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• A. J. Clark School of Engineering •Department of Civil and Environmental Engineering
Third EditionCHAPTER
1
Structural Steel DesignLRFD Method
ENCE 355 - Introduction to Structural DesignDepartment of Civil and Environmental Engineering
University of Maryland, College Park
INTRODUCTION TO
STRUCTURAL STEELDESIGN
Part II – Structural Steel Design and Analysis
FALL 2002By
Dr Ibrahim Assakkaf
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 1ENCE 355 ©Assakkaf
Advantages of Steel as aStructural Material
It is interesting to know that steel wasnot economically made in the UnitedStates until late in the nineteenthcentury.However, since then steel has becomethe predominate material for theconstruction of bridges, buildings,towers, and other structures.
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 2ENCE 355 ©Assakkaf
Steel exhibits desirable physicalproperties that makes it one of the mostversatile structural material in use.Its great strength, uniformity, lightweight, ease of use, and many otherdesirable properties makes it thematerial of choice for numerous
structures such as steel bridges, highrise buildings, towers, and otherstructures.
Advantages of Steel as a
Structural Material
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 3ENCE 355 ©Assakkaf
Advantages of Steel as aStructural Material
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 4ENCE 355 ©Assakkaf
Advantages of Steel as a
Structural Material
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 5ENCE 355 ©Assakkaf
Advantages of Steel as aStructural Material
Construction of Golden Gate Bridge (San Francisco, CA)
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 6ENCE 355 ©Assakkaf
Advantages of Steel as a
Structural Material
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 7ENCE 355 ©Assakkaf
Advantages of Steel as aStructural Material
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 8ENCE 355 ©Assakkaf
The many advantages of steel can besummarized as follows: – High Strength
• This means that the weight of structure thatmade of steel will be small.
– Uniformity• Properties of steel do not change as oppose to
concrete.
– Elasticity• Steel follows Hooke’s Law very accurately.
Advantages of Steel as a
Structural Material
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 9ENCE 355 ©Assakkaf
– Ductility• A very desirable of property of steel in which
steel can withstand extensive deformationwithout failure under high tensile stresses, i.e.,it gives warning before failure takes place.
– Toughness
• Steel has both strength and ductility. – Additions to Existing Structures
• Example: new bays or even entire new wingscan be added to existing frame buildings, andsteel bridges may easily be windened.
Advantages of Steel as aStructural Material
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 10ENCE 355 ©Assakkaf
Although steel has all this advantages asstructural material, it also has manydisadvantages that make reinforcedconcrete as a replacement for constructionpurposes.For example, steel columns sometimescan not provide the necessary strength
because of buckling, whereas R/Ccolumns are generally sturdy and massive,i.e., no buckling problems occurs.
Disadvantages of Steel as a
Structural Material
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 11ENCE 355 ©Assakkaf
The many disadvantages of steel canbe summarized as follows: – Maintenance Cost
• Steel structures are susceptible to corrosionwhen exposed to air, water, and humidity.They must be painted periodically.
– Fireproofing Cost• Steel is incombustible material, however, its
strength is reduced tremendously at hightemperatures due to common fires
Disadvantages of Steel as aStructural Material
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 12ENCE 355 ©Assakkaf
– Susceptibility to Buckling• For most structures, the use of steel columns is
very economical because of their high strength-to-weight ratios. However, as the length andslenderness of a compressive column isincreased, its danger of buckling increases.
– Fatigue• The strength of structural steel member can be
reduced if this member is subjected to cyclic
loading.
Disadvantages of Steel as a
Structural Material
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 13ENCE 355 ©Assakkaf
Disadvantages of Steel as aStructural Material
Figure 1. S - N Curves for Various Materials (Byars and Snyder, 1975)
S = stress range N = number of cycles
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 14ENCE 355 ©Assakkaf
– Brittle Fracture• Under certain conditions steel may lose its
ductility, and brittle fracture may occur at placesof stress concentration. Fatigue type loadingsand very low temperatures trigger the situation.
