Short Notes: Design of Steel Structures | Short Notes for Civil Engineering - Civil Engineering (CE) PDF Download

 Page 1


 
 
   
 
DESIGN OF STEEL STRUCTURES
 
INTRODUCTION: Advantages and Disadvantages of Steel structures, Loads and Load 
combinations, Design considerations, Limit State Method (LSM) of design, Failure criteria for 
steel, Codes, Specifications and section classification.        
             
Why Structural Design Courses?  
Anyone managing the construction process needs a basic understanding of the engineer’s 
environment and the basic understanding of how a structure behaves.  Constructors must be able 
to address a number of technical questions at the project site including structural issues that 
sometimes are not addressed by the design professionals.  Since the safety of construction 
workers as well as the strength and stability of structures during the construction phase is of 
paramount importance, construction mangers need this knowledge.  
Structural Design  
• Definition: Determination of overall proportions and dimensions of the supporting 
framework and the selection of individual members.  
• Responsibility:The structural engineer, within the constraints imposed by the architect 
(number of stories, floor plan,..) is responsible for structural design 
• Safety (the structure doesn’t fall down)  
• Serviceability (how well the structure performs in term of appearance and deflection)  
• Economy (an efficient use of materials and labor)  
Alternatives  
• Several alternative designs should be prepared and their costs compared 
Page 2


 
 
   
 
DESIGN OF STEEL STRUCTURES
 
INTRODUCTION: Advantages and Disadvantages of Steel structures, Loads and Load 
combinations, Design considerations, Limit State Method (LSM) of design, Failure criteria for 
steel, Codes, Specifications and section classification.        
             
Why Structural Design Courses?  
Anyone managing the construction process needs a basic understanding of the engineer’s 
environment and the basic understanding of how a structure behaves.  Constructors must be able 
to address a number of technical questions at the project site including structural issues that 
sometimes are not addressed by the design professionals.  Since the safety of construction 
workers as well as the strength and stability of structures during the construction phase is of 
paramount importance, construction mangers need this knowledge.  
Structural Design  
• Definition: Determination of overall proportions and dimensions of the supporting 
framework and the selection of individual members.  
• Responsibility:The structural engineer, within the constraints imposed by the architect 
(number of stories, floor plan,..) is responsible for structural design 
• Safety (the structure doesn’t fall down)  
• Serviceability (how well the structure performs in term of appearance and deflection)  
• Economy (an efficient use of materials and labor)  
Alternatives  
• Several alternative designs should be prepared and their costs compared 
 
 
 
 
Types of Load  
• Dead Loads (permanent; including self-weight, floor covering, suspended ceiling, 
partitions,..)  
• Live Loads (not permanent; the location is not fixed; including furniture, equipment, and 
occupants of buildings)  
• Wind Load (exerts a pressure or suction on the exterior of a building)  
Types of Load Continued  
• Earthquake Loads (the effects of ground motion are simulated by a system of horizontal 
forces)  
• Snow Load (varies with geographical location and drift)  
• Other Loads (hydrostatic pressure, soil pressure)  
 Types of Load Continued  
• If the load is applied suddenly, the effects of IMPACT must be accounted for.  
• If the load is applied and removed many times over the life of the structure, FATIGUE 
stress must be accounted for  
 Design Specifications  
• Provide guidance for the design of structural members and their connections.  
• They have no legal standing on their own, but they can easily be adopted, by reference, as 
part of a building code.  
• American Concrete Institute (ACI 318-99) Building Code Requirements for Structural 
Concrete  
• National Design Specifications for Wood Construction by American Forest and Paper 
Association.  
 Structural Steel  
• Steel is an alloy of primarily iron, carbon (1 to 2%) and small amount of other 
components (manganese, nickel, …)  
• Carbon contributes to strength but reduces ductility.  
 Steel Properties  
• The important characteristics of steel for design purposes are:  
o yield stress (F
y
)  
o ultimate stress (F
u
)  
o modulus of elasticity (E)  
o percent elongation (e)  
o coefficient of thermal expansion (a)  
Page 3


 
 
   
 
DESIGN OF STEEL STRUCTURES
 
INTRODUCTION: Advantages and Disadvantages of Steel structures, Loads and Load 
combinations, Design considerations, Limit State Method (LSM) of design, Failure criteria for 
steel, Codes, Specifications and section classification.        
             
