Showing posts with label Road and Highways. Show all posts
Showing posts with label Road and Highways. Show all posts


What Are The Types Of Suspension Bridges?

Several arrangements of suspension bridges are illustrated in Fig. 1. The main cable is continuous, over saddles at the pylons, or towers, from anchorage to anchorage.  

FIGURE 15.9 Suspension-bridge arrangements. (a) One suspended span, with pin-ended stiffening truss. (b) Three suspended spans, with pin-ended stiffening trusses. (c) Three suspended spans, with continuous stiffening truss. (d ) Multispan bridge, with pin-ended stiffening trusses. (e) Self-anchored suspension bridge.

When the main cable in the side spans does not support the bridge deck (side spans independently supported by piers), that portion of the cable from the saddle to the anchorage is virtually straight and is referred to as a straight backstay.

This is also true in the case shown in Fig. 1a where there are no side spans. Figure 1d represents a multispan bridge. This type is not considered efficient, because its flexibility distributes an undesirable portion of the load onto the stiffening trusses and may make horizontal ties necessary at the tops of the pylons.

Ties were used on several French multispan suspension bridges of the nineteenth century. However, it is doubtful whether tied towers would be esthetically acceptable to the general public. Another approach to multispan suspension bridges is that used for the San Francisco–Oakland Bay Bridge (Fig. 2). It is essentially composed of two three-span suspension bridges placed end to end.

This system has the disadvantage of requiring three piers in the central portion of the structure where water depths are likely to be a maximum. Suspension bridges may also be classified by type of cable anchorage, external or internal. Most suspension bridges are externally anchored (earth-anchored) to a massive external anchorage (Fig. 1a to d).

In some bridges, however, the ends of the main cables of a suspension bridge are attached to the stiffening trusses, as a result of which the structure becomes self-anchored (Fig. 1e). It does not require external anchorages.

The stiffening trusses of a self-anchored bridge must be designed to take the compression induced by the cables. The cables are attached to the stiffening trusses over a support that resists the vertical component of cable tension. The vertical upward component may relieve or even exceed the dead-load reaction at the end support. If a net uplift occurs, a pendulum link tie-down should be provided at the end support.

Self-anchored suspension bridges are suitable for short to moderate spans (400 to 1,000 ft) where foundation conditions do not permit external anchorages. Such conditions include poor foundation bearing strata and loss of weight due to anchorage submergence. Typical examples of self-anchored suspension bridges are the Paseo Bridge at Kansas City, with a main span of 616 ft, and the former Cologne-Mu¨lheim Bridge (1929) with a 1,033-ft span.

Another type of suspension bridge is referred to as a bridle-chord bridge. Called by Germans Zu¨gelgurtbru¨cke, these structures are typified by the bridge over the Rhine River at Ruhrort-Homberg (Fig. 15.11), erected in 1953, and the one at Krefeld-Urdingen, erected in 1950.  

It is a special class of bridge, intermediate between the suspension and cable-stayed types and having some of the characteristics of both. The main cables are curved but not continuous between towers. Each cable extends from the tower to a span, as in a cable stayed bridge. The span, however, also is suspended from the cables at relatively short intervals over the length of the cables, as in suspension bridges.

A distinction to be made between some early suspension bridges and modern suspension bridges involves the position of the main cables in profile at midspan with respect to the stiffening trusses. In early suspension bridges, the bottom of the main cables at maximum sag penetrated the top chord of the stiffening trusses and continued down to the bottom chord.

Because of the design theory available at the time, the depth of the stiffening trusses was relatively large, as much as 1⁄40 of the span. Inasmuch as the height of the pylons is determined by the sag of the cables and clearance required under the stiffening trusses, moving the midspan location of the cables from the bottom chord to the top chord increases the pylon height by the depth of the stiffening trusses.

In modern suspension bridges, stiffening trusses are much shallower than those used in earlier bridges and the increase in pylon height due to midspan location of the cables is not substantial (as compared with the effect in the Williamsburg Bridge in New York City where the depth of the stiffening trusses is 25% of the main-cable sag).

Although most suspension bridges employ vertical suspender cables to support the stiffening trusses or the deck structural framing directly, a few suspension bridges, for example, the Severn Bridge in England and the Bosporus Bridge in Turkey, have inclined or diagonal suspenders.

In the vertical-suspender system, the main cables are incapable of resisting shears resulting from external loading. Instead, the shears are resisted by the stiffening girders or by displacement of the main cables. In bridges with inclined suspenders, however, a truss action is developed, enabling the suspenders to resist shear.

