土木工程英文外文文献翻译分析预应力混凝土连续梁毕业设计

土木工程英文外文文献翻译分析预应力混凝土连续梁毕业设计内容摘要：

e most fundamental. Section design From the outset it was recognised that prestressed concrete has to be checked at both the working load and the ultimate load. For steel structures, and those made from reinforced concrete, there is a fairly direct relationship between the load capacity under an allowable stress design, and that at the ultimate load under an ultimate strength design. Older codes were based on permissible stresses at the working load。 new codes use moment capacities at the ultimate load. Different load factors are used in the two codes, but a structure which passes one code is likely to be acceptable under the other. 12 For prestressed concrete, those ideas do not hold, since the structure is highly stressed, even when unloaded. A small increase of load can cause some stress limits to be breached, while a large increase in load might be needed to cross other limits. The designer has considerable freedom to vary both the working load and ultimate load capacities independently。 both need to be checked. A designer normally has to check the tensile and pressive stresses, in both the top and bottom fibre of the section, for every load case. The critical sections are normally, but not always, the midspan and the sections over piers but other sections may bee critical ， when the cable profile has to be determined. The stresses at any position are made up of three ponents, one of which normally has a different sign from the other two。 consistency of sign convention is essential. If P is the prestressing force and e its eccentricity, A and Z are the area of the crosssection and its elastic section modulus, while M is the applied moment, then where ft and fc are the permissible stresses in tension and pression. cet fZMZPAPf  Thus, for any bination of P and M , the designer already has four in equalities to deal with. The prestressing force differs over time, due to creep losses, and a designer is 13 usually faced with at least three binations of prestressing force and moment。 • 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。 Guyon (1951a) denoted the limiting condition as the critical span. Shorter spans will be governed by tensile stresses in the two extreme fibres, while longer spans will be governed by the limiting eccentricity and tensile stresses in the bottom fibre. However, it does not take a large increase in moment ， at which point pressive stresses will govern in the bottom fibre under maximum moment. Only when much longer spans are required, and the feasible region moves as far down as possible, does the structure bee governed by pressive stresses in both fibres. Continuous beams The design of statically determinate beams is relatively straightforward。 the engineer can work on the。