A bridge too far
Crumbling infrastructure worries civil engineers
THE first bridges were likely to have been built by early man shoving a fallen tree across a stream. Since then, construction techniques have come on a bit—from wood to stone, wrought iron and then steel. In the 20th century, reinforced concrete appeared. Concrete is an immensely strong material, especially when coupled with steel. But the sudden collapse of the Morandi bridge in Genoa this week (pictured), with a tragic loss of life, adds to the concern of civil engineers that many bridges around the world which use reinforced concrete are deteriorating faster than was expected.
The Genoa bridge is based on a design called a cable-stayed bridge, although it is a somewhat unusual variant. Such a bridge uses one or more towers, from which run cables that support the deck of the bridge. This is different from a suspension bridge, such as the Golden Gate Bridge in San Francisco, in which the cables holding up the deck are suspended vertically from a main cable anchored at either end of the bridge. Cable-stayed bridges are widely used, mainly for spans shorter than those crossed in one go by a suspension bridge.
斜拉桥的一个常见特征是：从桥塔斜拉出的钢索形成一个像扇子的图案。通常情况下，如果出现某一根钢索损坏或断裂，剩余的钢索应该能够承担设计荷载，以保持桥体结构的稳定。 Morandi桥不同之处在于，它是由预应力钢筋混凝土作为拉索来支撑桥面，而不是钢索。由钢丝绳组成钢筋束，经过张拉后置入混凝土中，待混凝土凝固后对其产生压力，以提高混凝土的抗压强度。这座桥由该类桥梁的支持者Riccardo Morandi设计。世界上只建造了少数几个此类桥梁。
A familiar feature of a cable-stayed bridge is that the cables form a fan-like pattern emanating from the supporting tower. If one of the cables is damaged or breaks, it should be obvious; the loading on the bridge is calculated so that the remaining cables will be capable of holding the structure up. The Morandi bridge is different because it was supported by pre-stressed concrete tendons. The tendons are made from bundles of steel wires tightened to produce compressive strength and then encased in concrete. The bridge was designed by Riccardo Morandi, a proponent of this type of bridge. Only a few have been built around the world.
过去曾提出过对热那亚桥梁的担忧。据意大利媒体报道，2016年，热那亚大学钢筋混凝土专家Antonio Brencich将这座桥描述为“失败的工程”，迟早将被更换。英国斯特拉斯克莱德大学的土木工程专家Daniele Zonta表示，自1967年桥梁开通以来，钢筋束需要持续监测和维护。
Concerns about Genoa’s bridge had been raised in the past. The Italian media has reported that in 2016, Antonio Brencich, a specialist in reinforced concrete at the University of Genoa, described the bridge as a “failure of engineering” and that sooner or later it would have to be replaced. Daniele Zonta, a civil-engineering expert at the University of Strathclyde, in Britain, says that since the opening of the bridge in 1967 the tendons have required continuous monitoring and maintenance.
Although the design of the bridge is unusual, it is much too early to say if that played any fundamental part in the collapse. And in other respects, the Morandi bridge is far from atypical. All around the world bridges built long ago, particularly those using reinforced concrete, are deteriorating. Even back in 1999, a study found that around 30% of road bridges in Europe had some sort of defect, particularly corrosion of their steel reinforcing or pre-stressed tendons.
A report from the American Road & Transportation Builders Association in January is even more sobering. It reckoned that 54,259 of that country’s 612,677 bridges are “structurally deficient”. These problem bridges have an average age of 67 years and are crossed by vehicles 174m times every day. At the present rate of repair and replacement, it will take 37 years to remedy all the problems, says Alison Premo Black, the organisation’s chief economist.
What is going wrong with these bridges? The difficulty is that concrete, or rather the steel used to reinforce it, can fail in a number of ways. Salt, ice and the pounding of weather can cause tiny fractures in the concrete’s surface. As these cracks creep inward, they let in water. Once the water reaches the steel reinforcing or tendons, it corrodes them. This enlarges the cracks, which can cause the concrete to fall apart. That this is happening is evident from rusty streaks on crumbling concrete.
Other factors compound the deterioration of bridges, such as a constant cyclic vibration from traffic, says Mehdi Kashani, an expert in structural mechanics at the University of Southampton, in Britain. This is troublesome for bridges designed in the 1960s, when traffic flows were lower, cars were smaller and lorries much lighter. On top of that, extreme weather can take a toll, with heat and cold expanding and contracting the structure, floods eroding away foundations and high winds buffeting the bridge. This is why regular inspections and maintenance are essential.
