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Arch

Arch

An arch is a curved structure capable of spanning a space while supporting significant weight (e.g. a doorway in a stone wall). The arch was developed in Mesopotamia, Assyria, Egypt and Etruria. It was later refined in Ancient Rome. The arch became an important technique in cathedral building and is still used today in some modern structures as for example in bridges.

Technical aspects

The arch is significant because, in theory at least, it provides a structure which eliminates tensile stresses in spanning an open space. All the forces are resolved into compressive stresses. This is useful because several of the available building materials such as stone, cast iron and concrete can strongly resist compression but are very weak when tension, shear or torsional stress is applied to them. By using the arch configuration, significant spans can be achieved. This same principle holds when the force acting on the arch is not vertical such as in spanning a doorway, but horizontal, such as in arched retaining walls or dams. Even when using concrete, where the structure may be monolithic, the principle of the arch is used so as to benefit from the concrete's strength in resisting compressive stress. Where any other form of stress is raised, it has to be resisted by carefully placed reinforcement rods or fibres. (See Arch bridge.) The following gallery shows examples of arch forms displayed in roughly the order in which they were developed. Image:Treledsbåge.png|Triangular arch Image:Rundbåge.png|Round arch or Semi-circular arch Image:Segmentbåge.png|Segmental arch Image:Stigande båge.png|Unequal round arch or Rampant round arch Image:Lansettbåge.png|Lancet arch Image:Spetsbåge.png|Equilateral pointed arch Image:Skulderbåge.png|Shouldered flat arch Image:Trepassbåge.png|Three-foiled cusped arch Image:Hästskobåge.png|Horseshoe arch Image:Korgbåge.png|Three-centred arch Image:Ellipsbåge.png|Elliptical arch Image:Draperibåge.png|Inflexed arch Image:Kölbåge.png|Ogee arch Image:Karnisbåge.png|Reverse ogee arch Image:Tudorbåge.png|Tudor arch Image:Parabelbåge.png|Catenary or Parabolic arch Parabolic arch

Construction

An arch requires all of its elements to hold it together. This raises the interesting question of how an arch is actually constructed. One simple answer is to build a frame (historically, of wood) which exactly follows the form of the underside of the arch. This is known as a centre or centring. The voussoirs are laid on it until the arch is complete and self-supporting. For an arch higher than head height, scaffolding would in any case be required by the builders, so the scaffolding can be combined with the arch support. Occasionally arches would fall down when the frame was removed if construction or planning had been incorrect. (The A85 bridge at Dalmally, Scotland suffered this fate on its first attempt, in the 1940s).

History

Arches were used by the Egyptian, Babylonian, Greek and Assyrian civilizations for underground structures such as drains and vaults, but the ancient Romans were the first to use them widely above ground although it is thought that Romans learned it from the Etruscans. The so-called Roman arch is semicircular, and built from an odd number of arch bricks (in modern architectural parlance, these are called voussoirs). The capstone or keystone is the topmost stone in the arch. This shape is the simplest to build, but not the strongest. There is a tendency for the sides to bulge outwards, which must be counteracted by an added weight of masonry to push them inwards. The semicircular arch can be flattened to make an elliptical arch. The horseshoe arch is based on the semicircular arch, but its lower ends are extended further round the circle until they start to converge. It was used in Spanish Visigothic architecture, Islamic architecture and mudéjar architecture, as in the Great Mosque of Damascus and in later Moorish buildings. It was used for decoration rather than for strength. The semicircular arch was followed in Europe by the pointed Gothic arch or ogive, whose centreline more closely followed the forces of compression and which was therefore stronger. This design had been used by the Assyrians as early as 722 BC. The parabolic and catenary arches are now known to be the theoretically strongest forms. The arch was used in some bridges in China since the Song dynasty. Song dynasty design.]]

Other types

A dome is a three-dimensional application of the arch. Igloos are notable early structures making use of domes. A special form of the arch is the triumphal arch, usually built to celebrate a victory in war. The most famous example of this is the Arc de Triomphe in Paris, France. Natural rock formations may also be referred to as 'arches'. These natural arches are formed by erosion rather than being carved or constructed by man. See Arches National Park for examples.

References

- pp. 27-8 Category:Architectural elements Category:Arch bridges ja:アーチ th:อาร์ช

Wall

A wall is a usually solid structure that defines and sometimes protects space. Most commonly, a wall separates space in buildings into rooms, or protects or delineates a space in the open air. There are three principal types of structural walls: building walls, exterior boundary walls, and retaining walls. rooms Building walls have two main purposes: to support roofs and ceilings, and to divide space, providing security against intrusion and weather. Such walls most often have three or more separate components. In today's construction, a building wall will usually have the structural elements (such as 2×4 studs in a house wall), insulation, and finish elements, or surface (such as drywall or panelling). In addition, the wall may house various types of electrical wiring or plumbing. Electrical outlets are usually mounted in walls. Building walls frequently become works of art, such as when murals are painted on them. On a ship, the walls separating compartments are termed 'Bulkheads', whilst the thinner walls separating cabins are termed 'Partitions'. Boundary walls include privacy walls, boundary-marking walls, and city walls. These intergrade into fences; the conventional differentiation is that a fence is of minimal thickness and often is open in nature, while a wall is usually more than a nominal thickness and is completely closed, or opaque. More to the point, if an exterior structure is made of wood or wire, it is generally referred to as a fence, while if it is made of masonry, it is considered a wall. A common term for both is barrier, convenient if it is partly a wall and partly a fence, e.g. the Berlin Wall or the Israeli West Bank barrier. Before the invention of artillery, many European cities had protective walls. Since they are no longer relevant for defense, the cities have grown beyond their walls, and many of the walls have been torn down. Extreme examples of boundary walls include the Great Wall of China and Hadrian's Wall. Hadrian's Wall In areas of rocky soils around the world, farmers (and their slaves, as in the United States before slavery was abolished) have often pulled large quantities of stone out of their fields to make farming easier, and have stacked those stones to make walls that either mark the field boundary, or the property boundary, or both. Retaining walls are a special type of wall, that may be either external to a building or part of a building, that serves to provide a barrier to the movement of earth, stone or water. The ground surface or water on one side of a retaining wall will be noticeably higher than on the other side. A dike is one type of retaining wall, as is a levee. Special laws often govern walls shared by neighbouring properties. Typically, one neighbour cannot alter the common wall if it is likely to affect the building or property on the other side.

Assyria

:For the Assyrian people in Iraq and other countries, see Assyrian people. Assyria in earliest historical times referred to a region on the Upper Tigris river, named for its original capital, the ancient city of Ashur. Later, as a nation and Empire, it also came to include roughly the northern half of Mesopotamia (the southern half being Babylonia). Assyria proper was located in a mountainous region, extending along the Tigris as far as the high Gordiaean or Carduchian mountain range of Armenia, sometimes called the "Mountains of Ashur".

Early history

Of the early history of the kingdom of Assyria, little is positively known. According to some Judeo-Christian traditions, the city of Ashur (Aŝŝur) was founded by Ashur the son of Shem, who was deified by later generations as the city's patron god. Besides Ashur, the other three royal Assyrian cities were Calah (Nimrud), Khorsabad, and Nineveh. This region seems to have been ruled from Sumer, Akkad, and northern Babylonia in its earliest stages, being part of Sargon the Great's empire. Destroyed by barbarians in the Gutian period, it was rebuilt, and ended up being governed as part of the Empire of the 3rd dynasty of Ur. Assyria as an independent kingdom was perhaps founded ca. 1900 BC by Bel-kap-kapu.

