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外文翻譯
結(jié)構(gòu)設(shè)計(jì)
結(jié)構(gòu)設(shè)計(jì)
Augustine J.Fredrich
摘要：結(jié)構(gòu)設(shè)計(jì)是選擇材料和構(gòu)件類型，大小和形狀以安全有用的樣式承擔(dān)荷載。一般說(shuō)來(lái)，結(jié)構(gòu)設(shè)計(jì)暗指結(jié)構(gòu)物如建筑物和橋或是可移動(dòng)但有剛性外殼如船體和飛機(jī)框架的工廠穩(wěn)定性。設(shè)計(jì)的移動(dòng)時(shí)彼此相連的設(shè)備（連接件），一般被安排在機(jī)械設(shè)計(jì)領(lǐng)域。
關(guān)鍵詞：結(jié)構(gòu)設(shè)計(jì) ； 結(jié)構(gòu)分析 ； 結(jié)構(gòu)方案 ； 工程要求
Abstract: Structure design is the selection of materials and member type ,size, and configuration to carry loads in a safe and serviceable fashion .In general ,structural design implies the engineering of stationary objects such as buildings and bridges ,or objects that maybe mobile but have a rigid shape such as ship hulls and aircraft frames. Devices with parts planned to move with relation to each other(linkages) are generally assigned to the area of mechanical .
Key words: Structure Design ； Structural analysis ；structural scheme ； Project requirements
Structure Design
Structural design involved at least five distinct phases of work: project requirements, materials, structural scheme, analysis, and design. For unusual structures or materials a six phase, testing, should be included. These phases do not proceed in a rigid progression , since different materials can be most effective in different schemes , testing can result in change to a design , and a final design is often reached by starting with a rough estimated design , then looping through several cycles of analysis and redesign . Often, several alternative designs will prove quite close in cost, strength, and serviceability. The structural engineer, owner, or end user would then make a selection based on other considerations.
Project requirements. Before starting design, the structural engineer must determine the criteria for acceptable performance. The loads or forces to be resisted must be provided. For specialized structures, this may be given directly, as when supporting a known piece of machinery, or a crane of known capacity. For conventional buildings, buildings codes adopted on a municipal, county , or , state level provide minimum design requirements for live loads (occupants and furnishings , snow on roofs , and so on ). The engineer will calculate dead loads (structural and known, permanent installations ) during the design process.
For the structural to be serviceable or useful , deflections must also be kept within limits ,since it is possible for safe structural to be uncomfortable “bounce” Very tight deflection limits are set on supports for machinery , since beam sag can cause drive shafts to bend , bearing to burn out , parts to misalign , and overhead cranes to stall . Limitations of sag less than span /1000 ( 1/1000 of the beam length ) are not uncommon . In conventional buildings, beams supporting ceilings often have sag limits of span /360 to avoid plaster cracking, or span /240 to avoid occupant concern (keep visual perception limited ). Beam stiffness also affects floor “bounciness,” which can be annoying if not controlled. In addition , lateral deflection , sway , or drift of tall buildings is often held within approximately height /500 (1/500 of the building height ) to minimize the likelihood of motion discomfort in occupants of upper floors on windy days .
Member size limitations often have a major effect on the structural design. For example, a certain type of bridge may be unacceptable because of insufficient under clearance for river traffic, or excessive height endangering aircraft. In building design, ceiling heights and floor-to-floor heights affect the choice of floor framing. Wall thicknesses and column sizes and spacing may also affect the serviceability of various framing schemes.
Materials selection. Technological advances have created many novel materials such as carbon fiber and boron fiber-reinforced composites, which have excellent strength, stiffness, and strength-to-weight properties. However, because of the high cost and difficult or unusual fabrication techniques required , they are used only in very limited and specialized applications . Glass-reinforced composites such as fiberglass are more common, but are limited to lightly loaded applications. The main materials used in structural design are more prosaic and include steel, aluminum, reinforced concrete, wood , and masonry .
Structural schemes. In an actual structural, various forces are experienced by structural members , including tension , compression , flexure (bending ), shear ,and torsion (twist) . However, the structural scheme selected will influence which of these forces occurs most frequently, and this will influence the process of materials selection.
