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have been adequately funded and scheduled in the Project Management Plan ( PMP). GENERAL CONSTRUCTION COST ESTIMATING G Civil engineering-- . PDF Drive is your search engine for PDF files. As of today we have 78,, eBooks Fluid Mechanics for Civil Engineers - Department of Civil Engineering. The second edition of the Civil Engineering Handbook has been revised and updated to provide a comprehensive reference work and resource book covering .
BC2 Bridges constructed by incremental launching method are usually low in span depth ratio and typical values are 14 to When it is in finely divided form, they can react with calcium hydroxide in the presence of moisture to form compounds with cementing properties. Place and fix reinforcement based on design drawings. TB2 For elastomeric bearing to function as a soft spring, the bearing should be allowed for bulging laterally and the compression stiffness can be increased by limiting the amount of lateral bulging. In fact, aggregates in concrete would not cause shrinkage and helps to resist the deformation.
Then why waiting? Hicks for material for free. This website is not the original publisher of this book. This e-book has been collected from other websites on the internet. All the rights on this book belong to the respective publisher. Tyler G. Hicks, P. He has worked in plant design and operation in a variety of industries, taught at several engineering schools, and lectured both in the United States and abroad. He is the author of more than books in engineering and related fields. Download Pdf.
For more books and updates Like us on Facebook. Which of the following methods to reduce cantilever moment is better in incremental launching, i temporary nose, ii mast or iii auxiliary piers? BC2 The use of mast is an alternative to temporary nose. From practical point of view, the use of mast requires continual adjustment of forces in the guys when the superstructure is pushed forward.
On the other hand, the implementation of temporary nose system does not require much attention during operation. However, it may not be economical for auxiliary piers with height more than 40m. For incremental launching method, the span depth ratio of bridges is normally low. BC2 Bridges constructed by incremental launching method are usually low in span depth ratio and typical values are 14 to With low span depth ratio, the bridge segments are stiff in bending and torsion which is essential to cater for the launching process.
Such low span depth ratio could tolerate the discrepancy in vertical alignment on supports over which they slide. Such differential settlements may occur owing to the shortening of piers when the superstructure slides over them and the differential deformation of different piers. CHU 7. In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?
BC2 i During the launching process the leading edge of the superstructure is subject to a large hogging moment. In this connection, steel launching nose typically about 0.
Sometimes, instead of using launching nose a tower and stay system are designed which serves the same purpose. Normally, a central prestress is provided in which the compressive stress at all points of bridge cross section is equal. In this way, it caters for the possible occurrence of tensile stresses in upper and lower part of the cross section when subject to hogging and sagging moment respectively.
Later when the whole superstructure is completely launched, continuity prestressing is performed in which the location and design of continuity tendons are based on the bending moments in final completed bridge condition and its provision is supplementary to the central prestress.
Should special design be catered for in bridge piers upon jacking up of superstructure for installation of bearings in incremental launching method? BC2 After the completion of launching process, the superstructure has to be lifted up to allow for installation of bearings.
This is usually achieved by means of jacks to raise mm successively at each pier. In fact, it is anticipated that no special design is necessary for this operation because the effect of differential settlements at support should already be checked in bridge design.
Level readings should be checked to ensure that it does not deviate from the designed figure. Can the use of temporary nose in incremental launching method reduce the cantilever moment of superstructure to the value of inner support moment?
BC2 When the superstructure is pushed forward, a temporary nose is usually adopted at the front end of the superstructure to reduce the cantilever moment for which the central prestress is designed. The bending moment of self-weight for internal spans equal span of long bridge is However, without the use of temporary nose, the bending moment in the leading pier when the deck has to cantilever from one pier to another would be Theoretically speaking, it is possible to reduce the cantilever moment to the value of inner support moment i.
However, from economic point of view, it is would better to adopt temporary additional prestressing instead of longer nose. Hence, in actual site practice, the use of temporary nose would not reduce the cantilever moment of superstructure to the value of inner support moment but only to achieve What are the main design considerations for temporary nose in incremental launching?
BC2 There are two main design considerations for temporary nose: Why is creep a major concern in balanced cantilever method? BC3 In balanced cantilever method, the moment is balanced along the length of the piers.
However, along the extended cantilevers only a part of negative bending moment is balanced by prestressing bending moment arising from normal force induced by prestressing. As such, it results in large deflections induced by concrete creep. These partly unbalanced permanent loads generate creep which produces unexpected and unevaluated hyperstatic effect. In fact, the compressive stresses are very high in lower slab while they are very low in upper slab of bridges. In bridge widening projects, the method of stitching is normally employed for connecting existing deck to the new deck.
What are the problems associated with this method in terms of shrinkage of concrete? SBGB1 In the method of stitching, it is a normal practice to construct the widening part of the bridge at first and let it stay undisturbed for several months. After that, concreting will then be carried out for the stitch between the existing deck and the new deck. In this way, the dead load of the widened part of bridge is supported by itself and loads arising from the newly constructed deck will not be transferred to the existing deck which is not designed to take up these extra loads.