Old Assyrian city-state

The city-state of Ashur had extensive contact with cities on the Anatolian plateau. The Assyrians established "merchant colonies" in Cappadocia, e.g., at Kanesh (modern Kültepe) circa 1920 BC–1840 BC and 1798 BC–1740 BC. These colonies, called karum, the Akkadian word for 'port', were attached to Anatolian cities, but physically separate, and had special tax status. They must have arisen from a long tradition of trade between Ashur and the Anatolian cities; but no archaeological or written records show this. The trade consisted of metal (perhaps lead or tin; the terminology here is not entirely clear) and textiles from Assyria, that were traded for precious metals in Anatolia. The city of Ashur was conquered by Shamshi-Adad I (1813 BC–1791 BC) in the expansion of Amorite tribes from the Khabur delta. He put his son Ishme-Dagan on the throne of nearby Ekallatum, and allowed trade to continue. Only after the death of Shamshi-Adad and the fall of his sons, did Hammurabi of Babylon conquer Ashur. With Hammurabi, the various karum in Anatolia ceased trade activity, probably because the goods of Assyria were now being traded with the Babylonians' partners. In the 15th century BC, Saushtatar, king of "Hanilgalbat" (Hurrians of Mitanni), sacked Ashur and made Assyria a vassal. Assyria paid tribute to Hanilgalbat until Mitanni power collapsed from Hittite pressure, enabling Ashur-uballit I (1365 BC–1330 BC), to again make Assyria an independent and conquering power. Hanilgalbat was finally conquered under Adad-nirari I, who described himself as a "Great-King" (Sharru rabû) in letters to the Hittite rulers. Adad-nirari I's successor, Shalmaneser I, made Calah his capital, and followed up on expansion to the northwest, mainly at the expense of the Hittites, reaching as far as Carchemish. His son and successor, Tukulti-Ninurta, deposed Kadashman-Buriash of Babylon and ruled there himself, as king for seven years. Following this, Babylon revolted against Tukulti-Ninurta, and later even made Assyria tributary during the reigns of the Babylonian kings Melishipak II and Marduk-apal-iddin I, another weak period for Assyria.

Assyrian Empire

Carchemish

Assyrian empire-building

As the Hittite empire collapsed from onslaught of the Phrygians (called Mushki in Assyrian annals), Babylon and Assyria began to vie for Amorite regions, formerly under firm Hittite control. The Assyrian king Ashur-resh-ishi defeated Nebuchadnezzar I of Babylon in a battle, when their forces encountered one another in this region. In 1120 BC, Ashur-resh-ishi's son, Tiglath-Pileser I crossed the Euphrates, capturing Carchemish, defeated the Mushki and the remnants of the Hittites—even claiming to reach the Black Sea—and advanced to the Mediterranean, subjecting Phoenicia. He also marched into Babylon twice, assuming the old title "King of Sumer and Akkad", although he was unable to depose the actual Babylonian king on these occasions. He may be regarded as the founder of the first Assyrian empire. After Tiglath-Pileser I, the Assyrians were in decline for nearly two centuries, a time of weak and ineffective rulers, wars with neighboring Urartu, and encroachments by Aramaean nomads. This long period of weakness ended with the accession in 911 BC of Adad-nirari II. He brought the areas still nominally under Assyrian vassalage firmly under subjection, deporting populations in the north to far-off places. Apart from pushing the boundary with Babylonia slightly southward, he did not engage in actual expansion, and the borders of the empire he consolidated reached only as far west as the Khabur. He was succeeded by Tukulti-Ninurta II, who made some gains in the north during his short reign. The next king, Ashurnasirpal II (883 BC–858 BC), embarked on a vast program of merciless expansion, first terrorizing the peoples to the north as far as Nairi, then subjecting the Aramaeans between the Khabur and the Euphrates. His harshness prompted a revolt that was crushed decisively in a pitched, two-day battle. Following this victory, he advanced without opposition as far as the Mediterranean and exacted tribute from Phoenicia. Unlike any before, the Assyrians began boasting in their ruthlessness around this time. Ashurnasirpal II also moved his capital to the city of Kalhu (Nimrud). [There is ongoing discussion among academics over the nature of the Nimrud lens, a piece of rock crystal unearthed by John Layard in 1850, in the Nimrud palace complex in northern Iraq. A small minority believe that it is evidence for the existence of ancient Assyrian telescopes, which could explain the great accuracy of Assyrian astronomy.] Ashurnasirpal's son, Shalmaneser III (858 BC–823 BC), fought against Urartu, and in the reign of Ahab, king of Israel, he marched an army against an alliance of the Syrian states (a rare occasion in near-eastern history of an alliance between the Israeli state and the Aramaic Kingdom), whose allied army he encountered at Karkar in (854 BC). Despite Shalmaneser's description of 'vanquishing the opposition', it seems that the battle ended in a deadlock, as the Assyrian forces were withdrawn soon afterwards. Shalmaneser retook Carchemish in 849 BC, and in 841 BC marched an army against Hazael, King of Damascus, besieging and taking that city. He also brought under tribute Jehu of Israel, Tyre, and Sidon. His black obelisk, discovered at Kalhu, records many military exploits of his reign. [http://www.kchanson.com/ANCDOCS/meso/obelisk.html] In the following century, Assyria again experienced a relative decline, owing to weaker rulers (including the Queen Semiramis) and a resurgence in expansion by Urartu. The notable exception was Adad-nirari III (810 BC–782 BC), who brought Syria under tribute as far south as Edom and advanced against the Medes, perhaps even penetrating to the Caspian Sea. In 745 BC, the crown was seized by a military adventurer called Pul, who assumed the name of Tiglath-Pileser III. After subjecting Babylon to tribute and severely punishing Urartu, he directed his armies into Syria, which had regained its independence. He took Arpad near Aleppo in 740 BC after a siege of three years, and reduced Hamath. Azariah (Uzziah) had been an ally of the king of Hamath, and thus was compelled by Tiglath-Pileser to do him homage and pay yearly tribute. In 738 BC, in the reign of Menahem, king of Israel, Tiglath-Pileser III occupied Philistia and invaded Israel, imposing on it a heavy tribute (2 Kings 15:19). Ahaz, king of Judah, engaged in a war against Israel and Syria, appealed for help to this Assyrian king by means of a present of gold and silver (2 Kings 16:8); he accordingly "marched against Damascus, defeated and put Rezin to death, and besieged the city itself." Leaving part of his army to continue the siege, he advanced, ravaging with fire and sword the province east of the Jordan, Philistia, and Samaria; and in 732 BC took Damascus, deporting its inhabitants to Assyria. In 729 BC, he had himself crowned as "King Pul of Babylon". Tiglath-Pileser III died in 727 BC, and was succeeded by Shalmaneser V, who reorganized the Empire into provinces, replacing the troublesome vassal kings with Assyrian governors. However, King Hoshea of Israel suspended paying tribute, and allied himself with Egypt against Assyria in 725 BC. This led Shalmaneser to invade Syria (2 Kings 17:5) and besiege Samaria (capital city of Israel) for three years.

