buidling material and steel

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CHAPTER 18

Material Steel and Steel Components

Introduction

Because of its extensive use in industry, construction, and weaponry, iron is by far the most important of all metals. That is why metals are generally divided into two categories: Ferrous metals—metals that contain iron (the Latin term

for iron is ferrum )

Nonferrous metals—metals that do not contain iron (e.g., aluminum and copper)

Introduction – cont’d

Steel is the most important ferrous metal.

Its high strength in relation to its weight makes it the material of choice for skyscrapers and long-span structures, such as sports stadiums and bridges.

Its malleability and weldability allow it to be shaped, bent, and made into different types of components.

These characteristics provide the versatility that architects and engineers have exploited in creating a wide range of highly expressive structures.


Rock and Roll Hall of Fame and Museum, Cleveland, Ohio. The sloping glass skinframed with tubular steel trusses spans nearly 200 feet. The steel tower structure is seen in the background.


The interior of the steel dome of the German Bundestag, Berlin.


Wrought iron – the earliest form of iron

Cast iron – discovery of the blast furnace

Wrought iron (with nearly 0.02% carbon) and cast iron (with 2.5% to 4% carbon) represent two extremes of an iron-carbon alloy.

The perfect amount of carbon in iron is 0.1% to 1.7%, producing a metal called steel.

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Making of Modern Steel


Making of Modern Steel(Integrated Mill)

This illustration shows the process of manufacturing steel in an integrated mill. The blast furnace, and the basic oxygen furnace (BOF) are the two (of the three) major components of an integrated mill. Raw materials (iron ore, limestone, and coke) are fed into the blast furnace, which yields molten iron. Molten iron is used as a charge for the BOF. In the BOF, oxygen is blown into molten iron, which converts the carbon in iron to carbon dioxide, leaving behind molten steel. Molten steel is cast into billets, blooms, and slabs for rolling into finished steel sections. A rolling mill is the third major component of an integrated mill.

Making of Modern Steel(Integrated Mill) – cont’d (Hot Rolling)

Process of hot rolling to obtain finished steel sections. In this process, billets, blooms, and slabs, obtained from the basic oxygen furnace, are reheated to virtual softening and rolled to the required cross sections as shown.

Making of Modern Steel – Sustainable Steel Manufacturing (The Mini Mill)

An electric arc furnace (EAF) converts scrap steel into molten steel. This requires the use of lime and other chemicals, which are fed into the furnace as and when needed through a side door. The furnace has a retractable roof to allow the charging of steel scrap. Molten steel from the EAF is sent to a vessel (called a ladle) to modify itschemistry as needed. Molten steel from the ladle is cast into billets, blooms, and slabs, and subsequently reheated and rolled into finished cross-sections.


Some mini mills produce near-net shape billets (also called beam blanks). These are subsequently rolled into I-sections.


Rectangular billets used for rolling into channels, angles, bars and rods.

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The mini-mill method relies on scrap recycling.

A large scrap storage yard and a shredder are the two most important parts of a recycling plant.

After shredding, the scrap is separated into ferrous and nonferrous scrap using magnetic separation.

Nonferrous scrap is further separated into metals (copper, aluminum, etc.) and nonmetals (plastics, rubber, fabric, etc.).

After the scrap goes through shredding and separation, it is stored in the yard as feed for the EAF.

Most mini mills have a large scrap recycling facility within or close to the mill.


Crushed cars in the scrap yard.


A scrap shredder. After the scrap is shredded, it is separated into ferrous and non-ferrous parts through magnetic separation. The ferrous scrap is then sent by rail carts to storage, ready for feed into the electric arc furnace.


A view of the yard, showing the shredded ferrous scrap in the background and finished steel sections in the foreground.

Making of Modern Steel – Sustainable Steel Manufacturing (Slag)

Slag is a waste product from the blast furnace and the electric arc furnace.

It has many uses—as a lightweight aggregate, as a raw material in the manufacture of portland cement, and in the manufacture of insulation, called slagwool.

Slagwool is noncombustible and is similar to fiberglass in its thermal properties.

Steel Skeleton and Birth of Frame Structures


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Steel Skeleton and Birth of Frame Structures

A steel building under construction has the appearance of a skeleton. Therefore, when steel buildings first appeared in Chicago toward the end of the nineteenth century, the terms skeleton cage, steel skeleton and skeleton frame were coined to describe their appearance.

Classification of Steel Components


Classification of Steel Components

Steel may be classified in a number of ways.

