Aluminum is an important commercial metal possessing some very unique properties. It is very light (density about 2.7) and some of its alloys are very strong, so its strength-weight ratio makes it very attractive for aeronautical uses and other applications in which weight saving is important.
Aluminum, especially in the pure form, has very high electrical and thermal conductivities and is used as an electrical conductor in heat exchangers, etc. Aluminum has good corrosion resistance, is nontoxic, and has a pleasing silvery-white color; these properties make it attractive for applications in the food and container industry, architectural, and general structural fields.
Aluminum is very ductile and easily formed by casting and mechanical forming methods. Aluminum owes its good resistance to atmospheric corrosion to the formation of a tough, tenacious, highly insulating, thin oxide film, in spite of the fact that the metal itself is very anodic to other metals.
In moist atmospheres, this protective oxide may not form, and some caution must be taken to maintain this film protection. Although aluminum can be joined by all welding processes, this same oxide film can interfere with the formation of good bonds during both fusion and resistance welding, and\ special fluxing and cleaning must accompany welding operations.
Commercially pure aluminum (99+%) is very weak and ductile: tensile strength of 90 Mpa (13,000 lb/in2), yield strength of 34.5 MPa (5000 lb/in2), and shearing strength of 62 MPa (9500 lb/in2). Extrapure grades (electrical conductor grade) are 99.7+% pure, and are even weaker, but have better conductivity.
Heat Treatment of Aluminum Alloys.
Alloys of the 1000, 3000, and 5000 series cannot be hardened by heat treatment. They can be hardened by cold working and are available in annealed (recrystallized) and cold-worked tempers.
The 5000 series alloys are the strongest non-heat-treatable alloys and are frequently used where welding is to be employed, since welding will generally destroy the effects of hardening heat treatment. The remaining wrought alloys can be hardened by controlled precipitation of alloy phases.
The precipitation is accomplished by first heating the alloy to dissolve the alloying elements, followed by quenching to retain the alloy in supersaturation. The alloys are then “aged” to develop a controlled size and distribution of precipitate that produces the desired level of hardening. Some alloys naturally age at room temperature; others must be artificially aged at elevated temperatures.
Titanium alloys are important industrially because of their high strength-weight ratio, particularly at temperatures up to 427°C. The density of the commercial titanium alloys ranges from 4.50 to 4.85 g/cm3, or approximately 70% greater than aluminum alloy and 40% less than steel.
The purest titanium currently produced (99.9% Ti) is a soft, white metal. The mechanical strength increases rapidly, however, with an increase of the impurities present, particularly carbon, nitrogen, and oxygen.
The commercially important titanium alloys, in addition to these impurities, contain small percentages (1% to 7%) of (1) chromium and iron, (2) manganese, and (3) combinations of aluminum, chromium, iron, manganese, molybdenum, tin, or vanadium.
The thermal conductivity of the titanium alloys is low, about 15 W/m ⋅ K at 25°C, and the electrical resistivity is high, ranging from 54 mΩ ⋅ cm for the purest titanium to approximately 150 mΩ ⋅ cm for some of the alloys.
The coefficient of thermal expansion of the titanium alloys varies from 2.8 to 3.6 x 10–6 per degree Celsius, and the melting-point range is from 1371 to 1704°C for the purest titanium. The tensile modulus of elasticity varies between 100 to 120 GPa (15 to 17 # 106 lb/in2).
The mechanical properties, at room temperature, for annealed commercial alloys range approximately as follows: yield strength 760 to 965 MPa (110,000 to 140,000 lb/in2); ultimate strength 800 to 1100 MPa (116,000 to 160,000 lb/in2); elongation 5% to 18%; hardness 300 to 370 Brinell.
On the basis of the strength-weight ratio many of the titanium alloys exhibit superior short-time tensile properties as compared with many of the stainless and heat-resistant alloys up to approximately 427°C.
However, at the same stress and elevated temperature, the creep rate of the titanium alloys is generally higher than that of the heat-resistant alloys. Above about 482°C, the strength properties of titanium alloys decrease rapidly. The corrosion resistance of the titanium alloys in many media is excellent; for most purposes, it is the equivalent or superior to stainless steel.
Bronze is an alloy consisting principally of copper and tin and sometimes small proportions of zinc, phosphorus, lead, manganese, silicon, aluminum, magnesium, etc. The useful range of composition is from 3% to 25% tin and 95% to 75% copper.
Bronze castings have a tensile strength of 195 to 345 MPa (28,000 to 50,000 lb/in2), with a maximum at about 18% of tin content. The crushing strength ranges from about 290 MPa (42,000 lb/in2) for pure copper to 1035 MPa (150,000 lb/in2) with 25% tin content.
Cast bronzes containing about 4% to 5% tin are the most ductile, elongating about 14% in 5 in. Gunmetal contains about 10% tin and is one of the strongest bronzes.
Bell metal contains about 20% tin. Copper-tin-zinc alloy castings containing 75% to 85% copper, 17% to 5% zinc, and 8% to 10% tin have a tensile strength of 240 to 275 Mpa (35,000 to 40,000 lb/in2), with 20% to 30% elongation.
Government bronze contains 88% copper, 10% tin, and 2% zinc; it has a tensile strength of 205 to 240 MPa (30,000 to 35,000 lb/in2), yield strength of about 50% of the ultimate, and about 14% to 16% elongation in 2 in; the ductility is much increased by annealing for ½ h at 700 to 800°C, but the tensile strength is not materially affected.
Phosphor bronze is made with phosphorus as a deoxidizer; for malleable products such as wire, the tin should not exceed 4% or 5%, and the phosphorus should not exceed 0.1%. United States Navy bronze contains 85% to 90% copper, 6% to 11% tin, and less than 4% zinc, 0.06% iron, 0.2% lead, and 0.5% phosphorus; the minimum tensile strength is 310 MPa (45,000 lb/in2), and elongation at least 20% in 2 in.
Lead bronzes are used for bearing metals for heavy duty; an ordinary composition is 80% copper, 10% tin, and 10% lead, with less than 1% phosphorus. Steam or valve bronze contains approximately 85% copper, 6.5% tin, 1.5% lead, and 4% zinc; the tensile strength is 235 Mpa (34,000 lb/in2), minimum, and elongation 22% minimum in 2 in (ASTM Specification B61). The bronzes have a great many industrial applications where their combination of tensile properties and corrosion resistance is especially useful.
What causes corrosion on Iron and Steel?
Principles of Corrosion.
Corrosion may take place by direct chemical attack or by electrochemical (galvanic) attack; the latter is by far the most common mechanism. When two dissimilar metals that are in electrical contact are connected by an electrolyte, an electromotive potential is developed, and a current flows.
The magnitude of the current depends on the conductivity of the electrolyte, the presence of high resistance “passivating” films on the electrode surfaces, the relative areas of electrodes, and the strength of the potential difference. The metal that serves as the anode undergoes oxidation and goes into solution (corrodes).
When different metals are ranked according to their tendency to go into solution, the galvanic series, or electromotive series, is obtained. Metals at the bottom will corrode when in contact with those at the top; the greater the separation, the greater the attack is likely to be.
Table 4-14 is such a ranking, based on tests by the International Nickel Company, in which the electrolyte was seawater.
