Prestressed members are often classified by how the steel is stressed and anchored to the concrete.The member is said to be pretensioned if the steel is positioned in the form and stressed before the concrete is cast.A member is said to be post-tensioned when the steel is stressed after the concrete has hardened to a specific design strength.<\/p>\n
Pretensioning is used primarily in precasting plants to mass-produce members whose size and weight are small enough to permit shipment to the site by truck. If pretensioning is carried out in the plant, the contractor is not required to supply equipment and trained personnel to prestress members in the field. In precast plants, members are commonly constructed on a long slab. These casting beds, which may be 400 to500 ft(122 to152 m) long, permit a number of members to be pretensioned simultaneously (Fig.11.3). Large abutments, positioned at each end of the casting bed,are constructed with fittings to stress and anchor the tendons.After tendons have been tensioned and anchored to the abutments, forms are erected. Next, regular reinforcing steel required for carrying diagonal tension associated with shear, for controlling crack width produced by moment, or for strengthening the anchorage zones is inserted into the form.Then concrete is cast and compacted. After the concrete reaches the required design strength, the tendons are cut. As the steel contracts, the force in the cable is transferred, primarily at the ends of the member, to the concrete by bond and by friction. As cables are elongated by tensioning, a lateral contraction of the tendon, due to the Poisson\u2019s ratio effect, takes place. After the tendons are cut and the reinforcement tries to return to its original unstressed dimensions, lateral expansion occurs. Wherever the reinforcement is encased in concrete, the lateral expansion creates high radial pressures between the concrete and the reinforcement, termed the Hoyer effect.The radial pressures allow large values of friction to develop between the concrete and the tendon, thereby permitting effective anchorage of cable strand and small-diameter-wire tendons (Fig.11.4). This method cannot be used to anchor large diameter\uff5b3\/4 in(19 mm) and above\uff5dhigh-strength bars because the available friction is not adequate to anchor the large bar forces. Bearing plates must be used to anchor tendons with large forces.<\/p>\n
Since steel forms and the capital costs of a prestressed plant are high, high-early-strength cement and steam curing are often used to accelerate the development of the concrete\u2019s strength in order to permit forms to be removed and reused as rapidly as possible. Under plant conditions, concretes with a compressive strength of 3 to 4 kips\/in2 (20.68 to 27.58Mpa) can be routinely produced in approximately half a day.<\/p>\n
The other method of stressing the steel, posttensioning, is most logical when 1.\u00a0\u00a0 Structures are too large to be pretensioned and shipped to the site.<\/p>\n
To ensure that tendons are free to elongate when the steel is tensioned, post-tensioned construction requires the cable to remain unbonded until the concrete hardens. To prevent bond of the tendons to the concrete, cables may be enclosed in ducts that extend through the concrete or the tendons may be coated with grease or mastic and wrapped with paper. Ducts used to position tendons are often filled with cement grout after the tendons have been stressed and anchored. Grouting provides protection against corrosion and also raises the ultimate strength of the tendon at the section of maximum moment.<\/p>\n
A large variety of mechanical devices to anchor tendons to concrete have been developed for posttensioned construction by manufacturers of prestressing systems.Fittings using wedges that lock the tendons to anchor plates by friction are frequently used to anchor tendons made of wire or strand. To minimize cable slipping when the jacking force is released and the wedges forced into position, the surface of the wedge in contact with the tendon is grooved to produce sharp projections that dig into the cable surface.<\/p>\n