One of the main concerns is the effect of stress induced by shrinkage of newly widened part of the bridge on the existing bridge. To address this problem, the widened part of the bridge is constructed a period of time say months prior to stitching to the existing bridge so that shrinkage of the new bridge will take place within this period and the effect of shrinkage stress exerted on the new bridge is minimized.
Traffic vibration on the existing bridge causes adverse effect to the freshly placed stitches. To solve this problem, rapid hardening cement is used for the stitching concrete so as to shorten the time of setting of concrete. Moreover, the stitching work is designed to be carried out at nights of least traffic Saturday night and the existing bridge may even be closed for several hours e.
Sometimes, longitudinal joints are used in connecting new bridge segments to existing bridges. The main problem associated with this design is the safety concern of vehicles. CHU coefficients of bridge deck and longitudinal joints when vehicles change traffic lanes is very dangerous to the vehicles. Moreover, maintenance of longitudinal joints in bridges is quite difficult. Stitching refers to formation of a segment of bridge deck between an existing bridge and a new bridge.
In joints of precast concrete bridge segments, what are the functions of applying epoxy adhesive? In precast segmental box girder bridges, the bridge segments are usually formed by match casting. It is sometimes observed that a gap is formed between adjacent bridge segments. SBGB2 To enhance perfect fitting of bridge segments in precast segmental box girder bridges, segments are usually constructed by match casting so that it would not impair the serviceability and load bearing ability of the bridge.
The end face of completed segment is adopted as formwork for the new segment. During the concrete hardening process, the hydration effect of new segment induces a temperature rise and develops a temperature gradient in the completed segment.
Hence, the completed segment bows temporarily and the new segment sticks to this bowed shape when hardened. After match casting, the completed segment retains its original shape after cooling down while the new segment obtains the profile of bowed shape. Such bowing effect is even more significant for slender segments with large height to width ratio. Bearings and Expansion Joints 1. In a curved prestressed bridge, how should the guided bearings in piers of the curved region be oriented with respect to the fixed bearing in abutment?
B1 To determine the orientation of guided bearings, one should understand the movement of curved region of a prestressed bridge. Movement of prestress and creep are tangential to the curvature of the bridge or along longitudinal axis while the movement due to temperature and shrinkage effects are in a direction towards the fixed pier. If the direction of guided bearings is aligned towards the fixed bearing in the abutment, the difference in direction of pretress and creep movement and the guided direction towards fixed bearing would generate a locked-in force in the bridge system.
The magnitude of the lock-in force is dependent on the stiffness of deck and supports. If the force is small, it can be designed as additional force acting on the support and deck. However, if the force is large, temporary freedom of movement at the guided bearings has to be provided during construction. The diagram showing how the guided bearings in piers of the curved region is oriented with respect to the fixed bearing in abutment. CHU 2. What is preset in bridge bearing?
The normal length of an upper bearing plate should be composed of the following components: Initially the bearing is placed at the mid-point of the upper bearing plate without considering the directional effect of irreversible movement.
In this arrangement, the size of upper plate is minimized in which irreversible movement takes place in one direction only and there is no need to include the component of two irreversible movements in the upper plate. Preset in sliding bearing. What is the purpose of leveling pad in bridge bearing? Bridge bearings should be installed to lie horizontally on bridge piers and columns so that it would not induce eccentricity forces on substructure.
However, the bridge superstructure requires different longitudinal and transverse level and gradient in order to keep in line with the geometry of the road. CHU the bottom of the superstructure to join with bridge bearing.
Wedge-shaped leveling pad is commonly used for better concrete mobility at the bridge bearings. Under what situation should engineers use pot bearings instead of elastomeric bearings?
TB1 In the event of high vertical loads combined with large angle of rotations, rubber bearings are undesirable when compared with pot bearings.
For instance, elastomeric bearings require large bearing surfaces so that compression can be maintained between the contact surfaces between the bearings and piers. Moreover, it also leads to uneven distribution of stress on the piers and some of these highly induced stresses may damage the piers. Consequently, pot bearings are better alternatives than elastomeric bearings in such an scenario as suggested by David J.
Why do most elastomers used in pot bearing are usually contained? TB1 To specify space requirements, most pot bearings are designed for high contact pressures with small contact area with bridges. This also enhances lower friction values. Under the free state, most elastomers in pot bearings can hardly sustain this high pressure and hence they are most contained to prevent overstraining.
When properly constrained, the elastomer behaves like semi-viscous fluid and can safely accommodate angular displacement. Polytetrafluoroethylene PTFE is commonly used in sliding bearings. TB1 The choice of sliding surface of bearings is of vital importance because the sliding surfaces generate frictional forces which are exerted on the bearings and substructure of the bridge. For instance, PTFE and lubricated bronze are commonly choices of sliding surfaces for bearings.
PTFE is a flurocarbon polymer which possesses good chemical resistance and can function in a wide range of temperature. The most important characteristic of this material is its low coefficient of friction.
PTFE has the lowest coefficients of static and dynamic friction of any solid with absence of stick-slip movement David J. The coefficient of friction is found to decrease with an increase in compressive stress. However, PTFE do have some demerits like high thermal expansion and low compressive strength. Moreover, it is recommended that the stainless steel surface be positioned on top of the PTFE surface to avoid contamination of dirt and rubbish.
