Structural material

Last updated

Stress-strain curve for low-carbon steel. Hooke's law (see above) is only valid for the portion of the curve between the origin and the yield point (2).
1: Ultimate strength
2: Yield strength (yield point)
3: Rupture
4: Strain hardening region
5: Necking region
A: Apparent stress (F/A0)
B: Actual stress (F/A) Stress v strain A36 2.svg
Stress–strain curve for low-carbon steel. Hooke's law (see above) is only valid for the portion of the curve between the origin and the yield point (2).

Structural engineering depends on the knowledge of materials and their properties, in order to understand how different materials resist and support loads.

Contents

Common structural materials are:

Iron

Wrought iron

Wrought iron is the simplest form of iron, and is almost pure iron (typically less than 0.15% carbon). It usually contains some slag. Its uses are almost entirely obsolete, and it is no longer commercially produced.

Wrought iron is very poor in fires. It is ductile, malleable and tough. It does not corrode as easily as steel.

Cast iron

Cast iron is a brittle form of iron which is weaker in tension than in compression. It has a relatively low melting point, good fluidity, castability, excellent machinability and wear resistance. Though almost entirely replaced by steel in building structures, cast irons have become an engineering material with a wide range of applications, including pipes, machine and car parts.

Cast iron retains high strength in fires, despite its low melting point. It is usually around 95% iron, with between 2.1% and 4% carbon and between 1% and 3% silicon. It does not corrode as easily as steel.

Steel

The 630-foot (192 m) high, stainless-clad (type 304) Gateway Arch in Saint Louis, Missouri Gateway arch.jpg
The 630-foot (192 m) high, stainless-clad (type 304) Gateway Arch in Saint Louis, Missouri

Steel is an iron alloy with controlled level of carbon (between 0.0 and 1.7% carbon).

Steel is used extremely widely in all types of structures, due to its relatively low cost, high strength-to-weight ratio and speed of construction.

Steel is a ductile material, which will behave elastically until it reaches yield (point 2 on the stress–strain curve), when it becomes plastic and will fail in a ductile manner (large strains, or extensions, before fracture at point 3 on the curve). Steel is equally strong in tension and compression.

Steel is weak in fires, and must be protected in most buildings. Despite its high strength to weight ratio, steel buildings have as much thermal mass as similar concrete buildings.

The elastic modulus of steel is approximately 205 GPa.

Steel is very prone to corrosion (rust).

Stainless steel

Stainless steel is an iron-carbon alloy with a minimum of 10.5% chromium content. There are different types of stainless steel, containing different proportions of iron, carbon, molybdenum, nickel. It has similar structural properties to steel, although its strength varies significantly.

It is rarely used for primary structure, and more for architectural finishes and building cladding.

It is highly resistant to corrosion and staining.

Concrete

The interior of the Sagrada Familia, constructed of reinforced concrete to a design by Antoni Gaudi Barcelona sagrada familia interno.jpg
The interior of the Sagrada Familia, constructed of reinforced concrete to a design by Antoni Gaudí
A "cage" of reinforcing steel Trebar.jpg
A "cage" of reinforcing steel

Concrete is used extremely widely in building and civil engineering structures, due to its low cost, flexibility, durability, and high strength. It also has high resistance to fire.

Concrete is a non-linear, non-elastic and brittle material. It is strong in compression and very weak in tension. It behaves non-linearly at all times. Because it has essentially zero strength in tension, it is almost always used as reinforced concrete, a composite material. It is a mixture of sand, aggregate, cement and water. It is placed in a mould, or form, as a liquid, and then it sets (goes off), due to a chemical reaction between the water and cement. The hardening of the concrete is called hydration. The reaction is exothermic (gives off heat).

Concrete increases in strength continually from the day it is cast. Assuming it is not cast under water or in constantly 100% relative humidity, it shrinks over time as it dries out, and it deforms over time due to a phenomenon called creep. Its strength depends highly on how it is mixed, poured, cast, compacted, cured (kept wet while setting), and whether or not any admixtures were used in the mix. It can be cast into any shape that a form can be made for. Its colour, quality, and finish depend upon the complexity of the structure, the material used for the form, and the skill of the worker.

The elastic modulus of concrete can vary widely and depends on the concrete mix, age, and quality, as well as on the type and duration of loading applied to it. It is usually taken as approximately 25 GPa for long-term loads once it has attained its full strength (usually considered to be at 28 days after casting). It is taken as approximately 38 GPa for very short-term loading, such as footfalls.

