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1. CHANGES OVER TIME

Over the years, the changes in materials and reductions in safety factors make it more important to understand the behavior of reinforced concrete and provide more care. Rules of thumb and empirical methods may have been developed for different conditions and may not be applicable for today’s materials’ properties and design criteria need to be checked to determine whether they are still applicable. Another result of the development of and changes to material properties is that the ultimate limit state is often no longer critical, and a design now often depends on the serviceability limit states, apart from punching shear.

Concrete: There has been a continuous increase in the strength of concrete over the last hundred years; much of the increase has developed since 1980 (see Figure 1). Around that time, the value of blended cements and the use of admixtures was realized. Modern concretes have become complex with almost infinite variations available depending on the requirements.

The understanding of how to change the properties of concrete and reinforcement is developing rapidly. It includes:

• The use of admixtures and blended cements. Admixtures are essential for modern concrete. Self-compacting concrete is one important example. Blended cements allow the control of the rate of strength gain and the amount of heat created.
• The use of stainless steel will increase for situations where durability is paramount.
• The use of higher strength concrete will become more popular for floor slabs, particularly flat slabs. This will result in thinner and longer span slabs.
• Serviceability limit states have already become critical to flat slab design and it will become more common to check vibration of floors.
• The use of fibres will increase; the use of steel fibres has already been proven for ground floor slabs.

All these developments add complexity and cost to concrete construction. Above a cylinder strength of 50 MPa, the stress–strain properties change with increases in strength. Figure 2 shows this change diagrammatically. The concrete itself becomes more brittle as the strength increases, but it should be noted that in flexural members (beams and slabs), the ductility and brittleness are dependent mostly on the properties of the reinforcement. The increase in concrete strength and reduction in overall factor of safety (see Figure 3) have meant that, for many structural elements, the design for the serviceability limit state is becoming more critical than that for the ultimate limit state.

Reinforcement: A similar pattern of change has occurred for reinforcement both in strength and partial safety factors (see Figures 4 and 5).

Figure 5 Reduction in reinforcement overall safety factor during 20th century.

In 1964, the construction of a 35 m high dust bunker for a coal-fired power station included an external concrete cantilever staircase to be built on t the face of the outside wall of the bunker. The construction of the wall meant that the reinforcement required for the stairway would be cast flush with the wall and then bent out after removal of the formwork. At that time, proprietary reinforcement systems for such a situation did not exist and the bars were bent before fixing within the shutter. The radius of bend would have been to a standard of three times the bar diameter. 

After the formwork had been removed, the surface of the concrete was scabbled to expose these bars and the scaffold tubes were threaded over them. The scaffold tubes were then used to lever the bars out of the wall into their final positions. About 30% of all the bars bent out snapped off during the operation. The reason was a combination of factors:

• The bars should have been bent out with a special tool that ensured that the radius of bend was at least three times the bar diameter.
• The particular batch of reinforcement was found to be more brittle (less ductile) than specified.
• The work was carried out at a temperature just above freezing.

The remedial action taken was to drill holes into the concrete and grout in replacement bars.

Comment: The bending out of reinforcement cast into walls is a common procedure and, all too often is done with scaffold tubes that are readily accessible on site. It is regrettable that a proper re-bending tool is not often used which is a reflection of poor understanding of the material’s physical and chemical properties. In the manufacture of reinforcement, special procedures are in place to check the re-bending of bars to ensure that the reinforcement is sufficiently ductile. It is unfortunate that some manufacturers have continually tried to eliminate such tests from the reinforcement standard.

3. TACK WELDING OF REINFORCEMENT

The design of a building with large columns required 32 mm diameter starter bars projecting from the pile caps. The temporary works for construction included tack welding some small diameter bars to the starter bars. When the time came to fix the column reinforcement to the starter bars, the contractor attempted to bend the starter bars to ensure that they would fit into the column shutter with sufficient cover to the concrete face. A large sledge hammer was used to effect this. During this operation, two of the 32 mm diameter bars snapped off.

The reason was that tack welding the small bars onto the larger diameter starter bars changed the molecular structure of the latter. Unlike structural welding, tack welding heats just the local spot, and the heat sink of the main bar cools it very rapidly. The result was that the starter bars became brittle and required only a sharp blow to fail. In the past, tack welding on site was forbidden. Today it is sometimes permitted if carried out by a skilled specialist. Unfortunately once permitted, it is all too easy for a nonspecialist to do this work, believing that it will do no harm.

Comment: Too many people are unaware that tack welding can have significant structural effects. This is another case where a material’s chemical and physical behavior was not properly understood.

4. HIGH ALUMINA CEMENT

High alumina cement concrete has achieved a certain notoriety following the collapse of several buildings in the 1970s. By the end of 1974, up to 50,000 buildings had been reported as suspect and a major effort was made to check their safety. Fortunately, many of the affected beams stood in dry conditions and the chemical deterioration had not reached an advanced stage. The worst affected elements were positioned in damp environments.

Description: High alumina cement is manufactured from limestone or chalk and bauxite (the ore from which aluminium is obtained). The two materials are crushed and fired together using pulverised coal as a fuel. The materials fuse together, and after cooling are crushed and ground into a dark grey powder.