Sargonidae (dynasty)

Shalmaneser V was deposed in 722 BC in favour of Sargon the Tartan, or commander-in-chief of the army, who then quickly took Samaria, carrying 27,000 people away into captivity into the Israelite Diaspora, and effectively ending the northern Kingdom of Israel. (2 Kings 17:1–6, 24; 18:7, 9). He also overran Judah, and took Jerusalem (Isa. 10:6, 12, 22, 24, 34). In 721 BC, Babylon threw off the rule of the Assyrians, under the powerful Chaldean prince Merodach-baladan (2 Kings 20:12), and Sargon, unable to contain the revolt, turned his attention again to Syria, Urartu, and the Medes, penetrating the Iranian Plateau as far as Mt. Bikni and building several fortresses, before returning in 710 BC and retaking Babylon. Sargon also built a new capital at Dur Sharrukin ("Sargon's City") near Nineveh, with all the tribute Assyria had collected from various nations. In 705 BC, Sargon was slain while fighting the Cimmerians, and was succeeded by his son Sennacherib (2 Kings 18:13; 19:37; Isa. 7:17, 18), who moved the capital to Nineveh and made the deported peoples work on improving Nineveh's system of irrigation canals. In 701 BC, Hezekiah of Judah formed an alliance with Egypt against Assyria, so Sennacherib accordingly marched toward Jerusalem, destroying 46 villages in his path. This is graphically described in Isaiah 10; exactly what happened next is unclear (the Bible says an Angel of the Lord smote the Assyrian army at Jerusalem; Herodotus says they were destroyed by a plague of field mice at Egypt; modern historians suspect Plague in both instances); however what is certain, is that the besieging army was somehow decimated, and Sennacherib failed to capture Jerusalem. In 689 BC, Babylonia again revolted, but Sennacherib responded swiftly by opening the canals around Babylon and flooding the outside of the city until it became a swamp, resulting in its destruction, and its inhabitants were scattered. In 681 BC, Sennacherib was murdered, most likely by one of his sons. Sennacherib was succeeded by his son Esarhaddon (Ashur-aha-iddina), who had been governor of Babylonia under his father. As king, he immediately had Babylon rebuilt, and made it his capital. Defeating the Cimmerians and Medes (again penetrating to Mt. Bikni), but unable to maintain order in these areas, he turned his attention westward to Phoenicia—now allying itself with Egypt against him—and sacked Sidon in 677 BC. He also captured Manasseh of Judah and kept him prisoner for some time in Babylon (2 Kings 19:37; Isa. 37:38). Having had enough of Egyptian meddling, he next invaded that country in 674 BC, conquering it all by 670 BC. Assyria was also at war with Urartu and Dilmun (probably modern Qatar) at this time. This was Assyria's greatest territorial extent. However, the Assyrian governors Esarhaddon had appointed over Egypt were obliged to flee the restive populace, and while leading another army to pacify them, Esarhaddon died suddenly, in 669 BC. Assur-bani-pal or Ashurbanipal (Ashurbanapli, Asnappar), the son of Esarhaddon, succeeded him. He continued to campaign in Egypt, when not distracted by pressures from the Medes to the east, and Cimmerians to the north of Assyria. Unable to contain Egypt, he installed Psammetichus as a vassal king in 663 BC, but by 652 BC, this vassal king was strong enough to declare outright independence from Assyria with impunity, especially as Ashurbanipal's brother, Shamash-shum-ukin, governor of Babylon, began a civil war in that year that lasted until 648 BC, when Babylon was sacked and the brother set fire to the palace, killing himself. Elam was completely devastated in 646 BC and 640 BC.

Assyrian government

The anachronistic (Persian-Greek) term satrap has been used to describe Assyrian governors.

Downfall and heritage

Ashurbanipal had promoted art and culture and had a vast library of cuneiform tablets at Nineveh, but upon his death in 627 BC, the Assyrian Empire began to disintegrate rapidly. Babylonia became independent; their king Nabopolassar, along with Cyaxares of Media, destroyed Nineveh in 612 BC, and Assyria fell. A general called Ashur-uballit II, with military support from the Egyptian Pharaoh Necho II, held out as a remnant of Assyrian power at Harran until 609 BC, after which Assyria ceased to exist as an independent nation. However, the Assyrian people have managed to keep their identity, and still exist as a distinct ethnic group, mainly in northern Iraq, where they are distinguished from their Arab, Kurdish, and Turkmen neighbors by their traditions, politics, Christian religion, and Aramaic dialect.

Etruria

:This article is about the ancient Italian country. Etruria, Staffordshire is also a place in England. Etruria was an ancient country in Central Italy, located in an area that covered part of what now are Tuscany, Latium and Umbria. It is identified as the territory of Etruscans, and it was one of the most important city/states on the Italian peninsula before falling to the Roman Republic in the 3rd century BC. Etruria became dominant in the Italian peninsula after 650 BCE. Their expansion included the Po River Valley and Latium and continued south until they came in contact with the Greek colonies in Southern Italy. Etruscan kings most notably conquered and ruled Rome for 100 years until 509 BCE when the last Etruscan king Lucius Tarquinius Superbus was removed from power and the Roman Republic was established. The Etruscans are credited with changing Rome from a farming village into a large city. They are also responsible for building the first road on the main street of Rome, the Via Sacra, and also temples, housing, markets, etc. The Etruscans are responsible for much of the Culture of Greece imported into Rome, including the Twelve Olympians, the Latin alphabet (adopted from the Greek alphabet), the phalanx formation, and the growing of olives and grapes. The Kingdom of Etruria was a creation of Napoleon I of France in Tuscany which existed from 1801 to 1807.

Cathedral

---- A cathedral is a Christian church building, specifically of a denomination with an episcopal hierarchy (such as the Roman Catholic Church or the Lutheran or Anglican churches), which serves as the central church of a bishopric. As cathedrals are often particularly impressive edifices, the term is sometimes also used loosely as a designation for any large important church. Some pre-Reformation cathedrals in Scotland now within the Church of Scotland still retain the term cathedral, despite the Church's Presbyterian polity which does not have bishops. The term is not officially used in Eastern Orthodoxy, the church of a bishop being known as "the great church", though 'cathedral' is commonly used in English translations.

Designation

The word cathedral is derived from the Greek noun (cathedra) which translates as seat and refers to the presence of the bishop's (or archbishop's) chair or throne. In this sense therefore, the word cathedral, though grammatically used as a noun, is originally the adjective in the phrase cathedral church, from the Latin ecclesia cathedralis. The seat marks the place set aside in the prominent church of the diocese for the head of that diocese and is therefore a major symbol of authority. Although a cathedral may be amongst the grandest of churches in the diocese (and country), especially in the medieval and Renaissance times, this has never been a requirement and (especially in modern times, where functionality rather than grandeur is the foremost consideration) a cathedral church may be modest in structure. Certainly the early Celtic and Saxon cathedrals tended to be of diminutive size, and where they continued in use would have undergone expansion through the development of the bishopric. Cathedrals have either been founded and built as such, or were originally parish or monastic churches that were elevated to cathedral status. Missionary activity, ecclesiastical power and, more recently, demographic considerations have determined the creation or reorganisation of sees. One of the earliest instances of the term ecclesia cathedralis is said to occur in the acts of the council of Tarragona in 516. Another name for a cathedral church is ecclesia mater, indicating that it is the mother church from which other congregations have been established. As the one important church, it was also known as ecclesia major. As the chief house of God, it is called the Domus Dei in Latin, where the words domus (home) and dominus (lord) share a common origin. From this Latin root come the German word for a cathedral, Dom (a Domherr being a German canon), as well as the Italian Duomo. In Spanish it is customary to refer to la seo (the see). At Strasbourg and many places in Germany, and in many imposing churches of England, the cathedral is called a Minster (German: Münster), from Latin monasterium, because some cathedrals were served by canons living in community. The other kind of great church in Western Europe is the abbey.

