影响混凝土基础设计的因素毕业论文外文翻译.doc

《影响混凝土基础设计的因素毕业论文外文翻译.doc》由会员分享，可在线阅读，更多相关《影响混凝土基础设计的因素毕业论文外文翻译.doc（12页珍藏版）》请在淘文阁 - 分享文档赚钱的网站上搜索。

1、文献综述FootingsTypes and function of substructure, or foundation, is that part of a structure which is usually placed below the surface of the ground and which transmits the load to the underlying soil or rock. All soils compress noticeably when loaded and cause the supported structure to settle. The t

2、wo essential requirements in the design of foundations are that the total settlement of the structure shall be limited to a tolerably small amount and that differential settlement of the various parts of the structure shall be eliminated as nearly as possible. With respect to possible structural dam

3、age, the elimination of differential settlement, i.e., different amounts of settlement within the same structure, is even more important than limitations on uniform overall settlement.To limit settlement as indicated, it is necessary to transmit the load of the structure to a soil stratum of suffici

4、ent strength and to spread the load cover a sufficiently large area of that stratum to minimize bearing pressure. If adequate soil is not found immediately below the structure, it becomes necessary to use deep foundations such as piles or caissons to transmit the load to deeper, firmer layers. If sa

5、tisfactory soil directly underlies the structure, it is merely necessary to spread the load, by footings or other means. Such substructures are known as spread foundations, and it is mainly this type which will be discussed.Types of spread foundationsFootings generally can be classified as wall and

6、column footings. The horizontal outlines of the most common types are given. A wall footing is simply a strip of reinforced concrete, wider than the wall, which distributes pressure. single column footing are usually square, sometimes rectangular, and represent the simplest and most economical type.

7、 Their use under exterior columns meets with difficulties if property rights prevent the use of footing projecting beyond the exterior walls. In this case combined footings or strap footings are used enable one to design a footing which will not project beyond the wall column. Combined footings unde

8、r closely spaced, heavily loaded interior columns where single footings, if they were provided, would completely or nearly merge.Such individual or combined column footings are the most frequently used types of spread foundations on soils of reasonable bearing capacity. If the soil is weak and/or co

9、lumn loads are great, the required footing area become so large as to be uneconomical. In this case, unless a deep foundation is called for by soil conditions, a mat or raft foundation is resorted to. This consists of a solid reinforced-concrete slab which extend under the entire building and which

10、consequently distributes the load of the structure over the maximum available area. Such a foundation, in view of its own rigidity, also minimizes differential settlement. it consists, in its simplest form, of a concrete slab reinforced in both directions. A form which provides more rigidity and at

11、the same time is often more economical consists of an inverted beam-and girder floor. Girders are located in the column lines in one direction, which beams in the other, mostly at closer intervals. If the columns are arranged in a square pattern, girders are equally spaced in both directions and the

12、 slab is provided with two ways reinforcement. Inverted flat slab, with capitals at the bottoms of the columns, are also used for mat foundation.Factors affecting the design of concrete footings In ordinary constructions the load on a wall or column is transmitted vertically to the footing, which in

13、 turn is supported by the upward pressure of the soil on which it rests. If the load is symmetrical with respect to the bearing area, the bearing pressure is assumed to the uniformly distributed. It is known that this is only approximately true. Under footing resting on coarse-grained soils the pres

14、sure is larger the center of the footings and decrease toward the perimeter. This is so because the individual grains in such soils are somewhat mobile, so that the soil located close to the perimeter can shift very slightly outward in the direction of lower soil stresses. In contrast, in clay soils

15、 pressures are higher near the edge than at the center of the footing, since in such soils the load produces a shear resistance around the perimeter which adds to the upward pressure. It is customary to disregard these nonuniformities because their numerical amount is uncertain and highly variable,

16、depending on type of soil, and because their influence on the magnitudes of bending moments and shearing forces in the footing is relatively small.On compressible soils footings should be loaded concentrically to avoid tilting, which will result if bearing pressures are significantly larger under on

