This document provides an overview of structural steel design. It discusses steel as a structural material, its advantages, common sections and grades. It covers design philosophies like limit states, allowable stress design and load resistance factor design. Applications of steel and some key aspects of steel construction are presented. The history and role of codes are summarized. An overview of the LRFD manual is also provided.

1. Lecture
1
Prepared by :
Eng. Feysal Mohamed
Shirwa
STRUCTURAL STEEL DESIGN
Introduction

2. General Overview
Structural design may be defined as a mixture of art and science combining the
intuitive feeling for the behavior of a structure with rational principles of
mechanics (statics, solid mechanics, dynamics, etc.) and structural analysis to
produce a safe and economical structure to serve its intended purposes.
Steel is one of the most important building materials in the modern
era. It is used solely or in combination with other materials such as
concrete, timber, composites etc for a variety of purposes.

3. What is Steel?
Steel is an alloy in which iron is mixed with carbon and other
elements.
An Alloy is a homogeneous mixture of two or more elements, at least
one of which is a metal, and where the resulting material has metallic
properties.
An Alloy usually has different properties (sometimes significantly
different) from those of its components.

4. Steel as a Structural Material
The use of steel as structural material is a function of state of
industrial development, the reason for which steel structure
emerged at the post industrial era.
Although it has been used for long time, it is important to notice
that structural steel was produced in commercial quantities only in
the 18th century .
The 19th century witnessed the development of high strength steel,
the landmark structures were built during this period. eg, Verrazano
Bridge and Eiffel Tower.

6. Advantages of Steel as a Structural Material
Steel exhibits desirable physical properties that make one
of the most versatile structural material in use.
For its several desirable properties such as its great
Strength, uniformity, light weight and ease of erection
Steel is proven as attractive choice for a range of structural
application such as high rise buildings, bridges, towers etc.

7. Advantageous Properties of Steel
High Strength
Uniformity
Elasticity
Ductility
Toughness
Additions to Existing Structures
Environmental advantages

15. Disadvantages of Steel as a Structural Material

16. Environmental Disadvantages

17. Benefits of Structural Steel
Some benefits associated with use of structural steel for owners are:
Steel allows for reduced frame construction time and the ability to construct in all seasons
Steel makes larger spans and bay sizes possible, providing more flexibility for owners
Steel is easier to modify and reinforce if architectural changes are made to a facility over its
life
Steel is lightweight and can reduce foundation costs (AISC 1999)
Steel is durable, long-lasting and recyclable

18. Some Samples of Steel Structures
Seattle Public Library (2004)
St. Louis Arch, 192mx192m

19. Some Samples of Steel Structures
Steel building

20. Orient railway station, Lisbon, Portugal Architect: Santiago Calatrava
Steel gives the flexibility to create the desired
aesthetic effect using the structural members

21. New River Gorge Bridge the largest single span steel arch bridge
in the western hemisphere. It measures 876 ft. from the bridge to the
bottom of the gorge
Old Nanpu Bridge (double loop ramp), Shanghai,

22. Exposed steel will require special processing that will impact the
cost and schedule
Airport terminal, Lyon, France Architect: Santiago Calatrava

23. Architecturally Exposed Structural Steel (AESS)
Roof of the Santa Fe Opera Theatre, Santa Fe, New Mexico Architects: Polshek Partnership, LLP

24.  The entire structure or key portions may use AESS
 Popular applications include
 Hanging walkways
 Framing in atriums and lobbies
 Office interiors
 Airport terminals
Typical Applications

25. AESS Canopy and Hanging Walkway
Portland International Airport (PDX) Roadway Canopy and Pedestrian Bridges, Portland, Oregon,
Architects: Zimmer Gunsul Frasca Partnership

26. AESS Office Interiors
Herman Miller Marketplace, Zeeland, MI Architects: Integrated Architecture, Grand RapidsS, tMeeI lP Dheostoig bny - HDer.d Sriceks hBule Asdsliunrgi
Lindhout Associates Headquarters, Brighton, MI

27. AESS Airport Terminals
The United Airlines Terminal, Chicago, IL Architects: Murphy/Jahn and A. Epstein & Sons Int’l

28. Residual stresses
Stresses can be left behind in steel shapes after
certain events
 Hot-rolling (due to differential cooling)
 Welding (due to differential cooling)
 Cold-forming (due to plastic deformation)
 Excessive deformation
 Etc.