History and organization

It was early decreed that the cathedra of a bishop was not to be placed in the church of a village, but only in that of a city. This was not difficult on the continent of Europe, where towns were numerous and cities were the natural centres from which Christianity was diffused among the surrounding districts. In the British Isles, however, towns were few, and, instead of exercising jurisdiction over definite areas, many of the bishops were bishops of tribes or peoples, as the bishops of the south Saxons, the West Saxons, the Somersætas, etc. The cathedra of such a bishop was often migratory. In 1075 a council was held in London, under the presidency of Archbishop Lanfranc, which, reciting the decrees of the council of Sardica held in 347 and that of Laodicea held in 360 on this matter, ordered the bishop of the south Saxons to remove his see from Selsey to Chichester; the Wiltshire and Dorset bishop to remove his cathedra from Sherborne to Old Sarum, and the Mercian bishop, whose cathedral was then at Lichfield, to transfer it to Chester. Traces of the tribal and migratory system may still be noted in the designations of the Irish see of Meath (where the result has been that there is now no cathedral church) and Ossory, the cathedral church of which is at Kilkenny. Some of the Scottish sees were also migratory. By the canon law the bishop is regarded as the pastor of the cathedral church, the parochia of which is his diocese. In view of this, canon lawyers sometimes speak of the cathedral church as the one church of the diocese, and all others are deemed chapels in their relation to it. Occasionally two churches share the distinction of containing the bishop's cathedra. In such case they are said to be co-cathedrals. Cathedral churches may have different degrees of dignity: # the simple cathedral church of a diocesan bishop, # the metropolitical church to which the other diocesan cathedral churches of a province are suffragan, # the primatial church under which are ranged metropolitical churches and their provinces, # patriarchal churches to which primatial, metropolitical, and simple cathedral churches alike owe allegiance. The title of "primate" was occasionally conferred on metropolitans of sees of great dignity or importance, such as Canterbury, York, Rouen, whose cathedral churches remained simply metropolitical. Lyon, where the cathedral church is still known as La Primatiale, and Lund in Sweden, may be cited as instances of churches which were really primatial. Lyon had the archbishops of Sens and Paris and their provincial dioceses subject to it until the French Revolution, and Lund had the archbishop of Uppsala and his province subject to it. As with the title of primate, so also that of "patriarch" has been conferred on sees such as Venice and Lisbon, the cathedral churches of which are patriarchal in name alone. The Basilica di San Giovanni in Laterano, the cathedral church of Rome, alone in Western Europe possesses a patriarchal character among Roman Catholics, since the Pope is the Patriarch of the West. The removal of a bishop's cathedra from a church deprives that church of its cathedral dignity, although often the name clings in common speech, as for example at Antwerp, which was deprived of its bishop at the French Revolution. The history of the body of clergy attached to the cathedral church is obscure, and as in each case local considerations affected its development, all that can be attempted is to give a general outline of the main features which were more or less common to all. Originally the bishop and cathedral clergy formed a kind of religious community, which, in no true sense a monastery, was nevertheless often called a monasterium. The word did not have the restricted meaning which it afterwards acquired. Hence the apparent anomaly that churches like York Minster and Lincoln Cathedral, which never had any monks attached to them, have inherited the name of minster or monastery. In these early communities the clergy often lived apart in their own dwellings, and were not infrequently married. In the 8th century, however, Chrodegang, bishop of Metz (743-766), compiled a code of rules for the clergy of the cathedral churches, which, though widely accepted in Germany and other parts of the continent, gained little acceptance in England. According to Chrodegang's rule, the cathedral clergy were to live under a common roof, occupy a common dormitory and submit to the authority of a special officer. The rule of Chrodegang was, in fact, a modification of the Benedictine rule. Gisa, a native of Lorraine, who was bishop of Wells from 1061 to 1088, introduced it into England, and imposed its observance on the clergy of his cathedral church, but it was not followed for long there, or elsewhere in England. During the 10th and 11th centuries, the cathedral clergy became more definitely organized, and were divided into two classes. One was that of a monastic establishment of some recognized order of monks, often the Benedictines, while the other class was that of a college of clergy, bound by no vows except those of their ordination, but governed by a code of statutes or canons. Hence the name of canon. In this way arose the distinction between the monastic and secular cathedral churches. 11th centuries is Finland's most famous church]] In Germany, as in England, many of the cathedral churches were monastic. In Denmark all seem to have been Benedictine at first, except Børglum, which was Praemonstratensian till the Reformation. The others were changed to churches of secular canons. In Sweden, Uppsala was originally Benedictine, but was secularized about 1250, and it was ordered that each of the cathedral churches of Sweden should have a chapter of at least fifteen secular canons. In France monastic chapters were very common, but nearly all the monastic cathedral churches there had been changed to churches of secular canons before the 17th century. One of the latest to be so changed was that of Seez, in Normandy, which was Augustinian till 1547, when Pope Paul III dispensed the members from their vows, and constituted them a chapter of secular canons. The chapter of Senez was monastic till 1647, and others perhaps even later, but the majority were secularized about the time of the Reformation. In the case of monastic cathedral churches there were Dignitaries, the internal government was that of the order to which the chapter belonged, and all the members kept perpetual residence. The reverse of this was the case with the secular chapters; the dignities of provost, dean, precentor, chancellor, treasurer, etc., soon came into being, for the regulation and good Order of the church and its services, while the non-residence of the canons, rather than their perpetual residence, became the rule, and led to their duties being performed by a body of "vicars", who officiated for them at the services of the church. Abroad, the earliest head of a secular church seems to have been the provost (praepositus, Probst, etc.), who was charged, not only with the internal regulation of the church, and oversight of the members of the chapter and control of the services, but was also the steward or seneschal of the lands and possessions of the church. The latter often mainly engaged his attention, to the neglect of his domestic and ecclesiastical duties, and complaints were soon raised that the provost was too much mixed in worldly affairs, and was too frequently absent from his spiritual duties. This led, in many cases, to the institution of a new officer called the "dean", who had charge of that portion of the provost's duties which related to the internal discipline of the chapter and the services of the church. In some cases the office of provost was abolished, but in others it was continued: the provost, who was occasionally archdeacon as well, remaining head of the chapter. This arrangement was most commonly followed in Germany. In England the provost was almost unknown. Bishop Gisa introduced a provost as head of the chapter of Wells, but the office was afterwards subordinated to the other dignities, and the provost became simply the steward of certain of the prebendal lands. The provost of the collegiate church of Beverley was the most notable instance of such an officer in England, but at Beverley he was an external officer with authority in the government of the church, no stall in the choir and no vote in chapter. The provost of Eton, introduced by Henry VI of England, occupied a position most nearly approaching that of a foreign cathedral provost. In Germany and in Scandinavia, and in a few of the cathedral churches in the south of France, the provost was the ordinary head of the cathedral chapter, but the office was not common elsewhere. As regards France, of one hundred and thirty-six cathedral churches existing at the Revolution, thirty-eight only, and those either on the borders of Germany or in the extreme south, had a provost as the head of the chapter. In others the provost existed as a subordinate officer. There were two provosts at Autun, and Lyons and Chartres had four each, all as subordinate officers. Scandinavia, Germany, the tallest church in the world]] The normal constitution of the chapter of a secular cathedral church comprised four dignitaries (there might be more), in addition to the canons. The dean (decanus) seems to have derived his designation from the Benedictine dean who had ten monks under his charge. The dean, as already noted, came into existence to supply the place of the provost in the internal management of the church and chapter. In England the dean was the head of all the secular cathedral churches, and was originally elected by the chapter and confirmed in office by the bishop. He is president of the chapter, and in church has charge of the due performance of the services, taking specified portions of them by statute on the principal festivals. He sits in the chief stall in the choir, which is usually the first on the right hand on entering the choir at the west. Next to the dean (as a rule) is the precentor (primicerius, cantor, etc.), whose special duty is that of regulating the musical portion of the services. He presides in the dean's absence, and occupies the corresponding stall on the left side, although there are exceptions to this rule, where, as at St Paul's, the archdeacon of the cathedral city ranks second and occupies what is usually the precentor's stall. The third dignitary is the chancellor (scholasticus, écoldtre, capiscol, magistral, etc.), who must not be confounded with the chancellor of the diocese. The chancellor of the cathedral church is charged with the oversight of its schools, ought to read divinity lectures, and superintend the lections in the choir and correct slovenly readers. He is often the secretary and librarian of the chapter. In the absence of the dean and precentor he is president of the chapter. The easternmost stall, on the dean's side of the choir, is usually assigned to him. The fourth dignitary is the treasurer (custos, sacrisla, cheficier). He is guardian of the fabric, and of all the furniture and ornaments of the church, and his duty was to provide bread and wine for the Eucharist, and candles and incense, and he regulated such matters as the ringing of the bells. The treasurer's stall is opposite to that of the chancellor. These four dignitaries, occupying the four corner stalls in the choir, are called in many of the statutes the quatuor majores personae of the church. In many cathedral churches there were additional dignitaries, as the praelector, subdean, vice-chancellor, succentor-canonicorum, and others, who came into existence to supply the places of the other absent dignitaries, for non-residence was the fatal blot of the secular churches, and in this they contrasted very badly with the monastic churches, where all the members were in continuous residence. Besides the dignitaries there were the ordinary canons, each of whom, as a rule, held a separate prebend or endowment, besides receiving his share of the common funds of the church. For the most part the canons also speedily became non-resident, and this led to the distinction of residentiary and non-residentiary canons, till in most churches the number of resident canons became definitely limited in number, and the non-residentiary canons, who no longer shared in the common funds, became generally known as prebendaries only, although by their non-residence they did not forfeit their position as canons, and retained their votes in chapter like the others. This system of non-residence led also to the institution of vicars choral, each canon having his own vicar, who sat in his stall in his absence, and when the canon was present, in the stall immediately below, on the second form. The vicars had no place or vote in chapter, and, though irremovable except for offences, were the servants of their absent canons whose stalls they occupied, and whose duties they performed. Abroad they were often called demi-prebendaries, and they formed the bachcrur of the French churches. As time went on the vicars were themselves often incorporated as a kind of lesser chapter, or college, under the supervision of the dean and chapter. There was no distinction between the monastic cathedral chapters and those of the secular canons, in their relation to the bishop or diocese. In both cases the chapter was the bishop's consilium which he was bound to consult on all important matters and without doing so he could not act. Thus, a judicial decision of a bishop needed the confirmation of the chapter before it could be enforced. He could not change the service books, or "use" of the church or diocese, without capitular consent, and there are many episcopal acts, such as the appointment of a diocesan chancellor, or vicar general, which still need confirmation by the chapter, but the older theory of the chapter as the bishop's council in ruling the diocese has become a thing of the past, not in England only, but on the continent also. In its corporate capacity the chapter takes charge sede vacante of a diocese. In England, however (except as regards Salisbury and Durham), this custom has never obtained, the two archbishops having, from time immemorial, taken charge of the vacant dioceses in their respective provinces. When, however, either of the sees of Canterbury or York is vacant the chapters of those churches take charge, not only of the diocese, but of the province as well, and incidentally, therefore, of any of the dioceses of the province which may be vacant at the same time. All the English monastic cathedral chapters were dissolved by Henry VIII, and, except Bath and Coventry, were refounded by him as churches of secular chapters, with a dean as the head, and a certain number of canons ranging from twelve at Canterbury and Durham to four at Carlisle, and with certain subordinate officers as minor canons, gospellers, epistolers, etc. The precentorship in these churches of the "New Foundation", as they are called, is not, as in the secular churches of the "Old Foundation", a dignity, but is merely an office held by one of the minor canons.

