reinforced concrete design assistant vs 2015

7/23/2019 Reinforced Concrete Design Assistant vs 2015

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Heavens Light is Our Guide

Reinforced Concrete Design Assistant

This Thesis Submitted to the

Department of Civil Engineering,

Rajshahi University of Engineering & Technology,

In Partial Fulfillment of the Requirements of the Degree of

BACHELOR OF SCIENCE

IN

CIVIL ENGINEERING

Supervised By

Dr. Tohur Ahmed.

ProfessorDepartment of Civil Engineering,

Rajshahi University of Engineering

& Technology.

Rajshahi-6201.

Submitted by

Md. Shahoriaz Al Mani.

Roll: 100021.


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Acknowledgements

My almost gratitude to ALLAH, the Almighty without his mercy and blessing this work would

not been possible. I am grateful and would like to express my sincere gratitude to my supervisor,

Professor Dr. Tohur Ahmed, Department of Civil Engineering, Rajshahi University of Engineering

& Technology, for giving me this opportunity all his effort, time and patience in helping me to

complete this thesis. This thesis would not be possible without his guidance and encouragement.

He patiently guided me through the process of making the abstract idea and program.

I also gradually acknowledge towards my Teachers, my friends and well-wishers who helped me

suggested me with a view to accomplishing the work.

The author acknowledges the sacrifice of parents and others of the family that has enabled him to

attain this level.

RUET, Rajshahi

January, 2016

Author

Md. Shahoriaz Al Mani


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Abstract

Nowadays most of the Civil Engineering Structural problem is solved with the help of computer

because manually solving problem is not only time consuming and laborious but also it is difficult

to find out the required economical section and optimum requirement of reinforcing bar. Reason

for developing this computer program is to Building a software which is reliable for Civil

Engineers to design various structural component in simple, fast and easy to operate without any

complication. The program has four module beam, column, footing and stair. All the design is

performed as per ACI code and all the units are in FPS unit.

This computer program has been developed by using Microsoft Visual Studio 2015 with C sharp

(C#) programming Language.


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Content

Page No.

Acknowledgement ii

Abstract ii

Content iv

List of Figures vi

Chapter-1 Introduction

1.1 General 1

1.2 Statement of the project 2

1.3 Objective of the project 2

1.4 Computer Software 2

1.5 Software Engineering 3

1.6 Structural Design 3

1.7 Structural Design Process 4

1.8 Engineering Design Process 4

1.9 Reason for developing this Software 6

1.10 Reason for using Visual Studio 2015 and C Sharp 6

Chapter-2 Reinforce Concrete Structure 8

2.1 General 8

2.2 Safety 92.3 Building Code Requirement for Structural Concrete 9

2.4 Safety Provisions of the ACI Code 10

2.5 Design Methods of Reinforced Concrete Structure 11

2.5.1 Change of Design Methods according to ACI 318 Code 11


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2.5.2 The Working Stress Design (WSD) 11

2.5.3 The Ultimate Strength Design (USD) 12

2.6 Loads 14

2.6.1 Dead Load 14

2.6.2 Live Loads 14

2.6.3 Environmental Loads 14

2.7 Required Strength 15

2.8 Design Strength 17

2.9 Concrete Cover for Reinforcement 17

2.10 Selection of Bar and Bar Spacing 19

Chapter-3 Review of Structural Design on the ACI Code 20

3.1 Beam 20

3.1.1 Introduction 20

3.1.2 Types of Beam 20

3.1.3 Reinforced Concrete Beam Design Parameters 21

3.1.4 Design Procedure 24

3.2 Column 25


3.2.2 Types of Column 25

3.2.3 ACI Code Safety Provision for Column 27

3.2.4 Behavior of Axially Loaded Column 29

3.2.5 Biaxial Bending 30

3.2.5.1 Bresler Load Contour Method 34

3.2.5.2 Bresler Reciprocal Method 34


3.3 Footing 37


3.3.2 Types of Footings 37

3.3.3 Design Consideration 39



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3.4 Stair 40


3.4.2 Types of Staircases 40

3.4.3 Components of Stairs 42


Chapter-4 Reinforcement Concrete Structure Designer 44

4.1 General 44

4.2 Beam Module 44


4.2.2 Beam Design Module 45

4.2.3 T-Beam Module 46

4.3 Column Module 46


4.3.2 Column Design Module 47

4.4 Footing Module 47


4.4.2 Footing Design Module 48

4.5 Stair Module 48


4.5.2 Stair Design Module 48

Chapter-5: Conclusion and Recommendation 50

5.1 Conclusion 50

5.2 Recommendation 50

References 51

Appendix A Computer Program 52


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Fig. No. List of Figures Page No.

Fig. 3-1: Reinforced Rectangular Beam 20

Fig. 3-2: Common Shapes of Concrete Beam 21

Fig. 3-3: Column Types 26Fig. 3-4: The Column Types Depending on Applied Load 26

Fig. 3-5: Eccentric Loaded Conditions 27

Fig.3-6:ACI Safety Provisions Superimposed on Column Strength

Interaction Diagram29

Fig. 3-7: Behavior of Tied and Spiral Column 30

Fig. 3-8: Interaction diagram for Compression plus Biaxial Bending 32

Fig. 3-9: Interaction Surfaces for the Reciprocal Load Method 35

Fig. 3-10: Footing Types 38

Fig. 3-11: Types of Staircases 40

Fig. 3-12: Transversely Supported Stairs 41

Fig. 3-13: Longitudinally Supported Stairs 42

Fig. 3-14: Stairs Main Components 43

Fig. 4-1: Beam Design Module 45

Fig. 4-2: T Beam Design Module 46

Fig 4-3: Column Design module 47

Fig 4-4: Footing Design Module 48

Fig. 4-5: Stair Design Module 49


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Chapter-1

Introduction

1.1 General

RCC (Reinforced Cement Concrete) is a construction technology which evolved with the

evolution of different structural materials in the 18th century during the Industrial

Revolution. Industrial Revolution brought in new technology which helped in the

manufacture of various materials. Now a Days Reinforced concrete structures are one of

the most popular structure systems. Many Civil Engineering students are using reinforced

concrete structure systems for their designs. But there are many cases where they design

structurally questionable buildings because they are trying to express their design ideas

with limited knowledge about Reinforced Concrete Design. Frequently the structural

member design would not be their primary focus.

Although there is the possibility that excessive structural considerations may disturbing

their search for unique designs, basic structural calculation is important for design.

Structurally sound solutions can make their design concepts closer to reality. Unfortunately

most Civil Engineering Institute concentrate their curriculum on visual design education

rather than a balanced education of design and structure. The balanced education does not

mean equal class time for structural and design classes. But it is essential that students can

at least discriminate that their design has a reasonable structure. Many students use the

commonly available books on structural graphic standards as a reference. But they are not

applicable to many different conditions.

Furthermore, reinforced concrete structures need a lot of calculations and different

condition inputs because it is a composite material of concrete and steel. The Reinforced

Concrete Structure Designprogram (RCSD), which has been developed for this thesis, can

help Civil Engineering students and users to analyze their designs and understand structural

fundamentals. Although there are many reinforced concrete structure programs, most

programs are targeting advanced level users who have a background in structural

engineering. The RCSD program is for beginner level users such as Civil Engineering

undergraduate and graduate students with limited knowledge about structures. For this, it

provides a graphical input method and a step-by-step calculation procedure to help users.


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With this program, it is possible for the user to design basic structural parts such as slab,

beam, column and footing. Also the program is based on the American Concrete Institute

Code. The ultimate goal of this program is that users can analyze their own designs using

this program and determine structural proportions of their design idea.

The rapid development of the computer in the last decade has resulted in rapid adoption of

Computer Structural Design Software that has now replaced the manual computation. This

has greatly reduced the complexity of the analysis and design process as well as reducing

the amount of time required to finish a project.

1.2 Statement of the Study

This study involves the development of design software for Beam, Column, Footing and

Staircase.

1.3 Objective of the Study

1. To make the design Calculation simple, easier and rapid.

2. To get knowledge and to use the American Concrete Institute Code (ACI 318-05).

3. To develop a software for the design of several structural element (Beam, Column,

Stair, Footing) according to the provision & procedure of the American Concrete

Institute Code (ACI 318-05).

