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Concrete Column Design

Requires: Design Level

Concrete Column Capabilities

The concrete column module allows VisualAnalysis to design and check concrete columns based on the ACI 318-11 Code Provisions. The column may have one of four configurations:

The design module takes into account the effects of slenderness by using the ACI moment magnifier method. The moment magnifier for member strong and weak axes default to 1.0.  For columns in frames that may have sidesway, you must enter a cumulative moment magnifier directly, or you may use P-Delta analysis results per ACI  Checks are performed if slenderness needs to be considered.

The design module also calculates effective length factors (k factors) automatically. The calculations are based on section 10.12.3 of ACI 318.

Regular ties may be used for all columns and spirals may be chosen for the circular bar patterns. The module also suggests a reinforcing pattern and tie/spiral spacing.

Results tabulated include interaction equation results as well as a plot of the interaction diagram for the column. In the case of biaxial bending, the Parme interaction equation is used for checking adequacy.

The design checks are determined for those load cases that have been created with the strength check box checked. You should only select load cases that have appropriate ACI load factors defined to use with the ACI Column Design Module. The interaction equation value for the worst load case is shown in the Design View. A graphic interaction diagram is displayed in a report, showing the interaction point for each of the strength load cases.

Concrete Column Assumptions and Limitations

This section summarizes the assumptions made in the concrete column design software module. It also includes several discussions that are very critical to the proper use of the design module.

Column Forces

The columns are designed for compressive axial force plus either uniaxial or biaxial bending. Tension columns are not designed or checked. Furthermore, the effects of shear and torsion are neglected in the design checks.

Reinforcement Distribution

All column capacity calculations are based on the assumed location of the reinforcement.  Four face patterns are assumed to have an equal number of bars on each face, and are spaced evenly along     each face.  Interaction diagrams are created based on the selected details and the associated steel locations.  Points on the interaction diagrams are calculated by varying the neutral axis depth and solving for the axial and flexural capacities.  Phi factors are applied to the capacites based on the extreme tensile strain in the reinforment.  If the centroid of a reinforcing bar is located withing the concrete compression block, the stress in the bar is reduced by the magnitude of the concrete compression block.

Reinforcement Distribution and Member Local Axes

Bars specified as distributed to 2-faces are located on the +y and -y faces (local member axes) of the column.  This corresponds to conventional strong bending (bending about the z-z axes.  The picture on the right illustrates a typical distribution.  The beta angle will rotate the local axes, which will also rotate the orientation of the faces on which the rebar lies.  Note that a parametric rectangle can be defined as 12"wide x 24"high, or 24"wide x 12"high.  It is critical to pay attention to local local axes direction when specifying  reinforcement on two faces.  Turning on the Picture View filter and the Member Local Axes filter  will help you orient the parametric shape with the location of the reinforcement. Creating a couple of simple test cases and reviewing the Unity Ratios for a given loading can also help determine what reinforcement distribution exists.

Moment Magnifiers

When a member is allowed to have sidesway and is found to be slender, the design module requires you to enter the moment magnifier. Otherwise, the module determines the moment magnifier for you. When calculating the magnifier in cases where sidesway is prohibited, the larger of the EI values from ACI equations 10-11 and 10-12 is used.

Parme Equation

In the case of biaxial bending for rectangular and square sections, an interaction equation is used to determine the adequacy. The equation used by this module is the Parme equation.

See "Reinforced Concrete Design", by Wang and Salmon for more information. The Parme equation has the following form:
(Muy/Moy) exp + (Muz/Moz) exp < 1.0
Muy = Factored moment about section y axis.
Moy = Factored moment capacity (uniaxial) about section y axis.
Muz = Factored moment about section z axis
Moz = Factored moment capacity (uniaxial) about section z axis
exp = Exponent = -0.30103/log10(beta)
beta = Parme equation "Beta" (0.5 < beta < 1.0)

Column Design Length

The column design length is assumed to be the clear spacing between beams at each end. Note that any internal member forces beyond the column design length are ignored.

Member Section Axes

The column cross section is oriented relative to the lcoal y and z-axes. All concrete design checks are in terms of the section axes (only different for spandrels).

Concrete Column Parameters Overview

In selecting member sizes, reinforcing details and subsequently checking a concrete beam, several parameters must be defined. The design parameters are controlled completely through the Modify Tab of the Project Manager in the design view. After creating a design group, you should select one of the members that belongs to the design group in the Design View and go to the Modify Tab of the Project Manager and set up the design parameters. 

Keep in mind that the parameter information you enter applies to all members of the group, so it may be wise to choose the most conservative condition that applies to any member in the group.

Concrete Parameters

Member Type: Indicates the design shape (Beam or Column).

Overstrength?: Causes the member to be designed for overstrength forces per ASCE 7 and ACI 318

Disable Checks?: Causes selected design group to be omitted from design checks.

High Seismic Risk - (Use Reduced Phi Factors for Members Resisting Earthquake Effects) This tells the design module to apply the lower PHI factors as indicated by ACI Code section 9.3.4 for members which are designed to resist earthquake effects and are part of a structure that relies on special moment resisting frames or special reinforced concrete structural walls to resist earthquake effects. Note that the program relies solely on this parameter in determining whether or not to use the reduced f (phi) it does not attempt to calculate whether the shear capacity is greater than the shear corresponding to the development of the nominal flexural strength of the member. Only the Phi factor for the shear is affected by this entry.