Lubricants are sometimes introduced to reduce the friction between the PTFE surface and the upper stainless steel plate. Hence, the PTFE may be designed with dimples to avoid the lubricant from squeezing out under repeated translation movements. What is the purpose of dimples in PTFE in bridge bearings? TB1 PTFE is a flurocarbon polymer which possesses good chemical resistance and can function in a wide range of temperature. PTFE has the lowest coefficients of static and dynamic friction of any solid with absence of stick-slip movement .
However, PTFE do have some demerits like high thermal expansion and low compressive strength . Moreover, it is recommended that the stainless steel surface be positioned on top of the PTFE surface to avoid contamination by possible accumulation of dirt and rubbish on the larger lower plates. What is the purpose of dowel bar in elastomeric bearing? TB2 Elastomeric bearing is normally classified into two types: For fixed types, the bridge deck is permitted only to rotate and the horizontal movements of the deck are restrained.
On the other hand, for free types the deck can move horizontally and rotate. To achieve fixity, dowels are adopted to pass from bridge deck to abutment. Alternatively, in case there is limitation in space, holes are formed in the elastomeric bearings where anchor dowels are inserted through these holes.
CHU 9. How to determine the size of elastomeric bearings? TB2 For elastomeric bearing, the vertical load is resisted by its compression while shear resistance of the bearing controls the horizontal movements. The design of elastomeric bearings is based on striking a balance between the provision of sufficient stiffness to resist high compressive force and the flexibility to allow for translation and rotation movement.
The cross sectional area is normally determined by the allowable pressure on the bearing support. Sometimes, the plan area of bearings is controlled by the maximum allowable compressive stress arising from the consideration of delamination of elastomer from steel plates.
In addition, the size of elastomeric bearings is also influenced by considering the separation between the structure and the edge of bearing which may occur in rotation because tensile stresses deriving from separation may cause delamination. The thickness of bearings is designed based on the limitation of its horizontal stiffness and is controlled by movement requirements. The shear strain should be less than a certain limit to avoid the occurrence of rolling over and fatigue damage.
The vertical stiffness of bearings is obtained by inserting sufficient number of steel plates. In the design of elastomeric bearings, why are steel plates inserted inside the bearings? TB2 For elastomeric bearing to function as a soft spring, the bearing should be allowed for bulging laterally and the compression stiffness can be increased by limiting the amount of lateral bulging.
To increase the compression stiffness of elastomeric bearings, metal plates are inserted. After the addition of steel plates, the freedom to bulge is restricted and the deflection is reduced when compared with bearings without any steel plates under the same load. Tensile stresses are induced in these steel plates during their action in limiting the bulging of the elastomer.
This in turn would limit the thickness of the steel plates. However, the presence of metal plates does not affect the shear stiffness of the elastomeric bearings. Effect of steel plate in elastomeric bearing. Why do some engineers prefer to use neoprene instead of natural rubber in elastomeric bearings?
TB2 Some engineers may choose to design elastomeric bearings to sit on the piers without a connection. The bearing is held in place by frictional resistance only. Paraffin used in natural rubber would bleed out and result in significant decrease in friction. As such, elastomeric bearings would slip away and walk out from their original locations. To solve this problem, neoprene, instead of natural rubber, is used as elastomer because paraffin is absent in neoprene bearings.
What is the importance of shear stiffness in the design of elastomeric bearing? TB2 For elastomeric bearing, the shear stiffness is an important parameter for design because it influences the force transfer between the bridge and its piers.
In essence, elastomers are flexible under shear deformation but it is relatively stiff in compression. However, elastomeric bearings should not be used in tension. Elastomeric bearing should be designed in serviceability limit state only. The cross sectional area is normally determined by the compressive stress limit under serviceability limit state. The shape factor, i. In essence, higher capacity of bearings could be obtained with higher shape factor.
The long side of the bearing is usually oriented parallel to the principle axis of rotation because it facilitates rotational movement. CHU bearings is limited and controlled by shear strain requirements. In essence, the shear strain should be less than a certain limit to avoid the occurrence of rolling over at the edges and delamination due to fatigue.
Hence, it follows that higher rotations and translations require thicker bearing. On the other hand, the vertical stiffness of bearings is obtained by inserting sufficient number of steel plates. In addition, checks should be made on combined compression and rotation to guard against the possible occurrence of uplifting of corners of bearings under certain load combinations. For elastomeric bearings, which shape is better, rectangular or circular?
TB2 Circular bearings have the advantage for standardization because only one dimension can vary in plan. They are suitable for use in curved and large skewed bridge as they could accommodate movement and rotations in multiple directions. Rectangular bearings are suitable for low skewed bridges. In particular, it is best suited in bridges with large rotations and movements. What is the advantage of sliding bearings over roller bearings? In roller bearing for a given movement the roller bearing exhibit a change in pressure centre from its original position by one-half of its movement based on David J.
