical curves. Since the speed of vehicles vary with driver, terrain etc, a design speed is adopted for all the geometric design. Design speed is defined as the highest continuous speed at which individual vehicles can travel with safety on the highway when weather conditions are conducive. Design speed is different from the legal speed limit which is the speed limit imposed to curb a common tendency of drivers to travel beyond an accepted safe speed. Design speed is also different from the desired speed which is the maximum speed at which a driver would travel when unconstrained by either traffic or local geometry.

Since there are wide variations in the speed adopted by different drivers, and by different types of vehicles, design speed should be selected such that it satisfy nearly all drivers. At the same time, a higher design speed has cascading effect in other geometric designs and thereby cost escalation. Therefore, an 85th percentile design speed is normally adopted. This speed is defined as that speed which is greater than the speed of 85% of drivers. In some countries this is as high as 95 to 98 percentile speed.

Topography The next important factor that affects the geometric design is the topography. It is easier to construct roads with required standards for a plain terrain. However, for a given design speed, the construction cost increases multiform with the gradient and the terrain. Therefore, geometric design standards are different for different terrain to keep the cost of construction and time of construction under control. This is characterized by sharper curves and steeper gradients.

Other factors

In addition to design speed and topography, there are various other factors that affect the geometric design and they are briefly discussed below: Vehicle: The dimensions, weight of the axle and operating characteristics of a vehicle influence the design aspects such as width of the pavement, radii of the curve, clearances, parking geometrics etc. A design vehicle which has standard weight, dimensions and operating characteristics are used to establish highway design controls to accommodate vehicles of a designated type. Human: The important human factors that influence geometric design are the physical, mental and psychological characteristics of the driver and pedestrians like the reaction time. Traffic: It will be uneconomical to design the road for peak traffic flow. Therefore a reasonable value of traffic volume is selected as the design hourly volume which is determined from the various traffic data collected. The geometric design is thus based on this design volume, capacity etc. Environmental: Factors like air pollution, noise pollution etc. should be given due consideration in the geometric design of roads. Economy: The design adopted should be economical as far as possible. It should match with the funds allotted for capital cost and maintenance cost. Others: Geometric design should be such that the aesthetics of the region is not affected.

Pre stressing steel Pre-stressing raises both the quality and the resistance to tension and compression characteristics of the steel; the technique actually manages to create a state of co-action in which the tensions and deformations are opposed to those induced by the loads which will subsequently act upon the structure. It also raises the resistance to tension of reinforced concrete which is, in fact, negligible.

Forms of Pre-stressing Steel The development of pre-stressed concrete was influenced by the invention of high strength steel. It is an alloy of iron, carbon, manganese and optional materials. In addition to pre-stressing steel, conventional non-pre-stressed reinforcement is used for flexural capacity (optional), shear capacity, temperature and shrinkage requirements.

Wires. A pre-stressing wire is a single unit made of steel. The nominal diameters of the wires are 2.5, 3.0, 4.0, 5.0, 7.0 and 8.0 mm. The different types of wires are as follows: 1) Plain wire: No indentations on the surface. 2) Indented wire: There are circular or elliptical indentations on the surface.

Strands. A few wires are spun together in a helical form to form a pre-stressing strand. The different types of strands are as follows: 1) Two-wire strand: Two wires are spun together to form the strand. 2) Three-wire strand: Three wires are spun together to form the strand. 3) Seven-wire strand: In this type of strand, six wires are spun around a central wire. The central wire is larger than the other wires.

Tendons. A group of strands or wires are placed together to form a pre-stressing tendon. The tendons are used in posttensioned . The following figure shows the cross section of a typical tendon. The strands are placed in a duct which may be filled with grout after the post-tensioning operation is completed

Cables. A group of tendons form a pre-stressing cable. The cables are used in bridges. Bars. A tendon can be made up of a single steel bar. The diameter of a bar is much larger than that of a wire. Bars are available in the following sizes: 10, 12, 16, 20, 22, 25, 28 and 32 mm. Figure 2 shows the different forms of pre-stressing steel.

Types of Pre-stressing Steel The steel is treated to achieve the desired properties. The following are the treatment processes: 

Cold working (cold drawing) is being done by rolling the bars through a series of dyes. It re-aligns the crystals and increases the strength.

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Stress relieving is being done by heating the strand to about 350°C and cooling slowly. This reduces the plastic deformation of the steel after the onset of yielding.

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Strain tempering for low relaxation is being done by heating the strand to about 350°C while it is under tension. This also improves the stress-strain behavior of the steel by reducing the plastic deformation after the onset of yielding. In addition, the relaxation is reduced.

Properties of Pre-stressing Steel The steel in pre-stressed applications has to be of good quality. It requires the following attributes: 1) High strength 2) Adequate ductility 3) Bendability, which is required at the harping points and near the anchorage 4) High bond, required for pre-tensioned 5) Low relaxation to reduce losses 6) Minimum corrosion.

The tensile strength of pre-stressing steel is given in of the characteristic tensile strength (fpk). The characteristic strength is defined as the ultimate tensile strength of the coupon specimens below which not more than 5% of the test results are expected to fall.