SUBJECT:

STEEL STRUCTURES

• Course
Dr. Muhammad Habib
Instructor:
 BOOKS
• AISC 2005 Specifications
(Free on web site www. AISC.org).
• Steel Structures, 2nd Ed. by Zahid and Ashraf.
• LRFD Steel Design Aids, 3rd Ed. by Zahid
(Always bring in the practical classes).
• Steel Structures by John. E. Lothers.
• Any other book on Steel Structures, like Bowles,
McCormac, Salmon & Johnson, Gaylord &
Gaylord.
Introduction To Steel Structures
• Steel structures are assembly of structural
steel shapes joined together by means of
riveted / bolted or welded connections.
• Selection of a section out of those available
in the market.
• Concrete structures are easily joined
together by monolithic construction. But
special methods are required to join
individual members for steel structures.
Hot Rolled Sections
Cold Formed Sections
Steel Diamond Plate
Connections
Earthquake Resistant Connection
Beam Column Connections
• Steel construction is being used for almost
every type of structure including multi-
storey buildings, bridges, industrial
buildings, towers, etc.
• There are two main categories of steel
structures:-

- Framework or skeletal systems.

- Shell systems.
 Framework Or Skeletal Systems
• The main load carrying elements in this type are
one-dimensional or line elements (such as beams,
columns, etc.) forming two-dimensional or three-
dimensional frames.

• Examples are:-

- The frameworks of industrial buildings with

their internal members such as crane girders,
platforms, etc.
- Highway and railways large span bridges.
- Multi-storey buildings, large halls, domes
etc.
- Towers, poles, structural components of
hydraulic works.
- All other trusses and rigidly connected
frame structures.
 Shell Systems
• The main load carrying elements in this
category of structures are plates and sheets
besides some skeletal members.
• Examples are:-
- Gas tanks for the storage and
distribution of gases.
- Tanks and reservoirs for the storage of
liquids.
- Bins and bunkers for the storage of loose
material.
- Special structures such as blast furnaces,
air heaters, etc.
- Large diameter pipes.
- All other plate and shell structures.
Freedom of Expression
Creativity
Creativity
Easy Extension
Easy Fixing of Facade
Easy and Efficient Fabrication
Express Function
Large Span
Large Span
No Limit of Architectural Design
No Limits of Architectural Design
Recycling is possible
Slender columns, more space
Transparency
Visible Connections
Visible Connections
Weather Independent Construction
 Merits Of Steel Construction
1. Reliability
2. Industrial Behavior
3. Lesser ConstructionTime
4. High Strength And Light Weight Nature
5. Uniformity, Durability And Performance
6. Elasticity
7. Ductility And Warning Before Failure
8. Additions To ExistingStructures
9. Possible Reuse
10. Scrap Value
 Merits Of Steel Construction
11. Water-Tight And Air-Tight
Constructions
12. Long Span Construction
13. Long Span Construction
 Demerits Of Steel Construction
1. High Maintenance Costs And Corrosion
2. High Fireproofing Costs
3. Susceptibility To Buckling

4. High Initial Cost / Less Availability

5. Aesthetics
 Specifications
• The adequacy of a structural member is
determined by a set of design rules, called
specifications.
• These include formulas that guide the designer in
checking strength, stiffness, proportions and other
criteria that may govern the acceptability of the
member.
• There are a variety of specifications that have been
developed for both materials and structures.
• Each is based on years of research and experience
gained through actual structural usage.
 • Following specifications will be used in this
class quite often:
1- AISC: American Institute of Steel Construction.

2- AISI: American Iron and Steel Institute

3- AWS: American Welding Society.

4- AASHTO: American Association of State Highway and

Transportation Officials.
5- AREA: American Railway Engineering Association.

6- ASTM: American Society for Testing and Materials.