Large-diameter high-strength bars may be anchored by wedges or threaded connections. To prevent threading from lowering the strength of bar by reducing the area of the end sections, the ends of bars to be threaded are often enlarged by forging (termed upsetting) to ensure that the cross section through the roots of the threads will be equal to or greater than the cross section of the unthreaded sections of the bars.<\/p>\n
While engineers should be aware of the characteristics of the various types of prestressing systems so that their designs will provide adequate clearances for tendons and sufficient width for the end anchors, the designer typically specifies only the position of the centerline of the tendon and the magnitude of the prestress force. The contractor is then free to select the simplest and least expensive system supplying the required prestress.<\/p>\n
Under certain design conditions members are both pretensioned and posttensioned. For example, if many identical members are required in a structure, economy may be achieved by using a prestressing plant to produce the members.Pretensioning would be designed to carry all forces applied to the member during shipping and erection. After the members have been assembled in the field, additional tendons can be posttensioned to produce continuity or create additional strength.<\/p>\n
The structural design of building, whether of structural steel or reinforced concrete, requires the determination of the overall proportions and dimensions of the supporting framework and the selection of the cross sections of individual members.\u00a0In most cases the functional design, including the establishment of the number of stories and the floor plan, will have been done by an architect, and the structural engineer must work within the constraints imposed by this design.Ideally, the engineer and architect will collaborate throughout the design process so that the project is completed in an efficient manner.\u00a0In effect, however, the design can be summed up as follows:The architect decides how the building should look; the engineer must make sure that it doesn\u2019t fall down.Although this is an oversimplification, it affirms the first priority of the structural engineer: safety. Other important considerations include serviceability (how well the structure performs in terms of appearance and deflection) and economy.An economical structure requires an efficient use of materials and construction labor. Although this can usually be accomplished by a design that requires a minimum amount of material, savings can often be realized by using slightly more material if it results in a simpler, more easily constructed projects.<\/p>\n
Loads<\/p>\n
The forces the act on a structure are called\u00a0loads. They belong to one of two broad categories,\u00a0dead load and live load.\u00a0Dead loads are those that are permanent, including the weight of the structure itself, which is sometimes called the\u00a0self-weight.\u00a0Other dead loads in a building include the weight of nonstructural components such as floor coverings, suspended ceilings with light fixtures, and partitions.\u00a0All of the loads mentioned thus far are forces due to gravity and are referred to as gravity loads.Live loads, which can also be gravity loads, are those that are not as permanent as dead loads.This type may or may not be acting on the structure as any given time, and the location may not be fixed.Examples of live load include furniture, equipment, and occupants of buildings.\u00a0In general, the magnitude of a live load is not as well defined as that of a dead load, and it usually must be estimated. In many cases, a given structural member must be investigated for various positions of the live load so that a potential failure situation is not overlooked.<\/p>\n
Building codes<\/p>\n
Building must be designed and constructed according to the provisions of a building codes, which is a legal document containing requirements related to such things as structural safety, fire safety, plumbing, ventilation, and accessibility to the physically disabled.\u00a0A building code has the force of law and is administered by a governmental entity such as a city, a county, or, for some large metropolitan areas, a consolidated government.\u00a0Building codes do not give design provisions, but they do specify the design requirements and constraints that must be satisfied.\u00a0Of particular importance to the structural engineer is the prescription of minimum live loads for buildings.Although the engineer is encouraged to investigate the actual loading conditions and attempt to determine realistic values, the structure must be able to support these specified minimum loads.<\/p>\n
Design specifications<\/p>\n
In contrast to building codes, design specifications give more specific guidance for the design of structural members and their connections.\u00a0They present the guidelines and criteria that enable a structural engineer to achieve the objectives mandated by a building code.Design specifications represent what is considered to be good engineering practice based on their latest research.They are periodically revised and updated by supplements or by completely new editions.\u00a0As with model building codes, design specifications are written in a legal format by nonprofit organizations.They have no legal standing on their own, but by presenting design criteria and limits in the form of legal mandates and prohibitions, they can easily be adopted, by reference, as part of a building code.<\/p>\n
Concrete undergoes volume changes during hardening.\u00a0If it loses moisture by evaporation, it shrinks, but if the concrete hardens in water, it expands.\u00a0The causes of the volume changes in concrete can be attributed to changes in moisture content, chemical reaction of the cement with water, variation in temperature, and applied loads.<\/p>\n
Shrinkage<\/p>\n
The change in the volume of drying concrete is not equal to the volume of water removed. The evaporation of free water causes little or no shrinkage.\u00a0As concrete continues to dry, water evaporates and the volume of the restrained cement paste changes, causing concrete to shrink, probably due to the capillary tension that develops in the water remaining in concrete.\u00a0\u00a0Emptying of the capillaries causes a loss of water without shrinkage. But once the absorbed water is removed, shrinkage occurs.<\/p>\n
Many factors influence the shrinkage of concrete caused by the variations in moisture conditions.<\/p>\n
1.Cement and water content. The more cement or water content in the concrete mix, the greater the shrinkage.<\/p>\n
2.Composition and fineness of cement. High-early-strength and low-heat cements show more shrinkage than normal portland cement. The finer the cement, the greater is the expansion under moist conditions.<\/p>\n
3.Type, amount, and gradation of aggregate.\u00a0\u00a0The smaller the size of aggregate particles, the greater is the shrinkage. The greater the aggregate content, the smaller is the shrinkage.<\/p>\n
4.Ambient conditions, moisture, and temperature. Concrete specimens subjected to moist conditions undergo an expansion of 200 to 300\u00d710-6, but if they are left to dry in air, they shrink. High temperature speeds the evaporation of water and, consequently, increases shrinkage.<\/p>\n
5.Admixtures. Admixtures that increase the water requirement of concrete increase the shrinkage value.<\/p>\n
