Structural Steel – Preferred Material for Construction
Structural steel is not just a material which has only the technical competence. It has many other qualities that make it the preferred material for architects, designers and engineers. It is economical and provides great mechanical functionality; it permits the design of structures which are graceful, light and airy; it streamlines construction site processes; and it offers rapid execution. A major advantage, however, is the infinite freedom for creation which it provides to the architects, designers and engineers. The combination of structural steel with different materials lends themselves to rich and varied types of construction. When combined with glass, structural steel makes fabulous use of light and space.
Structural steel is the material ‘par excellence’ when it comes to inventing new structures and forms. All solutions are possible, from the very simplest to the most challenging. Structural steel can be used for small buildings as well as large structures, for routine construction projects, and those subject to complex urban constraints. No other material is used to make structures which are so thin, light and airy. Forms can be created using different structural effects and envelopes with pure or finely sculpted curves. Architects, designers and engineers can give free reign to their imagination and creativity with structural steels.
Structural steel is a standard construction material made from specific steel grades and is available in standard cross sectional shapes. This steel exhibits desirable physical properties such as strength, uniformity of properties, light weight and ease of use etc. This makes it one of the most versatile structural materials in use. Major applications for this steel is in high rise and tall multi-storey buildings, industrial buildings, towers, tunnels, bridges, road barriers, and industrial structures etc.
Within the overall architectural concept, the structural steelwork may be concealed or exposed to reveal its essence. In both cases various advantages remain such as facility for modular design, compactness, economy of material, freedom of use, and speed of assembly etc.
Structural steel is a material used for steel construction, which is formed with a specific shape following certain standards of chemical composition and strength. They can also be defined as hot rolled products, with a cross section of special form like angles, channels, tees, rounds, squares, beams, and hollow pipes both round and square etc. There has been an increasing demand for structural steel for construction purposes mainly because the selection of structural steel for the framing system of a building brings numerous benefits to the construction projects. All other materials are measured against the standard of structural steel and hence structural steel is the material of choice for the architects, designers, and engineers. Various benefits provided by the structural steel (Fig 1) are described below.
Fig 1 Benefits of structural steel
Speed of construction
Structures made from structural steel can be erected speedily, the predictability and accuracy of steel components speeds up the process. This deliver time savings in the construction programme compared to a concrete frame. In fact, speed of erection is often one of the main criteria for selecting structural steel. Short construction periods leads to savings in site preliminaries, earlier return on investment and reduced interest charges. Time related savings can be substantial and result into reduction in the overall project cost.
Materials other than structural steel may be able to start field work sooner, but the rapid design, fabrication and erection cycle with structural steel allow the framing system to finish sooner than in the case with other materials .
Structural steel enhances the productivity in construction since there is shop fabrication. This also helps in the maintenance of tight construction tolerances. Field placed materials always lag behind the productivity curve. Productivity improvements in construction occur due to shop based technology enhancements and not in labour based field activities.
Rapid erection in all seasons with close tolerances being maintained for integration with other systems of the building construction project and minimal construction site waste is achievable only with structural steel.
Lower costs of the construction project
Fabrication in controlled conditions results in high quality, defect free components that produce very little waste during the construction process. Furthermore, steel structures are durable and require little maintenance, extracting maximum value from the resources invested in the structure and minimizing its whole-life costs.
Cost savings in buildings constructed with the structural steel start at the foundations, where the loads imposed by a steel frame are up to 50 % less than those of a concrete alternative. That means foundations can be much smaller and therefore cost considerably less. Foundation costs constitutes a major component of the overall building costs, so lighter foundation loads can have a big impact on the overall costs
In the present day scenario structural steel remains the cost leader for the majority of construction projects when competing framing systems are evaluated using comparable and the current cost data. Comparative studies show that a structural steel framing system including decking and fire protection costs lesser than a concrete framing system.
Aesthetic appeal
General public praises the beauty of structural steel and is excited when it gets exposed in the design of the structures to emphasize grace, slenderness, strength and transparency of frame. Structural steel allows the designers a greater degree of expression and creativity in their design than any other construction material. Column-free clear spans, the use of colored coatings and the opportunity for natural lighting highlight the elegant simplicity of using structural steel. Structural steel sections can be bent and rolled to create non-linear members to further enhance the aesthetic appeal of the structure.
Design flexibility
The advantages that structural steel offers to the construction sector have long been recognized by the designers and specifiers. The versatility of steel gives architects, designer and engineers the freedom to achieve their most ambitious visions.
From the simplest, functional structure to the complex, signature design, structural steel can be readily used to accomplish the design intent of the designer and the structural engineer. No other framing material comes close to structural steel in terms of innovativeness, freedom of expression, and design creativity.
