Bridge Work Interview Questions

The design process of a bridge can be divided into four basic stages:

1. Conceptual Design Stage – the purpose of this stage is to come up with various feasible bridge schemes and to decide on one or more final concepts for further consideration.
2. Preliminary Design Stage – in this stage, selection of the best scheme from proposed concepts are finalized and ascertain the feasibility of the selected concept and refine its cost estimates.
3. Detailed Design Stage – in this stage, finalization of all the details of the bridge structure are completed so that the document is sufficient for tendering and construction.
4. Construction Design Stage – this design stage is important to provide step-by-step procedures for building the bridge.

The most prominent technical problems for long span bridges are:

1. Girder stiffness in the transverse direction.
2. Reduction in cable efficiency of very long cables in a cable-stayed bridge.
3. Torsional stiffness of the main girder.
4. Allowable stresses of the construction materials.

Mainly, all of these problems are, in some way, related to the construction materials.

By increasing the width of the bridge, we can increase lateral stiffness as it increases the stability of bridges

We can increase cable efficiency in following ways:

1. By providing tie ropes
2. By providing strut supports
3. By suspended by suspension cable.

In stitching method of bridge widening, the widening part is constructed first and let it undisturbed for several months and only after that concreting will be done for the stitch between the new deck and the old one. In this way, the dead load of the widened part of bridge is supported by itself and thus this load did not transfer to the existing deck as existing deck was not designed for that load.

There stress induced by shrinkage of newly widened deck of the bridge on the existing deck is the matter of concern, thus to counter this problem, the widened part of bridge is constructed much before (say about 7 to 9 months) the stitching process, so that the shrinkage effect generated by the new deck will take place within this period and thus the shrinkage stress exerted on the bridge is minimized.

Also, the traffic vibrations on the existing bridges caused adverse effect to the newly constructed stitches, thus to counter this issue, rapid handing cement is used instead of normal cement for stitching process and also the work should be done in nights and the existing bridge should be closed for at-least 6 hours to let the stitching works left undisturbed.

While designing road bridges and culverts, the following loads, should be considered, where applicable:

1. Dead load
2. Live load
3. Snow load
4. Impact or dynamic effect due to vehicles
5. Impact due to floating bodies or vessels
6. Wind load

The Indian Road Congress (IRC) has formulated Standard Specifications and Codes of Practice for road bridges with a view to establish a common procedure for the design and construction of road bridges in India. The specifications are collectively known as the Bridge Code. Prior to the formulation of the IRC Bridge Code, there was no uniform code for the whole country. Presently, we are following the IRC Bridge Code.

While designing road bridges, the following forces should be considered:

1. Longitudinal forces caused by the tractive effort of vehicles or by braking of vehicles.
2. Longitudinal forces due to frictional resistance of expansion bearings.
3. Centrifugal forces due to curvature.
4. Horizontal forces due to water currents.
5. Buoyancy
6. Earth pressure, including live load surcharge.
7. Forces and effects due to earthquake

There are four types of Standard Loadings for which road bridges are designed:

1. IRC Class AA Loading
2. IRC Class 70 R Loading
3. IRC Class A Loading
4. IRC Class B Loading

Construction Interview Questions and Answers

Highway Road Work Interview Questions and Answers

Civil Engineering Interview Questions and Answers

Railway bridges in India are to be built to conform to the Indian Railway Standards (IRS) laid down by the Ministry of Railways, Government of India, as below:

1. The loads to be considered in design are given in IRS Bridge Rules.
2. The details of design of steel bridge should conform to IRS Steel Bridge Code.
3. The details of design of bridge members in plain, reinforced and prestressed concrete should be in accordance with IRS Concrete Bridge Code.
4. Masonry ad plain concrete arch bridges should be detailed so as to conform to IRS Arch Bridge Code.
5. The substructure for bridges should be in accordance with IRS Bridge Substructure Code.

The usual types of reinforced concrete bridges are:

1. Slab bridges; 
2. Girder and slab (T-beam) bridges;
3. Hollow girder bridges;
4. Balanced cantilever bridges;
5. Rigid frame bridges;   
6. Arch bridges;
7. Bow string girder bridges.

The T-beam superstructure consists of the following components:

1. Deck slab
2. Cantilever portion
3. Footpaths; if provided, kerbs and handrails
4. Longitudinal girders, considered in design to be of T-section
5. Cross beams or diaphragms
6. Wearing course.

1. Uneven settlement of foundations may lead to disaster. Hence this type of structures should not be used in situations where unyielding foundations cannot be obtained at a reasonable cost.
2. The detailing and placing of reinforcements need extra care.
3. The sequence of placing concrete and the sequence of removing formwork have to be carefully planned.
4. Being statically indeterminate, the design is more complicated than simple beams.