4.

To get economical section without any arithmetic mistakes.

1.4 Computer software

Software is a program that enables a computer to perform a specific task, as opposed to the

physical components of the system (hardware).

This includes application software such as a word processor, which enables a user to

perform a task, and system software such as an operating system, which enables other

software to run properly, by interfacing with hardware and with other software. Practical

computer systems divide software into three major classes: system software, programming

software and application software, although the distinction is arbitrary, and often blurred.

Computer software has to be "loaded" into the computer's storage (such as a hard drive,

memory, or RAM). Once the software is loaded, the computer is able to execute the


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software. Computers operate by executing the computer program. This involves passing

instructions from the application software, through the system software, to the hardware

which ultimately receives the instruction as machine code. Each instruction causes the

computer to carry out an operation moving data, carrying out a computation, or altering the

control flow of instructions.

1.5 Software Engineering

Software engineering is the study and an application of engineering to the design,

development and maintenance of software.

Typical formal definitions of software engineering are:

Research, design, develop, and test operating systems-level software, compilers and

network distribution software for medical, industrial, military, communications,

aerospace, business, scientific, and general computing applications.

The systematic application of scientific and technological knowledge, methods, and

experience to the design, implementation testing, and documentation of software.

A software engineer is a licensed professional engineer who is schooled and skilled in the

application of engineering discipline to the creation of software. A software engineer is

often confused with a programmer, but the two are vastly different disciplines. While a

programmer creates the codes that make a program run, a software engineer creates the

designs the programmer implements. By law no person may use the title engineer (of any

type) unless the person holds a professional engineering license from a state licensing board

and are in good standing. A software engineer is also held accountable to a specific code

of ethics.

1.6 Structural Design

Structural design is the methodical investigation of the stability, strength and rigidity of

structures. The basic objective in structural analysis and design is to produce a structure

capable of resisting all applied loads without failure during its intended life. The primary

purpose of a structure is to transmit or support loads. If the structure is improperly designed

or fabricated, or if the actual applied loads exceed the design specifications, the device will


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probably fail to perform its intended function, with possible serious consequences. A well-

engineered structure greatly minimizes the possibility of costly failures

1.7 Structural design process

A structural design project may be divided into three phases, i.e. planning, design and

construction.

Planning: This phase involves consideration of the various requirements and factors

affecting the general layout and dimensions of the structure and results in the choice of one

or perhaps several alternative types of structure, which offer the best general solution. The

primary consideration is the function of the structure. Secondary considerations such as

aesthetics, sociology, law, economics and the environment may also be taken into account.

In addition there are structural and constructional requirements and limitations, which may

affect the type of structure to be designed.

Design:This phase involves a detailed consideration of the alternative solutions defined in

the planning phase and results in the determination of the most suitable proportions,

dimensions and details of the structural elements and connections for constructing each

alternative structural arrangement being considered.

Construction:This phase involves mobilization of personnel; procurement of materials

and equipment, including their transportation to the site, and actual on-site erection. During

this phase, some redesign may be required if unforeseen difficulties occur, such as

unavailability of specified materials or foundation problems.

1.8 Engineering Design Process

The engineering design process is a series of steps that engineers follow to come up with a

solution to a problem. Many times the solution involves designing a product (like a machine

or computer code) that meets certain criteria and/or accomplishes a certain task.

Define the criteria and constraints of a design problem with sufficient precision to

ensure a successful solution, taking into account relevant scientific principles and

potential impacts on people and the natural environment that may limit possible

solutions.


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Evaluate competing design solutions using a systematic process to determine how

well they meet the criteria and constraints of the problem.

Analyze data from tests to determine similarities and differences among several

design solutions to identify the best characteristics of each that can be combined

into a new solution to better meet the criteria for success.

Develop a model to generate data for iterative testing and modification of a

proposed object, tool, or process such that an optimal design can be achieved.

Engineering design process illustrated briefly in flow chart below-

Define the Problem

Brainstorm, Evaluate and

Choose Solution

Solution Meets

Re uirements

Solution Meets Requirements

Partially or Not at All

Communicate Result

Test Solution

Develop and Prototype

Solution

Do Background Research

Specify Requirements

Based on results and data

make design changes,

prototype, test again and

review new data


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Engineers do not always follow the engineering design process steps in order, one after

another. It is very common to design something, test it, find a problem, and then go back

to an earlier step to make a modification or change to your design. This way of working is

called iteration.

1.9 Reason for developing this Software

Beam, column, footing, stair are the important elements of the whole building. Engineers

should have to be careful and sincere to give an economic design within minimum time.

This software will serve following purpose;

1. It will not only give accurate result but also save time and money.

2. Design can be completed quickly, hence saving time it will increase the efficiency

of an engineer.

3. It will reduce the error due to arithmetic mistakes some error of mathematic number

and minimize the amount of manually handled data.

4.

Various types of building elements and mist of the cases, the engineers perform the

design from their experience, which is not accurate and not economical. This

software will reduce the labor and time and will ensure economical design.

1.10 Reason for using Visual Studio 2015 and C Sharp

C# (C Sharp) is an elegant, simple, type-safe, object-oriented language that allows

enterprise programmers to build a breadth of applications. It is a user friendly language. C#

is better than C++ because -

It has a huge standard library with so much useful stuff that's well-implemented and

easy to use.

It allows for both managed and native code blocks.

It allows you to treat class-methods' signatures as free functions (i.e. ignoring the

statically typed this pointer argument), and hence create more dynamic and flexible

relationships between classes.

Assembly versioning easily remedy DLL problems.

Microsoft Visual Studio is an integrated development environment (IDE) from Microsoft.

It is used to develop computer programs for Microsoft Windows, as well as web sites, web


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applications and web services. Visual Studio uses Microsoft software development

platforms such as Windows API, Windows Forms, Windows Presentation Foundation,

Windows Store and Microsoft Silverlight. It can produce both native code and managed

code. It has easy code navigation, fast builds, and quick deployment. Visual Studio

increases productivity and makes it easy to do work alone or as part of a larger team. Visual

C# is an implementation of the C# language by Microsoft. Visual Studio supports Visual

C# with a full-featured code editor, compiler, project templates, designers, code wizards, a

powerful and easy-to-use debugger, and other tools. The .NET Framework class library

provides access to many operating system services and other useful, well-designed classes

that speed up the development cycle significantly.


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Chapter-2

Reinforced Concrete Structure

2.1 General

Concrete is one of the most popular materials for buildings because it has high compressive

strength, flexibility in its form and it is widely available. The history of concrete usage

dates back for over a thousand years. Contemporary cement concrete has been used since

the early nineteenth century with the development of Portland cement. Despite the high

compressive strength, concrete has limited tensile strength, only about ten percent of its

compressive strength and zero strength after cracks develop. In the late nineteenth century,

reinforcing materials, such as iron or steel rods, began to be used to increase the tensile

strength of concrete. Today steel bars are used as common reinforcing material. Usually

steel bars have over 100 times the tensile strength of concrete; but the cost is higher than

concrete. Therefore, it is most economical that concrete resists compression and steel

provides tensile strength. Also it is essential that concrete and steel deform together and

deformed reinforcing bars are being used to increase the capacity to resist bond stresses.

Advantages of reinforced concrete can be summarized as follows (Hassoun, 1998).

1. It has a relatively high compressive strength.

2.

It has better resistance to fire than steel or wood

3. It has a long service life with low maintenance cost

4. In some types of structures, such as dams, piers, and footing, it is the most

economical structural material.

5. It can be cast to take any shape required, making it widely used in precast structural

components.

Also, disadvantages of reinforced concrete can be summarized as follows:

1. It has a low tensile strength (zero strength after cracks develop).

2. It needs mixing, casting, and curing, all of which affect the final strength of

concrete.

3. The cost of the forms used to cast concrete is relatively high. The cost of form

material and artisanry may equal the cost of concrete placed in the forms.


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4. It has a lower compressive strength than steel (about 1/10, depending on material),

which requires large sections in columns of multistory buildings.

5.

Cracks develop in concrete due to shrinkage and the application of live loads.

