PICTURES OF ARCH BRIDGE:
Picture 1: Shape of Arch Bridge
Picture 2: Peace Bridge (example of arch bridge)
DESCRIPTION OF BRIDGE:
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle. Designs of bridges vary depending on the function of the bridge, the nature of the terrain where the bridge is constructed, the material used to make it and the funds available to build it. [1]
Based on the bridge in Picture 2, it should use that material which results in the best bridge regarding shape, technical quality, economics and compatibility with the environment. The strength and stability structure for this bridge also different. Besides, the important thing to make the bridges become more strength and stable, we should know the material to use in this bridges.
Amongst bridge materials steel has the highest and most favorable strength qualities, and it is therefore suitable for the most daring bridges with the longest spans.
Normal building steel has compressive and tensile strengths of 370 N/mm2, about ten times the compressive strength of a medium concrete and a hundred times its tensile strength. A special merit of steel is its ductility due to which it deforms considerably before it breaks, because it begins to yield above a certain stress level. This yield strength is used as the first term in standard quality terms. [2]
For bridges high strength steel is often preferred. The higher the strength, the smaller the value difference between the yield strength and the tensile strength. This means that high strength steels are not as ductile as those with normal strength.
The fatigue strength rise proportion to the tensile strength. It is therefore necessary to have a profound knowledge of the behavior of these special steels before using them. For building purposes, steel is fabricated in the form of plates (6 to 80mm thick) by means of rolling when red hot. For bearings and some other items, cast steel is used. For members under tension only, like ropes or cables, there are special steels, processed in different ways which allow us to build bold suspension or cable-stayed bridges.
. Aluminum was occasionally used for bridges and the same form was used as for steel girders. Aluminum profiles are fabricated by the extrusion process which allows many varied hollow shapes to be formed, so that aluminum structures can be more elegant than those of steel. Aluminum profiles are popular for bridge parapets because they need no protective paint. [2]
BACKGROUND INFORMATION FOR STRUCTURE AND SHAPES OF ARCH BRIDGE:
Bridges make it possible to cross from one shore of a river to another. In earlier times, bridges allowed trade between farmers, and the movement of goods across a natural divide. Bridges over rivers allow the water to pass easily through the "holes in the bridge." Bridges allow traffic over obstacles (rivers, roads, ravines, park), without changing the obstacle. Arch shape is the common shape for the bridge. Other than the arch shape is beam and suspension shape.
Arch Bridge
"Pier": The bridge support how high do you make the piers? That depends on what has to travel underneath.
"Span": The distance pier to pier the span depends on the strength and weight of the arch. The thicker the material, the more weight it will hold and the further the span. But, the thicker the material, the heavier the bridge, so the more it takes to hold it up. A very strong arch could be too heavy for the "span" and sag into the river. We can lighten the arch by using a truss.
Arches are made up of wedges and a "keystone," the wedge at the top. An arch bridge stands up with "push." The "push" is in the form of a curve. Arch bridges can be very wide because you can have arch after arch in a continuous line. If the arch were very shallow, you would need to anchor the bottom wedges to the ground. This is usually done with a supporting wall.
Look at the Picture 2: Peace Bridge
The strongest shape of truss is triangle because it very stable. It is used to make very strong form. Besides, the tubes and squares shape also strong but less strong than triangle shape. Arch bridges can span a greater distance with a thicker beam. But there is a limit, because too thick an arch will sag into the river. An alternate solution is to build an arch of triangles, a "truss," which is much lighter than a beam of similar thickness. A truss is extremely strong and stable.
Picture 3: Types of Truss Bridge
Shape is important for strength, but so is the material. A steel beam will hold more weight that a wooden beam of equivalent size. Today, bridges are built of steel and reinforced concrete. Reinforced concrete beams are created by placing steel bars into the concrete before it hardens. This skeleton of steel helps the concrete withstand the tension that develops when long, unsupported beams are subjected to bending under heavy weight loads.
WEAK POINTS/ CRITICAL PART OF THE ARCH BRIDGE:
At the arch bridge, the critical part is at the beam member or truss member. At the members, it will buckle if the maximum load higher than the critical load at the bridge. Then, the bridge will collapse. To know how the bridges collapse, we can look at the Tacoma Narrow Bridge collapse. There have two different condition buckling occur which is flexural-torsion and lateral-torsion.