For design and construction professionals, three classification systems are important: classification based on steel’s application

classification based on steel strength

classification based on steel metallurgy

Classification of Steel Components– cont’d

Application-based classification – In this classification, steel may be classified asStructural steel

Cold-formed steel

Reinforcing steel

Pre-stressing (post-tensioning) steel


Structural steel members include steel cross sections, such as I-sections, H-sections, T-sections, C-sections (channels), L-sections (angles), plates, pipes, and rectangular tubes (hollow sections).

Cold-formed (or light-gauge) steel members are made from thin sheets of steel by bending sheets to various corrugated profiles at room temperature, hence the term cold-formed.

Reinforcing steel is in the form of deformed round bars (also called rebars) that are used in concrete slabs, beams, and columns.


Pre-stressing steel is used in precast concrete or post-tensioned concrete members as a replacement for (or in conjunction with) reinforcing steel.

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All components used in the structural frame of this building (columns, beams, and joists) are made of hot-rolled steel sections, referred to as structural steel. The floor deck is made of cold-formed steel.


All components (floor deck, joists, wall studs, and bracing members) used in the structural framing of this building are made of cold-formed steel. The framing of this building is similar to that of a wood light frame building.


Steel is alloyed during manufacturing with a small percentage of other metals to obtain steels that vary from each other with respect to a few important properties, such as the yield strength, tensile strength, and corrosion resistance.

The most commonly used steel for framing members (W-shape columns and beams) in contemporary buildings is A992, with a yield strength of 50 ksi.


The yield strength of steel is also referred to as the steel’s grade.

Thus, a steel with a yield strength of 50 ksi is called grade 50 steel.


A simple metallurgical distinction between the steels used in building construction is a) carbon steel and (b) alloy steel.

Increasing the amount of carbon increases steel’s strength but reduces its ductility and formability, and vice versa.

Sheet steel is made from low-carbon steel.

Most steel used in building construction is carbon steel.


Alloy steel contains other metals to change steel’s properties.

Weathering steel and stainless steel are alloy steels.

Chromium is the primary alloying metal in stainless steel.

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Structural Steel – Hot-Rolled Sections


Structural Steel – Hot-Rolled Sections

Because of the large financial outlay required in producing structural steel sections, design and construction professionals must select them from the standard shapes and sizes.

They are available in cross-sectional shapes of I, C, L, and T, pipes, tubes, round and rectangular bars, and plates.

Structural Steel – Hot-Rolled Sections– cont’d

Commonly used structural steel sections.

Structural Steel – Hot-Rolled Sections– cont’d (I-Sections)

Structural steel I-sections may be classified into four shapes: W-shapes

S-shapes

HP-shapes

M-shapes

Structural Steel – Hot-Rolled Sections– cont’d (W- and S-Shapes)

Comparison between W-shape and S-shape sections.

Structural Steel – Hot-Rolled Sections– cont’d (Wide-Flange)

Most wide-flange sections that share a common nominal depth designation have the same interior flange-to-flange dimension. This illustration shows three sections, all with a nominal depth of 14 in. Their overall depths are nearly 14 in., and several other dimensions are different from each other. However, the interior flange-to-flange dimensions are 12.60 in. for all sections. Several other sections with a nominal depth of 14 in. are available (see Table 18.1 in the next slide).

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Structural Steel – Hot-Rolled Sections– cont’d (C-Shapes)

Steel channels (C-shapes) are similar in profile to S-shapes, that is, their inner flange surfaces are inclined at an angle of 2:12, Next slide (a).

They are designated by two numbers after the letter C.

The first number gives the overall depth of the section, and the second number gives the weight of a 1-ft length of the section.

Thus, C8 x 11.5 means that the channel is 8 in. deep and weighs 11.5 lb/ft.

Miscellaneous channels do not have a standard slope on the inner flange surfaces.

They are designated in the same way as C-shapes, for example, MC12 x 50.

Structural Steel – Hot-Rolled Sections(C-Shapes) – cont’d

Structural Steel – Hot-Rolled Sections– cont’d (T-Shape Section)

A T-shape section is made by splitting a W-shape, M-shape, or S-shape into two equal parts, Next slide (b).

It is, therefore, called WT, MT, or ST, depending on its origin.

For example, two WT6 x 29 sections are obtained from one W12 x 58.

Structural Steel – Hot-Rolled Sections– cont’d (Angles)

Steel angles (L-shapes) may either be equal-leg angles or unequal-leg angles, Next slide (c).

The thickness of both legs is the same in an angle.