The nature of the electrolyte may affect the order to some extent. It also should be recognized that very subtle differences in the nature of the metal may result in the formation of anode-cathode galvanic cells: slight differences in composition of the electrolyte at different locations on the metal surface, minor segregation of impurities in the metal, variations in the degree of cold deformation undergone by the metal, etc.
It is possible for anode-cathode couples to exist very close to each other on a metal surface. The electrolyte is a solution of ions; a film of condensed moisture will serve.
An understanding of the mechanism of corrosion suggests possible ways of minimizing corrosion effects. Some of these include:
(1) avoidance of metal combinations that are not compatible,
(2) electrical insulation between dissimilar metals that have to be used together,
(3) use of a sacrificial anode placed in contact with a structure to be protected (this is an expensive technique but can be justified in order to protect such structures as buried pipelines and ship hulls),
(4) use of an impressed emf from an external power source to buck out the corrosion current (called cathodic protection),
(5) avoiding the presence of an electrolyte—especially those with high conductivities, and
(6) application of a protective coating to either the anode or the cathode.
What are steel strand and steel ropes?
Iron and Steel Wire. Annealed wire of iron or very mild steel has a tensile strength in the range of 310 to 415 MPa (45,000 to 60,000 lb/in2); with increased carbon content, varying amounts of cold drawing, and various heat treatments, the tensile strength ranges all the way from the latter figures up to about 3450 MPa (500,000 lb/in2), but a figure of about 1725 MPa (250,000 lb/in2) represents the ordinary limit for wire for important structural purposes.
For example, see the following paragraph on bridge wire. Wires of high carbon content can be tempered for special applications such as spring wire. The yield strength of cold-drawn steel wire is 65% to 80% of its ultimate strength. For examples showing the effects of drawing and carbon content on wire, see Making, Shaping, and Treating of Steel, U.S. Steel.
Galvanized-Steel Bridge Wire. The manufacture of high-strength bridge wire like that used for the cables and hangers of suspension bridges such as the San Francisco–Oakland Bay Bridge, the Mackinac Bridge in Michigan, and the Narrows Bridge in New York is an excellent example of careful control of processing to produce a quality material.
The wire is a high-carbon product containing 0.75% to 0.85% carbon with maximum limits placed on potentially harmful impurities. Rolling temperatures are carefully specified, and the wire is subjected to a special heat treatment called patenting.
The steel is transformed in a controlled-temperature molten lead bath to ensure an optimal microstructure. This is followed by cold drawing to a minimum tensile strength of 1550 MPa (225,000 lb/in2) and a 4% elongation.
The wire is given a heavy zinc coating to protect against corrosion. Joints or splices are made with cold-pressed sleeves which develop practically the full strength of the wire. Fatigue tests of galvanized bridge wire in reversed bending indicate that the endurance limit of the coated wire is only about 345 to 415 MPa (50,000 to 60,000 lb/in2).
Wire Rope. Wire rope is made of wires twisted together in certain typical constructions and may be either flat or round. Flat ropes consist of a number of strands of alternately right and left lay, sewed together with soft iron to form a band or belt; they are sometimes of advantage in mine hoists.
Round ropes are composed of a number of wire strands twisted around a hemp core or around a wire strand or wire rope. The standard wire rope is made of six strands twisted around a hemp core, but for special purposes, four, five, seven, eight, nine, or any reasonable number of strands may be used.
The hemp is usually saturated with a lubricant, which should be free from acids or corrosive substances;
this provides little additional strength but acts as a cushion to preserve the shape of the rope and helps to lubricate the wires. The number of wires commonly used in the strands are 4, 7, 12, 19, 24, and 37, depending on the service for which the ropes are intended.
When extra flexibility is required, the strands of a rope sometimes consist of ropes, which in turn are made of strands around a hemp core. Ordinarily, the wires are twisted into strands in the opposite direction to the twist of the strands in the rope. The makeup of standard hoisting rope is 6 X 19; extrapliable hoisting rope is 8 X 19 or 6 X 37; transmission or haulage rope is 6 X 7; hawsers and mooring lines are 6 X 12 or 6 X 19 or 6 X 24 or 6 X 37, etc.; tiller or hand rope is 6 X 7; highway guard-rail strand is 3 X 7; galvanized mast-arm rope is 9 X 4 with a cotton center.
The tensile strength of the wire ranges, in different grades, from 415 to 2415 MPa (60,000 to 350,000 lb/in2), depending on the material, diameter, and treatment. The maximum tensile efficiency of wire rope is 90%; the average is about 82.5%, being higher for 6 X 7 rope and lower for 6 X 37 construction.
The apparent modulus of elasticity for steel cables in service may be assumed to be 62 to 83 X 106 kPa (9 to 12 X 106 lb/in2) of cable section. Grades of wire rope are (from historic origins) referred to as traction, mild plow, plow, improved plow, and extra improved plow steel. The most common finish for steel wire is “bright” or uncoated, but various coatings, particularly zinc (galvanized), are used.
Iron-base alloys containing between 11% and 30% chromium form a tenacious and highly protective chrome oxide layer that gives these alloys excellent corrosion-resistant properties. There are a great number of alloys that are generally referred to as stainless steels, and they fall into three general classifications.
Austenitic stainless steels contain usually 8% to 12% nickel, which stabilizes the austenitic phase. These are the most popular of the stainless steels. With 18% to 20% chromium, they have the best corrosion resistance and are very tough and can undergo severe forming operations.
These alloys are susceptible to embrittlement when heated in the range of 593 to 816°C. At these temperatures, carbides precipitate at the austenite grain boundaries, causing a local depletion of the chromium content in the adjacent region, so this region loses its corrosion resistance.
Use of “extra low carbon” grades and grades containing stabilizing additions of strong carbide-forming elements such as niobium minimizes this problem. These alloys are also susceptible to stress corrosion in the presence of chloride environments.
Ferritic stainless steels are basically straight Fe-Cr alloys. Chromium in excess of 14% stabilizes the low-temperature ferrite phase all the way to the melting point. Since these alloys do not undergo a phase change, they cannot be hardened by heat treatment. They are the least expensive of the stainless alloys.
Martensitic stainless steels contain around 12% Cr. They are austenitic at elevated temperatures but ferritic at low; hence they can be hardened by heat treatment.
To obtain a significant response to heat treatment, they have higher carbon contents than the other stainless alloys. Martensitic alloys are used for tools, machine parts, cutting instruments, and other applications requiring high strength. The austenitic alloys are nonmagnetic, but the ferritic and martensitic grades are ferromagnetic.
The properties of steels can be greatly modified by thermal treatments, which change the internal crystalline structure of the alloy. Hardening of steel is based on the fact that iron undergoes a change in crystal structure when heated above its “critical” temperature.
Above this critical transformation temperature, the structure is called austenite, a phase capable of dissolving carbon up to 2%. Below the critical temperature, the steel transforms to ferrite, in which carbon is insoluble and precipitates as an iron carbide compound, FeeC (sometimes called cementite).
If a steel is cooled rapidly from above the critical temperature, the carbon is unable to diffuse to form cementite, and the austenite transforms instead to an extremely hard metastable constituent called martensite, in which the carbon is held in supersaturation. The hardness of the martensite depends sensitively on the carbon content.
Low-carbon steels (below about 0.20%) are seldom quenched, while steels above about 0.80% carbon are brittle and liable to crack on quenching. Plain carbon steels must be quenched at very fast rates in order to be hardened. Alloying elements can be added to decrease the necessary cooling rates to cause hardening; some alloy steels will harden when cooled in air from above the critical temperature.