7- ASCE: American Society of Civil Engineers

 TYPES OF LOAD
1. Dead Load
• It almost retains its magnitude and point of
application throughout the life of the structure
and is denoted by D.
• This load is usually the self weight of the
structure (not only this member but all other
members resting on it).
• This is estimated by multiplying volume of a
member with the standard density of the material
of construction.
• This load constitutes a bigger part of the total
load on a structure.
2. Live Load
• The load due to persons occupying the
structure and their belongings, denoted by L.
• Its magnitude and point of application
changes with time.
• In case of bridges, live load consists of weight
of vehicles moving over the bridge.
• Typical values for common occupancy types
are given in next slide.
Occupancy or Use Live Load
(kg/m2)
Private apartments, school class rooms 200
Offices 250
Fixed-seats, assembly halls, library reading 300
rooms
Corridors 400
Movable seats assembly hall 500
Wholesale stores, light storage warehouses 600
Library stack rooms 750
Heavy manufacturing, heavy storage 1200
warehouses, sidewalks and driveways subject to
trucking
3.Self Load
• This is type of dead load, which is due to
self weight of the member to be designed.
• For design, a reasonable value of self load
depending on past experience is assumed in
the start which is then compared with the
actual self weight at the end.
• Corrections in design are made if necessary.
• Other types of loads are wind load, earthquake
loads, water ad earth retaining loads and
temperature loads, etc.

4. Imposed / Superimposed Load

• This term is used for all external loads, leaving the
self weight, acting on the member to be designed.
• This includes live load, wind load, earthquake
load, etc. Part of dead load may also act as
imposed load.
5. Service Loads
• The maximum intensity of load expected during
the life of the structure depending upon a certain
probability of occurrence is called service load.
• No additional factor of safety or overload factor is
included in the service loads.

6. Factored Loads
• Service loads increased by some factor of safety or
overload factor are called factored loads.
 Mechanism Of Load Transfer
• The gravity load passes from top to bottom
through all the members of the structure until it
reaches the underneath soil.
• The load acts at the floor finish, goes to the
underneath slab and transfers to the beams and
walls.
• This is then accumulated in the columns, moves to
the foundations and then finally dissipates in the
soil.
• The terms member and support are defined
relative to each other.
• There are no separate supports in the
structure as is normally seen in the
structural analysis books.
• For the roof slab, beams and walls are
supports.
• For the beams, columns are acting like
supports, and for the columns, foundations
are acting as supports.
• Similarly, the underneath soil acts as
support for the foundations.
• This load path is only in one direction.
• The load of roof slab may act on the beams,
columns and foundations, but the load of
column is not acting on the beams.
• Similarly, the load of foundation can not act
on the columns.
What is design of structures?

Unknown cross-sectional details are to

be determined
Span lengths and basic dimensions are
taken from architectural drawings
Expected loads are determined using
handbooks and codes according to
occupancy of the structure

1
Continued from previous slide

Types of construction materials to be used

and their properties are decided
Sound knowledge of the principles of
statics, dynamics, mechanics of materials
and structural analysis is required for good
design
Construction practices, availability of
materials, labor and machinery, etc. are
also to be considered
Experience and intuitive feeling for the
structural behavior are also important
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Capacity Analysis of
Structures
Carried out to check already made design or
construction
Material properties, spans and cross-sectional
details are known
Load carrying capacity of members or
structure is evaluated
Capacity is compared with the applied loads
If applied load is lesser than capacity of
member, design is safe

3
Basic Design Equation
Used for design and capacity analysis in
all types of design and analysis methods
maximum internal
Load effects factor of resistance offered
x =
safety by material of
structure
Load effects may be axial force, shear
force, bending moment and torque

4
Continued from previous slide

Corresponding to each applied load action,

there is a resistive force such as resisting axial
force, resisting shear and resisting moment
In design, applied actions and material
resistances are equated to each other with
some FOS
A bending moment of Pl /4 may never be
obtained in a simply supported beam subjected
to a central point load if the member is not
sufficiently strong

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Factor of Safety

Value is always greater than one

Brings the structure from state of collapse
to a usable service state to avoid
excessive deformations, cracking, and
buckling, etc.
Covers uncertainties in loads within limits
Covers uncertainties in material strengths
up to certain extent

6
Continued from previous slide

Covers, in part, poor workmanship

Covers unexpected behavior in theory due to
simplifying assumptions or limited knowledge
Reduces the effect of natural disasters
Fabrication and erection stresses are taken
care of
Presence of residual stresses and local stress
concentrations are safely considered

7
 Comparison of FOS
FOS in ASD is about 1.67
FOS for LL in original LRFD is 1.7/0.9 or 1.889

FOS for DL in original LRFD is 1.4/0.9 or 1.556

FOS for LL in latest LRFD is 1.6/0.9 or 1.778

FOS for DL in latest LRFD is 1.2/0.9 or 1.333

Average FOS in latest LRFD is 1.63

(2 live:1dead ratio)

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Limit State
Stage in loading after which the structure
cannot fulfill its intended function
Limit state may be related with strength or
serviceability considerations
Actual collapse is not necessary
Strength limit states corresponds to
maximum strengths, such as ultimate
ductile flexural strength, ultimate shear
strength, buckling failure, fatigue, plastic
mechanism, overturning and sliding, etc.
9
Continued from previous slide

Serviceability limit states are concerned

with occupancy such as excessive
deflections, undesirable vibrations,
permanent deformations, excessive
cracking and behavior in fire, etc.

