高层结构与钢结构
近年来，尽管一般的建筑结构设计取得了很大的进步，但是取得显著成绩的还要属超高层建筑结构设计。
最初的高层建筑设计是从钢结构的设计开始的。钢筋混凝土和受力外包钢筒系统运用起来是比较经济的系统，被有效地运用于大批的民用建筑和商业建筑中。50层到100层的建筑被定义为超高层建筑。而这种建筑在美国得广泛的应用是由于新的结构系统的发展和创新。
这样的高度需要增大柱和梁的尺寸，这样以来可以使建筑物更加坚固以至于在允许的限度范围内承受风荷载而不产生弯曲和倾斜。过分的倾斜会导致建筑的隔离构件、顶棚以及其他建筑细部产生循环破坏。除此之外，过大的摇动也会使建筑的使用者们因感觉到这样的的晃动而产生不舒服的感觉。无论是钢筋混凝土结构系统还是钢结构系统都充分利用了整个建筑的刚度潜力，因此不能指望利用多余的刚度来限制侧向位移。
在钢结构系统设计中，经济预算是根据每平方英寸地板面积上的钢材的数量确定的。图示1中的曲线A显示了常规框架的平均单位的重量随着楼层数的增加而增加的情况。而曲线B显示则显示的是在框架被保护而不受任何侧向荷载的情况下的钢材的平均重量。上界和下界之间的区域显示的是传统梁柱框架的造价随高度而变化的情况。而结构工程师改进结构系统的目的就是减少这部分造价。
钢结构中的体系：钢结构的高层建筑的发展是几种结构体系创新的结果。这些创新的结构已经被广泛地应用于办公大楼和公寓建筑中。
刚性带式桁架的框架结构：为了联系框架结构的外柱和内部带式桁架，可以在建筑物的中间和顶部设置刚性带式桁架。1974年在米望基建造的威斯康森银行大楼就是一个很好的例子。
框架筒结构： 如果所有的构件都用某种方式互相联系在一起，整个建筑就像是从地面发射出的一个空心筒体或是一个刚性盒子一样。这个时候此高层建筑的整个结构抵抗风荷载的所有强度和刚度将达到最大的效率。这种特殊的结构体系首次被芝加哥的43层钢筋混凝土的德威特红棕色的公寓大楼所采用。但是这种结构体系的的所有应用中最引人注目的还要属在纽约建造的100层的双筒结构的世界贸易中心大厦。
斜撑桁架筒体： 建筑物的外柱可以彼此独立的间隔布置，也可以借助于通过梁柱中心线的交叉的斜撑构件联系在一起，形成一个共同工作的筒体结构。这种高度的结构体系首次被芝加哥的John Hancock 中心大厦采用。这项工程所耗用的刚才量与传统的四十层高楼的用钢量相当。
筒体： 随着对更高层建筑的要求不断地增大。筒体结构和斜撑桁架筒体被设计成捆束状以形成更大的筒体来保持建筑物的高效能。芝加哥的110层的Sears Roebuck 总部大楼有9个筒体，从基础开始分成三个部分。这些独立筒体中的终端处在不同高度的建筑体中，这充分体现出了这种新式结构观念的建筑风格自由化的潜能。这座建筑物1450英尺（442米）高，是世界上最高的大厦。
薄壳筒体系统：这种筒体结构系统的设计是为了增强超高层建筑抵抗侧力的能力（风荷载和地震荷载）以及建筑的抗侧移能力。薄壳筒体是筒体系统的又一大飞跃。薄壳筒体的进步是利用高层建筑的正面（墙体和板）作为与筒体共同作用的结构构件，为高层建筑抵抗侧向荷载提供了一个有效的途径，而且可获得不用设柱，成本较低，使用面积与建筑面积之比又大的室内空间。
由于薄壳立面的贡献，整个框架筒的构件无需过大的质量。这样以来使得结构既轻巧又经济。所有的典型柱和窗下墙托梁都是轧制型材，最大程度上减小了组合构件的使用和耗费。托梁周围的厚度也可适当的减小。而可能占据宝贵空间的墙上镦梁的尺寸也可以最大程度地得到控制。这种结构体系已被建造在匹兹堡洲的One Mellon银行中心所运用。
钢筋混凝土中的各体系：虽然钢结构的高层建筑起步比较早，但是钢筋混凝土的高层建筑的发展非常快，无论在办公大楼还是公寓住宅方面都成为刚结构体系的有力竞争对手。
框架筒：像上面所提到的，框架筒构思首次被43层的迪威斯公寓大楼所采用。在这座大楼中，外柱的柱距为5.5英尺（1.68米）。而内柱则需要支撑8英寸厚的无梁板。
筒中筒结构：另一种针对于办公大楼的钢筋混凝土体系把传统的剪力墙结构与外框架筒相结合。该体系由柱距很小的外框架与围绕中心设备区的刚性剪力墙筒组成。这种筒中筒结构（如插图2）使得当前世界上最高的轻质混凝土大楼（在休斯顿建造的独壳购物中心大厦）的整体造价只与35层的传统剪力墙结构相当。
