连接的优雅

张利/ZHANG Li



连接的优雅

The Elegance of Connecting

张利/ZHANG Li

作者单位：清华大学建筑学院 /《世界建筑》

从人类建造的历史看，桥实在是一种难得的建筑物类型，即无论何处，它始终与正向的、积极的含义联系在一起，受到技术、美学与伦理的多重眷顾。多少个世纪以来，桥帮助人们的足迹跨越鸿沟与阻隔，随之而来的是物品的交换、文化的沟通与知识的传播。

步行桥是较近时期的产物，它最初是应对现代城市中步行交通被机动车交通边缘化的问题，但随着时间的发展，它已远远超出了维系步行交通生存空间的意义。通过与若干当代进步理念的结合，步行桥极大地拓展了自古以来的桥的正面形象，造就了一系列当代城市与自然空间中令人难忘的景观。

步行桥是慢行系统在当代城市中重获优先地位的见证。如果说出现于20世纪的大都市商业区的步行街仅仅是对步行者的经济效益所进行的开发的话，21世纪所出现的大量非商业区的步行桥则是对步行作为优先的城市移动方式的真正推崇。20世纪初期以后，现代功能主义城市教条所倡导的基础设施优先与工具理性在不同的大洲把城市的地面步行系统切割得支离破碎，那些被动的、用以取代人行横道而存在的过街天桥也无非是机动车对步行者空间的再一次压榨。这些都必然地激起了人文主义者的愤怒。早在1960年代，德·波的心理地理漂移、史密斯森夫妇的第二层城市地面理想就形成了今天慢行城市理念的先导。虽然其乌托邦式的解决方案不大可能被带入到现实之中，但优先于机动车交通的连续步行网络逐渐成为了人们的共识，也成为了21世纪城市以人为本的前提。最先代表了这一共识在当代城市开花结果的，便是跨越机动交通干线的或连通被隔绝的步行系统的步行桥。在这方面，我们不仅能看到2006年都灵奥运村引桥、西雅图奥林匹克雕塑公园之类的高显现度的案例，也能看到像本期收录的布鲁日铁匠城门步行桥、斯特拉斯堡的米罗步行桥、韦德步行桥这样的真正服务于市民日常生活的案例。

步行桥是当代人工环境对自然环境表达谦恭的方式。在短短的一个多世纪的时间内，人对在城市环境中引入自然或将人工环境从城市延伸至自然的态度发生了迅速的转变。从绝对的人定胜天——如19世纪西方各大都市里的风格化花园，到并置式的共存——如波士顿的“翡翠项链”公园带，再到最小化的轻触式干预——如新加坡的“林中漫步”公园，可以认为，21世纪的今天，人们最希望看到的是人工环境对自然环境的绝对谦恭。不仅建造的材料从厚重走向轻盈，而且干预的位置也从地面移到空中——显然，把地面留给生态系统的原貌是件非常重要的事。由此可见，自然景观中的步行桥是必要的选择。本斯所收录的盖尔森基兴的莱因–赫恩运河步行桥、加德门的特里夫特山中悬索步行桥和温哥华的卡谱兰诺悬崖步道皆是这方面的例子。

步行桥是当代结构技术与建造工艺的结晶。桥梁本身就是结构工程师的最高舞台，而步行桥因其相对较小的规模、相对简单的功能与荷载需求，以及高度视觉化的景观属性，勿庸质疑地成为了桥梁工程中最精致的领域。我们看到，无论是在城市还是在自然之中，步行桥的设计与建造者们都在不知疲倦地诠释着一个可望而不可及的理想——一道悬浮于空中的优雅线条。压力与弯矩被最大限度地转换为拉力，从而使重力的传递得以隐晦地表达。从以显著的悬索柱的纵向存在换来水平桥面的轻盈与动感，如本期收录的德利-伦敦德利的和平桥；到以上翻的弯矩图形成的桥体外形，如巴克南的布莱希维泽步行桥；到以自然的悬垂线定义桥身，再利用山谷的空间装置对拉的侧向稳定索，如德国的盖尔利桥；等等。