2.2 Safety

A structure must be safe against collapse; strength of the structure must be adequate for all

loads that might act on it. If we could build buildings as designed, and if the loads and their

internal effects can be predicted accurately, we do not have to worry about safety. But there

are uncertainties in:

Actual loads;

Forces/loads might be distributed in a manner different from what we assumed;

The assumptions in analysis might not be exactly correct;

Actual behavior might be different from that assumed etc.

Finally, we would like to have the structure safe against brittle failure (gradual failure with

ample warning permitting remedial measures is preferable to a sudden or brittle failure).

2.3 Building Code Requirement for Structural Concrete

Buildings must be designed and constructed according to the provisions of a building code,

which is a legal document containing requirements related to such things as structural

safety, fire safety, plumbing, ventilation, and accessibility to the physically disabled. A

building code has the force of law and is administered by a governmental entity such as a

city, a county, or for some large metropolitan areas, a consolidated government. Building

codes do not give design procedures, but specify the design requirements and constraints

that must be satisfied. Of particular importance to the structural engineer is the prescription

of minimum live loads for buildings. While the engineer is encouraged to investigate the

actual loading conditions and attempt to determine realistic values, the structure must be

able to support these specified minimum loads. Many countries have their own structural

design codes, codes of practice or technical documents which perform a similar function.It

is necessary for a designer to become familiar with local requirements or recommendations

in regard to correct practice. In this chapter some examples are given, occasionally in a

simplified form, in order to demonstrate procedures. They should not be assumed to apply


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to all areas or situations. However, the Uniform Building Code (UBC) and other model

codes are adapted by jurisdictions, such as Cities, or States as governing codes. Material

and methods are tested by private or public organizations. They develop, share, and

disseminate their result and knowledge for adoption by jurisdictions. The American

Concrete Institute (ACI) is leading the development of concrete technology. The ACI has

published many references and journals. Building Code Requirement for Structural

Concrete (ACI 318 Code) is a widely recognized reinforced concrete design and

construction guide. Although the ACI Code does not have official power of enforcement,

it is generally adapted as authorized code by jurisdictions not only in United States but also

many countries. The ACI318 Code provides the design and construction guide of reinforced

concrete. ACI has been providing new codes depending on the change of design methods

and strength requirement.

2.4 Safety Provisions of the ACI Code

Load factors are applied to the loads, and a member is selected that will have enough

strength to resist the factored loads. In addition, the theoretical strength of the member is

reduced by the application of a resistance factor. The criterion that must be satisfied in the

selection of a member is

Factored Strength Factored Load

In this expression, the factored load is actually the sum of all working loads to be resisted

by the member, each multiplied by its own load factor. For example, dead loads will have

load factors that are different from those for live loads. The factored strength is the

theoretical strength multiplied by a strength reduction factor. Equation (1.3) can therefore

be written as

Nominal Strength X Strength Reduction Factor Load X Load Factors

Since the factored load is a failure load greater than the actual working loads, the load

factors are usually greater than unity. On the other hand, the factored strength is a reduced,

usable strength and the resistance factor is usually less than unity. The factored loads are

the loads that bring the structure or member to its limit.


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2.5 Design Methods of Reinforced Concrete Structure

Two major calculating methods of reinforced concrete have been used from early 1900s

to current. The first method is called Working Stress Design (WSD) and the second is called

Ultimate Strength Design (USD). Working Stress Design was used as the principal method

from early 1900s until the early 1960s. Since Ultimate Strength Design method was

officially recognized and permitted from ACI 318-56, the main design method of ACI 318

Code has gradually changed from WSD to USD method. The program of this thesis is based

on ACI 318-05 Code Which published in 2005.

2.5.1 Change of Design Methods according to ACI 318 Code (PCA, 1999).

ACI 318-56: USD was first introduced (1956)

ACI 318-63: WSD and USD were treated on equal basis.

ACI 318-71: Based entirely on strength Method (USD) WSD was called Alternate Design

Method (ADM).

ACI 318-77: ADM relegated to Appendix B ACI 318-89: ADM back to Appendix A

ACI 318-95: ADM still in Appendix A Unified Design Provision was introduced in

Appendix B

ACI 318-02: ADM was deleted from Appendix A (ACI, 2002).

2.5.2 The Working Stress Design (WSD)

Traditionally, elastic behavior was used as basis for the design method of 16 reinforcedconcrete structures. This method is known as Working Stress Design (WSD) and also called

the Alternate Design Method or the Elastic Design Method or Allowable stress design. This

design concept is based on the elastic theory that assumes a straight-line stress distribution

along the depth of the concrete section. To analyze and design reinforced concrete

members, the actual load under working conditions, also called service load condition, is

used and allowable stresses are decided depending on the safety factor. For example

allowable compressive bending stress is calculated as 0.45fc. If the actual stresses do not

exceed the allowable stresses, the structures are considered to be adequate for strength. The


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WSD method is easier to explain and use than other method but this method is being

replaced by the Ultimate Strength Design method. ACI 318 Code treats the WSD method

just in a small part.

The working stress method may be expressed by the following:

f allowable stresses (fallowable) (1)

where, f = an elastically computed stress, such as by using the flexure formula f = Mc/I for

beam.

fallow= A limiting stress prescribed by a building code as a percentage of the compressive

strength fc for concrete, or of the yield stress f y for the steel reinforcing bars.

2.5.3 The Ultimate Strength Design (USD)

The Ultimate Strength Design method, also called Strength Design Method (SDM), is

based on the ultimate strength, when the design member would fail. Since 1971 the ACI

Code has been totally a strength code with strength meaning ultimate. Select concrete

dimensions and reinforcements so that the member strength are adequate to resist forces

resulting from certain hypothetical overload stages, significantly above loads expected

actually to occur in service. The design concept is known as strength design. Based on

strength design the nominal strength of a member must be calculated on the basis of

inelastic behavior of material. In other words, both reinforcing steel and concrete behave

in elastically at ultimate strength condition.

The strength design method may be expressed by the following,

Strength provide Strength required to carry factored loads

where the strength provided such as moment strength is computed in accordance with

rules and assumptions of behavior prescribed by a building code, and the strength

required is that obtained by performing a structural analysis using the factored loads. The

design procedure is roughly as follows:

Multiply the working loads by the load factor to obtain the failure loads.


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Determine the cross sectional properties needed to resist failure under these loads. (A

member with these properties is said to have sufficient strength, and would be at the verge

of failure when subjected to the factored loads.)

Proportion your members that have these properties.

Basic Assumptions for Concrete in Ultimate Strength Design method (ACI):

l. Sections perpendicular to the axis of bending that arc plane before bending remains plane

after bending.

2. A perfect bond exists between the reinforcement and the concrete such that the strain in

the reinforcement is equal to the strain in the concrete at the same level.

3. The strains in both the concrete and reinforcement are assumed to be directly

proportional to the distance from the neutral axis (ACI 10.2.2).

4. Concrete is assumed to fail when the compressive strain reaches 0.003 (ACI 10.2.3).

5. The tensile strength of concrete is neglected (ACI 10.2.5).

6. The stresses in the concrete and reinforcement can be computed from the strains using

stress-strain curves for concrete and steel, respectively.

7. The compressive stress-strain relationship for concrete may be assumed to be

rectangular, trapezoidal, parabolic, or any other shape that results in prediction of strength

in substantial agreement with the results of comprehensive tests (ACI 10.2.6). ACI 10.2.7

outlines the use of a rectangular compressive stress distribution which is known as the

Whitney rectangular stress block.

8. Reinforcing steel will yield when strain is equal to Ey and stress after yield is always fy.

2.6 Loads

Loads that act on structures can be divided into three general categories:


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2.6.1 Dead Loads:Dead loads are those that are constant in magnitude and fixed in location

throughout the lifetime of the structure such as: floor fill, finish floor, and plastered ceiling

for buildings and wearing surface, sidewalks, and curbing for bridges.

2.6.2 Live Loads:Live loads are those that are either fully or partially in place or not

present at all, may also change in location; the minimum live loads for which the floors and

roof of a building should be designed are usually specified in building code that governs at

the site of construction

2.6.3 Environmental Loads:Environmental Loads consist of wind, earthquake, and snow

loads. Such as wind, earthquake, and snow loads.