钢结构与混凝土结构的联合体系也有所发展。Skidmore ,Owings 和Merrill共同设计的混合体系就是一个好例子。在此体系中，外部的混凝土框架筒包围着内部的钢框架，从而结合了钢筋混凝土体系与钢结构体系各自的优点。在新奥尔良建造的52层的独壳广场大厦就是运用了这种体系。
钢结构是指在建筑物结构中钢材起着主导作用的结构，是一个很宽泛的概念。大部分的钢结构都包括建筑设计，工程技术、工艺。通常还包括以主梁、次梁、杆件，板等形式存在的钢的热轧加工工艺。上个世纪七十年代，除了对其他材料的需求在增长，钢结构仍然保持着对于来自美国、英国、日本、西德、法国等国家的钢材厂钢材的大量需求。
发展历史：早在Bessemer和Siemens-Marton(开放式炉)工艺出现以前，钢结构就已经有几十年的历史了。而直到此工艺问世之后才使得钢材可以大批生产出来供结构所用。对钢结构诸多问题的研究开始于铁结构的使用，当时很著名的研究对象是1977年在英国建造的横跨斯沃河的Coalbrook dale 大桥。这座大桥以及后来的铁桥设计再加上蒸汽锅炉、铁船身的设计都刺激了建筑安装设计以及连接工艺的发展。铁结构对材料的需求量较小是优胜于砖石结构的主要方面。长久以来一直用木材制作的三角桁架也换成铁制的了。承受由直接荷载产生的重力作用的受压构件常用铸铁制造，而承受由悬挂荷载产生的推力作用的受拉构件常用熟铁制造。
把铁加热到塑性状态，使之从卷状转化为扁平状与圆状之间的某一状态的工艺，早在1800年就得以发展了。随后，1819年角钢问世，1894年第一个工字钢被建造出来作为巴黎火车站的顶梁。此工字钢长17.7英尺）（5.4米）。
1851年英国的Joseph Paxtond为伦敦博览会建造了水晶宫。据说当时他已有这样的骨架结构构思：用比较细的铁梁作为玻璃幕墙的骨架。此建筑的风荷载抵抗力是由对角拉杆所提供的。在金属结构的发展历史中，有两个标志性事件：首先是从木桥发展而来的格构梁由木制转化为铁制；其次是锻铁制的受拉构件与铸铁制的受压构件受热后通过铆钉连接工艺的发展。
十九世纪五六十年代，Bessemer 与 Siemens-Martin工艺的发展使钢材的生产能满足结构的需求。钢的受拉强度与受压强度都好于铁。这种新型的金属常被有想象力的工程师所利用，尤其倍受那些参与过英国、欧洲以及美国的道桥建设的工程师的喜爱。
其中一个很好的例子就是Eads大桥（也被称为路易斯洲大桥）（1867-1874）。在这座大桥中，每隔500英尺（152.5米）设有由钢管加强肋形成的拱。英国的Firth of Forth悬索桥设有管件支撑，直径大约为12英尺（3.66米），长度为350英尺（107）米。这些大桥以及其他结构在引导钢结构的发展，规范的实施，许用应力的设计方面起到了很重要的作用。1907年Quebec悬索大桥的偶然破坏揭露了二十世纪初期由于缺乏足够的理论知识，甚至是缺乏足够的理论研究的基础知识，而导致在应力分析方面出现了很多的不足。但是，这样的损坏却很少出现在金属骨架的办公大楼中。因为尽管在缺乏缜密的分析的情况下，这些建筑也表现出了很高的实用性。在上个世纪中叶，没有经过任何特殊合金强化、硬化过的普通碳素钢已经被广泛地使用了。
在1889年巴黎召开的世界博览会上，金属结构表现出了在超高层建筑运用上的内在潜力。在这次会上，法国著名的桥梁设计师埃非尔展示了他的杰作-300米高的露天开挖的铁塔。无论是它的高度（比著名的金字塔的两倍还高），架设的速度-人数不多的工作人员仅用几个月的时间就完成了整个工程任务，还是很低的工程造价都使它脱颖而出。