17、e side of the footing than under the opposite side. This means that single footings should be placed concentrically under the columns and wall footing concentrically under the walls and that for combine footings the controlled of the footings area should coincide with the resultant of the column loa

18、ds. Eccentrically loaded footings can be used on highly compacted soils and on rock. It follow that one should count on rotational restraint of the column by a single footing only when such favorable soil conditions are present and when the footing is designed both for the column load and the restra

19、ining moment. Even then, less than full fixity should be assumed, except for footings on rock.The accurate determination of stresses, particularly in single-column footings, is not practical, since they represent relatively massive blocks which cantilever from the column in all four directions. Unde

20、r uniform upward present they deform in a bowl shape, a fact which would greatly complicate an accurate stress analysis. For this reason present procedures for the design of such footings are based almost entirely on the results of two extensive experimental investigations, both carried out at the u

21、niversity of Illinois. These tests have been reevaluated, particularly in the light of newer concepts of strength in shear and diagonal tension.The AISC specification allows three types of construction in steel frames based on the type and behavior of the connections. Type 1, commonly designated as

22、“rigidframe”(continuous frame), assumes that beantocolumn connections have sufficient rigidity to hold virtually unchanged the original angles between intersecting members. Type 2, commonly designated as “simple” framing (unrestrained,free-ended), assumes that, in so far as gravity loading is concer

23、ned, the ends of beams and girders are connected for shear only, and are free to rotate under gravity load. Type 3, commonly designated as”semi-rigid” framing (partially restrained), assumes that the connections of beams and girders possess a dependent and known moment capacity 1 and the complete fl

24、exibility for Type 2.To provide lateral stiffness to the building, especially to those with frames of Type 2 construction, a system of bracing may be designed to resist lateral forces due to wind for earthquake . Wind loads acting on the exterior walls are transmitted by the floor system to braced b

25、ents or the shear walls in the steel framing system. If the floor is not sufficiently rigid to transform the loads, a horizontal bracing system muat be provided between bents.The two approaches used to obtain stiffness in regular-shaped buildings are shown in Fig.25-1.In the core-wall type, the late

26、ral load resisting system is integrated with a central utility core. In the bearing-wall type, the exterior walls are braced and stiffened such that the building acts like a huge cantilevered box. The use of the bearing-wall type of bents was a major factor in reducing the cost of the structural fra

27、ming for both the World Trade Center in New York and the John Hancock Building in Chicago.Bents may be braced in a number of ways. The full diagonal bracing is the most economical type of wind bracing. Its use is somewhat restricted by frequent requirements for window and door openings. Portal braci

28、ng is usually more costly and its use is limited to special cases. The Vierendeel type of braced bents has the advantage of being easily treated architecturally and can be used on the outside walls as part of the building faade.Floor system. The floor system of the multi-story building is a more ela

29、borate construction than the type used in industrial buildings. It includes a framing of girder and beams which supports the floor deck. Several types of floor decks are availability to the application of finished floors and ceilings and to the installation of utilities. The selection of a suitable

30、type of floor deck depends on the occupancy requirements, structural adequacy, and cost considerations.One of the lightest types of floor construction is the open-web joist system. The systems consist of a 2 to 20.5. Concrete slab poured in place over ribbed-wire fabric backed by heavy paper laid di

31、rectly on the joists. The satisfactory design and construction of the joist is assured by adhering to the specifications of the steel Joist Institute. Several types of prefabricated joists are available.Cellular steel floors have weights comparable to the open-web joist system. The concrete on Altho

32、ugh there have been many advancements in building construction technology in general. Spectacular archievements have been made in the design and construction of ultrahigh-rise buildings.The early development of high-rise buildings began with structural steel framing.Reinforced concrete and stressed-

33、skin tube systems have since been economically and competitively used in a number of structures for both residential and commercial purposes.The high-rise buildings ranging from 50 to 110 stories that are being built all over the United States are the result of innovations and development of new str

34、uctual systems.Greater height entails increased column and beam sizes to make buildings more rigid so that under wind load they will not sway beyond an acceptable limit.Excessive lateral sway may cause serious recurring damage to partitions, ceilings.and other architectural details. In addition,exce

35、ssive sway may cause discomfort to the occupants of the building because their perception of such motion.Structural systems of reinforced concrete,as well as steel,take full advantage of inherent potential stiffness of the total building and therefore require additional stiffening to limit the sway.

