steal strength

8/9/2019 steal strength

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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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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




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Structural Material



Construction of Golden Gate Bridge (San Francisco, CA)


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Structural Material




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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.


Structural Material


– 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.



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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


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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– 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.


Structural Material


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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– 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.


Structural Material


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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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


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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Steel Sections


Steel Sections


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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.


Steel SectionsRolled SectionsI-Shaped Sections

Flange

Web Slope 0 to 5%

W section S section

slope%32

16


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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.


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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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.



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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


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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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


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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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.



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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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.


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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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.


– 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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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


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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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.


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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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.

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