design of a reinforced concrete structure

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SAFI Sample Projects

Design of a Reinforced Concrete Structure

SAFI Quality Software Inc.3393, chemin Sainte-Foy

Ste-Foy, Quebec, G1X 1S7Canada

Contact: Rachik Elmaraghy, P.Eng., M.A.Sc.

Tel.: 1-418-654-94541-800-810-9454

Fax: 1-418-653-9475

Internet Site: http://www.safi.comE-mail: [email protected]



Design of a Reinforced Concrete Structure

Introduction

This sample problem presents the design of a reinforced concrete structure based on the CAN/CSA

A23.3-1994 code. The procedures presented in this example apply to any other design code

supported by the program. The model used for this example can be found in the Samples\Concrete

sub-directory, the name of the file is RC-Ex-1.str .

Creating the model

The model is a simple 2D structure representing a building with three bays and six floors. The

exterior bays are 9m wide and the interior bay is 6m wide. The first floor 4.85m high and the

remaining floors are 3.65m high. The beams are extended outside the exterior columns by a

distance of 1.5m.

The edition of models is discussed in the User's Guide and will not be repeated here. We will

assume the following material properties for the elements of the model and the reinforcement bars.

Concrete Rebars Stirrups

Yield stress (MPa) 3.28 400.00 300.00

Ultimate stress (MPa) 30.00 600.00 450.00

Elastic modulus (MPa) 27000.00 200000.00 200000.00Poisson ratio 0.15 0.27 0.27

Voluminal mass (kg/m3) 2400.00 7850.00 7850.00

Thermal expansion (1/°C) 1.10e-005 1.17e-005 1.17e-005

The beams and columns of the model should be defined as physical elements to obtain a continuity

in the reinforcement bars. The physical elements consist of several connected members grouped

together as a continuous element.



Design of a Reinforced Concrete Structure 2

Geometry

The cross sections of the elements of the structure are presented below.

Exterior columns Interior columns




First three floors

Other floors

Editing the loads

The loads acting on the structure are presented below. In this example, the self weight of the

structure is modeled by uniform and concentrated loads. It is also possible to let the program

calculate the self weight of the elements by using the Gravity Loads command. Here again, the

method for editing the loads is discussed in the User's Guide and will not be repeated here.




Analysis model with self weight loads




Analysis model with additional dead loads




Analysis model with live loads




Analysis model with positive and negative wind loads




Analysis model with seismic loads

The load combinations used to analyze the structure are created using the load combination wizard

according to the NBC-95 building code. The load combinations are summarized below.




Summary of the load combinations

The last load combination which includes the factored dead loads only is not generated by the

wizard as it is not required by the NBC-95 code. However, it is necessary to calculate the sustained

loads for the second order effects. This combination should be created manually.

Selecting the design code

The first step required to design a reinforced concrete structure is to select the desired design code.

The program allows to design a reinforced concrete structure according to the Canadian A23.3-

M94 and S6-2000 codes, the American ACI-2002 code and the Egyptian ECC-95 code.

Select the Codes command from the Analysis menu. In the dialog box displayed, activate theReinforced Concrete tab. In the code list, select the CAN/CSA A23.3 code and click on the OK

button to accept the changes.




Creating the design groups

In order for the program to calculate the reinforcement of the elements of the model, it is required

to specify some parameters for the reinforcement bars (Library, size, material, etc.). This is done by

creating groups of parameters and by assigning these groups of parameters to the elements of the

model.

First, we will create a design group for the beams. Select the Beam Design Groups command fromthe Application-Reinforced concrete sub-menu. In the dialog box displayed, click on the button

to create a new group.

It is not required to create a design group for each beam of the model, if all beams share the same

types of reinforcement bars and stirrups, only one group is required. However, the forces acting on

the various elements are not the same for all the elements so the required reinforcement will be

different for each beam. Thus, the Design all elements in the group separately option should bechecked. For larger projects, it is often better to have the same reinforcement bars in several similar

beams. For example, we could have designed the three first floors beams to be identical and the

three last floors beams to be identical. This could have been achieved by creating two design

groups with the Design all elements in the group separately option unchecked.




Fill the data of the dialog box as shown below and click on the OK button.

The left part of the dialog box refers to the beams longitudinal reinforcement bars and the right part

refers to the stirrups.

The design group for the column is created in the same manner using the Column Design Groups

command from the Application-Reinforced concrete sub-menu.

The left part of the dialog box refers to the column longitudinal reinforcement bars and the right part refers to the spirals and ties.

Assigning the design groups to the elements

The allow the program to design the concrete element, the design groups created above must be

attached to the appropriate elements of the model. This is done as for any other member parameters

using the Member Attributes command found in the Edit-Member sub-menu.




Select all the beams of the model using the selection tools and activate the Member Attributes

command. In the dialog box displayed, activate the Concrete tab.

Check the Change beam design group option and select the beam design group created earlier in

the list below. When this is done, click on the OK button to accept the changes. Repeat these

operations for the columns of the model.




Defining the member end offsets

The connection between a beam and a column forms a rigid joint where the real acting forces arelower than the calculated forces. These regions must be defined for the calculated reinforcement to

be accurate.

