Structural steel
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Structural steel is steel construction material, a profile, formed with a specific shape or cross section and certain standards of chemical composition and strength. Structural steel shape, size, composition, strength, storage, etc, is regulated in most industrialised countries.
Structural steel, such as I-beams, have a large polar moment of inertia, which allows the beam to be very stiff in respect to its cross-sectional area.
Common structural shapes
In most developed countries, the shapes available are set out in published standards, although a number of specialist and proprietary cross sections are also available.
- I-beam (I-shaped cross-section - in Britain these include Universal Beams (UB) and Universal Columns (UC); in Europe it includes the IPE, HE, HL, HD and other sections; in the US it includes Wide Flange (WF) and H sections)
- Z-Shape (half a flange in opposite directions)
- HSS-Shape (Hollow structural section also known as SHS (structural hollow section) and including square, rectangular, circular (pipe) and elliptical cross-sections)
- Angle (L-shaped cross-section)
- Channel (C-shaped cross-section)
- Tee (T-shaped cross-section)
- Rail profile (asymmetrical I-beam)
- Bar, a piece of metal, rectangular cross sectioned (flat) and long, but not so wide so as to be called a sheet.
- Rod, a round or square and long piece of metal or wood, see also rebar and dowel.
- Plate, sheet metal thicker than 6 mm or 1/4 in.
- Open web steel joist
While many sections are made by hot or cold rolling, others are made by welding together flat or bent plates (for example, the largest circular hollow sections are made from flat plate bent into a circle and seam-welded).
Structural steels
Most industrialised countries prescribe a range of standard steel grades with different strengths, corrosion resistance and other properties.
Standard structural steels (Europe)
Most steels used throughout Europe are specified to comply with the European standard EN 10025. However, many national standards also remain in force.
Typical grades are described as 'S275J2' or 'S355K2W'. In these examples, 'S' denotes structural rather than engineering steel; 275 or 355 denotes the yield strength in newtons per square millimetre or the equivalent megapascals; J2 or K2 denotes the materials toughness by reference to Charpy impact test values; and the 'W' denotes weathering steel. Further letters can be used to designate normalised steel ('N' or 'NL'); quenched and tempered steel ('Q' or 'QL'); and thermomechanically rolled steel ('M' or 'ML').
The normal yield strength grades available are 195, 235, 275, 355, 420, and 460, although some grades are more commonly used than others e.g. in the UK, almost all structural steel is grades S275 and S355. Higher grades are available in quenched and tempered material (500, 550, 620, 690, 890 and 960 - although grades above 690 receive little if any use in construction at present).
Standard structural steels (USA)
Steels used for building construction in the US use standard alloys identified and specified by ASTM International. These steels have an alloy identification beginning with A and then two, three, or four numbers. The four-number AISI steel grades commonly used for mechanical engineering, machines, and vehicles are a completely different specification series.
The standard commonly used structural steels are: [1]
Carbon steels
- A36 - structural shapes and plate
- A53 - structural pipe and tubing
- A500 - structural pipe and tubing
- A501 - structural pipe and tubing
- A529 - structural shapes and plates
High strength low alloy steels
- A441 - structural shapes and plates
- A572 - structural shapes and plates
- A618 - structural pipe and tubing
- A992 - W shapes beams only
Corrosion resistant high strength low alloy steels
- A242 - structural shapes and plates
- A588 aka Cor-ten - structural shapes and plates
Quenched and tempered alloy steels
Steel vs. concrete
As raw material prices fluctuate, often so does building design. During times of lower steel prices, more steel and less concrete is used, and vice versa. Each set of vendors and users typically maintain national industry associations that advocate the use of its materials versus the other. However, both materials are typically used together. Concrete without steel reinforcement (usually ribbed round bars called Rebar) crumbles under tensile loads. Steel on its own, without solid concrete floors, is likewise not a preferred building method.
While rebar is almost always steel, it is not considered a structural steel and is described separately in the Rebar and Reinforced concrete articles.
Thermal properties
The properties of steel vary widely, depending on its alloying elements.
The austenizing temperature, the temperature where a steel transforms to an austenite crystal structure, for steel starts at 900°C for pure iron, then, as more carbon is added, the temperature falls to a minimum 724°C for eutectic steel (steel with only .83% by weight of carbon in it). As 2.1% carbon (by mass) is approached, the austenizing temperature climbs back up, to 1130°C. Similarly, the melting point of steel changes based on the alloy.
The lowest temperature at which a plain carbon steel can begin to melt, its solidus, is 1130 °C. Steel never turns into a liquid below this temperature. Pure Iron ('Steel' with 0% Carbon) starts to melt at 1492 °C (2720 °F), and is completely liquid upon reaching 1539 °C (2802 °F). Steel with 2.1% Carbon by weight begins melting at 1130 °C (2066 °F), and is completely molten upon reaching 1315 °C (2400 °F). 'Steel' with more than 2.1% Carbon is no longer Steel, but is known as Cast iron. http://www.msm.cam.ac.uk/phase-trans/images/FeC.gif
Fireproofing of structural steel
In order for a fireproofing product to qualify for a certification listing of structural steel, through a fire test, the critical temperature is set by the national standard, which governs the test. In Japan, this is below 400°C. In China, Europe and North America, it is set at ca. 540°C. The time it takes for the steel element that is being tested to reach the temperature set by the national standard determines the duration of the fire-resistance rating.
Care must be taken to ensure that thermal expansion of structural elements does not damage fire-resistance rated wall and floor assemblies. Penetrants in a firewalls and ferrous cable trays in organic firestops should be installed in accordance with an appropriate certification listing that complies with the local building code.
Open Web Steel Joists (OWSJ) require a great deal of spray fireproofing because they are not very massive and also because they are so open, that a lot of the sprayed plaster flies right past its constituent parts during the coating process.
Structural steel requires external insulation (fireproofing) in order to prevent the steel from weakening in the event of a fire. When heated, steel expands and softens, eventually losing its structural integrity. Given enough energy, it can also melt. Heat transfer to the steel can be slowed by the use of fireproofing materials. While concrete structures that comprise buildings are able to achieve fire-resistance ratings with out additional fireproofing, concrete can be subject to severe spalling, particularly if it has an elevated moisture content. Fireproofing is available for concrete but this is typically not used in buildings. Instead, it is used in traffic tunnels and locations where a hydrocarbon fire is likely to break out. Thus, steel and concrete compete against one another not only on the basis of the price per unit of mass but also on the basis of the pricing for the fireproofing that must be added in order to satisfy the passive fire protection requirements that are mandated through building codes. Common fireproofing methods for structural steel include intumescent, endothermic and plaster coatings as well as drywall and calcium silicate cladding.
See also
References
- ^ Manual of Steel Construction, 8th Edition, 2nd revised printing, American Institute of Steel Construction, 1987, ch 1 page 1-5
External links
- The American Institute of Steel Construction, Inc.
- Product Data Standards for Structural Steel (CIS/2, IFC) The product data standards facilitate interoperability between CAD software