Angles are designated by three numbers. The first two numbers give the length of each leg, and the third

number gives the thickness of the legs. L4 x 4 x 12 is an example of an equal-leg angle with legs equal to 4 in.

each; the thickness of each leg is 12 in.

In an unequal-leg angle, the longer leg is mentioned first, as in L4 x 3 x 14.

Angles have various uses, such as for masonry lintels and members of steel trusses.


(a) Channel section profile. (b) WT-sections obtained from a W-section. (c) Equal-leg and unequal-leg angles.

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Structural Steel – Hot-Rolled Sections– cont’d (Pipes)

Pipes are designated by their nominal diameter and by whether the pipe is a standard weight, extra strong, or double-extra strong.

These three designations refer to the pipe’s wall thickness.

Pipes are generally used as columns or as members of a truss.

Structural Steel – Hot-Rolled Sections– cont’d (HSS)

A tube is referred to as a hollow structural section (HSS) and is made by bending a steel plate and welding it seamlessly.

That is why the edges of a tube are rounded (see next slide).


Bent-plate angle and built-up sections.

Structural Steel – Hot-Rolled Sections– cont’d (HSS)

An HSS may be square, rectangular, or round.

Square or round HSSs are generally used as columns, and rectangular HSSs are used as beams.

Like pipe trusses, HSS member trusses are fairly common for long-span structures.

Structural Steel – Hot-Rolled Sections– cont’d (Plate Girder)

(a) Anatomy of a typical plate girder. (b) One of the several uses of a plate girder.

Steel Joists and Joist Girders – From Hot-Rolled Sections


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Steel Joists and Joist Girders – From Hot-Rolled Sections

In addition to the standard steel shapes and built-up sections, two types of prefabricated steel members are commonly used for roof and floor structures in buildings.

These are truss-like, open-web members, called joists and joist girders.

There is no fundamental difference between a joist and a joist girder, except that a joist girder is a heavier member and spans from column to column, whereas a joist is a lighter member that spans between the girders.

Steel Joists and Joist Girders – From Hot-Rolled Sections – cont’d

A steel frame structure showing the use of steel joists and steel joist girders. Observe that a joist girder spans from column to column. A joist, on the other hand, spans from joist girder to joist girder.


The Steel Joist Institute classifies joists into three categories: K-series joists (joist depth ranges from 8 in. to 30 in.)

LH-series joists (joist depth ranges from 18 in. to 48 in.)

DLH-series (joist depth ranges from 52 in. to 72 in.)

Steel Joists and Joist Girders – From Hot-Rolled Sections – cont’d (K-Series)

Steel Joists and Joist Girders – From Hot-Rolled Sections (K-Series) – cont’d


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Steel joists and joist girders are slender elements and are, therefore, unstable and prone to overturning.

As per SJI’s specifications, the joists must be stabilized by rows of continuous horizontal members, referred to as horizontal bridging members.

Horizontal bridging members are used in rows: One row is welded to the top chord of the joist, and the other row is welded to the bottom chord



Horizontal bridging of steel joists to provide stability against overturning.


Diagonal bridging of steel joists is an alternative to horizontal bridging. Regardless of the bridging system used, the bridging members in the end spans must be securely connected to the exterior wall (as shown here) or the spandrel beam.

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Steel Roof and Floor Decks – Cold-Formed Steel


Steel Roof and Floor Decks – Cold-Formed Steel

Steel decks are made from sheet steel by pressing the sheets into various cross-sectional profiles at room temperature, hence the term cold-formed steel.

They are available in two categories: roof decks

floor decks

The primary difference between them is that a roof deck is generally topped with rigid insulation and a roofing membrane (for waterproofing), and a floor deck is topped with structural concrete fill.

Steel Roof and Floor Decks – Cold-Formed Steel – cont’d (Deck)

Deck terminology.

Steel Roof and Floor Decks – Cold-Formed Steel – cont’d (Roof Deck)

Commonly used roof deck types.

Steel Roof and Floor Decks – Cold-Formed Steel (Roof Deck) – cont’d

Commonly used roof deck types.

Steel Roof and Floor Decks – Cold-Formed Steel (Roof Deck) – cont’d

Anchorage of a roof deck to supporting elements.

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Steel Roof and Floor Decks – Cold-Formed Steel – cont’d (Floor Deck)

A steel floor deck functions as a working platform as well as permanent formwork for the concrete fill, so that the deck and the fill form a concrete floor slab.

Two types of floor decks are used: A form deck functions as permanent formwork only. Thus,

the concrete slab must be reinforced with conventional reinforcement.

In a composite deck , the deck also functions as steel reinforcement for the concrete slab, reducing the need for conventional concrete reinforcing bars.

