Cracking Load and Ultimate Moment 开裂荷载和极限弯矩.ppt

当剪切和粘结比弯曲强的预应力受弯构件受荷失效 时，它们会以下列方式中的一种失效：
cracks in the tensile flange. (The linearity（直线性）is a function of the rate at which the load is applied.) For this reason, normal elastic-design relationships（关系式）can be used in computing the cracking load by
（压力线）for beams in the elastic range. This is a fundamental principle （规律）of prestressed construction. In a normal prestressed beam, this shift（移动）in the location of the pressure line continues at a relatively
uniform rate（速度）, as（随着）the external load is increased, to the
point where cracks develop in the tension fiber. After the cracking load has
been exceeded, the rate of movement in the pressure line decreases as
desirable（想要）to make a precise estimate of the cracking load, such as is required in some research work, this effect（影响）should be considered.
应该承认有粘结的预应力构件的性能实际上是一个 换算截面的函数，不是混凝土毛截面的函数。如果想要对 开裂荷载作一精确估计，例如在一些研究工作中需要的， 该影响应该被考虑。
the applied loads, as described above, as well as to plastic
strains that begin to take place in the steel and the concrete when stressed to high levels（受到很大压力）.
of rupture（断裂模量）of the concrete when computing the cracking load.
很多试验证明，在引起受拉翼缘最初开裂的荷载以内或稍微超过时， 预应力梁的荷载-挠度曲线是近似直线的。（直线性是荷载施加速度的函 数。）因此，通过简单地确定导致受拉翼缘中产生一个净的受拉应力的荷 载（预应力减去施加荷载的效应 ），普通的弹性设计关系式能用来计算开 裂荷载，其值等于混凝土的抗拉强度。当计算开裂荷载时，习惯上假定混 凝土的受弯抗拉强度等于混凝土的断裂模量。
在一些结构中，保持受弯构件不开裂可能是必要的，甚至在明显的超 载下。这可能是由于结构在它们的有效寿命期间被暴露在特别腐蚀的空气 中的原因。当设计预应力构件用于这种类型的特殊结构时，可能有必要计 算引起受拉翼缘开裂的荷载，以确保该设计提供足够抵抗开裂的安全性。 也有必要计算会导致开裂的弯矩，以确保符合一些设计标准。
In should be recognized that the performance（性 能）of bonded（有粘结的）prestressed member is actually a function of the transformed section（换算截面）
rather than the groIf it is
超载（-开裂荷载）下的反应
It has been shown that a variation in the external load acting on a
prestressed beam results in a change in the location of the pressure line
Cracking Load and Ultimate Moment
开裂荷载和极限弯矩
教学目标
了解预应力梁在开裂荷载下的反应 了解有（无）粘结力的预应力构件的极限弯矩能力 熟悉预应力构件中的专业词汇 熟悉科技类文献中的常用句型
Action Under Overload-Cracking Load
the load is increased above the cracking load. The curvature （弯曲）of the load-deflection curve for loads over the cracking load is due to（由于..所造成的）the change in the basic internal resisting moment action that counteracts（抵消）
Many tests have demonstrated（证明）that the load-deflection
curves of prestressed beams are approximately linear（近似直线的）up to and slightly in excess of（稍微超过）the load that causes the first
应该注意到荷载挠度曲线在开裂荷载以内是接近直线的， 且在超过开裂荷载后，该曲线随着荷载的增加逐渐变得更弯曲。 正如上面所描述的，当超过开裂荷载时，其荷载-挠度曲线的弯 曲是由于抵消施加荷载的基本内部抵抗弯矩作用的变化以及当 受到很大压力时钢筋和混凝土中开始发生的塑性应变所造成的。
In some structures it may be essential that the flexural members remain crack free（保持构件不开裂）even under significant（明显的） overloads. This may be due to the structures’ being exposed to exceptionally corrosive（特别腐蚀）atmospheres during their useful life （有效寿命）. In designing prestressed members to be used in special structures of this type, it may be necessary to compute the load that causes cracking of the tensile flange（受拉翼缘）, in order to ensure that adequate safety against cracking is provided by the design. The computation of the moment that will cause cracking is also necessary to ensure compliance with（符合）some design criteria（criterion的复数形式， 标准）.
simply determining the load that results in a net（净）tensile stress in the tensile flange (prestress minus the effects of the applied loads) that is equal to the tensile strength of the concrete. It is customary to assume that the flexural tensile strength of the concrete is equal to the modulus
It should be noted that the load deflection curve is
close to（接近于）a straight line up to the cracking load and that the curve becomes progressively（逐渐）more curved as
additional load is applied, and a significant（显著）increase in the stress in the prestressing tendon（预应力钢筋束）and the resultant concrete force begins to take place.
已经显示作用在一根预应力梁上的外部荷载的变化会导致梁在弹性阶段 时其压力线位置的变化。这是预应力结构的基本规律。在一根普通的预应力 梁中，随着外力的增加，其压力线位置以一个相对均匀的速度不断移动直至 受拉纤维形成开裂的位置。在已经超过开裂荷载后，随着附加荷载的施加， 压力线移动的速度便会降低，且使预应力钢筋束中的应力和合成的混凝土力 开始发生显著的增加。
内部弯矩的作用不断变化直到所有压力线的移动都停止。其后施加的荷 载引起的弯矩完全通过相应的、且按比例的内力变化来抵消，正如在钢 筋混凝土的结构中。这个事实，即弹性阶段和塑性阶段的力由基本不同 的作用来传送是非常重要的，且使强度计算变得对所有的设计都是必要 的，以确保存在足够的安全系数。这是正确的，即使弹性阶段的应力可 能符合公认的弹性设计准则。
Principles of Ultimate Moments Capacity for Bonded members
有粘结的构件极限弯矩能力的规则
When prestressed flexural members that are stronger in shear and bond than in bending are loaded to failure, they fail in one of the following modes（方 式）: