A structure is an assembly of components which are fastened together through some type of connection. If connections are not designed properly and fabricated with care, there may be a source of weakness in the finished structure, not only in their structural action but also because they may be the focus of corrosion and aesthetically unpleasing. The design of main members has reached an advanced stage, based upon theories which have been developed and refined, the behaviour of connections is often so complex that theoretical considerations are of little use in practical design. The connections are a jumble of local effects. Most connections with the distribution of stress are highly indeterminate depending upon the deformation of fasteners and the detailed material. Local restraints may prevent the deformation necessary for desirable stress redistribution. Following are the requirements of a good connection in steelwork:
- it should be rigid, to avoid fluctuating stresses which may cause fatigue failure
- It should be such that there is the least possible weakening of the parts to be joined
- It should be such that it can be easily installed, inspected and maintained.
The following are the common types of connections used for structural steelwork;
- Riveted connections
- Bolted connections
- Pinned connections
- Welded connections
Rivets, bolts and welds are used extensively, and frequently the economic advantage of one over the other two is so small to be uncertain. However, riveting has been superseded in importance by welding and high-strength bolting. Riveting is joining of structural steel components by inserting ductile metal pins, called rivets, into holes of the components to be connected from coming apart. A rivet consists of
(i) a shank of given length and diameter
(ii) ahead known as manufactured head.
The size of the rivet is defined by the diameter of the shank. Riveting is essentially a forging process during which a hot rivet is driven in its plastic state and ahead is formed at the other end. The head so formed at the other end of the rivet with the help of a riveting hammer and a buckling bar is known as driven head. Rivets driven in the field during the erection of a structure are known as field rivets. Rivets driven in the fabricating shop are known as shop rivets. Both these types are known as hot driven rivets since the rivets are heated to a temperature ranging between 1000o F to 1950o F before driving. Field rivets are driven by a hand-operated pneumatic riveting hammer, while the shop rivets are driven by “bull” riveter. Some rivets are driven at atmospheric temperature. They are known as cold driven rivets which are squeezed or driven to fill the holes and to form the heads by application of large pressure.
Steel is a widely-used material in multi-storied residential buildings, skyscrapers, bridges, commercial buildings etc. due to the strength, durability, usability, low cost, flexibility, aesthetics, low weight and performance characteristics. The extensive usage of steel has made it an important issue to evaluate steel structures in the life cycle on environmental aspects. The structural system constitutes often more than 50% of a building’s embodied energy which is a standardized quantity and an important indicator allows you to make a comparison in terms of environmental impact. In this context, it can be also said that for sustainable buildings and to reduce environmental impacts, reducing the embodied energy of the system plays an important role. There also occur considerably energy consumption and carbon dioxide emissions during the production process of steel. On average, it is known that during the production of 1.8 tons of steel, there occurs one ton of Carbon dioxide. According to the International Energy Agency’s 2010data, iron and steel industry constitute a 6.7% rate in total carbon dioxide (CO2) emissions on the earth. The steel industry is also responsible for resource consumption on the earth. According to US Geological Survey, 2.2 billion tons of iron ore is extracted from the ground on an annual to produce steel used in buildings, bridges and other infrastructure and also it is estimated that currently, accessible iron ore reserves are 150 billion tons on the earth. As such the accessible iron ore reserves equate to 64 years based on an annual steel demand rate in extraction. As well as depletion of iron reserves, in the literature survey, it has been found that the production phase of structural steel plays an important role in energy consumption and environmental impacts. For this reason, evaluation of the recovery facilities of structural steel is thought to be so significant in environmental burden. The advantage of structural steel is Architectural Advantages, Structural Advantages, Constructional Advantages and Sustainable Advantages. For sustainable development, it is declared that cyclic processes must replace linear ones in UNESCO Conference. Steel can be reprocessed as it is a unique component. Steel is considered to have a closed-loop material cycling. Closed-loop refers to a process of keeping materials in the life cycle by reuse and recycling rather than disposing of.
