Recent questions tagged bridge-works

Description : What is sucker deck principle for variable depth bridge decks?

Last Answer : For a variable depth bridge deck, the depth of continuous multi­span bridge deck is increased in pier supports and this absorbs sagging moments in the ... of arches and curved soffit rather than boring uniform deck alignment. Reference is made to Brian Pritchard (1992).

Description : How do engineer determine the number of cells for concrete box girder bridges?

Last Answer : If the depth of a box girder bridge exceeds 1/6 or 1/5 of the bridge width, then it is recommended to be designed as a single cell box girder bridge. However, if the bridge depth is ... improvement in transverse load distribution when the number of cells of box girder is increased to three or more.

Description : Should raking piles of a bridge abutment be placed under an embankment?

Last Answer : 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 ... experience severe bending moment and damage the piles as recommended by Dr. Edmund C Hambly (1979).

Description : Polytetrafluoroethylene (PTFE) is commonly used in sliding bearings. Why?

Last Answer : 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 ... be designed with dimples to avoid the lubricant from squeezing out under repeated translation movements.

Description : What are the shortcomings of grillage analysis which is commonly used in structural analysis of bridges?

Last Answer : Grillage analysis suffers from the following shortcomings based on E. C. Hambly: (i) For coarse mesh, torques may not be identical in orthogonal directions. Similarly, twists may differ in ... moment in an element depends on the curvatures in the beam's direction and its orthogonal direction.

Description : Under what situation should engineers use pot bearings instead of elastomeric bearings?

Last Answer : 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 are better alternatives than elastomeric bearings in such an scenario as suggested by David J. Lee.

Description : In joints of precast concrete bridge segments, what are the functions of applying epoxy adhesive?

Last Answer : Epoxy adhesive is applied in these joints for the following purposes according to International Road Federation (1977): (i) It seals up the joints completely between precast concrete segments to protect the ... in transferring of shear between the joints in case a large single shear key is used.

Description : What are the functions of grout inside tendon ducts?

Last Answer : Grout in prestressing works serves the following purposes: (i) Protect the tendon against corrosion. (ii) Improve the ultimate capacity of tendon. (iii) Provide a bond between the structural member and the tendon. (iv) In case of failure, the anchorage is not subject to all strain energy.

Description : In the design of a simply supported skew bridge, which direction of reinforcement should be provided?

Last Answer : In the conventional design of steel reinforcement for a simply supported skew bridge, a set of reinforcement is usually placed parallel to free edge while the other set is designed parallel ... ). In this way, considerable savings would be obtained from the orthogonal arrangement of reinforcement.

Description : Why is the span length ratio of end span/approach span to its neighboring inner spans usually about 0.75?

Last Answer : From aesthetic point of view, an odd number of spans with a decrease in length in the direction of abutment is desirable. Moreover, spans of equal length are found to be boring. However, the ... the last span in a continuous bridge while approach span refers top the first span of a bridge.

Description : What are the three major types of reinforcement used in prestressing?

Last Answer : i) Spalling reinforcement Spalling stresses are established behind the loaded area of anchor blocks and this causes breaking away of surface concrete. These stresses are induced by strain incompatibility with ... of action of the load. Reinforcement is needed to resist these lateral tensile forces.

Description : What is the advantage of sliding bearings over roller bearings?

Last Answer : In roller bearing for a given movement the roller bearing exhibit a change in pressure centre from its original position by one­half of its movement based on David J. Lee. However, with sliding ... built in the substructure. It follows that there is no change in pressure center after the movement.

Description : Are diaphragms necessary in the design of concrete box girder bridges?

Last Answer : 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 ... In fact, diaphragms also contribute to the provision of torsional restraint to the bridge deck.

Description : In the construction of a two­span bridge (span length = L) by using span­by­span construction, why is a length of about 1.25L bridge segment is constructed in the first phase of construction?

Last Answer : Basically, there are mainly three reasons for this arrangement: (i) The permanent structure is a statically indeterminate structure. During construction by using span­by­span construction, if the ... 25L from the first pier where there is relatively more space to accommodate prestressing works.

Description : 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?

Last Answer : 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 ... large, temporary freedom of movement at the guided bearings has to be provided during construction.

Description : How to determine the size of elastomeric bearings?

Last Answer : For elastomeric bearing, the vertical load is resisted by its compression while shear resistance of the bearing controls the horizontal movements. The design ... and fatigue damage. The vertical stiffness of bearings is obtained by inserting sufficient number of steel plates.

Description : In the design of elastomeric bearings, why are steel plates inserted inside the bearings?

Last Answer : 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 ... , the presence of metal plates does not affect the shear stiffness of the elastomeric bearings.

Description : In prestressing work, if more than one wire or strand is included in the same duct, why should all wires/strands be stressed at the same time?

Last Answer : If wires/strands are stressed individually inside the same duct, then those stressed strand/wires will bear against those unstressed ones and trap them. Therefore, the friction of the trapped wires is high and is undesirable.

Description : Sometimes the side of concrete bridges is observed to turn black in colour. What is the reason for this phenomenon?

Last Answer : In some cases, it may be due to the accumulation of dust and dirt. However, for the majority of such phenomenon, it is due to fungus or algae growth on concrete bridges. After rainfall, the ... help to solve this problem. Reference is made to Sandberg Consulting Engineers Report 18380/X/01.

Description : What are the advantages of assigning the central pier and the abutment as fixed piers?

Last Answer : (i) For abutment pier to be assigned as fixed pier while the bridge is quite long, the longitudinal loads due to earthquake are quite large. As the earthquake loads are resisted by fixed piers, the size of ... Therefore, the size of movement joints can be reduced significantly.

Description : 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?

Last Answer : 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 ... Stitching refers to formation of a segment of bridge deck between an existing bridge and a new bridge.

Description : In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?

Last Answer : 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 0.6­0.65 times span length is ... . iii)For very long span bridge, temporary piers are provided to limit the cantilever moment.

Description : What is “preset” during installation of bridge bearings?

Last Answer : Preset is a method to reduce the size of upper plates of sliding bearings in order to save the material cost. The normal length of a upper bearing plate should be composed of the ... of a certain distance of sliding bearings with respect to upper bearing plates during installation of bearings.

Description : Under what situation shall engineers use jacking at one end only and from both ends in prestressing work?

Last Answer : 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 ... if the total span length is sufficiently long, jacking from both ends should be considered.