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. Although steel is non-combustible it starts to lose strength when heated above 400°C.
Nearly all structural steel is formed by the hot rolling process. Steel roof and floor deck panels are fabricated from sheet metal by further cold rolling.
Structural steel is primarily used to form a skeleton for the building or structure. The skeletal framework of the steel frame buildings carries all the loads to which the building is subjected. Steel is most often used for structures where loads and spans are large and therefore is not often used for domestic architecture. (Dennis Lam, 2004)
The standard roll shapes are frequently used as beams and columns, the wide flange or W shape being the most common. The widely separated flanges give it the best profile for resisting the bending action of beams or the buckling action of columns.
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The connections of steel shapes are two types:
Those made in the workshop
Those made at the building site.
Workshop connections are usually welded and site connections are usually made with bolts.
Steel has many advantages when compared to other structural building materials such as concrete, timber, plastics and newer composite materials. According to the American iron and steel institute, steel is one of the friendliest environmental building materials. Steel is 100% recyclable and is the most recycled material in the United States reducing burden on today's landfills. (Berman, 2006)
Some other advantages of steel are high strength, high stiffness and good ductility combined with relative ease of fabrication and competitive cost. (Purkiss, 2008)
Structural steel must comply with the European Standard EN10025 a Governed by the ECISS (European Committee for Iron and Steel Standardisation a subset of CEN (European Committee for Standardisation.
Structural Steel Properties (Philip Quinn)
Steel is composed of about 98 percent of iron with the main alloying elements carbon, silicon and manganese. Structural steel is basically produced in three strength grades S275, S355 and S460. The important design properties are:
The stress strain curves for the three grades of steel are shown in Fig.2 below and these are the basis for the design methods used for steel. Elastic design is kept within the elastic region and because steel is almost perfectly elastic, design based on elastic theory is a very good method to use.
There are two types of strength:
Yield strength of structural steel measures the minimum force required to create a permanent deformation in the steel
Tensile strength of structural steel relates to the point at which permanent deformation occurs when the material is pulled or stretched laterally along its length.
This is a measure of the amount of plastic flow that can be absorbed before failure. It is assessed by measuring the elongation at fracture. Low carbon steels have the highest ductility.
This is the ability of the two pieces of steel to be joined by fusion in a localised region at their boundary.
Design Considerations (Philip Quinn)
Special problems occur with steelwork and good practice must be followed to ensure satisfactory performance in service. The following factors are:
We will now discuss the following headings briefly.
This failure can occur in members subjected to fluctuating loads such as crane girders, bridges and offshore structures.
Failure occurs through initiation and propagation of a crack that starts at a fault and the failure load may be well below its static value.
Weld connections have the greatest effect on the fatigue strength of steel structures. Bolted connections do not reduce the strength under fatigue loading. (Dennis Lam, 2004)
Structural steel is ductile at temperatures above 10°C but becomes more brittle as the temperature falls and fracture can occur at low stresses below 0°C. The Charpy impact test is used to determine the resistance of steel. In design brittle fracture should be avoided by using steel quality grade with adequate impact toughness.
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In addition to taking care in the selection of steel grade to be used it should also be necessary to pay attention to the design detail to reduce the likelihood of brittle fracture.
Thin plates are more resistant than thick ones. Abrupt changes of the sections should be avoided. Fillet welds should not be laid down across tension flanges and intermittent welding should not be used. (Lam, 2004)
Structural steelwork performs badly in fires. The strength decreases with the increase in temperature. At 550°C the yield stress has fallen to about 0.7 of its value at normal temperature, this means it has reached its working stress and failure occurs under working loads.
The requirements of fire protection are usually set out clearly in the approved documents from Building Regulations. These shown or lay down the fire resistance period that any load bearing element in a building must have and also give the fire resistance periods for different types of fire protection.
Fire protection can be provided by encasing the member in concrete, fire board or cementations fibre materials. The main types of fire protection for columns and beams are shown below:
Intumescent paint is being used especially for exposed steelwork.
Exposed steelwork can severely affected by corrosion in the atmosphere. The type of protection depends on the surface conditions and the length of life required. The main types of protective coatings are:
Metallic coatings can be either a sprayed on in line coating of aluminium or zinc is used or the member is coated by hot dipping in a bath of molten zinc.
This is achieved by using a primer of zinc chromate followed by finishing coats of micaceous iron oxide.
The most important factor in achieving good corrosion protection coating is surface protection. Steel is covered with a mill scale and this must be removed before protection is applied.