The predominant compounds are calcium aluminates; calcium silicates account for no more than a few percent. The calcium aluminates react with water and the primary product is calcium aluminate decahydrate (CAH₁₀).

One of its main characteristics is that the concrete made with it achieves its full strength after 24 hours compared with 28 days for a concrete with Portland cement. However, its crystal structure is unstable and changes to tricalcium aluminate hexahydrate (C₃AH₆) spontaneously (albeit slowly). This process occurs at room temperature and is accelerated by an increase in temperature. The crystal structure transforms itself to a more compact form, with the result that the cement matrix of the concrete becomes porous and weaker. The extent to which this conversion, as it is known, occurs is largely a function of the:

Degree of conversion: It was found that at the time of the collapses most HAC concrete used in buildings was 90% or more converted. A concrete from a wet mix exposed subsequently to the sun was found to have its strength reduced from 40 MPa at 24 hours to an average of about 10 MPa after less than 10 years. In contrast, concrete from prestressed precast beams with a low water-to-cement ratio and hence a 24 hour strength of 65 MPa from the same building but in a dry environment was found to have retained a strength of about 35 MPa.

5. CALCIUM CHLORIDE

Many reinforced concrete structures have suffered from too much chloride in the concrete mix. This causes the breakdown of the high level of alkalinity. When moisture and oxygen are present, carbonation occurs. This allows the reinforcement to rust and leads to spalling of the concrete surface.

Before 1980, calcium chloride was used extensively for in situ concrete works, frequently without adequate supervision. It was used principally for frost protection and to facilitate the rapid stripping of shutters. However, all too often, too much was added. In the 1980s, the codes of practice and concrete specifications were tightened to ensure that the rusting and spalling should not happen again. The following three examples describe where too much chloride in concrete caused structural failures.

Example 1: A primary school (built in 1952) was shut in 1973 due to extensive corrosion of the reinforcement of factory-made precast concrete beams. This was due to the presence of too much calcium chloride added during the manufacture of the beams to hasten the hardening of the cement. The condensation under the beams accelerated the corrosion by combining with the calcium chloride to produce hydrochloric acid.

Example 2: In 1974, the concrete roof of a school collapsed. The reason was found to be too much calcium chloride in the concrete, causing the reinforcement to deteriorate and eventually fail.

Example 3: An independent investigation of the collapse of a 100 m long pedestrian bridge found the cause to be high levels of calcium chloride in the grout used in the ducts for the prestressed tendons. This led to corrosion and failure of the prestressing tendons.

6. ALKALI–SILICA REACTION

The alkali–silica reaction (ASR) is a heterogeneous chemical reaction that takes place in aggregate particles between the alkaline pore solution of cement paste and silica in the aggregate particles. Hydroxyl ions penetrate the surface regions of the aggregate and break the silicon–oxygen bonds. Positive sodium, potassium, and calcium ions in the pore liquid follow the hydroxyl ions so that electro-neutrality is maintained. Water is imbibed into the reaction sites and eventually an alkali–calcium–silica gel is formed.

Figure 6 Examples of alkali–silica reaction. (Top: From the US Department of Transportation Highway Administration; middle: From Dr. Ideker, http://web.engr. oregonstate.edu/~idekerj/; bottom: From the US Department of Transportation Highway Administration.)

The reaction products occupy more space than the original silica so the surface reaction sites are put under pressure. The surface pressure is balanced by tensile stresses in the centres of the aggregate particles and in the ambient cement paste. At a certain point, the tensile stresses may exceed the tensile strength and brittle cracks propagate. The cracks radiate from the interior of the aggregate out into the surrounding paste.

The cracks are empty (not gel-filled) when formed. Small or large amounts of gel may subsequently exude into the cracks. Small particles may undergo complete reaction without cracking. Formation of the alkali–silica gel does not cause expansion of the aggregate. Observation of gel in concrete is therefore no indication that the aggregate or concrete will crack. ASR is diagnosed primarily by four main features

• Presence of alkali–silica reactive aggregates
• Crack pattern (often appearing as three-pointed star cracks)
• Presence of alkali–silica gel in cracks and/or voids
• Ca(OH)2 depleted paste

In mainly unidirectional reinforced members, the cracks become linear and parallel to the reinforcement. The degree of cracking depends on the amount of confining reinforcement, i.e., links, etc. One major concern was that ASR caused cracking that led bits of concrete to fall off structural elements and hit people below. This led to demolition of the structures in some cases. Examples of ASR effects are given in Figure 6.

7. LIGHTWEIGHT AGGREGATE CONCRETE

During the 1960s, a medium-size civil engineering contractor wanted to join the housing drive, then at its peak. At the time, an Austrian construction firm used crushed brick rubble as aggregate in un-reinforced concrete walls for six- and seven-storey blocks of flats. Inspired by this, it was decided to try to develop a similar form of load-bearing wall with adequate thermal insulation, made of lean-mix plain concrete with light expanded clay aggregate (LECA). A 12-storey block was constructed as a pilot project.

The strength of the wall concrete was reduced in stages—about 2000 psi (14 MPa) at 28 days for the four bottom storeys, 1600 psi (15 MPa) for the next four, and 1200 psi (8 MPa) for the top storeys. The floor slabs were of traditional reinforced concrete, but the roof slab was reinforced LECA concrete with a strength of 3000 psi (21 MPa).

