【導讀】識并用適當?shù)募夹g分析結(jié)構(gòu)的組成，有助于有效地利用預應力混凝土。尋求解決的方案。有一些根本性的分歧在預應力混凝土和其他材料之間。在沒有作用荷載下結(jié)。起不同的自應力，所有這些要素都是受到徐變和溫度效應的影響。開裂可以減少，如果可以很好的粘結(jié)在一起，這將增加剛度和提高耐久性。承受能力；關于這些情況Leonhardt和Abeles已做出了嘗試。這是Freyssi對三座橋梁的觀察結(jié)果，它坐落在維希附近的Allier河上，定的，但他承認說由于變形的存在，這也解釋為何在早期的檢測預應力已經(jīng)損失。預應力，而且同時使用了高質(zhì)量的混凝土，因此這可減少總體的徐變。Freyssi的早期預應力混凝土研究是被寫在其他地方。容許應力設計哲學認為，預應力混凝土的一種方式，靠消除拉應力避免開裂；索的下垂對梁產(chǎn)生上升的力，導致梁產(chǎn)生反作用力。舊規(guī)范是根據(jù)在正常工作負荷下的容許應力規(guī)定的；新規(guī)范是使用短期的極限荷載。所允許的拉應力范圍內(nèi)。力將控制在梁底最大極限彎矩之內(nèi)。

　　

【正文】 induced by the construction sequence. These trapped moments can be large, and obey the same rules as the secondary moments, in that they are brought about by a redistribution of the deadload support reactions. The designer may deliberately choose to use the continuity cables to induce a secondary moment that reduces the trapped moment. Further trapped moments can be induced by the use of temporary prestressing cables which are introduced when the structure is in one configuration, and then removed later after the support conditions have changed. For example, in spanbyspan construction, where a long viaduct is built one span at a time, it is sometimes necessary to introduce temporary cables to resist sagging bending moments that occur during construction but which will be removed later. Putting a cable into a twospan structure (for example), and then removing it once the structure is more indeterminate, does not leave a zero stress state。 these effects should not be overlooked. Creep effects The final effect that needs to be considered is appropriately enough creep (Bazant and Wittmann 1982). It was Freyssi’s original observation of creep that made prestressed concrete possible since he managed to reduce the loss of force caused by creep. In simply supported beams creep causes some loss of prestress and increased deflections, which may need to be taken into account, but it does not alter the distribution of bending moments so the design remains relatively straightforward. 19 If the structure is indeterminate there is always the possibility that the bending moments may be altered by redistribution of the support reactions. If the structure is built in one piece, all the concrete will be of the same age, and its effective modulus will change uniformly throughout the structure. No redistribution of forces is to be expected under these circumstances. However, if the concrete is of different ages, the amount of creep that can occur in the various parts of the structure will vary, which allows redistribution of moments. It is now wellestablished that the structure will creep towards the monolithic state, and the designer can take the asbuilt condition (including trapped moments) and the monolithic state as limiting conditions for the behaviour of the beam. This simplifies the design process. England has studied the effect of temperature variation through the depth of the beam. Creep is temperature dependent and takes place more quickly on the warmer side of a structure than on the colder side, which can significantly alter the load distribution. This work was originally applied to nuclear reactor containment vessels, where the temperature variation across the thickness can be of the order of 100176。 C The work makes use of the concept of a steadystate, when creep can continue but without redistribution of stress. More recently, it has been shown that the much smaller temperature variations that can be expected through the depth of a bridge deck, which may be of the order of 5176。 C, can also have a significant effect. The speed with which creep occurs is very heavily dependent on the relative ages of the concrete in different parts of the structure. Conclusion The successful design of continuous prestressed concrete beams cannot be divorced from the techniques used to analyse the structure, and the way these have developed in the 60 years since the first indeterminate structures were built is a fascinating reflection on the way structural analysis has developed over the same period. It remains the case that designers cannot blindly use analysis programs with out fundamental understanding of the way prestressed concrete behaves. References 20 Abeles, P. W. (1964). An introduction to prestressed concrete. London: Concrete Publications. Andrew, R. P. and P. Witt (Eds.) (1951). Prestressed Concrete Statically Indeterminate Structures. Cement and Concrete Association. Burgoyne, C. J. (1988). Cable design for continuous prestressed concrete bridges. Proc. Inst. Civ. Engrs 85, 161{184. Finsterwalder, U. (Sept 1939). Eisenbetontr196。ager mit selbstt196。atiger vorspannung (reinforced concrete beams with selfacting prestressing). Der Bauinginieur. Freyssi, E. (1956). Birth of prestressing. Library translation 59, Cement and Concrete Association. Translated from French. Published by Travaux, JulyAugust 1954. Grote, J. and B. Marrey (2020). Freyssi, Prestressing and Europe 19301945. Paris: Editions du Linteau. La Grange, L. E. (1961). Moment redistribution in prestressed concrete beams and frames. Ph. D. thesis, University of Cambridge. Lin, T. Y. (1963). Load balancing method for design and analysis of prestressed concrete structures. Journ. Amer. Conc. Inst. 60/6, 719{742. Low, A. M. (1982). The preliminary design of prestressed concrete viaducts. Proc. Inst. Civ. Engrs 73, 351{364. Mattock, A. H., J. Yamazaki, and B. T. Kattula (1971). Comparative studyof prestressed concrete beams, with and without bond. ACI Journal 68, 116{125. Sch196。onberg, M. and F. Fichter (Feb 1939). Die AdolfHitlerBr196。ucke in Aue (Sa) (The Adolf Hitler Bridge at Aue (Saxony)). Die Bautechnik. Xu, Q. and C. Burgoyne (2020). E174。ect of temperature and construction sequence on creep of concrete bridges. Proc. Inst. Civ. Engrs, Bridge Engineering (in Press).