High strength
Structural steel is always more preferred to concrete because it offers better tension and compression properties thus resulting in lighter construction. The strength of the structural steel is much higher than the strength of the other construction materials. All other materials talk about high strength, but their strength is still less than that of structural steel even when enhanced by steel reinforcing. Yield stress of the structural steel is typically 350 N/sq mm in both compression and tension. By comparison, a normal concrete mix has a yield stress of 20 to 35 N/sq mm in compression only and a high strength concrete may have a compressive yield stress of 80 to 100 N/sq mm. Structural steel is not only a stronger material but it also has a much higher strength to weight ratio when compared to other materials. This results in the building being lighter and lighter buildings require less extensive and costly foundations. Unlike concrete, erected structural steel does not shrink or creep. The various other mechanical properties of the structural steels are as follows.
Elasticity – Structural steel follows hooks law very accurately.
Ductility – it is a very desirable property of structural steel, in which steel can withstand extensive deformation without failure under high tensile stresses, i.e., it gives warning before failure takes place.
Toughness – Structural steel has both strength and ductility.
Ease of design
Structural steel is the most desirable material for the structural engineers for designing since there are a variety of tools available for structural steel design. Also these tools are a quantum step ahead of tools being used for designing with other materials. Full integration between analysis, design, detailing and fabricating software is in use today. The structural steel industry is actively associated with ‘Building Information Modeling’. This innovative blending of technology is not just for simple boxes, but also for complex structures requiring innovative design approaches and the cost saving techniques of 3-D modeling with full involvement of the steel specialty contractor in the design process.
Now the specifications for structural steel buildings integrate the load and resistance factor design methodology and the allowable stress and plastic design methodology into a single uniform design methodology.
Normally structural steel uses three dimensional trusses hence making it larger than its concrete counterpart. There are different new techniques which enable the production of a wide range of structures and shapes. These techniques are (i) high-precision stress analysis, (ii) computerized stress analysis, and (iii) innovative jointing technology.
Sustainable structural steel
Structural steel is the leading construction material for sustainability, offering exceptional environmental, social and economic benefits. The balance between the three elements sees structural steel deliver a sound sustainability case.
Sustainability is in the core of use of the structural steel. Structural steel is the most recycled material. In the present day scenario most of the recycled steel goes into the production of the structural steel. Further structural steel can be reused without further processing. The carbon footprint of structural steel has been reduced substantially since 1990. Energy used in the production of structural steel has been reduced in the past three decades.
Structural steel can be recycled endlessly with no detrimental effect on its properties. When a steel-framed building is demolished, its components can be reused or returned to the steelmaking process to create brand new components. The recycling rate of structural steel and at the end of its life is 100 %.
Structural steel is also highly durable. Steel piles extracted from an area with a high water table are generally found to be fit for reuse.
The production of structural steel also conserves the most valuable resource which is water. A small amount of water is used in the production of structural steel and no water is used in the fabrication process and during erection at the construction site.
Innovative approach
New systems approaches (girder slab, guide plate etc.) are now available for the project designers with the structural steels. These new approaches join ongoing innovations addressing issues such as long-span deck systems, fire protection, connection optimization, coating systems, and progressive collapse.
The structural steel industry continues to pioneer new innovations for both the material and the use of structural steel. There is improvement in the material specification for hot rolled structural sections with the yield stress increasing by 40 % from 250 N/sq mm to 350 N/sq mm. Design tools continue to mature to allow more efficient steel designs. Further there is better understanding of the behaviour of structural steel through research activities with respect to the increasing of the productivity while bringing greater economy to the construction projects.
The structural steel industry pioneered the movement towards open standards and inter-operable software that has most recently resulted in the growth in popularity of ‘Building Information Modeling’. The ‘CIMSteel Integration Standards (CIS/2)’ is a product model and electronic data exchange file format for structural steel project information. CIS/2 facilitates data exchange through seemingly stand-alone programs, such as structural analysis, CAD and detailing systems by allowing them to communicate with each other. By providing a neutral data format, CIS/2 allows data exchange between a varieties of program types—as long as these programs have translators written to interpret the CIS/2 neutral data into the programs’ native format.
In 1990s the industry in USA adopted CIS/2 as a standard data protocol for the exchange of information between structural design, detailing and manufacturing/fabrication programs. The result was that software programs from different vendors were suddenly able to exchange model based information beyond simple geometry. Projects taking advantage of this vertical integration within the structural steel industry were able to demonstrate cost savings of up to 20 % on the structural package. The success of the CIS/2 implementation within the structural steel industry has encouraged the broader design and construction marketplace to pursue similar open standards and data protocols for the exchange of model based data between design disciplines and in other industries vertical supply chains.
Feasibility of modification
The inherent adaptability and flexibility of the structural steel means that future changes or extensions – even vertically – can be carried out with minimal disruption and cost.
The requirements from a building always changes with time. A composite steel frame can be easily modified to satisfy existing or new changed requirements such as increased floor loads for storage and equipment, new openings for mechanical equipment and vertical shafts for floor-to-floor staircases. Existing steel columns and beams can be strengthened through the attachment of steel plate to the flanges or web of sections allowing for greater loads. New stairways can be added to existing steel framed buildings by removing a portion of the floor decking, bracing a single bay and adding the desired stair structure. These types of changes can be accomplished with little disruption while the building is still occupied.