There was no significant adverse feedback from the tenants nor the building authority. The block remained standing and in use for over 40 years. Encouraged by the apparent success, the contractor started promoting the ‘system.’ About the same time, lightweight aggregate concrete was included in the code of practice and a minimum strength of 3000 psi (21 MPa) was

stipulated. This required a richer mix than that used for the walls of the earlier block. The resulting effects of this on the thermal insulation and shrinkage properties of the LECA concrete appear to have been overlooked by the design team.

A few blocks were built for local authorities outside the London County Council area. These were higher than the first block utilising the higher concrete strength required by the code in the walls. Many of the flats were allocated to tenants in poor financial circumstances, who could not afford the charges for the underfloor heating and used paraffin heaters instead.

This, combined with the reduced thermal insulation of the external walls, led to severe condensation. Structurally more important, however, were the diagonal cracks that developed on the top floor of one of the blocks within a short time after hand-over. From their geometry, they appeared to be due to lower shrinkage and greater thermal expansion of the roof slab relative to the wall concrete.

Definitely alarming was the occurrence of horizontal cracks in one of the 200 mm thick internal cross walls connected to the 300 mm thick external wall at right angles. One of these cracks on the 13th floor of a 16-storey block opened suddenly with a noise like a gun shot. The wall was 200mm thick and, according to the design assumptions, carried the floor slabs that spanned about 3.5 m on either side. This meant that the building contained three storeys of unreinforced concrete cross wall, with the load from approximately 3.5 m width of floors plus the roof hanging or cantilevering off the external wall!

Structurally, the only explanation for these cracks seemed to be that the internal wall was drying out, and therefore shrinking and shortening, while the external wall with very little load to carry (at least initially) and exposed to the British weather was not shortening at the same rate.

Discussion: The porous LECA pellets were soaked just before the mixing of the concrete, to prevent them from absorbing water from the fresh mix and thus making it too stiff. They therefore constituted a reservoir of water, over and above that required for the hydration of the cement. This extra water meant that the LECA concrete needed more time to dry out and the 300 mm external walls would have a slower rate of drying out than the 250 mm internal walls even if they had the same environment on both faces.

Comment: These cracks were due to the changed properties of the wall concrete. A proper study of the properties of the materials along with a review of the design would have shown that the two-stage extrapolation from medium rise to high rise and from lean mix, brick rubble, unreinforced concrete to dense, albeit lightweight aggregate, concrete could not be sustained.

This dictionary contains some 14,000 English terms together with their German, French, Dutch and Russian equivalents, which are used in the main branches of civil engineering and relate to the basic principles of structural design and calculations (the elasticity theory, strength of materials, soil mechanics and other allied technical disciplines); to buildings and installations, structures and their parts, building materials and prefabrications, civil engineering technology and practice, building and road construction machines, construction site equipment, housing equipment and fittings (including modern systems of air conditioning); as well as to hydro technical and irrigation constructions.

The Dictionary also includes a limited number of basic technical expressions and terms relating to allied disciplines such as architecture and town planning, as well as airfield, railway and underground construction. The Dictionary does not list trade names of building materials, parts and machines or the names of chemical compounds. Nor does it give adverbial, adjective or verbal terms.

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Introduction

A key function of the envelope of a building is to act as a passive climate modifier to help maintain an indoor environment that is more suitable for habitation than the outdoors. However, besides providing shelter from stormy weather, the building envelope alone can hardly ensure that the indoor environmental conditions will always be comfortable to the occupants, or be suitable for the intended purposes of the indoor spaces, particularly during periods of unfavourable outdoor conditions, such as in the night or when the outdoor air is stagnant.

The need for building design features that would facilitate use of natural ventilation and daylight has diminished, as active means of environmental control, such as central heating, ventilating and air-conditioning (HVAC) systems and electric lights, can be used instead to maintain adequate indoor thermal and visual environmental conditions, and air quality. This has also helped to remove the restrictions imposed on the design of buildings, particularly to maximisation of the amount of floor area that can be built upon a given piece of land.

The increased reliance on HVAC and lighting systems for active control over the indoor environmental conditions, however, has made buildings the dominant energy consumers in modern cities worldwide. This has not only accelerated the depletion of the limited reserve of fossil fuels on earth; it has also exacerbated global warming and environmental pollution due to the emissions of combustion products resulting from burning of fuels for generating electricity, steam, hot water or chilled water for use in buildings. Buildings also contribute to other environmental problems, such as the use of CFC as refrigerants in HVAC plants and halons as fire extinguishing agents, which are causes of the depletion of the stratospheric ozone layer. Therefore, besides fulfilment of the functional needs and aesthetics, the environmental performance of buildings, which includes energy efficiency, has become an essential attribute of environmentally friendly buildings.

Measures that can be used to enhance the energy efficiency of a building include the adoption of building design features that can help reduce the frequency and intensity of use of the HVAC plants and the lighting installations, and the use of more efficient HVAC and lighting system designs and equipment. For instance, the cooling or heating load due to heat transmission through the building envelope can be reduced through the use of thermal insulation at external walls and roofs, and high performance glazing and shading devices at windows. The use of energy-efficient boilers and chillers, variable speed motor drives in heating and air-conditioning systems, and energy-efficient lamps and electronic ballasts can lead to very significant energy saving.