Advantages of Steel as a Structural Material (Philip Quinn)
Steel exhibits desirable physical properties that make it one of the most versatile structural materials in use. Its great strength, uniformity, light weight, ease of use and many other desirable properties makes it the material of choice for numerous structures such as steel bridges, high rise buildings, towers and other structures.
The many advantages of steel can be summarised as follows:
High strength: This means that the weight of the structure that is made of steel will be small.
Uniformity: Properties of steel do not change as oppose to concrete
Elasticity: Steel follows Hook's Law very accurately
Ductility: a very desirable property of steel in which steel can withstand extensive deformation without failure under high tensile stresses
Toughness: Steel has both strength and ductility
Additions to existing structures: New bays or even new wings can be added to existing frame buildings and steel bridges can be widened.
Disadvantages of Structural Steel (Philip Quinn)
Although steel has its advantages as a structural material it also has many disadvantages. The many disadvantages are as follows:
Maintenance: Steel structures are susceptible to corrosion when exposed to air, water and humidity. They must be painted periodically
Fireproofing Cost: Steel is incombustible material however its strength is reduced tremendously at high temperatures due to fires
Susceptibility to Buckling: For most structures the use of steel columns is very economical because of their strength to weight ratio. However as the length and slenderness of a compressive column is increased its danger of buckling increases.
Fatigue: The strength of a structural steel member can be reduced if this member is subjected to cyclic
Brittle Fracture: Under certain conditions steel may loose its ductility and brittle fracture may occur at places of stress concentration. Fatigue type loadings and very low temperatures trigger the situation.
Typical Standard Steelwork Sections (Liam Doran)
There are various types of standard steel sections available to use for construction. There are a few different ways of forming steel sections. These include the following:
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Hot rolled sections
Cold rolled sections
Hot rolled sections
Hot rolled sections are produced in steel mills from steel billets by passing them through a series of rolls. Different members are produced by varying the flange, web, and leg or wall thickness. There are various components of hot rolled sections which include the following:
Equal and Unequal Angles
Circular and rectangular hollow sections
Compound Sections are formed by strengthening a rolled section such as a UB by welding cover plates, combining two separate rolled sections or connecting two members together to form a strong combined member. Various profiles are possible, which can be designed specifically for extreme situations such as very high loads and long spans, where standard sections alone would be insufficient. Some popular combinations of standard sections include:
Beam or column
The formation of these steel compound sections can be seen in the image below with the various combinations.
Figure : Typical steel box sections (Source from (Greeno, 2010)
Built up sections are made by welding plates together to form I, H or box members which are termed plate girders, built up columns box girders or columns respectively
These members are used where heavy loads have to be carried and in the case of plate and box girders where long spans may required.
Cold rolled sections
Cold rolled steel sections are a lightweight alternative to the relatively heavy, hot rolled steel sections that have been traditionally used in sub-framing situations, e.g. purlins, joists and sheeting rails. Cold rolled sections are generally only a few millimetres in wall thickness, saving on material and handling costs and building dead load. They are also produced in a wide variety of section profiles that can be seen below.
Figure : Typical section profiles. (Source from (Greeno, 2010)
Dimensions vary considerably and many non-standard profiles are made for particular situations. A range of standard sections are produced to:
BS EN 10162: Cold rolled steel sections. Technical delivery conditions. Dimensional and cross sectional tolerances.
Cold-formed steel sections are fabricated by means of folding, press-braking of plates or cold-rolling of coils made from carbon steel. Sheet steel used in cold formed sections is typically 0.9-8mm thick. It is usually supplied pre-galvanized in accordance with European Standard EN 10142. Galvanizing gives adequate protection for internal members or those adjacent to the boundaries of the building envelope. Cold working of the steel increases its yield strength but also lowers its ductility.
The main benefits of using a cold-formed section are not only its high strength to- weight ratio but also its lightness, which can save costs on transport, erection and the construction of foundation, and flexibility that the members can be produced in a wide variety of sectional profiles, which can result in more cost effective designs.
Steel construction (Les Brown)
Constructing with steel has numerous benefits to a project in the words of (Structural steel solutions , 2012) states that structural steel is the best choice material and the reason for this is as follows:
Speed of construction
Lower project costs
Ease of design
Reliable and predicable
All of the above benefits will be explained throughout this part of the report; the above benefits are important and should be examined to prove that steel is more beneficial than other materials when it comes to the above benefits.