The load factors are 1.7 for live load and 1.4 for dead load. Other factors are given in Table

Table 2-1: Factored load combinations for determining required strength U

Condition Factored load or load effect U

Basic U = 1.4D + 1.7L

Winds

U = 0.75(1.4D + 1.7L + 1.7W)

U = 0.9D + 1.3W

U = 1.4D + 1.7L

Earthquake

U = 0.75(1.4D + 1.7L + 1.87E)

U = 0.9D + 1.43E

U = 1.4D + 1.7L

Earth pressure

U = 1.4D + 1.7L + 1.7H

U = 0.9D + 1.7H

U = 1.4D + 1.7L

Settlement, creep, shrinkage, or

temperature change effects

U = 0.75(1.4D + 1.4T + 1.7L)

U = 1.4(D + T)

2.7 Required Strength

The required strength U is expressed in terms of factored loads, or related internal moments

and forces. Factored loads are the loads specified in the general building code multiplied

by appropriate factors. The factor assigned is influenced by the degree of accuracy to which

the load effect can be determined and the variation which might be expected in the load

during the lifetime of the structure. Dead loads are assigned a lower load factored than live

load because they can be determined more accurately. Load factors also account forvariability in the structural analysis used to compute moments and shears. The code gives


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load factors for specific combinations of loads. In assigning factors to combinations of

loading, some consideration is given to the probability of simultaneous occurrence. While

most of the usual combinations of loadings are included, the designer should not assume

that all cases are covered. Various load combinations must be considered to determine the

most critical design condition. This is particularly true when strength is dependent on more

than one load effect, such as strength for combined flexure and axial load or shear strength

in members with axial load. Since the ACI 318 Building Code is a national code, it has to

conform to the International Building Code, IBC2012 and in turn be consistent with the

ASCE-7 Standard on Minimum Design Loads for Buildings and Other structures. These

two standards contain the same probabilistic values for the expected safety resistance

factors iRn where is a strength reduction factor, depending on the type of stress being

considered in the design such as flexure, shear, or compression, etc.

Factored Load Combinations for Determining Required Strength U in ACI Code

U = 1.4(D + F) (1)

U = 1.2(D + F + T) + 1.6 (L + H) + 0.5(Lror S or R) (2)

U = 1.2D + 1.6 (Lror S or R) + (1.0L or 0.8W) (3)

U = 1.2D + 1.6W + 1.0L + 0.5(Lror S or R) (4)

U = 1.2D + 1.0E + 1.0L + 0.2S (5)

U = 0.9D + 1.6W + 1.6H (6)

U= 0.9D + 1.0E + 1.6H (7)

Where,

D= Dead Load

L= Live Load

E =Earthquake Load

W= Wind Load

T= Self-Straining force such as Creep, Shrinkage & Temperature Effect

H=Load due to the weight & lateral pressure of soil and water in soil


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Lr = Roof Load

R= Rain Load

S= Snow Load

F= Lateral fluid pressure Load

Due Regard is to be given to sign in determining U for combinations of loadings, as one

type of loading may produce effects of opposite sense to that produced by another type.

The load combinations with 0.9D are specifically included for the case where a higher dead

load reduces the effects of other loads. The loading case may also be critical for tension

controlled column sections. In such a case a reduction in axial load and an increase in

moment may result in critical load combination.

Except for

The load factor on L in Equation (3) to (5) shall be permitted to be reduced to 0.5 except

for garages, areas occupied as places of public assembly, and all areas where the live load

L is greater than 100 lb/ft2.

Where wind load W has not been reduced by a directionality factor, it shall be permitted

to use 1.3W in place of 1.6W in Equations (4) and (6)

Where earthquakeload E is based on service-level seismic forces, 1.4E shall be used in

place of 1.0E in Equations (5) and (7).

The load factor on H shall be equal to zero in Equation (6) and (7) if the structural action

due to H counteracts that due to W or E. Where lateral earth pressure provides resistance

to structural actions from other forces. It shall not be included in H but shall be included inthe design resistance.

2.8 Design Strength

The strength of a particular structural unit calculated using the current established

procedures is termed nominal strength. For example, in the case of a beam the resisting

moment capacity of the section calculated using the equations of equilibrium and properties

of concrete and steel is called the nominal moment capacity Mn of the section.


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The purpose of the strength reduction factor f are (MacGregor, 1976; and Winter, 1979):

To allow for under-strength members due to variations in material strengths and

dimensions

To permit for inaccuracies in the design provisions

To reflect the degree of ductility and required probability of the member under the load

effects being considered

To reflect the importance of the member in the structure.

Strength Reduction Factors, F, of the ACI Code-

Tension controlled sections ..0.90

Compression controlled sections

i. Members with spiral reinforcement ...0.75

ii.

Other members ...0.65

Shear and torsion ..0.75

Bearing on Concrete .0.65

Plain Concrete ...0.55

2.9 Concrete Cover for Reinforcement

Concrete cover for reinforcement is required to protect the rebar against corrosion and to

provide resistance against fire. The thickness of cover depends on environmental conditions

and type of structural member. The minimum thickness of reinforcement cover is indicated

in the drawings, or shall be obtained from the relevant code of practice. Below are the

specifications for reinforcement cover for different structural members in different

conditions.

a) At each end of reinforcing bar, net less than 1 inch or 25 mm or less than twice the

diameter of the bar.

b) For a longitudinal reinforcing bar in a column, not less than 8/5 inch or 40 mm not less

than the diameter of such bar. In case of columns of minimum dimension of 8 in or 20 cm

under, whose reinforcing bards do no not exceed in or 12 mm a cover of 1 inch or 25mm to be used.


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c) For longitudinal reinforcing bars in a beam, not less than 6/5 inch or 30 mm or less than

the diameter of the bar.

d) For tensile, compressive shear or other reinforcements in a slab or wall not less than 3/5

inch or 15 mm, not less that the diameter of such bar.

e) For any other reinforcement not less than 3/5 inch or 15 mm, not less than the diameter

of such bar.

f) For footings and other principal structural members in which the concrete is deposited

directly against the ground, cover to the bottom reinforcement shall be 3 inch or 75 mm. If

concrete is poured on a layer of lean concrete, the bottom cover maybe reduced to 2 inch

or 50 mm.

g) For concrete surfaces exposed to the weather or the ground after removal of forms, such

as retaining walls, grade beams, footing sides and top etc. not less than 2 inch or 50 mm.

h) Increased cover thickness shall be provided as indicated on the drawings, for surfaces

exposed to the action of harmful chemicals (or exposed to earth contaminated by such

chemicals), acid, alkali, saline atmosphere, sulphorone, smoke etc.

i) For liquid retaining structures, the minimum cover to all steel shall be 8/5 inch or 40 mm

or the diameter of the main bar, whichever is greater. In the presence of sea water and oils

and waters of a corrosive character the covers, shall be increased by 2/5 inch or 10 mm.

j) Protection to reinforcement in case of concrete exposed to harmful surroundings may

also be given by providing a dense impermeable concrete with approved protective

coatings. In such a case the extra cover mentioned in (b) & (i) above may be reduced.

k) The correct cover shall be maintained by cement mortar cubes (blocks) or other approved

means. Reinforcements for footings, grade beams and slabs on a sub-grade shall be

supported on re-cast concrete blocks as approved by EIC. The use of pebbles or stones shall

not be permitted.

l) The minimum clear distance between reinforcing bars shall by in accordance with IS:

4562000 or as shown in drawing.


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2.10 Selection of Bar and Bar Spacing

Common reinforcing bar sizes range from No. 3 to No. 11 (No. 10 to No. 36), the bar

number corresponding closely to the number of eighth-inches (millimeters) of bar diameter.

The two larger sizes, No. 14 (No. 43) [1.75 inch. (43 mm) diameter] and No. 18 (No. 57)

[2.25 inch. (57 mm) diameter] are used mainly in columns.

It is often desirable to mix bar sizes to meet steel area requirements more closely. In general,

mixed bars should be of comparable diameter, for practical as well as theoretical reasons,

and generally should be arranged symmetrically about the vertical centerline. Many

designers limit the variation in diameter of bars in a single layer to two bar sizes, using,

say, No. 10 and No. 8 (No. 32 and No. 25) bars together, but not Nos. 11 and 6 (Nos. 36and 19). There is some practical advantage to minimizing the number of different bar sizes

used for a given structure.