Structure should not cross any strength

or serviceability limit for a perfect design

10
Strength and Ductility
In general, structures are designed for
strength against loads.
Strength of a material means what
maximum stresses may be developed
Ductility means how much deformations
are produced before final collapse

11
Continued from previous slide

Sometimes, the design is more based on

ductility than strength such as for
earthquake loading
For heaviest earthquakes, only ductility is
provided for safety of life and perhaps not
for complete safety of structure
Time available before final collapse due to
ductility is called warning before failure,
persons may escape

12
Types of Design
Allowable Stress Design (ASD)

Strength Design, Load and Resistance

Factor Design (LRFD), or Limit State
Design

Plastic Design

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Allowable Stress Design
Factor of safety is taken on RS of basic
design equation, called safety factor, and
denoted by .
material
External load resistances
=
effects safety
factor

Ra  Rn / 

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Advantages of ASD
Elastic analysis for loads and elastic material
behavior become compatible for design
Senior engineers are used to this method
Old famous books are according to this method
Was the only method of design in past

Is included as alternate method of design in

AISC-05 Specifications.

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Disadvantages of ASD
Latest research and literature is very much
limited
Same factor of safety is used for different
loads
The failure mode is not directly predicted
With some overloading, the material
stresses increase but do not go to collapse
(How to observe failure mode?)
16
Continued from previous slide

The ductility and warning before failure

cannot be studied precisely

Results cannot be compared with

experimental tests up to collapse

17
Strength Design or LRFD
or Limit State Design
Major part of FOS is applied on load
actions called overload factor
Minor part of FOS is taken on RS of
design equation, becomes reciprocal of
FOS, called resistance factor or capacity
reduction factor ()

18
Continued from previous slide

Resistance factor () is lesser than or equal to

1.0 and is applied on material strength

The design equation is checked for each

strength and serviceability limit states one-by-
one

The design equation becomes:

Ru  Rn

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Advantages of Using LRFD

Behavior at collapse including ductility,

warning before failure and strain-hardening,
etc. may be considered directly

Every type of load may be given a different

FOS depending upon its probability of
overload, number of severe occurrences
and changes in point of application

20
Continued from previous slide

More safe structures result due to better

awareness of structural behaviour near
collapse
Results can be compared with experiments
up to collapse and with structural failures in
the past
Latest research and literature is available in
this method
Even if using ASD, this method provides a
second alternative to check the designs
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Continued from previous slide

Economical in case dead loads are

larger, such as in concrete structures

More safety than ASD if live loads are

greater in magnitude

Plastic design may be incorporated with

very few changes

22
Continued from previous slide

The convenient elastic analysis for loads

is generally used in this method
The design procedure is similar to ASD
with only slight modifications
Using LRFD, steel and concrete design
become consistent with each other

23
Disadvantages of LRFD
Elastic behavior considered for load
analysis and ultimate plastic behavior
for material strengths are not
compatible, however, percentage
difference is less
Engineers experienced in ASD have to
become familiar with this technique

24
 Continued from previous slide

Old books and design aids become

ineffective
Validity of previous designs is still to be
checked according to ASD

25
Plastic Design
Same as LRFD with the difference that
plastic analysis is used for load analysis
Best available method
Incompatibility in load analysis and
material behavior is removed
Very lengthy even for computer
application due to plastic analysis

26
 Objectives Of Structural Designer
Design is a process by which an optimum
solution is obtained satisfying certain criteria.
Some typical criteria are:-
a. minimum cost
b. minimum weight
c. minimum construction time
d. minimum labour
e. maximum efficiency of operation to
owner, etc.
1
1. The structure must safely support the
loads to which it is subjected.

The deflections and vibrations should not

be so excessive as to frighten the
occupants or cause unsightly cracks.

2. The designer must keep the

construction, operation, and
maintenance costs at the lowest level
without sacrificing the strength.