首批摩天大厦：在刚结构发展的同时，美国的另一个是也蓬勃的发展起来了。1884-1885年，芝加哥的工程师Maj.William Le Baron Jennny设计了家庭保险公司大厦。这座大厦也是金属结构的，有十层高。大厦的梁是钢制的，而柱是铸铁所制。铸铁制的过梁支撑着窗洞口上方的砌体，同时也需要铸铁制的柱支撑着。实心砌体的天井与界墙提供抵抗风载的侧向支撑。不到十年的功夫，芝加哥和纽约已经有超过30座办公大楼是利用这种结构。钢材在这些结构中起了非常大的作用。这种结构利用铆钉把梁与柱连接在一起。有时为了抵抗风荷载还是在竖向构件和横向构件的连接点出贴覆上节点板来加固结构。此外，轻型的玻璃幕墙结构代替了老式的重质砌体结构。
尽管几十年来之中建筑形式主要是在美国发展的，但是它却影响着全世界钢材工业的发展。十九世纪的最后几年，基本结构形状工字型钢的厚度已经达到20英寸（0.508米），非对称的Z字型钢和T型钢可以与有一定宽度和厚度的板相联结，使得构件具体符合要求的尺寸和强度。1885年最重的型钢通过热轧生产出来，每英寸不到100磅（45千克）。到二十世纪六十年代这个数字已经达到每英寸700磅（320千克）。
紧随着钢结构的发展，1988年第一部电梯问世了。安全载客电梯诞生，以及安全经济的钢结构设计方法的发展促使建筑高度迅猛增加。1902年在纽约建造的高286英寸（87.2米）的Flatiron大厦不断地被后来的建筑所超越。这些建筑分别是高375英尺（115米）的时代大厦（1904），（后来改名为联合化工制品大厦）。1908年在华尔街建造的高468英尺（143米）的城市投资公司大厦，高612 英尺（187米）的星尔大厦，以及700英尺（214米）的都市塔和780英尺高（232米）的Woll worth大厦。
房屋高度与高宽比的不断增加也带来了许多的问题。为了控制道路的阻塞，要对建筑的缩进设计进行限定。侧向支撑的设置也是其中一项技术问题，例如，埃非尔铁塔所采用的对角支撑体系对于要靠太阳光来照明的办公大厦就不实用了。而只有考虑到具体的单独梁与单独柱的抗弯能力以及梁柱相交处的刚度的框架设计才是可靠的。随着现代内部采光体系的不断发展，抵抗风荷载的对角支撑又重新被利用起来了。芝加哥的John Hancock 中心就是一个很显著的例子。外部的对角支撑成为此结构立面的一个很显眼的部分。
第一次世界大战暂时中断了所谓摩天大厦（当时这个词并没有确定）的蓬勃发展，但是二十世纪二十年代又恢复了这一趋势。1931年建造的帝国大厦把词潮流推向了顶峰。102层高1250英尺（381米）的帝国大厦在后来的40年一直保持着世界最高的地位。它的建造速度充分证明了这种新的结构形式已经被当时的技术所掌握。次项工程所需要的梁是由Bayonne海湾对岸的军械库所提供的。是由用精密仪器控制的驳船和卡车负责运输的。由九架起重机将这些梁提升到指定的位置。由工业轨道装置把钢材和其他材料移到每一层上去。先是螺栓连接紧接着铆钉连接，最后是装修，整个工程的最终完成只用了一年零45天。
二十世纪三十年代席卷全世界的大萧条以及第而次世界大战使钢结构的发展又一次受到了阻碍。但是与此同时，焊接代替了铆钉连接则是一个很重要的发展。
十九世纪末，利用焊接把各个钢零件相连接已取得了很好的成绩，并在第一次世界大战中被运用于救生船的修理。但直到第二次世界大战后才用于建筑结构中。同时在连接领域中又一进步就是高强螺栓代替了铆钉。
二战结束后，欧洲，美国，日本等国都扩大了对在不定应力（包括超过屈服点的情况）作用下各种结构钢的性质的研究，并进行了更为精确、系统的分析。此后，许多国家采用了一些更为自由灵活的设计规范和更为理想化的弹性设计规范。计算机在工程上的运用代替了冗长的手工计算，从而更加促进了钢结构的发展，并大大的减低了造价。
Talling building and Steel construction
Although 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-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 structual 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,excessive 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.