Why Structural Design Courses?  
Anyone managing the construction process needs a basic understanding of the engineer’s 
environment and the basic understanding of how a structure behaves.  Constructors must be able 
to address a number of technical questions at the project site including structural issues that 
sometimes are not addressed by the design professionals.  Since the safety of construction 
workers as well as the strength and stability of structures during the construction phase is of 
paramount importance, construction mangers need this knowledge.  
Structural Design  
• Definition: Determination of overall proportions and dimensions of the supporting 
framework and the selection of individual members.  
• Responsibility:The structural engineer, within the constraints imposed by the architect 
(number of stories, floor plan,..) is responsible for structural design 
• Safety (the structure doesn’t fall down)  
• Serviceability (how well the structure performs in term of appearance and deflection)  
• Economy (an efficient use of materials and labor)  
Alternatives  
• Several alternative designs should be prepared and their costs compared 
 
 
 
 
Types of Load  
• Dead Loads (permanent; including self-weight, floor covering, suspended ceiling, 
partitions,..)  
• Live Loads (not permanent; the location is not fixed; including furniture, equipment, and 
occupants of buildings)  
• Wind Load (exerts a pressure or suction on the exterior of a building)  
Types of Load Continued  
• Earthquake Loads (the effects of ground motion are simulated by a system of horizontal 
forces)  
• Snow Load (varies with geographical location and drift)  
• Other Loads (hydrostatic pressure, soil pressure)  
 Types of Load Continued  
• If the load is applied suddenly, the effects of IMPACT must be accounted for.  
• If the load is applied and removed many times over the life of the structure, FATIGUE 
stress must be accounted for  
 Design Specifications  
• Provide guidance for the design of structural members and their connections.  
• They have no legal standing on their own, but they can easily be adopted, by reference, as 
part of a building code.  
• American Concrete Institute (ACI 318-99) Building Code Requirements for Structural 
Concrete  
• National Design Specifications for Wood Construction by American Forest and Paper 
Association.  
 Structural Steel  
• Steel is an alloy of primarily iron, carbon (1 to 2%) and small amount of other 
components (manganese, nickel, …)  
• Carbon contributes to strength but reduces ductility.  
 Steel Properties  
• The important characteristics of steel for design purposes are:  
o yield stress (F
y
)  
o ultimate stress (F
u
)  
o modulus of elasticity (E)  
o percent elongation (e)  
o coefficient of thermal expansion (a)  
 
 
  
 
Standard Cross-Sectional Shapes 
 Refer steel table 
Design Philosophies  
• Allowable Stress Design Method (ASD)  
• Load and Resistance Factor Design (LRFD)  
 A member is selected such that the max stress due to working loads does not exceed an 
allowable stress.  
• It is also called elastic design or working stress design.  
o allowable stress=yield stress/factor of safety  
o actual stress ? allowable stress  
 LRFD –Load and Resistance Factor Design 
• A member is selected such that its factored strength is more than the factored loads.  
o S(loads x L factors) ? resistance x R factor  
• Each load effect (DL, LL, ..)has a different load factor which its value depends on the 
combination of loads under consideration.  
Load Factors  
• The values are based on extensive statistical studies  
o DL only    1.4D  
o DL+LL+SL (LL domin.) 1.2D+1.6L+0.5S  
o DL+LL+SL (SL domin.) 1.2D+0.5L+1.6S  
o In each combination, one of the effects is considered to be at its “lifetime” max 
value and the others at their “arbitrary point in time “ values.  
 Resistance Factor  
• The resistance factors range in value from 0.75 to 1.0 depending on the type of resistance 
(tension, bending, compression, ..)  
• These factors account for uncertainties in material properties, design theory, and 
fabrication and construction practices.  
 History  
• ASD has been the primary method used for steel design since the first AISC 
specifications was issued in 1923.  
• In 1986, AISC issued the first specification for LRFD.  
• The trend today is toward LRFD method, but ASD is still in use. 
Page 4


 
 
   
 
DESIGN OF STEEL STRUCTURES
 
INTRODUCTION: Advantages and Disadvantages of Steel structures, Loads and Load 
combinations, Design considerations, Limit State Method (LSM) of design, Failure criteria for 
steel, Codes, Specifications and section classification.        
             
