Structural steel

Various structural steel shapes

Structural steel is a category of steel used as a construction material for making structural steel shapes. A structural steel shape is a profile, formed with a specific cross section and following certain standards for chemical composition and mechanical properties. Structural steel shapes, sizes, composition, strengths, storage practices, etc., are regulated by standards in most industrialized countries.

Structural steel members, such as I-beams, have high second moments of area, which allow them to be very stiff in respect to their cross-sectional area.

Common structural shapes

The shapes available are described in many published standards worldwide, and a number of specialist and proprietary cross sections are also available.

A steel I-beam, in this case used to support timber joists in a house.

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).

Standards

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 fine grain steel ('N' or 'NL'); quenched and tempered steel ('Q' or 'QL'); and thermomechanically rolled steel ('M' or 'ML').


1. S275JOH Specification S275JOH is steel grade in EN 10219 specification, EN 10210 standard. And the most widely used specification is EN10219 standard, which is Cold formed welded structural hollow sections of non-alloy and fine grain steels.
EN10219-1 specifies the technical delivery conditions for cold formed welded structural hollow sections of circular, square or rectangular forms and applies to structural hollow sections formed cold without subsequent heat treatment.
Requirements for S275JOH pipe tolerances, dimensions and sectional s275 pipe properties are contained in EN 10219-2.
2. S275JOH Steel Pipes manufacture Process
The steel manufacturing process shall be at the discretion of the steel producer. S275JOH carbon steel pipes can be made in ERW, SAW or seamless process. All S275JOH steel material and S275JOH pipes should conform to EN10219 standards. [1]


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).

A set of euronorms define the shape of a set of standard structural profiles:

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:[2]

Carbon steels

High strength low alloy steels

Corrosion resistant high strength low alloy steels

Quenched and tempered alloy steels

Non-preload bolt assembly (EN 15048)
Pre-load bolt assembly (EN 14399)

CE marking

The concept of CE marking for all construction products and steel products is introduced by the Construction Products Directive (CPD). The CPD is a European Directive that ensures the free movement of all construction products within the European Union.

Because steel components are “safety critical”, CE Marking is not allowed unless the Factory Production Control (FPC) system under which they are produced has been assessed by a suitable certification body that has been approved to the European Commission.[3]

In the case of steel products such as sections, bolts and fabricated steelwork the CE Marking demonstrates that the product complies with the relevant harmonized standard.[4]

For steel structures the main harmonized standards are:

The standard that covers CE Marking of structural steelwork is EN 1090-1. The standard has come into force in late 2010. After a transition period of two years, CE Marking will become mandatory in most European Countries sometime early in 2012.[5] The official end date of the transition period is July 1, 2014.

Steel vs. concrete

Choosing the ideal structural material

Most construction projects require the use of hundreds of different materials. These range from concrete of all different specifications, structural steel of different specifications, clay, mortar, ceramics, wood, etc. In terms of a load bearing structural frame, they will generally consist of structural steel, concrete, masonry, and/or wood, using a suitable combination of each to produce an efficient structure. Most commercial and industrial structures are primarily constructed using either structural steel or reinforced concrete. When designing a structure, an engineer must decide which, if not both, material is most suitable for the design. There are many factors considered when choosing a construction material. Cost is commonly the controlling element; however, other considerations such as weight, strength, constructability, availability, sustainability, and fire resistance will be taken into account before a final decision is made.

Reinforced concrete

Structural steel

The tallest structures today (commonly called "skyscrapers" or high-rise) are constructed using structural steel due to its constructability, as well as its high strength-to-weight ratio. In comparison, concrete, while being less dense than steel, has a much lower strength-to-weight ratio. This is due to the much larger volume required for a structural concrete member to support the same load; steel, though denser, does not require as much material to carry a load. However, this advantage becomes insignificant for low-rise buildings, or those with several stories or less. Low-rise buildings distribute much smaller loads than high-rise structures, making concrete the economical choice. This is especially true for simple structures, such as parking garages, or any building that is a simple, rectilinear shape.[13]

Structural steel and reinforced concrete are not always chosen solely because they are the most ideal material for the structure. Companies rely on the ability to turn a profit for any construction project, as do the designers. The price of raw materials (steel, cement, coarse aggregate, fine aggregate, lumber for form-work, etc.) is constantly changing. If a structure could be constructed using either material, the cheapest of the two will likely control. Another significant variable is the location of the project. The closest steel fabrication facility may be much further from the construction site than the nearest concrete supplier. The high cost of energy and transportation will control the selection of the material as well. All of these costs will be taken into consideration before the conceptual design of a construction project is begun.[8]

Combining steel and reinforced concrete

Structures consisting of both materials utilize the benefits of structural steel and reinforced concrete. This is already common practice in reinforced concrete in that the steel reinforcement is used to provide steel's tensile strength capacity to a structural concrete member. A commonly seen example would be parking garages. Some parking garages are constructed using structural steel columns and reinforced concrete slabs. The concrete will be poured for the foundational footings, giving the parking garage a surface to be built on. The steel columns will be connected to the slab by bolting and/or welding them to steel studs extruding from the surface of the poured concrete slab. Pre-cast concrete beams may be delivered on site to be installed for the second floor, after which a concrete slab may be poured for the pavement area. This can be done for multiple stories.[13] A parking garage of this type is just one possible example of many structures that may use both reinforced concrete and structural steel.

A structural engineer understands that there are an infinite number of designs that will produce an efficient, safe, and affordable building. It is the engineer's job to work alongside the owner(s), contractor(s), and all other parties involved to produce an ideal product that suits everyone's needs.[8] When choosing the structural materials for their structure, the engineer has many variables to consider, such as the cost, strength/weight ratio, sustainability of the material, constructability, etc.

