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REINFORCED CONCRETE DESIGN（P332-348 12.2.2-12.4.3）
This method, which is the most suitable for in situ construction, involves the stressing against the hardened concrete of tendons or steel bars which are not bonded to the concrete. The tendons are passed through a flexible sheathing, which is cast into the concrete in the correct position. They are tensioned by jacking against the concrete and anchored mechanically by means of steel thrust plates or anchorage blocks at each end of the member.
这种最适合原位施工的方法涉及将未粘结到混凝土上的受力筋或钢筋压在硬化的混凝土上。 钢筋束穿过柔性护套，然后在正确的位置浇铸到混凝土中。 通过顶起混凝土使它们张紧。 并通过钢制止推板或锚定块将其机械固定在构件的两端。
Alternatively, steel bars threaded at their ends may be tensioned against bearing plates by means of tightening nuts. It is of course usually necessary to wait a considerable time between casting and stressing to permit the concrete to gain sufficient strength under in situ conditions.
可替代地，可以通过拧紧螺母将两端拧有螺纹的钢筋张紧在轴承板上。当然，通常需要在浇铸和应力之间等待相当长的时间，以使混凝土在原定条件下获得足够的强度。
The use of tendons consisting of a number of strands passing through flexible sheathing offers considerable advantages in that curved tendon profiles may be obtained. A post - tensioned structural member may be constructed from an assembly of separate pre-cast units which are constrained to act together by means of tensioned cables which are often curved as illustrated in figure 12.6. Alternatively, the member may be cast as one unit in the normal way but a light cage of untensioned reinforcing steel is necessary to hold the ducts in their correct position during concreting.
使用由穿过柔性护套的许多股线组成的受力筋供了明显的优点，因为可以获得弯曲的受力筋轮廓。后张紧的结构构件可以由单独的预制单元的组件构造而成，这些单独的预制单元通过通常如图12.6所示弯曲的张紧的电缆被约束成一起作用。另外，该构件也可以按通常的方式铸成一个整体，但是在混凝土浇筑时，必须使用轻型无拉力钢筋笼将管道固定在正确的位置。
After stressing, the remaining space in the ducts may be left empty (unbonded construction), or mote usually will be filled with grout under high pressure (lsquo;bonded construction). Although this grout assists in transmitting forces between the steel and concrete under live loads. and improves the ultimate strength of the member, the principal use is to protect the highly stressed strands from corrosion. The bonding of the highly stressed steel with the surrounding concrete beam also greatly assists demolition, since the beam may then safely be rsquo;chopped-up into small lengths without releasing the energy stored in the steel.
施加压力后，导管中的剩余空间可能会留空（“未粘合”结构），或者通常会在高压下用水泥浆填充微粒（“粘合”结构）。尽管这种灌浆有助于在活荷载下在钢和混凝土之间传递力。并提高了构件的极限强度，其主要用途是保护高应力钢绞线不受腐蚀。高应力钢与周围混凝土梁的粘结也极大地有助于拆除，因为随后可以安全地将梁“切成”小段而不会释放存储在钢中的能量。
12.3 Analysis of Concrete Section under Working Loads
Since the object of prestressing is to maintain favorable stress conditions in a concrete member under load, the lsquo;working load for the member must be considered in terms of both maximum and minimum values. Thus at any section, the stresses produced by the prestress force must be considered in conjunction with the stresses caused by maximum and minimum values of applied moment.
Unlike reinforced concrete, the primary analysis of prestressed concrete is based on service conditions, and on the assumption that stresses in the concrete are limited to values which will correspond to elastic behavior . In this section, the following assumptions are made in analysis:
12.3工作荷载下混凝土截面分析
由于预应力的目的是在荷载作用下保持混凝土构件的良好应力状态，因此必须从最大值和最小值两个角度考虑构件的“工作荷载”。因此，在任何截面上，必须结合由预应力的最大值和最小值引起的应力来考虑预应力所产生的应力。
与钢筋混凝土不同，预应力混凝土的主要分析基于使用条件，并假设混凝土中的应力被限制在与弹性性能相对应的值范围内。在本节中，在分析中进行了以下假设：
- Plane sections remain plane.
- Stress-strain relationships are linear,
- Bending occurs about a principal axis.
- The prestressing force is the value remaining after all losses have occurred
- Changes in tendon stress due to applied loads on the member have negligible effect on the behavior of the member.
- Section properties are generally based on the gross concrete cross-section.
The stress in the steel is unimportant in the analysis of the concrete section under working conditions, it being the force provided by the steel that is considered in the analysis.
The sign conventions and notations used for the analysis are indicated in figure.
For direct and bending stresses, compression is taken as positive — and a positive moment is defined as one which causes a numerically greater stress to occur in fibre 2 than in fibre 1, that is f1greater than f2 corresponds to a positive moment, hence sagging is positive. To fit this convention, the eccentricity of the prestress.
（1）平面截面保持平面。
（2）应力-应变关系是线性的，
（3）绕主轴发生弯曲。
（4）预应力是发生所有损失后的剩余值
（5）由于在构件上施加载荷而导致的腱应力变化对构件的行为的影响可以忽略不计。
（6）截面特性通常基于混凝土的总截面。
在工作条件下的混凝土截面分析中，钢中的应力并不重要，分析中考虑的是钢所提供的力。
分析中使用的符号约定和符号如图所示
对于直接应力和弯曲应力，将压缩视为正值，将正力矩定义为一个正力矩，该力矩使光纤2中产生的应力大于光纤1中的应力，即f1大于f2表示正力矩，因此下垂为 正。 为了符合这一惯例，预应力的偏心率
12. 3. 1 Member Subjected to Axial Prestress Force
lf section BB of the member shown in figure l 2.8 is subjected to moments ranging between Mmax and Mmin.the net stresses at the outer fibres of the beam are given by where z1 and z2 are the elastic section moduli and P is the final prestress force.
For this value of prestress force, substitution in the other equations will yield the stresses in the beam under maximum load and also under minimum load. Similarly the stresses immediately after prestressing,before losses have occurred , may be calculated if the value of losses is known.
For example, the maximum stress in the top of the member is given by equation 12.1
It can be seen from the stress distributions in figure 12.8 that the top fibre is generally in considerable compression,while the bottom fibre is generally at lower stresses. Much better use of the concrete could be made if the stresses at both top and bottom can be caused to vary over the full range of
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