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 steel 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 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 vertical 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,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 concrete 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 York
Column-diagonal truss tube. The exterior columns of a building can be spaced reasonably far apart and yet be made 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 40-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 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 world’s tallest 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 development of the stressed-skin tube utilizes the façade 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 floor area.
Because of the contribution of the stressed-skin façade, 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 requirement 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 steel 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 concrete slabs.
Tube in tube. Another system in reinforced concrete for office buildings combines the traditional shear wall construction with an exterior framed tube. The system consists of an outer framed tube of very closely spaced columns and an interior rigid shear wall tube enclosing the central service area. The system (Fig .2), known as the tube-in-tube system , made it possible to design the world’s present tallest (714ft or 218m)lightweight concrete building ( the 52-story One Shell Plaza Building in Houston) for the unit price of a traditional shear wall structure of only 35 stories.
Systems combining both concrete and steel have also been developed, an examle of which is the composite system developed by skidmore, Owings &Merril in which an exterior closely spaced framed tube in concrete envelops an interior steel framing, thereby combining the advantages of both reinforced concrete and structural steel systems. The 52-story One Shell Square Building in New Orleans is based on this system.
Steel construction refers to a broad range of building construction in which steel plays the leading role. Most steel construction consists of large-scale buildings or engineering works, with the steel generally in the form of beams, girders, bars, plates, and other members shaped through the hot-rolled process. Despite the increased use of other materials, steel construction remained a major outlet for the steel industries of the U.S, U.K, U.S.S.R, Japan, West German, France, and other steel producers in the 1970s.
Early history. The history of steel construction begins paradoxically several decades before the introduction of the Bessemer and the Siemens-Martin (openj-hearth) processes made it possible to produce steel in quantities sufficient for structure use. Many of problems of steel construction were studied earlier in connection with iron construction, which began with the Coalbrookdale Bridge, built in cast iron over the Severn River in England in 1777. This and subsequent iron bridge work, in addition to the construction of steam boilers and iron ship hulls , spurred the development of techniques for fabricating, designing, and jioning. The advantages of iron over masonry lay in the much smaller amounts of material required. The truss form, based on the resistance of the triangle to deformation, long used in timber, was translated effectively into iron, with cast iron being used for compression members-i.e, those bearing the weight of direct loading-and wrought iron being used for tension members-i.e, those bearing the pull of suspended loading.
The technique for passing iron, heated to the plastic state, between rolls to form flat and rounded bars, was developed as early as 1800;by 1819 angle irons were rolled; and in 1849 the first I beams, 17.7 feet (5.4m) long , were fabricated as roof girders for a Paris railroad station.
Two years later Joseph Paxton of England built the Crystal Palace for the London Exposition of 1851. He is said to have conceived the idea of cage construction-using relatively slender iron beams as a skeleton for the glass walls of a large, open structure. Resistance to wind forces in the Crystal palace was provided by diagonal iron rods. Two feature are particularly important in the history of metal construction; first, the use of latticed girder, which are small trusses, a form first developed in timber bridges and other structures and translated into metal by Paxton ; and second, the joining of wrought-iron tension members and cast-iron compression members by means of rivets inserted while hot.
In 1853 the first metal floor beams were rolled for the Cooper Union Building in New York. In the light of the principal market demand for iron beams at the time, it is not surprising that the Cooper Union beams closely resembled railroad rails.
The development of the Bessemer and Siemens-Martin processes in the 1850s and 1860s suddenly open the way to the use of steel for structural purpose. Stronger than iron in both tension and compression ,the newly available metal was seized on by imaginative engineers, notably by those involved in building the great number of heavy railroad bridges then in demand in Britain, Europe, and the U.S.
A notable example was the Eads Bridge, also known as the St. Louis Bridge, in St. Louis (1867-1874), in which tubular steel ribs were used to form arches with a span of more than 500ft (152.5m). In Britain, the Firth of Forth cantilever bridge (1883-90) employed tubular struts, some 12 ft (3.66m) in diameter and 350 ft (107m) long. Such bridges and other structures were important in leading to the development and enforcement of standards and codification of permissible design stresses. The lack of adequate theoretical knowledge, and even of an adequate basis for theoretical studies, limited the value of stress analysis during the early years of the 20th century,as iccasionally failures,such as that of a cantilever bridge in Quebec in 1907,revealed.But failures were rare in the metal-skeleton office buildings;the simplicity of their design proved highly practical even in the absence of sophisticated analysis techniques. Throughout the first third of the century, ordinary carbon steel, without any special alloy strengthening or hardening, was universally used.