Design Parameters

Minimum Depth: The first value for depth that the program tries as it iterates over sizes in search of a satisfactory section. It will start from this size and go up. If the width needs to be fixed at a certain size, that size should be specified as the Minimum and Maximum Depth and the Increment Depth parameter should be set to zero.

Maximum Depth: The maximum value for depth that the program tries as it iterates over sizes in search of a satisfactory section.

Increment Depth: The amount by which the program increases its "trial" depth as it iterates over sizes in search of a satisfactory section. If the depth needs to be a specified value, enter zero for this increment (the depth value will be that specified by the Starting Depth parameter).

Minimum Width: The first value for width that the program tries as it iterates over sizes in search of a satisfactory section. It will start from this size and go up. If the width needs to be fixed at a certain size, that size should be specified for Minimum and Maximum Width and the Width Increment parameter should be set to zero. For Tees and Spandrels, this is the Stem Width.

Maximum Width: The maximum value for width that the program tries as it iterates over sizes in search of a satisfactory section.

Increment Width: The amount by which the program increases its "trial" width as it iterates over sizes in search of a satisfactory section. If the width needs to be a specified value, enter zero for this increment (the width value will be that specified by the Starting Width parameter).

Rebar Pattern:  Specifies if bars are to lie on 2-faces or 4-faces (Rectangular and Square sections) , Round Spiral (Square sections),and Round Spiral or Tied (Circular  sections)

Main Steel Fy: Specified yield strength of the main longitudinal steel in the column. 

F'c: The specified compressive strength of the concrete.

Cover: Clear cover between outer concrete surface and outer surface of ties or spiral.

Confinement Parameters 

Tie Steel Fy: Specified yield strength of the ties or spiral.

Tie Steel Size: Size of reinforcing steel to use for ties and spirals. 

Columns Parameters

Parme Eqn. Beta: The beta coefficient to be used when evaluating the Parme interaction equation for combined compression and biaxial bending. This value must be between 0.5 and 1.0. This parameter will only be available in space frame models because biaxial bending is not possible in a plane model. 

Allow Moment Magnification: Option allowing slender columns to be designed per the limit of ACI equation  10-7. If you choose to not consider moment magnification, column sizes will be chosen so that the slenderness does not exceed the limit of ACI equation 10-7 or section 10.13.2.

My/Mz Magnifier: The combined delta-ns and delta-s moment magnifier. Used only when sidesway is allowed (for braced frames, ACI equations 10-8 through 10-12 are used). 

See common Design Parameters for a description of other settings.

Concrete Column Design Process

1. Specify preliminary member size and Material (Model View) , and Design Parameters (Design View)

Concrete members are created using Standard Parametric cross sections.  You will also need to apply loads and set up strength design load combinations.  For more details on these 'basic' steps, please see: The Design Process)

2. Analyze your model, and select the Design View Tab to check the Unity Ratio.

If the cross section created in the Model View meets strength requirements, reinforcement will be sized to meet demands. 

3. Manually adjust reinforcement (optional)

The sized longitudinal reinforcement and tie spacing can then be adjusted to your preference while in the Design View (Modify Tab, Details Heading, Adjust Rebar [...] button).  Unity checks will then be updated in the Design View.

You can iterate steps 1-3 to obtain a final design, or you can use the Design feature in VisualAnalysis for optimizing design by continuing with the following steps:

4. Design The Group

Select a group from the Design View window. Then use the menu item Design | Design Members… .

5. Select a Column Size

From the dialog you a may select an appropriate column size. Unity Ratios are shown in the table to indicate just how close you are to code allowed maximums. Once you chose a size, a suggested set of reinforcement will be shown if possible. The ~ symbol indicates unity ratios must be validated with another analysis (because member stiffness may have changed).

6. Adjust Suggested Reinforcing Details

Initially the suggested reinforcing patterns that should meet code requirements are presented. If you are not satisfied with the suggestions, change the values as you see fit (Similar to step 3).

7. Synchronize Design Changes

Analysis results exist for the modeled shape, and are affected by member stiffness.  In order to provide final verification of designed shapes, the menu item Design | Synchronize Design Changes must be selected.  This will update the modeled shape to that selected in the design process.

8. Verify Unity Ratios in the Design View

The final step in the design process is to verify that Unity Ratios (Design View)  are less than unity.  If member sizes were drastically changed during the design process, final unity ratios can differ from predicted unity ratios because the analysis results may vary significantly. 

Concrete Column Reports (Interaction Diagrams)

The design reports produced by the design module are available by simply double clicking on the member while in the Design View. The right click context menu also allows quick generation of design reports.

This will report section material, reinforcement and code checks, as well as an interaction diagram.  The interaction diagram is generated with reports and plots factored axial load vs. factored moment. Each strength load case result appears as a point on the diagram and the values plotted. The "Key" to these data points is in the Load Cases table, which numbers each load case and load combination. A .01 notation indicates the 1st set of results (e.g. linear) a .02 indicates P-Delta results. If a point falls within the diagram, it satisfies strength criteria.