1. Flexural-torsion buckling –
Occurs in compression members only and it can be described as a combination of bending and twisting of a member. And it must be consider for design purposes, since the shape and cross sections are very critical. This mostly occurs in channels, structural tees, double-angle shapes, and equal-leg single angles. [3]
2. Lateral-torsion buckling –
When a simple beam is loaded in flexure, the top side is in compression, and the bottom side is in tension. When a slender member is subjected to an axial force, failure takes place due to bending or torsion rather than direct compression of the material.
If the beam is not supported in the lateral direction (i.e., perpendicular to the plane of bending), and the flexural load increases to a critical limit, the beam will fail due to lateral buckling of the compression flange. In wide-flange sections, if the compression flange buckles laterally, the cross section will also twist in torsion, resulting in a failure mode known as lateral-torsion buckling. [3]
OVERCOME THE ARCH BRIDGE PROBLEMS:
There are a few way to overcome the arch bridge problems such as buckling. From the formula of critical force/ buckling force, we can overcome the beam at the arch bridge problems. The formula for buckling force is (Formula I). If want the bridge don’t buckling, the value of maximum force should be less than buckling force, (Formula II).
[Formula I]
[Formula II]
If the bridge not stable (buckling), we should change the material. To choose the good material, we should look at the yield strength of material use for bridge. If the values yield strength of material higher, the critical buckling also will higher. Then, the probability the bridge want buckling is less.
2- Length of bridge
Length of beam at the arch bridge should be short if want the bridge stable and probability to buckling is less. Based on the formula above, we know that the buckling force inversely proportional to length of beam at the arch bridge.
3- Geometry of column cross-section
Geometry of column cross-section is same as with second moment of area which is property of a cross section that can be used to predict the resistance of beams to bending and deflection, around an axis that lies in the cross-sectional plane. [4]
To make the bridge stable and strength, we must choose the bigger cross-section shape for beam. If the value for second moment inertia high, the buckling force also will high. Then, the capacity load for bridge will higher.
4- Dimension for beam
Based on the second moment of area, there have different formula for different shape. For rectangular cross-section, the formula is (Formula III). Then, for circular cross-section is (Formula IV). If our shape of beam is rectangular, we should increase the dimension for base, b and height, h. But, if circular shape of beam, we should increase of radius, r. This is because; we want the high value for second moment of area.
[Formula III]
[Formula IV]
IMPROVEMENT THE STRENGTH OR STABILITY AT ARCH BRIDGE:
By changing structural design parameters such as height of main girder, restrained conditions of the arch, and arch vertical and lateral stiffness, it will improve the arch bridge stability. [5]
1- Rise-Span Ratio
When the rise-span ratio is excessively large, increases of height of arch rib will lead to increases of length of arch rib, thereby reducing structural rigidity and increasing weight, finally leading to reductions of arch stability. When the rise-span ratio is excessively small, the arch will be too flat and straight.
2- Height of main girder
Increase of main girder height can improve the overall structural stability but that effects are limited.
3- Restraints of Arch Springing
When arch springing are completely restrained, structural destabilization is in-plane buckling mode.
4- Flexural Stiffness of Arch Rib
Increasing the vertical flexural stiffness of the arch can effectively increase the stability of the overall structure, as the destabilization of the arch is in-plane buckling mode, increases in vertical flexural stiffness has a significant impact on increasing the stability of the arch.
Increasing the lateral flexural stiffness of the arch will increase the stability of the overall structure, but the impact is very little when the lateral flexural stiffness has a large value. This is because the stability of the arch is determined by the vertical flexural stiffness of the arch. Increasing the lateral flexural stiffness does not have a significant impact on the occurrence of in-plane buckling.
CONCLUSION:
As a conclusion, to make the arch bridge strength or stable, we should change the material which is the higher value of yield strength. Besides, for the length of beam at the bridge, we should make it short. So, it will difficult to buckling. Lastly, we must make the bridge with bigger value of cross-section area. When the cross-section area larger, so the bridge will be stable and strength. To set the shape of beam structure, we should make it with triangle shape because the triangle shape is more stable than other shape.
REFERENCES:
[5] Tamkang Journal of Science and Engineering (2010), Vol. 13, No. 4, Stability Analysis of Special-Shape Arch Bridge, Wen-Liang Qiu, Chin-Sheng Kao, Chang-Huan Kou, Jeng-Lin Tsai and Guang Yang, pp. 365-373.
Picture 1: Webpage Visual Dictionary Online
Picture 2: Blog Word press (Buffalo Blood Donor)
Picture 3: Webpage “How Bridge are Built”
