4.4 Reinforced Concrete Column
4.4.1 Types of Columns
Columns are defined as members that carry loads chiefly in compression. Usually columns carry bending moment as well, about one or both axes of the cross-section, and the bending action may produce tensile force over a part of the cross-section. Even in such cases, columns are generally referred to as compression members, because the compression forces dominate their behavior. In addition to the common type of compression members, i.e., vertical elements in structures, compression members include arch ribs, inclined rigid frame members, compression elements in trusses or shell.
Columns may be classified based on the following different categories:
- Based on loading, columns maybe classified as follows:
- Axially loaded columns, where loads are assumed acting at the centroid of column section.
- Eccentrically loaded columns, where the loads are acting at a distance e from the centroid of column section. The distance e could be along the x or y axis, causing moments either about the x axis or y axis.
- Biaxially loaded columns, where the loads are applied at any point on the column section, causing moments about both the x axis and y axis simultaneously.
- Based on length, colums can be classified as follows:
- Short columns, where the columnrsquo;s failure is due to the crushing of concrete or due to the yielding of the steel bars under the full load capacity of the column.
- Long columns (slender columns),where the buckling effect and he slenderness ratio must be taken into consideration in the design, thus reducing the load capacity of the column relative to that of a short column.
- Based on lateral reinforcement, columns can be classified as follows:
- Columns reinforced with longitudinal bars and lateral ties.
- Columns reinforce with longitudinal bars and continuous spirals.
- Composite compression members reinforced longitudinally with structural steel shapes, pipe, or tubing, with or without additional longitudinal bars, and various types of lateral reinforcement.
The main reinforcement in columns is longitudinal, parallel to the direction of the load, and consists of bars arranged in a square, rectangular, or circular pattern.
Lateral reinforcement, in the form of individual relatively widely spaced ties or a continuous closely spaced spirals, serves several functions. For one, such reinforcement is needed to hold the longitudinal bars in positions in the forms while the concrete is being placed. For this purpose, longitudinal and transverse steel are wired together to form cages, which are then moved into the forms and properly positioned before placing the concrete. For another, transverse reinforcement is needed to prevent the highly stressed, slender longitudinal bars from bucking outward by bursting the thin concrete cover.
4.4.2 Short Columns
1. Behavior of Axially Loaded Short Column
When an axial load is applied to a reinforced concrete short column, the concrete can be considered to behave elastically up to a low stress of about (1/3) fc. Two different types of failure occur in columns, depending on whether ties or spirals are used. A tied column fails at the load Na. At this load, the concrete fails by crushing or shearing outward along inclined planes, and the longitudinal steel bars fail by buckling outward between ties. The column failure occurs suddenly, much like the failure of a concrete cylinder.
In a spirally reinforced column, when the same load Na is reached, the longitudinal steel and the concrete within the core are prevented from moving outward by the spiral. The concrete in the outer shell, however, not being so confined, does fail; i.e, the outer shell spalls off when Na is reached. It is at this stage that the confining action of the spiral has a significant effect, and if sizable spiral steel is provided, the load will ultimately fail the column by causing the spiral steel to yield. The axial strain when the column fails can be much larger than that at which the shell spalls off.
2. Short Columns under Combined Axial Force and Moment
When a member is subjected to combined axial compression N and moment M, it is usually convenient to replace the axial load and moment with an equal load N applied at eccentricity e=M/N. The two loading are statically equivalent. All columns may then be classified in terms of the equivalent eccentricity.
- Failure modes of columns
Under the combined actions of axial force and moment, there are two types of failure modes of column, balanced failure, compression failure, when the neutral axis is outside the section, causing compression throughout the section, and tension failure, when the neutral axis is within the section, developing tensile strain on the left of the neutral axis.
Balanced failure
Under this mode of failure, yielding of the tensile steel on the far side of the column occurs simultaneously with the attainment of limit compressive strain of 0.0033 in concrete.
Compression failure
Compression failure of the column occurs when the relative eccentricity e0/h0 is small, or although the relative eccentricity e0/h0 is large, the tension reinforcement ratio of large. Column having small relative eccentricity are generally characterized by compression over the entire concrete section, but columns with large eccentricity are subjected to tension over at least a part of the section. Column of this type fail by crushing of the concrete and compressive yielding of the steel on the side near the load, while the reinforcement on the side farthest from the load does not yield.
Tension failure
Tension failure occurs when the relative eccentricity e0/h0 is large, and the tension reinforcement ratio is moderate. The longitudi
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4.4 Reinforced Concrete Column
4.4 钢筋混凝土柱
4.4.1 Types of Columns
Columns are defined as members that carry loads chiefly in compression. Usually columns carry bending moment as well, about one or both axes of the cross-section, and the bending action may produce tensile force over a part of the cross-section. Even in such cases, columns are generally referred to as compression members, because the compression forces dominate their behavior. In addition to the common type of compression members, i.e., vertical elements in structures, compression members include arch ribs, inclined rigid frame members, compression elements in trusses or shell.
Columns may be classified based on the following different categories:
- Based on loading, columns maybe classified as follows:
- Axially loaded columns, where loads are assumed acting at the centroid of column section.