6.Size and shape of specimen. As shrinkage takes place in a reinforced concrete member, tension stresses develop in the concrete, and equal compressive develop in the steel. These stresses are added to those developed by the loading action. Therefore, cracks may develop in concrete when a high percentage of steel is used. Proper distribution of reinforcement, by producing better distribution of tensile stresses in concrete, can reduce differential internal stresses.<\/p>\n
Since steel forms and the capital costs of a prestressed plant are high, high-early-strength cement and steam curing are often used to accelerate the development of the concrete\u2019s strength in order to permit forms to be removed and reused as rapidly as possible. Under plant conditions, concretes with a compressive strength of 3 to 4 kips\/in2 (20.68 to 27.58Mpa) can be routinely produced in approximately half a day.<\/p>\n
\u4e2d\u6587\u7ffb\u8bd1\uff1a<\/p>\n
\u9884\u5e94\u529b\u65b9\u6848<\/p>\n
\u9884\u5e94\u529b\u6784\u4ef6\u901a\u5e38\u90fd\u662f\u6839\u636e\u5176\u65bd\u52a0\u9884\u5e94\u529b\u7684\u65b9\u6cd5\u548c\u5728\u6df7\u51dd\u571f\u4e0a\u7684\u951a\u56fa\u65b9\u6cd5\u5206\u7c7b\u7684\u3002 \u5982\u679c\u5728\u6d47\u7b51\u6df7\u51dd\u571f\u524d\u5148\u5c06\u94a2\u7b4b\u653e\u7f6e\u5728\u6a21\u677f\u5185\u5f20\u62c9\uff0c\u6784\u4ef6\u5c31\u79f0\u4e3a\u5148\u5f20\u6cd5\u6784\u4ef6\uff0c\u800c\u5f53\u6df7\u51dd\u571f\u8fbe\u5230\u4e00\u5b9a\u5f3a\u5ea6\u540e\u5bf9\u94a2\u7b4b\u8fdb\u884c\u5f20\u62c9\uff0c\u5219\u79f0\u6784\u4ef6\u4e3a\u540e\u5f20\u6cd5\u6784\u4ef6\u3002<\/p>\n
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\u7531\u4e8e\u9884\u5236\u5382\u7684\u94a2\u6a21\u677f\u548c\u8d44\u672c\u6210\u672c\u90fd\u8f83\u9ad8\uff0c\u5e38\u91c7\u7528\u65e9\u5f3a\u6df7\u51dd\u571f\u548c\u84b8\u6c14\u517b\u62a4\u4ee5\u4fbf\u5c3d\u53ef\u80fd\u5feb\u7684\u62c6\u9664\u548c\u91cd\u590d\u4f7f\u7528\u6a21\u677f\uff0c\u5728\u9884\u5236\u5382\u60c5\u51b5\u4e0b\uff0c\u901a\u5e38\u5927\u7ea6\u534a\u5929\u5c31\u80fd\u751f\u4ea7\u51fa\u6297\u538b\u5f3a\u5ea6\u9ad8\u8fbe3\u81f35\u5343\u78c5\/\u82f1\u5bf8\uff0820.68\u81f327.58Mpa\u7684\u6df7\u51dd\u571f\uff09\u3002<\/p>\n
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1.\u6c34\u7070\u6bd4\uff1a\u6c34\u7070\u6bd4\u8d8a\u5927\uff0c\u6536\u7f29\u8d8a\u5927\uff1b<\/p>\n
2.\u6c34\u6ce5\u7684\u6210\u5206\u548c\u7ec6\u5ea6\uff1a\u65e9\u5f3a\u548c\u4f4e\u70ed\u6c34\u6ce5\u7684\u6536\u7f29\u5927\u4e8e\u666e\u901a\u6c34\u6ce5\uff0c\u6c34\u6ce5\u8d8a\u7ec6\uff0c\u5176\u5728\u6f6e\u6e7f\u73af\u5883\u4e2d\u7684\u81a8\u80c0\u8d8a\u5927\u3002<\/p>\n
3.\u9aa8\u6599\u7684\u7c7b\u578b\u3001\u542b\u91cf\u53ca\u5176\u7ea7\u914d\uff1a\u9aa8\u6599\u7684\u7c92\u5f84\u8d8a\u5c0f\uff0c\u6536\u7f29\u8d8a\u5927\uff1b\u9aa8\u6599\u542b\u91cf\u8d8a\u5927\uff0c\u6536\u7f29\u5219\u8d8a\u5c0f\u3002<\/p>\n
4.\u5916\u90e8\u6761\u4ef6\uff0c\u6c34\u5206\u4e0e\u6e29\u5ea6\uff1a\u6f6e\u6e7f\u73af\u5883\u4e0b\u7684\u6df7\u51dd\u571f\u8bd5\u4ef6\u7684\u81a8\u80c0\u91cf\u4e3a200 to 300\u00d710-6\uff0c\u4f46\u5982\u679c\u8ba9\u5176\u5728\u7a7a\u6c14\u4e2d\u5e72\u71e5\uff0c\u5b83\u4eec\u5c06\u6536\u7f29\u3002\u9ad8\u6e29\u52a0\u901f\u4e86\u6c34\u5206\u7684\u84b8\u53d1\uff0c\u56e0\u6b64\u4e5f\u52a0\u5feb\u4e86\u6536\u7f29\u3002<\/p>\n
5.\u6dfb\u52a0\u5242\uff1a \u4f7f\u7528\u6c34\u91cf\u589e\u52a0\u7684\u5916\u52a0\u5242\u4e5f\u589e\u52a0\u4e86\u6536\u7f29\u503c\u3002<\/p>\n
6.\u8bd5\u4ef6\u7684\u5c3a\u5bf8\u548c\u5f62\u72b6\uff1a\u5f53\u6536\u7f29\u5728\u94a2\u7b4b\u6df7\u51dd\u571f\u6784\u4ef6\u4e2d\u53d1\u751f\u65f6\uff0c\u6df7\u51dd\u571f\u4e2d\u4ea7\u751f\u62c9\u5e94\u529b\uff0c\u540c\u6837\u5927\u5c0f\u7684\u538b\u529b\u4ea7\u751f\u4e8e\u94a2\u7b4b\u4e2d\uff0c\u8fd9\u4e9b\u529b\u4e0e\u8377\u8f7d\u5f15\u8d77\u7684\u529b\u76f8\u8fed\u52a0\u3002\u56e0\u6b64\uff0c\u5f53\u94a2\u7b4b\u7684\u914d\u7b4b\u7387\u9ad8\u65f6\uff0c\u53ef\u80fd\u4f1a\u4f7f\u6df7\u51dd\u571f\u5f00\u88c2\u3002\u94a2\u7b4b\u7684\u5408\u7406\u5206\u5e03\u3001\u4f1a\u4f7f\u6df7\u51dd\u571f\u4e2d\u7684\u62c9\u5e94\u529b\u5206\u5e03\u66f4\u6709\u5229\uff0c\u53ef\u51cf\u5c0f\u5185\u90e8\u5e94\u529b\u5dee\u3002<\/p>\n
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<\/p>\n