TRAFFIC DENSITY

CIVIL_ENGINEERING

Traffic density

Traffic density is the number of vehicles occupying a given length of the highway in a traffic lane. 

It is expressed as vehicle/mile or vehicle/kilometer.

For example 4 vehicles are present in 500 feet. So traffic density in one mile will be 42.24 vehicles.

k = 42.24 vehicles per mile.

Density is inversely proportional to the volume of traffic. If density is less, then speed will be more and traffic volume will be more. And if the density is greater, then the speed will be low and traffic volumewill be less.

Whenever, traffic jam problem create at certain point, then by the consideration of the peak hour traffic volume, we may proceed towards widening of road, fly over or under pass.

Headway and Spacing

Consider two successive vehicles in a lane.

Spacing

Spacing is defined as the distance between successive vehicles in a traffic lane as measured from front bumper to front bumper in a traffic stream. 

It is measured in feet. It depends upon the speed.

Headway

It is the corresponding time between two successive vehicles as they pass a given point on the highway.

Clearance and Gap

The clear distance between two successive vehicles in feet is known as clearance. Time between the two successive vehicles in seconds is known as Gap.

LANE OCCUPANCY

CIVIL_ENGINEERING

Lane occupancy

Lane occupancy is the ratio of the sum of the lengths of the vehicles to the length of the road section in which those vehicles are present.

To find lane occupancy, we are concerned with the standard dimensions of the vehicles.

For example 4 vehicles of lengths 18′,20′,22′,24′ are present over a length of the road of 500′.

Lane occupancy = (18+20+22+24)/500 = 0.168

More is the lane occupancy, more will be the trafficdensity.

TRAFFIC CONTROLLING DEVICES

CIVIL_ENGINEERING

Traffic controlling devices

The devices which are used to control and regulate the flow of traffic are called traffic controlling devices.

These are of three types:

1. Pavement markings.
2. Traffic signs.
3. Traffic signals.

Pavement markings

Pavement markings are in the form of bands or strips of paints, which are physically drawn on the pavement.

Purposes

1. Pavement markings are used to mark the center lineof the road in case of 2 lane highway.
2. To make the pavement and shoulder distinct by means of shoulder lines drawn at the pavement edges.
3. To make lane boundaries for multi lane highway.
4. To make cross walks for pedestrians.
5. To make stop lines at the road surface close to signals.
6. Turning movement.
7. These are used in parking places where parking is permitted.
8. These are used in no passing zones. These are the areas where we do not over take i.e, on steep curve, on narrow bridge continuous line is drawn there.

Width of paint line = 4 inches.

Length of paint line = 10 feet dashes separated by 30 feet spaces.

Color

Yellow for distinguish one surface to other, white for controlling.

Cat’s eyes or reflectors

Sometimes we use rays reflectors or cat’s eyes instead of paint lines. These are illuminated by head lights and they are raised less than 1 inches from the surface. It warns the inattentive driver as the driver feels the jerk when passes over it.

Below is the picture of cat’s eyes along with pavement marking on the road.

Cat’s eyes along with pavement markings

TYPES OF TRAFFIC CONTROL DEVICES AND TRAFFIC SIGNS

CIVIL_ENGINEERING

Traffic Signs

Traffic signs are means of warning, regulating and informing the drivers.

Material for traffic signs

Material should be such that

1. Which does not lost its color often.
2. Rust resistant.
3. Usually a sheet metal which is protected by a coating of some rust resisting material is used.
4. Nuts, bolts are all galvanized.

Types of signs

Traffic signs are of three types.

1. Warning signs.
2. Mandatory signs.
3. Informative signs.

Warning signs

Warning signs call the attention of the driver to some approaching hazard. For example;

1. Level crossing.
2. Road functions.
3. Narrow bridges.
4. Sharp curves.
5. Bends.
6. U-turns.

Mandatory signs

These are regulatory signs or prohibitory signs enforced by the law. For example

1. Speed limit.
2. No parking.
3. No blowing horns.

Non observance of these signs constitute as penal offence, crime punishable under law. Such a sign is called mandatory sign.