It should be noted, however, that it is the amount of carbon that primarily determines the properties of the alloy; the alloying elements serve to make the response to heat treatment possible.
Normalizing is a treatment in which the steel is heated over the critical temperature and allowed to cool in still air. The purpose of normalizing is to homogenize the steel. The carbon in the steel will appear as a fine lamellar product of cementite and ferrite called pearlite.
Annealing is similar to normalizing, except the steel is very slowly cooled from above the critical. The carbides are now coarsely divided and the steel is in its softest state, as may be desired for cold-forming or machining operations.
Process annealing is a treatment carried out below the critical temperature designed to recrystallize the ferrite following a cold-working operation. Metals become hardened and embrittled by plastic deformation, but the original state can be restored if the alloy is heated high enough to cause new strain-free grains to nucleate and replace the prior strained structure. This treatment is commonly applied as a final processing for low-carbon steels where ductility and toughness are important, or as an intermediate treatment for such products as wire that are formed by cold working.
Stress-relief annealing is a thermal treatment carried out at a still lower temperature. No structural changes take place, but its purpose is to reduce residual stresses that may have been introduced by previous nonuniform deformation or heating.
Tempering is a treatment that always follows a hardening (quenching) treatment. After hardening, steels are extremely hard, but relatively weak owing to their brittleness. When reheated to temperatures below the critical, the martensitic structure is gradually converted to a ferrite-carbide aggregate that optimizes strength and toughness.
When steels are tempered at about 260°C, a particularly brittle configuration of precipitated carbides forms; steels should be tempered above or below this range. Another phenomenon causing embrittlement occurs in steels particularly containing chromium and manganese that are given a tempering cycle that includes holding at or cooling through temperatures around 567 to 621°C. Small molybdenum additions retard this effect, called temper brittleness. It is believed to be caused by a segregation of trace impurity elements to the grain boundaries.
Labels: Steel Works
Iron and steel may be classified on the basis of composition, use, shape, method of manufacture, etc. Some of the more important ferrous alloys are described in the sections below.
Ingot iron is commercially pure iron and contains a maximum of 0.15% total impurities. It is very soft and ductile and can undergo severe cold-forming operations. It has a wide variety of applications based on its formability.
Its purity results in good corrosion resistance and electrical properties, and many applications are based on these features. The average tensile properties of Armco ingot iron plates are tensile strength 320 MPa (46,000 lb/in2); yield point 220 MPa (32,000 lb/in2); elongation in 8 in, 30%; Young’s modulus 200 GPa (29 # 106 lb/in2).
Plain carbon steels are alloys of iron and carbon containing small amounts of manganese (up to 1.65%) and silicon (up to 0.50%) in addition to impurities of phosphorus and sulfur. Additions up to 0.30% copper may be made in order to improve corrosion resistance.
The carbon content may range from 0.05% to 2%, although few alloys contain more than 1.0%, and the great bulk of steel tonnage contains from 0.08% to 0.20% and is used for structural applications.
Medium-carbon steels contain around 0.40% carbon and are used for constructional purposes—tools, machine parts, etc. High-carbon steels have 0.75% carbon or more and may be used for wear and abrasion-resistance applications such as tools, dies, and rails.
Strength and hardness increase in proportion to the carbon content while ductility decreases. Phosphorus has a significant hardening effect in low-carbon steels, while the other components have relatively minor effects within the limits they are found.
It is difficult to generalize the properties of steels, however, since they can be greatly modified by cold working or heat treatment.
High-strength low-alloy steels are low-carbon steels (0.10% to 0.15%) to which alloying elements such as phosphorus, nickel, chromium, vanadium, and niobium have been added to obtain higher strength.
This class of steel was developed primarily by the transportation industry to decrease vehicle weight, but the steels are widely used. Since thinner sections are used, corrosion resistance is more important, and copper is added for this purpose.
What is Elastic Strength?
To the user and the designer of machines or structures, one significant value to be determined is a limiting stress below which the permanent distortion of the material is so small that the structural damage is negligible and above which it is not negligible. The amount of plastic distortion which may be regarded as negligible varies widely for different materials and for different structural or machine parts.
In connection with this limiting stress for elastic action, a number of technical terms are in use; some of them are
1. Elastic Limit. The greatest stress which a material is capable of withstanding without a permanent deformation remaining on release of stress. Determination of the elastic limit involves repeated application and release of a series of increasing loads until a set is observed upon release of load.
Since the elastic limit of many materials is fairly close to the proportional limit, the latter is sometimes accepted as equivalent to the elastic limit for certain materials. There is, however, no fundamental relation between elastic limit and proportional limit. Obviously, the value of the elastic limit determined will be affected by the sensitivity of apparatus used.
2. Proportional Limit. The greatest stress which a material is capable of withstanding without a deviation from proportionality of stress to strain. The statement that the stresses are proportional to strains below the proportional limit is known as Hooke’s Law. The numerical values of the proportional limit are influenced by methods and instruments used in testing and the scales used for plotting diagrams.
3. Yield Point. The lowest stress at which marked increase in strain of the material occurs without increase in load. If the stress-strain curve shows no abrupt or sudden yielding of this nature, then there is no yield point. Iron and low-carbon steels have yield points, but most metals do not, including iron and low-carbon steels immediately after they have been plastically deformed at ordinary temperatures.
4. Yield Strength. The stress at which a material exhibits a specified limiting permanent set. Its determination involves the selection of an amount of permanent set that is considered the maximum amount of plastic yielding which the material can exhibit, in the particular service condition for which the material is intended, without appreciable structural damage.
A set of 0.2% has been used for several ductile metals, and values of yield strength for various metals are for 0.2% set unless otherwise stated. The yield strength is generally used to determine the elastic strength for materials whose stress-strain curve in the region pr is a smooth curve of gradual curvature.
Stress is the intensity at a point in a body of the internal forces or components of force that act on a given plane through the point. Stress is expressed in force per unit of area (pounds per square inch, kilograms per square millimeter, etc.).
There are three kinds of stress: tensile, compressive, and shearing.
Flexure involves a combination of tensile and compressive stress. Torsion involves shearing stress. It is customary to compute stress on the basis of the original dimensions of the cross section of the body, though “true stress” in tension or compression is sometimes calculated from the area of the time a given stress exists rather than from the original area.
Strain is a measure of the change, due to a force, in the size or shape of a body referred to its original size or shape. Strain is a nondimensional quantity but is frequently expressed in inches per inch, etc.
Under tensile or compressive stress, strain is measured along the dimension under consideration. Shear strain is defined as the tangent of the angular change between two lines originally perpendicular to each other.
A stress-strain diagram is a diagram plotted with values of stress as ordinates and values of strain as abscissas. Diagrams plotted with values of applied load, moment, or torque as ordinates and with values of deformation, deflection, or angle of twist as abscissas are sometimes referred to as stress-strain diagrams but are more correctly called load-deformation diagrams.
The stress-strain diagram for some materials is affected by the rate of application of the load, by cycles of previous loading, and again by the time during which the load is held constant at specified values; for precise testing, these conditions should be stated definitely in order that the complete significance of any particular diagram may be clearly understood.
Modulus of Elasticity.
The modulus of elasticity is the ratio of stress to corresponding strain below the proportional limit. For many materials, the stress-strain diagram is approximately a straight line below a more or less well-defined stress known as the proportional limit.