Normally, it is necessary to maintain a certain minimum distance between adjacent bars to

ensure proper placement of concrete around them. Air pockets below the steel are to be

avoided, and full surface contact between the bars and the concrete is desirable to optimize

bond strength. ACI Code 7.6 specifies that the minimum clear distance between adjacent

bars not be less than the nominal diameter of the bars, or 1 inch. (For columns, these

requirements are increased to 1.5 bar diameters and 1.5 inch.) Where beam reinforcement

is placed in two or more layers, the clear distance between layers must not be less than 1

inch, and the bars in the upper layer should be placed directly above those in the bottom

layer.


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Chapter-3

Review of Structural Design on the ACI Code

3.1 Beam

3.1.1 Introduction

Beams are structural elements carrying transverse external loads that cause bending

moment, shear forces and in some cases torsion across their length. Concrete is strong in

compression and very weak in tension. Steel reinforcement is used to take up tensile

stresses in reinforced concrete beams. When the bending moment acts on the beam, bending

strain is produced. The resisting moment is developed by internal stresses. Under positive

moment, compressive strains are produced in the top of beam and tensile strains in thebottom. Concrete is a poor material for tensile strength and it is not suitable for flexure

member by itself. The tension side of the beam would fail before compression side failure

when beam is subjected a bending moment without the reinforcement. For this reason, steel

reinforcement is placed on the tension side. The steel reinforcement resists all tensile

bending stress because tensile strength of concrete is zero when cracks develop. In the

Ultimate Strength Design (USD), a rectangular stress block is assumed (Fig. 3-1).

Fig 3-1: Reinforced rectangular beam (Ambrose, 1997)

As shown Fig. 3-1, the dimensions of the compression force is the product f beam width,

depth and length of compressive stress block. The design of beam is initiated by the

calculation of moment strengths controlled by concrete and steel.

3.1.2 Types of Beam

Fig. 3-2 shows the most common shapes of concrete beams: single reinforced rectangular

beams, doubly reinforced rectangular beams, T-shape beams, spandrel beams, and joists.

In castin-place construction, the single reinforced rectangular beam is uncommon. The T-

shape and L-shape beams are typical types of beam because the beams are builtmonolithically with the slab. When slab and beams are poured together, the slab on the


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beam serves as the flange of a T-beam and the supporting beam below slab is the stem or

web. For positive applied bending moment, the bottom of section produces the tension and

the slab acts as compression flange. But negative bending on a rectangular beam puts the

stem in compression and the flange is ineffective in tension. Joists consist of spaced ribs

and a top flange.

Fig. 3-2: Common shapes of concrete beam (Spiegel, 1998)

3.1.3 Reinforced Concrete Beam Design Parameters

a. Reinforcement Ratio:

The amount of steel reinforcement in concrete members should be limited. Over reinforcing

(the placement of too much reinforcement) will not allow the steel to yield before the

concrete crushes and there is a sudden failure. The reinforcement ratio in concrete beam

design is the following fraction:

=

The reinforcement ratio, ,must be less than a value determined with a concrete strain of

0.003 and tensile strain of 0.004 (minimum). When the strain in the reinforcement is 0.005

or greater, the section is tension controlled. (For smaller strains the resistance factor reduces

to 0.65 because the stress is less than the yield stress in the steel.)


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b. Maximum Reinforcement:

Based on the limiting strain of 0.005 in the steel,x(or c) = 0.375d so

= 1(0.375d) to find As-max

The values of 1are presented in the following Table 4.1:

c. Minimum Reinforcement:

Minimum reinforcement is provided even if the concrete can resist the tension, in order to

control cracking.

Minimum required reinforcement:

=

But not less than

=

200

where:

fyis the yield strength in psi

bwis the width of the web of a concrete T-Beam cross section

d = the effective depth from the top of a reinforced concrete beam to the centroid of the

tensile steel.


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d. Cover for Reinforcement:

Cover of concrete over/under the reinforcement must be provided to protect the steel from

corrosion. For indoor exposure, 1.5 inch is typical for beams and columns, 0.75 inch is

typical for slabs, and for concrete cast against soil, 3 inch minimum is required.

e. Bar Spacing:

Minimum bar spacing are specified to allow proper consolidation of concrete around the

reinforcement. The minimum spacing is the maximum of 1 in, a bar diameter, or 1.33 times

the maximum aggregate size.

f. Effective width beff:

In case of T-Beams or Gamma-Beams, the effective slab can be calculated as follows:

i.

For interior T-sections, beffis the smallest of:

L/4, bw+ 16t, or center to center of beams

ii. For exterior T-sections, beff

is the smallest of

bw + L/12, bw+ 6t, or bw+ (clear distance to next beam)

When the web is in tension the minimum reinforcement required is the same as for

rectangular sections with the web width (bw) in place of b.

When the flange is in tension (negative bending), the minimum reinforcement required is

the greater value of-

= 6

or =

where:

fyis the yield strength in psi

bwis the width of the web of a concrete T-Beam cross section


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beffis the effective flange width

3.1.4 Design Procedure

Rectangular Beam

1. Assume the depth of beam using the ACI Code reference, minimum thickness

unless consideration the deflection.

2.

Assume beam width (ratio of with and depth is about 1:2).

3. Compute self-weight of beam and design load.

4.

Compute factored load

5. Compute design moment (Mu).

6.

Compute maximum possible nominal moment for singly reinforced beam(Mn).

7. Decide reinforcement type by Comparing the design moment (Mu) and the

maximum possible moment for singly reinforced beam (Mn). If Mn is less

than Mu, the beam is designed as a doubly reinforced beam else the beam can

be designed with tension steel only.

8. Determine the moment capacity of the singly reinforced section.(concrete-steel

couple)

9. Compute the required steel area for the singly reinforced section.

10.Find necessary residual moment, subtracting the total design moment and the

moment capacity of singly reinforced section.

T-shape Beam

1. Compute the design moment (Mu).

2.

Assume the effective depth.

3. Decide the effective flange width (b) based on ACI criteria.

4. Compute the practical moment strength (Mn) assuming the total effective

flange is supporting the compression.

5. If the practical moment strength (Mn) is bigger than the design moment(Mu),

the beam will be calculated as a rectangular T-beam with the effective flange

width b. If the practical moment strength (Mn) is smaller than the design

moment (Mu), the beam will behave as a true T-shape beam.


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6. Find the approximate lever arm distance for the internal couple.

7. Compute the approximate required steel area.

8.

Design the reinforcement. 9. Check the beam width.

9. Compute the actual effective depth and analyze the beam.

3.2 Column

3.2.1 Introduction

Columns support primarily axial load but usually also some bending moments. The

combination of axial load and bending moment defines the characteristic of column and

calculation method. A column subjected to large axial force and minor moment is design

mainly for axial load and the moment has little effect. A column subjected to significant

bending moment is designed for the combined effect. The ACI Code assumes a minimal

bending moment in its design procedure, although the column is subjected to compression

force only. Compression force may cause lateral bursting because of the low-tension stress

resistance. To resist shear, ties or spirals are used as column reinforcement to confine

vertical bars. The complexity and many variables make hand calculations tedious which

makes the computer-aided design very useful.

3.2.2 Types of Columns

Reinforced concrete columns are categorized into five main types; rectangular tied column,

rectangular spiral column, round tied column, round spiral column, and columns of other

geometry (Hexagonal, L-shaped, T-Shaped, etc.).

Fig. 3-3 shows the rectangular tied and round spiral concrete column. Tied columns have

horizontal ties to enclose and hold in place longitudinal bars. Ties are commonly No. 3 or

No.4 steel bars. Tie spacing should be calculated with ACI Code.


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Fig. 3-3: Column types

The columns are also categorized into three types by the applied load types. The column

with small eccentricity, the column with large eccentricity (also called eccentric column)

and biaxial bending column. Fig 3-4 shows the different column types depending on

applied load.

Fig. 3-4: The column types depending on applied load.

Eccentricity is usually defined by location:

Interior columns usually have


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Exterior columns usually have large eccentricity

Corner column usually has biaxial eccentricity.

Fig. 3-5: Eccentric loaded conditions (Spiegel, 1998)

But eccentricity is not always decided by location of columns. Even interior columns can

be subjected by biaxial bending moment under some load conditions Fig. 3-5 shows some

examples of eccentric load conditions.