The arch axis is generally governed by three considerations:

1. Span and rise from the road gradient and navigation or traffic clearances below,
2. The economical shape from point of view of saving of materials, and
3. The beauty of the intrados.

If continuous spans are used, the governing bending moments can be minimized and hence the individual span lengths can be increased. But unyielding supports are required for continuous construction. If supports settle, the net moments get modified in magnitude as well as in sense, resulting in distress to the structure. Hence for medium spans in the range of about 35 to 60 m, a combination of supported spans, cantilevers and suspended spans may be. The bridge with this type of superstructure is known as balanced cantilever bridge.

Continuous girder bridges have the following advantages over simply supported girder bridges: 

• The depth of decking at midspan will be much smaller. This is particularly important in the case of over bridges where the headroom available is generally restricted.
• As a corollary to the above, the quantities of steel and concrete will be less, resulting in reduced cost. Also reduced depth of deck leads to decrease in cost of approach ramps and earthwork.
• Fewer bearings are required. At each pier, only one bearing is needed, as against two bearings required for simply supported designs. Hence the piers can be narrower. Although the cost of individual bearings will be higher, the total cost on bearings will be lower.
• Fewer expansion joints will be required. For a continuous girder design, only two joints are needed at the ends, while the simply supported girder design will require one joint on each abutment and pier. Elimination of joints enhances the riding quality over the bridge.
• Since the bearings are placed on the center lines of the piers, the reactions of the continuous girder are transmitted centrally to the piers.
• The continuous girder bridge suffers less vibration and deflection.

The disadvantages of continuous girder designs over the simply supported girder designs may be listed as below:

• Uneven settlement of foundations may lead to disaster. Hence this type of structures should not be used in situations where unyielding foundations cannot be obtained at a reasonable cost.
• The detailing and placing of reinforcements need extra care.
• The sequence of placing concrete and the sequence of removing formwork have to be carefully planned.
• Being statically indeterminate, the design is more complicated than simple beams.

From the point of view of bridge construction, the basic differences between pre-tensioning and post-tensioning are listed below:

• Post-tensioning is well suited for prestressing at a construction site without the need for costly factory-type installations.
• Cast-in-place structures can be conveniently stressed by post-tensioning, which would not be possible with pre-tensioning.
• With post-tensioning, tendons can have curved trajectories, which lead to structural advantages, particularly for shear resistance.
• The need for individual tensioning, special anchorages, sheath and grouting results in a higher unit cost (cost per KN of effective prestressing force) for post-tensioning than for pre-tensioning.
• Many of the post-tensioning devices are covered by patents, restricting the   user to purchase materials and equipment from the patent holders. This difficulty is not present in pre-tensioning.
• It is possible to fabricate a beam with a number of precast elements, which are post-tensioned together to form one structural unit.

The segmental cantilever method of construction of prestressed concrete bridges has the following advantages:

• Centering and false work are avoided, enabling construction of structures with tall piers and over deep valleys.
• The speed of construction is enhanced, typically at 1 m per day per CFT for cast-in-place construction and possibly 3 m per day with the use of prefabricated segments.
• Enhanced levels of quality and workmanship are facilitated due to mechanization of repetitive tasks and
• The cost of construction permits competition with alternative design of a steel superstructure of long span.

Steel bridges can be classified under the following groups:

1. Beam bridges
2. Plate girder bridges
3. Box girder bridges
4. Truss brides
5. Arch bridges
6. Cantilever bridges
7. Cable stayed bridges
8. Suspension bridges.

Following Table gives the range of span in meters for different types of Bridges:

Steel bridges could be preferred option in Build Operate Transfer (BOT) projects, where speed in construction is crucial. Steel structures may also prove advantageous for urban flyover/elevated road projects as they cause fewer disturbances to traffic through faster construction and possible prefabrication.

Plate girder bridges are of two types:

(a) deck type; and

(b) half-through type.

Deck type is normally preferred. Half-through type is adopted when the cost of additional embankment to raise the rail level is high. A plate girder highway bridge will consist of the deck slab (normally of reinforced concrete) and stringers running longitudinally and resting on transverse floor beams, which in turn rest on the plate girders.

Truss bridges have been used economically in the span range of 100 to 200 m. A bridge truss derives its economy from its two major structural advantages

• The primary forces in its members are axial forces, and
• Greater overall depths permissible with its open web construction leads to reduced self-weight when   compared with solid web systems.