Concrete has very favourable properties in fire – it is not adversely affected by fire until it reaches very high temperatures. It also has very high mass, so it is good for providing sound insulation and heat retention (leading to lower energy requirements for the heating of concrete buildings). This is offset by the fact that producing and transporting concrete is very energy intensive. To study the material behavior plenty of numerical models were developed, e.g. the microplane model for constitutive laws of materials.

Reinforced concrete

Reinforced concrete is concrete in which steel reinforcement bars ("rebars"), plates or fibers have been incorporated to strengthen a material that would otherwise be brittle. In industrialised countries, nearly all concrete used in construction is reinforced concrete. Due to its weakness in tension capacity, concrete will fail suddenly and in brittle manner under flexural (bending) or tensile force unless adequately reinforced with steel.

Prestressed concrete

Prestressed concrete is a method for overcoming the concrete's natural weakness in tension. [1] [2] It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. Prestressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that offsets the tensile stress that the concrete compression member would otherwise experience due to a bending load.

Aluminium

Stress vs. strain curve typical of aluminium
1. Ultimate strength
2. Yield strength
3. Proportional Limit Stress
4. Rupture
5. Offset strain (typically 0.002). Stress v strain Aluminum 2.png
Stress vs. strain curve typical of aluminium
1. Ultimate strength
2. Yield strength
3. Proportional Limit Stress
4. Rupture
5. Offset strain (typically 0.002).

Aluminium is a soft, lightweight, malleable metal. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa. Aluminium has about one-third the density and stiffness of steel. It is ductile, and easily machined, cast, and extruded.

Corrosion resistance is excellent due to a thin surface layer of aluminium oxide that forms when the metal is exposed to air, effectively preventing further oxidation. The strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper.

Aluminium is used in some building structures (mainly in facades) and very widely in aircraft engineering because of its good strength to weight ratio. It is a relatively expensive material.

In aircraft it is gradually being replaced by carbon composite materials.

Composites

Light composite aircraft Rutan.variEze.g-veze.arp.jpg
Light composite aircraft

Composite materials are used increasingly in vehicles and aircraft structures, and to some extent in other structures. They are increasingly used in bridges, especially for conservation of old structures such as Coalport cast iron bridge built in 1818. Composites are often anisotropic (they have different material properties in different directions) as they can be laminar materials. They most often behave non-linearly and will fail in a brittle manner when overloaded.

They provide extremely good strength to weight ratios, but are also very expensive. The manufacturing processes, which are often extrusion, do not currently provide the economical flexibility that concrete or steel provide. The most commonly used in structural applications are glass-reinforced plastics.

Masonry

A brick wall built using Flemish bond Flemish Bond.jpg
A brick wall built using Flemish bond

Masonry has been used in structures for thousands of years, and can take the form of stone, brick or blockwork. Masonry is very strong in compression but cannot carry tension (because the mortar between bricks or blocks is unable to carry tension). Because it cannot carry structural tension, it also cannot carry bending, so masonry walls become unstable at relatively small heights. High masonry structures require stabilisation against lateral loads from buttresses (as with the flying buttresses seen in many European medieval churches) or from windposts.

Historically masonry was constructed with no mortar or with lime mortar. In modern times cement based mortars are used. The mortar glues the blocks together, and also smooths out the interface between the blocks, avoiding localised point loads that might have led to cracking.

Since the widespread use of concrete, stone is rarely used as a primary structural material, often only appearing as a cladding, because of its cost and the high skills needed to produce it. Brick and concrete blockwork have taken its place.

Masonry, like concrete, has good sound insulation properties and high thermal mass, but is generally less energy intensive to produce. It is just as energy intensive as concrete to transport.

Timber

The reconstructed Globe Theatre, London, by Buro Happold London, UK (August 2014) - 156.JPG
The reconstructed Globe Theatre, London, by Buro Happold

Timber is the oldest of structural materials, and though mainly supplanted by steel, masonry and concrete, it is still used in a significant number of buildings. The properties of timber are non-linear and very variable, depending on the quality, treatment of wood, and type of wood supplied. The design of wooden structures is based strongly on empirical evidence.

