Structural steel

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Structural steel is steel construction material, a profile, formed with a specific shape and certain standards of chemical composition and strength. Structural steel shape, size, composition, strength, storage, etc, is regulated in most industrialised countries.

A steel I-beam, in this case used to support wooden beams in a house. The I-beam is probably the most recognizable structural steel element: I-beams and related shapes are used widely in all-steel construction and composite construction with concrete, wood, or other structural materials.
A steel I-beam, in this case used to support wooden beams in a house. The I-beam is probably the most recognizable structural steel element: I-beams and related shapes are used widely in all-steel construction and composite construction with concrete, wood, or other structural materials.
Steel is sometimes described as a sea of electrons. Protons are virtually surrounded by electrons. It is easy to see how the addition of heat first causes expansion and then softening, to the point of liquification. That is how steel is manufactured and that is how it acts as a structural element in a building fire. Proper fireproofing mitigates this. Still, care must be taken to ensure that expansion of structural elements does not damage wall and floor assemblies required to have a fire-resistance rating. Of particular concern are any penetrants in a firewalls and ferrous cable trays in organic firestops.
Steel is sometimes described as a sea of electrons. Protons are virtually surrounded by electrons. It is easy to see how the addition of heat first causes expansion and then softening, to the point of liquification. That is how steel is manufactured and that is how it acts as a structural element in a building fire. Proper fireproofing mitigates this. Still, care must be taken to ensure that expansion of structural elements does not damage wall and floor assemblies required to have a fire-resistance rating. Of particular concern are any penetrants in a firewalls and ferrous cable trays in organic firestops.
Structural steel in construction: A primed steel beam is holding up the floor above, which consists of a metal deck (Q-Deck), upon which a concrete slab has been poured. The masonry wall to the right stops short of the deck and is joined to the deck with a firestop system, consisting of stuffed rockwool and silicone caulking, in a manner consistent with the bounding requirements of passive fire protection. Once the firestopping is complete, fireproofing of the beam and the deck may follow.
Structural steel in construction: A primed steel beam is holding up the floor above, which consists of a metal deck (Q-Deck), upon which a concrete slab has been poured. The masonry wall to the right stops short of the deck and is joined to the deck with a firestop system, consisting of stuffed rockwool and silicone caulking, in a manner consistent with the bounding requirements of passive fire protection. Once the firestopping is complete, fireproofing of the beam and the deck may follow.
Steel beam through-penetration. The firestop surrounding the beam is incomplete - packing only, sealant is yet to be applied. The beam itself must be treated with fireproofing to prevent it from twisting and damaging the wall during a fire. The beam is the penetrant.
Steel beam through-penetration. The firestop surrounding the beam is incomplete - packing only, sealant is yet to be applied. The beam itself must be treated with fireproofing to prevent it from twisting and damaging the wall during a fire. The beam is the penetrant.
Metal deck and OWSJ (Open Web Steel Joist), receiving first coat of spray fireproofing plaster, made of polystyrene leavened gypsum, all subject to bounding on the basis of Underwriters Laboratories product certification listings. 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.
Metal deck and OWSJ (Open Web Steel Joist), receiving first coat of spray fireproofing plaster, made of polystyrene leavened gypsum, all subject to bounding on the basis of Underwriters Laboratories product certification listings. 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.

Contents

[edit] 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)
  • Railway rail (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.
  • Plate, sheet metal thicker than 6 mm or 1/4 in.

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).

[edit] Structural steels

Most industrialised countries prescribe a range of standard steel grades with different strengths, corrosion resistance and other properties.

[edit] 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).

[edit] 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]

[edit] 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

[edit] 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

[edit] Corrosion resistant high strength low alloy steels

[edit] Quenched and tempered alloy steels

  • A514 - structural shapes and plates
  • A517 - boilers and pressure vessels

[edit] 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 really needed together. Concrete without steel re-enforcement (usually ribbed round bars called Rebar) is not structurally sound. 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.

[edit] Fire protection with steel vs. competition

As the critical temperature for steel is around 540°C (give or take, depending on whose country's test standards one reads at the time), and design basis fires reach this temperature within a few minutes, structural steel requires external insulation in order to prevent the steel from absorbing enough energy to reach this temperature. First, steel expands, when heated, and once enough energy has been absorbed, it softens and loses its structural integrity. This is easily prevented through the use of fireproofing. Likewise, although concrete structures on their own are able to achieve fire-resistance ratings, concrete is also subject to severe spalling, especially with elevated moisture inside the concrete at the time it becomes exposed to fire. There is also fireproofing 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.

[edit] See also

[edit] References

  1. ^ Manual of Steel Construction, 8th Edition, 2nd revised printing, American Institute of Steel Construction, 1987, ch 1 page 1-5


[edit] External links

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