3.2.3 ACI Code Safety Provision for Column

For columns, as for all members designed according to the ACI Code, adequate safety

margins are established by applying load factors to the service loads and strength reduction

factors to the nominal strengths. Thus, for columns, Pn Puand Mn Muare the basic

safety criteria. For most members subject to axial compression or compression plus flexure

(compression controlled members the ACI Code provides basic reduction factors:

= 0.65 for tied columns

= 0.75 for spirally reinforced columns

The spread between these two values reflects the added safety furnished by the greater

toughness of spirally reinforced columns.

There are various reasons why the values for columns are lower than those for flexure or

shear (0.90 and 0.75, respectively). One is that the strength of under reinforced flexural


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members is not much affected by variations in concrete strength, since it depends primarily

on the yield strength of the steel, while the strength of axially loaded members depends

strongly on the concrete compressive strength. Because the cylinder strength of concrete

under site conditions is less closely controlled than the yield strength of mill-produced steel,

a larger occasional strength deficiency must be allowed for. This is particularly true for

columns, in which concrete, being placed from the top down in a long, narrow form, is

more subject to segregation than in horizontally cast beams. Moreover, electrical and other

conduits are frequently located in building columns; this reduces their effective cross

sections, often to an extent unknown to the designer, even though this is poor practice and

restricted by the ACI Code. Finally, the consequences of a column failure, say in a lower

story, would be more catastrophic than those of a single beam failure in the same building.

For high eccentricities, as the eccentricity increases from ebto infinity (pure bending), the

ACI Code recognizes that the member behaves progressively more like a flexural member

and less like a column. As described in Chapter 3, this is acknowledged in ACI Code 9.3.2

by providing a linear transition in from values of 0.65 and 0.75 to 0.90 as the net tensile

strain in the extreme tensile steel t increases from fy/Es(which may be taken as 0.002 for

Grade 60 reinforcement) to 0.005.

At the other extreme, for columns with very small or zero calculated eccentricities, the ACI

Code recognizes that accidental construction misalignments and other unforeseen factors

may produce actual eccentricities in excess of these small design values. Also, the concrete

strength under high, sustained axial loads may be somewhat smaller than the short-term

cylinder strength. Therefore, regardless of the magnitude of the calculated eccentricity, ACI

Code 10.3.6limits the maximum design strength to 0.80cfJP 0 for tied columns (with =

0.65) and to 0.85P0 for spirally reinforced columns (with = 0.75), where P0 is the

nominal strength of the axially loaded column with zero eccentricity.

The effects of the safety provisions of the ACI Code are shown in Fig.3-2.and represents

the actual carrying capacity, as nearly as can be predicted. The smooth curve shown

partially dashed, then solid, then dashed, represents the basic design strength obtained by

maximum design load stipulated in the ACI Code for small eccentricities, i.e., large axial

loads, as just discussed. At the other end, for large eccentricities, i.e., small axial loads, the


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Fig.3-6: ACI safety provisions superimposed on column strength interaction diagram.

ACI Code permits a linear transition of from 0.65 or 0.75, applicable for t fy/Es(or

0.002 for Grade 60 reinforcement) to 0.90 at t= 0.005. By definition, t= fy/Esat thebalanced condition. The effect of the transition in is shown at the lower right end of the

design strength curve.

3.2.4 Behavior of Axially Loaded Column

When an axial load is applied to a reinforced concrete short column, the concrete can be

considered to behave elastically up to a low stress of about3fc If the load on the column is

increased to reach its ultimate strength, the concrete will reach the maximum strength and

the steel will reach its yield strength, fy, The nominal load capacity of the column can be

written as follows:

P0= 0.85fcAn+ Astfy

Where, Anand Ast= the net concrete and total steel compressive areas, respectively.

An= AgAst


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Ag= Gross concrete area

Two different types of failure occur in columns, depending on whether ties or spirals are

used. For a tied column, the concrete fails by crushing and shearing outward, the

longitudinal steel bars fail by buckling outward between ties, and the column failure occurs

suddenly. Much like the failure of a concrete cylinder.

Fig. 3-7: Behavior of Tied and Spiral Column

A spiral column undergoes a marked yielding, followed by considerable deformation

before complete failure. The concrete in the outer shell fails and spalls off. The concrete

inside the spiral is confined and provides little strength before the initiation of column

failure, A hoop tension develops in the spiral, and for a closely spaced spiral` the steel may

yield A sudden failure is not expected Figure 3-, shows typical load deformation curves for

tied and spiral columns. Up to point a, both columns behave similarly. At point a, the

longitudinal steel bars of the column yield, and the spiral column shell spalls off, after the

factored load is reached, a tied column fails suddenly (curve b), whereas a spiral column

deforms appreciably before failure (curve c).

3.2.5 Biaxial Bending

The design of eccentrically loaded columns using the strain compatibility method of

analysis described requires that a trial column be selected. The trial column is then


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investigated to determine if it is adequate to carry any combination of Puand Muthat may

act on it should the structure be overloaded, if Puand Mu from the analysis of the structure,

when plotted on a strength interaction diagram such as Fig. 3-7, fall within the region

bounded by the curve labeled "ACI design strength." Furthermore, economical design

requires that the controlling combination of Puand Mu be close to the limit curve. If these

conditions are not met, a new column must be selected for trial. This Method permit

rectangular or square columns to be designed if bending is present about only one of the

principal axes. There are situations, by no means exceptional, in which axial compression

is accompanied by simultaneous bending about both principal axes of the section. Such is

the case, for instance, in corner columns of buildings where beams and girders frame into

the columns in the directions of both walls and transfer their end moments into the columns

in two perpendicular planes. Similar loading may occur at interior columns, particularly if

the column layout is irregular.

The situation with respect to strength of bi-axially loaded columns is shown in Fig. 3-8. Let

X and Y denote the directions of the principal axes of the cross section. In Fig. 3-8(a), the

section is shown subject to bending about the Y axis only, with load eccentricity ex

measured in the X-direction .The corresponding strength interaction curve is shown as case

(a) in the three-dimensional sketch in Fig. 3-8(d) and is drawn in the plane defined by the

axes Pnand Mny. Such a curve can be established by the usual methods for uniaxial The

situation with respect to strength of bi-axially loaded columns is shown in Fig. 3-8. Let X

and Y denote the directions of the principal axes of the cross section. In Fig. 3-8(a), the

section is shown subject to bending about the Y axis only, with load eccentricity ex

measured in the X-direction .The corresponding strength interaction curve is shown as case

(a) in the three-dimensional sketch in Fig. 3-8(d) and is drawn in the plane defined by the

axes Pn and Mny . Such a curve can be established by the usual methods for uniaxialbending. Similarly, Fig.3-8(b) shows bending about the X axis only, with eccentricity ey

measured in the Y direction. The corresponding interaction curve is shown as case (b) in

the plane of Pn and Mnxin Fig. 3-8(d). For case (c), which combines X and Y axis bending,

the orientation of the resultant eccentricity is defined by the angle [3]:

= tan

= tan


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Fig 3-8: Interaction diagram for compression plus biaxial bending:

a.

uniaxial bending about Y axis;

b. uniaxial bending about X axis;

c. biaxial bending about diagonal axis;

d. Interaction surface.

Bending for this case is about an axis defined by the angle with respect to the X axis.

The angle in Fig. 3-8(c) establishes a plane in Fig. 3-8(d), passing through the vertical Pn

axis and making an angle with the Mnxaxis, as shown. In that plane, column strength is

defined by the interaction curve labeled case (c). For other values of A, similar curves are

obtained to define a failure surface for axial load plus biaxial bending, such as shown in

Fig. 3-8(d). The surface is exactly analogous to the interaction curve for axial load plus

uniaxial bending. Any combination of Pu, Mux, and Muyfalling inside the surface can be

applied safely, but any point falling outside the surface would represent failure. Note that


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the failure surface can be described either by a set of curves defined by radial planes passing

through the Pnaxis, such as shown by case (c), or by a set of curves defined by horizontal

plane intersections, each for a constant Pn, defining load contours.

The nominal ultimate strength of a section under biaxial bending and compression is a

function of three variables Pn,Mnxand Mnywhich may also be expressed as Pn acting at

eccentricities ey=Mnx/Pnand ex= Mny/PnWith respect to the X and Y axis.

