BEAUTIFUL BRIDGES OF THE 20TH CENTURY
2.5 CHINA BRIDGE AESTHETICS DESIGN PROBLEM ANALYSIS
2.5.1 On the Rationality of Bridge Main Hole Span
Main hole span of bridge is the most important aspect in long-span bridges scale, which determines the bridge type selection. Main hole (Navigable) navigation bridge span must first meet the requirements, taking into account the main pier anti-ship collision safety. China’s inland waterways is no clear uniform standard, they use a bridge deliberations, after the Ministry of Transport Water Transport Department to conduct feasibility studies depending on the circumstances at the bridge site fairways make case decisions. In this case, since the revetment work is limited to China waterway near the city zone, resulting in a large and stable enough swing waterway, plus there are still a large number of small vessels and large tugs hit pier accident, waterway sector is often calls for increasing the main span of the bridge to ensure navigation safety. There are also the owners in order to pursue “the span of the first”, indicating the use of excessive force span, which brought the ratio between the bridge across the main navigable height and disorders, but create a sense of oppression.
The normal ration of main span (two-way navigation) to the navigable clear height falls between 15 m to 18 m, the maximum should not be exceed 20 m. An arrangement of two navigable holes (top and bottom rows of holes navigation), the main span and the clear height ratio should be 10 or less. However, there are many domestic large-span bridges have exceeded this proportion. Since the main crossing points is too large, so that cable-stayed bridge pylon height above deck and below deck height imbalance, especially when used in deck elevation elected at the legs of the gem-shaped pylon, it shows the lower part too short and tall enough to affect the aesthetics of the bridge tower shape.
A Case Study of Nanjing Yangtze River Bridge, the bridge main span of 628 m, while the navigation bridge is only 24 m in height restricted Nanjing Bridge upstream, resulting ratio between the two is 26.2. Tower high above the deck for 150 m, above and below deck tower higher than 150/24 = 6.25, which creates the effect of short leg pylon gem type (Fig. 2.45), make cable-stayed bridge in overall lack of beauty. In contrast, the Shanghai Nanpu Bridge and Yangpu Bridge pylon shaped gem because there upright sense of proper proportion.
In the middle reaches of the Yangtze River (Wuhan–Nanjing section), in order to meet the requirements of navigable inland level (clear height 24 m), the use of cable-stayed bridges with the main span of 400~500 m is a reasonable choice for the economy. If the underlying low construction cost can also be used across 160~200 m of multi-beam bridge to meet up and down the rows of holes navigation. However, in the blind pursuit of large- span, but there have been some misguided thinking across a river 800~1000 m span cable- stayed bridge, and even super-kilometer suspension bridge. At this point, span and bridge clearance height ratio much higher than the 20 : 1, even upto 50 : 1 entire deck “lying” on the water, completely lost beauty. Moreover, the cost is very expensive, and the adjacent smaller bridge span is also very coordinated. France, Greece, in the design of the Strait of Rion-Antirion bridge, in order to meet the requirements of 180000t of navigation, the use of multi-span cable-stayed bridge 560 m (Fig. 2.46). Between Denmark and Germany Fei Manen Channel Bridge, for the navigation 260,000t seagoing vessel, the recommended solution is multi-span 780 m double rail-cum Cable Stayed Bridge, bridge truss Composite Beam (Fig. 2.47), a solution worth learning.
Fig. 2.46 Rion-Antirion Channel Bridge, Greece.
Fig. 2.47 Fei Manen Channel Bridge.
2.5.2 Facade Layout Symmetry
Arranged in accordance with the position of the main channel fairway hole layout principles is a quite natural layout. However, some owners prefer regardless of position in the water channel width symmetrically arranged, or even ask to move the channel by channel diversion works and dredging, which is very unreasonable.
Facades of foreign long-span bridges are mostly arranged to determine the position of the main hole span and in accordance with channel centerline, while the edge of the hole
at the actual water depth and geological conditions are arranged left-right asymmetry (left and right edges of different pore size). As the water widens, it can also be arranged in a symmetrical side hole, plus about the length of different non-navigable water hole bridge, to form an asymmetric distribution holes in general, and to indicate to the navigation of the ship channel the water side of the actual bias location. Sometimes, you can also employ single tower cable-stayed bridge asymmetric arrangement or the use of collaborative systems to adapt to nature, and to achieve the purpose of the economic fabric of the hole.
Take the design of a bridge to the program as an example [Fig. 2.48(a)], because the owners unreasonably required by the symmetrical arrangement of the water and had to increase the span of the main hole to 428 m, and the need to move the channel 46.5 m, in order to meet shipping requirements, navigable but still biased side of the main pier, increasing the risk of ship collision. Moreover, increasing the main hole is not conducive to the stability of the whole economy and arch bridges, but also increases the difficulty of construction.
Instead, the use of asymmetric arrangement [Fig. 2.48(b)] not only conforms to the natural, to the economy and security in general, but also in the aesthetic point of view, a reasonable asymmetrical arrangement can also be presenting a beauty. Fig. 2.48 Facade arrangement of a bridge (size unit: m) (a) Arranged symmetrically; (b) Are arranged asymmetrical.
2.5.3 The Side Holes Scales in Cable-stayed Bridge
The ratio of side hole and central hole in the twin tower cable-stayed bridge is a question of the overall arrangement must focus consideration. According to earlier studies, Professor Leonhardt of Germany, in order to control the magnitude of stress tail cord to ensure its anti-fatigue properties, the tail rope must reserve sufficient internal forces of dead-load, live-load so that the positive and negative changes caused by the internal forces without causing too much stress amplitude. One important measure is appropriate to shorten the length of side spans of dead-load to increase the tail rope tension, and dead- load and live-load than the smaller side span and is also smaller than the mid-span.At anchor when the ratio of maximum stress to minimum stress is under kac = 0. 4, relationship shown in Table 2.1 can be drawn from Fig. 2.49.
Domestic cable-stayed bridge, a strong side beams and pouring of concrete at the ends of the hole plus the weight of the transition are commonly used to to provide balance weights, there will be a mutation in the shape of the beam, affect the appearance. It is mostly used in foreign countries made a long pull anchor seat, anchor pier on the use of weight to balance the tail rope pull force, and set the continuous transition pore (transition span), the joints move to the rear end of the transition pore to optimize the structure at the anchor pier, as shown in Fig. 2.50.
Fig. 2.49 Beam stiffness affect the relationship between la/lm and p/g of (kac = 0. 4 time).
Fig. 2.50 End processing in cable-stayed bridge. Note: The solid line is ignoring curvature, shaded conditions included bending stiffness. Table 2.1 Effect of the beam stiffness on the la/lm and p/g ratio Deck type Deck live-load, constant load ratio p/g Side-span to cross-span ratio la/lm Steel deck 0.4 0.35~0.39 Bond beam deck 0.2 0.40~0.45 PC deck 0.125 0.46~0.5