External links

- [http://www.newadvent.org/cathen/03438a.htm Cathedral] - Catholic Encyclopedia
- [http://www.sacred-destinations.com/sacred-sites/cathedrals.htm Cathedrals of the World] - Sacred Destinations
- [http://perrin.olivier.free.fr/Cathedrale%20Evry/index.html Gallery of photographs on the Cathedral Evry]
- ja:大聖堂 simple:Cathedral

Bridge

:This article is about the edifice (it is mostly an index to articles concerning specific bridge types). For other meanings, see bridge (disambiguation). bridge (disambiguation) A bridge is a structure built to span a gorge, valley, road, railroad track, river, body of water, or any other physical obstacle. Designs may be built higher than otherwise needed in order to allow other traffic (particularly ship traffic) beneath. The purpose of a bridge is to allow easier passage by providing a continuous more uniform more easily navigable route via what would otherwise be an uneven or impossible path for the particular kind of thing travelling or being transported, whether people, vehicles, trains, ships, liquids or whatever else.

History

The first bridges were spans made of wooden logs or planks and eventually stones, using a simple support and crossbeam arrangement. The arch was first used by the Roman Empire for bridges and aqueducts, some of which still stand today. The Romans also had cement, which reduced the variation of strength found in natural stone. Brick and mortar bridges were built after the Roman era, as the technology for cement was lost then later rediscovered. Rope bridges, a simple type of suspension bridge, were used by the Inca civilization in the Andes mountains of South America, just prior to European colonization in the 1500s. During the 18th century there were many innovations in the design of timber bridges by Hans Ulrich, Johannes Grubenmann, and others. The first engineering book on building bridges was written by Hubert Gautier in 1716. With the rise of the Industrial Revolution in the 19th century, truss systems of wrought iron were developed for larger bridges, but iron did not have the tensile strength to support large loads. With the advent of steel, which has a high tensile strength, much larger bridges were built, many using the ideas of Gustave Eiffel.

Etymology

The Oxford English Dictionary traces the origin of the word bridge to an Old Norse word bryggja, meaning "landing stage, gangway, or movable pier".

Types of bridges

Bridges may be classified by their use or by their structure.

By use

A bridge is usually designed for trains, pedestrian or road traffic, a pipeline or waterway for water transport or barge traffic. In some cases there may be restrictions in use. For example, it may be a bridge carrying a highway and forbidden for pedestrians and bicycles, or a pedestrian bridge, possibly also for bicycles. An aqueduct is a bridge that carries water, resembling a viaduct.

Decorative and ceremonial bridges

To create a beautiful image, some bridges are built much taller than necessary. This type, often found in east-asian style gardens, is called a Moon bridge, evoking a rising full moon. Other garden bridges may cross only a dry bed of stream washed pebbles, intended only to convey an impression of a stream. Often in palaces a bridge will be built over an artificial waterway as symbolic of a passage to an important place or state of mind. A set of five bridges cross a sinuous waterway in an important courtyard of the Forbidden City in Beijing, China. The central bridge was reserved exclusively for the use of the Emperor, Empress, and their attendants.

Index to types of bridges and bridge related topics

Image:NagasakiMeganebashi.jpg|Arch bridge Image:Pont_du_gard.jpg|Aqueduct Image:Baileycoppename.jpg|Bailey bridge Image:UniversityBridge-1Clip.jpg|Bascule bridge Image:Small footbridge.jpg|Beam bridge Image:Concrete box girder bridge.JPG|Box girder bridge Image:ThreeTwrBrCenter.jpg|Cable-stayed bridge Image:CaissonSchematic.jpg|Caisson Image:ForthBridgeEdinburgh.jpg|Cantilever bridge Image:Puente del Alamillo.jpg|Cantilever spar cable-stayed bridge Image:Tarr steps clapper bridge.jpg|Clapper bridge Image:Australia sydney-404.jpg|Compression arch suspended-deck bridge Image:Guilford vermont covered bridge 20040820.jpg|Covered bridge Image:CurlingBridgeClip.jpg|Curling bridge Image:Fort ticonderoga drawbridge to demilune.jpg|Drawbridge Image:Hoernbruecke.jpg|Folding bridge Image:IRBSideViewClip.jpg|Inca rope bridge Image:JetwayAtVancouverBC.jpg|Jetway Image:Guilford vermont bridge covered bridge interior 20040820.jpg|Lattice bridge Image:BNSFBridgeClip.jpg|Lift bridge Image:Vallorcine footpath bridge 2003-12-13.jpg|Log bridge Image:SFTGMoonBridge.jpg|Moon bridge Image:PlateGirderUnderTracks.jpg|Plate girder bridge Image:ArmyPontoonBr.jpg|Pontoon bridge Image:Uppsala Ultunabron02 2005-06-16.jpg|Retractable bridge
(Thrust bridge) Image:ProposedSFOBBEasternSpan.jpg|Self-anchored suspension bridge Image:SegmentalBridgeFtLauderdale.jpg|Segmental bridge Image:WinnepegBridge.jpg|Side-spar cable-stayed bridge Image:CapilanoBridge.jpg|Simple suspension bridge Image:StepStoneBridge.jpg|Step-stone bridge Image:BridgeSubmerging4.jpg|Submersible bridge Image:suspension.bridge.bristol.arp.750pix.jpg|Suspension bridge Image:Railway swing bridge.jpg|Swing bridge Image:Millenium_bridge_close.jpg|Tilt bridge Image:Paying Toll on passing a Bridge From a Painted Window in the Cathedral of Tournay Fifteenth Century.png|Toll bridge Image:Newport.transporter.750pix.jpg|Transporter bridge Image:AlhambraTrestle.jpg|Trestle Image:Eastbound_over_SCB.jpg|Truss arch bridge Image:RRTrussBridgeSideView.jpg|Truss bridge Image:Conwy Castle 2.jpg|Tubular bridge Image:Toronto-bloorviaduct.jpg|Viaduct Image:NoImageYetRectFramed.png|Weigh bridge Image:BoxerwoodDotComZigZag.jpg|Zig-zag bridge