Speed of construction (Les Brown)
(Structural steel solutions , 2012) States that there are materials that will be able to start work sooner than steel would be, but as for rapid design, fabrication and erection cycle with structural steel will allow the forming system to finish the project sooner than any other material. The productivity of construction can be enhanced or made easier due to structural steel because of its fabrication while maintaining tight construction tolerances, there is a new technology on the market at the moment called 3D interoperability and building information modelling which allows close communication between the steel contractors and the design team, this technology allows time and money to be saved in the construction process by simply combining fabrication and erection efficiencies in the design process. Structural steel will maintain close tolerances and minimal construction waste in all seasons.
Lower project costs (Les Brown)
Presently according to (Structural steel solutions , 2012) remains the cost leader for the majority of construction projects. Previously, some studies have shown that a structural steel framing system including decking and fire protection will cost 5% to 7% less than constructing a concrete frame, structural steel has remained the cost leader for the past 30years, which can be research from the Bureau of labour statistics that the price for fabricated structural steel prices for commercial buildings increased by 62% but whereas concrete prices have increased by 114% during that same period of time.
Aesthetic appearance (Les Brown)
Coming from (Structural steel solutions , 2012) they say that the beauty of structural steel appeals to architects and structural engineers. They feel like they have to expose it in their design, to emphasize grace, slenderness and strength, and relate this while designing structures of complex design. Structural steel allows the architect a greater deal of expression and creativity in their design than any other construction material. Column free clear spans, the use of coloured coatings and the opportunity for natural sunlight increase the architect's view of using structural steel for design.
Design Flexibility (Les Brown)
As stated by (Structural steel solutions , 2012) structural steel can be produced for the most simplex or complex design, depending on the architect or structural designers style. No other material comes close to the flexibility, expression and design creativity of structural steel.
High strength (Les Brown)
You will hear about construction materials for example (Structural steel solutions , 2012) stating that they are trying to equal the strength of structural steel but their strength will always be less than that of structural steel even if that material is reinforced with steel. Structural steel is typically 50ksi material indicating that the steel has a yield stress of 50,000 pounds per square inch in both compression and tension where as a normal concrete mix has a yield stress of 3 to 5ksi, not only is steel stronger material, it also has a much greater strength to weight ratio than other materials, this means the building will be lighter therefore lighter buildings need less extensive and costly foundations.
Ease of design (Les Brown)
Structural steel makes design more flexible than any other material due to the way it can be bent in all different shapes along with the advantage of spanning distances. There are many different types of bending methods like compound bending, helical bending, mandrel bending, and multi radius bending and many more. The structural steel industry are the poster child for collaborative building information modelling this technique is not just for simple design but for complex design also requiring innovative design ideas from the design team.
Figure Tube bending for rock 'n' roll McDonald's, Chicago, IL http://www.cmrp.com/commercial-buildings
Sustainable (Les Brown)
As indicated by (Structural steel solutions , 2012) Sustainability is structural steels middle name, it is the most recycled material on the planet, todays structural steel is made from 88% recycled product. The carbon footprint of structural steel has decreased by 47% since the year 1990, and energy producing the material has been reduced by 9% since the same period of time. The production of steel saves our most important resource water, the only water that is used when producing structural steel is the make-up water.
Steel Constructions role in sustainable development (Les Brown)
As stated by (Pro. Patrick Dowling , 2006) The use of steel construction can have a lot to offer sustainable development, previously, in the year 2002 the launch of the steel construction sectors sustainability strategy was an important public affirmation of the sectors commitment to sustainability, it was designed to produce a healthier future for the sector, it actually sets out how steel can be used to deliver more sustainable construction at the design, execution, in-use and construction stage.
The construction phase (Les Brown)
On the word of (Pro. Patrick Dowling , 2006) One of the most effective ways of reducing local community impacts is by maximising off site prefabrication, which not only minimises site activity, but also can provide efficient, safe, high quality and fast construction. All steel products are manufactured off site and with a degree of fabrication increasing from linear elements, to infill panels to complete the modules which are fully finished and fitted out in the factory, ready for assembly as soon as the hit the site.
Prefabrication and factory based work decreases waste, not just on site but throughout the design and manufacturing process. For steel construction, wastage rates will vary depending on the complexity of the manufacturing process and the product in question.
Life extension (Les Brown)
Extending the life of existing buildings through refurbishment and re use achieves a number of sustainable development benefits, fewer resources are required compared with new build, construction and demolition waste will be reduced. The re use of steel has obvious advantages over recycling, it requires less energy and it preserves the value added during manufacturing.