In the design process, the effectiveness of individual energy efficiency enhancement measures, particularly the possible energy and running cost saving, would need to be quantified. The financial benefit, derived from the difference in the annual energy consumption of the building with and without a particular measure, would be essential input to a financial appraisal for determining whether or not to adopt individual measures, and for selecting the most viable choices.

Quantification of the annual energy use in a building requires prediction of the space cooling loads of individual rooms in the building that would arise at different times in the operating periods throughout the year. This involves determination of the heat and mass transfer through the building envelope that are significant parts of the heat and moisture gains or losses of an indoor space. The other sources of heat and moisture gains include occupants, equipment and appliances present within the air-conditioned spaces, and infiltration.

Methods for modelling the heat and mass transfer through the building envelope is a key starting point in the prediction of the annual energy consumption in a building. In most such analyses, the mass transfer modelled would be limited to the bulk air transport into or out of buildings through infiltration and exfiltration, while the moisture transport through the building fabric elements would be ignored.

Heat and mass transfer processes in buildings

The range of heat and mass transfer processes that would take place in buildings is as illustrated in Fig.1, which shows a perimeter room on an intermediate floor in a multi-storey office building. The room is separated from the outdoors by an external wall and a window, and from adjoining rooms at the sides by internal partitions, and at above and below by a ceiling and a floor slab. The room is equipped with a HVAC system that would supply heating or cooling to the room by circulating air between the room and the air-handling unit via the supply and return air ducts.

(a) conduction heat transfer through the building fabric elements, including the external walls, roof, ceiling and floor slabs and internal partitions;
(b) solar radiation transmission and conduction through window glazing;
(c) infiltration of outdoor air and air from adjoining rooms;
(d) heat and moisture dissipation from the lighting, equipment, occupants and other materials inside the room; and
(e) heating or cooling and humidification or dehumidification provided by the HVAC system.

The conduction heat transfer through an opaque building fabric element, such as an external wall as shown in Fig. 2, is the effect of the convective heat that the surface at each side of the element is exchanging with the surrounding air, and the radiant heat exchanges with other surfaces that the surface is exposed to. For an external wall or a roof, the radiant heat exchange at the external side includes the absorbed solar radiation, including both direct and diffuse radiation.

The heat transfer through a window is shown in Fig. 3. The window glass will transmit part of the incident solar radiation into the indoor space. While the solar radiation penetrates the glass pane, some of the energy will be absorbed by the glass, leading to an increase in the glass temperature, which, in turn, will cause heat to flow in both the indoor and the outdoor directions, first by conduction within the glass and then by convection and radiation at the surfaces at both sides. The heat flows through a building fabric element resulting from the absorbed solar radiation and the outdoor to indoor temperature difference are often treated together through the use of an equivalent outdoor air temperature, called ‘sol-air temperature’, that will, in the absence of the radiant heat exchange, cause the same amount of conduction and convection heat flow through the element. A similar parameter, called ‘environmental temperature’, is used to account for the combined effects of the convective heat transfer from the internal surface to the room air and the radiant energy gain at the surface.

The transmitted solar radiation from the windows will be imparted to the indoor air and become cooling load only after it has been absorbed by the internal surfaces. Consequently, the temperature at such surfaces will rise, leading to convective heat flow from the surfaces into the room air. It is this convective heat flow that will affect the indoor air temperature and constitutes a component of the space cooling load. This cooling load component will differ in magnitude and in the time of occurrence of its peak value from those of the radiant heat gain, as shown in Fig. 4, which is the result of the thermal capacitance of the fabric elements or furniture materials that are subject to thermal irradiation. Besides the short wave radiation from the sun, radiant heat gains from the lighting and equipment and the long wave radiation exchange among the internal surfaces within the space will need to undergo a similar process to become a cooling load.

When there are air movements into and out of an indoor space, heat and moisture will be brought into or out of the room if the airs that enter the space are at thermodynamic states different from that of the indoor air. The air movements can be set up by pressure differences between the room and the adjoining rooms and the outdoor, due to wind or stack effect, or imbalance in the supply and extract flow rates maintained by the ventilation system.

The thermodynamic state of the air within the room would vary with the net heat and moisture gains experienced by the room air, resulting from heat and moisture exchanges with the enclosure surfaces, air transport into or out of the room bringing with it heat and moisture, heat and moisture gains from sources present within the room, and heating, cooling, humidification or dehumidification provided by the HVAC system serving the room. These heat and moisture transfer processes would need to be modelled for the prediction of the indoor air condition or the rate of heating or cooling, and humidification or dehumidification required for maintaining the indoor air condition at the set point state.

The estimating team will consider construction methods and employ planning techniques to:

1. Highlight any critical or unusual activities.
2. Examine alternative ways of tackling the work.
3. Calculate optimum durations for temporary works and plant.
4. Reconcile the labour costs in the estimate with a programme showing resources.
5. Determine the general items and facilities priced in the preliminaries section of the bill.
6. Check whether the time for completion is acceptable.

The effort needed will depend on the size and complexity of the project, the proposed use of heavy plant and the design of major temporary works. Estimating for civil engineering work in particular is dependent on an examination of alternative methods and pre-tender programmes. A civil engineering estimator usually produces a resourced programme to price major aspects of the work operationally. 