(Since the cables can support loads only in tension, design of such bridges should ensure that there always is a residual tension in the suspenders; that is, the magnitude of the compression generated by live-load shears should be less than the dead-load tension.) A further advantage of the inclined suspenders is the damping properties of the system with respect to aerodynamic oscillations.


What Are The Different Types of Asphalt Products?

Asphalt used in pavements is produced in three forms: asphalt cement, asphalt cutback, and asphalt emulsion. Asphalt cement is a blend of hydrocarbons of different molecular weights.

The characteristics of the asphalt depend on the chemical composition and the distribution of the molecular weight hydrocarbons. As the distribution shifts toward heavier molecular weights, the asphalt becomes harder
and more viscous.

At room temperatures, asphalt cement is a semisolid material that cannot be applied readily as a binder without being heated. Liquid asphalt products, cutbacks and emulsions, have been developed and can be used without heating (The Asphalt Institute, 2007).

Although the liquid asphalts are convenient, they cannot produce a quality of asphalt concrete comparable to what can be produced by heating neat asphalt cement and mixing it with carefully selected aggregates. Asphalt cement has excellent adhesive characteristics, which make it a superior binder for pavement applications. In fact, it is the most common binder material used in pavements.

A cutback is produced by dissolving asphalt cement in a lighter molecular weight hydrocarbon solvent. When the cutback is sprayed on a pavement or mixed with aggregates, the solvent evaporates, leaving the asphalt residue as the binder.

In the past, cutbacks were widely used for highway construction. They were effective and could be applied easily in the field.

However, three disadvantages have severelylimited the use of cutbacks. First, as petroleum costs have escalated, the use of these expensive solvents as a carrying agent for the asphalt cement is no longer cost effective.

Second, cutbacks are hazardous materials due to the volatility of the solvents. Finally, application of the cutback releases environmentally unacceptable hydrocarbons into the atmosphere. In fact, many regions with air pollution problems have outlawed the use of any cutback material.

An alternative to dissolving the asphalt in a solvent is dispersing the asphalt in
water as emulsion. In this process the asphalt cement is physically broken down into micron-sized globules that are mixed into water containing an emulsifying agent.

Emulsified asphalts typically consist of about 60% to 70% asphalt cement, 30% to 40% water, and a fraction of a percent of emulsifying agent. There are many types of emulsifying agents; basically they are a soap material.

The emulsifying molecule has two distinct components, the head portion, which has an electrostatic charge, and the tail portion, which has a high affinity for asphalt. The charge can be either positive to produce a cationic emulsion or negative to produce an anionic emulsion.

When asphalt is introduced into the water with the emulsifying agent, the tail portion of the emulsifier attaches itself to the asphalt, leaving the head exposed. The electric charge of the emulsifier causes a repulsive force between the asphalt globules, which maintains their separation in the water.

Since the specific gravity of asphalt is very near that of water, the globules have a neutral buoyancy
and, therefore, do not tend to float or sink. When the emulsion is mixed with aggregates or used on a pavement, the water evaporates, allowing the asphalt globs to come together, forming the binder.

The phenomenon of separation between the asphalt residue and water is referred to as breaking or setting. The rate of emulsion setting can be controlled by varying the type and amount of the emulsifying agent.

Since most aggregates bear either positive surface charges (such as limestone) or negative surface charges (such as siliceous aggregates), they tend to be compatiblewith anionic or cationic emulsions, respectively.

However, some emulsion manufacturers can produce emulsions that bond well to aggregate-specific types, regardless of the surface charges. Although emulsions and cutbacks can be used for the same applications, the use of emulsions is increasing because they do not include hazardous and costly solvents.

What is the function of longitudinal joints in concrete road pavements?

Civil Engineering Tutorials

A longitudinal joint consists of a tie bar placed at the mid-depth of a concrete pavement and it is not intended for joint lateral movement.

Then one may doubt the reasons of placing longitudinal joints in concrete pavements. In fact, longitudinal joints are normally designed at a regular spacing e.g. 4.5m to accommodate the effect of differential settlement
of pavement foundation.

When uneven settlement occurs, the tie bars in longitudinal joints perform as hinges (Ministry of Transport (1955)) which allow for the settlement of concrete carriageway.

Moreover, it also serves to cater for the effect of warping of concrete due to moisture and temperature gradients by permission of a small amount of angular movement to occur so that stresses induced by restrained warping can be avoided.

Dowel bars are provided in longitudinal joints for the following reasons:

(i) In case of the occurrence of uneven settlement between adjacent panels, it helps to maintain a level surface by transfer of loads through dowel bars.

(ii) Keep the longitudinal joints close.