An example is where a building needs more usable space, this can be achieved by either vertical or horizontal roof top extensions, generally using lightweight construction e.g. Empress state building in London had a combination of both horizontal and vertical extensions, it was extended horizontally by twenty seven floors and three stories vertically and was constructed by using a braced steel frame. Sometimes the external appearance of buildings can lead to demolition, the structure of the building may be sound and all that is needed is to upgrade the building envelope. Over cladding can extend the buildings life from deteriorating.
In order to maximise life extension, a building should be designed for future flexibility to avoid replacement, buildings that are future proofed in this way are commonly referred to as adoptable buildings, they can be easily extended structurally, they are characterised by long spanning steel members and steel columns spaces to enable the internal space to be remodelled and which can be upgraded easily.
Figure Empress state building in London Figure 2 Twin towers New York
The end of life phase (Les Brown)
In the words of (Pro. Patrick Dowling , 2006) Sustainable development requires that the end of life impact on buildings is minimised. This means reducing waste and ensuring that materials are recovered and either re used or recycled. Steel products lend themselves to both and the potential for reuse of steel components has been enhanced by the standardisation of the components and the connections in which the steel industry have achieved. Products that are commonly used include sheet and bearing piles, structural members, purlins, rails and modules. At the larger scale with proper consideration at the design stage, whole buildings can be dismantled and re-erected elsewhere, such design considerations include:
Minimising the use of wet trades to make it easier for buildings to be dismantled.
Use of standard structural components and bolted connections.
Ensuring that the structure is easily assessable for inspection and subsequent dismantling without damage.
The ability to readily identify the properties of the materials that are proposed to be re used.
The longer the structural member, the greater the scope for re using it by cutting down to a new length and forming new connection details.
And finally, where the life of a building cannot be extended, and its components cannot be re used the steel content can be recycled. The sustainable development advantages of recycling steel are as follows:
Use of natural resources is minimised.
Energy usage is reduced.
Reduction in waste generation.
All new steel has a recycled content which can vary between 10% and 100% depending upon the availability and price scrap, the specification of the steel and the steel production route, but, global demand for new steel exceeds the supply of scrap steel by a good factor and so it is currently possible to meet the demand for all new steel to be made entirely from scrap.
For steel, the process of improvement and innovation to contribute to sustainable construction is a continuous one, but today the steel construction sector is pointing in the right direction towards the following issues:
Advances in process and product technology.
Factory based construction that minimises site impact and creates a good quality working condition.
Fabrication methods that enable buildings to be connected.
Jointing techniques for building to be easily dismantled and components re used.
And an industry with global infrastructure for recycling all steel construction components.
Innovative (Les Brown)
In the words of (Structural steel solutions , 2012) there have been new systems developed over the past few years for producing structural steel, they are as follows:
These inventions where produced due to the fact the designer demands and new approaches, there are also on going innovative ideas to address issues such as long span deck systems, fire protection, connection optimisation, coating systems, and progressive collapse. The structural steel industry continues to grow innovations for both the material and the use of structural steel.
Modifiable (Les Brown)
It has been stated by (Structural steel solutions , 2012) that structural steel buildings can be modified for future use, loading conditions, vertical expansions and changes to suit owners ideas. Home owners or building owners always face changing requirements and a composite steel frame can be easily modified to satisfy existing or new tenants such as increased floor loads for storage and equipment. Existing steel columns and steel plates can be fixed to the flange or web of sections allowing for greater load pressure.
It is not unusual as previously stated that structural steel buildings to have additional floors added years after the building was originally built for e.g. the blue cross blue shell building in Chicago was enlarged by 24 stories to the structure.
Efficient (Les Brown)
As mentioned by (Structural steel solutions , 2012) efficacy of steel means that the structural steel buildings can optimise building space through the use of slender columns maximising useable floor space, longer spanning members open column free spaces. The typical steel column takes up 75% less than the equivalent concrete column, nevertheless structural steel allows longer spans that eliminate columns creating open floor areas which are ideal for the layout of offices in the present time, maximising space.
Parking facilities benefit from smaller structural steel columns and longer spans as well, structural steel framing systems for parking facilities will generally span 60 feet allowing for a drive lane and 2 parking bays without any columns getting in the way.
Readily available (Les Brown)
Before the year 2008 when the construction industry was at an all-time high the structural steel industry had the domestic production to meet the foreseeable construction demands of the Irish market place, but now that so many steel manufacturers have closed down due to the economic downturn better known as the recession, it might be necessary to pre order in advance, previous years they would have stocked up to 3 months of structural steel fabrications for new projects within days from the time of order.