2
3. Designers need to understand fabrication
methods and should try to fit their work to
the available fabrication facilities, available
materials and the general construction
practices.

Some designers lack in this very important

aspect and their designs cause problems
during fabrication and erection.

3
Designers should learn everything possible about
the detailing, the fabrication, and the field erection
of steel besides the loads, mechanics, and the
expected material strengths.

The designer must have information concerning

the transportation of the materials to site, labour
conditions, equipment for erection, problems at
site, field tolerances and the required clearances
at the site.

This knowledge helps to produce reasonable,

practical and economical designs.

4
Procedure Of The Structural Design
The structural framework design is the
selection of the arrangement and sizes of
structural elements so that service loads may
be safely carried.

The important steps in the design of separate

members are shown in the form of a flow
chart in Figure 1.1.

5
 C o l l e c t a n d list all t h e k n o w n d a t a

S e l e c t trial s e c t i o n
b a s e d o n a s s u m e d stresses/
effectiveness of cross-section.
Alternatively, selection tables m a y b e u s e d

A p p l y all stability c h e c k s

Perform strength checks

P e r f o r m serviceability c h e c k s

A c c e p t s e c t i o n if a l l c h e c k s a r e
satisfied, other-wise revise

W rite final selection

6
 LOAD FACTORS AND
LOAD COMBINATIONS
It is almost impossible that all loads like live load,
snow load, wind load and earthquake all occur
together with their maximum intensity.
A load combination combines different types of loads
depending on the probability of occurrence of these
loads acting simultaneously, considering their
expected intensity in the combination compared with
the maximum load intensity.

7
LRFD Load Combinations

When the loads S, R, H, F, E and T are taken

equal to zero and wind loads are taken from the
previous codes, the load combinations are
reduced to the following form:
1. 1.4 D
2. 1.2 D + 1.6 L + 0.5Lr
3. 1.2 D + 1.6Lr + (L or 0.8 W)
4. 1.2 D + 1.3 W + L + 0.5 Lr
5. 0.9 D + 1.3 W

8
ASD Load Combinations
The simplified ASD load combinations are as
follows:
1. D
2. D+L
3. D + Lr
4. D + 0.75L + 0.75Lr
5. D + 0.8W
6. D + 0.6W + 0.75L + 0.75Lr
7. 0.6 D + 0.8W
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TYPES OF STRUCTURAL STEEL

Steels are divided into four categories

depending on the carbon percentages (C) as
follows:
1 Low carbon steel C < 0.15%

2 Mild carbon steel C = 0.15 − 0.29%

3 Medium carbon steel C = 0.30 − 0.59%

4 High carbon steel C = 0.60 −1.70%

10
E-value for steel = 185 GPa to 230 GPa
(Average 200 GPa)

Unit weight = 7850 kg/m3

= 77 kN/m3

For comparison, the unit weight of concrete is

23.6 kN/m3

11
Most of the structural steel falls into the mild carbon
steel or simply mild steel (MS) category.
Hot rolled structural shapes may be made to
conform to A36M, A529M, A572M, A588M, A709M,
A913M and A992M.
Sheets are manufactured according to the standards
ASTM A606, A1011MSS, HSLAS and HSLAS-F.
Bolts are made according to ASTM standards A307,
A325M, A449, A40M and F1852.

12
Most commonly used structural steel is A36M
having the following properties:
Fy = 250 MPa
Fu = 400 MPa E
= 200 GPa
Weld electrodes are classified as E60, E70, E80,
E100 and E110. The letter E denotes electrode.
The two digits indicate the ultimate tensile strength
in ksi. The corresponding SI equivalents are E425,
E495, E550, E690, E690 and E760.
13
HOT ROLLED STRUCTURAL SHAPES

These are the steel cross-sectional shapes

that are hot rolled in the mills. Some of these
shapes are shown in Figure 1.2, whereas, the
steel bars, plates and hollow sections are
reproduced in Figure 1.3.
HSS are hollow structural sections that are
prismatic square, rectangular or round
products of a pipe or tubing.

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 Slope  0o 16.7% Slope

W Section S-Section Angle-Section

Thicker than
flange
16.7% Slope

Channel Section Tee Section

HP-Section

Figure 1.2. Common Steel Structural Shapes.

15
 Plates
Bars

Pipe Section Structural

Tubing

Figure 1.3. Hollow Steel Sections, Bars and Plates.