However, with sliding bearing a sliding plate is attached to the upper superstructure and the moving part of bearing element is built in the substructure.
It follows that there is no change in pressure center after the movement. CHU laminar layer divided by area of perimeter free to bulge, affects the relation between shear stress and the compressive load. The thickness of bearings is limited and controlled by shear strain requirements. Why are excessive movement joints undesirable in bridges? EJ1 Movement joints are normally added to bridge structures to accommodate movements due to dimensional changes arising from temperature variation, shrinkage, creep and effect of prestress.
In essence, the structural reserve provided by a continuous bridge is destroyed by the multiple-span statically determinate structure resulting from the addition of excessive joints. How will inclined bridge deck affect joint continuity? EJ1 Bearings are usually designed to sit in a horizontal plane so as to avoid the effect of additional horizontal force and uneven pressure distribution resulting from non-horizontal placing of bearings .
For an inclined bridge deck subject to a large longitudinal movement, a sudden jump is induced at the expansion joint and discontinuity of joint results. To solve this problem, an inclined bearing instead of a truly horizontal bearing is adopted if the piers can take up the induced horizontal forces.
The effect of inclined bridge deck on joint discontinuity. How does the position of bearing affect the continuity of joints? Expansion joints in a bridge structures cater for movements in transverse, longitudinal, vertical and rotational forms. The layout and position of expansion joins and bearings have to be carefully designed to minimize the future maintenance problem. The position of bearings affects the discontinuity of a joint .
If the location of a bearing is too far away from a bridge joint, discontinuity of the joint would be experienced when there is an excessive angular rotation at the joint.
Hence, by keeping the bearings and movement joints close in position, the discontinuity in the vertical direction can be avoided. The effect of position of bearing to the discontinuity of joint. Bridge Structure 1.
What are the advantages of assigning the central pier and the abutment as fixed piers? As the earthquake loads are resisted by fixed piers, the size of fixed piers will be large and massive.
In this connection, for better aesthetic appearance, the selection of abutment as fixed piers could accommodate the large size and massiveness of piers. Normally abutments are relatively short in height and for the same horizontal force, the bending moment induced is smaller. However, if the fixed pier is located at the abutment, the amount of movement to be incorporated in each bearing due to temperature variation, shrinkage, etc.
Therefore, the size of movement joints can be reduced significantly. What are the functions of diaphragms in bridges? BS1 Diaphragm is a member that resists lateral forces and transfers loads to support. Some of the diaphragms are post-tensioned and some contain normal reinforcement. It is needed for lateral stability during erection and for resisting and transferring earthquake loads.
Based on past research, diaphragms are ineffective in controlling deflections and reducing member stresses.
Moreover, it is commonly accepted that diaphragms aided in the overall distribution of live loads in bridges. The main function of diaphragms is to provide stiffening effect to deck slab in case bridge webs are not situated directly on top of bearings. Therefore, diaphragms may not be necessary in case bridge bearings are placed directly under the webs because loads in bridge decks can be directly transferred to the bearings . On the other hand, diaphragms also help to improve the load-sharing characteristics of bridges.
In fact, diaphragms also contribute to the provision of torsional restraint to the bridge deck. Are diaphragms necessary in the design of concrete box girder bridges? BS1 Diaphragms are adopted in concrete box girder bridges to transfer loads from bridge decks to bearings. Since the depth of diaphragms normally exceeds the width by two times, they are usually designed as deep beams.
This arrangement suffers from the drawback that changing of bearings during future maintenance operation is more difficult.
How do engineer determine the number of cells for concrete box girder bridges? CHU 5. What are the problems of using transition slabs in bridges? BS2 In some designs, transition slabs are provided on the approach to bridges. For instance, soils in embankment supporting the roads may settle due to insufficient compaction and sharp depressions would be developed at the junction with the relatively rigid end of bridge decks .
This creates the problem of poor riding surfaces of carriageway and proper maintenance has to be carried out to rectify the situation. As a result, transition slabs are sometimes designed at these junctions to distribute the relative settlements between the approaching embankments and end of bridge decks so that the quality of riding surface between these junctions could be significantly improved and substantial savings could be obtained by requiring less maintenance.
What is the purpose of overlays on concrete bridge deck? After years of servicing, some overlays may be applied on the top surface of bridges. Overlays on concrete bridge decks achieve the following purposes : Hence, it may be applied during the maintenance operation to hide the uneven and spalling deck surface and offers a smoother surface for road users.
What are the purposes of waterproofing in bridge decks?
Waterproofing materials like membranes are applied on top of bridge deck surface because: The chemicals easily penetrate and cause the deterioration of concrete bridge decks. In case waterproofing is not provided, the salt solution penetrates through the concrete cracks of the bridge and causes the corrosion of reinforcement. CHU 8. Why are split piers sometimes used when piers are built directly into the deck? BS3 When the piers are built directly into deck without bearings, the monolithic construction creates a portal structure which modifies the bending moment envelope in the deck when compared with bridges with bearings.
For instance, hogging moments are increased in supports with the decrease in sagging moments in mid-span of bridge deck. On the other hand, the shear stiffness of piers is a major concern because it tends to resist length changes of bridge deck which could not expand and contract readily.