29. Unique Aspects of Steel Construction
Procurement and management of structural steel is similar to other materials, but there are some
unique aspects to steel construction:
Steel is fabricated off-site (above left)
On-site erection is a rapid process (above right)
This gives use of structural steel some scheduling advantages
Coordination of all parties is essential for achieving potential advantages

30. History of Steel construction
 Ancient Use:
Beginning 5th Cent. B.C., Weaponry, Ornamentals and Bridge
construction in India (small suspension bridges), Middle East and
China
 Early Use:
1777-79 First Cast iron bridge in England
 1780-1820 Several bridges all over Europe,
preliminary rolled shapes manufactured
•1820 -Rails manufactured
•1840 -advent of wrought iron
Iron bridge-Coalbrookdale, UK, 1789 By
Thomas Telford

31. History of steel construction
First modern
suspension bridge -
James
Finley’s -Jacob’s Creek,
Pennsylvania, 1801

32. History

33. History

35. Applications of Steel as a Structural Material
Beam Column Steel Buildings
Thin Corrugated Sheets
Cables in Suspension Bridges
Girders In Highway Bridges
others

36. Steel Building
Beam –Column System

39. Common Steel Sections
Structural steel can be classified into two groups:
1. Cold Formed Steel: The steel is first manufactured in rods or roles
then the desired shapes and sections are produced through special
treatments
2.Hot rolled steel: The desired sections are produced directly by
pouring the melted iron into standard mold. The type of steel usually
contains residual stresses. Most of steel section fall under this
category, and the most desirable shapes are those having large
moment of inertia in proportion to their area.

44. Common Structural Steel Shapes

45. Terminology of Steel Section

46. Grades of Steel

47. Grades of Steel

48. Design philosophies
A general statement assuming safety in
engineering design is:
Resistance ≥ Effect of applied loads ---(1)
In eq(1) it is essential that both sides are
evaluated for same conditions and units e.g.
compressive stress on soil should be
compared with bearing capacity of soil

49. Design philosophies
Resistance of structures is composed of its members
which comes from materials & cross-section
Resistance, Capacity, and Strength are somewhat
synonym terms.
Terms like Demand, Stresses, and Loads are used to
express Effect of applied loads.

50. Limit States
When particular loading reaches its limit, failure is
the assumed result, i.e. the loading condition
become failure modes, such a condition is referred
to as limit state and it can be defined as
“A limit state is a condition beyond which a
structural system or a structural component ceases
to fulfill the function for which it is designed.”

51. Limit States
There are three broad classification of limit
states:
1. Strength limit states
2. Serviceability limit states
3. Special limit states

52. Limit States
Strength Limit States:
• Flexure
• Torsion
• Shear
• Fatigue
• Settlement
• Bearing

53. Limit States
Serviceability Limit States:
• Cracking
• Excessive Deflection
• Buckling
• Stability

54. Limit States
Special Limit States:
Damage or collapse in extreme earthquakes.
Structural effects of fire, explosions, or
vehicular collisions.

55. Design Considerations
•Structure and Structural Members should have
adequate strength, stiffness and toughness to
ensure proper functioning during service life
•Reserve Strength should be available to cater for:
– Occasional overloads and underestimation of
loads
–Variability of strength of materials from those
specified
–Variation in strength arising from quality of
workmanship and construction practices

56. Design Considerations
Structural Design must provide adequate margin
of safety irrespective of Design Method
Design Approach should take into account the
probability of occurrence of failure in the design
process

57. Design Considerations
An important goal in design is to prevent limit
state from being reached.
It is not economical to design a structure so that
none of its members or components could ever
fail. Thus, it is necessary to establish an acceptable
level of risk or probability of failure.