Pre-tender programmes are prepared by either the estimator or planning engineer, or more likely by working together.The choice depends on company policy, size of project and type of work. The planning engineer’s contribution can be seen as producing an appraisal of labour and plant resources and general items – in other words the estimator expresses his solutions in terms of cash, the programmer deals with time. The aim is to reconcile one with the other.

In a competitive market it is important to look for ways to construct the project more economically. Applying planning techniques can have opposite consequences. Increasing the value of the tender when problems are identified and reducing the estimate when methods can be adopted which reduce individual and overall durations. The team must, however, look for the solution, which reflects the ‘true’ cost of construction.The role of the planning engineer is wider than just producing a programme. His input to a tender can also include:

1. Producing site layout drawings, which are used to locate temporary facilitates, such as concrete batching plant, cranage, access routes, restrictions, areas for accommodation and storage, location of services, overhead service, temporary spoil heaps, and areas which will need reinstatement.
2. Examining the most suitable methods in relation to the design and the temporary works required.
3. Preparing method statements not only for pricing purposes but also for submission to clients or consultants when requested.
4. Producing cashflow forecast charts for management and clients who need them.
5. Providing staff structure and resource histograms for general labour, production labour and plant.

The planning engineer will often have a better understanding of current site practice and will be better placed to collect data from monitoring exercises on site. His experience of completed work will be important especially where the overall duration of a project could be reduced. Shorter contract periods can have a substantial effect on the cost of preliminaries where time-related costs (mainly staff, site accommodation, cranage and scaffolding) account for as much as 12–20% of a tender figure.

An electrical device which accelerates charged atomic or subatomic particles to high energies. The particles may be charged either positively or negatively. If subatomic, the particles are usually electrons or protons and, if atomic, they are charged ions of various elements and their isotopes throughout the entire periodic table of the elements.
 

Accelerators that produce various subatomic particles at high intensity have many practical applications in industry and medicine as well as in basic research. Electrostatic generators, pulse transformer sets, cyclotrons, and electron linear accelerators are used to produce high levels of various kinds of radiation that in turn can be used to polymerize plastics, provide bacterial sterilization without heating, and manufacture radioisotopes which are utilized in industry and medicine for direct treatment of some illnesses as well as research. They can also be used to provide high-intensity beams of protons, neutrons, heavy ions, pi mesons, or x-rays that are used for cancer therapy and research.
 

The x-rays used in industry are usually produced by arranging for accelerated electrons to strike a solid target. However, with the advent of electron synchrotron storage rings that produce x-rays in the form of synchrotron radiation, many new industrial applications of these x-rays have been realized, especially in the field of solid-state microchip fabrication and medical diagnostics.

Particle accelerators fall into two general classes—electrostatic accelerators that provide a steady dc potential, and varieties of accelerators that employ various combinations of time-varying electric and magnetic fields.

Electrostatic accelerators

 

Electrostatic accelerators in the simplest form accelerate the charged particle either from the source of high voltage to ground potential or from ground potential to the source of high voltage. All particle accelerations are carried out inside an evacuated tube so that the accelerated particles do not collide with air molecules or atoms and may follow trajectories characterized specifically by the electric fields utilized for the acceleration. The maximum energy available from this kind of accelerator is limited by the ability of the evacuated tube to withstand some maximum high voltage.

Time-varying field accelerators. In contrast to the highvoltage- type accelerator which accelerates particles in a continuous stream through a continuously maintained increasing potential, the time-varying accelerators must necessarily accelerate particles in small discrete groups or bunches.
 

An accelerator that varies only in electric field and does not use any magnetic guide or turning field is customarily referred to as a linear accelerator or linac. In the simplest version of this kind of accelerator, the electrodes that are used to attract and accelerate the particles are connected to a radio-frequency (rf) power supply or oscillator so that alternate electrodes are of opposite polarity. In this way, each successive gap between adjacent electrodes is alternately accelerating and decelerating. If these acceleration gaps are appropriately spaced to accommodate the increasing velocity of the accelerated particle, the frequency can be adjusted so that the particle bunches are always experiencing an accelerating electric field as they cross each successive gap. In this way, modest voltages can be used to accelerate bunches of particles indefinitely, limited only by the physical length of the accelerator construction.

All conventional (but not superconducting) research linacs usually are operated in a pulsed mode because of the extremely high rf power necessary for their operation. The pulsed operation can then be adjusted so that the duty cycle or amount of time actually on at full power averages to a value that is reasonable in cost and practical for cooling. This necessarily limited duty cycle in turn limits the kinds of research that are possible with linacs; however, they are extremely useful (and universally used) as pulsed high-current injectors for all electron and proton synchrotron ring accelerators. Superconducting linear accelerators have been constructed that are used to accelerate electrons and also to boost the energy of heavy ions injected from electrostatic machines. These linacs can easily operate in the continuouswave (cw) rather than pulsed mode, because the rf power losses
are only a few watts.
 