16
1. W Shapes

The letter ‘W’ stands for an I-shape with wide

flange. The cross-section is doubly symmetric
in the form of the letter “I” (Figure 1.4). The
width / depth ratio varies from about 0.3 to
1.0.
The US Customary designation W16 x 40
means that the nominal depth of the section is
16 in. and the weight per unit length of the
section is 40 lbs/ft.
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 Flange
Less or no slope

Web

Figure 1.4. Typical W-section.

The equivalent SI designation W410 x 60 means that

the W-section has a nominal depth of 410 mm and a
weight of 60 kgf/m.
This kilogram-force weight per unit length may be
converted in kN/m by multiplying it with the factor
9.81/1000.
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Nominal height is the rounded off height to
be used for common use. Actual depth of
the section may be in decimals and
somewhat different from this depth.
2. S Shapes

16.7 Figure 1.5. Typical S-section.

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* Doubly symmetric I-shapes.
* Previously called standard I-beams or
American Standard Beams.
* The inner edge of the flange has a slope of
approximately 16.7.
* An S510 x 112 section means that the section
is S-shape having nominal depth of 510 mm
and weight of 112 kgf/m.
* The width / depth ratio varies from about 0.25
to 0.85.

20
3. M Shapes

* Miscellaneous I-shapes.
* Doubly symmetric I-shapes not classified as
W or S shapes.
* Relatively lightweight used for smaller spans
and lesser loads.
* An M310 x 17.6 means that it is M-shape
section having nominal depth of 310 mm and
weight of 17.6 kgf/m.

21
4. C Shapes

The C-shapes have the following

distinguishing features (Figure 1.6):

16.7

Figure 1.5. Typical C-section.

22
* Channel shapes with standard
proportions.
* Inner flange slope is the same as that
for the S shapes (16.7).
* Previously called Standard or
American Standard Channels.
* A C150 x 19.3 is a standard channel
shape with a nominal depth of 150mm
and a weight of 19.3 kgf/m.
23
5. MC Shapes
These sections have the following properties:
* Channels not classified as C-shapes.
* Previously called Shipbuilding or
Miscellaneous Channels.
6. L Shapes or Angle Sections
The various types of angle sections are shown
in Figure 1.6 and their salient features are
given below:
24
 a

b Figure 1.6. Typical Angle-Sections.

* The single angle sections are in the form

of letter ‘L’.
* If a = b, these are called equal angle
sections.
* If a  b, these are called unequal angle
sections.
25
Sides of the angle are called ‘legs’ or ‘arms’.
L89 x 76 x 12.7 is an unequal leg angle with
longer leg dimension of 89mm and shorter
leg dimension of 76mm with a leg thickness
of 12.7mm.
Double angle sections are combination of
two angles with longer or shorter sides close
to each other.
Double angle sections are denoted by 2Ls.

26
2L89 x 76 x 12.7 means two angles 2L89 x
76 x 12.7 placed side by side in one of the
ways shown in the figure.

7. T Shapes

Figure 1.7. Typical Tee-Section.

27
* These are called structural tees.
* These are obtained by splitting W, S or
M shapes and are called WT, ST or MT
shapes, respectively.
* A WT205 x 30 is a structural tee with a
nominal depth of 205mm and a weight
of 30 kgf/m and is obtained by splitting
the W410 x 60 section.

28
COLD - FORMED SHAPES

These sections are formed from thin high

strength steel alloy plates under normal
temperature.

Some of the common shapes of these

sections are drawn in Figure 1.8.

29
Channels Zees I-Shaped Double Angle
Channels

Hat Sections

Figure 1.8. Commonly Used Cold Formed Shapes.

30
BUILT-UP SECTIONS
Sections made by combining two or more standard
hot rolled sections, joined together at intervals with the
help of direct welding, stay plates or lacing, are called
built-up sections.
Examples are four angles section, double angle
section and double channel section shown in Fig. 1.9.
However, double angle section is sometimes excluded
from built-up section category and is considered as a
regular hot rolled member because of difference of its
behaviour from other built-up sections.

31
4-Angle Box
Section

Figure 1.9. Some Examples of Built-Up Sections.

32
CLADDING

The exterior covering of the structural

components of a building that are made up of
steel sections is called cladding.
This covering may be made up of reinforced
concrete, wood, aluminium or any other
architectural and lightweight material.

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End of this file

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