Steel Roof and Floor Decks – Cold-Formed Steel (Floor Deck) – cont’d

This illustration shows the placement of concrete on a floor deck (a form deck in this case). Observe the steel reinforcing bars placed over the deck.


Two commonly used composite deck types.


Comparison between the general profiles, depths, and gauges of composite decksand form decks.

Steel Roof and Floor Decks – Cold-Formed Steel – cont’d (Composite Deck)

Both a composite deck and a form deck can be made to act compositely with the supporting beams by using shear studs.

Shear studs prevent slippage of the deck under bending of the underlying beam. They are similar to nails that connect a plywood deck with supporting wood joists.

Steel Roof and Floor Decks – Cold-Formed Steel (Composite Deck) – cont’d

Use of shear studs for composite action between floor slab and beam.

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Steel Roof and Floor Decks – Cold-Formed Steel – cont’d (Pour Stop)

Use of shear studs for composite action between floor slab and beam.

Corrosion Protection of Steel


Corrosion Protection of Steel

Because steel (unlike aluminum) does not automatically form a protective oxide coating, it must be protected against corrosion.

However, structural steel members enclosed by the building envelope do not require any protective coating unless they are in a corrosive environment.

In other words, interior structural steel members can be left bare (mill-finished state) in most situations.

Corrosion Protection of Steel – cont’d

Although bare steel is acceptable, almost all structural steel members generally receive a prime coat in the fabricator’s shop before being delivered to the construction site.

The prime coat provides temporary protection until the steel is wrapped by the building envelope.

Corrosion Protection of Steel – cont’d

Several protective coatings are available for steel to suit different environmental conditions, aesthetic requirements and budgets.

These include acrylics, epoxies, polyurethanes, and zinc coating. For exposed structural steel members, polyurethane

coatings are the hardest, toughest, and most versatile.

For cold-formed and light structural steel members, zinc coating (referred to as galvanizing) is a cost-effective solution.

Fire Protection of Steel


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Fire Protection of Steel

The fire endurance of steel is poor, and its inherent incombustibility gives a false sense of security.

Exposed (unprotected) steel, unless it is very thick, cannot withstand long exposure to fire.

Fire Protection of Steel – cont’d

There are basically two ways to protect steel against fire:

Insulate the steel component with a noncombustible thermal insulation. Spray-applied fire protection and intumescent paints fall in

this category.

Encase the steel component with a noncombustible material with high thermal capacity, such as concrete, gypsum board, or water. These materials retard the buildup of temperature

providing the same end result as thermal insulation.

Fire Protection of Steel – cont’d (Concrete Encasement)

Encasing steel members in concrete (or masonry) is one of the oldest methods of protecting steel against fire. The use of this method is relatively uncommon in contemporary construction because of the availability of more efficient and economical alternatives.

Fire Protection of Steel – cont’d (Gypsum Board Encasement)

Details of a steel column covered with gypsum board layers. Details (a) and (b) are identical except that the column in detail (b) is heavier than that of the column in detail (a). Because the thickness of steel also affects the member’s fire resistance, in addition to the thickness and type of gypsum board, the fire resistance rating of column (b) is higher than that of column (a).

Fire Protection of Steel – cont’d (Spray-On Fire Protection)

Spray-on fire protection of a steel roof assembly. Observe that both joists as wellas the roof deck are protected.

Fire Protection of Steel (Spray-On Fire Protection) – cont’d

Spray-on fire protection of a steel floor assembly. Observe that only the floor beams are protected. Fire protection is generally not necessary for the floor deck because of the concrete topping.

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Fire Protection of Steel – cont’d (Intumescent Paints)

An alternative to gypsum board encasement is intumescent paint on steel members.

Intumescent paint is typically 20 to 50 mil (0.5 to 1.3 mm) thick.

When exposed to the heat of fire, the paint intumesces, or swells, yielding an insulating char cover on steel that is 2 in. to 4 in. thick.

It is this char layer that protects steel from fire.

Fire Protection of Steel (Intumescent Paints) – cont’d

A steel truss painted with intumescent paint providing a 1-h fire rating,

Fire Protection of Steel – cont’d (Suspended Ceilings)

Suspended ceilings consisting of gypsum lath and plaster, gypsum boards, or acoustical tiles are also used to provide fire protection to otherwise unprotected steel beams or trusses in roof-ceiling or floor-ceiling assemblies.

The ceiling grid may be directly attached to, or hung from, the bottom flanges of beams or the bottom chords of trusses.

buidling material and steel

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