Wood is strong in tension and compression but can be weak in bending due to its fibrous structure. Wood is relatively good in bonfires as it chars, which provides the wood in the centre of the element with some protection and allows the structure to retain some strength for a reasonable length of time.

Other structural materials

Bamboo scaffolding can reach great heights. BambooConstructionHongKong.jpg
Bamboo scaffolding can reach great heights.

Related Research Articles

Alloy Mixture or metallic solid solution composed of two or more elements

An alloy is an admixture of metals, or a metal combined with one or more other elements. For example, combining the metallic elements gold and copper produces red gold, gold and silver becomes white gold, and silver combined with copper produces sterling silver. Combining iron with non-metallic carbon or silicon produces alloys called steel or silicon steel. The resulting mixture forms a substance with properties that often differ from those of the pure metals, such as increased strength or hardness. Unlike other substances that may contain metallic bases but do not behave as metals, such as aluminium oxide (sapphire), beryllium aluminium silicate (emerald) or sodium chloride (salt), an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, opacity, and luster. Alloys are used in a wide variety of applications, from the steel alloys, used in everything from buildings to automobiles to surgical tools, to exotic titanium alloys used in the aerospace industry, to beryllium-copper alloys for non-sparking tools. In some cases, a combination of metals may reduce the overall cost of the material while preserving important properties. In other cases, the combination of metals imparts synergistic properties to the constituent metal elements such as corrosion resistance or mechanical strength. Examples of alloys are steel, solder, brass, pewter, duralumin, bronze, and amalgams.

Concrete Composite construction material

Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement that hardens (cures) over time. In the past, lime based cement binders, such as lime putty, were often used but sometimes with other hydraulic cements, such as a calcium aluminate cement or with Portland cement to form Portland cement concrete. Many other non-cementitious types of concrete exist with other methods of binding aggregate together, including asphalt concrete with a bitumen binder, which is frequently used for road surfaces, and polymer concretes that use polymers as a binder.

Metallurgy Domain of materials science that studies the physical and chemical behavior of metals

Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are called alloys. Metallurgy encompasses both the science and the technology of metals; that is, the way in which science is applied to the production of metals, and the engineering of metal components used in products for both consumers and manufacturers. Metallurgy is distinct from the craft of metalworking. Metalworking relies on metallurgy in a similar manner to how medicine relies on medical science for technical advancement. A specialist practitioner of metallurgy is known as a metallurgist.

Masonry The building of structures from individual units of stone, brick, or block

Masonry is the building of structures from individual units, which are often laid in and bound together by mortar; the term masonry can also refer to the units themselves. The common materials of masonry construction are brick, building stone such as marble, granite, and limestone, cast stone, concrete block, glass block, and adobe. Masonry is generally a highly durable form of construction. However, the materials used, the quality of the mortar and workmanship, and the pattern in which the units are assembled can substantially affect the durability of the overall masonry construction. A person who constructs masonry is called a mason or bricklayer. These are both classified as construction trades.

Steel Metal alloy made by combining iron with other elements

Steel is an alloy of iron with typically a few percent of carbon to improve its strength and fracture resistance compared to iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant need typically an additional 11% chromium. Because of its high tensile strength and low cost, steel is used in buildings, infrastructure, tools, ships, trains, cars, machines, electrical appliances, and weapons. Iron is the base metal of steel. Depending on the temperature, it can take two crystalline forms : body-centred cubic and face-centred cubic. The interaction of the allotropes of iron with the alloying elements, primarily carbon, gives steel and cast iron their range of unique properties.

Structural engineering

Structural engineering is a sub-discipline of civil engineering in which structural engineers are trained to design the 'bones and muscles' that create the form and shape of man-made structures. Structural engineers need to understand and calculate the stability, strength and rigidity and earthquake of built structures for buildings and nonbuilding structures. The structural designs are integrated with those of other designers such as architects and building services engineer and often supervise the construction of projects by contractors on site. They can also be involved in the design of machinery, medical equipment, and vehicles where structural integrity affects functioning and safety. See glossary of structural engineering.

Reinforced concrete Concrete with rebar

Reinforced concrete (RC), also called reinforced cement concrete (RCC), is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having higher tensile strength or ductility. The reinforcement is usually, though not necessarily, steel bars (rebar) and is usually embedded passively in the concrete before the concrete sets.