Disadvantages of Steel as a
Structural Material
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 15ENCE 355 ©Assakkaf
Early Uses of Iron and Steel
1777-1779: Metal as structural materialbegan with cast iron, used on a 100-ft(30-m) arch span, which was built inEngland.1780 –1820: A number of cast-iron
bridges were built during this period.1846 -1850: The Brittania Bridge overMenai Strait in Wales was built.
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 16ENCE 355 ©Assakkaf
1840: Wrought iron began replacingcast iron soon.1855: Development of the Bessemerprocess, which help producing steel inlarge quantities and at cheaper prices.1989: Steel shapes having yieldstrength of 24,000 to 100,000 psi were
produced.
Early Uses of Iron and Steel
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 17ENCE 355 ©Assakkaf
Steel Sections
Rolled Sections – Structural steel can be economically rolled
into a wide variety of shapes and sizeswithout appreciably changing its physicalproperties.
– Usually the most desirable members arethose with large moments of inertia inproportion to their areas.
– The I, T, and C shapes, so commonlyused, fall into this class.
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 18ENCE 355 ©Assakkaf
Steel Sections
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 19ENCE 355 ©Assakkaf
Steel Sections
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 20ENCE 355 ©Assakkaf
Steel Sections
Rolled Sections – Steel section are usually designated by the
shapes of their cross sections. – As examples, there are angles, tees, zees,
and plates. – It is necessary, however, to make a definite
distinction between American standardbeams (called S beams) and wide-flangebeams (called W beams) as they are both Ishaped.
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 21ENCE 355 ©Assakkaf
Steel SectionsRolled SectionsI-Shaped Sections
Flange
Web Slope 0 to 5%
W section S section
slope%32
16
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 22ENCE 355 ©Assakkaf
Steel Sections
Designation System – Structural shapes are abbreviated by a
certain system usually described in LRFDmanual for use in drawings, specifications,and designs.
– This system has been standardized so thatall steel mills can use the sameidentification for purposes of ordering,
billing, etc.
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 23ENCE 355 ©Assakkaf
Steel SectionsDesignation System
Some examples of this abbreviation systemare as follows:
1. A W17 × 117 is a W section approximately27 in. deep weighing 114 b/ft.
2. An S12 × 35 is an S section 12 in. deep
weighing 35 lb/ft.3. An HP12 × 74 is bearing pile section which
is approximately 12 in. deep weighing 74lb/ft.
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 24ENCE 355 ©Assakkaf
Steel Sections
Designation System4. A C10 × 30 is a channel section 10 in.
deep weighing 30 lb/ft.5. An MC18 × 58 is a miscellaneous
channel 18 in. deep weighing 58 lb/ft,which cannot be classified as a C shapebecause of its dimensions.
6. An L6 × 6 × ½ is an equal leg angle, each
leg being 6 in long and ½ in. thick.
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 25ENCE 355 ©Assakkaf
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 26ENCE 355 ©Assakkaf
Stress-Strain Relationships in
Structural SteelIdealized Relationships
Strain
S t r e s s
F y
Elasticregion
ε y
(b) IdealizedStrain
S t r e s s
F y
Elasticregion
ε y
(a) As Determined by Tensile Test
y
y F E ε
== slope
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 27ENCE 355 ©Assakkaf
Modern Structural SteelsProperties of Modern Steels – The properties of steel used can be greatly
changed by varying the quantities of carbonpresent and adding other elements such as
• Silicon• Nickel• Manganese, and
• Copper
– A steel having a significant amount of theseelements is referred to as an alloy steel .
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 28ENCE 355 ©Assakkaf
Modern Structural Steels
Yield Point of Modern Steels – In the past, a structural carbon steel
designated as A36 and having yield stressof F y = 36 ksi was the commonly usedstructural steel.
– Today, a steel having F y = 50 ksi can beproduced and sold at almost the sameprice as 36 ksi steel.