Typical forms of box girders are given below:

• Rectangular box with wide cantilevering span on either side
• Trapezoidal box sections
• Two box sections which are connected together by bracing for the integral action of the deck
• One wide box section subdivided into three cells
• Two box sections kept wide apart an
• One middle box section with one longitudinal girder on either side.

It was developed in Germany in the post-war years in an effort to save steel which was then in short supply. Since then, many cable stayed bridges have been built all over the world, chiefly because they are economical over a wide range of span lengths and they are aesthetically attractive.

The towers may take any one of the following forms

1. Single free-standing tower, as in Norderelbe bridge
2. Pair of free-standing tower shafts, as in Dusseldort North bridge
3. Portal frame, as in Stromsund bridge and Second Hooghly bridge
4. A-frame as in Severins bridge or inverted Y-shape as in Yangpu bridge
5. Diamond configuration as in Globe Island bridge, Sydney.

The components of a suspension bridge are:

• flexible main cables,
• towers;
• anchorages,
• hangers, deck, and       
• stiffening systems

The towers support the main cables and transfer the bridge loads to the foundations. Besides the primary structural function, the towers have a secondary function in giving the entire bridge a robust, graceful and soaring visual image. While earlier bridges had steel towers, concrete towers have been used in the Humber bridge and the Great Belt East bridge. Anchorages are usually massive concrete structures which resist the tension of the main cables.

The arch form is best suited to deep gorges with steep rocky banks which furnish efficient natural abutment to receive the heavy trust exerted by the ribs. In the absence of these natural conditions, the arch usually suffers a disadvantage, because the construction of a suitable abutment is expensive and time consuming.

Howrah bridge with a main span of 457 m was the third longest span cantilever bridge in the world at the time of its construction (1943). The bridge was erected by commencing at the two anchor spans and advancing towards the center with the use of creeper cranes moving along the upper chord. The closure at the middle was obtained by means of sixteen hydraulic jacks of 800 Ton capacity each. The construction was successfully completed with very close precision.

Piers are structures located at the ends of bridge spans at intermediate points between the abutments. The function of the piers is two-fold: to transfer the vertical loads to the foundation, and to resist all horizontal forces and transverse forces acting on the bridge. Being one of the most visible components of a bridge, the piers contribute to the aesthetic appearance of the structure.

Single column piers are increasingly used in urban elevated highway applications, and also for river crossings with a skew alignment. In an urban setting, single column piers provide an open and free-flowing perception to the motorists using the road below. Such piers when used for a skew bridge across a river results in least obstruction to passage of flood below the bridge.

An abutment is the substructure which supports one terminus of the superstructure of a bridge and, at the same time, laterally supports the embankment which serves as an approach to the bridge. For a river bridge, the abutment also protects the embankment from scour of the stream. Bridge abutments can be made of masonry, plain concrete or reinforced concrete.

Bearings are provided in bridges to transmit the load from the superstructure to the substructure in such a manner that the bearing stresses induced in the substructure are within permissible limits. They also accommodate certain relative movements between the superstructure and the substructure.

Expansion bearings for girder bridges are of the following types:

1. Sliding plate bearing 
2. Sliding-cum-rocker bearing
3. Steel roller-cum-rocker bearing
4. R.C. rocker expansion bearing
5. Elastomeric bearing

Since metallic bearings are expensive in cost and maintenance, the recent trend is to favour elastomeric bearings. An elastomeric bearing accommodates both rotation and translation through deformation of the elastomeric, these bearings are easy to install, low in cost and require practically no maintenance. They do not freeze, corrode or deteriorate. Barring an earthquake, the only probable causes for failure of an elastomeric bearing are inferior materials, incorrect design or improper installation. Elastomeric bearings can tolerate loads and movements exceeding the design values.

The joint is the weakest and most vulnerable area in bridge design. Unless properly designed, the distress at bridge joints will lead to many maintenance problems, ranging from spelling of concrete edges at the joint to deterioration of pier caps. With the extremely high density of traffic occurring on most major bridges, maintenance work on the bridge should be restricted to a minimum length of time. Hence the joints on a bridge should be so designed as to perform satisfactorily for a long time without requiring repair or replacement.

The main causes of reconstruction include:

1. Inadequate carriageway for the volume of traffic
2. Structural inadequacy due to deterioration or increase in design loadings
3. Insufficient waterway for river bridges and
4. Inadequate clearances for road under bridges.

A wearing course (sometimes referred as wearing coat) is provided over concrete bridge decks to protect the structural concrete from the direct wearing effects of traffic and also to provide the cross camber required for surface drainage. The wearing course may be of asphaltic concrete or cement concrete.