Constructing such an interaction surface for a given column would appear to be an obvious

extension of uniaxial bending analysis. In Fig. 3-8(c), for a selected value of , successive

choices of neutral axis distance c could be taken. For each, using strain compatibility and

stress-strain relations to establish bar forces and the concrete compressive resultant, thenusing the equilibrium equations to find Pn, Mnx, and Mnyone can determine a single point

on the interaction surface. Repetitive calculations, easily done by computer, then establish

sufficient points to define the surface. The triangular or trapezoidal compression zone, such

as shown in Fig. 3-8(c), is a complication, and in general the strain in each reinforcing bar

will be different, but these features can be incorporated.

The main difficulty, however, is that the neutral axis will not, in general, be perpendicular

to the resultant eccentricity, drawn from the column center to the load Pn- For each

successive choice of neutral axis, there are unique values of Pn, Mnx, and Mnyand only for

special cases will the ratio of Mn/Mnxbe such that the eccentricity is perpendicular to the

neutral axis chosen for the calculation. The result is that, for successive choices of c for any

given , the value of in Fig.3-8(c) and d will vary. Points on the failure surface established

in this way will wander up the failure surface for increasing Pn, not representing a plane

intersection, as shown for case (c) in Fig. 3-8(d).

In practice, the factored load Pu and the factored moments Muxand Muyto be resisted are

known from the frame analysis of the structure. Therefore, the actual value of

=arctan(Muy/Mux) is established, and one needs only the curve of case (c), Fig. 8.16d, to

test the adequacy of the trial column. Alternatively, simple approximate methods Bresler

load contour method and Reciprocal method are widely used.


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3.2.5.1 Bresler load contour method

The load contour method is based on representing the failure surface of Fig. 3-8(d) by a

family of curves corresponding to constant values of Pn. The general form of these curves

can be approximated by a non-dimensional interaction equation [3]:

(0

) + (0

)2 = 1

Where,

Mnx=Pney;

Mnx0=Mnx; when Mny= 0.

Mny=Pnex;

Mny0=Mny. When, Mnx= 0.

The exponents 1 and 2 are exponents depending on column dimensions, amount and

distribution of steel reinforcement, stress-strain characteristics of steel and concrete,

amount of concrete cover, and size of lateral ties or spiral.

3.2.5.2 Bresler reciprocal method

A simple, approximate design method developed by Bresler has been satisfactorily verified

by comparison with results of extensive tests and accurate calculations It is noted that the

column interaction surface in Fig. 3-9(d) can, alternatively, be plotted as a function of the

axial load Pn and eccentricities ex=Mny/Pnand ey=Mnx/Pn, as is shown in Fig. 3-9(a). The

surface S1of Fig. 3-9(a), can be transformed into an equivalent failure surface S2, as shown

in Fig.3-9(b), where ex and ey are plotted against 1/Pn rather than Pn. Thus, ex= ey= 0

corresponds to the inverse of the capacity of the column if it were concentrically loaded P0,

and this is plotted as point C. For ey= 0 and any given value of ex, there is a load Pny0

(corresponding to moment Mny0) that would result in failure. The reciprocal of this load is

plotted as point A. Similarly, for ex= 0 and any given value of ev, there is a certain load

Pnx0(corresponding to moment Mnx0) that would cause failure, the reciprocal of which is

point B. The values of Pnx0 and Pny0 are easily established, for known eccentricities of

loading applied to a given column, using the methods already established for uniaxialbending, or using design charts for uniaxial bending.


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Fig 3-9: Interaction surfaces for the reciprocal load method.

An oblique plane S2is defined by the three points: A, B, and C. This plane is used as an

approximation of the actual failure surface S2.Note that, for any point on the surface S2(for

any given combination of ex and e), there is a corresponding plane S2. Thus, the

approximation of the true failure surface S2 involves an infinite number of planes S2

determined by particular pairs of values of exand ey, i.e., by particular points A, B, and C.

The vertical ordinate 1/Pn,exact to the true failure surface will always be conservatively

estimated by the distace 1/Pn,approx to the oblique plane ABC (extended), because of the

concave upward eggshell shape of the true failure surface. In other words, 1/Pn,approx is

always greater than 1/Pn,exact.which means that Pn,approxis always less than Pn,exact.

Bresler's reciprocal load equation [3] derives from the geometry of the approximating

plane. It can be shown that

1P

= 1P0

+ 1P0

1P0


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Where,

Pn= approximate value of nominal load in biaxial bending with eccentricities exand ey

Pnyo= nominal load when only eccentricity exis present (ey= 0)

Pnxo= nominal load when only eccentricity eyis present (ex= 0)

P0= nominal load for concentrically loaded column.

Test result indicate that above equation may be inappropriate when small values of axial

load are involvef such as when Pn/P0 is in the range of 0.06 or less.For such cases the

member should be desined for flexure only.

3.2.3 Design Procedures

Short Columns with small eccentricities

1. Establish the material strength and steel area.

2.

Compute the factored axial load.

3. Compute the required gross column area.

4.

Establish the column dimensions.

5. Compute the load on the concrete area.

6. Compute the load to be carried by the steel.

7. Compute the required steel area.

8. Design the lateral reinforcing (ties or spiral).

9. Sketch the design.

Short Columns with large eccentricities

1.

Establish the material strength and steel area.

2. Compute the factored axial load (Pu) and moment (Mu).

3. Determine the eccentricity (e).

4.

Estimate the required column size based on the axial load and 10% eccentricity.

5. Compute the required gross column area.

6. Establish the column dimensions.


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7. Compute the ratio of eccentricity to column dimension perpendicular to the

bending axis.

8.

Compute the ratio of a factored axial load to gross column area.

9. Compute the ratio of distance between centroid of outer rows of bars to

thickness of the cross section, in the direction of bending.

10.

Find the required steel area using the ACI chart.

11.Design the lateral reinforcing (ties or spiral).

12.Sketch the design.

3.3 Footing

3.3.1 Introduction

The foundation of a building is the part of a structure that transmits the load to ground to

support the superstructure and it is usually the last element of a building to pass the load

into soil, rock or piles. The primary purpose of the footing is to spread the loads into

supporting materials so the footing has to be designed not to be exceeded the load capacity

of the soil or foundation bed. The footing compresses the soil and causes settlement. The

amount of settlement depends on many factors. Excessive and differential settlement can

damage structural and nonstructural elements. Therefore, it is important to avoid or reduce

differential settlement. To reduce differential settlement, it is necessary to transmit load of

the structure uniformly. Usually footings support vertical loads that should be applied

concentrically for avoid unequal settlement. Also the depth of footings is an important

factor to decide the capacity of footings. Footings must be deep enough to reach the

required soil capacity.

3.3.2 Types of Footings

The most common types of footing are strip footings under walls and single footings under

columns. Common footings can be categorized as follow:

1. Individual column footing (Fig 3-6a):This footing is also called isolated or single

footing. It can be square, rectangular or circular of uniform thickness, stepped, or


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sloped top. This is one of the most economical types of footing. The most common

type of individual column footing is square of rectangular with uniform thickness.

2. Wall footing (Fig3-6b):Wall footings support structural or nonstructural walls.

This footing has limited width and a continuous length under the wall.

3. Combined footing (Fig3-6e):They usually support two or three columns not in a

row and may be either rectangular or trapezoidal in shape depending on column. If

a strap joins two isolated footings, the footing is called a cantilever footing.

Fig 3-10: Footing types (Spiegel, 1998)

4. Mat foundation (Fig3-6f): Mats are large continuous footings, usually placed

under the entire building area to support all columns and walls. Mats are used when

the soil-bearing capacity is low, column loads are heavy, single footings cannot be


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used, piles are not used, or differential settlement must be reduced through the entire

footing system.

5. Pile footing (Fig3-6g): Pile footings are thick pads used to tie a group of piles

together and to support and transmit column loads to the piles.

3.3.3 Design Consideration

Footing must be designed to carry the column loads and transmit them to the soil safety

while satisfying code limitation. The design procedure must take the following strength

requirements into consideration:

The area of the footing based on the allowable bearing soil capacity

Two-way shear or punching shear

One-way shear

Bending moment and steel reinforcement required

Dowel requirements

Development length of bars


Individual column footing

1. Compute the factored loads.

2. Assume the total footing thickness.

3. Compute the footing self-weight, the weight of earth on top of the footing.

4.

