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What is a concrete column? [Usage and its types]
Concrete column design in ITBS software
Manual design of concrete column
Study time: 6 minutes Concrete column is one of the most important building elements, which is used to transfer loads from the roof to the foundation. In fact, concrete or steel columns should be used to support the loads that have entered the building.
The design of the concrete column has different stages and is determined based on the dimensions, the number of reinforcements, side restraints, etc. There are two manual and software methods for designing this element, and in this article from the Isfahan Ahan Encyclopedia, we explain the training steps of each.
What is a concrete column? [Usage and its types]
ETABS software is a structural engineering program that is used to analyze and design building structures. This software was developed by Computers and Structures (CSI) and is one of the most popular structural engineering software in the world.
This powerful and flexible software is used to analyze and design building structures with different materials such as concrete, steel and wood; It can also consider the effect of different loads including dead and live loads.
ITBS has a simple and user-friendly graphical user interface that makes it easy to model and analyze structures. This software also has a wide library of sections, materials and ready loads that can save structural engineers time and money.
Here are some of the special features of ITBS:
Accurate 3D modeling of building structures
Static and dynamic analysis of structures
Designing structures according to various structural engineering standards
Generating various reports from analysis and design results
Concrete column design steps in ITBS
The design of these columns in the software has different steps that must be done in order.
First, the specifications of the column, including the type of cross section, cross section dimensions, concrete strength and rebars must be defined in the software. To define these specifications, use the Column Properties window. In this window, you must enter items such as Type, Dimensions, Concrete, and Reinforcement.
In the second step, you should use the Load Cases window to load the column. In this window, in the Name field, enter the loading name, Type, and the loading value in the Value field.
The third step is column analysis, which is done using the Analysis Options window. In this step, there are two options, Load Combinations to enter load combinations and Analysis Type field to enter the type of analysis.
In the fourth step, design the longitudinal bars using the Reinforcement Design window. In this window, the Column field is for entering the desired column, the Reinforcement Type field is for determining the type of longitudinal reinforcement, the Number of Bars option is for the number of longitudinal bars, and the Bar Diameter field is for entering the diameter of the longitudinal bars.
In this window, there is another section for the design of transverse bars, whose fields are:
Column
Reinforcement Type
Number of Bars
Bar Diameter
Spacing
Important points of concrete column design in ITBS software
When defining the specifications of the column, pay attention to the cross-sectional dimensions of the column and its ability to withstand the incoming loads. Also, when loading the column, define the loading combinations correctly so that accurate analysis results are calculated.
When designing the longitudinal bars, it should be ensured that the number and diameter of the bars are suitable so that it can provide the compressive strength of the column. When designing the transverse bars, it should be ensured that the number and diameter of the bars are proportional to its tolerance against the lateral failure of the column.
Manual design of concrete column
Although Ibbets software is widely used for column design today, calculating this operation manually is still an essential knowledge for engineers. Because the calculations and results obtained from this software must be controlled by an engineer and confirm its accuracy.
Designing this element manually requires knowing the basic concepts that we will explain below.
PCA or covariance curve method
PCA is one of the manual methods of concrete column design. In this method, first the action curve of the column is calculated according to the characteristics of the column and concrete strength. Then, according to the loads on the column, a point of the action curve is selected. This point should be within the bending capacity of the column.
The action curve of the column is a figure that shows the relationship between the axial force and the bending moment of the column. This curve can be drawn using the following relations:
P = f'c * Ac – M / (L / 2)
M = f'c * As * √(h/d)
In these relationships, P is the axial force, M is the bending moment, f’c is the concrete compressive strength, Ac is the cross-sectional area of the column, As is the cross-sectional area of the rebars, h is the height of the column, and d is the diameter of the column.
To select a point from the action curve, you must also use the following relationship:
P = Pcr – P
In this regard, Pcr is the bending capacity of the column and P is the load on the column.
According to the selected point of the action curve, determine the characteristics of the longitudinal bars as follows:
As = Pcr / ƒy
In this regard, As is the cross-sectional area of the rebars and ƒy is the yield stress of the rebars.
Example: Assume that the specification of a concrete column includes a square cross-section with dimensions of 200 x 200 mm, concrete strength of 30 MPa, and the load on the column is 500 kilonewtons. First, you need to draw the column action curve using the following relations.
Pcr = 30 * 0.04 * 200 * 200 = 6000 kN
Mcr = 30 * 0.002 * 200 * √(200/200) = 600
According to this point, the specifications of the longitudinal bars are determined as follows:
As = 6000 / 420 = 14.28 square mm
According to this value, 14 mm rebars can be used.
The PCA method for concrete column design is simple, understandable and applicable, but it not only requires very high accuracy in calculations, but it is not suitable for thin columns.
Reverse charge method
This method is based on stress and strain analysis in the column. In this method, first the axial load on the column is determined. Then, according to the load on the column, the resulting bending moment is calculated.
To determine the characteristics of longitudinal bars, the following relationship should be used:
As = 0.85 * f’c * Ac / ƒy – M / (L / 2)
In this regard, As is the cross-sectional area of the rebars, f’c is the compressive strength of concrete, Ac is the cross-sectional area of the column, ƒy is the yield stress of the rebars, M is the bending moment and L is the length of the column.
In the first step, the axial load on the column is determined. This load can include dead loads, live loads, wind loads and earthquake loads.
In the second step, the bending moment resulting from the axial load is determined. This is done using the following relationship. In this regard, M is the bending moment, P is the axial load and L is the length of the column.
M = P * L / 2
In the third step, according to the load on the column and the resulting bending moment, the specifications of the longitudinal bars can be determined as follows. In this regard, As is the cross-sectional area of the rebars, f’c is the compressive strength of concrete, Ac is the cross-sectional area of the column, ƒy is the yield stress of the rebars, M is the bending moment and L is the length of the column.
As = 0.85 * f’c * Ac / ƒy – M / (L / 2)
Example: Assume that the specifications of a concrete column include a square cross-section with dimensions of 200 x 200 mm, concrete strength of 30 MPa, load on the column of 500 kilonewtons, and column length of 3000 mm. In this example, the bending moment resulting from the axial load should be calculated through the following equation:
M = 500 kN * 3000 mm / 2 = 7500000 mm N
According to these values, the specifications of the longitudinal bars are calculated according to the formula and finally 14 mm bars should be used.
In this article, we investigated one of the specialized topics in the construction field and explained the concrete column design methods. As mentioned, these calculations are done by manual and software methods; But it is necessary for a civil engineer to have sufficient mastery of both methods.