The erection of steelwork is reliant on crane hook time; therefore, multiple small beams will have a disproportionately high erection cost when compared against a large single beam. Tower cranes are the main source of lifting on site. Crane capacity should be factored into the logistics strategy as any individual members that exceed the tower crane capacity will need to be erected by utilizing mobile cranes (with their associated road closures and space requirement). In cost planning buildings, allowances should be made for tower cranes with sufficient capacity to lift the majority (if not all) of the components necessary to construct the building. Where specific specialist lifts are required then allowance needs to be included within the overall building budget (this is not specific to steel framing and should be taken into account when considering the building as a whole). Design requirements to strengthen the frame in response to the building’s security assessment rating will mean increased structural demands on connection details and edge beams, particularly at the lower levels of the building, like provision for column removal without progressive collapse of the building. In most commercial buildings, straightforward steel construction will meet the required vibration performance criteria without modification. However, stiffening may be required to meet particularly onerous floor vibration design criteria, in which case deeper and heavier beams would be needed. In an optimum structural zone where beams work efficiently. However, with the introduction of services and the desire to increase floor-to-ceiling heights, this zone can become compromised. The reduced structural zone leads to making the frameless efficient and increased steel member weights. Site conditions have a direct impact on costs which manifests itself in the erection and package-specific preliminaries costs. In extreme cases the site conditions determine the design solution, eg constructing above railway lines, sites adjacent to or over rivers, or sites with restricted access (double handling). Site-specific preliminaries are influenced by tower crane availability, building height, uniformity of grid, on-site welding requirements, delivery timings and quiet periods. External factors such as currency exchange rates, buoyancy of the market, labour availability and commodity prices all influence market dynamics and as such should be considered at the time of developing the cost plan. It is advisable always to include exchange rates in the basis and assumptions of the cost document.
The Steel manufacturers have brought about a rise in the prices of steel for the first time in the last seven months in November, as the inventory at both the consumer and steel manufacturers’ end shrank after the recent pick-up in demand.
One of the major steel companies like JSW Steel has hiked prices of the hot-rolled coil by Rs 500-Rs 750 a tonne to Rs 35,500 and TMT bars by Rs 750-Rs 1,000 depending on quality specifications.
Jayant Acharya, Director (sales and marketing), JSW Steel, said, globally, steel prices have gone up by $20-$30 a tonne due to a production shut-down by most steel companies, as their selling prices had fallen below production costs. Inventory in China has also come down by 18 per cent or two million tonnes in the last two months. Steel production in China may slow down with the beginning of winter. As such the cost index is more or less similar to the 2017 levels, steel prices have fallen by over $100 a tonne to $430, from the level of $545 that prevailed two years back, he said.
Domestic prices at a discount to import prices
Besides the Rs 500-Rs 750 hike, domestically, HR coil is being sold at a discount to imports, he added. In India, the inventory with the dealer has dropped by 0.5 million tonnes in October, as steel demand increased by 3.9 per cent. Vehicle sales in October increased to 2, 85, 000 from 2, 23,000 in September; similarly, two-wheelers and commercial vehicles sales were up 6 per cent and 16 per cent every month, while the yearly comparison still remains a concern. After the monsoon, the construction activity has increased with higher allocation for both state and Central government-funded projects. On the packaging side, it is said the ban on single usage plastics has stoked demand for tin plates, where JSW Steel hiked capacity recently. On steel imports, also, globally, steel prices have fallen below the anti-dumping duty price of $489 a tonne and some shipments booked at higher levels were hitting Indian shores. It will be difficult for the user-industry to import steel when domestic prices are trading at a discount to the import price.
All beams in steel structures, including beams designed as simply supported, experience a certain degree of axial and rotational restraints at their ends. Beam internal forces are strongly related to the degree of end restraint and beam temperature. End restraints generate axial force and moment within the supported member at elevated temperature. At the initial stage of a fire (100°C – 400°C), a steel beam starts to experience compressive internal forces due to the restraint to thermal expansion and it starts to bow towards the fire. When the beam temperature reaches 400°C, the steel begins to lose its strength. the beam internal force begins to Progressive collapse is the collapse of a building due to a
failure of a single vertical load-carrying element, or a small number, which are disproportionate to the complete building failure. Element failure could occur as a result of any of several extreme loading events on buildings, including strong earthquakes, blast, vehicle impact, fire, or similar incidents.turn from compression into tensile force, and then the connections begin to support the steel beam by resisting pull-in forces as well as vertical shears. At the advanced level, a steel beam hangs as a suspension cable from the cooler end connections. Such a phenomenon is called “Catenary Action” Consequently, providing the end connections’ resistance still exists, the tensile horizontal axial force grows progressively, as the beam mid-span deflection grows. Furthermore, beam end connections are required to resist an additional moment, resulting from rotation due to excessive mid-span deflections. This leads to reductions in the beam mid-span moment. In essence, the behaviour of the beam is affected by the connections’ ductility and stiffness. In this manner, catenary action helps the beam by reducing the mid-span moment and it hangs from the cooler connections which need to sustain the tensile axial force of the beam and prevent the collapse. Therefore, Catenary Action works to improve the survival time of steel beams in a fire provided that the beam end connections still function. Beam-to- column joints in structural steel buildings transfer the floor and beam loads to the columns and provide the link between the principal structural elements for the overall structural stability. By this means, the forces transmitted through the joints can be axial and shear forces, bending and torsional moments. The effect of torsion on individual members can be neglected in plane frames, as a result of the lateral restraint provided by composite action with floor slabs. Nevertheless, the magnitudes of the other three actions transferred by the connections to the supporting columns depend on the connection type and the surrounding conditions. In the case of steel frame structures with moment-resistant connections, such as bolted end plates, the bending moments are predominant compared to axial and shear force.