Cast iron

Cast iron is a group of iron-carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its colour when fractured: white cast iron has carbide impurities which allow cracks to pass straight through, grey cast iron has graphite flakes which deflect a passing crack and initiate countless new cracks as the material breaks, and ductile cast iron has spherical graphite "nodules" which stop the crack from further progressing.

Rebar Steel reinforcement

Rebar, known when massed as reinforcing steel or reinforcement steel, is a steel bar or mesh of steel wires used as a tension device in reinforced concrete and reinforced masonry structures to strengthen and aid the concrete under tension. Concrete is strong under compression, but has weak tensile strength. Rebar significantly increases the tensile strength of the structure. Rebar's surface is often "deformed" with ribs, lugs or indentations to promote a better bond with the concrete and reduce the risk of slippage.

Carbon steel Steel in which the main interstitial alloying constituent is carbon

Carbon steel is a steel with carbon content from about 0.05% up to 2.1% by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

Prestressed concrete Form of concrete used in construction

Prestressed concrete is a form of concrete used in construction. It is substantially "prestressed" (compressed) during production, in a manner that strengthens it against tensile forces which will exist when in service.

Seismic retrofit Modification of existing structures to make them more resistant to seismic activity

Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with our recent experiences with large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged. Prior to the introduction of modern seismic codes in the late 1960s for developed countries and late 1970s for many other parts of the world, many structures were designed without adequate detailing and reinforcement for seismic protection. In view of the imminent problem, various research work has been carried out. State-of-the-art technical guidelines for seismic assessment, retrofit and rehabilitation have been published around the world – such as the ASCE-SEI 41 and the New Zealand Society for Earthquake Engineering (NZSEE)'s guidelines. These codes must be regularly updated; the 1994 Northridge earthquake brought to light the brittleness of welded steel frames, for example.

Ductile iron

Ductile iron, also known as ductile cast iron, nodular cast iron, spheroidal graphite iron, spheroidal graphite cast iron and SG iron, is a type of graphite-rich cast iron discovered in 1943 by Keith Millis. While most varieties of cast iron are weak in tension and brittle, ductile iron has much more impact and fatigue resistance, due to its nodular graphite inclusions.

Tempering (metallurgy) Process of heat treating used to increase toughness of iron-based alloys

Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered at much higher temperatures.

Titanium alloys are alloys that contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness. They are light in weight, have extraordinary corrosion resistance and the ability to withstand extreme temperatures. However, the high cost of both raw materials and processing limit their use to military applications, aircraft, spacecraft, bicycles, medical devices, jewelry, highly stressed components such as connecting rods on expensive sports cars and some premium sports equipment and consumer electronics.

Structural steel is a category of steel used for making construction materials in a variety of shapes. Many structural steel shapes take the form of an elongated beam having a profile of a specific cross section. Structural steel shapes, sizes, chemical composition, mechanical properties such as strengths, storage practices, etc., are regulated by standards in most industrialized countries.

This is an alphabetical list of articles pertaining specifically to structural engineering. For a broad overview of engineering, please see List of engineering topics. For biographies please see List of engineers.

Architectural metals

Metals used for architectural purposes include lead, for water pipes, roofing, and windows; tin, formed into tinplate; zinc, copper and aluminium, in a range of applications including roofing and decoration; and iron, which has structural and other uses in the form of cast iron or wrought iron, or made into steel. Metal alloys used in building include bronze ; brass ; monel metal and nickel silver, mainly consisting of nickel and copper; and stainless steel, with important components of nickel and chromium.

Carbon fiber reinforced polymer, Carbon fibre reinforced polymer, or carbon fiber reinforced plastic, or carbon fiber reinforced thermoplastic, is an extremely strong and light fiber-reinforced plastic which contains carbon fibers. The spelling 'fibre' is typically used outside the US. CFRPs can be expensive to produce, but are commonly used wherever high strength-to-weight ratio and stiffness (rigidity) are required, such as aerospace, superstructures of ships, automotive, civil engineering, sports equipment, and an increasing number of consumer and technical applications.

This glossary of structural engineering terms pertains specifically to structural engineering and its sub-disciplines. Please see glossary of engineering for a broad overview of the major concepts of engineering.

References

  1. Nawy, Edward G. (1989). Prestressed Concrete. Prentice Hall. ISBN   0-13-698375-8.
  2. Nilson, Arthur H. (1987). Design of Prestressed Concrete. John Wiley & Sons. ISBN   0-471-83072-0.

Further reading