Major steel bridge "Akaski Kaikyo Bridge" and details of structural steel members utilized for construction with their specifications and features

Introduction to Akaski Kaikyo Bridge:

Facts of Akashi Kaikyo Suspension Bridge:

1. The bridge is so long, it would take eight years Towers laid end to end to span the same distance.
2. The length of the cables used in the bridge totals 300,000 kilometres. That's enough to circle the earth 7 times!
3. The bridge was originally designed to be 12,825 feet. But on January 17, 1995, the Great Hanshin Earthquake stretched the bridge an additional three feet.
4. The bridge holds three records: it is the longest, tallest, and most expensive suspension bridge ever built.
5. Over 2 million workers, billions of dollars, 181,000 tons of steel and 1.4 million cubic meters of concrete were used in its construction
6. Its foundation is as deep as a 20 storey apartment blocks, towers almost as tall as the Eiffel towers in Paris.
7. Its span is nearly 2 km (1/3 times more than any other suspension bridge built ever before)
8. Theory of suspension bridge design
9. Two main cables suspended across the water, held up by two towers

 

 

Statistics / Facts:

1. Location: Kobe and Awaji-shima, Japan
2. Completion Date: 1998
3. Cost: $4.3 billion
4. Length: 12,828 feet
5. Type: Suspension
6. Purpose: Roadway
7. Materials: Steel
8. Longest Single Span: 6,527 feet
9. Engineer(s): Honshu-Shikoku Bridge Authority

Construction Phase

Now

v  On identifying one of the major steel bridges namely “Akashi Kaikyo Suspension Bridge” I have found the following structural members that were involved in its construction:

1.    1. The Main Cables

2.    2. Pylons

3.     3.Stiffening Girder

4.   4. Anchorages

5.   5. Connections: Hangers & Cable Bands

ü Specifications and features of Structural Members utilized for the construction of the bridge:

1-    The Main Cables

The main cables run from anchorage to anchorage. A cable is composed of a number of strands. These strands can be made in situ by cable spinning or they are prefabricated. The number of strands builds up in a simple arithmetic progression. The cable thus consists of a central strand surrounded by 6 strands, 6 + 12 strands, 6 + 12 + 18 strands, etc.

The cable in Figure 4 contains 37 strands and this is a common number in bridges with spun cables. (The numbers indicate the sequence of spanning the individual strands.) Thus, in the Humber Bridge the main cables are composed of 37 strands: each having 404 wires.

In bridges with main cables made of parallel wire strands, the number of strands is generally higher as each strand has to contain fewer wires for reasons of handling.

Before starting the main cable erection, a catwalk is made, running from anchorage to anchorage via the pylon tops. A tramway is mounted on this catwalk if the cables are to be spun. The tramway is not necessarily required for the transport of prefabricated strands. To protect the catwalk to some, extend against the wind, a pretensioning system of counter parabolas is used.

The essentials of the cable spinning process are shown in Figure 5. The catwalk and tramway are indicated. Two spinning wheels, connected to a loop, carry separate wires from one side to the other. At one of the anchors blocks the dead wire is fixed. On the other side the wire is taken off the spinning wheel and looped around a strand shoe (Figure 6). Next the live wire is fixed. When a strand, consisting of 300 to 400 wires, is finished, this strand is located carefully in its correct place. This operation is carried out at night when the temperature is nearest to uniform.

The optimum wire diameter is 5,0-5,5mm. A larger diameter makes the wire too stiff, while a smaller diameter requires more wires and more labour. The wire material has an ultimate strength up to 1600 - 1800N/mm².

With span cables, each working shift can mount about 160 wires a day, and in most cases two shifts per day per cable is used. Prefabricated strands can be mounted in a shorter period.

After having erected all wires the bundle of strands are compacted into a circular shape. As a protective treatment, the cable is finally wrapped with a mild steel wire. During this wrapping a protective paint is added.

Where the cable passes over the pylon tops and the splay saddles, they lose their circular shape. Special provision is necessary at these locations to keep corrosion under control.

Technical features of cable

2 Pylons:

The foundation of the pylon is a concrete massive slab with base dimensions of 67.4 x 28.0 m and thickness variable from 2.5 to 6.5 m, placed on 160 reinforced concrete piles, 18.0 m long of 1.5 m diameter. The piles were drilled in drawn steel tubes with cement injection of the base. It was assumed that both the piles and the soil under the foundation slab were load carrying element. The pylon’s structure is hybrid (Fig. 3): - the pylon’s legs and the lower parts of the arms are made of reinforced concrete; - the upper part of the arms is a hollow composite structure; - the lower crossbeam is a post-tensioned concrete element; - the upper crossbeam is a post-tensioned steel-concrete box structure. Inside the pylon, a steel core, presented in Fig. 3, was placed. It formed the inner formwork during erection of the pylon and it interacts with the reinforced concrete shell, transferring vertical forces and bending moments. In the cable anchorage zones the core carries horizontal forces transmitted to the walls of the pylon by cable stays. The steel core and the reinforced concrete shell act as a composite section due to shear studs welded to the core’s side plates. An important load carrying element of the pylon is the upper crossbeam (compressed and torsioned). It was vertically and longitudinally post-tensioned, and heavily reinforced (Fig. 4).