Informative signs

These are the signs which convey some sort of information to the drivers. For example;

1. Sign showing the location of gasoline pump.
2. Bus stops.
3. Distance of cities.
4. Rest houses.
5. Motels etc.

They are not mandatory signs, and there is no prosecution if we ignore these signs.

Placement of traffic signs on roads

Warning and mandatory signs should be installed 5 feet above the road way and at a distance of 6′-12′ from the edge of the pavement. The distance of sign from hazard for a high speed highway is 1500 feet.

JOISTS

CIVIL_ENGINEERING

Joists

Joists are any of the small timbers or metalbeams. These small beams ranges parallelfrom wall to wall in a structure to support a floor or ceiling.

Or

Joists are less important beams that are closely spaced and are frequently having truss-type webs.

These are closely spaced smaller beams.. In most of cases joists rest on secondary beams. With the presence of joists, the strength requirements of the roof sheathing or slabare greatly reduced.

PURLINS

CIVIL_ENGINEERING

Purlins

The roof beams spanning between the trusses are known as purlins.

or

A horizontal member in the roof.

Roof sheets are connected with purlins. Purlins are connected with panel points of the truss. There is no direct connection between roof sheathing and truss top chord. Uniformly distributed roof load is carried by purlins. This UDL is converted into point loads acting at panel points of truss.

The load acting at purlins is inclined that is why these beams subjected to bi-axial bending along with torsion.

DEFINITION OF LINTELS AND SPANDRELS

CIVIL_ENGINEERING

Lintels

Lintels are members supporting a wall over window or door openings.

Or

 Lintel is a horizontal structural member spanning and usually carrying load above an opening.

Opening may be like the opening of doors and windows. Lintels are provided above the doors and windows to support load of the wall above it.

Spandrels

In case of high rise buildings, the masonry walls are usually not able to withstand their self weight and the slabweight. In such cases, the beams are provided exterior walls at each floor level to support the wall load and perhaps some roof load also. These beams are termed as spandrels.

BRIDGE GIRDER

CIVIL_ENGINEERING

Bridge girder

Bridge girder is a main structural member laying horizontally as in buildings or bridges. It supports vertical loads and consists of a single piece or consists of more than one piece bound together.

Girder is a primary beams that are used at the place where wide space is available. This main beam which is also called as bridge girder in case of bridges, supports smaller beams and other structural components. Girders are made of built up sections and carry heavier loads over larger spans. Bridge girders or girders are supported directly on columns.

STIFFENERS

CIVIL_ENGINEERING

Stiffeners

Stiffeners are the members which are connected to the thin webs to prevent loss of strength due to web buckling.

It is usually necessary to stiffen the webs of plategirders to prevent loss of strength due to web buckling. For riveted or bolted girders, angles are connected to the webs while for welded girders, plates are connected to the webs.

Image taken from John Brandon’s ‘Fly Safe!’ tutorials

These are divided into two groups:

Bearing stiffeners

Bearing stiffeners are provided just above the reactions or just below the heavy concentrated loads. These transfer heavy reactions or concentrated loads to the full depth of the web providing a uniform shear transfer. Bearing stiffeners are always provided in pairs, i.e., on both sides of the web.

Intermediate stiffeners

Intermediate stiffeners are also called as non-bearing or stability stiffeners. These are placed at various intervals along the web to counteract against the buckling due to diagonal compression.

These are provided in pairs on both sides of the web or as a single unit alternatively on each side of the web.

Tension field action

The intermediate may also provide post-buckling shear strength by the development of mechanism called Tension field action.

DEFINITION OF CLADDING AND BUILT-UP SECTIONS

CIVIL_ENGINEERING

Cladding

Cladding is the exterior covering of the structural components of a building that are made up of steel sections.

This covering may be made up of reinforced concrete, wood, aluminium or any other architectural and light weight material.

Built-up sections

Sections made by combining two or more hot rolled sections, joined together at intervals are called built-up sections.

This joining is done with the help of direct welding, stay plates or lacing.

Examples are four angles section, double angles section and double channel section.

However, double angle section is sometimes excluded from built-up member and is included in hot rolled section. This is because of difference of its behavior with respect to other built up sections.