Compute the effective allowable soil pressure for superimposed service loads.

5. Compute required footing area.

6. Compute the factored soil pressure from superimposed loads.

7. Assume the effective depth for the footing.

8. Check the punching shear and beam shear.

9.

Compute the design moment at the critical section.

10.Compute the required steel area.

11.Check the ACI Code minimum reinforcement requirement.

12.Check the development length.

13.Check the concrete bearing strength at the base of the column


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3.4 Stair

3.4.1 Introduction

Staircase is an important component of a building providing access to different floors and

roof of the building. It consists of a flight of steps (stairs) and one or more intermediate

landing slabs between the floor levels. Different types of staircases can be made by

arranging stairs and landing slabs. Staircase, thus, is a structure enclosing a stair.

3.4.2 Types of Staircases

There are different types of Stairs, which depend mainly on the type and function of the

building and on the architectural requirements. Some of the common types of staircases

based on geometrical configurations:

Fig 3-11: Types of Staircases


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(a) Single flight staircase (Fig 3-7 a)

(b) Two flight staircase (Fig 3-7 b)

(c)

Open-well staircase (Fig 3-7 c)

(d) Spiral staircase (Fig 3-7 d)

(e) Helical staircase (Fig 3-7 e)

Architectural considerations involving aesthetics, structural feasibility and functional

requirements are the major aspects to select a particular type of the staircase. Other

influencing parameters of the selection are lighting, ventilation, comfort, accessibility,

space etc.

Fig 3-12: Transversely Supported Stairs

For purpose of design, stairs are classified into two types; transversely, and longitudinally

supported.

A.

Transversely supported (transverse to the direction of movement):Transversely supported stairs include:


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a. Simply supported steps supported by two walls or beams or a combination of

both.

b.

Steps cantilevering from a wall or a beam.

c. Stairs cantilevering from a central spine beam.

B.

Longitudinally supported (in the direction of movement):

These stairs span between supports at the top and bottom of a flight and

unsupported at the sides. Longitudinally supported stairs may be supported in any

of the following manners:

Fig 3-13: Longitudinally Supported Stairs

a. Beams or walls at the outside edges of the landings.

b. Internal beams at the ends of the flight in addition to beams or walls at the

outside edges of the landings.

c. Landings which are supported by beams or walls running in the longitudinal

direction. d. A combination of (a) or (b), and (c).

Stairs with quarter landings associated with open-well stairs.

3.4.3 Components of Stairs

The definitions of some technical terms, which are used in connection with design of stairs,

are given.

a. Tread or Going:horizontal upper portion of a step.

b.

Riser:vertical portion of a step.


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c. Rise:vertical distance between two consecutive treads.

d. Flight:a series of steps provided between two landings.

e. Landing:a horizontal slab provided between two flights.

f. Waist:the least thickness of a stair slab.

g. Winder:radiating or angular tapering steps. h. Soffit: the bottom surface of a stair

slab.

h. Nosing:the intersection of the tread and the riser.

i. Headroom: the vertical distance from a line connecting the nosings of all treads

and the soffit above.

Fig. 3-14: Stairs main Components


Design procedure foe single flight Stair

1. First calculate the loads.

2. Then calculate maximum moment.

3.

Check the depth. If ok then go to next steps otherwise change the section.

4. Calculate reinforcement.

5. Check for bond and development length.

6.

Calculate reinforcement of first flight and spacing.

7. Sketch reinforcement details.


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Chapter 4

Reinforced Concrete Structure Designer (RCSD)

4.1 General

RCSD is a computer program for reinforced concrete structure design according to the ACI

Code. It includes beam, column, stair and footing design. Its main purpose is to help

architecture students who do not have enough structural background but need a structural

calculation to design their building. So this program is developed with easy to use interface

based on ACI Code procedures. RCSD provides step by step calculations and is composed

of separate modules for beam, stair, column and footing design. The step by step design

method is considered one of the best methods to help beginning users, like civil engineering

students. For example, users do not need to input the all required data at once. The program

asks the minimum required data and provides default-input data. The user can use the

default data or select other data.

The modular RCSD program structure also has the advantage that each module is

executable separately and the user can add other modules. RCSD is programmed using

Microsoft Visual Studio 2015. Visual Studio is much easier to learn than other languages

and provides good graphic user interface (GUI). Each module is composed of multiple

pages that have been organized using Microsoft Tabbed Control Dialog Component. Each

module is executed step by step along the tabs. Tabs are divided into frames for better

organization of different category of input and output data.

RCSD is a computer program for reinforced concrete structure design according to the ACI

Code. It includes beam, column, stair and footing design. Its main purpose is to help

architecture students who do not have enough structural background but need a structural

calculation to design their building. So this program is developed with easy to use interface

based on ACI Code procedures. RCSD provides step by step calculations and is composed

of separate modules for beam, stair, column and footing design. The step by step design

method is considered one of the best methods to help beginning users, like civil engineering

students. For example, users do not need to input the all required data at once. The program

asks the minimum required data and provides default-input data. The user can use the

default data or select other data.


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The modular RCSD program structure also has the advantage that each module is

executable separately and the user can add other modules. RCSD is programmed using

Microsoft Visual Studio 2015. Visual Studio is much easier to learn than other languages

and provides good graphic user interface (GUI). Each module is composed of multiple

pages that have been organized using Microsoft Tabbed Control Dialog Component. Each

module is executed step by step along the tabs. Tabs are divided into frames for better

organization of different category of input and output data.

4.2Beam Module

4.2.1 Introduction

RCSD provides single and double reinforced beam design method in one module in both

WSD and USD method.

4.2.2 Rectangular Beam Design Module

The beam design module has INPUT, RESULT and REINFORCEMENT DETAIL. The

INPUT tab contain Material Strength, Moment, Shear and Dimension.

Fig. 4.1 Beam Design Module


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4.2.3 T Beam Design Module

The beam design module has INPUT, RESULT and REINFORCEMENT DETAIL. The

INPUT tab contain Material Strength, Moment, Shear and Dimension.

Fig. 4.2: T Beam Design Module

4.3 Column Module

4.3.1 Introduction

Column is classified into two types spiral column and tied Column. The Tied Column canbe classified into two types Uniaxial and Biaxial Bending. This program provides all three

types of column design. The design of column carrying small eccentricity is calculated by

simple method computed by the ACI method for axial load with small eccentricity. If axial

load is applied with eccentricity the column is sunjected to moment and more bending

strength.


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4.3.2 Column Design Module

The column design module has contains three tabs tied column for uniaxial, biaxial bending

and spiral column. The tied portion designs for biaxial bending, uniaxial bending, axial

load. The spiral design portion for axial load as it is weak in bending. Each design tab

contains INPUT, RESULT and REINFORCEMENT DETAILS.

Fig 4.3: Column design module.

4.4 Footing Module

4.4.1 Introduction

This program provides design module foe individual column footing. The thickness of the

footing is calculated from two-way and one way shear check and the thickness is checked

with the bending moment at the face of the column.


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4.4.2 Footing Design module

Individual column footing module has INPUT, OUTPUT REINFORCEMENT DETAILS

tabs. The INPUT tab contains load, material, column size and soil condition, based on this

data the program calculates footing size and thickness to resist shear.

Fig 4.4; Footing Design Module

4.5 Stair module

4.5.1 Introduction

In stair design module some material property and loading data has to input and it gives the

required section for design reinforcement.

4.5.2 Stair Design Module

Stair module has INPUT, OUTPUT and REINFORCEMENT DIAGRAM tabs. The

INPUT tab requires dimension, material strength, Load. Based on the input data this

program calculates possible section for reinforcement


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Fig. 4.5: Stair Design Module


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Chaptre-5

Conclusion and Recommendation

5.1 Conclusion

This simplified reinforced concrete structure design program for civil engineering students,

based on the American Concrete Institute Code (ACI 318), is expected to help engineering

students to design sound concrete structures. The ultimate goal of this program is to assist

students in the reinforced concrete structures design and guide them to design structurally

safe buildings. ACI Code is the most common code of Reinforce Concrete structure design,

but it is difficult to use for beginner users. This program will help engin

reinforced concrete design assistant vs 2015

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