Since there are three kinds of stress, there are three moduli of elasticity for a material, that is, the modulus in tension, the modulus in compression, and the modulus in shear.
The value in tension is practically the same, for most ductile metals, as the modulus in compression; the modulus in shear is only about 0.36 to 0.42 of the modulus in tension.
The modulus is expressed in pounds per square inch (or kilograms per square millimeter) and measures the elastic stiffness (the ability to resist elastic deformation under stress) of the material.
Bentonite is a clay formed by the decomposition of volcanic ash with a high content of montmorillonite. It exhibits the properties of clay to an extreme degree.
Varved Clays consist of thin alternating layers of silt and fat clays of glacial origin. They possess the undesirable properties of both silt and clay. The constituents of varved clays were transported into fresh water lakes by the melted ice at the close of the ice age.
Kaolin, China Clay are very pure forms of white clay used in the ceramic industry.
Boulder Clay is a mixture of an unstratified sedimented deposit of glacial clay, containing unsorted rock fragments of all sizes ranging from boulders, cobbles, and gravel to finely pulverized clay material.
Calcareous Soil is a soil containing calcium carbonate. Such soil effervesces when tested with weak hydrochloric acid.
Marl consists of a mixture of calcareous sands, clays, or loam.
Hardpan is a relatively hard, densely cemented soil layer, like rock which does not soften when wet.
Boulder clays or glacial till is also sometimes named as hardpan.
Caliche is an admixture of clay, sand, and gravel cemented by calcium carbonate deposited from ground water.
Peat is a fibrous aggregate of finer fragments of decayed vegetable matter. Peat is very compressible and one should be cautious when using it for supporting foundations of structures.
Loam is a mixture of sand, silt and clay.
Loess is a fine-grained, air-borne deposit characterized by a very uniform grain size, and high void ratio. The size of particles ranges between about 0.01 to 0.05 mm. The soil can stand deep vertical cuts because of slight cementation between particles. It is formed in dry continental regions and its color is yellowish light brown.
Shale is a material in the state of transition from clay to slate. Shale itself is sometimes considered a rock but, when it is exposed to the air or has a chance to take in water it may rapidly decompose.
HOW THE SOILS ARE FORMED?
Soil is defined as a natural aggregate of mineral grains, with or without organic constituents, that can be separated by gentle mechanical means such as agitation in water. By contrast rock is considered to be a natural aggregate of mineral grains connected by strong and permanent cohesive forces.
The process of weathering of the rock decreases the cohesive forces binding the mineral grains and leads to the disintegration of bigger masses to smaller and smaller particles. Soils are formed by the process of weathering of the parent rock.
The weathering of the rocks might be by mechanical disintegration, and/or chemical decomposition.
Mechanical weathering of rocks to smaller particles is due to the action of such agents as the expansive forces of freezing water in fissures, due to sudden changes of temperature or due to the abrasion of rock by moving water or glaciers.
Temperature changes of sufficient amplitude and frequency bring about changes in the volume of the rocks in the superficial layers of the earth's crust in terms of expansion and contraction. Such a volume change sets up tensile and shear stresses in the rock ultimately leading to the fracture of even large rocks.
This type of rock weathering takes place in a very significant manner in arid climates where free, extreme atmospheric radiation brings about considerable variation in temperature at sunrise and sunset.
Erosion by wind and rain is a very important factor and a continuing event. Cracking forces by growing plants and roots in voids and crevasses of rock can force fragments apart.
Chemical weathering (decomposition) can transform hard rock minerals into soft, easily erodable matter.
The principal types of decomposition are hydmtion, oxidation, carbonation, desilication and leaching. Oxygen and carbon dioxide which are always present in the air readily combine with the elements of rock in the presence of water.
Labels: Soil Mechanics
Most estimators begin their career doing quantity takeoff; as they develop experience and judgment, they develop into estimators. A list of the abilities most important to the success of an estimator follows, but it should be more than simply read through.
Any weaknesses affect the estimator’s ability to produce complete and accurate estimates. If individuals lack any of these abilities, they must (1) be able to admit it and (2) begin to acquire the abilities they lack. Those with construction experience, who are subsequently trained as estimators, are often most successful in this field.
To be able to do quantity takeoffs, the estimator must
1. Be able to read and quantify plans.
2. Have knowledge of mathematics and a keen understanding of geometry. Most measurements and computations are made in linear feet, square feet, square yards, cubic feet, and cubic yards. The quantities are usually multiplied by a unit price to calculate material costs.
3. Have the patience and ability to do careful, thorough work.
4. Be computer literate and use computer takeoff programs such as On-Screen Takeoff or Paydirt.
To be an estimator, an individual needs to go a step further. He or she must
1. Be able, from looking at the drawings, to visualize the project through its various phases of construction. In addition, an estimator must be able to foresee problems, such as the placement of equipment or material storage, then develop a solution and determine its estimated cost.
2. Have enough construction experience to possess a good knowledge of job conditions, including methods of handling materials on the job, the most economical methods of construction, and labor productivity. With this experience, the estimator will be able to visualize the construction of the project and thus get the most accurate estimate on paper.
3. Have sufficient knowledge of labor operations and productivity to thus convert them into costs on a project. The estimator must understand how much work can be accomplished under given conditions by given crafts. Experience in construction and a study of projects that have been completed are required to develop this ability.
4. Be able to keep a database of information on costs of all kinds, including those of labor, material, project overhead, and equipment, as well as knowledge of the availability of all the required items.
5. Be computer literate and know how to manipulate and build various databases and use spreadsheet programs and other estimating software.
6. Be able to meet bid deadlines and still remain calm. Even in the rush of last-minute phone calls and the competitive feeling that seems to electrify the atmosphere just before the bids are due, estimators must “keep their cool.”
7. Have good writing and presentation skills. With more bids being awarded to the best bid, rather than the lowest bid, being able to communicate what your company has to offer, what is included in the bid, and selling your services is very important. It is also important to communicate to the project superintendent what is included in the bid, how the estimator planned to construct the project, and any potential pitfalls.
People cannot be taught experience and judgment, but they can be taught an acceptable method of preparing an estimate, items to include in the estimate, calculations required, and how to make them. They can also be warned against possible errors and alerted to certain problems and dangers, but the practical experience and use of good judgment required cannot be taught and must be obtained over time.
How closely the estimated cost will agree with the actual cost depends, to a large extent, on the estimators’ skill and judgment. Their skill enables them to use accurate estimating methods, while their judgment enables them to visualize the construction of the project throughout the stages of construction.
COMPOSITION OF CLAY MINERALS
The word 'clay' is generally understood to refer to a material composed of a mass of small mineral particles which, in association with certain quantities of water, exhibits the property of plasticity.
According to the clay mineral concept, clay materials are essentially composed of extremely small crystalline particles of one or more members of a small group of minerals that are commonly known as clay minerals.
These minerals are essentially hydrous aluminum silicates, with magnesium or iron replacing wholly or in part for the aluminum, in some minerals. Many clay materials may contain organic material and water-soluble salts.
Organic materials occur either as discrete particles of wood, leaf matter, spores, etc., or they may be present as organic molecules adsorbed on the surface of the clay mineral particles. The water-soluble salts that are present in clay materials must have been entrapped in the clay at the time of accumulation or may have developed subsequently as a consequence of ground water movement and weathering or alteration processes.
