Steel consists of layers of iron ore, coke (a form of carbon), and limestone.
1) Cast iron= 3-4% carbon
2) High carbon steel= less than 2% carbon
3) Milled steel= 3/10 of 1% carbon
1) Approximates proportion: 7 parts iron ore, to 3 parts coke, to 1 part limeston, to 16 parts air.
2) Coke is burned.
3) Oxidation with air produces carbon monoxide.
4) Limestone is combined with the ore, reacting with impurities in the ore to produce a slag.
5) Carbon monoxide from step 3 reacts with the ore to produce elemental iron.
6) Result is molten steel, which is cast into ingots.
7) Hot ingots are squeezed between rollers into rectangular
shapes called blooms.
8) Blooms are rolled into structural shapes (at 1260 degrees C).
1) Rolling into shapes under heat decreases the thickness of the steel pieces while increasing their length.
2) Reduction and shaping breaks down crystals and makes them smaller and more compact.
3) The result is steel which is both stronger and more malleable.
1) Steel elements are named for their characteristic section.
2) Because steel is relatively expensive, sections are designed to carry the maximum load with the minimum cross sectional area.
The typical types of steel elements:
1) Wide Flange: The I- bean, or H-beam. Flange refers to the horizontal parts of the I, the vertical part is known as the web. Flanges range between 8 and 30 in height. If the web height exceeds the flange width the section is an I, if the flange width exceeds the web height, the section is an H.
2) Channel: The C flange. Flanges range between 2 and 4 while webs range between 3 and 24.
3) Angle: The L shape. The angle can be equal (typical sizes are 8x8, 1x1), or unequal (typical sizes are 6x8, and 1x2).
5) Solid bar: these are available in square, rectangular, and round sections, of varying sizes, but are generally small.
6) Steel Plate: a section where the thickness is minimal compared with the width.
· W12x26. The letter refers to the type of cross section (in this case, wide flange); the first number refers to the nominal depth of the section (web height plus flange thicknesses). The second number refers to weight per foot of length.· Thus W12x26 refers to a wide flange beam, with a 12 depth weighing 26lbs. For each foot of length.
1) W= wide flange
2) L= angle
3) C=channel
4) Wt= structural tee
1) Members made in factory- assembled on site.
2) Steel members are used to frame buildings (much like wood framing). The basic members used are beams, columns, and joists.
3) Shapes are standardized depending on use.
4) Steel is very hard, and strong in both tension and compression.
5) Because steel is so strong, it can be used to create large open spaces. The openess of space in steel frame buildings makes for increased flexibility in partitioning interiors.
6) Strength of steel is compromised by extreme heat, which results in buckling.
The most common method of joining members in a steel frame is with bolts, using smaller steel angles as transitional elements.
1) Beam into column flange
2) Beam into column web
Welding:
The process of melting two steel elements together to form a monolithic form. It is very expensive, and rarely used.
· The major form of joining steel members before bolts came into wide use following the Second World War.· With rivets, adjoining steel members are joined with a common hole.
· Rivets are like round head screws with no threads, that are heated and inserted in the hole.
· The end without a head is hammered flat, until the rivet is permanently attached to the hole.
· Under extreme heat, stell is subject to buckling and twisting, which can result in structural failure.· The building code specifies that steel frame buildings of more than one storey must have a certain amount of sheathing over structural steel elements, which provides a greater amount of time until heat reaches the member.
1) Masonry, or concrete surrounding the members. This was the first method used, but is not often used today because of cost, and the extra weight which must be compensated for in a stronger structure.
2) Plaster covering: this is the most common method for fire protecting steel frames today.
a. Lath and plaster covering
b. Gypsum board: the same drywall used for walls
c. Spray on plaster: consisting of a fiber combined with a binding agent. This method is very light in weight.
Floors and roofs in steel frame buildings:
1) Metal corrugated decking with
a. Poured concrete, shich foroms the basic subfloor.
b. Wood or timber decking on top.
c. Gypsum board slabs on top (for roofing).
2) Precast concrete decking suspended on steel beams.
3) Poured in place concrete slabs, with or without lost forms.
· Another common use of steel is as wall studs for partition walls, and light framing.· The members typically used are between 12 and 20 gauge (refers to the thickness of the steel), and are formed in channel sections.
· Because of their thin character, they can be easily bent, screwed or welded.
These are prefabricated lightweight flat trusses. They are typically spaced between 3 and 7 apart depending on the type of decking used. These are commonly used in conjunction with suspended ceiling systems.
1) S type:
a. 8 to 24 deep
b. used for 4 and 48 spans
c. The lower member consists of two rectangular section solid steel bars.
2) L type:
a. 18 to 48 deep
b. used for 25 to 96 spans
c. The lower members consist of two L section angles.
· Sheets are made from low carbon steel, which is passed through rollers to form various shapes.
· Round and geometric corrugation, ribbed sheets, and crimped sheets.
· As well as its use in steel decking, corrugated sheets are also used as a cladding material (roofing and siding).
· For weather protection, the sheets are galvanized, that is, coated with zinc. The thickness of the coating is important for determining the weather protection.
1) Hot dipping
2) Electro-galvanizing (based on electrolytic action of dissolved ions subjected to a current).
3) Sherardizing (the application of zinc dust in a heated chamber).
4) Metallic spraying
Three main forms of steel reinforcement:
1) Hot rolled reinforcing rods: which are given a textured surface to provide friction with the concrete. These rods may be galvanized, or epoxy coated where the concrete is likely to be frequently damp.
2) Drawn steel wire: is made into a rope or meshwork (sometimes using welding to secure fibers). This material is frequently used where the reinforcement is to be prestressed.
3) Steel pans: which serve as formwork as well as part of the roofing.
· Very soft metal, and is therefore quite malleable
· Very corrosion-resistant
· Often used for flashing, or for the entire roof covering
Aluminium
· Soft and light, and consequently not strong enough to be used where any load is present.
· Aluminium is sometimes used as a decorative cladding material (or locally as a vapor barrier).
· Duralumin is a copper-aluminum alloy which is somewhat stronger than pure aluminium.
· Stainless steel refers to an alloy of steel with nickel and/or chromium.
· It is resistant to oxidation and corrosion.
· Steel is used for doors and windows, as flashing, for couonters,
and as a cladding in curtain walls.