无论何处，总会有一座步行桥在向我们展示连接的优雅。

特别感谢本期的客座编辑古斯塔夫·安布罗西尼教授，他的鼎力支持使本期《世界建筑》成为可能。

Looking at the history of human construction,it is not hard to see that bridges are an unusual type,a type which has no negative associations at all.For centuries,bridges have been blessed by technology,aesthetics and ethics,all at the same time.This is something that can hardly be said to other types of buildings.Wherever and whenever there is a bridge,it is immediately supposed that there is a connection over a certain gap,and there are expected positive qualities such as trade,communication and the spread of knowledge.

Footbridge is a rather recent thing.Originally made to tackle the marginalisation of pedestrians in modern cities,it has made a huge leap in a short time,way beyond the initial role of keeping survival of pedestrian mobility.Over the last few decades,the concept of footbridge has pleasantly cross-fertilised with a series of progressive ideas and has dramatically expanded the positive persona of all bridges,delivering impressive intervention in urban or natural setups time after time.

Footbridge is a key evidence of the prioritisation of slow mobility in contemporary cities.If in shopping streets of the last century we saw an exploitation of the pedestrian economy,in footbridges of this century we see a genuine endorsement of the pedestrian-centred urban life.In early 20th century,modernists' prioritised infrastructure and mechanics with foolhardy technocracy,even the footbridges at crossings were poor devices to yield more ground space to vehicular traffic.Angered humanists took revenge.Debord's psychogeographic drift and the Smithsons' second urban ground were radical ideas pioneering today's slow mobility thought.Although utopianism never had a chance in reality,it helped to create the consensus of continuous pedestrian network today.Representing the blooming of this consensus in the 21st century are the footbridges in world cities,those that either bridge a gap made by vehicular passage or connecting broken pedestrian lines.In this regard,we not only see high profile examples such as the Olympic village footbridge of Turin 2006 and the Olympic Sculpture Garden in Seattle,but also see convincing example that are well integrated in day to day city life,such as the Smedenpoort Footbridges in Bruges,and the Mirò Footbridge in Strasbourg,both included in this issue.

Footbridge is a good way of expressing to nature the humility of the artificial.Man's attitude to nature has changed dramatically over the last two centuries.It used to be a sheer pride of control,as presented by the 19th century geometric gardens.It turned to be a delicate juxtaposition,as presented by the Emerald Necklace in Boston.It is now simply a minimisation of human footprint,as presented by the Forrest Walk in Singapore.We can safely assume that today people want to see nature being respected as much as possible.Not only constructions in a natural setup get lighter and lighter,the use of ground surface also gets less and less.Leaving more surface area of the earth to its native ecology is important.Thus,footbridges become an indispensable option if you want to host pedestrians in nature.A number of examples are included in this issue,such as the Rhine-Herne Canal Footbridge in Gelsenkirken and the Trift Bridge in Gadmen.

Footbridge is a condensation of stateof-the-art structural technology and cuttingedge design and making.Bridges are high playgrounds to engineers.Footbridges,due to their relatively small size,simple load-bearing and functional requirements,and unmistakable visual importance,have become true delicacies of bridge design.In the cities and in nature,we witness bridge designers relentlessly reaching for one unachievable dream – to make a bridge an elegant line free-floating in the air.Pressures and momentums are converted to tensions as much as possible.The path gravity is made in the most unexpected way.There are bridges emphasizing powerful vertical masts trading for the light dynamics of the deck,such as the Peace Bridge in Derry-Londonderry.There are bridges adopting the flipped momentum graph as their shapes,such as the Bleichwiese Footbridge in Backnang.There are bridges minimising their masses utilising the natural suspension curve,assisted by stabling cables in tension from the valley below,such as the Geierley Bridge.

Wherever you go,there will be a footbridge demonstrating the elegance of connecting.

Our special thanks to our guest editor,Prof.Gustavo Ambrosini,whose full support has made this issue of World Architecture possible.

收稿日期：2016-03-10