Clays can be divided into three general groups on the basis of their crystalline arrangement and it is observed that roughly similar engineering properties are connected with all the clay minerals belonging to the same group.
STRUCTURE OF CLAY MINERALS
Clay minerals are essentially crystalline in nature though some clay minerals do contain material which is non-crystalline (for example allophane). Two fundamental building blocks are involved in the formation of clay mineral structures.
1. Tetrahedral unit.
2. Octahedral unit.
The tetrahedral unit consists of four oxygen atoms (or hydroxyls, if needed to balance the structure) placed at the apices of a tetrahedron enclosing a silicon atom which combines together to form a shell like structure with all the tips pointing in the same direction. The oxygen at the bases of all the units lie in a common plane.
Bridges have been categorized in many ways.
They have been categorized by their principal use as highway, railroad, pedestrian, pipeline, etc.; by the material used in their construction as stone, timber, wrought iron, steel, concrete, and prestressed concrete; by their structural form as girder, box-girder, moveable, truss, arch, suspension, and cable-stayed; by structural behavior as simple span, continuous, and cantilever; and by their span dimension as short, intermediate, and long-span. The last classification, specifically long-span, is the one of
primary interest in this Section.
The span of a bridge is defined as the dimension (length), along the longitudinal axis of the bridge, between two supports. However, what defines a ‘‘long-span’’? In other words, how long is long?
It should be understood that the word ‘‘long’’ is a relative term. Throughout the history of bridge construction and technology, as our methods of analysis improved and as we moved from one material to another more appropriate material, the span length has been constantly pushed forward to a new frontier.
Therefore, what was considered a long-span in the eighteenth and nineteenth centuries may not be considered as such in the twentieth century. What is considered a long-span today may not be considered as such in the twenty-first century.
It is conceptually simple to understand this concept of the relativity of span length, however, in of itself it does not define ‘‘long-span.’’
Perhaps the best definition of ‘‘long-span’’ is that presented by Silano as ‘‘if a bridge has a span too long to design from standard handbooks, you call it a long-span bridge.’’ The current AASHTO Standard Specifications for Highway Bridges states that ‘‘They apply to ordinary highway bridges and supplemental specifications may be required for unusual types and for bridges with spans longer than 500 ft.’’
Therefore, by the above criteria, the lower bound of long-span may be considered to be 500 ft, at least for highway bridges. (Silano, L. G., ‘‘Design of Long-Span Bridges,’’ reprinted from the Structural Group Lecture Series of the Boston Society of Civil Engineers/ASCE, April 1990, Parsons Brinckerhoff, New York.)
Suspension bridges with cables made of high-strength, zinc-coated, steel wires are suitable for the longest of spans. Such bridges usually become economical for spans in excess of 1000 ft, depending on specific site constraints.
Nevertheless, many suspension bridges with spans as short as 300 or 400 ft have been built, to take advantage of their esthetic features. The basic economic characteristic of suspension bridges, resulting from use of high strength materials in tension, is lightness, due to relatively low dead load.
But this, in turn, carries with it the structural penalty of flexibility, which can lead to large deflections under live load and susceptibility to vibrations. As a result, suspension bridges are more suitable for highway bridges than for the more heavily loaded railroad bridges.
Nevertheless, for either highway or railroad bridges, care must be taken in design to provide resistance to wind- or seismic-induced oscillations, such as those that caused collapse of the first Tacoma Narrows Bridge in 1940.
A pure suspension bridge is one without supplementary stay cables and in which the main cables are anchored externally to anchorages on the ground. The main components of a suspension bridge are illustrated in Fig. 15.8.
Most suspension bridges are stiffened; that is, as shown in Fig. 15.8, they utilize horizontal stiffening trusses or girders. Their function is to equalize deflections due to concentrated live loads and distribute these loads to one or more main cables.
The stiffer these trusses or girders are, relative to the stiffness of the cables, the better this function is achieved. (Cables derive stiffness not only from their crosssectional dimensions but also from their shape between supports, which depends on both cable tension and loading.)
For heavy, very long suspension spans, live-load deflections may be small enough that stiffening trusses would not be needed. When such members are omitted, the structure is an unstiffened suspension bridge.
Thus, if the ratio of live load to dead load were, say, 1:4, the\ midspan deflection would be of the order of 1⁄100 of the sag, or 1/1,000 of the span, and the use of stiffening trusses would ordinarily be unnecessary. (For the George Washington Bridge as initially constructed, the ratio of live load to dead load was approximately 1:6. Therefore, it did not need a stiffening truss.)
FIGURE 15.8 Main components of a suspension bridge.
1. Evaluate the results obtained with different forms of tendons The capacity of a given member is increased by using deflected rather than straight tendons, and the capacity is maximized by using parabolic tendons. (However, in the case of a pretensioned beam, an economy analysis must also take into account the expense incurred in deflecting the tendons.)
2. Evaluate the prestressing force For a given ratio of yj/ye the prestressing force that is required to maximize the capacity of a member is a function of the cross-sectional area and the allowable stresses. It is independent of the form of the trajectory.
3. Determine the effect of section moduli If the section moduli are in excess of the minimum required, the prestressing force is minimized by setting the critical values offbf and/, equal to their respective allowable values.
4. Determine the most economical short-span section For a short-span member, an I section is most economical because it yields the required section moduli with the minimum area. Moreover, since the required values of Sb and St differ, the area should be disposed unsymmetrically about middepth to secure these values.
5. Consider the calculated value of e Since an increase in span causes a greater increase in the theoretical eccentricity than in the depth, the calculated value of e is not attainable in a long-span member because the centroid of the tendons would fall beyond the confines of the section. For this reason, long-span members are generally constructed as T sections. The extensive flange area elevates the centroidal axis, thus making it possible to secure a reasonably large eccentricity.
6. Evaluate the effect of overload
A relatively small overload induces a disproportionately large increase in the tensile stress in the beam and thus introduces the danger of cracking. Moreover, owing to the presence of many variable quantities, there is not a set relationship between the beam capacity at allowable final stress and the capacity at incipient cracking. It is therefore imperative that every prestressed-concrete beam be subjected to an ultimate-strength analysis to ensure that the beam provides an adequate factor of safety.
A pile is a slender column made of wood, concrete or steel. A pile is either driven into the soil or formed in situ by excavating a hole and then filling it with concrete. A group of piles are driven to the required depth and are capped with R.C.C. slab, over which super structure is built.
The pile transfer the load to soil by friction or by direct bearing, in the latter case, piles being taken up to hard strata. This type of foundations is used when top soil is not capable of taking the load of the structure even at 3–4 m depth. Pile foundations are classified according to the materials used and also on the nature of load transfer.
Classification According to Materials Used:
Piles may be classified as:
(a) Timber piles
(b) Concrete piles
(c) Steel piles and
(d) Composite piles.
(a) Timber piles: Circular seasoned wood can be used as piles. Their diameter may vary from 200 mm to 400 mm. Similarly square piles of sizes 200 mm to 400 mm are also used. The length of timber pile should not be more than 20 times its lateral dimension.
The bottom of the pile is sharpened and is provided with iron shoe, so that it can be driven in the ground easily by hammering. These piles should be always kept below water table; otherwise alternating wet and dry condition cause the decay.
These piles are cheap and can be easily driven rapidly. The main disadvantage is their load carrying capacity is low and are likely to be damaged during driving in the soil.