Why Structural Design Courses?  
Anyone managing the construction process needs a basic understanding of the engineer’s 
environment and the basic understanding of how a structure behaves.  Constructors must be able 
to address a number of technical questions at the project site including structural issues that 
sometimes are not addressed by the design professionals.  Since the safety of construction 
workers as well as the strength and stability of structures during the construction phase is of 
paramount importance, construction mangers need this knowledge.  
Structural Design  
• Definition: Determination of overall proportions and dimensions of the supporting 
framework and the selection of individual members.  
• Responsibility:The structural engineer, within the constraints imposed by the architect 
(number of stories, floor plan,..) is responsible for structural design 
• Safety (the structure doesn’t fall down)  
• Serviceability (how well the structure performs in term of appearance and deflection)  
• Economy (an efficient use of materials and labor)  
Alternatives  
• Several alternative designs should be prepared and their costs compared 
 
 
 
 
Types of Load  
• Dead Loads (permanent; including self-weight, floor covering, suspended ceiling, 
partitions,..)  
• Live Loads (not permanent; the location is not fixed; including furniture, equipment, and 
occupants of buildings)  
• Wind Load (exerts a pressure or suction on the exterior of a building)  
Types of Load Continued  
• Earthquake Loads (the effects of ground motion are simulated by a system of horizontal 
forces)  
• Snow Load (varies with geographical location and drift)  
• Other Loads (hydrostatic pressure, soil pressure)  
 Types of Load Continued  
• If the load is applied suddenly, the effects of IMPACT must be accounted for.  
• If the load is applied and removed many times over the life of the structure, FATIGUE 
stress must be accounted for  
 Design Specifications  
• Provide guidance for the design of structural members and their connections.  
• They have no legal standing on their own, but they can easily be adopted, by reference, as 
part of a building code.  
• American Concrete Institute (ACI 318-99) Building Code Requirements for Structural 
Concrete  
• National Design Specifications for Wood Construction by American Forest and Paper 
Association.  
 Structural Steel  
• Steel is an alloy of primarily iron, carbon (1 to 2%) and small amount of other 
components (manganese, nickel, …)  
• Carbon contributes to strength but reduces ductility.  
 Steel Properties  
• The important characteristics of steel for design purposes are:  
o yield stress (F
y
)  
o ultimate stress (F
u
)  
o modulus of elasticity (E)  
o percent elongation (e)  
o coefficient of thermal expansion (a)  
 
 
  
 
Standard Cross-Sectional Shapes 
 Refer steel table 
Design Philosophies  
• Allowable Stress Design Method (ASD)  
• Load and Resistance Factor Design (LRFD)  
 A member is selected such that the max stress due to working loads does not exceed an 
allowable stress.  
• It is also called elastic design or working stress design.  
o allowable stress=yield stress/factor of safety  
o actual stress ? allowable stress  
 LRFD –Load and Resistance Factor Design 
• A member is selected such that its factored strength is more than the factored loads.  
o S(loads x L factors) ? resistance x R factor  
• Each load effect (DL, LL, ..)has a different load factor which its value depends on the 
combination of loads under consideration.  
Load Factors  
• The values are based on extensive statistical studies  
o DL only    1.4D  
o DL+LL+SL (LL domin.) 1.2D+1.6L+0.5S  
o DL+LL+SL (SL domin.) 1.2D+0.5L+1.6S  
o In each combination, one of the effects is considered to be at its “lifetime” max 
value and the others at their “arbitrary point in time “ values.  
 Resistance Factor  
• The resistance factors range in value from 0.75 to 1.0 depending on the type of resistance 
(tension, bending, compression, ..)  
• These factors account for uncertainties in material properties, design theory, and 
fabrication and construction practices.  
 History  
• ASD has been the primary method used for steel design since the first AISC 
specifications was issued in 1923.  
• In 1986, AISC issued the first specification for LRFD.  
• The trend today is toward LRFD method, but ASD is still in use. 
 