The Continuous Electron Beam Accelerator Facility (CEBAF) uses two 400-MeV superconducting linacs to repeatedly accelerate electrons around a racetrack-like arrangement where the two linacs are on the opposite straight sides of the racetrack and the circular ends are a series of recirculation bending magnets, a different set for each of five passes through the two linacs in succession. The continuous electron beam then receives a 400-MeV acceleration on each straight side or 0.8 GeV per turn, and is accelerated to a final energy of 4 GeV in five turns and extracted for use in experiments. The superconducting linacs allow for continuous acceleration and hence a continuous beam rather than a pulsed beam. This makes possible many fundamental nuclear and quark structure measurements that are impossible with the pulsed electron beams from conventional electron linacs. 

 

As accelerators are carried to higher energy, a linac eventually reaches some practical construction limit because of length. This problem of extreme length can be circumvented conveniently by accelerating the particles in a circular path maintained by either static or time-varying magnetic fields. Accelerators utilizing steady magnetic fields as guide paths are usually referred to as cyclotrons or synchrocyclotrons, and are arranged to provide a steady magnetic field over relatively large areas that allow the particles to travel in an increasing spiral orbit of gradually increasing size as they increase in energy.

Practical limitations of magnet construction and cost have kept the size of circular proton accelerators with static magnetic fields to the vicinity of 100 to 1000 MeV. For even higher energies, up to 400 GeV per nucleon in the largest conventional (not superconducting) proton synchrotron in operation, it is necessary to vary the magnetic field as well as the electric field in time. In this way the magnetic field can be of a minimal practical size, which is still quite extensive for a 980-GeV accelerator (6500 ft or 2000min diameter). This circular magnetic containment region, or “racetrack,” is injected with relatively low-energy particles that can coast around the magnetic ring when it is at minimum field strength. The magnetic field is then gradually increased to stay in step with the higher magnetic rigidity of the particles as they are gradually accelerated with a time-varying electric field.

Superconducting magnets

 

The study of the fundamental structure of nature and all associated basic research require an ever increasing energy in order to allow finer and finer measurements on the basic structure of matter. Since the voltage-varying and magnetic-field-varying accelerators also have limits to their maximum size in terms of cost and practical construction problems, the only way to increase particle energies even further is to provide higher-varying magnetic fields through superconducting magnet technology, which can extend electromagnetic capability by a factor of 4 to 5. Large superconducting cyclotrons and superconducting synchrotrons are in operation.

 

Storage rings

 

Beyond the limit just described, the only other possibility is to accelerate particles in opposite directions and arrange for them to collide at certain selected intersection regions around the accelerator. The main technical problem is to provide adequate numbers of particles in the two colliding beams so that the probability of a collision is moderately high. Such storage ring facilities are in operation for both electrons and protons.
 

Besides storing the particles in circular orbits, the rings can operate initially as synchrotrons and accelerate lower-energy injected particles to much higher energies and then store them for interaction studies at the beam interaction points.
 

Large proton synchrotrons have been used as storage-ring colliders by accelerating and storing protons in one direction around the ring while accelerating and storing antiprotons (negative charge) in the opposite direction. The proton and antiproton beams are carefully programmed to be in different orbits as they circulate in opposite directions and to collide only when their orbits cross at selected points around the ring where experiments are located. The antiprotons are produced by high-energy proton collisions with a target, collected, stored, cooled, and eventually injected back into the synchrotron as an antiproton beam.

Electron-positron synchrotron accelerator storage rings have been in operation for many years in the basic study of particle physics, with energies ranging from 2 GeV + 2 GeV to 104 GeV+104 GeV. The by-product synchrotron radiation from many of these machines is used in numerous applications. However, the synchrotron radiation loss forces the machine design to larger and larger diameters, characterized by the Large Electron
 

Positron Storage Ring (LEP) at CERN, near Geneva, Switzerland (closed down in 2000), which was 17 mi (27 km) in circumference. Conventional rf cavities enable electron-positron acceleration only up to 50–70 GeV (limited by synchrotron radiation loss) while higher energies of 100–150 GeV require superconducting cavities.

 

Advanced linacs

 

Although circular machines with varying magnetic fields have been developed because linacs of comparable performance would be too long (many miles), developments in linac design and utilization of powerful laser properties may result in a return to linacs that will outperform present ring machines at much lower cost. As a first example, the 20-GeV electron linac at Stanford University, Palo Alto, California, has been modified to provide simultaneous acceleration of positrons and electrons to energies as high as 50 GeV, while operating in what is called the SLED mode. After acceleration the electrons and positrons are separated by a magnet, and the two beams are magnetically directed around the opposite sides of a circle so that they collide at one intersection point approximately along a diameter extending from the end of the linac across the circle. This collider arrangement is much less expensive than the 17-mi (27-km) ring at CERN and provides electron-positron collisions of comparable energies but at lower intensities.

Flexibility implies the availability of right facilities at the right time. Its a challenge to utilize the best value in mega projects and to cope with the barriers or limitations that might arise with time in doing so. Flexibility in design holds significance for a variety of professionals and stakeholders associated with the project. These include but are not limited to designers, investors and lenders, managers, users, regulators and clients. There exist uncertainties in future both in terms of opportunities and limitations. Flexible systems open the gates to utilizing these uncertainties for the benefit of the project. Uncertainties in technological systems can only be exploited by utilizing flexibility in the initial design. In doing so , the stakeholders should try to predict the future and try to accommodate a range of possibilities in the design rather than going for fixed expectations.

