Navigation:  Step-By-Step Examples > Example 14: Design of a Continuous Concrete Beam

Problem Description

The following sub-frame [Ref 17, pp 7-43], which consists of one continuous beam plus top and bottom columns framing into the beam, is used to perform flexural and shear design of the continuous beam under vertical loads.

 

Member sizes: beam = 36 x 19.5 in; exterior columns =16 x 16 in; interior columns = 18 x 18 in.  Story height = 13 ft.

 

Service Loads: Dead = 3.9 kips/ft (including self-weight); Live = 1.8 kips/ft

 

 

Design the continuous beam according to ACI 318-02/05.  Use fc = 4 ksi, fy = 60 ksi

 

Suggested Modeling Steps

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

▪Generate rectangular frame by Create > Templates > Rectangular Frames as follows:

 

 

▪Select and delete top and bottom beam elements (element 1, 2, 3, 7, 8 and 9 that were generated).  

▪Select far end nodes of columns and assign fixed supports to them.

▪Select all members and renumber each selected member by running Modify > Renumber > Renumber Selected Members, as shown below

 

 

▪Define three rectangular sections 36 x 19.5 in, 18 x 18 in and 16 x 16 in using Regular Section in Modify > Member Properties > Sections.  Assign each of these sections to appropriate elements

▪Define 4.0 ksi material using Std Material in Modify > Member Properties > Materials.  Assign this material to all.

▪Define five load cases: Dead, Live1, Live2, Live3 and Live4 by Create > Load Cases.  Note Live1, 2, 3 and 4 cases are used for live load patterning.  Live1 loading is applied to element 1 and 2.  Live2 loading is applied to elements 1 and 3.  Live3 is applied to element 2 only. Live4 is applied to elements 2 and 3.

▪Define four new load combinations: a). 1.2Dead + 1.6Live1, b). 1.2Dead + 1.6Live2 and c). 1.2Dead + 1.6Live3.  d). 1.2Dead + 1.6Live4.  Make sure “Perform Concrete Design using this Load Combination” is checked.  Also enter sustained load factor (1.2 in this case).

▪Define and apply line loads for Dead, Live1, 2, 3 and 4 cases Create > Draw Loads > Line Loads.  Use View > Load Diagram to check that the loads are applied correctly.

▪Set the analysis options by Analysis > Analysis Options.  Choose the model type “2D Frame”.  Uncheck “Consider shear deformation on members”.  Run Static Analysis to make sure the model is correct before we proceed to the concrete design.

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

▪Define and assign beam design criteria by Concrete Design > RC Design Criteria > RC Beam Design Criteria.  

 

 

▪Set model concrete design criteria by Concrete Design > RC Model Design Criteria.  Make sure to select the checkbox “Automatically compute support widths”.

 

 

▪Select all columns and exclude them from concrete design by Concrete Design > Exclude Elements.

 

 

▪Perform concrete design by Concrete Design > Perform Concrete Design.

▪To view the beam design results in tabulated form, run Concrete Design > Concrete Design Output > RC Beam Results for flexural design and Concrete Design > Concrete Design Output > Member Shear Results for shear design.

 

 

 

▪To view the beam design result in graphics, run Concrete Design > Concrete Design Diagrams > RC Member Envelope Diagram.  The following shows the member moment envelope diagram.

 

 

 

Results

The following table compares the design moments between the program and the reference [Ref 17, pp 7-43]:

 

 

Comments

The reference [Ref 17, pp 7-43] uses the approximate coefficients method while the program uses the exact stiffness method.  It is apparent the former method is quite conservative.