The sketch below represents a typical beam in the model with the corresponding end offsets which

are equal to half the size of the column connected to the beam end. These offsets must be defined

for all beams of the model.




Select the left cantilever beams and activate the Member Attributes command. In the lower part of

the Concrete tab, specify an end offset for node j of 225 mm.

Specify the end offsets for the other beam elements.




Running the analysis

To run the analysis, select the Run command from the analysis menu. In the displayed dialog box,

select the desired type of analysis and the Design - Reinforced concrete option in the Design list. If

second order effects should be considered when designing the columns, activate the corresponding

options in the Second Order Effects section. When performing a linear first order analysis like it isthe case here, the second order effects (lateral drift and member stability) need to be accounted for

by an alternate method like the one presented in the design codes.




Viewing the reinforcement results

After the analysis is completed, various results are available. First, we can verify the reinforcement

of the beams and columns which are available as reinforcement layouts. These layouts may be

further edited and additional analyses can be performed to verify the edited reinforcement. These

results are available as input data to allow the engineer to edit the reinforcement provided or toallow the verification of existing structures where the reinforcement bars are known.

Click on the Rebars Layouts command from the Applications-Reinforced concrete sub-menu. The

dialog box displayed should contain 6 tabs, one for each beam (physical beam element) in the

model.




To have a graphical display of the longitudinal bars in physical element 5, click on the Preview

Layout button from the PHYS-5 layout tab. In the dialog box displayed, select the physical

element P5 in the element list. For a better preview, select 3:1 in the Vertical Scale list.




PHYS-5 in Physical Member 5

6 1 .

0 5

5 3 8 .

9 5

6 1 .

0 5

5 3 8 .

9 5

(5) 2-30M 5100 (8) 1-30M 3100 (9) 1-30M 4000

(6) 2-30M 15100 (7) 2-30M 4600

(4) 2-30M 29000

(2) 2-20M 7600 (3) 2-20M 7600

(1) 4-20M 26200

S3 1500 S3 9000 S3 6000 S3 9000 S3 1500

[mm]

2250

400

1 1 0 6

0 0

S3 - T-600x400x2250x110[mm]

In the same manner, we can verify the shear reinforcement of the beams. Click on the Stirrups

Layouts command from the Applications-Reinforced concrete sub-menu. The dialog box displayed

should contain 6 tabs, one for each beam (physical beam element) in the model.




To have a graphical display of the stirrups in physical element 10, click on the Preview Layout

button from the PHYS-10 layout tab. In the dialog box displayed, select the physical element P10

in the element list. For a better preview, select 5:1 in the Vertical Scale list.




PHYS-10 in Physical Member 10

(2) 11-10M@270

(3) 1-10M@3020

(4) 13-10M@195

(5) 1-10M@500

(6) 7-10M@315

(7) 1-10M@1090

(8) 7-10M@315

(9) 1-10M@500

(10) 12-10M@215

(11) 1-10M@2945

(12) 10-10M@300

S4 1500 S4 9000 S4 6000 S4 9000 S4 1500

[mm]

2250

400

1 1 0 5

5 0

S4 - T-550x400x2250x110[mm]

Column reinforcement can be visualized by the Peripheral bars Layouts command from the

Applications-Reinforced concrete sub-menu. The dialog box displayed should contain 4 tabs, one

for each column (physical column element) in the model.




To have a graphical display of the column reinforcement in physical element 1, click on the

Preview Layout button from the PHYS-1 layout tab. In the dialog box displayed, select the

physical element P1 in the element list. Then, move the mouse cursor over the graphic in the left

part of the dialog box to determine the position in the column where to view the reinforcement bars.

PHYS-1 in Physical Member 1 @ 0 mm

450

Z

Y

[mm]

PHYS-1 in Physical Member 1 @ 15800 mm

450

Z

Y

[mm]




Viewing the analysis, verification and design results

The analysis results may be checked either graphically or in tables using the various results

commands from the Analysis menu. We will look at the reinforced concrete design results which

includes the bending, shear and compression-bending resistance curves as well as the interaction

diagrams for the columns.

Click on the Bending command from the Analysis-Chart-Reinforced concrete sub-menu and then

click on one of the beam of the first floor of the structure. The following dialog box should be

displayed presenting the bending resistance curves for physical member 5.

In the same manner, we can visualize the shear resistance of the beams and compression-bending

resistance of the columns.




The interaction diagrams can also be visualized using the Interaction diagram command from the

Analysis-Charts-Reinforced concrete sub-menu. Activate this command and then click on one of

the member of the leftmost column.

The graphic at the left is used to determine the critical section on the column where to view the

interaction diagrams and forces acting at this critical section. When the dialog box is opened, the

critical section at which the ultimate limit state is the largest is selected by default.

Note that although the 2D analysis will give null bending moments on the weak axis, the design on

the weak axis is made using a minimum eccentricity as required by the design code. The bending

moments on the weak axis listed in the results dialog box correspond to this minimum eccentricity.

design of a reinforced concrete structure

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