Structural steel is designed for a variety of construction uses. Ductility is an essential property of structural steel which allows for redistribution of stresses. Because of the high ductility structural steel can be customized into a wide variety of shapes, sizes and thicknesses. There are strict standards in most countries that regulate steel shapes, sizes, chemical compositions and mechanical properties.
Many structural steel shapes are designed for a good strength to weight ratio and therefore they are able to support extremely heavy loads without sagging making steel great reinforcement material.
Shapes Used in Structural Steel
The following steel shapes are commonly found worldwide but are given in different standards throughout the world.
- An I-beam has two horizontal elements called flanges and a vertical element called the web. In Britain an I-beam is known as a Universal Beam (UB) or Universal Column (UC), In Europe it includes the IPE, HE, HL, HD and other sections; in the US it includes Wide Flange (WF or W-Shape) and H sections)
- All construction industries commonly use I-beams as the construction is very effective for carrying bending loads.
- Z-Shape contains two half flanges going in opposite directions
- Hollow structural section also known as HSS-Shape includes square, rectangular, elliptical and circular cross sections
- L-shaped cross-section also known as angle
- C-beam, or C cross-section also known as structural channel
- T-shaped cross-section also known as tee
- Rail profile an asymmetrical Universal Beam also known as
- Railway rail, Vignoles rail, Flanged T-rail, Grooved rail
- Bar, a long piece with a rectangular cross section
- Rod, a round or square section with more length than width compared to its width, also called rebar and dowel.
- Metal sheets thicker than 6 mm or 1⁄4 in are known as plates
- Open web steel joist
Structural steel shapes may be made by hot rolling, cold rolling or welding together sections.
For decades the terms angle iron, channel iron, and sheet iron have been commonly used to describe wrought iron. Nowadays steel has replaced iron, but these terms are still informally used however it is incorrect to refer to steel using these old terms. The correct terms are angle stock, channel stock, and sheet.
Standards in Structural Steel
Standards found in Europe
There are many national standards in force when it comes to steel manufacture, and most steels used in Europe comply to the European standard EN 10025.
S275J2 and S355K2W are typical designations for grades of steel, with the S designating them as structural steel, the following three digits describing the yield strength in newtons per square millimetre or in megapascals, and the following letter-number combination is the toughness rating on the Charpy impact test. The final letter W on the second example indicates that the item is comprised of weathering steel. Designations may also include letters that indicate fine grain steel (N or NL), quenched and tempered steel (Q or QL) and, finally, thermomechanically rolled steel is indicated by M or ML.
This is the EN 10219 specification, EN 10210 standard and is very common. It is sometimes written without the space, as EN10219. The designation (in either form) indicates an item that is cold formed welded structural hollow sections of fine grain, non-alloy steels.
EN 10219-1 includes the additional digit at the end, which specifies the delivery conditions of structural hollow sections that were formed cold without any heat treatment following that stage of the process.
EN 10219-2 contains the requirements for S275JOH pipe tolerances and dimensions and s275 pipe properties.
Manufacture Process for S275JOH Steel Pipes
There is discretion available on the part of the steel producer. S275JOH carbon steel pipes might be made via processes including ERW, SAW or seamless techniques, but all S275JOH steel and S275JOH pipes do need to conform to EN 10219 standards.
Yield strength for steel materials varies, and grades include 195, 235, 275, 355, 420 and 460. Almost all structural steel in the UK is of either S275 or S355 grading. Quenching and tempering the steel can produce higher grades of steel strength, including 500, 550, 620, 690, 890 and 960. Currently, little or no construction is using grades higher than 690.
The shape of a set of standard structural profiles are defined by five Euronorms:
- European I-beam: IPE – Euronorm 19-57
- European I-beam: IPN – DIN 1025-1
- European flange beams: HE – Euronorm 53-62
- European channels: UPN – DIN 1026-1
- European cold formed IS IS 800-1
In the United States, alloyed steels used in constructing buildings are identified and specified by a body called ASTM International. The designation for building construction material begins with an A, followed by between two and four numbers. The four-number designations are usually for mechanical engineering. A separate system of naming is used for steel used for machines and vehicles.
The kinds of structural steel most commonly used include:
- A36 – for structural shapes and plate.
- A53 – for structural pipe and tubing.
- A500 – for structural pipe and tubing.
- A501 – for structural pipe and tubing.
- A529 – for structural shapes and plate.
- A1085 – for structural pipe and tubing.
High strength low alloy steels
- A441 – for structural shapes and plates – (Superseded by A572)
- A572 – for structural shapes and plates.
- A618 – for structural pipe and tubing.
- A992 – for applications such as W or S I-Beams.
- A913 – for quenched and self tempered (QST) W shapes.
- A270 – for structural shapes and plates.
Corrosion resistant high strength low alloy steels
- A243 – for structural shapes and plates.
- A588 – for structural shapes and plates.
Quenched and tempered alloy steels
- A514 – for structural shapes and plates.
- A517 – for boilers and pressure vessels.
- Eglin steel – for lower-cost aerospace and weaponry items.
- A668 – for steel forgings
What is a CE marking?