Bridge structural and evolutionary taxonomy

Zig-zag bridge Bridges may be classified by how the four forces of tension, compression, bending and shear are distributed through their structure. Most bridges will employ all of the principle forces to some degree, but only a few will predominate. The separation of forces may be quite clear, as in a suspension or cable-stayed span; the elements in tension are distinct in shape and placement. In other cases the forces may be distributed among a large number of members, as in a truss, or not clearly discernible to a casual observer as in a box beam. Bridges can also be classified by their lineage, which is shown as the vertical axis on the diagram to the right.

Efficiency

A bridge's structural efficiency may be considered to be the ratio of load carried to bridge weight, given a specific set of material types. In one common challenge young students are to be divided into groups of two or three and then to be given a fixed quantity of wood sticks, a specific distance to span, and a given glue, and then to construct a bridge that will be tested to destruction by the progressive addition of load at the center of the span. The bridge taking the greatest load is by this test the most structurally efficient. A bridge's economic efficiency will be site and traffic dependent, the ratio of savings by having a bridge (instead of, for example, a ferry, or a longer road route) compared to its cost. For a given site, kind of bridge employed and the materials used determine the total cost, a lifetime cost composed of materials, labor, machinery, engineering, cost of money, maintenance, refurbishment, risk potential, and ultimately, demolition and associated disposal, recycling, and reuse. Bridges employing only compression are relatively inefficient structurally, but may be highly cost efficient where suitable materials are available near the site. For medium spans, trusses or box beams are usually most economical, while in some cases, the appearance of the bridge may be more important than its cost efficiency. The longest spans usually require suspension bridges.

Notable bridges

- Akashi-Kaikyo Bridge - Japan, with the longest section span of 1.9 km.
- Forth Railway Bridge - Scotland, one of the most famous cantilever bridges in the world.
- Golden Gate Bridge - USA, arguably the most beautiful of its type.
- The Iron Bridge - England, the world's first iron bridge.
- Confederation Bridge - Canada, world's longest bridge over waters that freeze.
- Jamuna Bridge- Bangladesh, longest rail-road bridge in south asia , 2nd longest in world.
- Lake Pontchartrain Causeway - USA, spanning Lake Pontchartrain in south Louisiana, it is the longest bridge in the world at 23.87 miles (38.41 km).
- Lupu Bridge- China, longest single steel arch.
- Mackinac Bridge - USA, Opened to traffic in 1957, connecting the two peninsulas of Michigan; held the title of the world's longest two tower suspension bridge between anchorages until the 1990s.
- Mahatma Gandhi Setu - India, the longest river bridge in the world.
- Menai Suspension Bridge - Wales, first road suspension bridge in the world.
- Millau Viaduct - France, tallest bridge in the world.
- Overtoun Bridge, - Scotland, dogs have leaped to their deaths from this bridge, leading to urban legends.
- Penang Bridge - Malaysia, longest bridge in Southeast Asia.
- Québec Bridge - Canada, largest cantilever bridge in the world.
- San Francisco-Oakland Bay Bridge - USA, especially for seismic retrofit and eastern span replacement.
- Sundial Bridge - USA, a dramatic single cantilever spar cable stayed span for pedestrians.
- Sydney Harbour Bridge - Australia, arguably the best-known suspended-deck compression arch bridge.
- Tacoma Narrows Bridge - USA, famous for its collapse due to aerodynamic effects.
- Tatara Bridge - Japan, largest span cable-stayed bridge.
- Tower Bridge - London, England, and a symbol of this city.
- Tyne Bridge - England, one of Northern England's most iconic structures.
- Trajan's bridge - Romania, ancient Roman bridge over the river Danube, only fragments visible.
- Vasco da Gama Bridge - Portugal, the longest bridge in Europe at 17.2 km.
- Victoria Falls Bridge - linking Zimbabwe to Zambia, built in 1905 as part of the projected Cape-Cairo railway.
- Zakim Bunker Hill Bridge - USA, built during Boston's Big Dig, the widest cable-stayed bridge.

External links

- [http://www.structurae.de/en/ Structurae] - International Database and Gallery of Structures.
- [http://www.asce.org/history/hp_bridges.html American Society of Civil Engineers] History and Heritage of Civil Engineering - Bridges
- [http://www.historicbridges.org/index.htm Historic Bridges of Michigan and Elsewhere] Photos, information, and maps of historic bridges in and around Michigan. Also has a links page with links to bridge databases in other regions of the U.S.A.
- [http://www.brantacan.co.uk/bridges.htm Bridge Building — Art and Science] Comprehensive explanations about bridges.
- [http://www.garrettsbridges.com/index.html Model Bridge Building]
- [http://pghbridges.com/basics.htm Bridge Basics] A guide to bridge terminology and styles
- [http://www.chinapage.com/bridge/shanghai/lupu/lupu.html Shanghai lupu] Chinese bridge site showing suspended deck arch construction and completion.
- [http://www.bridgebuilder-game.com BridgeBuilder] - Great game where you design a bridge and see if a train will drive over it safely.
- [http://finance.groups.yahoo.com/group/RRbridge/ RRbridge]- Discussion group focused on railroad bridges, trestles, and viaducts. Hosted by Yahoo! Groups
- [http://filebox.vt.edu/users/aschaeff/titlepage.html Bridge disasters]
- [http://www.nireland.com/bridgeman/Dictionary.htm A dictionary of bridge terms]
- [http://bridges.lib.lehigh.edu/BookListpage.html/ Digital Bridge Library at Lehigh University]
- [http://bobjagendorf.smugmug.com/gallery/119358 Bridge Photos]
- [http://flickr.com/photos/tags/bridge Flickr: Photos tagged with bridge] Category:Buildings and structures
- Category:Coastal construction ko:다리 ms:Jambatan ja:橋 simple:Bridge th:สะพาน

Tensile stress

Tensile stress (or tension) is the stress state leading to expansion; that is, the length of a material tends to increase in the tensile direction. The volume of the material stays constant; therefore in a uniaxial material the length increases in the tensile stress direction and the other two directions will decrease in size (see Poisson's ratio for detail). In the uniaxial manner of tension, tensile stress is induced by pulling forces across a bar, specimen, etc. Tensile stress is the opposite of compressive stress. Structural members in direct tension are ropes, soil anchors and nails, bolts, etc. Beams subjected to bending moments may include tensile stress as well as compressive stress and/or shear stress. Tensile stress may be increased until the reach of tensile strength, namely the limit state of stress. The formula for tensile stress is: Tensile Stress = Force / Cross-sectional Area

Rock (geology)

, plutonic, metamorphic rock types of North America. ]] Rock is a naturally occurring aggregate of minerals and/or mineraloids. Rocks are classified by mineral and chemical composition; the texture of the constituent particles; and also by the processes that formed them. These indicators separate rocks into igneous, sedimentary, and metamorphic. Igneous rocks are formed from molten magma, and are divided into two main categories: Plutonic rock and Volcanic rock. Plutonic rocks result when the magma cools and crystallises slowly within the Earth's crust, while Volcanic rocks result from the magma reaching the surface either as lava or fragmental ejecta. Sedimentary rocks are formed by deposition of either detrital or organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation. The latter can occur at or near the earth's surface, especially in the case of carbonate-rich sediments. Metamorphic rocks are formed by subjecting any rock type (including previously-formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the earth's surface, and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals (e.g. by recrystallisation). The transformation of one rock type to another is described by the geological model called the rock cycle. The Earth's crust (including the lithosphere) and mantle are formed of rock.

