Navigation:  Step-By-Step Examples > Example 13: Design of Two Braced Concrete Columns

Problem Description

Two concrete columns A and B are part of a braced frame [Ref 16, pp568].  The frame is analyzed and the results of the two columns are listed below.

 

 

Design the columns according to ACI 318-02/05 .  Use fc = 3 ksi, fy = 60 ksi

 

Suggested Modeling Steps

▪Set proper units from Settings and Tools > Units & Precisions.

▪Create two beam elements:  element 1 – 20 ft, element 2– 24 ft.

▪Select element 1 and 2, define and assign the standard material (Concrete fc = 3.0 ksi) by Modify > Member Properties > Materials. Make sure “Assign active material to currently selected elements” is checked in the dialog.

▪Select element 1 and 2, define and assign the standard section (Rectangle 14 x 14 inch) by Modify > Member Properties > Sections.  Make sure “Assign active section to currently selected members” is checked in the dialog.

▪Select and assign pinned support to the start node of each member by Create > Boundary Conditions > Support.  Select and assign roller support to the end node of each member by Create > Boundary Conditions > Support.

▪Define Dead and Live load cases by Create > Load Cases.

▪Define two load combinations:  one with 1.0Dead and the other with 1.2Dead + 1.6Live.  The former combination contains only the sustained load cases and will be used to calculate βd. Combination two will be used to perform the actual design.  Make sure “Perform Concrete Design using this Load Combination” is checked.  Also enter sustained load factor (1.2 in this case).

▪Define and apply nodal loads and moments for Dead and Live cases by Create > Draw Loads > Nodal Loads.

▪Set the analysis options by Analysis > Analysis Options.  Choose the model type “2D Frame”.  Uncheck “Consider shear deformation on members”.  

▪Select ASTM_615 (English) rebar database by Concrete Design > Concrete Design Tools > Rebar Database.

 

▪Define and assign two column design criteria by Concrete Design > RC Design Criteria > RC Column Design Criteria.  Make sure “Assign active criteria to selected members” is checked in the dialog box.

 

 

▪Set model concrete design criteria by Concrete Design > RC Model Design Criteria.  Make sure the sustained load combination is selected for computing βd.

▪Perform the static analysis by Analysis > Static Analysis.

▪Perform concrete design by Concrete Design > Perform Concrete Design.  Concrete sections will be generated automatically based on column design criteria.  Exact 3D P-Mx-My capacity surfaces will be generated and are used to check against the column internal forces and moments.

 

 

▪View column design results by Concrete Design > Concrete Design Output > RC Column Results.  Detailed column section results such as interaction diagrams may be viewed or printed by Concrete Design > Flexural/Axial Interaction.

 

 

Results

The following table shows some intermediate results during the design.  The program gives comparable results with the reference [Ref 16].

 

 

The program chooses 6#8 bars for column A and 4#8 bars for column B.  The reference gives 4#8 bars for both column A and column B.  The program gives the unit check of 1.038 for column A if 4#8 bars were used.  For practical applications, a unit check of slightly over 1.0 is probably acceptable.

 

Comments

This example shows the program can be used to design multiple concrete columns in a fast fashion.  The loads are applied as nodal forces and moments.  These loads are usually obtained from analysis results.  For columns that are part of an unbraced frame, second-order analysis must be used, with consideration to stiffness adjustment according to ACI 318-02/05.