It should be considered that there are many factors influencing the choice of the material for pylons, e.g. soil conditions, speed of erection, stability during their construction, etc. Therefore, the choice should not be based entirely on a quantity-based cost estimate.

Functions of Tower/Pylon  of Cable Suspension Bridge or Cable Stayed Bridge: 

The main function of tower of cable stayed bridge was to safely support bridge loads and traffic loads. Towers should be aesthetically pleasing and demonstrate satisfactory survivability performance. Figures shows free body diagram of suspension bridge and cable stayed bridge and illustrates how towers support bridge loads and traffic loads.

 

3. Stiffening Girder

The choice of the cross-section for the stiffening girder is a very important step, since this influences the behaviour of the total structural system.

The developments in girder design have been closely connected to the development of calculation methods, in particular the deflection theory which is especially relevant for lighter girders.

Technical Features of Stiffening Girder:

  To determine the type of the stiffening girder, several types of girders as shown in Fig. 5 were investigated. Fig. 5 also shows the relationships between the onset wind speed of flutter and deck weight for the investigated girders. From these results, the truss girder and compounds stiffness box girder were selected as prospective types. The compound stiffness box girder is a bridge system that arranges high-tortionally-stiffened girders around the tower and aerodynamically-well flat girders at the central portion of the bridge. From the comparison of these two types of girders, truss was finally selected due to its ease of its erection on the international navigation channel, because the erection of the truss can be done by not using the navigation channel.

The dynamic behaviour of suspension bridges due to the wind has been investigated mainly by wind tunnel tests using sectional models so far. For the Akashi Kaikyo Bridge, however, a full model test in a large wind tunnel facility was required for the following reasons. 1 The deflection of the bridge due to wind is large. 2 Along the bridge axis with the length of 4 km, there is a large variation of wind properties. 3 The effect of the main cables cannot be neglected. 4 The effect of the turbulent flow cannot be neglected. Therefore, we built a large boundary layer wind tunnel facility with width of 41m, height of 4 m, and length of 30 m. As a result of the experiment of a scale of 1/100, it was confirmed that the required wind resistance could be obtained by installing some gratings on the road deck and a vertical stabilizing device along the truss girder as shown in Fig. 6

Due to its flexible structure, vibration, especially flutter due to wind, was the most important problem at the design stage of the stiffening girder. In the design code, it is specified that flutter must not occur under the wind speed of 78 m/s in the wind tunnel test within the attack angle from -3 deg. to +3 deg..

4.Anchorages

The main cable force is composed of two components, the vertical trying to lift the anchor block, and the horizontal trying to pull the anchor block towards the centre of the bridge. The method of anchoring the main cable depends largely on the local soil conditions.

In gravity anchorages equilibrium between the cable force and soil pressure has to be carefully considered during erection. During erection the vertical component of the cable force increases, exerting a vertical lifting force on the anchorage. In order to limit the variations in soil pressure it might prove advantageous to increase the mass of the anchorage in pace with the erection. This procedure reduces subsidence problems and could make the construction, in particular the foundation, cheaper.

 

 5-Hangers and Cable Bands

Socketed hangers connect the main cable and girder.

The hanger is connected to the main cable via a cable band consisting of two semi-cylindrical halves, connected together by high tensile steel bolts to develop the necessary friction. The hanger is connected to this cable band via a pin connection or it may be looped over the cable band.

The cable bands are firmly tightened onto the main cable and get their load carrying resistance mainly from friction and compression of the cable (Figure 17). The cable bands are carefully machined, taking into account an air void of approximately 20% in the cable.

 

The main cable is subject to an axial loading that increases during erection of the bridge. The elongation of the cable from the anchor block to the pylon should be taken into account, e.g. by giving the pylons a pull-back (Figure 18).

 

             For the cable bands, the transverse contraction of the cable section is of the utmost importance. It causes the friction between cable band and cable to decrease and, as a result, the load carrying resistance goes down. Precautions should be taken to measure the relaxation and to tighten the bolts during erection, e.g. by making a backlash (Figure 19). For reasons of maintenance, the remaining gap is filled with rubber. In view of the contraction, the cable wrapping should be carried out after the bridge carries almost all of its full dead load.

Vertical hangers are usual. For a period of about 15 years inclined hangers were popular (Figure 20). The use of inclined hangers started with the Severn Bridge (1966) and was concluded with the Humber Bridge (1981).

 

The idea was to make the bridge more rigid (» 25%), due to the truss behavior, and to reduce the tendency to oscillate (flutter). The aim was to increase damping by utilizing the hysteresis of the spiral wires forming the hangers. However, the constantly changing forces in the hangers can create fatigue problems, and this is one of the reasons why designers returned to apply vertical hangers only.

By: Shaib Shabir

B.tech (Civil Engineering)

M.tech (Construction Technology & Management)

dar_shoaib_shabir