Tension is the most efficient way to resist applied loads ,since the entire member cross section is acting to full capacity and bucking is not a concern . Any tension scheme must also included anchorages for the tension members . In a suspension bridge , for example ,the anchorages are usually massive dead weights at the ends of the main cables . To avoid undesirable changes in geometry under moving or varying loads , tension schemes also generally require stiffening beams or trusses.
Compression is the next most efficient method for carrying loads . The full member cross section is used ,but must be designed to avoid bucking ,either by making the member stocky or by adding supplementary bracing . Domed and arched buildings ,arch bridges and columns in buildings frames are common schemes . Arches create lateral outward thrusts which must be resisted . This can be done by designing appropriate foundations or , where the arch occurs above the roadway or floor line , by using tension members along the roadway to tie the arch ends together ,keeping them from spreading . Compression members weaken drastically when loads are not applied along the member axis , so moving , variable , and unbalanced loads must be carefully considered.
Schemes based on flexure are less efficient than tension and compression ,since the flexure or bending is resisted by one side of the member acting in tension while the other side acts in compression . Flexural schemes such as beams , girders , rigid frames , and moment (bending ) connected frames have advantages in requiring no external anchorages or thrust restrains other than normal foundations ,and inherent stiffness and resistance to moving ,variable , and unbalanced loads .
Trusses are an interesting hybrid of the above schemes . They are designed to resist loads by spanning in the manner of a flexural member, but act to break up the load into a series of tension and compression forces which are resisted by individually designed tension and have excellent stiffness and resistance to moving and variable loads . Numerous member-to-member connections, supplementary compression braces ,and a somewhat cluttered appearance are truss disadvantages .
Plates and shells include domes ,arched vaults ,saw tooth roofs , hyperbolic paraboloids , and saddle shapes .Such schemes attempt to direct all force along the plane of the surface ,and act largely in shear . While potentially very efficient ,such schemes have very strict limitations on geometry and are poor in resisting point ,moving , and unbalanced loads perpendicular to the surface.
Stressed-skin and monologue construction uses the skin between stiffening ribs ,spars ,or columns to resist shear or axial forces . Such design is common in airframes for planes and rockets, and in ship hulls . it has also been used to advantage in buildings. Such a design is practical only when the skin is a logical part of the design and is never to be altered or removed .
For bridges , short spans are commonly girders in flexure . As spans increase and girder depth becomes unwieldy , trusses are often used ,as well as cablestayed schemes .Longer spans may use arches where foundation conditions ,under clearance ,or headroom requirements are favorable .The longest spans are handled exclusively by suspension schemes ,since these minimize the crucial dead weight and can be erected wire by wire .
For buildings, short spans are handled by slabs in flexure .As spans increase, beams and girders in flexure are used . Longer spans require trusses ,especially in industrial buildings with possible hung loads . Domes ,arches , and cable-suspended and air –supported roofs can be used over convention halls and arenas to achieve clear areas .
Structural analysis . Analysis of structures is required to ensure stability (static equilibrium ) ,find the member forces to be resisted ,and determine deflections . It requires that member configuration , approximate member sizes ,and elastic modulus ; linearity ; and curvature and plane sections . Various methods are used to complete the analysis .
Final design . once a structural has been analyzed (by using geometry alone if the analysis is determinate , or geometry plus assumed member sizes and materials if indeterminate ), final design can proceed . Deflections and allowable stresses or ultimate strength must be checked against criteria provided either by the owner or by the governing building codes . Safety at working loads must be calculated . Several methods are available ,and the choice depends on the types of materials that will be used .
Pure tension members are checked by dividing load by cross-section area .Local stresses at connections ,such as bolt holes or welds ,require special attention . Where axial tension is combined with bending moment ,the sum of stresses is compared to allowance levels . Allowable : stresses in compression members are dependent on the strength of material, elastic modulus ,member slenderness ,and length between bracing points . Stocky members are limited by materials strength ,while slender members are limited by elastic bucking .
Design of beams can be checked by comparing a maximum bending stress to an allowable stress , which is generally controlled by the strength of the material, but may be limited if the compression side of the beam is not well braced against bucking .