36、In a steel structure,for example,the economy can be defined in terms of the total average quantity of steel per square foot of floor area of the building.Curve A in Fig .1 represents the average unit weight of a conventional frame with increasing numbers of stories. Curve B represents the average st

37、eel weight if the frame is protected from all lateral loads. The gap between the upper boundary and the lower boundary represents the premium for height for the traditional column-and-beam frame.Structural engineers have developed structural systems with a view to eliminating this premium.Systems in

38、 steel. Tall buildings in steel developed as a result of several types of structural innovations. The innovations have been applied to the construction of both office and apartment buildings.Frame with rigid belt trusses. In order to tie the exterior columns of a frame structure to the interior vert

39、ical trusses,a system of rigid belt trusses at mid-height and at the top of the building may be used. A good example of this system is the First Wisconsin Bank Building (1974) in Milwaukee.Framed tube. The maximum efficiency of the total structure of a tall building, for both strength and stiffness,

40、to resist wind load can be achieved only if all column element can be connected to each other in such a way that the entire building acts as a hollow tube or rigid box in projecting out of the ground. This particular structural system was probably used for the first time in the 43-story reinforced c

41、oncrete DeWitt Chestnut Apartment Building in Chicago. The most significant use of this system is in the twin structural steel towers of the 110-story World Trade Center building in New YorkColumn-diagonal truss tube. The exterior columns of a building can be spaced reasonably far apart and yet be m

42、ade to work together as a tube by connecting them with diagonal members interesting at the centre line of the columns and beams. This simple yet extremely efficient system was used for the first time on the John Hancock Centre in Chicago, using as much steel as is normally needed for a traditional 4

43、0-story building.Bundled tube. With the continuing need for larger and taller buildings, the framed tube or the column-diagonal truss tube may be used in a bundled form to create larger tube envelopes while maintaining high efficiency. The 110-story Sears Roebuck Headquarters Building in Chicago has

44、 nine tube, bundled at the base of the building in three rows. Some of these individual tubes terminate at different heights of the building, demonstrating the unlimited architectural possibilities of this latest structural concept. The Sears tower, at a height of 1450 ft(442m), is the worlds talles

45、t building.Stressed-skin tube system. The tube structural system was developed for improving the resistance to lateral forces (wind and earthquake) and the control of drift (lateral building movement ) in high-rise building. The stressed-skin tube takes the tube system a step further. The developmen

46、t of the stressed-skin tube utilizes the faade of the building as a structural element which acts with the framed tube, thus providing an efficient way of resisting lateral loads in high-rise buildings, and resulting in cost-effective column-free interior space with a high ratio of net to gross floo

47、r area.Because of the contribution of the stressed-skin faade, the framed members of the tube require less mass, and are thus lighter and less expensive. All the typical columns and spandrel beams are standard rolled shapes, minimizing the use and cost of special built-up members. The depth requirem

48、ent for the perimeter spandrel beams is also reduced, and the need for upset beams above floors, which would encroach on valuable space, is minimized. The structural system has been used on the 54-story One Mellon Bank Center in Pittburgh.Systems in concrete. While tall buildings constructed of stee

49、l had an early start, development of tall buildings of reinforced concrete progressed at a fast enough rate to provide a competitive chanllenge to structural steel systems for both office and apartment buildings.Framed tube. As discussed above, the first framed tube concept for tall buildings was used for the 43-story DeWitt Chestnut Apartment Building. In this building, exterior columns were spaced at 5.5ft (1.68m) centers, and interior columns were used as needed to support the 8-in . -thick (20-m) flat-plate