Expansion joint

The expansion joints are constructed in the transverse direction to provide space for the thermal expansion of the slab, thus preventing cracking owing to compressive stresses that developed when restricted. 

At the contraction joint, there is a complete discontinuity of concrete slab. The gap width introduced depends on the slab thickness, the coefficient of thermal expansion of concrete, the developed friction with the sub-base and the spacing between two expansion joints. The gap width is usually 20 to 25 mm. The top part of this gap is always filled with appropriate sealant. The spacing between expansion joints is determined by the pavement design methodology followed. 

The use of dowel bars is necessary in the expansion joints, because of the gap created between slabs, ensuring smooth load transfer from one slab to the other and increasing concrete s resistance to bending and shear. 

The dowel bars for expansion joints are slightly longer than those used in the contraction joints, typically 500-600 mm, and are usually placed every 300 mm. The bar diameter is always greater than that of the dowel bars placed in the contraction joints, usually 25 or 32 mm. As in contraction joints, the dowel bars are made of smooth steel and coated by a corrosion-resistant material. 

For the exact determination of the diameter, length, spacing length, steel quality of the dowel bars or any other related construction detail, the designer should follow precisely the instructions of the methodology used. 

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The movement-absorbing devices used in semirigid and nonrigid piping systems are usually called expansion joints. Based on the method by which the pressure seal is effected, joints are either of the packed type or the packless or bellows type. Selection depends not only on the required movement but also on the severity of service in terms of pressure and temperature, tolerabiUty of leakage, and the number of service cycles. 

The selection and appHcation of an expansion joint is not as simple as selecting a pipe fitting or a valve and requires a sound understanding of the joint s capabihties and limitations. Improper appHcation of any type of joint can result in serious or damaging effects. However, when properly selected and integrated into the piping system, satisfactory service and safe operation can be expected. Selection and appHcation of beUows expansion joints require special attention to design and installation. 

Dual-beUows assembhes, ie, universal-type expansion joints, are particularly vulnerable to squirm, and can experience elastic squirm at one-fourth the pressure of an individual bellows. When large amounts of offset are encountered, as is often the design basis, a pinwheel effect occurs because of unbalanced pressure forces. This effect tends to rotate the center-spool pipe which may lead to bellows mpture. Eor this reason the center spool should always be stabilized by hinges or tie-rod lugs to prevent such rotation. 

Standards ofi the Expansion Joint Manufiacturers Association 6th ed., EJMA Inc., White Plains, N.Y., 1993. 

The coefficient of linear expansion of these alloys in the temperature range of 21 to 100°C (70 to 212°F) is 12.2 X lO C (6.8 X 10"V°F), which is slightly above that of cast iron (National Bureau of Standards). Since these loys have practically no elasticity, it is necessary to use expansion joints in relatively short pipe hnes. Connections for flanged pipe, fittings, valves, and pumps are made to 125-lb American Standarci drilling. 

Bending or torsional flexibihty may be provided by bends, loops, or offsets by corrugated pipe or expansion joints of the bellows type or by other devices permitting rotational movement. These devices must be anchored or otherwise suitably connected to resist end forces from fluid pressure, frictional resistance to pipe movement, and other causes. 

Expansion Joints All the foregoing applies to stiff piping systems, i.e., systems without expansion joints (see detail 1 of Fig. 10-169). When space hmitations, process requirements, or other considerations result in configurations of insufficient flexibihty, capacity... 

Expansion joints for free-movement systems can be designed for axial or offset movement alone, or for combined axial and offset movements (see Fig. 10-171). For offset movement alone, the end load due to pressure and weight can be transferred across the joint by tie rods or structural members (see Fig. 10-172). For axial or combined movements, anchors must be provided to absorb the imbalanced pressure load and force bellows to deflect. 

FIG. 10-170 Hinged expansion joint. (From Kellogg, Design of Piping Systems, Wiley, New York, 1965.)... 

To protect terminal equipment or other (weaker) portions of the system, restraints (such as anchors and guides) shall be provided where necessary to control movement or to direct expansion into those portions of the system that are adequate to absorb them. The design, arrangement, and location of restraints shall ensure that expansion-joint movements occur in the directions for which the joint is designed. In addition to the other thermal forces and moments, the effects of friction in other supports of the system shall be considered in the design of such anchors and guides. 

Anchors for Expansion Joints Anchors (such as those of the corrugated, omega, disk, or shp type) shall be designed to withstand the algebraic sum of the forces at the maximum pressure and temperature at which the joint is to be used. These forces are ... 

Pressure thrust, which is the product of the effective thrust area times the maximum pressure to which the joint will be subjected during normal operation. (For shp joints the effective thrust area shall be computed by using the outside diameter of the pipe. For corrugated, omega, or disk-type joints, the effective thrust area shall be that area recommended by the joint manufacturer. If this information is unobtainable, the effective area shall be computed by using the maximum inside diameter of the expansion-joint bellows.)... 

The force required to overcome the static friction of the pipe in expanding or contracting on its supports, from installed to operating position. The length of pipe considered should be that located between the anchor and the expansion joint. 

Provision for differential expansion Expansion joint in shell Individual tubes free to expand floating head Floating head I loating head Floating head... 

For odd niimher of tube side passes, floating head requires packed joint or expansion joint. 

Differential expansion between the shell and the tubes can develop because of differences in length caused by thermal expansion. Various types of expansion joints are used to ehminate excessive stresses caused by expansion. The need for an expansion joint is a function of both the amount of differential expansion and the cycling conditions to be expected during operation. A number of types of expansion joints are available (Fig. 11-37). 

The foregoing designs were discussed as ring expansion joints by Kopp and Sayre, Expansion Joints for Heat Exchangers (ASME Misc. Pap., vol. 6, no. 211). All are statically indeterminate but are subjected to analysis by introducing various simplifying assumptions. Some joints in current industrial use are of lighter wall constniction than is indicated by the method of this paper. 

Chemical-plant exchangers requiring expansion joints most commonly have used the flanged-and-flued-head type. There is a trend toward more common use of the light-wall-bellows type. 

Flexible expansion Joints of aluminium or copper are essential after every three or four standard lengths... 

An expansion Joint connects two straight, normally aligned sections of the same run of busbars. 

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