- Eccentrically loaded columns, where the loads are acting at a distance e from the centroid of column section. The distance e could be along the x or y axis, causing moments either about the x axis or y axis.
- Biaxially loaded columns, where the loads are applied at any point on the column section, causing moments about both the x axis and y axis simultaneously.
- Based on length, colums can be classified as follows:
- Short columns, where the columnrsquo;s failure is due to the crushing of concrete or due to the yielding of the steel bars under the full load capacity of the column.
- Long columns (slender columns),where the buckling effect and he slenderness ratio must be taken into consideration in the design, thus reducing the load capacity of the column relative to that of a short column.
- Based on lateral reinforcement, columns can be classified as follows:
- Columns reinforced with longitudinal bars and lateral ties.
- Columns reinforce with longitudinal bars and continuous spirals.
- Composite compression members reinforced longitudinally with structural steel shapes, pipe, or tubing, with or without additional longitudinal bars, and various types of lateral reinforcement.
The main reinforcement in columns is longitudinal, parallel to the direction of the load, and consists of bars arranged in a square, rectangular, or circular pattern.
Lateral reinforcement, in the form of individual relatively widely spaced ties or a continuous closely spaced spirals, serves several functions. For one, such reinforcement is needed to hold the longitudinal bars in positions in the forms while the concrete is being placed. For this purpose, longitudinal and transverse steel are wired together to form cages, which are then moved into the forms and properly positioned before placing the concrete. For another, transverse reinforcement is needed to prevent the highly stressed, slender longitudinal bars from bucking outward by bursting the thin concrete cover.
4.4.1 柱的类型
柱被定义为受压构件。通常，柱还承载弯曲力矩，该力矩一般出现在横截面的一个或两个轴，并且弯曲可以在横截面的一部分上产生张力。即使在这种情况下，柱通常被称为压缩构件，因为压缩力主导其行为。除了常见类型的压缩构件，即结构中的垂直元件之外，压缩构件包括拱形肋，倾斜的刚性框架构件，桁架或外壳中的压缩元件。
柱可以根据以下不同类别进行分类：
-
- 根据载荷，柱可划分为：
- 轴心受力柱，其中负载假定在柱段的质心处。
- 偏心受力柱，其中负载与柱部分的质心距离为e。距离e可以沿x或y轴，引起围绕x轴或y轴的力矩。
-
双轴加载柱，其中负载施加在柱部分上的任何点处，同时引起绕x轴和y轴的力矩。
- 根据长度，柱可分为：
- 短柱，柱的失效是由于混凝土的破碎或由于柱的满载能力下钢筋的屈服而引起的。
-
长柱（细长柱）在设计中必须考虑弯曲效应和细长比，从而降低柱相对于短柱的载荷能力。
- 基于侧向加固，柱可分为：
- 纵杆和侧关节加强柱。
- 用纵向条和连续螺旋进行加固的色谱柱。
- 复合纵向加强压缩构件，具有结构钢形状，管道或管道，具有或不具有附加的纵向杆和各种类型的横向加强件。
柱中的主要加固件是纵向的，平行于负载方向，并且由以矩形，矩形或圆形图案排列的杆组成。
单个相对宽间隔的连接件或连续紧密间隔的螺旋形式的侧向加强件具有若干功能。一方面，需要这种加固，以便在放置混凝土时将纵向杆保持在形式中的位置。为此，纵向和横向钢丝连接在一起形成笼子，然后将其移动到形式中并在放置混凝土之前进行适当的定位。另一方面，需要横向钢筋，以防止高应力情况下，细长的纵向钢筋向外抵抗而顶破较薄的混凝土保护层层。
4.4.2 Short Columns
1. Behavior of Axially Loaded Short Column
When an axial load is applied to a reinforced concrete short column, the concrete can be considered to behave elastically up to a low stress of about (1/3) fc. Two different types of failure occur in columns, depending on whether ties or spirals are used. A tied column fails at the load Na. At this load, the concrete fails by crushing or shearing outward along inclined planes, and the longitudinal steel bars fail by buckling outward between ties. The column failure occurs suddenly, much like the failure of a concrete cylinder.
In a spirally reinforced column, when the same load Na is reached, the longitudinal steel and the concrete within the core are prevented from moving outward by the spiral. The concrete in the outer shell, however, not being so confined, does fail; i.e, the outer shell spalls off when Na is reached. It is at this stage that the confining action of the spiral has a significant effect, and if sizable spiral steel is provided, the load will ultimately fail the column by causing the spiral steel to yield. The axial strain when the column fails can be much larger than that at which the shell spalls off.
2. Short Columns under Combined Axial Force and Moment
When a member is subjected to combined axial compression N and moment M, it is usually convenient to replace the axial load and moment with an equal load N applied at eccentricity e=M/N. The two loading are statically equivalent. All columns may then be classified in terms of the equivalent eccentricity.
- Failure modes of columns
Under the combined actions of axial force and moment, there are two types of failure modes of column, balanced failure, compression failure, when the neutral axis is outside the section, causing compression throughout the section, and tension failure, when the neutral axis is within the section, developing tensile strain on the left of the neutral axis.
Balanced failure
Under this mode of failure, yielding of the tensile steel on the far side of the column occurs simultaneously with the attainme
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