In steel industrial building, the usage of elements with web-tapered cross-section, made of welded thin plates is common. These low structure elements are generally designed using gravitational load combinations. Rafters and columns help shape the bending moment in gravitational load combination thus reducing the material consumption and reducing the structural self-weight. In case of an earthquake, low inertia forces are used in single-span industrial buildings. Thus not allowing the seismic design concept to impose a high structural ductility class. Within the framework of an RFCS research program, a simple chart based procedure to select the best seismic design concept for portal frames have been proposed. Depending on the frame geometry we may use a low-dissipative or dissipative concept. The method mentioned above indicates the fact that for most cases of portal frames with variable column and beam cross-sections, the optimal choice will be a low-dissipative concept. This helps eliminate the strict anti-seismic conditions referring to cross-section class, elements slenderness, imposed detailing conditions, etc. and conduct to a more effective cost of the building. The tapered shape of the element and high cross-section class of web wall is obtained at the end with the maximum height. The buckling capacity of such a slender member will be determined by the efficiency and position of the restraining elements, end support conditions and initial geometrical and material imperfections. At cross-sections members, used generally for columns elements, restrained against lateral or/and torsional buckling, the coupling between sectional capacity and overall elastic buckling of the members in compression and/or in bending may occur. In certain section members, found at the rafter highest section of the tapered web, local buckling of the walls or distortion may appear in elastic domain. For member which are not laterally restrained or the restrains are not effective, the global failure mode of the members will be characterized by the lateral-torsional mode, either alone or in interaction with local buckling.
A joint can be considered an assembly of several components which individually have their characteristic strength, stiffness and deformation capacity. Such a joint is a combination of “springs”. Combining these springs will result in the overall behaviour of the whole joint. This so-called “component method” is the basis of the design of joints in Euro code 3. Simplified and more sophisticated spring modelling can be assumed. In scientific context the sophisticated spring model is available. However, there is an urgent need for such models in framework programs for use in practice. The use of the simplified model leads to iterations to determine the correct distribution of moments in the structure. For steel structures, the development of new types of joints among these “plug and play” joints need a boost to meet the goals described in the introduction. Ideas for these joints need theoretical and experimental research before these joints can be applied in practice. The philosophy for using these joints is that in the early erection phase the joints are plug and play, resulting in joint behaviour that is sufficient for the loading conditions in the erection phase. When the access of the joints is better because of the presence of parts of the floor system, the joints can be finalized by adding bolts resulting in the final mechanical behaviour needed for the loading conditions in the final stage of the building. Semi-rigid and partial strength design consideration will also lead to substantial economic advantages in the detailing and fabrication of the joints. The flooring structure should be as light as possible before finalizing the joint in the end-stage of the building process, such as thin-walled profiled sheeting for composite slabs or steel-board flooring systems. But, it is also possible using prefabricated thin concrete slabs on which the final concrete floor is poured without props in the end-stage of the building process. The columns can be continuous or bolted at floor level, just underneath the floor finishing. Further creativity is needed to develop “plug and play” type joints which can be considered as finalized once the beams are remotely controlled positioned and connected to the columns.
The steel frame structures with traditional joints with connecting parts as bolts and welds are designed based on forces resulting from plane frame analyses. The spatial behaviour of the structure due to the loading is most times neglected because the resistance of the traditional joints against these resulting forces is normally sufficient. The wind loading on a building can produce tensile forces in the joints, while eccentric loading on the floor can produce torsional moments on the joints. Traditional joints have by themselves sufficient resistance against these tensile forces and torsional moments. Because bolts in holes, designed primarily for shear and or bending moments, are almost always capable of carrying these “secondary” forces and moments. These forces are not “secondary” but primary forces and moments due to the spatial behaviour of the structures under the loading actions and most times neglected in design. In designing new concepts for plug and play joints the designer needs to be explicitly aware of these forces and moments, which can implicitly be neglected in designing traditional joints. Also, loading that can appear in the onsite construction phase produces most times tension and torsion in the beams and joints. The main task of the designer is to design the joints such that robustness to prevent premature and progressive collapse can be demonstrated explicitly. In traditional design, it is assumed that the joints are stiff and strong and that the forces and moments in the structure are determined using the linear-elastic theory. It is assumed that the joints were stiff, it needs to be checked whether the joints are stiff. In many cases in practice, this is neglected. The strength of the joints is adjusted as per the requirement. As a result, most joints have low deformation capacity. Last but not least, fabrication costs are very high. In the latest designs, the joints are considered as structural components such as columns and beams with properties as stiffness, strength and deformation capacity. These structural properties of the joints are incorporated into the design on the same level as those of columns and beams. The joint layout is influenced by fabrication considerations and considerations for easy and safe construction on-site. The structural safety verification of all components is dependent on the design method used to determine the distribution of forces and moment in the structure a. In case that the linear elastic theory is used, the beams need to be checked for strength and lateral-torsional buckling, the columns need to be checked for strength, beam-column stability and the connecting parts of the joints to have sufficient strength to transfer bending moments, shear forces and tensile forces resulting from the linear elastic theory adjusted for the second-order effects. b. In case that the elastic-plastic-non-linear theory is used, the beams and columns need to be checked for lateral-torsional buckling only and the joints need to be checked to have sufficient deformation (in fact rotation) capacity.