58. Design Considerations
Brittle behavior is to be avoided as it will imply a
sudden loss of load carrying capacity when elastic
limit is exceeded.
Reinforced concrete can be made ductile by
limiting the steel reinforcement.

59. Design Considerations
To determine the acceptable margin of safety,
opinion should be sought from experience and
qualified group of engineers.
In steel design AISC manuals for ASD & LRFD
guidelines can be accepted as reflection of such
opinions.

60. Design Considerations
Any design procedure require the confidence of
Engineer on the analysis of load effects and
strength of the materials.
The two distinct procedures employed by
designers are Allowable Stress Design (ASD) &
Load & Resistance Factor Design (LRFD).

61. Allowable Stress Design (ASD)
Safety in the design is obtained by specifying, that the effect
of the loads should produce stresses that is a fraction of the
yield stress fy, say one half.
•This is equivalent to:
• FOS = Resistance, R/ Effect of load, Q
= fy/0.5fy
= 2

62. Allowable Stress Design (ASD)
Since the specifications set limit on the stresses, it
became allowable stress design (ASD).
It is mostly reasonable where stresses are
uniformly distributed over X-section (such on
determinate trusses, arches, cables etc.)

63. Allowable Stress Design (ASD)
Mathematical Description of A S D

 i
n
Q
R


Rn = Resistance or Strength of the component being designed
Φ = Resistance Factor or Strength Reduction Factor
= Overload or Load Factors


= Factor of Safety FS
Qi = Effect of applied loads

64. Load and Resistance Factor Design (LRFD)
To overcome the deficiencies of ASD, the LRFD
method is based on:
Strength of Materials
It consider the variability not only in resistance
but also in the effects of load.
It provides measure of safety related to
probability of failure.

65. Load and Resistance Factor Design (LRFD)
 Safety in the design is obtained by specifying that the
reduced Nominal Strength of a designed structure is less
than the effect of factored loads acting on the structure
Rn = Resistance or Strength of the component being designed
Qi = Effect of Applied Loads
n = Takes into account ductility, redundancy and operational imp.
Φ = Resistance Factor or Strength Reduction Factor
= Overload or Load Factors
= Factor of Safety

 i
n
Q
n
R 




66. The role of various codes
It is very difficult to devise a design code that is
applicable to all uses and all types of structures
such as buildings, highway bridges, railway bridges
and transmission towers
The responsibility of infrastructure on roads,
bridges and electrical transmission towers rests
with the organization responsible for approving,
operating and maintaining these facilities

67. The role of various codes
Uses and critical loads may be different in
different types of structures and no one code can
cater to all the different important considerations
For above reasons different codes prevail and will
continue to do so
AISC ASD Code and LRFD Code primarily is
pertinent to Building Structures.

68. Overview of LRFD Manual
Part 1: Dimensions and properties
Part 2: General Design considerations
Part 3: Design of flexural members
Part 4: Design of compression members
Part 5: Design of Tension members
Part 6: Design of members subject to
combined loading

69. Overview of LRFD Manual
Part 7: Design considerations for bolts
Part 8: Design considerations for welds
Part 9: Design of connecting elements
Part 10: Design of simple shear connections
Part 11: Design of flexible moment
connections

70. Overview of LRFD Manual
Part 12: Design of fully restrained (FR)
moment connections
Part 13: Design of Bracing connections and
truss connections
Part 14: Design of Beam bearing plates,
Column base plates, anchor rods,
and column splices.

71. Overview of LRFD Manual
Part 15: Design of Hanger connections,
Bracket plates, and Crane-rail
connections
ANSI/LRFD Specifications for structural steel
Buildings.