The future is unpredictable no matter how precise our analysis becomes. It is thus desirable to design and develop our projects by incorporating flexibility to accommodate any changes needed to mitigate the risks in future or to make modifications to increase the project’s value. Explanations of the engineering design procedures are wide in the literature. The models are inflexible and signify step-wise procedures of an iterative nature. Cross [1] discusses the four main stages of the most basic design model model as specification, concept, schematic design and detail. It is of vital importance that members of the project team should collaborate over time to benefit from such a flexibility. System management also holds key importance in this regard. The system will not be able to profit from the incorporated flexibility unless the managers manage it smartly and make changes as necessary. The timing and prediction of requirements hold key importance in making a project successful in the long run.

The aspects related to the need of flexibility in designs and its significance will be briefly discussed in this article.

Uncertainty in Future

Technology is evolving at a very fast rate. State-of-the-art today may become obsolete in the near future. Numerous examples can be seen from near past in this aspect. The distribution of music on vinyl records shifted towards and then to CDs and DVDs to end up being distributed wirelessly across multiple platforms and through online music stores. The development in computer hardware and software can also be seen as one of the extremely rapidly changing products. The uncertainties associated with any project or product are often only translated as risks. This approach however addresses the negative aspect only. Uncertainties also create numerous opportunities and expansions can be made to make the project more lucrative. The changes in user requirements as well as industry standards can benefited from. Upside potential can be kept in mind to harvest the full potential of a project.

A very interesting case study is the Iridium fleet of communication satellites where sensitivity of technological projects to changes in context is illustrated. Motorola designed this satellite system to provide wireless telephone service all over the globe. This no doubt was a major technological breakthrough but it failed miserably financially. The reason being that the system was designed almost a decade prior to its actual execution. The cell phone technology had already become very popular when this technology was launched at a very high expected number users. This clearly shows the change in context and user requirements which were not incorporated in design which lead to its bankruptcy and eventual sale at about half percent of its initial $4 billion investment. Global Positioning System (GPS) is a technology initially developed by U.S military to track long range missiles. GPS chips are very popular these days and are available in cell phones, cars, aircrafts etc. The U.S military did not anticipate such a commercial success of this technology and did not incorporate a way to charge for the services. This clearly deprives them from the benefit they might have from the commercial users.

Thus technology affects the value of investments either directly or indirectly. Productivity is also related to the uncertainties in future and strongly affects the benefits. The future uncertainties can not be predicted accurately and the challenge is equally confronted by all the stakeholders.

Categories of Flexible Designs

As discussed in the previous sections, flexibility can be viewed from many perspectives. Flexible designs can be broadly classified in to following three categories,

Changes in size and shape

This kind of flexible design allows future expansion or contraction or the same in short terms. This can be seen in many modular systems e.g in formwork in construction industry, modular construction methods which allow for expansion or capability of a facility like an airport or a public venue to be partially closed. Space shuttles, solar dishes, antennas and other aerospace equipment also have the flexibility to change their size but this flexibility is mainly utilized during the launching phase. Flying cars also change in size to perform the required functions. Foldable guns, foldable bridges, origami inspired structures etc have the ability to change their shape.

Changes in function

Flexibility can also be seen as a change in intended function of a system. USB ports and other connection types which allow the users to connect state-of-the art equipment with their computers is a very common case. A sports stadium with a deployable roof and folding chairs can also be utilized off season for parties, concerts and other public events rather than being closed and wait for the revenue generating opportunities in the next sports season.

Protection against unwanted events and accidents

The systems are usually capable of dealing with the associated risks. Seat belts and air bags are among the flexibility added to deal with the risk of collision in cars. The stadiums with retractable roofs also feature such a kind of flexibility by allowing the events to be conducted under closed roof conditions in rainy or snowy weather.

Bibliography
[1] Cross, N. (2000). Engineering design methods: strategies for product design. Wiley.

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Stepan Prokopovych Timoshenko was born on December 22, 1878 in the village of Shpotivka in the Ukrain. Timoshenko’s early life seems to have been a happy one in pleasant rural surroundings. He studied at a “realnaya” school from 1889 to 1896.

Timoshenko continued his education towards a university degree at the St. Petersburg Institute of Engineering. After graduating in 1901, he stayed on teaching in this same institution from 1901 to 1903 and then worked at the St. Petersburg Polytechnic Institute under Viktor Kyrpychov 1903–1906. His restlessness and discontent with the educational system extant in Russia at that time motivated the young Timoshenko to venture out to explore, examine, and assimilate diverse pedagogical views and cultures in France, Germany, and England. In 1905 he was sent for 1 year to the University of Göttingen where he worked under Ludwig Prandtl.

In the fall of 1906 he was appointed to the Chair of Strengths of Materials at the Kyiv Polytechnic Institute. Thanks to his tormented spirit at this institute, Timoshenko took the plunge to writing his maiden Russian classic, Strength of Materials in 1908 (Part I) and 1910 (Part II). From 1907 to 1911 as a professor at the Polytechnic Institute he did research in the area of finite element methods of elastic calculations, and did excellent research work on buckling. He was elected dean of the Division of Structural Engineering in 1909.