(b) Concrete piles: These piles may be further classified as precast piles and cast in situ piles. Precast piles are reinforced with steel and are manufactured in factories. The cross-section diameter/dimension varies from 200 mm to 500 mm.
Square, circular and octagonal sections are commonly used. The length of piles may be up to 20 m. They are provided with steel shoe at the lowest end. These piles can carry fairly large loads. These piles are highly resistant to biological and chemical actions of the soil. The disadvantage of these piles is they need more time to manufacture and are heavy to handle.
Cast in situ concrete piles are formed first by boring the holes in the soil and then concreting them. Concreting is usually made using casing tubes. If the hole is filled with only plain concrete it is pressure pile.
The load carrying capacity of the piles may be increased by providing enlarged base. The reinforcement caging may be inserted in the bored holes and to increase load carrying capacity one or two under reams may be formed. After that concreting may be carried out.
Such piles are known as under reamed piles. These piles are provided at regular interval of 2 to 4 m and capping beam is provided over them.
(c) Steel Piles: A steel pile may be a rolled steel I sections, tubes or fabricated in the form of box. These piles are mostly used as bearing piles since surface available for friction is less and also the coefficient of friction is less. If tubes are used the soil inside the tube is driven out by compressed air and concrete is filled. These piles are very useful for driving close to existing structures since they disturb the soil least.
(d) Composite Piles: Composite piles may be of concrete and timber or of concrete and steel. Wooden piles should not be subjected to alternating wet and dry conditions. Hence they are preferred for the portion below water table.
The portion above water table are built with cast in situ concrete piles. If the required length of steel piles is less than the depth of pile, many times upper portions are built with concrete. Thus steel and concrete composite piles are sometimes used.
Classification of Piles According to Load Transfer:
According to the load transfer to the soil piles may be classified as
(a) Bearing piles and
(b) Friction piles.
Bearing piles rest on hard strata and transfer the load by bearing. Such piles are preferred. These piles are used if the hard strata is available at reasonable depth.
Friction piles transfer the load to the soil by the friction between soil and the pile. Such piles are used if hard strata is not available to a considerable depth. The friction developed is to be properly assessed before deciding the length of the pile. The surface of such piles is made rough to increase the skin friction so that required length of pile is reduced.
The planning and construction of a building should be aimed at fulfilling the following requirements:
1. Strength and stability
2. Dimensional stability
3. Resistance to dampness
4. Resistance to fire
5. Heat insulation
6. Sound insulation
7. Protection against termite attack
9. Security against burglary
10. Lighting and ventilation
11. Comforts and convenience
1. Strength and Stability: Building should be capable of transferring the expected loads in its life period safely to the ground. Design of various structural components like slabs, beams, walls, columns and footing should ensure safety. None of the structural components should buckle, overturn and collapse.
2. Dimensional Stability: Excessive deformation of structural components give a sense of instability and result into crack in walls, flooring etc. All structural components, should be so designed that deflections do not exceed the permissible values specified in the codes.
3. Resistance to Dampness: Dampness in a building is a great nuisance and it may reduce the life of the building. Great care should be taken in planning and in the construction of the building to avoid dampness.
4. Resistance to Fire: Regarding achieving resistance to fire, the basic requirements laid down in the codes are:
(a) the structure should not ignite easily.
(b) building orientation should be such that spread of fire is slow.
(c) In case of fire, there should be means of easy access to vacate building quickly.
5. Heat Insulation: A building should be so oriented and designed that it insulates interior from heat.
6. Sound Insulation: Buildings should be planned against outdoor and indoor noises.
7. Protection from Termite: Buildings should be protected from termites.
8. Durability: Each and every component of the building should be durable.
9. Security against Burglary: This is the basic need the owner of the building expects.
10. Lighting and Ventilation: For healthy and happy living natural light and ventilations are required. Diffused light and good cross ventilation should be available inside the building.
11. Comforts and Conveniences: Various units in the building should be properly grouped and integrated keeping in mind the comfort and convenience of the user.
12. Economy: Economy without sacrificing comfort, convenience and durability is another basic requirement of the building.
(a) Soundness Test: It is conducted by sieve analysis. 100 gms of cement is taken and sieved through IS sieve No. 9 for fifteen minutes. Residue on the sieve is weighed. This should not exceed 10 per cent by weight of sample taken.
(b) Setting Time: Initial setting time and final setting time are the two important physical properties of cement. Initial setting time is the time taken by the cement from adding of water to the starting of losing its plasticity. Final setting time is the time lapsed from adding of the water to complete loss of plasticity. Vicat apparatus is used for finding the setting times [Ref. Fig. 1.5].
FIG 1.5 – PARTS OF VICAT APPARATUS
Vicat apparatus consists of a movable rod to which any one of the three needles shown in figure can be attached. An indicator is attached to the movable rod. A vicat mould is associated with this apparatus which is in the form of split cylinder.
Before finding initial and final setting time it is necessary to determine water to be added to get standard consistency. For this 300 gms of cement is mixed with about 30% water and cement paste prepared is filled in the mould which rests on non porous plate. The plunger is attached to the movable rod of vicat apparatus and gently lowered to touch the paste in the mould. Then the plunger is allowed to move freely.
If the penetration is 5 mm to 7 mm from the bottom of the mould, then cement is having standard consistency. If not, experiment is repeated with different proportion of water fill water required for standard consistency is found. Then the tests for initial and final setting times can be carried out as explained below:
Initial Setting Time: 300 gms of cement is thoroughly mixed with 0.85 times the water for standard consistency and vicat mould is completely filled and top surface is levelled. 1 mm square needle is fixed to the rod and gently placed over the paste. Then it is freely allowed to penetrate.
In the beginning the needle penetrates the paste completely. As time lapses the paste start losing its plasticity and offers resistance to penetration. When needle can penetrate up to 5 to 7 mm above bottom of the paste experiment is stopped and time lapsed between the addition of water and end if the \ experiment is noted as initial setting time.
Final Setting Time. The square needle is replaced with annular collar. Experiment is continued by allowing this needle to freely move after gently touching the surface of the paste. Time lapsed between the addition of water and the mark of needle but not of annular ring is found on the paste. This time is noted as final setting time.
(c) Soundness Test: This test is conducted to find free lime in cement, which is not desirable. Le Chatelier apparatus is used for conducting this test. It consists of a split brass mould of diameter 30 mm and height 30 mm. On either side of the split, there are two indicators, with pointed ends.
The ends of indicators are 165 mm from the centre of the mould. another glass plate and a small weight is placed over it. Then the whole assembly is kept under water for 24 hours. The temperature of water should be between 24°C and 50°C.
Note the distance between the indicator. Then place the mould again in the water and heat the assembly such that water reaches the boiling point in 30 minutes. Boil the water for one hour. The mould is removed from water and allowed to cool.
The distance between the two pointers is measured. The difference between the two readings indicate the expansion of the cement due to the presence of unburnt lime. This value should not exceed 10 mm.
(d) Crushing Strength Test: For this 200 gm of cement is mixed with 600 gm of standard sand confirming to IS 650–1966. After mixing thoroughly in dry condition for a minute distilled potable water P/4 + 3 percentage is added where P is the water required for the standard consistency.
They are mixed with trowel for 3 to 4 minutes to get uniform mixture. The mix is placed in a cube mould of 70.6 mm size (Area 5000 mm2) kept on a steel plate and prodded with 25 mm standard steel rod 20 times within 8 seconds.
