Prestressing Methods

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.

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’s 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｛3/4 in(19 mm) and above｝high-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.

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’s 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.

The other method of stressing the steel, posttensioning, is most logical when 1.   Structures are too large to be pretensioned and shipped to the site.

2. The required cable shape (often called the cable profile) cannot be produced, e.g., a curved cable, if the cable is heavily tensioned since a tensioned cable tends to straighten between the points at which the tension is applied.
3. The design requires that the tendons be stressed in stages. 4.  A structure is fabricated in sections to limit the weight of the element and then joined to other components by posttensioning to form a unit.

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.

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.

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.

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.

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.

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. In 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. In 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’t 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.

Loads

The forces the act on a structure are called loads. They belong to one of two broad categories, dead load and live load. Dead loads are those that are permanent, including the weight of the structure itself, which is sometimes called the self-weight. Other dead loads in a building include the weight of nonstructural components such as floor coverings, suspended ceilings with light fixtures, and partitions. All 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. In 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.

Building codes

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. A 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. Building codes do not give design provisions, but they do specify the design requirements and constraints that must be satisfied. Of 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.

Design specifications

In contrast to building codes, design specifications give more specific guidance for the design of structural members and their connections. They 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. As 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.

Concrete undergoes volume changes during hardening. If it loses moisture by evaporation, it shrinks, but if the concrete hardens in water, it expands. The 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.

Shrinkage

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. As 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.  Emptying of the capillaries causes a loss of water without shrinkage. But once the absorbed water is removed, shrinkage occurs.

Many factors influence the shrinkage of concrete caused by the variations in moisture conditions.

1.Cement and water content. The more cement or water content in the concrete mix, the greater the shrinkage.

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.

3.Type, amount, and gradation of aggregate.  The smaller the size of aggregate particles, the greater is the shrinkage. The greater the aggregate content, the smaller is the shrinkage.

4.Ambient conditions, moisture, and temperature. Concrete specimens subjected to moist conditions undergo an expansion of 200 to 300×10-6, but if they are left to dry in air, they shrink. High temperature speeds the evaporation of water and, consequently, increases shrinkage.

5.Admixtures. Admixtures that increase the water requirement of concrete increase the shrinkage value.

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.

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’s 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.

中文翻译：

预应力方案

预应力构件通常都是根据其施加预应力的方法和在混凝土上的锚固方法分类的。 如果在浇筑混凝土前先将钢筋放置在模板内张拉，构件就称为先张法构件，而当混凝土达到一定强度后对钢筋进行张拉，则称构件为后张法构件。

先张法主要用于在预制厂批量生产可用卡车运送到工地的重量轻和尺寸小的构件，如果在预制厂采用先张法，承包商无需提供现场张拉设备和训练有素的工作人员在预制厂，构件通常都是在一个较长的台上张拉。这种混凝土台，可长达400到500英尺（122m到152m），能同时用来张拉许多构件  张拉台两端的大型台座配有张拉和锚固装置。张拉完筋束并将其锚固在台座上后，就立模。接着，再将用来抵抗剪切斜裂缝、控制弯曲裂缝或加强锚固区的一般钢筋放入模板，然后浇筑并振捣混凝土。待混凝土达到要求的强度后，剪断钢束。随着钢筋的回缩，钢束中的力主要在构件两端，通过粘结和摩擦传至混凝土。随着钢筋的回缩，钢束中的力主要在构件两端，通过粘结和摩擦传至混凝土。随着钢束的张拉伸长，由于波松比效应，钢束出现横向收缩。钢束被切断后，它便试图回缩到其未张拉时尺寸，从而产生横向膨胀。 在钢筋埋入混凝土的地方，这种横向膨胀便在钢筋和混凝土间产生很高的径向压力，称为霍友E.Hoyer效应（德国人）这种径向压力使混凝土和钢束间产生较大的摩擦力，从而能有效地锚固钢丝绳和小直径钢绞线。此方法不能用来锚固大直径高强钢筋（直径不小于3/4英寸，19mm），因为这种有限的摩擦力不足以锚固大直径钢筋的拉力， 必须用端承板来锚固拉力大的钢束。

由于预制厂的钢模板和资本成本都较高，常采用早强混凝土和蒸气养护以便尽可能快的拆除和重复使用模板，在预制厂情况下，通常大约半天就能生产出抗压强度高达3至5千磅/英寸（20.68至27.58Mpa的混凝土）。

在以下情况下，用另一张拉方法（后张法）将更为合理。 结构太大，张拉后不能运送到工地；

由于张紧的钢束趋于在张拉点之间变直，如果钢束张力很大，就不能生产出需要的束筋形状（束筋线形），如曲线束;设计要求分阶段张拉钢束结构分段制作以限制构件自重，然后通过后张与其它构件相连接而形成整体。

为了确保在钢筋张拉时能自由伸长，后张结构要求在混凝土硬结前钢束与混凝土之间无粘结，为了防止钢束与混凝土粘结，可将钢束穿过贯穿混凝土的管道，或在钢束表面涂上润滑油脂再用纸加以包裹。在钢束张拉并锚固好后，常用水泥砂浆灌实固定钢束的管道 压浆不但能防止钢束锈蚀，而且还能提高最大弯矩截面处钢束的极限承载力。

预应力系统制造商已经研制出了许多用于后张法的将钢束锚固到混凝土上的机械装置以摩擦方式将钢束固定到锚固钢板的楔块装置常用于锚固由钢丝或钢丝绳组成的束筋，为了尽可能减小千斤顶力释放后楔块定位时的钢束滑移，在与钢束相接触的楔块表面加了槽口以形成能卡入钢束表面的尖利突齿。