Structural steel buildings can be modified in the future for new applications, loading conditions, vertical expansions and changes as per the requirement which the framing systems with other materials can never accomplish.
Structural steel provides the flexibility needed to enable a building to evolve throughout its working life. The building can be initially designed in order to facilitate the following future evolutions.
Modification of applied loads due to change of use of the building
Floor plan morphology in order to retain the possibility to create new openings
Horizontal and vertical movements, exits: appropriate measures can be taken in order to limit any impact on the primary building structure during alterations.
It is not unusual for a structural steel building to have additional floors added even years after the building was originally completed. This is feasible while it is still occupied.
Provides space efficiency
Structural steel buildings optimize building space efficiency through the use of slender columns maximizing useable floor space, longer spans for open, column-free spaces and the integration of heating, ventilation and air conditioning (HVAC) systems into structural spaces.
The typical steel column occupies 75 % less floor space than an equivalent concrete column. At the same time structural steel allows longer spans that eliminate intermediate columns creating larger open floor areas.
Parking structures benefit from smaller structural steel columns and longer spans as well. Structural steel framing systems for parking structures are typically having span of 18 m to 20 m allowing for a drive lane and 2 parking bays without any intervening columns. The use of the smaller footprint steel columns at the front of the parking bays create less intrusion into the parking space than larger concrete columns.
Structural steel is reliable and predictable
Structural steel has consistently high quality standards, precision products and guaranteed strength and durability in the most challenging environments. The quality assurance of structural steel work is always assured since it is produced to the most exacting specifications under highly controlled conditions, eliminating the risks of onsite variability. Rigorous quality assurance processes give full traceability at all stages in the supply chain, from steelmaking to fabrication and erection at site.
Structural steel is reliable and has predictable properties since it is manufactured and fabricated under controlled conditions using modern quality assurance processes. The final strength of the structural steel is verified at the point of production, not after the material is already placed in the frame of the building. Structural steel is shop fabricated to close tolerances impossible for site cast materials. Various international standards closely define the properties of structural steel whereas the actual properties of cast-in-place concrete are a function of concrete mix design, local aggregates and delivery conditions requiring testing of samples 28 days after the material has already been placed in the structure.
The strength and ductility of structural steel make it highly resistant to accidental damage. If any damage does occur, it can easily be repaired by cutting, welding or bolting to restore its full strength. The erection of structural steel on site is not restricted by weather conditions, other than high winds, and can continue year round, with no need for special protection measures in any weather conditions.
Types of structural steel and its availability
The structural steel is readily available. Hot rolled structural steel is a common product for all the steel plants. The common shapes in which structural steels are available consist of sections (beams, channels, tees section and angles), squares and rounds, hexagons, plates, pipes, hollow square sections, steel cable, Z sections and cold formed sections etc.
The structural steel all over the world pre-dominates the construction scenario. This material has been exhaustively used in various constructions all over the world because of its various specific characteristics that are very much ideally suited for construction. Structural steel is durable and can be well molded to give the desired shape and to give an ultimate look to the structure that has been constructed. Steel products made from structural steel demonstrate specific characteristics according to their grade and form and is defined by various national and international standards.
There are many types of structural steels. These are classified, according to their composition. There are three main categories of steel namely (i) non-alloy or carbon steel grades, (ii) stainless steel grades, and (iii) other alloy steel grades. Non-alloy or carbon steel grades are commonly used in the construction sector. The steel grades with yield strengths of 250 N/sq mm and 350 N/sq mm are generally used for structural members. However the higher strength steel with yield strength around 450 N/sq mm is now being used more and more in the construction.
Structural steel exhibits desirable physical properties that make it one of the most versatile structural materials in use. Its great strength, uniformity, light weight, easy to 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 structure. It also finds use in the manufacturing of automotive vehicles, wagons, and ships etc.
Disadvantages of structural steel
Although structural steel has all the above advantages as structural material, it also has some disadvantages that make reinforced concrete as a replacement for construction purposes. For example steel columns sometimes cannot provide the necessary strength because of buckling, whereas reinforced concrete columns are generally sturdy and massive, i.e., no buckling problem occurs. Other disadvantages are given below.
Maintenance cost – Structural steel structures are susceptible to corrosion when exposed to air.
Fire proofing cost – Structural steel is an incombustible material; however, its strength is reduces at high temperature caused by a fire.
Fatigue – The strength of structural steel member can be reduced if this member is subjected to cyclic loading.
Brittle fracture – Under certain conditions structural steels lose its ductility, and brittle fracture may occur at places of stress concentration. Fatigue type loadings and very low temperature trigger the situation.
The response of structures made from structural steel to fire is well understood, and extensive best-practice guidance is available. The performance of steel components and steel structures in fire has been researched more extensively than any other building material. The steel sector has invested decades of research into understanding the behaviour of structural steel components in fire – giving designers the confidence to engineer the buildings safely in steel.