Design of beam-columns ,or compression members with bending moment ,must consider two items . First ,when a member is bowed due to an applied moment ,adding axial compression will cause the bow to increase .In effect ,the axial load has magnified the original moment .Second ,allowable stresses for columns and those for beams are often quite different .
Members that are loaded perpendicular to their long axis, such as beams and beam-columns, also must carry shear. Shear stresses will occur in a direction to oppose the applied load and also at right angles to it to tie the various elements of the beam together. They are compared to an allowable shear stress. These procedures can also be used to design trusses, which are assemblies of tension and compression members. Lastly, deflections are checked against the project criteria using final member sizes.
Once a satisfactory scheme has been analyzed and designed to be within project criteria, the information must be presented for fabrication and construction. This is commonly done through drawings, which indicate all basic dimensions, materials, member sizes, the anticipated loads used in design, and anticipated forces to be carried through connections.
結(jié)構(gòu)設(shè)計(jì)
結(jié)構(gòu)設(shè)計(jì)包含至少5個(gè)不同方面的工作：工程要求，材料，結(jié)構(gòu)方案，分析和設(shè)計(jì)。對(duì)于不一般的結(jié)構(gòu)或材料，又包含一個(gè)方面：試驗(yàn)。這些方面不是嚴(yán)格按步驟進(jìn)行，因?yàn)椴煌牧显诓煌桨复蠖鄶?shù)是有效的，試驗(yàn)會(huì)導(dǎo)致設(shè)計(jì)變更，最終設(shè)計(jì)由初步估計(jì)設(shè)計(jì)開(kāi)始，然后經(jīng)過(guò)分析和再設(shè)計(jì)幾個(gè)循環(huán)后完成。通常，可替代的設(shè)計(jì)證明在費(fèi)用，強(qiáng)度和使用性上十分接近。結(jié)構(gòu)工程師，業(yè)主或最后住戶基于其它的考慮選擇一種。
工程要求。在開(kāi)始設(shè)計(jì)前，結(jié)構(gòu)工程師必須決定容易接受的執(zhí)行標(biāo)準(zhǔn)。必須提供承擔(dān)的荷載或力。對(duì)于一些專門(mén)結(jié)構(gòu)，當(dāng)支持一臺(tái)已知載重的機(jī)器或起重機(jī)時(shí)，這可能直接給出，對(duì)于普通建筑物，采用市政，縣，州的建筑規(guī)范，提供了設(shè)計(jì)所需活載（人群荷載和設(shè)備，屋頂雪荷載，等等）的最小值。工程師將計(jì)算出設(shè)計(jì)期間的恒載（結(jié)構(gòu)和已知永久性設(shè)備）。