In order to retain the bending stiffness of piers and to destroy the shear stiffness of pier simultaneously, the piers are split into two parts. Split pier 9. What is the significance of spacing of split piers?
BS3 Live loads on one span tend to cause uplift of outer column of the split piers twin leaf piers. When the two split piers are designed too close, the uplift may be greater than the dead load reaction of the outer pier so that tension is induced in the outer pier. CHU leaving both piers in compression.
As such, this allows for the use of pinned bearing. What is the purpose of providing a barrier around the bridge piers? Accidental collision of heavy vehicles such as tractor-trailer with bridge piers is not uncommon around the world. The consequence of such collision is catastrophic which may involve the collapse of bridges and loss of human lives.
As such, suitable provisions are made to protect bridge piers against these accidental collisions. The most common way is to install a crashworthy barrier which should be designed to be capable of resisting the impact of heavy vehicles. Alternatively, in some countries such as the United States, they tend to revise the design of bridge barriers by requiring the bridge piers to be able to resist the collision of 1,kN static force at 1.
In bridge columns, why are stirrups be placed around the vertical reinforcement? In uniaxial compression test of concrete, upon reaching the ultimate load failure of concrete occurs where major cracks line up in the vertical direction and the concrete cube would be split up. The development of vertical cracks involves the expansion of concrete in lateral directions. In case the concrete is confined in lateral directions, it was observed that the formation of vertical cracks would be hindered as indicated in past experiments.
As a result, the concrete strength is increased with also a rise in failure strain. The above theory is often used in the design of bridge columns. Steel stirrups are installed at around the vertical main reinforcement. Other than the function of shear reinforcement, it helps to avoid the lateral deformation of interior concrete core so that the strength of concrete column is increased.
What are the effects of bridge piers across a stream? The presence of bridge piers across a stream causes constricted flow in the openings because of the decrease of width of stream owing to the presence of the piers. Moreover, it creates the following problems from hydraulic point of view: CHU i Local scouring at the piers and bed erosion may take place.
To avoid the damage to the foundation of piers, some protective layers of stone or concrete apron could be provided around the piers. Consequently, this may result in flooding in upstream areas. What is the mechanism of scouring at obstructions e. When the water flow in river is deflected by obstructions like bridge piers, scouring would occur arising from the formation of vortexes.
The mechanism of formation of vortices is as follows: When the flow reaches the seabed, it would move in a direction opposite to its original flow direction before hitting the bridge piers.
Hence, this movement of flow before the bridge piers results in the formation of a vortex. Owing to the formation of this vertical vortex, seabed material is continuously removed so that holes are formed at the seabed and this result in local scour at bridge piers. What is the purpose of installation of shear keys in bridge abutment? BS4 In small and medium sized bridges, shear keys are often designed in bridge abutments to provide transverse support to the bridge superstructure under lateral loads.
They are not intended to carry vertical loads and they have important applications in resisting seismic loads. Shear keys in bridge abutment are divided into two types, exterior or interior. Exterior shear keys have the demerit of the ease of inspection and repair.
The shear keys are designed as sacrificial and it is assumed that once their capacity has been exceeded, the shear keys would not provide further support. As such, the bridge columns should be designed to provide transverse support once the shear keys fail to function. Which bridge parapet is better, steel parapet or aluminum parapet?
CHU galvanizing as they are prone to corrosion and they are normally the cheapest choice for normal containment level of vehicles. The initial material and setup cost of aluminum parapet is high. They are free of the problem of corrosion and the design of aluminum parapets does not require surface protection.
However, owing to their high material price, care should be taken on the design to prevent stolen of parts of parapet. Moreover, aluminum parapet is lighter than steel and has weight savings over steel parapets.
When should engineers consider using truss with K-bracing? In the arrangement of triangulated framework in truss structures, it is more economical to design longer members as ties while shorter ones as struts e. Pratt truss. As such, the tension forces are taken up by longer steel members whose load carrying capacities are unrelated to their lengths.
However, the compression forces are reacted by shorter members which possess higher buckling capabilities than longer steel members . For heavy loads on a truss structure, the depth of the truss is intentionally made larger so as to increase the bending resistance and to reduce deflection. With the increase in length of the vertical struts, buckling may occur under vertical loads. Therefore, K-truss is designed in such as way that the vertical struts are supported by compression diagonals.
What are the characteristics of Vierendeel girder? T1 The Vierendeel girder design is sometimes adopted in the design of footbridges. In traditional truss design, triangular shape of truss is normally used because the shape cannot be changed without altering the length of its members. By applying loads only to the joints of trusses, the members of truss are only subjected to a uniform tensile or compressive stress across their cross sections because their lines of action pass through a common hinged joint.
Hence, bending, shear and axial capacity of these members contribute to the resistance to external loads. The use of this girder enables the footbridge to span larger distances and present an attractive outlook. CHU . Vierendeel Truss. T2 A truss is a simple structure whose members are subject to axial compression and tension only and but not bending moment.
