【正文】 的有效應力 的概念。 提出加 載平衡的哲學，利用預應力 對 永久荷載 的 反效果（林 1963） 。 對于 預應力混凝土，這些想法不 太對的 ，由于結構是高 應力的 ，即使 沒有負荷 。 cet fZMZPAPf ???? 因此，對于任何組合的 P 和 M，設計師 都用四分之一來 處理?？紤]應力的 極限狀態(tài) ，對于不同負荷情況下，預應力 的 影響可 以被 忽略 ，留下的表達形式： 4 ra ng e s t re s s eP e rm is s ibl Ra ng eM om e nt ?Z 這些 不等式 ， 不是太困難 ， 這樣 截面 的最 小 容許 尺寸 就可以 確定。改變這兩個最高和最低彎矩，但保持 在一定 的范圍 內(nèi) ，同時，提高和降低可行的區(qū)域。 3 連續(xù)梁 設計靜定梁是相對比較 簡單 的 ； 工程師 會根據(jù)特殊的斷面進行設計 ，正如上文所述。這些都是常被稱為次 內(nèi)力 ，其值 并不總是小 ， 但 也 并不總是 不好的 。符合吻合線時是沒有次內(nèi)力矩， es 和 ep 是重合的，所有的內(nèi)力線都是它本身的吻合線。 作用物彎矩是可以估算的，對于預應力混凝土梁自平衡引起的受力也是一個重要的問題。當兩懸臂梁端合攏在一起使得它們完全地連續(xù)。 徐變的影響 最后需要考慮的是徐變的影響， Freyssi 發(fā)現(xiàn)用預應力混凝土可以減少由于徐變所引起強度損失。這個研究是利用穩(wěn)態(tài)的這一概念。 the unwary will either make mistakes or spend inordinate time trying to extract a solution from the various equations. There are a number of fundamental differences between the behaviour of prestressed concrete and that of other materials. Structures are not unstressed when unloaded。 ? the applied moment at the time the prestress is first applied, before creep losses occur, ? the maximum applied moment after creep losses, and ? the minimum applied moment after creep losses. Figure 4: Gustave Magnel Other binations may be needed in more plex cases. There are at least twelve inequalities that have to be satisfied at any crosssection, but since an Isection can be defined by six variables, and two are needed to define the prestress, the problem is overspecified and it is not immediately obvious which conditions are superfluous. In the hands of inexperienced engineers, the design process can be very longwinded. However, it is possible to separate out the design of the crosssection from the design of the prestress. By considering pairs of stress limits on the same fibre, but for different load cases, the effects of the prestress can be eliminated, leaving expressions of the form: ra ng e s t re s s eP e rm i s s i bl Ra ng eM om e nt ?Z These inequalities, which can be evaluated exhaustively with little difficulty, allow the minimum size of the crosssection to be determined. 14 Once a suitable crosssection has been found, the prestress can be designed using a construction due to Magnel (). The stress limits can all be rearranged into the form: ? ?MfZPAZe ???? 1 By plotting these on a diagram of eccentricity versus the reciprocal of the prestressing force, a series of bound lines will be formed. Provided the inequalities (2) are satisfied, these bound lines will always leave a zone showing all feasible binations of P and e. The most economical design, using the minimum prestress, usually lies on the right hand side of the diagram, where the design is limited by the permissible tensile stresses. Plotting the eccentricity on the vertical axis allows direct parison with the crosssection, as shown in Fig. 5. Inequalities (3) make no reference to the physical dimensions of the structure, but these practical cover limits can be shown as well A good designer knows how changes to the design and the loadings alter the Magnel diagram. Changing both the maximum and 15 minimum bending moments, but keeping the range the same, raises and lowers the feasible region. If the moments bee more sagging the feasible region gets lower in the general, as spans increase, the dead load moments increase in proportion to the live load. A stage will be reached where the economic point (A on ) moves outside the physical limits of the beam。 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 1001