– Structural steels are generally grouped intoseveral major ASTM classifications:
ASTM = American Society for Testing and Materials
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 29ENCE 355 ©Assakkaf
Modern Structural SteelsYield Point of Modern Steels – The carbon steels A36, A53, A500, A501,
and A529. – The high-strength low alloy steels A572,
A618, A913, and A992. – The corrosion resistant high-strength low-
alloy steels A242, A588, and A847
Considerable information is presented foreach of these steels in Part 2 of The LRFDManual.
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 30ENCE 355 ©Assakkaf
Uses of High-Strength Steels
There are indeed ultra-high-strengthsteels that have yield strengths from160 to 300 ksi. These steels have notbeen included in the LRFD Manualbecause they have not been assigned
ASTM numbers.The steel industry is now experimenting
with steels with yield stresses from 200to 300 ksi.
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 31ENCE 355 ©Assakkaf
Uses of High-Strength Steels
It is believed hat steels with 500 ksiyield strength will be made availablewithin few years.The theoretical biding force betweeniron atoms has been estimated to be in
excess of 4000 ksi.
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 32ENCE 355 ©Assakkaf
Uses of High-Strength Steels
Factors that Lead to the Use of High-strength Steels:
1. Superior corrosion resistance.2. Possible savings in shipping, erection,
and foundation costs caused by weightsavings.
3. Use of shallow beams permitting smallerfloor depths.
4. Possible savings in fireproofing becausesmaller members can be used.
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 33ENCE 355 ©Assakkaf
Responsibilities of the StructuralDesigner and Engineer
The structural designer or engineermust learn to arrange and proportionthe parts of structures so that they canbe practically erected and will havesufficient strength and reasonable
economy. Some of the items that mustbe considered include – Safety – Cost – Practicality
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 34ENCE 355 ©Assakkaf
Responsibilities of the Structural
Designer and Engineer – Safety• Not only must the frame of a structure safely
support the loads to which it is subjected, butalso it must support them in such a manner thatdeflections and vibrations are not so great as tofrighten the occupants or to cause unsightlycracks.
– Cost• The engineer or designer needs to keep in mind
the factors that can lower cost without sacrificingthe strength, e.g., the use of standard-sizemembers, simple connections, etc.
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 35ENCE 355 ©Assakkaf
– Practicality• Designers and engineers need to understand
fabrication methods, and should try to fit theirwork to the fabrication facilities available.
• The more the designer knows about theproblems, tolerances, and clearances in shopand field the more probable it is thatreasonable, practical, and economical designswill be produced.
Responsibilities of the StructuralDesigner and Engineer
Could I get this thing together if I were sentout to do it??
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 36ENCE 355 ©Assakkaf
Computers and Structural Steel
DesignPersonal computers have drasticallychanged the way steel structures areanalyzed and designed.Many of the commercial structuralsoftware packages can perform – Structural Analysis, and – Structural Design
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 37ENCE 355 ©Assakkaf
Computers and Structural SteelDesign
The need for these programs stemsfrom the fact that the calculationsinvolved in both the design and analysisof an engineering system are quite time-consuming.
With the use of a computer, the designengineer greatly can reduce the timerequired to perform these calculations.
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 38ENCE 355 ©Assakkaf
Computers and Structural Steel
Design Although computers do increase designproductivity, they also tend to reducethe engineer’s “feel” for the structure.This can be a particular problem foryoung engineers with very little designexperience.Computers should not be looked at asblack boxes that can do powerful thingsfor us.
CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 39ENCE 355 ©Assakkaf
Computers and Structural SteelDesign
Knowledge and understanding of thebasic engineering principals areprerequisites for the effectiveimplementation of any design.No matter how impressive your tool
chest, you will be hard-pressed to repaira car if you do not understand how itworks.
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CHAPTER 1. INTRODUCTION TO STRUCTURAL STEEL DESIGN Slide No. 40ENCE 355 ©Assakkaf
Computers and Structural Steel
DesignThis specially true when usingcomputers to perform structural designsand analyses.
Although they have powerful potentialutility, computers are particularlyuseless without a fundamentalunderstanding of how engineering
systems work.