【正文】 tainmeng tanks , pipes , and pressure reactor vessels , essentially follows the same basic principles as does linear prestressing . The circumferential hoop . or “hugging” stress on the cylindrical or spherical structure , neutralizes the tensile stresses at the outer fibers of the curvilinear surface caused by the internal contained pressure . From the preceding discussion , it is plain that permanent stresses in the prestressed structural member are created before the full dead and live loads are applied in order to eliminate or considerably reduce the tensile stresses caused by these loads . With reinforced concrete , it is assumed that the tensile strength of the concrete is negligible and disregarded . This is because the tensile forces resulting from the bending moments are resisted by the bond created in the reinforcement process . Cracking and deflection are therefore essentially irrecoverable in reinforced concrete once the member has reached its limit state at service load . The reinforcement in the reinforced concrete member does not exert any force of its own on the member , contrary to the action of prestressing steel . The steel required to produce the prestressing force in the prestressed member actively preloads the member , permitting a relatively high controlled recovery of cracking and deflection . Once the flexural tensile strength of the concrete is exceeded , the prestressed member starts to act like a reinforced concrete element . Prestressed members are shallower in depth than their reinforced concrete counterparts for the same span and loading conditions . In general , the depth of a prestressed concrete member is usually about 65 to 80 percent of the depth of the equivalent reinforced concrete member . Hence , the prestressed member requires less concrete , and about 20 to 35 percent of the amount of reinforcement. Unfortunately , this saving in material weight is balanced by 濰坊學院本科畢業(yè)論文 6 the higher cost of the higher quality materials needed in prestressing . Also, regardless of the system used , prestressing operations themselves result in an added cost : formwork is more plex ， since the geometry of prestressed sections is usually posed of flanged sections with thin webs . In spite of these additional costs, if a large enough number of precast units are manufactured, the difference between at least the initial costs of prestressed and reinforced concrete systems is usually not very large. And the indirect longterm savings are quite substantial, because less maintenance is needed, a longer working life is possible due to better quality control of the concrete, and lighter foundations are achieved due to the smaller cumulative weight of the superstructure. Once the bean span of reinforced concrete exceeds 70 to 90 feet ( to m), the dead weight of the beam bees excessive, resulting in heavier membersand,consequently,greater longterm deflection and cracking. Thus,for larger spans,prestressed concrete bees mandatory since arches are expensive to construct and do not perform as well due to thesevere longterm shrinkage and creep they large spans such as segmental bridges or cablestayed bridges can only be constructed through the use of prestressing . Prestressed concrete is not a new concept, dating back to 1872,when . Jackson ,an engineer from California, patented a prestressing system that used a tie rod to construct beams or arches from individual block. After a long lapse of time during which little progress was made because of the unavailability of highstrength steel to overe prestress losses, . Dill of Alexandria, Nebraska , recognized the effect of the shrinkage and creep(transverse material flow) of concrete on the loss of prestress. He subsequently developed the idea that successive posttensioning of unbonded rods would pensate for the timedependent loss of stress in the rods due to the decrease in the length of the member because of creep and shrinkage. In the early 1920s, of Minneapolis developed the principles of circular prestressing He hoopstressing horizontal reinforcement around walls of concrete tanks through the use of turnbuckles to prevent cracking due to internal liquid pressure , thereby achieving watertightness . thereafter , prestressing of tanks and pipes developed at an accelerated pace in the United States,with thousands of tanks for water,liquid,and gas storage built and much mileage of prestressed pressure pipe laid in the two to three decades that followed. Linear prestressing continue to develop in Europe and in France,in particular through the ingenuity of Eugene Freyssi,who proposed in 192328 methods to overe prestress losses through the use of highstrength and highductility ,he introduced the now wellknown and wellaccepted Freyssi system. . Abeles of England introduced and developed the concept of partial prestressing between the 1930s and 1960s . F. Leonhardt of Germany , V. Mikhailov of Russia, and . Lin of the United States also contributed a great deal to the art and science of the design of prestressed concrete .Lin39。 房屋是人類居住。我們主要感興趣的安排和房間 的大小和臥室數(shù)目。然而，它往往很難達到最佳的要求，同時對環(huán)境的考慮和位置，的道路。獲得 226Ra的， 232Th和 40K的活度濃度的平均值，在不同濰坊學院本科畢業(yè)論文 9 類型的水泥比報道科委出版物的全球相應值低。根據(jù)化學成分和每一個水力特性，有許多類型的水泥。平均室內(nèi)從地面的放射性源的空氣中吸收劑量率估計為 70 NGY H？1。知識的發(fā)生與濃度等重要材料的天然放射性是一般檢查其質(zhì)量和對周圍環(huán)境，特別是水泥生產(chǎn)工廠明知其效果的關鍵。與其他工廠的產(chǎn)品進行比較， 8個樣品取自普通硅酸鹽水泥（赫勒萬基納， ELkawmya，托拉）和白水泥（西奈半島和赫勒萬）， 4個樣本。密封裝配用的 NaI（ Tl）晶體耦合的 PCMCA（坎培拉 Accuspes）。 3。從選擇合適的材料在水泥生產(chǎn)中使用的角度來看，結果可能是重要的。涉及的預應力類型，連同它的大小，主要取決于系統(tǒng)建設跨度和所需的細長型的基礎上。然后創(chuàng)建一個向上的傾角?；颉皳肀А钡膱A柱形或球形結構上的壓力，中所載的內(nèi)部壓力所造成的曲線表面的外層纖維的拉伸應力。 預應力成員在深度較淺的比相同跨度和荷載條件下的鋼筋混凝土同行。因此，較大跨度預應力混凝土成為強制性的，因為拱門是昂貴的建設和不執(zhí)行以及由于嚴重的長期收縮和徐變，他們?nèi)缍螛蛄夯?。五米哈伊洛夫，俄羅斯，德國，樓倫哈德和 TY美國林也作出了很大貢獻預應力混凝土設計的藝術