The Construction Products Directive (CPD), a European directive, introduced CE marking for all steel and construction products. The CPD ensures uniformity of grading and description, facilitating the free movement of products and materials throughout the EU.
The Factory Production Control (FPC) system of the factory must be assessed by a suitable certification body, one that has been approved by the European Commission, in order to be allowed to add a CE Marking to the items and/or materials. This ensures that these ‘safety critical’ items are in fact of the quality stated on the labelling. For example, the CE Marking on products such as fabricated steelwork and bolts, verifies that the product’s manufacture and final attributes comply with the relevant harmonised standard (see below).
When it comes to steel structures, the standards are indicated by the following descriptive designations:
- For steel sections and plate, it is: EN 10025-1
- For hollow sections, it is: EN 10219-1 and EN 10210-1
- For pre-loadable bolts, it is: EN 14399-1
- For non-preloadable bolts, it is: EN 15048-1
- For fabricated steel, it is: EN 1090 -1
The CE Marking standard for structural steelwork is EN 1090-1.
The standard that covers CE Marking of structural steelwork, as of late 2010, is EN 1090-1. CE Marking became standard in the EU, after a 2-year transition period, in 2014.
Concrete or Steel?
Steel and concrete are not the only materials used in construction of course, but they are among the most plentiful and widely used materials in most modern building construction. Steel of various grades and attributes, concrete of various grades and attributes, and other materials, such as clay, mortar, ceramics, wood, masonry are all commonly used.
For load-bearing purposes, such as structural framing and weight-bearing crossbeams, the materials usually used include some combination of structural steel, concrete, masonry, and/or wood. Depending on the conditions and desired performance of the structural component, different combinations, grades and designs will be employed. By far the most common and plentiful component materials in these situations are reinforced concrete and steel. The best grade, material combination, and design for the purpose is determined by an engineer. Factors that influence these decisions include weight, strength, constructability, sustainability, availability, longevity, fire resistance, appearance and cost.
Let’s take a more detailed look at a few of these factors:
This will depend on several factors, such as the location of the build, the size of the order made, transport costs, availability and cost of supporting machinery, components and skilled and unskilled labour. Reinforced concrete, for example, requires form work prior to pouring, which accounts for roughly half of the finished cost. Prep work demands are high, but once this work is properly completed, the concrete can be poured in and allowed to cure. It them forms a solid, strong material that has conformed to the desired shape it took when in its pre-cured, liquid form. Precast concrete has become a popular way to reduce costs (through factory manufacturing methods) and maintain greater regularity of shape and form. Manufacture is swift and so, assuming transport is available and efficient, using preform methods can speed up other aspects of a build as well, saving costs over a wider range of factors than just the concrete itself. Since the steel (which is used to reinforce the concrete from with) is sold by weight, structural designers determine the lightest and least amount of steel that will still produce the required strength and other properties needed for the components. Bulk buying identical components (even though some may be over-engineered for their purpose) can greatly reduce costs as compared to buying each component with properties specific to the job at hand.
Strength-to-Weight Ratios, or specific strengths, are common ways to categorize construction materials. The strength is divided by the density, and the resulting rating is used to indicate how useful the material would be in a given situation of for a given purpose. For example, concrete is ten times greater for compression strength than for tension strength, so its strength-weight ratio is much higher for situations in which compression strength is the main attribute needed.
As environmental concerns grow in importance and urgency, many construction companies and materials vendors are listing sustainability attributes as major features of their products. Using sustainable and sustainably-manufactured materials usually does not significantly affect the performance or cost of structures, and some of them are actually less expensive. Currently, for example, more than 80% of structural steel members are made from recycled materials (A992 steel). It is cheaper and has a higher strength to weight ration than A36 grade steel members. Concrete primarily uses naturally-occurring materials as components, and is now being made to be permeable, which decreases the need for drainage and overflow infrastructure, as the water can pass through the surfaces themselves. Disposing of old concrete is also less environmentally harmful, as it can be used as aggregate in other construction projects, rather than simply thrown into a landfill.
Fire can be one of the most frightening and dangerous risks to a structure and those living and working within it. In climates where the weather is dry and windy, a fire can mean a roaring inferno in minutes, and wooden structures are especially susceptible to this danger. Even structural steel can be in danger of failure in such conditions. Using reinforced concrete, both as a primary part of the structure and as a firebreak or shield for other materials, is a great way to mitigate these risks.
Corrosion from exposure to water, heat, humidity, salt and other substances can pose a long-term problem to some building materials, causing damage to the appearance of the materials, but also to structural integrity. When installing some materials, special steps must be taken to ensure such materials are protected from potentially harmful elements, and maintenance of such materials needs to be done regularly and in compliance with recommended care procedures. Structural steel may rust if exposed to water, wood may rot, and mould may infiltrate cracks and cavities in the structure, causing danger to those living and working in the vicinity of the structure. These are all well-known risks, however, and both materials manufacturers and construction companies take steps to mitigate risks and educate users as to best practices to stay safe and extend the usable life of these products and structures.