The most common truss types are Warren truss, Pratt truss and Howe truss. Warren truss contains a series of isosceles triangles or equilateral triangles. To increase the span length of the truss bridge, verticals are added for Warren Truss. Pratt truss is characterized by having its diagonal members except the end diagonals slanted down towards the middle of the bridge span.
Under such structural arrangement, when subject to external loads tension is induced in diagonal members while the vertical members tackle compressive forces. Hence, thinner and lighter steel or iron can be used as materials for diagonal members so that a more efficient structure can be enhanced. The design of Howe truss is the opposite to that of Pratt truss in which the diagonal members are slanted in the direction opposite to that of Pratt truss i.
Hence, it is not economical to use steel members to handle compressive force. A typical Howe Truss. Warren Truss and Pratt Truss What is the difference between dry joint and wet joint in precast segmental bridges? Dry joints have been properly used in the past in which the bridge segments are formed by match casting. The prevalence in the past is due to it lower cost and time for construction.
There is no gluing material to seal up the joint. As such, leakage through the joint into the box culvert occurs from time to time and this may affect the durability of external post-tensioning tendons. Moreover, owing to the effect of seismic, temperature and creep, the joints are found to open under these conditions. Spalling of top concrete slab at bridge joint was also reported.
Wet joint involves the use of epoxy glue at the mating precast segments. After the application of epoxy glue, a temporary precompression pressure of 0. The epoxy sets under the applied pressure. The use of epoxy joints provides lubrication to help in the fit-up and alignment of the mating segments and minimizes the effect of hard point contact between segments.
Why are precast concrete piers seldom used in seismic region? The use of precast concrete elements enhances faster construction when compared with cast-in-situ method. Moreover, it enhances high quality of piers because of stringent control at fabrication yards. The environmental impact is reduced especially for bridges constructed near waterways. In particular, for emergency repair of bridges owing to bridge collapse by earthquake and vehicular collision, fast construction of damaged bridge is of utmost importance to reduce the economic cost of bridge users.
The precast bridge piers are mostly used in non-seismic region but not in seismic region because of the potential difficulties in creating moment connections between precast members and this is essential for structures in seismic region. Why are coatings sometimes provided at the back faces of abutments? BS6 There are different views on the necessity of the application of protective coatings may be in the form of two coats of paint to the back faces of bridge abutment . The main purpose of this coating serves to provide waterproofing effect to the back faces of abutments.
By reducing the seepage of water through the concrete, the amount of dirty materials accumulating on the surface of concrete would be significantly decreased.
Engineers tend to consider this as an inexpensive method to provide extra protection to concrete. However, others may consider that such provision is a waste of money and is not worthwhile to spend additional money on this. Coatings at back faces of an abutment. What is shock transmission unit in bridges? BS5 Shock transmission unit is basically a device connecting separate structural units.
It is characterized by its ability to transmit short-term impact forces between connecting structures while permitting long-term movements between the structures. If two separate structures are linked together to resist dynamic loads, it is very difficult to connect them structurally with due allowance for long-term movements due to temperature variation and shrinkage effect .
Instead, large forces would be generated between the structures. However, with the use of shock transmission unit, it can cater for short-term transient loads while allowing long-term movements with negligible resistance. It benefits the bridge structures by acting as a temporary link between the structures to share and transfer the transient loads. Shock transmission unit.
Should raking piles of a bridge abutment be placed under an embankment? BS6 For a bridge abutment to be supported on raking piles with different orientations, the movement between the ground and the pile group is difficult to predict.
For instance, if some of the raking piles of the bridge abutment are extended beneath an embankment, then the settlement of embankment behind the abutment may cause the raking piles to experience severe bending moment and damage the piles as recommended by Dr. Sometimes the side of concrete bridges is observed to turn black in colour.
What is the reason for this phenomenon? In some cases, it may be due to the accumulation of dust and dirt. CHU for the majority of such phenomenon, it is due to fungus or algae growth on concrete bridges.
After rainfall, the bridge surface absorbs water and retains it for a certain period of time. Hence, this provides a good habitat for fungus or algae to grow. Moreover, atmospheric pollution and proximity of plants provide nutrients for their growth. Improvement in drainage details and application of painting and coating to bridges help to solve this problem.
Reference is made to Sandberg Consulting Engineers Report 18 Are there any problems associated with Integral Abutment Bridge? BS6 Integral Abutment Bridges are bridges without expansion joints in bridge deck. The superstructure is cast integrally with their superstructure. The flexibility and stiffness of supports are designed to take up thermal and braking loads.
The design of Integral Abutment Bridges is simple as it may be considered as a continuous fame with a single horizontal member with two or more vertical members. The main advantage of this bridge form is jointless construction which saves the cost of installation and maintenance of expansion joints and bearings. It also enhances better vehicular riding quality.
Moreover, uplift resistance at end span is increased because the integral abutment serves as counterweight. As such, a shorter end span could be achieved without the provision of holding down to expansion joints. The overall design efficiency is increased too as the longitudinal and transverse loads on superstructure are distributed over more supports.
