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Using the General Column Design Module

What is the difference between General Column Design and the other concrete column design modules? Which module should I be using?

PROKON features three different concrete column design modules: Rectangular, Circular and General Column Design. There is some overlap in their design scope, but the General Column Design module stands out (as its name implies) for its ability to design columns of any general shape. However, its ability to design complex geometries does not necessarily make it the best tool for all situations.

The Code Approach to Column Design

The design codes address the complex topic of axial force and bi-axial bending moment interaction by breaking it down into a simplified design procedure. Not going into the exact detail of each code, the procedure for designing a concrete column is as follows:

Determine if the column is slender by evaluating whether the length exceeds some predefined slenderness limit for the given end fixities. If the column classifies as slender, the design bending moment about one or both principal axes are increased by adding a slenderness moment.

Quantify the effect of biaxial bending using some interaction formula, or by determining a design axis and calculating an effective design moment of that axis.

Determine the amount of reinforcement required to resist the design axial force and bending moment.  Verify that the amount of reinforcement falls within the allowed minimum and maximum reinforcement levels.

This approach is suitable for designing columns with regular shapes, i.e., rectangular, and circular. It also lends itself to simplifying the design procedure even further to a point where one can compile a set of design charts, and then read off reinforcement values for different column sizes and design loads.

PROKON’s Approach to Column Design

The Rectangular and Circular Column Design modules use the simplified design procedure presented in the codes and assume the reinforcement layout to be symmetrical. It works well for rectangular column sections with relatively small aspect ratios (less than 1:4) and circular columns (designed as square columns of equivalent size). However, one cannot use this procedure for designing irregular-shaped columns, e.g., L-shaped columns and columns with rainwater pipes. The General Column Design module bridges this constraint.

Designing Columns of General Shape

General Column Design works around the limitations of the simplified column design procedure in the codes by reverting to the first principles:

Using the code provisions, check if the column is slender. If slender, consider the section geometry to determine the orientation of the weakest axis. (This is the axis about which the column would buckle under high axial load.)

Calculate the additional (slenderness) moment of the weakest axis, and vectorially add this to the initial design moments to obtain the final design moment.

Analyse the column section for the entered reinforcement using stress-strain compatibility. Verify that the amount of reinforcement is within the code limits.

The program analyses the column section by performing an iterative analysis. The neutral axis is orientated at an angle perpendicular to the design moment direction and positioned such that the internal concrete and reinforcement stresses are in equilibrium with the design axial force and bending moment.

From version 2.6.13 onwards, the program can also consider orientations of the neutral axis that are not perpendicular to the design moment direction. With this option selected, the module finds the neutral axis orientation that gives equilibrium in the direction perpendicular to the design direction. In some cases, this option will yield a slightly lower safety factor, i.e., a more conservative design.

For each of the design load cases, analyses are performed at three locations along the column – top, middle and bottom.

For the case where the concrete is in compression, the rectangular stress block method is used as the stress-strain relationship. Concrete in tension is ignored, while reinforcement bars can work in tension or compression depending on their position within the concrete cross-section.

Figure 1: General Column Design

Interpreting the Design Results

The General Column Design module calculates a safety factor for the entered design loads and graphically presents it as a dot on a column design curve (axial force and moment capacities for the given geometry and reinforcement layout).

A safety factor of 1.0 or slightly larger suggests an optimal design. With a safety factor lower than 1.0, you must increase the size of the reinforcement bars or add more bars. Likewise, you may want to reduce the amount of reinforcement when the safety factor is much higher than 1.0.

To estimate the required reinforcement, you can divide the current amount or reinforcement (entered reinforcement) by the safety factor:

           As,req = As / SF

Note: The estimate assumes that the column capacity is directly proportional to the reinforcement area; this is not accurate when the safety factor deviates from unity by a wide margin.

Comparing Design Results

The different design approaches of the General Column Design module (compared to Rectangular and Circular Column Design) may yield different design results for similar column input. The difference can be significant in some cases. This is true, especially for slender columns, and mainly due to the following factors

The General Column Design module calculates the additional moment of the weakest axis only. Using the code method, additional moments are added to one or both main axes.

The General Column Design module uses a rational approach to determine the concrete and reinforcement stress distribution throughout the column section. In comparison, the design procedure in some codes simplifies the situation by using an inflated effective design moment about one of the main axes.

Figure 2: General column design interaction curve.

Recommended Use of The Column Design Modules

You can use the General Column Design module to design any shape column, including simple rectangular and circular columns. However, for such simple cases, it may be better to use the Rectangular and Circular Column Design modules instead for the following reasons:

The Rectangular and Circular Column Design modules follow the exact design procedures set out by the various codes. The Calcsheets set out every step of the design, making auditing your design calculations quite easy.

When using the General Column Design module, you need to enter the reinforcement during the input phase already. (The program requires knowledge of the size and position of each bar for the section analysis.) You may also need more than one attempt at designing the column before you feel satisfied with the results, potentially making it a time-intensive exercise. This is not the case for the other two modules where you enter the concrete geometry without reinforcement and arrive at the required reinforcement immediately after analysis.

You can use General Column Design to create bending schedules for columns with irregular shapes.  However, when detailing rectangular and circular columns, several more steps are needed to place rebar in the section compared to when using the Rectangular and Circular Column Design modules. In the interest of productivity and easy auditing, it is usually best to use the Rectangular Column Design and Circular Column Design modules for “normal” columns, and reserve General Column Design for special situations.