【正文】 if necessary. Expansion Hinges From the seismic point of view, it is desirable to reduce the number of expansion hinges (EH) to a minimum. If EHs are needed, the most bene?cial location from the seismic point of view is at midspan. This can be explained by observing Figure , where the superstructure bending midspan and for an EH at quarterspan. For the latter, it can be seen that the moment at the face The location of expansion hinges within a span, and its characteristics, depends also on the stiffness of the substructure and the type of connection of the superstructure to the piers. presents general guidelines intended to assist in the selection of location of expansion hinges. Precast Segmental Piers Precast segmental piers are usually hollow cross section to save weight. From research in other areas it can be extrapolated that the precast segments of the pier would be joined by means of unbonded prestressing tendons anchored in the footing. The advantage of unbonded over bonded tendons is that for the former, the prestress force would not increase signi?cantly under high column displacement demands, and would therefore not cause inelastic yielding of the strand, which would otherwise lead to a loss of prestress. The detail of the connection to the superstructure and foundation would require some insight into the dynamic characteristics of such a connection, which entails joint opening and closing providing that dry joints are used between segments. This effect is similar to footing rocking, which is well known to be bene?cial to the response of a structure in an earthquake. This is due to the period shift and the damping of the soil. The latter effect is clearly not available to the precast columns, but the period shift is. Details need to be developed for the bearing areas at the end of the columns, as well as the provision for clearance of the tendons to move relative to the pier during the the upper column segment is designed to be connected monolithically to the superstructure, yielding of the reinforcement should be expected. In this case, the expected plastic hinge length should be detailed ductile, using closely spaced ties [3,5]. Casting and Erection Casting There are obvious major differences in casting and erection when working with castinplace cantilever in travelers or in handling precast segments. There are also mon features, which must be kept in mind in the design stages to keep the projects simple and thereby economic and ef?cient,such as ? Keeping the length of segments equal and segments straight, even in curved bridges。 ? De?ection of cantilever arms after construction and before continuity。 the speed of erection and overall construction schedule. It falls into three categories, independent lifting equipment such as cranes,deckmounted lifting equipment such as beam and winch or swivel crane, and launching girder principle of the method is to erect or cast the pier segment ?rst, then to place typical segments one by one from each side of the pier, or in pairs simultaneously from both sides. Each newly placed precast segment is ?xed to the previous one with temporary PT bars, until the cantilever tendons are installed and stressed. The closure joint between cantilever tips is poured in place and continuity tendons installed and order to carry out this erection scheme, segments must be lifted and installed at the proper location. The simplest way is to use a crane, either on land or barge mounted. Many bridges have Bridge with erected with cranes as they do not require an investment in special lifting equipment. This method is slow. Typically, two to four segments per day are placed. It is used on relatively short bridges. An alternative is to have a winch on the last segment erected. The winch is mounted on a beam ?xed to the segment. It picks up segments from below, directly from truck or barge. After placing the segment, the beam and winch system is moved forward to pick up the next segment and so on. Usually, a beamandwinch system is placed on each cantilever tip. This method is also slow。因此，可以認(rèn)為，由于假定的改變，結(jié)構(gòu)的實(shí)際撓度會和設(shè)計(jì)的不同。因此，結(jié)構(gòu)通常會做成拱形，從而是變形接近零。對于行人和駕駛員使用的結(jié)構(gòu)，這種限制更嚴(yán)格。特變盛行于逐跨施工法的結(jié)構(gòu)中，后張拉置于箱梁箱室中而不是沿混凝土結(jié)構(gòu)的長度布置。外部預(yù)應(yīng)力已經(jīng)用于歐洲，美國和亞洲的項(xiàng)目，并且用的很好。外部未粘接鋼筋被使用著，從而保證混凝土中的管道不是開著的。很少在地震活躍的加利福利亞發(fā)現(xiàn)節(jié)段性結(jié)構(gòu)，那兒有美國大多數(shù)的抗震研究。 內(nèi)部鋼筋至少要有 50%的預(yù)應(yīng)力。下面的是不同可能的一個(gè)簡短描述。然后聯(lián)合區(qū)從本質(zhì)詳細(xì)情況是和現(xiàn)澆橋相同的。 圖 板 /墩的現(xiàn)澆接頭 圖 預(yù)應(yīng)力現(xiàn)澆墩 板 /上部結(jié)構(gòu)通過軸承連接 通常情況下對于跨越到 45米逐跨施工法，上層建筑將會通過軸承支撐。應(yīng)該指出的是 ,地震高發(fā)區(qū) ,對于有高墩和軟弱下部結(jié)構(gòu)的建筑，軸承更加為人所需，整體板和上部結(jié)構(gòu)連接是必需的。如果 EH是必需的 ,從地震的觀點(diǎn)看，最有利的位置是中跨。表 。懸臂法建造復(fù)雜結(jié)構(gòu) 連續(xù)梁橫截面內(nèi)部 相鄰橋墩產(chǎn)生適中上部結(jié)構(gòu) 地震是最小化上部結(jié)構(gòu) 上部結(jié)構(gòu)和基礎(chǔ)連接的細(xì)節(jié)需要洞察這個(gè)連接的動態(tài)特點(diǎn)，這就需要接頭的開端和結(jié)尾有干燥的接頭用于各部分。在這種情況下，期望的塑性鉸鏈長度應(yīng)該用空間關(guān)系 [3,5]詳細(xì)延展。保持各段的長度相等和直，曲線橋也如此 有個(gè)平衡力推動起重機(jī)向前。建造的比例通常是一個(gè)一個(gè)起重機(jī)一個(gè)節(jié)段。施工前后有五類撓度： 連續(xù)構(gòu)建的短期和長期撓度 各節(jié)段一塊一塊的在澆筑場地澆筑。節(jié)段的集合精確性是絕對的重點(diǎn)，并且充分的測量方法必須用來保證接下來的幾何關(guān)系。環(huán)氧樹脂材料的抗拉強(qiáng)度比鋼筋的大，但即使如此，環(huán)氧樹脂的抗拉強(qiáng)度在連接處結(jié)構(gòu)行為也不考慮。 澆筑方法 有兩種澆筑節(jié)段的方法。 (見圖 ) 幾何控制 筑和配合澆筑位置簡單平移造就一座直橋 (見圖 )。要獲得不同的超高，連接單元要繞頂板中建的水平軸線轉(zhuǎn)動 (見圖 )。 懸臂平衡法 這種方法的原則是先豎起或澆筑橋墩節(jié)段，然后從墩的一邊一個(gè)一個(gè)放置典型節(jié)段，或者從兩邊同時(shí)進(jìn)行。最簡單的方法是用起重機(jī)，不管是在陸地上還是駁船上。這種方法用于相對較短的橋。放置好節(jié)段后，梁和卷揚(yáng)機(jī)系統(tǒng)向前移動來拾起下一個(gè)節(jié)段。 這種建造方法有兩種基 本的自頂推構(gòu)臺。然后，中心支腿想懸臂端移動，該支腿必須承受構(gòu)架和橋墩節(jié)段重量的和。每個(gè)建造周期由所有類型懸臂節(jié)段和下一個(gè)懸臂段的安裝，同時(shí)不改變構(gòu)架的位置。 通常，平衡懸臂法用于 60到 110米 的頂推法中。 圖