張利/ZHANG Li

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連接的優(yōu)雅

The Elegance of Connecting

張利/ZHANG Li

作者單位：清華大學建筑學院 /《世界建筑》

從人類建造的歷史看，橋?qū)嵲谑且环N難得的建筑物類型，即無論何處，它始終與正向的、積極的含義聯(lián)系在一起，受到技術(shù)、美學與倫理的多重眷顧。多少個世紀以來，橋幫助人們的足跡跨越鴻溝與阻隔，隨之而來的是物品的交換、文化的溝通與知識的傳播。

步行橋是較近時期的產(chǎn)物，它最初是應對現(xiàn)代城市中步行交通被機動車交通邊緣化的問題，但隨著時間的發(fā)展，它已遠遠超出了維系步行交通生存空間的意義。通過與若干當代進步理念的結(jié)合，步行橋極大地拓展了自古以來的橋的正面形象，造就了一系列當代城市與自然空間中令人難忘的景觀。

步行橋是慢行系統(tǒng)在當代城市中重獲優(yōu)先地位的見證。如果說出現(xiàn)于20世紀的大都市商業(yè)區(qū)的步行街僅僅是對步行者的經(jīng)濟效益所進行的開發(fā)的話，21世紀所出現(xiàn)的大量非商業(yè)區(qū)的步行橋則是對步行作為優(yōu)先的城市移動方式的真正推崇。20世紀初期以后，現(xiàn)代功能主義城市教條所倡導的基礎(chǔ)設(shè)施優(yōu)先與工具理性在不同的大洲把城市的地面步行系統(tǒng)切割得支離破碎，那些被動的、用以取代人行橫道而存在的過街天橋也無非是機動車對步行者空間的再一次壓榨。這些都必然地激起了人文主義者的憤怒。早在1960年代，德·波的心理地理漂移、史密斯森夫婦的第二層城市地面理想就形成了今天慢行城市理念的先導。雖然其烏托邦式的解決方案不大可能被帶入到現(xiàn)實之中，但優(yōu)先于機動車交通的連續(xù)步行網(wǎng)絡(luò)逐漸成為了人們的共識，也成為了21世紀城市以人為本的前提。最先代表了這一共識在當代城市開花結(jié)果的，便是跨越機動交通干線的或連通被隔絕的步行系統(tǒng)的步行橋。在這方面，我們不僅能看到2006年都靈奧運村引橋、西雅圖奧林匹克雕塑公園之類的高顯現(xiàn)度的案例，也能看到像本期收錄的布魯日鐵匠城門步行橋、斯特拉斯堡的米羅步行橋、韋德步行橋這樣的真正服務于市民日常生活的案例。

步行橋是當代人工環(huán)境對自然環(huán)境表達謙恭的方式。在短短的一個多世紀的時間內(nèi)，人對在城市環(huán)境中引入自然或?qū)⑷斯きh(huán)境從城市延伸至自然的態(tài)度發(fā)生了迅速的轉(zhuǎn)變。從絕對的人定勝天——如19世紀西方各大都市里的風格化花園，到并置式的共存——如波士頓的“翡翠項鏈”公園帶，再到最小化的輕觸式干預——如新加坡的“林中漫步”公園，可以認為，21世紀的今天，人們最希望看到的是人工環(huán)境對自然環(huán)境的絕對謙恭。不僅建造的材料從厚重走向輕盈，而且干預的位置也從地面移到空中——顯然，把地面留給生態(tài)系統(tǒng)的原貌是件非常重要的事。由此可見，自然景觀中的步行橋是必要的選擇。本斯所收錄的蓋爾森基興的萊因–赫恩運河步行橋、加德門的特里夫特山中懸索步行橋和溫哥華的卡譜蘭諾懸崖步道皆是這方面的例子。

步行橋是當代結(jié)構(gòu)技術(shù)與建造工藝的結(jié)晶。橋梁本身就是結(jié)構(gòu)工程師的最高舞臺，而步行橋因其相對較小的規(guī)模、相對簡單的功能與荷載需求，以及高度視覺化的景觀屬性，勿庸質(zhì)疑地成為了橋梁工程中最精致的領(lǐng)域。我們看到，無論是在城市還是在自然之中，步行橋的設(shè)計與建造者們都在不知疲倦地詮釋著一個可望而不可及的理想——一道懸浮于空中的優(yōu)雅線條。壓力與彎矩被最大限度地轉(zhuǎn)換為拉力，從而使重力的傳遞得以隱晦地表達。從以顯著的懸索柱的縱向存在換來水平橋面的輕盈與動感，如本期收錄的德利-倫敦德利的和平橋；到以上翻的彎矩圖形成的橋體外形，如巴克南的布萊希維澤步行橋；到以自然的懸垂線定義橋身，再利用山谷的空間裝置對拉的側(cè)向穩(wěn)定索，如德國的蓋爾利橋；等等。

無論何處，總會有一座步行橋在向我們展示連接的優(yōu)雅。

特別感謝本期的客座編輯古斯塔夫·安布羅西尼教授，他的鼎力支持使本期《世界建筑》成為可能。

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Originally made to tackle the marginalisation of pedestrians in modern cities,it has made a huge leap in a short time,way beyond the initial role of keeping survival of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