【正文】 e (1695)and Belidor (1792) about the theoretical analysis of structures provided the basis in the field of mechanics of materials . Kuzmanovic(1977) focuses on stone and wood as the first bridgebuilding materials. Iron was introduced during the transitional period from wood to steel .According to recent records , concrete was used in France as early as 1840 for a bridge 39 feet (12 m) long to span the Garoyne Canal at Grisoles, but reinforced concrete was not introduced in bridge construction until the beginning of this century . Prestressed concrete was first used in 1927. Stone bridges of the arch type (integrated superstructure and substructure) were constructed in Rome and other European cities in the middle ages . These arches were halfcircular , with flat arches beginning to dominate bridge work during the Renaissance period. This concept was markedly improved at the end of the eighteenth century and found structurally adequate to acmodate future railroad loads . In terms of analysis and use of materials , stone bridges have not changed much ,but the theoretical treatment was improved by introducing the pressureline concept in the early 1670s(Lahire, 1695) . The arch theory was documented in model tests where typical failure modes were considered (Frezier,1739).Culmann(1851) introduced the elastic center method for fixedend arches, and showed that three redundant parameters can be found by the use of three equations of coMPatibility. Wooden trusses were used in bridges during the sixteenth century when Palladio built triangular frames for bridge spans 10 feet long . This effort also focused on the three basic principles og bridge design : convenience(serviceability) ,appearance , and endurance(strength) . several timber truss bridges were constructed in western Europe beginning in the 1750s with spans up to 200 feet (61m) supported on stone substructures .Significant progress was possible in the United States and Russia during the nieenth century ,prompted by the need to cross major rivers and by an abundance of suitable timber . Favorable economic considerations included initial low cost and fast construction . The transition from wooden bridges to steel types probably did not begin until about 1840 ,although the first documented use of iron in bridges was the chain bridge built in 1734 across the Oder River in Prussia . The first truss pletely made of iron was in 1840 in the United States , followed by England in 1845 , Germany in 1853 , and Russia in 1857 . In 1840 , the first iron arch truss bridge was built across the Erie Canal at Utica . The Impetus of Analysis The theory of structures The theory of structures ,developed mainly in the niheenth century,focused on truss analysis, with the first book on bridges written in 1811. The Warren triangular truss was introduced in 1846 , supplemented by a method for calculating the correcet forces .Ibeams fabricated from plates became popular in England and were used in shortspan bridges. In 1866, Culmann explained the principles of cantilever truss bridges, and one year later the first cantilever bridge was built across the Main River in Hassfurt, Germany, with a center span of 425 feet (130m) . The first cantilever bridge in the United States was built in 1875 across the Kentucky most impressive railway cantilever bridge in the nieenth century was the First of Forth bridge , built between 1883 and 1893 , with span magnitudes of 1711 feet (). At about the same time , structural steel was introduced as a prime material in bridge work , although its quality was often poor . Several early examples are the Eads bridge in 。 the force method redefined by Mohr。而在 15世紀(jì)至 16世紀(jì)期間，工程師們似乎并沒有注意到這些文字記載，只是單憑經(jīng)驗(yàn)和傳統(tǒng)來建造橋梁和渡槽。在從木材到鋼材的轉(zhuǎn)變過程中，鐵作為一種過渡材料被用于橋梁建筑中。這種觀念在 18 世紀(jì)末有了明顯的改進(jìn)，并發(fā)現(xiàn)其在結(jié)構(gòu)上能適應(yīng)后來的鐵路荷載。 當(dāng) palladio 建造了一座跨度為 10 英尺的三角形木制框架橋后， 16 世紀(jì)開始，木桁架在橋梁中得到應(yīng)用。木制橋梁具有良好的經(jīng)濟(jì)性，因?yàn)槠涑跗谕顿Y較低，施工速度較快。 理論分析的推動(dòng)作用 主要從 19 世紀(jì)發(fā)展起來的機(jī)構(gòu)分析理論著重于桁架的分析，首部關(guān)于橋梁工程的著作于1811年出版。美國(guó) 的首座懸臂梁橋于 1875 年建于Kentucky河上。由于混凝土可以較容易地澆注成各種形狀的結(jié)構(gòu)物，因此它在建筑上的使用價(jià)值幾乎是無限的。 Calisev (1923)的工作使得框架結(jié)構(gòu)的分析有可能進(jìn)一步簡(jiǎn)化，他利用逐步近似的方法將方程組的求解簡(jiǎn)化為一個(gè)簡(jiǎn)單表達(dá)式的迭代計(jì)算。第一次世界大戰(zhàn)以后，塑性的概念吸引著研究人員和工程師們的注意力，開始主要是在德國(guó)。這本設(shè)計(jì)準(zhǔn)則是與“美國(guó)各州公路工作者協(xié)會(huì)（ AASHO）” 1969 年的設(shè)計(jì)規(guī)范聯(lián)合使用的，它的表達(dá)方式使其很容易適應(yīng)極限狀態(tài)設(shè)計(jì)規(guī)范的發(fā)展。在正常使用極限狀態(tài)下，需驗(yàn)算構(gòu)件得撓度、最大裂縫寬度和疲勞強(qiáng)度。 隨著州際體系得建立和結(jié)構(gòu)等級(jí)分類的需要，某些橋型在橋梁界占有重要的地位。 1954 年，美國(guó)聯(lián)邦最高法院認(rèn)為，立法機(jī)關(guān)有權(quán)決定公共場(chǎng)所不但要有利于公眾健康，還要做到賞心悅目；不但要干凈，還要寬敞；不但要通暢，還要布局均衡。有關(guān)顏色、光線、質(zhì)地、形狀、比例以及其他感知形態(tài)的美學(xué)研究已經(jīng)展開，這個(gè)方向無論在理論上還是經(jīng)驗(yàn)上都是明確的。 荷載及荷