Then the mould is placed on a standard vibrating table that vibrates at a speed of 12000 ± 400 vibration per minute. A hopper is secured at the top and the remaining mortar is filled. The mould is vibrated for two minutes and hopper removed. The top is finished with a knife or with a trowel and levelled. After 24 ± 1 hour mould is removed and cube is placed under clean water for curing.
After specified period cubes are tested in compression testing machine, keeping the specimen on its level edges. Average of three cubes is reported as crushing strength. The compressive strength at the end of 3 days should not be less than 11.5 N/mm2 and that at the end of 7 days not less than 17.5 N/mm2.
Design values for lumber are contained in grading rules established by the National Lumber Grades Authority (Canadian), Northeastern Lumber Manufacturers Association, Northern Softwood Lumber Bureau, Redwood Inspection Service, Southern Pine Inspection Bureau, West Coast Lumber Inspection Bureau, and Western Wood Products Association.
The rules and the design values in them have been approved by the Board of Review of the American Lumber Standards Committee. They also have been certified for conformance with U.S. Department of Commerce Voluntary Product Standard PS 20-94 (American Softwood Lumber Standard).
In addition, design values for visually graded lumber may be established in accordance with ASTM D1990, ‘‘Standard Practice for Establishing Allowable Properties for Visually-Graded Dimensional Lumber from In-Grade Tests of Full- Size Specimens.’’
Design values for visually graded timbers, decking, and some species and grades of dimension lumber are based on provisions of ‘‘Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber,’’ ASTM D245.
ASTM D245 also specifies adjustments to be made in the strength properties of small clear specimens of wood, as determined in accordance with ‘‘Establishing Clear Wood Strength Values,’’ ASTM D2555, to obtain design values applicable to normal conditions of service.
The adjustments account for the effects of knots, slope of grain, splits, checks, size, duration of load, moisture content, and other influencing factors. Lumber structures designed with working stresses derived from D245 procedures and standard design criteria have a long history of satisfactory performance.
Design values for machine stress-rated (MSR) lumber and machine-evaluated lumber (MEL) are based on nondestructive tests of individual wood pieces. Certain visual-grade requirements also apply to such lumber.
The stress rating system used for MSR lumber and MEL is checked regularly by the responsible grading agency for conformance with established certification and quality-control procedures.
Samuel Johnson famously wrote that ‘to build is to be robbed’. Facing the same challenges, but with the benefit of hindsight, Pope Pius II praised his architect for ‘lying about the costs’ following budget overruns on the building of Pienza Cathedral, which threatened at the time to bankrupt the Vatican.
Both of these experiences suggest that clients have been and continue to be exposed to a significant degree of cost risk when undertaking construction projects. Invariably, they also pick up much of the financial consequences of decisions, omissions and mistakes made by others working on their behalf.
Decisions made at the outset of a project: investing in land, selecting one project opportunity in favour of others; confirming a brief; or establishing project governance could all potentially have a substantial impact on project outcomes, and as a result carry significant risk. Unfortunately, many of these early decisions have to bemade without the benefit of a considered design response and may, as a result, be sub-optimal.
Whilst it is important that advice given to clients early in a project should give the team some ‘wiggle room’ to develop a preferred solution, it is also important to work within project disciplines once these are established. Effective teamwork during the design development process between the designer and cost consultant can help to mitigate many of these potential risks.
As a client’s brief and concept designs are developed, a greater degree of fixity in terms of the design solution and predicted costs can be provided by the project team. This process is discussed in more detail in the section focused on cost planning.
However, as the design develops and cost certainty increases, so does the cost of changing the design, and the client and project team’s resistance to change.
Risk and risk transfer
As a project progresses to the appointment of contractors, the client’s overall financial commitment becomes better defined. More risk can also be transferred to third parties if the client so wishes.
Whilst under most procurement routes the client is required to accept risks associated with design performance, they will generally seek to transfer commercial and construction risks to the contractor through some form of a fixed price, lump sum contract.
Quite clearly, if the design information upon which the client obtains a contractual commitment is not complete, is ambiguous or is not fully coordinated then, not only will the client retain outstanding design risk, but will also find that the basis of his commercial risk transfer to the contractor is weakened.
Evidence from Construction Key Performance Indicators, published by the DTI, indicates the scale of this potential problem, showing that fewer than 80% of projects are completed with #10% of their original tender sum. Moreover, only around 50% of projects are completed within #5% of the tender sum.
Whilst some of this cost variation may reflect client changes, or problems on site, it is likely that some of these increases will have resulted from the consequences of continuing design development. In order to mitigate the client’s risk, it is incumbent upon the team to ensure that the design is completed to the appropriate level of detail and fixity required by the procurement route. To do otherwise risks rendering some of the effort expended in design development and cost-planning abortive.
In theory, it is entirely possible to design and construct a building made of totally independent components. The separate pieces of such a building could be designed in isolation, each part having an autonomous role to play.
Someone who proposes this idea may note that a beam is a beam and a duct is a duct, after all, and there is no need to confuse one for the other. For every function or role to be performed in a building, there are a host of competing and individualized products to choose from. As long as the final assembly has already been worked out, the independent pieces can fulfill their single-purpose roles simply by fitting in place and not interfering with other pieces.
Most architects would quickly denounce this isolationist approach to design. Where, they would ask, is the harmony, the beauty, or even the practicality in such an absurdly fragmented method? Surely there is some sympathy and order among the parts that lead to a comprehensive whole?
Architects are, in fact, inherently prone to take exactly the opposite approach: Starting with carefully considered ideas about the complete and constructed building, they would then explore inward, working through intricate relationships between all the parts and functions. But how far does this concern for relationships go, and how inclusive is the complete idea?
Equally important, what sort of thinking is required to comprehend and resolve all the issues that arise in the process? This is where the topic and discipline of integration fits in—providing an explicit framework for selecting and combining building components in purposeful and intentional ways.
Integration among the hardware components of building systems is approached with three distinct goals: Components have to share space, their arrangement has to be aesthetically resolved, and at some level, they have to work together or at least not defeat each other. These three goals are physical, visual, and performance integration The following sections serve as a brief overview of how these goals are attained.
Building components have to fit. They share space and volume in a building, and they connect in specific ways. CAD drawing layers offer a useful way to think about how complicated these networks of shared space and connected pieces can become. Superimposing structure and HVAC (heating, ventilating, and air-conditioning) layers provides an example: Are there problems where large ducts pass under beams? Do the reflected ceiling plan and furniture layouts put light fixtures where they belong?
Physical integration is fundamentally about how components and systems share space, how they fit together. In standard practice, for example, the floor-ceiling section of many buildings is often subdivided into separate zones: recessed lighting in the lowest zone, space for ducts next, and then a zone for the depth of structure to support the floor above.
These segregated volumes prevent “interference” between systems by providing adequate space for each individually remote system. Meshing the systems together, say, by running the ducts between light fixtures, requires careful physical integration. Unifying the systems by using the ceiling cavity as a return air plenum and extracting return air through the light fixtures further compresses the depth of physical space required. If the structure consists of open web joists, trusses, or a space frame, then it is possible that all three systems may be physically integrated into a single zone by carefully interspersing ducts and light fixtures within the structure.