 
 
 
Advantages of LRFD  
• It provides a more uniform reliability in all structures subjected to many types of loading 
conditions. It does not treat DL and LL as equivalent, thereby leading to a more rational 
approach.  
• It provides better economy as the DL make up a greater percentage on a given 
structure. Because DLs are less variable by nature than live loads, a lower load 
factor is used.  
This may lead to a reduction in member size and therefore better economy 
 
STEEL AS A STRUCTURAL MATERIAL 
1.1General 
Structural steel is a material used for steel construction, which is formed with a specific 
shape following certain standards of chemical composition and strength. They can also be 
defined as hot rolled products, with a cross section of special form like angles, channels and 
beams/joints. There has been an increasing demand for structural steel for construction purposes 
in the United States and India.   
 
 
 
 
 
 
 
 
Measures are been taken by the structural steel authority for ready availability of structural 
steel on time for the various projects. The people at every level are working hard to realize the 
purpose of producing steel on time, like, service centers, producers, fabricators and erectors 
along with the general contractors, engineers and architects are all working hand in hand. Steel 
has always been more preferred to concrete because steel offers better tension and compression 
thus resulting in lighter construction. Usually structural steel uses three dimensional trusses 
hence making it larger than its concrete counterpart. There are different new techniques which 
Page 5


 
 
   
 
DESIGN OF STEEL STRUCTURES
 
INTRODUCTION: Advantages and Disadvantages of Steel structures, Loads and Load 
combinations, Design considerations, Limit State Method (LSM) of design, Failure criteria for 
steel, Codes, Specifications and section classification.        
             
Why Structural Design Courses?  
Anyone managing the construction process needs a basic understanding of the engineer’s 
environment and the basic understanding of how a structure behaves.  Constructors must be able 
to address a number of technical questions at the project site including structural issues that 
sometimes are not addressed by the design professionals.  Since the safety of construction 
workers as well as the strength and stability of structures during the construction phase is of 
paramount importance, construction mangers need this knowledge.  
Structural Design  
• Definition: Determination of overall proportions and dimensions of the supporting 
framework and the selection of individual members.  
• Responsibility:The structural engineer, within the constraints imposed by the architect 
(number of stories, floor plan,..) is responsible for structural design 
• Safety (the structure doesn’t fall down)  
• Serviceability (how well the structure performs in term of appearance and deflection)  
• Economy (an efficient use of materials and labor)  
Alternatives  
• Several alternative designs should be prepared and their costs compared 
 
 
 
 
Types of Load  
• Dead Loads (permanent; including self-weight, floor covering, suspended ceiling, 
partitions,..)  
• Live Loads (not permanent; the location is not fixed; including furniture, equipment, and 
occupants of buildings)  
• Wind Load (exerts a pressure or suction on the exterior of a building)  
Types of Load Continued  
• Earthquake Loads (the effects of ground motion are simulated by a system of horizontal 
forces)  
• Snow Load (varies with geographical location and drift)  
• Other Loads (hydrostatic pressure, soil pressure)  
 Types of Load Continued  
• If the load is applied suddenly, the effects of IMPACT must be accounted for.  
• If the load is applied and removed many times over the life of the structure, FATIGUE 
stress must be accounted for  
 Design Specifications  
• Provide guidance for the design of structural members and their connections.  
• They have no legal standing on their own, but they can easily be adopted, by reference, as 
part of a building code.  
• American Concrete Institute (ACI 318-99) Building Code Requirements for Structural 
Concrete  
• National Design Specifications for Wood Construction by American Forest and Paper 
Association.  
 Structural Steel  
• Steel is an alloy of primarily iron, carbon (1 to 2%) and small amount of other 
components (manganese, nickel, …)  
• Carbon contributes to strength but reduces ductility.  
 Steel Properties  
• The important characteristics of steel for design purposes are:  
o yield stress (F
y
)  
o ultimate stress (F
u
)  
o modulus of elasticity (E)  
o percent elongation (e)  
o coefficient of thermal expansion (a)  
 