In 1911 he was awarded the D. I. Zhuravski prize of St. Petersburg; he went there to work as a Professor in the Electro-technical Institute and the St. Petersburg Institute of the Railways (1911–1917). During that time he developed the theory of elasticity and the theory of beam deflection, and continued to study buckling. In 1922 Timoshenko moved to the United States where he worked for the Westinghouse Electric Corporation from 1923 to 1927, after which he became a faculty professor at the University of Michigan where he created the first bachelor’s and doctoral programs in engineering mechanics. His textbooks have been published in 36 languages. His first textbooks and papers were written in Russian; later in his life, he published mostly in English.

It is a well-known fact that diamond is the hardest material on the planet. However, diamond lost its status of the hardest substance when scientists at the North Carolina State University discovered a new, harder-than-diamond form of carbon.

The research team found a new phase of carbon, called the Q-carbon.




The new phase of carbon is quite different from graphite and diamond. Q-Carbon shows some unique and quite unexpected properties like ferromagnetic nature. Another unusual feature of this phase of carbon is that it begins to glow in the presence of energy. See more images from various experiments below:

The research has been published in the Journal of Applied Physics. The Q-phase of carbon was created by focusing a laser beam on amorphous carbon for 200-nanoseconds. The temperature of the amorphous carbon sample rose quickly and was cooled down by the process of quenching to create Q-Carbon.

This recent discovery will be a massive breakthrough in the domain of structures and materials. The new hardest material on the planet might be used for the fabrication of prosthetic structures, improvement of the equipment used for deep drilling, as well as developing new, brighter screens for the smartphones and televisions.

If you grew up memorizing multiplication tables, you know the struggle of elementary mathematics students. Math isn’t meant to be about memorizing, it is supposed to be about problem-solving.

One Japanese method of multiplication teaches kids to both visualize how numbers multiply together and provides a quick and easy method to solve large multiplication problems. The best part? This technique involves absolutely no numbers in the solving stage, so no more forgetting to carry the one and getting the answer wrong. Check out the short video below to learn a little more.

Most commonly, this method is believed to have originated in Japan, but it is an underutilized tool in the realm of teaching kids mathematics. By drawing parallel lines for each number slot in one number then drawing perpendicular lines that are parallel to each other in another direction you end up with a series of intersection points. By separating these intersections into sections, just count the points and you have your final answer. For problems that involve multiplication of numbers involving tens and hundreds places, this method proves faster than doing it the old fashioned way.

Not only is this a cool trick, but it actually visualizes what is happening when you multiply 2 numbers together. Each place (ones, tens, hundreds) is symbolized by an intersection location created by the actual numbers in the problem. To explain further, for the problem of 123×321, the 1 line crosses the 3 lines creating a total of 3 intersections in the thousands place. This means that the first number is 3, and you know the answer will be to the magnitude of thousands by the number of intersection locations. Still not getting it? Check out the explanatory video below for further help.

If you have decided to start an engineering business, you are about to embark on an exciting journey into the unknown. Working for yourself is different to working for a boss. You are now the master of everything you do. You decide what happens to you and how much money you make. For some, that freedom is liberating, for others, it is terrifying. Becoming a boss won’t be easy. You have loads of things to consider before you make it big. Here are five super tips that you should keep in mind.

Make a financial budget before every quote

When you get your first job, it is easy to let it excite you. Many new businessmen rush into quoting people for a job, before doing their research. Don’t think that taking a little extra time to complete your budget will lose you a client. Many people worry that if they take too long getting back to the client, the client will go with another company. It is your duty to make a budget before you set a quote. If you don’t do that, you could end up losing money on jobs. 

Time management will be your best tool 

Engineering jobs always run longer than they should. You can set yourself aside from the majority of engineering companies by completing jobs on time. Time management is not easy. You need to have great organizational and planning skills. You should make sure that you make a detailed schedule of each day. That means you can plan everybody’s roles and responsibilities before they start work. Usually, engineering companies get just one fee for the entire job. That means that when you are wasting time, you’re also wasting money. 

Every employee plays an integral role 

Don’t dismiss anyone’s ability or skills. In an engineering company, there is a hierarchy system that works within every group. There is no such thing as a small job. Make sure that you appreciate the hard work that everybody is putting into your company. It is vital that you let your staff know how much you appreciate them. It can be easy to take staff for granted. If you do that, your staff will likely find another company for which to work. Take the time to get to know your staff so that you create a strong working relationship with them. 

Choose your material and tools with care 

When you are buying your materials and tools for each job, you might find buying the cheapest items is tempting. Remember, cheap materials never last. You want people to remember you as a quality company. That means that you need to ensure that you get quality materials so that they last. For example, when you’re buying a drawn seamless tube for a job, make sure that it comes with great reviews. Finding the right material for each job is vital to the success of your business, and so you need to make sure that you choose with care. 

Start small and think big later 

You might want to dive into large engineering jobs and take on the world. As with everything, though, you’re going to need to start small. You can’t expect to land a massive job when you first start your business. Your clients want to see a track record of great work from you. That means that you won’t get the big jobs until you have proved you can handle them. No job is ever too small for you to take on when you start your company. Thinking that you’re above certain jobs won’t win you friends or clients. Take any job you can get so that you can start building your portfolio.