监控结构的新方法可以帮助工程师防微杜渐。无人机可以轻松拍摄桥梁任何部分的细节照片，而无需攀爬桥梁或架设脚手架。 电子传感器可以定期读取结构中的任何位移。 激光扫描仪能够呈现清晰的细节并三维成像。 所有这一切都应该有所帮助，但前提是必须有制度来确保详细监测和预防性维护的执行。 如果由于某种原因而跳过此类任务，结果可能是灾难性的。 “热那亚桥不是倒塌的第一座，” Mehdi Kashani博士说， “不幸的是，它将不会是最后一个。
New methods of monitoring structures are available to help engineers spot problems before they become critical. Instead of the arduous task of climbing up bridges or erecting scaffolding, camera drones can easily take a close-up picture of just about any part of a bridge. Electronic sensors can provide regular readings of any movement in the structure. And laser scanners are capable of picking up fine details and displaying them as a three-dimensional image. All this should help, but only if regimes exist to ensure that careful monitoring and preventive maintenance take place. If such tasks are skipped, for whatever reason, the result could be disaster. “The Genoa bridge is not the first to fall down,” says Dr Kashani. “And unfortunately it will not be the last.”
Repair or replace?
监控和维修不是唯一的选择。 当桥梁在20世纪50年代，60年代和70年代建造时，许多人预计其寿命将超过100年。 但钢筋混凝土的腐蚀促使一些土木工程师认为这种桥梁的寿命可能只有50 – 60年。 这意味着成千上万的桥梁即将结束它们的使命。 翻新是一种可能的解决方案，但即慢又昂贵，投资可能超过建造一座新桥。
Monitoring and repair are not the only options. When bridges were being built in the 1950s, 60s and 70s, many were expected to last for more than 100 years. But the decay of reinforced concrete leads some civil engineers to think that such bridges may have a life of only 50-60 years. That means thousands of bridges are coming to the end of their days. Refurbishment is possible, but it is slow and very costly. It might end up being more expensive than building a new bridge.
工程技术的进步推动了新型结构的涌现。随着材料科学的突飞猛进步，甚至可以修补结构的内部材料，使混凝土更坚固、钢筋更防锈。 一些国家已经在制造超高性能混凝土，使建筑物能够抵抗地震作用和炸弹冲击。除了沙子和水泥之外，其它成分也添加到这些超级混凝土中，例如石英和各种增强材料。 在一些试验中，植物纤维的添加已经生产出了更高强度的混凝土。
New structures can also take advantage of advances in engineering. There has been huge progress in materials science, so much so that it is now possible to tinker with the internal structure of substances to make concrete more robust and steel better at resisting rust. Ultra-high-performance concrete is already being made in some countries to toughen buildings against such things as earthquakes and bombs. Apart from just sand and cement, other ingredients are added to these super concretes, such as quartz and various reinforcing materials. In some tests, the addition of plant fibres has been shown to produce markedly stronger concrete.
Self-healing concrete is also being explored. Different methods can be used, but the basic idea is that, should cracks appear in the surface, they will trigger a chemical reaction that seals them up again.
然而，整体更换旧桥梁代价高昂。跨越纽约哈德逊河的Governor Mario M Cuomo大桥将会替代老的Tappan Zee大桥，预计将于今年晚些时候全面投入运营。 它是以经典方式设计的斜拉桥， 预计耗资约40亿美元。 在20世纪50年代，这座古老的桥梁主要由钢和混凝土建造而成，耗资约6000万美元，按今天的价格计算，仅相当于5.64亿美元。 Tappan Zee 桥的预期寿命为50年，但却使用了将近62年。新桥预计将有100年的寿命，让我们拭目以待。
Wholesale replacement of elderly bridges would be an expensive exercise, however. The Governor Mario M Cuomo Bridge, which opened as a replacement for the old Tappan Zee Bridge which crosses the Hudson River in New York, is expected to become fully operational later this year. It is also a cable-stayed bridge, but one of a more traditional design. It is expected to cost some $4bn. The old bridge, built largely from steel and concrete in the 1950s, was knocked up for some $60m, which in today’s terms would be a bargain $564m. The Tappan Zee Bridge was predicted to have a lifetime of only 50 years; it managed nearly 62. Its replacement is supposed to last for a century. Time will tell.
左侧为Governor Mario M Cuomo大桥