對(duì)要正常使用的結(jié)構(gòu)，也必須控制其撓度，因?yàn)榘踩慕Y(jié)構(gòu)可能會(huì)存在令人不安的振動(dòng)。機(jī)器的支座有嚴(yán)格的變形限制，因?yàn)榱合鲁習(xí)?dǎo)致驅(qū)動(dòng)軸彎曲，燒毀，部件錯(cuò)位和上面的吊車(chē)熄火。撓度限制在跨度/1000 (梁長(zhǎng)的1/1000）以下是很普通的。在傳統(tǒng)建筑里，支持板的梁撓度限制在跨度1/360以避免粉刷開(kāi)裂或跨度1/240以避免人的擔(dān)憂（保持在可感知的變動(dòng)范圍內(nèi)）。梁的剛度也影響板“振動(dòng)”，如果不能控制會(huì)令人很頭疼。另外，高層建筑的側(cè)面變形，位移或搖擺通常限定在高度/500（建筑物高度的1/500）里，把在有風(fēng)的日子里上面樓層的人移動(dòng)的不舒服降到最小。構(gòu)件尺寸在結(jié)構(gòu)設(shè)計(jì)里起主要作用。例如，由于下面留作水上交通的凈空不夠或過(guò)高威脅到飛機(jī)的特定類型的橋是不可接受的。在建筑設(shè)計(jì)里，天花板高度和樓板之間高度影響樓板框架的選擇。墻厚和柱子尺寸和跨度也影響不同框架方案的適用性。
選擇材料。技術(shù)的進(jìn)步創(chuàng)造了許多新材料，如碳纖維加強(qiáng)復(fù)合材料和硼纖維加強(qiáng)復(fù)合材料，它們都具有極好的強(qiáng)度，剛度和強(qiáng)度重量比特性。然而，由于費(fèi)用高和非通常的制造要求，它們僅用在有限特殊領(lǐng)域。強(qiáng)化玻璃合成物如玻璃纖維是很普遍，但被限制應(yīng)用在小荷載情況下。用在結(jié)構(gòu)設(shè)計(jì)上的主要材料更多是普通的，包括鋼材，鋁，鋼筋混凝土，木材，砌體。
結(jié)構(gòu)方案。在一個(gè)實(shí)際方案里，結(jié)構(gòu)構(gòu)件承擔(dān)很多力，包括拉，壓，彎，剪和扭。然而所選擇的方案將會(huì)影響這些力產(chǎn)生的概率，也會(huì)影響材料選擇過(guò)程。
抗拉是有效的承擔(dān)荷載的方法，整個(gè)構(gòu)件的橫截面性能得到發(fā)揮，并且不涉及到彎曲變形。任何抗拉方案必須也對(duì)抗拉構(gòu)件的錨固。例如，在懸索橋里，錨固體通常是位于主要繩索尾段的強(qiáng)大自重。為了避免在荷載移動(dòng)或變形時(shí)有不期望的幾何變形，抗拉方案通常要求是剛性梁和桁架。
抗壓是另一個(gè)很有效的承擔(dān)荷載方法。全部桿件截面發(fā)揮了作用，但是設(shè)計(jì)時(shí)必須避免彎曲，或者是做成粗短構(gòu)件或者是增加附加支撐。圓頂和拱形建筑，拱橋和柱是很普遍的建筑方案。拱產(chǎn)生了必須抵擋住的水平外推力。這靠設(shè)計(jì)合適的基礎(chǔ)或建在車(chē)道或樓板的上面的拱解決，靠沿著車(chē)道用抗拉構(gòu)件把兩端的拱連接起來(lái)，阻止他們拉開(kāi)。當(dāng)荷載不是作用在構(gòu)件軸線上時(shí)，抗壓構(gòu)件顯著地被削弱。所以，必須認(rèn)真考慮移動(dòng)，變化和不平衡的荷載。
基于受彎的方案的效率比受拉和壓低，因?yàn)閺澢强繕?gòu)件一邊受拉另一邊受壓來(lái)抵抗。受彎方案如主梁，次梁，剛架和受彎框架在外部錨固或推力限制，與一般基礎(chǔ)不同，靠?jī)?nèi)部剛度阻擋可移動(dòng)，變化和不平衡的荷載的情況下有利。
桁架是上面方案的混合體。它們?cè)O(shè)計(jì)成荷載橫跨在受彎構(gòu)件上，但是分解成一系列拉力和壓力，由抗拉和抗壓構(gòu)件承擔(dān)。桁架方案設(shè)計(jì)時(shí)不需要特殊錨固或推力的限制，并且有很好的剛度抵抗移動(dòng)或變化的荷載。大量的構(gòu)件之間連結(jié)和抗壓構(gòu)件的附加支撐，看起來(lái)有點(diǎn)雜亂，這就是桁架的不利處。