External links

- [http://www.geol.lsu.edu/henry/Geology3041/2IgneousClassify/IgneousClassFlow.htm Classification of Igneous Rocks] Category:Geology Category:Rocks ja:岩石 ms:Batu th:หิน

Cast iron

:
- For cookware, go to cast iron cookware. Cast iron usually refers to grey cast iron, but can mean any of a group of iron-based alloys containing more than 2% carbon (alloys with less carbon are carbon steel by definition). It is made by remelting pig iron, often along with substantial quantities of scrap iron and scrap steel, and taking various steps to remove undesirable contaminants such as phosphorus and sulfur, which weaken the material. Carbon and silicon content are reduced to the desired levels, which may be anywhere from 2% to 3.5% for carbon and 1% to 3% for silicon depending on the application. Other elements are then added to the melt before the final form is produced by casting. casting.]] The iron-carbon eutectic point lies at 1403 kelvin and 4.3 mass % carbon. Since cast iron has nearly this composition, its melting temperature of 1420 to 1470 K is about 300 K lower than the melting point of pure iron. Cast iron tends to be brittle, unless the name of the particular alloy suggests otherwise. The color of a fracture surface can be used to identify an alloy: carbide impurities allow cracks to pass straight through, resulting in a smooth, "white" surface, while graphite flakes deflect a passing crack and initiate countless new cracks as the material breaks, resulting in a rough surface that appears grey.

Grey cast iron

Silicon is essential to making of grey cast iron as opposed to white cast iron. Silicon causes the carbon to rapidly come out of solution as graphite, leaving a matrix of relatively pure, soft iron. Weak bonding between planes of graphite lead to a high activation energy for growth in that direction, resulting in thin, round flakes. This structure has several useful properties. The metal expands slightly on solidifying as the graphite precipitates, resulting in sharp castings. The graphite content also offers good corrosion resistance. Graphite acts as a lubricant, improving wear resistance. The exceptionally high speed of sound in graphite gives cast iron a much higher thermal conductivity. Since ferrite is so different in this respect (having heavier atoms, bonded much less tightly) phonons tend to scatter at the interface between the two materials. In practical terms, this means that cast iron tends to “damp” mechanical vibrations (including sound), which can help machinery to run more smoothly. All of the properties listed in the paragraph above ease the machining of grey cast iron. The sharp edges of graphite flakes also tend to concentrate stress, allowing cracks to form much more easily, so that material can be removed much more efficiently. Easier initiation of cracks can be a drawback once an item is finished, however: grey cast iron has less tensile strength and shock resistance than steel. It is also difficult to weld. Grey cast iron's high thermal conductivity and specific heat capacity are often exploited to make cast iron cookware.

Other cast iron alloys

With a lower silicon content and faster cooling, the carbon in white cast iron precipitates out of the melt as the metastable phase cementite, Fe₃C, rather than graphite. These precipitates inhibit plastic deformation by impeding the movement of dislocations through the ferrite matrix, offering hardness at the expense of toughness. Since carbide makes up a large fraction of the material, white cast iron could reasonably be classified as a cermet. It is too brittle for most uses, but with good hardness and abrasion resistance and relatively low cost, it finds use in such applications as balls for rolling-element bearings and the teeth of a backhoe's digging bucket. It is difficult to cool thick castings fast enough to solidify the melt as white cast iron all the way through. However, rapid cooling can be used to solidify a shell of white cast iron, after which the remainder cools more slowly to form a core of grey cast iron. The resulting casting, called a “chilled casting”, has the benefits of a hard surface and a somewhat tougher interior. Malleable Iron starts as a white iron casting, then is held at about 900 °C for some time. Graphite separates out much more slowly in this case, so that surface tension has time to form it into spheroidal particles rather than flakes. Due to their lower aspect ratio, spheroids are relatively short and far from one another, and have a lower cross section vis-a-vis a propagating crack or phonon. They also have blunt boundaries, as opposed to flakes, which alleviates the stress concentration problems faced by grey cast iron. In general, the properties of malleable cast iron are more like mild steel. There is a limit to how large a part can be cast in malleable iron, since it is made from white cast iron. A more recent development is nodular or ductile cast iron. Tiny amounts of magnesium or cerium added to these alloys slow down the growth of graphite precipitates by bonding to the edges of the graphite planes. Along with careful control of other elements and timing, this allows the carbon to separate as spheroidal particles as the material solidifies. The properties are similar to malleable iron but parts can be cast with larger sections.

Concrete

In construction, concrete is a composite building material made from the combination of aggregate and cement binder. The most common form of concrete is Portland cement concrete, which consists of mineral aggregate (generally gravel and sand), Portland cement and water. It is commonly believed that concrete dries after mixing and placement. Actually, concrete does not solidify because water evaporates, but rather cement hydrates, gluing the other components together and eventually creating a stone-like material. When used in the generic sense, this is the material referred to by the term concrete. Concrete is used to make pavements, building structures, foundations, motorways/roads, overpasses, parking structures, bases for gates/fences/poles, and cement in brick or block walls. An old name for concrete is liquid stone.

History

The Assyrians and Babylonians used clay as cement in their concretes. The Egyptians used lime and gypsum cement. In the Roman Empire, cements made from pozzolanic ash/pozzolana and an aggregate made from pumice were used to make a concrete very similar to modern portland cement concrete. In 1756, British engineer John Smeaton pioneered the use of portland cement in concrete, using pebbles and powdered brick as aggregate. In the modern day, the use of recycled/reused materials as concrete ingredients is gaining popularity due to increasingly stringent environmental legislation. The most conspicuous of these is pulverized fuel ash, recycled from the ash by-products of coal power plants. This has a significant impact in reducing the amount of quarrying and the ever-attenuating landfill space.

Characteristics

During hydration and hardening, concrete needs to develop certain physical and chemical properties, among others, mechanical strength, low permeability to ingress of moisture, and chemical and volume stability. Concrete has relatively high compressive strength, but significantly lower tensile strength (about 10% of the compressive strength). As a result, concrete always fails from tensile stresses - even when loaded in compression. The practical implication of these facts is that concrete elements that are subjected to tensile stresses must be reinforced. Concrete is most often constructed with the addition of steel bar or fiber reinforcement. The reinforcement can be by bars (rebars), mesh, or fibres to produce reinforced concrete. Concrete can also be prestressed (reducing tensile stress) using steel cables, allowing for beams or slabs with a longer span than is practical with reinforced concrete. The ultimate strength of concrete is related to water/cement ratio, the proportion and type of cement to fillers, and the size, shape, and strength of the aggregate used. Concrete with lower water/cement ratio (down to 0.35) makes a stronger concrete than a higher ratio. Concrete made with smooth pebbles is weaker than that made with rough-surfaced broken rock pieces for example. Certain shapes are very strong in compression, such as arches and vaults, and are therefore preferred for concrete construction. Concrete is placed in a wet or plastic state, and therefore can be manipulated and molded as needed. Hydration and hardening of concrete may lead to tensile stresses at a time when it has not yet gained significant strength, resulting in shrinkage cracks. However, when concrete mix is placed in accordance with the best recommended practice, cracking may be minimal. vault

Additives

Additives are organic or non-organic materials in form of solids or fluids that are added to the concrete to give it certain characteristics. In normal use the additives make up less than 5% of the cement weight. The most used types of additives are:
- Accelerators: Speed up the hydration (strengthening) of the concete.
- Retarders: Slow the hydration of concrete.
- Air-entrainers: Add and distribute air to the concrete.
- Plasticizers: Increase the workability of concrete.