However, there are potential problems regarding the settlement and heaving of backfill in bridge abutment. Settlement of backfill continues with daily temperature cycles and it does not stabilize.
Active failure of upper part of backfilling material also occurs with wall rotations. This leads to backfill densification and can aggravate settlement behind the abutment. What are the functions of sleepers in railway? The functions of sleepers  in railway works are as follows: CHU longitudinal and lateral track movement.
Long Bridge 1. What are the advantages of cable-stayed bridges over suspension bridges for span less than 1,m? LBT1 The advantage of cable-stayed bridges lies in the fact that it can be built with any number of towers but for suspension bridges it is normally limited to two towers.
With span length less than 1,m, suspension bridges require more cables than cable-stayed bridges. Moreover, cable-stayed bridges possess higher stiffness and display smaller deflections when compared with suspension bridges.
Generally speaking, the construction time is longer for suspension bridges. When is single plane or multiple plane used in cable-stayed bridges? LBT1 For one cable plane to be adopted, the requirement of high torsional stiffness of bridge deck is necessary to enhance proper transverse load distribution.
Moreover, owing to the higher stiffness of bridge deck to cater for torsional moment, it possesses higher capacity for load spreading. As a result, this avoids significant stress variations in the stay and contributes to low fatigue loading of cables. On the other hand, the use of one cable plane enhances no obstruction of view from either sides of the bridges. For very wide bridge, three cable planes are normally adopted so as to reduce the transverse bending moment.
What is the difference between gravity anchorage and tunnel anchorages in suspension bridges? LBT2 Gravity anchorages consist of three main parts, namely the base block, anchorage block and weight block. The weight block sits on top of anchor block and its weight is not used for resisting the pull of cables. Instead, its vertical action presses the cables vertically downward so as to turn the pull of cables against the foundation. CHU friction between the embedded concrete anchorage and the surrounding foundation.
How does the shape of bridge deck affect the aerodynamic behaviour? Two types of bridge vibration that are of special concern are: One of the important features affecting the aerodynamic behaviour of a bridge is the shape of bridge deck.
The shape which provides maximum stability against wind effects is that of an airplane wing, on which the wind flows smoothly without creating turbulence and there is no separation of boundary layers. To improve the aerodynamic behaviour of a bridge, addition of wind fairings and baffle plates could be considered.
How do vortex-induced vibrations affect the stability of long bridges? AB2 When wind flows around a bridge, it would be slowed down when in contact with its surface and forms boundary layer. At some location, this boundary layer tends to separate from the bridge body owing to excessive curvature. This results in the formation of vortex which revises the pressure distribution over the bridge surface.
The vortex formed may not be symmetric about the bridge body and different lifting forces are formed around the body. As a result, the motion of bridge body subject to these vortexes shall be transverse when compared with the incoming wind flow. As the frequency of vortex shedding approaches the natural frequencies of the bridges, resonant vibrations often occur, the amplitude of which depends on the damping in the system and the motion of the wind relative to the bridges.
How does flatter affect the stability of long bridges? AB1 Flutter is a potentially destructive vibration and it is self-feeding in nature. The aerodynamic forces on a bridge, which is in nearly same natural mode of vibration of the bridge, cause periodic motion.
CHU so that a positive feedback occurs between the aerodynamic forces and natural vibration of the bridge. In essence, the vibration movements of the bridge increase the aerodynamic load which in turns cause further movement of the bridge. Consequently, it results in large magnitude of movement and may cause bridge failure. How does deck equipment median dividers and parapets affect the aerodynamic response of long-span bridges? Bridge parapets raise the overall level of bluffness of long-span bridges.
When the solidity ratio of barriers increases, the effect of increasing the bluffness also becomes more significant. The principal effects of deck equipment such as median dividers and parapets is that it enhances an increase in drag forces and a reduction in average value of lift force. In wind tunnel test, why are similarity of Reynolds Number between real bridge and model is often neglected?
Wind tunnel test is often conducted to check aerodynamic stability of long-span bridges. To properly conduct wind tunnel test, aerodynamic similarity conditions should be made equal between the proposed bridge and the model. Reynolds Number is one of these conditions and is defined as ratio of inertial force to viscous force of wind fluid. With equality of Froude Number, it is difficult to achieve equality in Reynolds Number.
As such, similarity of Reynolds Number between real bridge and model is often neglected. Prestressing Works 1. What are parasitic forces for prestressing?
In statically determinate structures, prestressing forces would cause the concrete structures to bend upwards. Hence, precambering is normally carried out to counteract such effect and make it more pleasant in appearance.
However, for statically indeterminate structures the deformation of concrete members are restrained by the supports and consequently parasitic forces are developed by the prestressing force in addition to the bending moment generated by eccentricity of prestressing tendons .
The developed forces at the support modify the reactions of concrete members subjected to external loads and produces secondary moments or parasitic moments in the structure. Why type of prestressing is better, external prestressing or internal prestressing? PT1 At several locations in the span i. The advantages of external prestressing are listed below: This is essential for bridges in urban areas where traffic disruption is undesirable.