可用楔块或螺丝端杆锚固大直径高强钢筋。为了防止套丝导致两端截面尺寸减小而使钢筋强度降低，通常将要套丝的钢筋端部锻粗（称为镦粗）以确保螺杆根部截面不小于未套丝的钢筋截面。

尽管工程师应通晓各种预应力体系的特点以便其设计不但能为钢束提供适当的公差，也能为端部锚具提供足够的宽度，设计者一般只规定钢束的中线位置和预加力的大小，承包人便可自由选取能提供所需要预应力的最简单和廉价的预应力施加体系。

在某些设计中，构件不但要进行先张，也要进行后张。 例如，当某一结构中需要有许多同样的构件时，通常在预制厂生产便可达到节约的目的。先张力用于承受运输和安装时的所有力， 当所有构件运至工地后，可再后张一些钢束以形成连续结构或增加结构强度。

建筑结构设计，不论是钢结构还是钢筋混凝土结构，都需要确定其支承结构的整体比例和尺寸以及各构件的截面尺寸。在大多数情况下，功能设计，包括楼层层数和楼层平面的确定，将要由建筑师来完成，因而结构工程师必须在此约束条件下工作。在理想状态下，工程师和建筑师将在整个设计过程中协同工作从而高效地完成设计工作。 然而，事实上，设计过程可概括如下： 建筑师确定建筑物的外观，工程师必须确保其不会倒塌。 尽管这样说过分简单，但它明确了工程师的第一个主要任务，即，确保安全。其它要考虑的因素包括适用性（就外观和挠曲而言其工作性能如何）。 经济的结构要求对材料和人工的有效使用，尽管这通常都能通过要求最少材料来取得，但通过采用稍多的材料，但能使建筑物更简单和更容易建造常常会实现节约的目的。

作用在结构物上的各种力称为荷载，它们属于一两种广义类型，恒载和活载。恒载是那些永久荷载，包括结构自身的重量，有时也称为自重。其它建筑物恒载包括非结构构件的重量，如楼面面层、带有灯具的吊顶以及隔墙。 至此所提的各种荷载都是由重力所引起，因而称为重力荷载。 活载也可以是重力荷载，它们是那些不如恒载那样永久的荷载。 这类荷载可能也可能不总是作用在结构物上，且作用位置也可能不是固定的。活荷载包括家具、设置和建筑物的居住者。通常，活荷载的大小不如恒载那样确定，常常必须估计。在许多情况下，必须研究活荷载作用在一给定的结构构件的各个位置以便不会漏掉每个可能的破坏情形。

建筑物必须根据各种建筑规范的条款设计和建造，规范是一种法律文件，包含各种要求，如建筑安全、防火安全、上下水、通风和体残人的可达性等。建筑规范具有法律效力，由政府部位发布，如城市、县、对于大的城区，如联合政府。建筑规范并不给出设计规定，但却规定设计必须满足的各种要求和约束条款。对结构工程师特别重要的是建筑物的最小活荷载规定。尽管鼓励工程师研究实际荷载工况以确定真实的荷载值，结构必须能支承这些规定的最小荷载。

与建筑规范不同，设计规程给出结构构件及其连接的更具体的指南。它们给出各种方针和标准，使结构工程师能建筑规范所规定的目标。 根据其最新研究，设计规程结出认为是好的工程作法。它们通过补充或通过发布新版本得到定期修订和更新。 如同一般建筑规范，设计规程由非赢利组织编写。尽管它们本身并无法律地位，但却以法令和禁令的形式给出设计准则和限制，以参考文献的形式，它们可容易地被录入，并作为建筑规范的一部分。

混凝土在硬结过程中会经历体积变化。如果蒸发失去水分，混凝土会收缩；但如果在水中硬结，它便膨胀。混凝土体积变化的原因可归结为含水量的变化、水泥与水的水化反应、温度变化和所施加的荷载。

混凝土干燥时的体积变化量不等于它所失去的水的体积。自由水的蒸发基本不产生收缩。随着混凝土的不断变干，水分蒸发，受约束水泥浆的体积也变化，导致了混凝土的收缩，这多半是由于残留在混凝土中的水的毛细张力所致。毛细管变空导致无收缩的水分丢失，但一旦失去吸收的水分，收缩便发生。

许多因素都会影响因水分环境发生变化而产生的混凝土收缩。

1.水灰比：水灰比越大，收缩越大；

2.水泥的成分和细度：早强和低热水泥的收缩大于普通水泥，水泥越细，其在潮湿环境中的膨胀越大。

3.骨料的类型、含量及其级配：骨料的粒径越小，收缩越大；骨料含量越大，收缩则越小。

4.外部条件，水分与温度：潮湿环境下的混凝土试件的膨胀量为200 to 300×10-6，但如果让其在空气中干燥，它们将收缩。高温加速了水分的蒸发，因此也加快了收缩。

5.添加剂： 使用水量增加的外加剂也增加了收缩值。

6.试件的尺寸和形状：当收缩在钢筋混凝土构件中发生时，混凝土中产生拉应力，同样大小的压力产生于钢筋中，这些力与荷载引起的力相迭加。因此，当钢筋的配筋率高时，可能会使混凝土开裂。钢筋的合理分布、会使混凝土中的拉应力分布更有利，可减小内部应力差。

由于预制厂的钢模板和资本成本都较高，常采用早强混凝土和蒸气养护以便尽可能快的拆除和重复使用模板，在预制厂情况下，通常大约半天就能生产出抗压强度高达3至5千磅/英寸（20.68至27.58Mpa的混凝土）。