板和殼包括圓頂，拱頂，有齒屋頂，雙曲拋物面和馬鞍形。這樣的方案把所有的力直接作用在平板表面并且作用有巨大的剪力。盡管可能效率很高，但是這樣的方案對(duì)幾何有嚴(yán)格的限制，并且在移動(dòng)，和不平衡垂直作用在表面的荷載的能力很弱。
薄殼結(jié)構(gòu)和硬殼結(jié)構(gòu)利用加勁肋，梁之間的殼板抵抗剪力和軸向力。這樣的設(shè)計(jì)在飛機(jī)機(jī)體和火箭，船體方面很普遍。它在建筑方面也是有利的。這樣的設(shè)計(jì)僅僅在殼是設(shè)計(jì)的邏輯部分并且永遠(yuǎn)不會(huì)被替代和移除時(shí)才實(shí)際些。
對(duì)于橋梁，短跨是很普遍受彎的梁。當(dāng)跨度增加和梁高變得很大時(shí)，通常用桁架和斜拉結(jié)構(gòu)。更長(zhǎng)跨時(shí)也許用拱，要考慮基礎(chǔ)條件和凈空要求。最長(zhǎng)的跨靠懸索方案處理，因?yàn)檫@可把關(guān)鍵性的自重降到最小并且能索連索地建造起來(lái)。
對(duì)于橋，短跨靠板承擔(dān)彎矩。當(dāng)跨度增加時(shí)，主梁和次梁被用來(lái)承擔(dān)彎曲。更長(zhǎng)的跨要求用桁架，尤其是在工業(yè)建筑有吊車(chē)荷載時(shí)，圓頂，拱和懸索和充氣屋頂被用在傳統(tǒng)的大廳和競(jìng)技場(chǎng)里以獲得凈面積。
結(jié)構(gòu)分析。結(jié)構(gòu)分析要求確定穩(wěn)定性（靜力平衡），構(gòu)件承擔(dān)的力和變形。它需要構(gòu)件形狀，大概尺寸，已知或假設(shè)的材料特性。分析包括：平衡，應(yīng)力，應(yīng)變和彈性模量，線形，塑性和彎曲和板截面。很多方法可以完成分析過(guò)程。
最終設(shè)計(jì)。一旦結(jié)構(gòu)分析完成（如果分析是正確的，只用幾何方法；反之附加構(gòu)件尺寸和材料假設(shè)）。最終設(shè)計(jì)可以進(jìn)行，必須對(duì)照業(yè)主或政府建筑規(guī)范標(biāo)準(zhǔn)來(lái)檢查變形和允許應(yīng)力或極限強(qiáng)度。必須計(jì)算工作荷載下的安全性。一般方法是可行的，依據(jù)所使用的材料類型做出選擇。
純抗拉構(gòu)件檢查橫截面應(yīng)力。特別注意螺栓孔或焊接處的應(yīng)力。拉彎構(gòu)件中，用應(yīng)力之和與分析應(yīng)力作比。受壓構(gòu)件中的允許應(yīng)力取決于構(gòu)件強(qiáng)度，彈性模量，長(zhǎng)細(xì)比和支點(diǎn)間距離。粗短構(gòu)件由材料強(qiáng)度決定，然而長(zhǎng)細(xì)構(gòu)件由彈性彎曲決定。
梁的設(shè)計(jì)由對(duì)于最大彎曲應(yīng)力和允許應(yīng)力來(lái)檢驗(yàn)，通常由材料強(qiáng)度控制，但是如果受壓一邊沒(méi)有側(cè)向支撐就會(huì)被限制。
梁，柱或有彎矩的受壓構(gòu)件的設(shè)計(jì)必須考慮兩項(xiàng)。首先，當(dāng)構(gòu)件由于承受彎矩而彎曲時(shí)，軸力會(huì)增加彎曲量，實(shí)際上，軸壓放大了原始彎矩。其次，對(duì)于柱和梁的允許應(yīng)力是不同的。
承受垂直于長(zhǎng)軸的荷載的構(gòu)件。如梁和梁——柱，也必須承擔(dān)剪力。剪應(yīng)力和荷載的方向相反并且在其右邊，把梁的不同部分連接起來(lái)。它們與允許剪應(yīng)力作對(duì)比。這些步驟也能用來(lái)設(shè)計(jì)由受拉和受壓構(gòu)件組成的桁架。最后，用工程標(biāo)準(zhǔn)檢驗(yàn)變形，使用最后的構(gòu)件。
一旦被分析和在工程標(biāo)準(zhǔn)內(nèi)的設(shè)計(jì)方案是令人滿意的，必須提出制造和建立信息。通過(guò)作圖，指明所以基本尺寸，材料和構(gòu)件大小。設(shè)計(jì)中預(yù)期荷載和節(jié)點(diǎn)承擔(dān)的預(yù)期力。
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