Moreover, the use of polyethylene sheathing with external prestressing has lower friction coefficient than corrugated metal ducts in internal prestressing. CHU vii Improvement of concrete placing in bridge webs owing to the absence of ducts. The major distinction between internal prestressing and external prestressing lies in the variation in cable eccentricity.
The deflected shape of external tendons is not exactly the same as beams because the displacement of external tendons is controlled by deviators. This is a second order effect at working load and it is very important at ultimate load. Based on past research, for small span with shallow cross section i. However, for deeper bridge cross section, the employment of external prestressing results in smaller amount of steel reinforcement. Under what situation shall engineers use jacking at one end only and from both ends in prestressing work?
During prestressing operation at one end, frictional losses will occur and the prestressing force decreases along the length of tendon until reaching the other end. These frictional losses include the friction induced due to a change of curvature of tendon duct and also the wobble effect due to deviation of duct alignment from the centerline.
Therefore, the prestress force in the mid-span or at the other end will be greatly reduced in case the frictional loss is high. Consequently, prestressing, from both ends for a single span i. In fact, stressing at one end only has the potential advantage of lower cost when compared with stressing from both ends.
For multiple spans e. On the contrary, jacking from the end of the shorter span would be conducted if the negative moment at the intermediate support controls the prestress force. However, if the total span length is sufficiently long, jacking from both ends should be considered.
Which one is better, one-way prestressing or two-way prestressing? CHU the other end. What are the three major types of reinforcement used in prestressing? PC1 i Spalling reinforcement Spalling stresses are established behind the loaded area of anchor blocks and this causes breaking away of surface concrete. Reinforcement is needed to resist these lateral tensile forces. Why is spalling reinforcement needed for prestressing works in anchor blocks?
CHU reinforcement. Bursting reinforcement is used where tensile stresses are induced during prestressing operation and the maximum bursting stress occurs where the stress trajectories are concave towards the line of action of the load. For equilibrium reinforcement, it is required where there are several anchorages in which prestressing loads are applied sequentially.
During prestressing, spalling stresses are generated in the region behind the loaded faces of anchor blocks . At the zone between two anchorages, there is a volume of concrete surrounded by compressive stress trajectories. Forces are induced in the opposite direction to the applied forces and it forces the concrete out of the anchor block. What are the functions of grout inside tendon ducts? PC2 Grout in prestressing works serves the following purposes: Therefore, the friction of the trapped wires is high and is undesirable.
What is the optimum size of cable duct for prestressing? PC1 The cross sectional area of duct is normally 2. The size of ducts should be not designed to be too small because of the followings: What is stress corrosion of prestressing steel?
PC1 Stress corrosion is the crystalline cracking of metals under tensile stresses in the presence of corrosive agents . The conditions for stress corrosion to occur are that the steel is subjected to tensile stresses arising from external loading or internally induced stress e.
Moreover, the presence of corrosive agents is essential to trigger stress corrosion. One of the main features of stress corrosion is that the material fractures without any damage observed from the outside.
Hence, stress corrosion occurs without any obvious warning signs. Concrete Material 1. Can grout replace concrete in normal structure? The mixture of cement and water alone cannot replace concrete Longman Scientific and Technical because: What are the differences between epoxy grout, cement grout and cement mortar? In fact, there are various types of resin used in construction industry like epoxy, polyester, polyurethane etc. Though epoxy grout appears to imply the presence of cement material by its name, it does not contain any cement at all.
On the other hand, epoxy hardener serves to initiate the hardening process of epoxy grout. It is commonly used for repairing hairline cracks and cavities in concrete structures and can be adopted as primer or bonding agent. Cement grout is formed by mixing cement powder with water in which the ratio of cement of water is more or less similar to that of concrete . Owing to the relatively high water content, the mixing of cement with water produces a fluid suspension which can be poured under base plates or into holes.
Setting and hardening are the important processes which affect the performance of cement grout. Moreover, the presence of excessive voids would also affect the strength, stiffness and permeability of grout. It is versatile in application of filling voids and gaps in structures.
Cement mortar is normally a mixture of cement, water and sand typical proportion by weight is 1: It is intended that cement mortar is constructed by placing and packing rather than by pouring.
They are used as bedding for concrete kerbs in roadwork. They are sometimes placed under base plates where a substantial proportion of load is designed to be transferred by the bedding to other members. What is the difference between no-fines concrete, lightweight concrete and lean concrete? CM1 Pervious concrete is sometimes called "no fines" concrete. It is designed with high porosity and allows water to pass though.
It is commonly used in concrete pavement so as to reduce surface runoff and allow the recharging of ground water. The high porosity is achieved by a network of interconnected void. In some cases, even the fine aggregates are also lightweight too. The primary use of lightweight concrete is to reduce the dead load of concrete structures.
Lean concrete, which is also known as cement bound material, has low cementitious material content. It has low concrete strength and is commonly used as roadbase material.
What is the difference between foam concrete and cement grout? CM1 Foam concrete is mainly composed of cement, water and air pores with filler such as PFA, sand etc. The air pores are formed by agitating air with a foaming agent.
The typical size of air bubbles is around 0. For cement grout, it mainly consists of cement and water.