Structural steel had a high compressive strength and a high tensile strength. This means that it is tough, strong in a variety of situations and functions, has excellent ductile properties, high stiffness, and is therefore widely used in a variety of construction situations.
Structural steel can be forged, formed and bent into a variety of shapes, bolted together, welded, cut and formed for nearly any situation or need. Structural steel can also be used immediately upon delivery to the site, whereas some other materials – such as concrete – must be mixed, poured, and then cured for at least 1-2 weeks before construction can continue upon it, and this time period can be affected one way or the other by the weather. This makes use of steel much better for scheduling.
Steel is non-combustible, but it can lose some of its strength, stiffness, and other characteristics when heated to high enough temperatures, which can lead to structural failure in the long-term, or even during the fire itself. For this reason, the International Building Code requires streel structural components to be enveloped in fire-resistance materials – which in turn can increase costs of its use in construction.
Steel can corrode when in contact with water. Brief, or minor contact is not a problem, but prolonged contact with sufficient water – especially in the presence of salt – can cause potentially dangerous problems and weaknesses in the structure. Prevention methods include protective paint and other shields, and placement of steel components where they will be out of reach from water.
Mould can be a problem in building structures, especially in some climates, and such problems are often difficult to detect, as they occur out of sight. Since mould grows better on porous surfaces, wood is more susceptible to it than steel or concrete.
Today’s skyscrapers, high-rises, and other very tall buildings are mainly constructed of structural steel. This is because steel is strong (both tensile and compression), rigid and can be used immediately, so building schedules are more predictable. Because steel has a high strength to weight ratio, it is well-suited to tall buildings that need to sustain structural integrity from the subterranean base to the towering top floors.
Lower buildings, however, do not require this high strength to weight ratio, and contain fewer floors (storeys) and so they are often made of concrete – which is cheaper to buy.
Structural steel and reinforced concrete are the top choices of building material, but builders do not always choose the best one for the purpose. Often the choice is made from a financial profit point of view. Both builders and designers know that they need to turn a profit to stay in business, so if the cheaper product will do the job – even if the other one would do the job better – they will choose it on that basis. Because the cost of materials, transport, and labour surrounding these materials is also in constant flux, the cheaper material for a job one year, might not be the cheapest for the same job the next year. Most of these factors are taken into consideration at the design stage, and companies must hope conditions have not significantly changed in the time between design and implementation of actual construction.
Steel and Reinforced Concrete
In most cases, building designers combine the strengths and weaknesses of both steel and concrete, using both of them for different purposes within the same structure. For example, steel may be used as the vertical and horizontal beams of a structure, and then floors of reinforced concrete are poured into moulds supported by the steel girders. While the floor of one storey is curing, construction of the next storey can continue on the bracing provided by the steel girders and beams. At the base of such buildings, usually well underground, reinforced concrete may be used as the footings or foundations of the structure. The high compression strength of concrete is excellent in these conditions, and it will not corrode if exposed to moisture.
The variety of designs available to a structural engineer or designer are nearly infinite. The goal is to combine these design features and materials choices to result in an affordable, efficient – and especially safe – building that will last a long time and look good. Input from owners, financiers, interior design experts and other specialists is combined and worked into the design. The end goal is that everyone’s needs are met, their limitations accounted for, and the end result matches or exceeds the initial need.
Resistance to Fire
Steel is manufactured by melting and forming the initial elements, so exposure to fire and other significant heat must be taken into account when planning to use it as a major structural component. The temperature at which steel cannot safely support its load is called the critical temperature of that steel. This is usually the point at which it can only support 60% or its non-heated maximum. This can get complicated very quickly, as various load amounts, load angles, and other factors affect this ability differently. Accepted calculation practices can be used to assess theoretical steel strength in certain conditions, or a fire test can be performed. Critical temperatures vary from country to country as well. In Japan, for example, it is below 400 degrees C; in China, Europe and North America it is below 530-810 degrees C (1000-1300 degrees F). Fireproofing methods can slow the transfer of heat to the component, lowering the temperature of the steel and increasing its reliability under extreme heat conditions.
Concrete buildings have fewer issues regarding fire regulations, mainly because they tend to meet fire resistance requirements without additional modifications. The thickness of the concrete over the rebar is usually sufficient fire protection. Concrete does have one issue in this regard, however, in that it can be subject to spalling, especially in conditions of elevated moisture content. In situations when hydrocarbon fuel fire is more likely, or in high traffic tunnels, additional fire-retardant steps may be used.
Bandsaws are usually used to cut workpieces to length. Drilling holes, milling slots into beams, channels and HSS elements, is normally done using a beam drill line. CNC beam drill lines equipped with feed conveyers and position sensors (to probe for position and move the element into its proper place) are used to drill holes and slots.
A cutting torch is an excellent tool for cutting irregular openings and non-uniform ends on dimensional (non-plate) elements. The most common are oxyfuel. These can be hand-held or fixed to automated CNC coping machines which accurately move the torch head according to pre-programmed specifications.
Plate elements are cut on a stationary table using cutting heads that range from punches, drills and torches. A gantry style arm – sometimes called a bridge – supports and moves the cutting heads across the plate as necessary.