Workability

Workability is the ability of a fresh (plastic) concrete mix to fill the form/mould properly with the desired work (vibration) and without reducing the concrete's quality. Workability depends on water content, additives, aggregate (shape and size distribution) and age (level of hydration). Raising the water content or adding plasticizer will increase the workability. Too much water will lead to bleeding (loss of water) and/or segregation (concrete starts to get heterogeneous) and the resulting concrete will have reduced quality. Workability is normally tested by slump measurement. High flow concrete, like self compacting concrete, are normaly tested by one of several flow measuring methods. Concrete slump is a simplistic measure of fresh (plastic) concrete's workability. Slump is normally determined by the ASTM C 143 or EN 12350-2 slump test standards, using the Abrams cone, into which concrete is placed for testing. When the cone is carefully lifted off the enclosed material, it will slump a certain amount due to its water content. A relatively dry sample will slump very little, and be given a slump of one or two inches (25 or 50 mm), while a relatively wet concrete sample may slump as much as six or seven inches (150 to 175 mm). To increase the slump, the rule of thumb is:
- US units :Add 1 US gallon of water per cubic yard of concrete in the mixer truck to increase slump by 1 inch. Adding 27 US gallons to 9 cubic yards of batched concrete will therefore increase the slump by about 3 inches.
- Metric units (converted from US rule of thumb) :Add 2 litres of water per cubic metre of concrete in the mixer truck to increase slump by 1 cm. Adding 54 litres to 9 cubic metres of batched concrete will therefore increase the slump by about 3 cm. Slump can also be increased by adding a plasticizer, without changing the water/cement ratio.

Self compacting concretes

During the 1980s a number of countries including Japan, Sweden and France developed a range of concretes that were self-compacting. These 'SCC's are characterised by their extreme fluidity (using plasticizers), behaving more like water than the traditional viscous concrete. SCCs are characterized by
- extreme fluidity measured by flow or slump, typically measured between 700-750 mm.
- no need for vibrators to compact the concrete, which can be noisy
- no or little need for expensive concrete pumping equipment
- no bleed water (excess water draining out of the concrete) SCC can offer benefits of up to 50% in labour costs, due to it being poured up to 80% faster and having reduced wear and tear on formwork. As of 2005, self compacting concretes account for 10-15% of concrete sales in some European countries.

Shotcrete / sprayed concrete

Main article: Shotcrete

Shotcrete uses compressed air to shoot (cast) concrete to a frame or structure. Shotcrete is mostly used for rock support, especially in tunnelling. Today there are two application methods for shotcrete: the dry-mix and the wet-mix procedure. In Dry-mix the dry mixture of cement and aggregates is filled into the machine and conveyed with compressed air through the hoses. The water needed for the hydration is added at the nozzle. In Wet-mix the mixes are prepared with all necessary water for hydration. The mixes are pumped through the hoses. At the nozzle compressed air is added for spraying. For both methods additives such as plasticizers and accelerators may be used. Shotcrete is normally reinforced by fibers.

External link

- [http://inventors.about.com/library/inventors/blconcrete.htm History of Concrete] Category:Concrete Category:Civil engineering Category:Materials Category:Construction Category:Pavements Category:Heterogeneous mixtures ms:Konkrit ja:コンクリート simple:Concrete

Shear stress

In physics, shear stress is a stress state in which the shape of a material tends to change (usually by "sliding" forces -- torque by transversely-acting forces) without particular volume change. The shape change is evaluated by measuring the relative change in the angle between initially perpendicular sides of a differential element of material (shear strain). A simple definition of shear stress is the components of stress at a point that act parallel to the plane in which they lie. In laboratory testing, shear stress is achieved by torsion of a specimen. Direct shear of a specimen by a moment induces shear stress, as well as tensile and compressive stress. Structural members in pure shear stress are the torsion bars and the driveshafts in automobiles. Riveted and bolted joints may also be mainly subjected to shear stress. Cantilevers, beams, consoles, and column heads are subject to composite loading, consisting of shear, tensile, and compressive stress. column.]] Also constructions in soil can fail due to shear; e.g., the weight of an earth-filled dam or dike may cause the subsoil to collapse, like a small landslide. Shear stress vectors are relevant to the motion of fluids upon surfaces, which result in the generation of shear stress. Particularly, the laminar flux on the surface has a 0 m/s velocity, and the shear stress appears. The biological importance of shear stress relies on blood flux. The endothelial cells recognise shear stresses and transduce signals to vascular muscular cells and others in order to modify the vessel structure. It is necessary, because high shear stress vessel regions must have larger vessel walls.

Force

:For other senses of this word, see force (disambiguation). In physics, a force is an external cause responsible for any change of a physical system. For instance, a person holding a dog by a rope is experiencing the force applied by the rope on their hand, and the cause for its pulling forward is the force exercised by the rope. The kinetic expression of this change is, according to Newton's second law, acceleration, non kinetic expressions such as deformation can also occur. The SI unit for force is the newton.

Elementary concepts

Force in its most primitive definition can be thought of as that which when acting alone causes an object to accelerate. In a practical sense forces can be divided into two groups: contact forces and field forces. Contact forces require the physical contact of one object with other such as a hammer striking a nail or the force exerted by a gas under pressure - gas produced by exploding gunpowder forces a heavy ball out of a cannon. Field forces on the other hand need no physical medium of contact. Gravity and magnetism are examples of such forces. It should be noted however, that fundamentally all forces are in fact field forces. The force of hammer striking the nail in the previous example turns out to be a clash of the electric forces in both hammer and nail. Nevertheless it is appropriate in some cases to maintain these two classifications for ease of understanding.

Quantitative definition

In physics models, the point-like system is used, where objects are represented as one-dimensional points at their centre of mass. The only change the system can experience is a change of its momentum (its speed). Since the rise of the atomic theory, any physical system has been considered in classical physics as composed of point-like systems called atoms or molecules. Therefore, all forces can be defined by their effect; that is, by the change of movement they induce on point-like systems. This change of movement can be quantified by the acceleration (the derivative of velocity). The discovery by Isaac Newton that a given force will induce an acceleration in inverse proportion to a quantity called the mass of inertia or inertial mass which is independent of the speed of the system is Newton's second law. This law allows us to predict the effect of a force on any point-like system whose mass is known. It is usually written as: :F = dp/dt = d(m·v)/dt = m·a (in the case where m does not depend on t) where :F is the force (a vector quantity), :p is the momentum, :t is the time, :v is the velocity, :m is the mass, and :a=d²x/dt² is the acceleration, the second derivative with respect to t of the position vector x. If the mass m is measured in kilograms and the acceleration a is measured in metres per second squared, then the unit of force is kilogram × metre/second squared. This unit is called the newton: 1 N = 1 kg x 1 m/s². This equation is a system of three second-order differential equations with respect to the three-dimensional position vector which is an unknown function of time. This equation can be solved if F is a known function of x and some of its derivatives and if the mass m is known. Morevover the boundary conditions are required; for example, the values of the position vector and x and the velocity v at the starting time, say t=0. Of course, this formula is only useful if one knows the numerical values of F and m. The definition above is an implicit definition, arrived at as follows. One defines a reference system (one litre of water) and a reference force (the gravitational force applied by the Earth on it at the altitude of Paris). One takes Newton's second law for granted (one postulates that it is true) and measures the acceleration induced by the reference force on the reference system. This gives us a mass unit (1 kg) and a force unit (the older unit of 1 kilogram-force = 9.81 N). Once this is done, one can measure any force by the acceleration it induces on the reference system and measure the inertial mass of any system by measuring the acceleration induced on this system by the reference force. Force is often considered a fundamental quantity in physics, but there are more fundamental quantities, such as momentum (p = mass m x velocity v). Energy, measured in joules, is still less fundamental than force and momentum, because it is defined as work, and work is defined in terms of force. The

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