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 (1,650 °F) for pure iron, then, as more carbon is added, the temperature falls to a minimum 724 °C (1,335 °F) 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 1,130 °C (2,070 °F). 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 1,130 °C (2,070 °F). Steel never turns into a liquid below this temperature. Pure Iron ('Steel' with 0% Carbon) starts to melt at 1,492 °C (2,718 °F), and is completely liquid upon reaching 1,539 °C (2,802 °F). Steel with 2.1% Carbon by weight begins melting at 1,130 °C (2,070 °F), and is completely molten upon reaching 1,315 °C (2,399 °F). 'Steel' with more than 2.1% Carbon is no longer Steel, but is known as Cast iron.[14]

Fire resistance

Metal deck and open web steel joist receiving spray fireproofing plaster, made of polystyrene-leavened gypsum.

Steel loses strength when heated sufficiently. The critical temperature of a steel member is the temperature at which it cannot safely support its load. Building codes and structural engineering standard practice defines different critical temperatures depending on the structural element type, configuration, orientation, and loading characteristics. The critical temperature is often considered the temperature at which its yield stress has been reduced to 60% of the room temperature yield stress.[15] In order to determine the fire resistance rating of a steel member, accepted calculations practice can be used,[16] or a fire test can be performed, the critical temperature of which is set by the standard accepted to the Authority Having Jurisdiction, such as a building code. In Japan, this is below 400 °C. In China, Europe and North America (e.g., ASTM E-119), this is approximately 1000–1300 °F[17] (530-810 °C). The time it takes for the steel element that is being tested to reach the temperature set by the test standard determines the duration of the fire-resistance rating. Heat transfer to the steel can be slowed by the use of fireproofing materials, thus limiting steel temperature. Common fireproofing methods for structural steel include intumescent, endothermic, and plaster coatings as well as drywall, calcium silicate cladding, and mineral wool insulating blankets.[18]

Concrete building structures often meet code required fire-resistance ratings, as the concrete thickness over the steel rebar provides sufficient fire resistance. However, concrete can be subject to spalling, particularly if it has an elevated moisture content. Although additional fireproofing is not often applied to concrete building structures, it is sometimes used in traffic tunnels and locations where a hydrocarbon fuel fire is more likely, as flammable liquid fires provides more heat to the structural element as compared to a fire involving ordinary combustibles during the same fire period. Structural steel fireproofing materials include intumescent, endothermic and plaster coatings as well as drywall, calcium silicate cladding, and mineral or high temperature insulation wool blankets. Attention is given to connections, as the thermal expansion of structural elements can compromise fire-resistance rated assemblies.

Manufacturing

Cutting workpieces to length is usually done with a bandsaw.

A beam drill line (drill line) has long been considered an indispensable way to drill holes and mill slots into beams, channels and HSS elements. CNC beam drill lines are typically equipped with feed conveyors and position sensors to move the element into position for drilling, plus probing capability to determine the precise location where the hole or slot is to be cut.

For cutting irregular openings or non-uniform ends on dimensional (non-plate) elements, a cutting torch is typically used. Oxy-fuel torches are the most common technology and range from simple hand-held torches to automated CNC coping machines that move the torch head around the structural element in accordance with cutting instructions programmed into the machine.

Fabricating flat plate is performed on a plate processing center where the plate is laid flat on a stationary 'table' and different cutting heads traverse the plate from a gantry-style arm or "bridge." The cutting heads can include a punch, drill or torch.

See also

References

  1. ↑ "EN10219 S275JOH Carbon Steel Pipe". CHINA HYSP PIPE.
  2. ↑ Manual of Steel Construction, 8th Edition, 2nd revised printing, American Institute of Steel Construction, 1987, ch 1 page 1-5
  3. ↑ The website of the British Constructional Steelwork Association Ltd. - SteelConstruction.org:CE-Marking.08/02/2011.
  4. ↑ Guide to the CE Marking of Structural Steelwork, BCSA Publication No. 46/08. p.1.
  5. ↑ Manufacturer Certification in Compliance with EN 1090, 09.08.2011
  6. 1 2 Levitt, M. Precast Concrete. ISBN 978-0-85334-994-5.
  7. ↑ Popescu, Calin. Estimating Building Costs.
  8. 1 2 3 4 5 6 7 8 9 10 Handbook of Structural Engineering. CRC Press. 1997. ISBN 978-0-8493-2674-5.
  9. ↑ Zaharia, Raul. Designing Steel Structures for Fire Safety. ISBN 978-0-415-54828-1.
  10. ↑ Russ, Tom. Sustainability and Design Ethics. ISBN 978-1-4398-0854-2.
  11. 1 2 Chen, Wai-Fah (2005). Principles of Structural Design. ISBN 978-0-8493-7235-3.
  12. ↑ Armstrong, Robert (7 March 2014). "Properties and Prevention of Household Mold". Absolute Steel. Retrieved 2 November 2014.
  13. 1 2 Taranath, Bungale. Reinforced Concrete Design of Tall Buildings. ISBN 978-1-4398-0480-3.
  14. ↑ http://www.msm.cam.ac.uk/phase-trans/images/FeC.gif
  15. ↑ Industrial fire protection engineering, Robert G. Zalosh, copyright 2003 pg.58
  16. ↑ Zalosh, Pg. 70
  17. ↑ Zalosh, Table 3.3
  18. ↑ Best Practice Guidelines for Structural Fire Resistance Design of Concrete and Steel Buildings, NIST Technical Note 1681, L. T. Phan, J. L. Gross, and T. P. McAllister, 2010. (View report)

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