 
  
 
Standard Cross-Sectional Shapes 
 Refer steel table 
Design Philosophies  
• Allowable Stress Design Method (ASD)  
• Load and Resistance Factor Design (LRFD)  
 A member is selected such that the max stress due to working loads does not exceed an 
allowable stress.  
• It is also called elastic design or working stress design.  
o allowable stress=yield stress/factor of safety  
o actual stress ? allowable stress  
 LRFD –Load and Resistance Factor Design 
• A member is selected such that its factored strength is more than the factored loads.  
o S(loads x L factors) ? resistance x R factor  
• Each load effect (DL, LL, ..)has a different load factor which its value depends on the 
combination of loads under consideration.  
Load Factors  
• The values are based on extensive statistical studies  
o DL only    1.4D  
o DL+LL+SL (LL domin.) 1.2D+1.6L+0.5S  
o DL+LL+SL (SL domin.) 1.2D+0.5L+1.6S  
o In each combination, one of the effects is considered to be at its “lifetime” max 
value and the others at their “arbitrary point in time “ values.  
 Resistance Factor  
• The resistance factors range in value from 0.75 to 1.0 depending on the type of resistance 
(tension, bending, compression, ..)  
• These factors account for uncertainties in material properties, design theory, and 
fabrication and construction practices.  
 History  
• ASD has been the primary method used for steel design since the first AISC 
specifications was issued in 1923.  
• In 1986, AISC issued the first specification for LRFD.  
• The trend today is toward LRFD method, but ASD is still in use. 
 
 
 
 
Advantages of LRFD  
• It provides a more uniform reliability in all structures subjected to many types of loading 
conditions. It does not treat DL and LL as equivalent, thereby leading to a more rational 
approach.  
• It provides better economy as the DL make up a greater percentage on a given 
structure. Because DLs are less variable by nature than live loads, a lower load 
factor is used.  
This may lead to a reduction in member size and therefore better economy 
 
STEEL AS A STRUCTURAL MATERIAL 
1.1General 
Structural steel is a material used for steel construction, which is formed with a specific 
shape following certain standards of chemical composition and strength. They can also be 
defined as hot rolled products, with a cross section of special form like angles, channels and 
beams/joints. There has been an increasing demand for structural steel for construction purposes 
in the United States and India.   
 
 
 
 
 
 
 
 
Measures are been taken by the structural steel authority for ready availability of structural 
steel on time for the various projects. The people at every level are working hard to realize the 
purpose of producing steel on time, like, service centers, producers, fabricators and erectors 
along with the general contractors, engineers and architects are all working hand in hand. Steel 
has always been more preferred to concrete because steel offers better tension and compression 
thus resulting in lighter construction. Usually structural steel uses three dimensional trusses 
hence making it larger than its concrete counterpart. There are different new techniques which 
 
 
  
 
enable the production of a wide range of structures and shapes, the procedures being the 
following: 
• High-precision stress analysis 
• Computerized stress analysis 
• Innovative jointing 
The structural steel all over the world pre-dominates the construction scenario. This material 
has been exhaustively used in various constructions all over the world because of its various 
specific characteristics that are very much ideally suited for construction. Structural steel is 
durable and can be well molded to give the desired shape to give an ultimate look to the structure 
that has been constructed. There is a mention of The Super dome situated in the United States 
and The Fukuoka Dome of Japan; both speak the unique language of the unique capabilities of 
the structural steel.  
 
1.2 Types of structural steel:  
Various types of structural steel sections and their technical specifications are as follows:  
• Beams 
• Channels 
• Angles 
• Flats