The possibilities inherent in metal construction for high-rise building was demonstrated to the world by the Paris Exposition of 1889.for which Alexandre-Gustave Eiffel, a leading French bridge engineer, erected an openwork metal tower 300m (984 ft) high. Not only was the height-more than double that of the Great Pyramid-remarkable, but the speed of erection and low cost were even more so, a small crew completed the work in a few months.
The first skyscrapers. Meantime, in the United States another important development was taking place. In 1884-85 Maj. William Le Baron Jenney, a Chicago engineer , had designed the Home Insurance Building, ten stories high, with a metal skeleton. Jenney’s beams were of Bessemer steel, though his columns were cast iron. Cast iron lintels supporting masonry over window openings were, in turn, supported on the cast iron columns. Soild masonry court and party walls provided lateral support against wind loading. Within a decade the same type of construction had been used in more than 30 office buildings in Chicago and New York. Steel played a larger and larger role in these , with riveted connections for beams and columns, sometimes strengthened for wind bracing by overlaying gusset plates at the junction of vertical and horizontal members. Light masonry curtain walls, supported at each floor level, replaced the old heavy masonry curtain walls, supported at each floor level , replaced the old heavy masonry.
Though the new construction form was to remain centred almost entirely in America for several decade, its impact on the steel industry was worldwide. By the last years of the 19th century, the basic structural shapes-I beams up to 20 in. ( 0.508m) in depth and Z and T shapes of lesser proportions were readily available, to combine with plates of several widths and thicknesses to make efficient members of any required size and strength. In 1885 the heaviest structural shape produced through hot-rolling weighed less than 100 pounds (45 kilograms) per foot; decade by decade this figure rose until in the 1960s it exceeded 700 pounds (320 kilograms) per foot.
Coincident with the introduction of structural steel came the introduction of the Otis electric elevator in 1889. The demonstration of a safe passenger elevator, together with that of a safe and economical steel construction method, sent building heights soaring. In New York the 286-ft (87.2-m) Flatiron Building of 1902 was surpassed in 1904 by the 375-ft (115-m) Times Building ( renamed the Allied Chemical Building) , the 468-ft (143-m) City Investing Company Building in Wall Street, the 612-ft (187-m) Singer Building (1908), the 700-ft (214-m) Metropolitan Tower (1909) and, in 1913, the 780-ft (232-m) Woolworth Building.
The rapid increase in height and the height-to-width ratio brought problems. To limit street congestion, building setback design was prescribed. On the technical side, the problem of lateral support was studied. A diagonal bracing system, such as that used in the Eiffel Tower, was not architecturally desirable in offices relying on sunlight for illumination. The answer was found in greater reliance on the bending resistance of certain individual beams and columns strategically designed into the skeletn frame, together with a high degree of rigidity sought at the junction of the beams and columns. With today’s modern interior lighting systems, however, diagonal bracing against wind loads has returned; one notable example is the John Hancock Center in Chicago, where the external X-braces form a dramatic part of the structure’s façade.
World War I brought an interruption to the boom in what had come to be called skyscrapers (the origin of the word is uncertain), but in the 1920s New York saw a resumption of the height race, culminating in the Empire State Building in the 1931. The Empire State’s 102 stories (1,250ft. [381m]) were to keep it established as the hightest building in the world for the next 40 years. Its speed of the erection demonstrated how thoroughly the new construction technique had been mastered. A depot across the bay at Bayonne, N.J., supplied the girders by lighter and truck on a schedule operated with millitary precision; nine derricks powerde by electric hoists lifted the girders to position; an industrial-railway setup moved steel and other material on each floor. Initial connections were made by bolting , closely followed by riveting, followed by masonry and finishing. The entire job was completed in one year and 45 days.
The worldwide depression of the 1930s and World War II provided another interruption to steel construction development, but at the same time the introduction of welding to replace riveting provided an important advance.
Joining of steel parts by metal are welding had been successfully achieved by the end of the 19th century and was used in emergency ship repairs during World War I, but its application to construction was limited until after World War II. Another advance in the same area had been the introduction of high-strength bolts to replace rivets in field connections.
Since the close of World War II, research in Europe, the U.S., and Japan has greatly extended knowledge of the behavior of different types of structural steel under varying stresses, including those exceeding the yield point, making possible more refined and systematic analysis. This in turn has led to the adoption of more liberal design codes in most countries, more imaginative design made possible by so-called plastic design ?The introduction of the computer by short-cutting tedious paperwork, made further advances and savings possible.