The steel frame structures with connecting parts as bolts and welds are designed on the basis of forces resulting from plane frame analyses. The spatial behaviour of the structure due to the loading is most times neglected because the resistance of the traditional joints against these resulting forces is normally sufficient. The wind loading on a building can produce tensile forces in the joints, while eccentric loading on the floor can produce torsional moments on the joints. Traditional joints have by themselves sufficient resistance against these tensile forces and torsional moments. Because bolts in holes, designed primarily for shear and or bending moments, are almost always capable of carrying these “secondary” forces and moments. In fact, these forces are not “secondary” but primary forces and moments due to the spatial behaviour of the structures under the loading actions and most times neglected in design. In designing new concepts for plug and play joints the designer needs to be explicitly aware of these forces and moments, which can implicitly be neglected in designing traditional joints. Also, loading that can appear in the onsite construction phase produces most times tension and torsion in the beams and joints. The main task of the designer is to design the joints such that robustness, in order to prevent premature and progressive collapse, can be demonstrated explicitly. The distribution of forces and moments in the structure due to the loading is due to the strength and stiffness distribution in the structure. Thus it can be said that the stiffness, strength and rotation capacity, together with those of the structural components like beams and columns, produce these forces in the joints. The designing of the joints include the connecting parts are of direct influence on the level of forces and moments in these joints. Joining components such as columns and beams together is construction, while designing is making choices for components taking the structural properties such as strength and stiffness into account. In traditional design the joints are stiff and strong and that the forces and moments in the structure are determined using the linear-elastic theory. In many cases in practice, this is neglected. The joints strength is adjusted to the level needed. As a result, most joints have low deformation capacity. Last but not least, fabrication costs are very high. In modern design, the joints are columns and beams with properties as stiffness, strength and deformation capacity. These structural properties of the joints are incorporated into the design on the same level as those of columns and beams. The joint layout is influenced by the fabrication considerations and considerations for easy and safe construction on-site. The structural safety of all components depends on the design method used to determine the distribution of forces and moment in the structure.
Extended end-plate connections are widely used in steel structures as moment resistance connections and as an alternative to fully welded ones that have been considered for use in steel frames. These connections consist of end-plates welded to the end of beams and field bolted to the connecting columns. The understanding of the behaviour of joints is enhanced by developing analytical models. Various forms of analysis and mathematical modelling methods have been suggested to study the semi-rigid characteristics of beam-to-column joints and their influence on the response of the rest of the structural members. There is a simplified analytical model to represent the semi-rigid connection behaviour based on the moment-rotation characteristics of the joint. The component method does not apply to end-plate joints with pre-tensioned bolts and research into these types of end-plate connections are therefore much needed. Many types of research on end-plate steel connections have been carried out analytically and experimentally to determine an accurate method for predicting the connection rotational behaviour. Two experimental tests were conducted on beam-to-beam and beam-to-column joints. Both tests failed the column flange in the compression zone. There were also tests on three specimens to enable the prediction of the moment resistance and the rotation capacity about the minor axis of the beam-to-column semi-rigid joints. Thus it motivated the development of a mechanical model to assess the structural response of joints. Furthermore, connection types and details are numerous and innovative with many parameters, that must be accounted collectively to characterize the behaviour of the joint. Such parameters include whether the end plate is flush or extended, whether the end plate extends beyond one or both of the beam flanges, the diameter and the grade of the bolts, the number of bolt rows, the vertical and horizontal spacing of the bolts, the end-plate thickness, the stiffening of steel and the coefficient of friction at the contact surface. Because of this, it is almost impossible to study the behaviour of these joints comprehensively except by physical tests. However, due to a limited number of tests, the numerical simulations are widely used currently. The more accurate finite element model for parametric analysis is particularly important. The interaction between the different components of the joint should be taken into consideration in the modelling. The contact between these components, viz. end plate-column flange, bolt-column flange and bolt-end plate were represented using the surface-to-surface contact interaction technique.