Connections between components and among systems in general constitute another aspect of physical integration. This is also where architectural details are generated. The structural, thermal, and physical integrity of the joints between different materials must be carefully considered. How they meet is just as important as how they are separated in space.
Exposed and formally expressive components of a building combine to create its image. This is true of the overall visual idea of the building as well as of the character of rooms and of individual elements, down to the smallest details.
The manner in which components share in a cumulative image is decided through acts of visual integration. Color, size, shape, and placement are common factors that can be manipulated in order to achieve the desired effect, so knowledge of the various components’ visual character is essential to integrating them.
Visual harmony among the many parts of a building and their agreement with the intended visual effects of design often provide some opportunities for combining technical requirements with aesthetic goals. Light fixtures, air-conditioning, plumbing fixtures, and a host of other elements are going to have a presence in the building anyway.
Ignoring them or trying to cover them with finishes or decoration is futile. Technical criteria and the systems that satisfy those functional demands require large shares of the resources that go into building. It follows that architects should be able to select, configure, and deploy building elements in ways that satisfy both visual and functional objectives.
If physical integration is “shared space” and visual integration is “shared image,” then performance integration must have something to dowith shared functions. A load-bearing wall, for example, is both envelope and structure, so it unifies two functions into one element by replacing two columns, a beam, and the exterior wall. This approach can save cost and reduce complexity if it is appropriate to the task at hand.
Performance integration is also served by meshing or overlapping the functions of two components, even without actually combining the pieces. This may be called “shared mandates.” In a direct-gain passive solar heating system, for example, the floor of the sunlit space is sharing in the thermal work of the envelope and the mechanical heating system by providing thermal storage in its massive heat capacity, which limits indoor temperature swings from sunlit day to cold starry night. The envelope, structure, interior, and services are integrated by the shared thermal mandate of maintaining comfortable temperatures.
In practice one has to make a choice between the use of vertical piles used singly or in groups to carry such loads or of groups incorporating at least some piles installed to an angle of rake. The capacity of a pile as a structural unit to carry shear loads at its head depends on the strength of the section, and when the forces become high, one is impelled to find some structurally acceptable solution which keeps stresses within reasonable limits.
However, in choosing the possible option of raking piles one should be aware of the problems and limitations that may be involved. Some of the factors involved are as follows:
1 Raking piles are usually more expensive than vertical piles. This is partly involved with extra time taken to set up and maintain the equipment in position, the less efficient use of hammers in the case of driven piles, and the difficulties of concrete placing in bored piles.
2 The standards of tolerance that can be maintained in the installation of raking piles are not as good as for vertical piles. Most analyses of pile groups of this kind ignore the effect of tolerances, but if tolerances are properly taken into account they can have a significant effect on calculated pile loads, depending on pile grouping and numbers, with small groups being usually most sensitive.
3 Where the upper part of a raking pile is embedded in a soil that is likely to suffer time-dependent settlement, the pile will in due course be subject to bending stresses unrelated to the structural design load conditions. This may require increase of strength of the section, which is in turn reflected in costs.
4 Many machines used for pile installation carry the pile driving or forming equipment on a long mast, so that they become intrinsically less stable, particularly as the line of the pile gets further from the vertical position. In certain cases, when working close to river banks or railway lines, for example, there is a major limitation on how machinery can be positioned to produce the desired end result.
5 Design of groups involving raking and vertical piles and with loads that are both vertical and horizontal should have regard to the constancy of the relationship between these. If, for example, the vertical load is near constant, but the horizontal force varies greatly, then it is better to employ groupings with rakers balanced in two opposed directions rather than to have an arrangement of vertical piles plus piles raking in one direction only. This is simply to minimize the shears in the
heads of the piles when horizontal load falls to a minimum value.
6 The use of raking piles to ‘spread’ load under vertically loaded foundations, where the piles are fully embedded in the soil mass and where the whole foundation is expected to undergo significant consolidation/creep settlement, must lead to large bending stresses being developed in the piles. In certain cases this can lead to such stress levels in the piles that the section will suffer damage, which may in turn lead to severe problems in the supported structure.
It should, however, be said that where groups of raking piles derive their axial capacity from strata that are hard and relatively non-deformable, they provide a stiffness in terms of laterally applied forces which can be very desirable. The main issue in design is to avoid large and unquantifiable secondary stresses, and provided this can be achieved all will be well.
Where there are very heavy lateral loads to be carried and neither raking piles nor single piles other than perhaps those of very large diameter are suitable, then diaphragm piers or ‘barrettes’ have a useful potential application. They can be given high stiffness in the direction of applied horizontal loading without fear of the problem of major secondary stresses.
Pre-boring is a commonly referenced method for easing the passage of some driven piles into the ground. However, its use can also be misunderstood or misguided. It is not a satisfactory way of overcoming significant obstructions to enable piles to be driven because that which impedes the driven pile will also in general impede progress of the pre-boring tool.
Pre-boring in sand and gravel presents a problem because of the inherent instability of the soil through which the pre-bore passes. When such soil is dense, pre-bores may stand open temporarily because of arching and the influence of temporary pore water suction.
However, as soon as a piling tube or pile enters the bore and the hammer begins striking, the upper granular soil collapses into the lower part of the bore. The lower section of the bore will possibly not collapse in this circumstance at the initial driving strokes because the soil is relatively more dense and the hammer influence more remote.
The result is frequently that because of re-compacted debris in the lower bore, piles will not drive back to the same depth as originally bored. Only if the bore is temporarily cased to prevent collapse, and if the casing is of large enough diameter to allow access for the final pile, can a satisfactory load bearing unit be inserted, albeit with loss of potential friction resulting from loss of displacement effects and the need for in-filling around the pile.
As an alternative to trying to form an open hole in sand soils, the pre-boring tool is sometimes used simply to stir up the ground, leaving disturbed soil in position. This may be sufficient to deal with dense soil near ground level.
However, if deep bores are attempted after this manner, again when a piling tube or pile is entered and driving begins, the loosened material is compacted down into the lower part of the bore and becomes virtually indistinguishable from the original natural soil. Piles will frequently not drive back to the depth of the pre-bore or may behave inconsistently under applied load.
It is therefore not generally satisfactory to use deep pre-bore methods in sands, for example, for the purpose of ensuring that piles reach a deeper stratum such as rock unless special temporary casing methods are adopted.
Pre-boring sockets into rock or very hard soils for the supposed purpose of enhancing end bearing or reaching strong soil, where there are overlying fill, sand or clay layers, is also generally futile. For the same reasons as stated above, it will be found that without guaranteed bore stability and measures to prevent soil from collapsing into the socket, a satisfactory load bearing and consistent unit cannot be formed because of debris falling before the pile arrives.
Pre-bores are satisfactory only under specific circumstances:
1 To loosen dense upper crust soils and enable long piles to be driven without breakage. Long piles struck at the head are really slender columns and so the possibilities of buckling failure can be very real.
2 To make an open hole in stiff clays or similar cohesive soils into which a pile is pre-entered. The purpose in this instance is to avoid or diminish soil heave. If using the method for the purpose of eliminating ground heave, it is generally legitimate to choose the area of the bore so that the pile cross-sectional area is just slightly larger.
Jobs with pre-boring are frequently associated with claims and cost overruns, partly because it is difficult to synchronize the activities of boring and driving machines with consequent delay, and partly because, where the motivation is to achieve stringent ‘sets’ this may be a major source of damage to equipment.
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