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Importance of Cold-Formed Steel in Seismic-Resistant Design of Structures
Finding References. Email this article Login required. Post a Comment Login required. About The Authors Milap D. At this point, our experts in the field of fire protection, thermal building physics, structural engineering, building services, Decontamination, among others, make the first step towards securing the overall success of the project in ensuring that everything will go smoothly during the construction phase later.
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Carrying out a renovation during the business hours of the building is a matter of trust. We develop a project-specific solution that ensures the maximum safety of all occupants and site staff while maintaining the use of as many rooms as possible during the progressive work. The reactions of iron and water include several end products, depending on the presence or absence of air, the temperature and the concentrations of salts in solution.
The chemical reactions here are for illustration: you don't have to remember them.
Note that in the Fe 2 O 3 produced, the ratio of iron to oxygen is The volume of red-brown rust is about 50 per cent greater than that of the metal, and so can enhance crack growth. The hydrated oxide is a very weakly protective film because it tends to spall away from the underlying surface in lamellar flakes, exposing a fresh surface to further attack.
The volume change associated with producing the hydrated oxide is larger than for the oxide itself owing to the water molecules in the atomic structure. The hydrogen gas may represent a danger if the reactions occur in an enclosed environment, such as a steel tank, for example. Many welders have been injured and killed by explosions when the welding torch penetrates to the interior: the hydrogen is released to mix with air and then explodes. They are usually removed by treatment with sulphuric acid in large-scale manufacture, a process known as pickling.
Black oxide is also formed in central heating systems, since the system is closed to the outer air and oxygen is depleted in the closed water supply by reaction. Hydrogen gas accumulates at the top of the system, and is liberated when the system is bled. To explore the problem further, the eye bar was examined for signs of further cracks. The mechanism that caused the critical crack was probably at work at other points on the inner surface of the eye bar, so could be tested by several techniques.
Many such sub-critical cracks were found Figure 40 , showing that there was a single mechanism at work. The interior of many of the cracks was filled with iron oxides, often present in a lamellar form showing successive and intermittent phases of formation. An adjacent eye bar on the next joint down along the chain was also found to be cracked in a similar way at roughly the same point.
Hint: bear in mind that a hole in a component represents a stress concentration factor of about three. The other eye bar of the same joint was located, and showed damage to the hole consistent with having been pulled off the pin. A large burr existed on one side of the hole only, showing that the end had been subjected to a large force in the accident.
Many sections were taken of the steel near the fracture to examine its microstructure, and were compared with different parts of the same eye bar as well as with other eye bars. The sections showed a steel core surrounded by a zone that could be identified as being of higher strength due to the presence of martensite.
Martensite is a strong, hard phase of steel usually formed by rapid quenching from a high temperature. XPS, X-ray photoelectron spectroscopy, gives information about the elements on the surface of a material.
It does this by analysis of the X-ray spectrum emitted by the surface when impinged by the electron beam. The fracture surface was also analysed by XPS for trace elements that might give a clue to the corrosion processes at work over the year lifetime of the bridge. In addition to small traces of manganese present in the original metal, the researchers found significant traces of sulphur present within the cracks; this is an element not present in the metal itself, indicating an unknown, external source. The sulphur concentration was greatest at the mouth of the crack.
The steel had a carbon content of 0.
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It was possible that the tiny cracks present on the inner surface of eye bar initiated the collapse by causing brittle fracture. It therefore became important to determine the strength of the steel and, also, its fracture toughness. Steel from eye bar was tested, as well as from other eye bars from the Silver Bridge.
Hardness tests across a section though an eye bar showed a soft outer zone, followed by a harder zone and then a softer core Figure The hard zone extended from about 2. This represented the hardened zone produced by quenching the steel during manufacture. The outer layer of the bar showed loss of carbon due to the heat treatment during manufacture.
The investigators wanted to know about the fatigue properties of the component, to find a feasible explanation of why it took 39 years for the eye bar to break. They needed information on the several stress corrosion mechanisms that were possible in the material, including hydrogen embrittlement, the effects of sulphur compounds such as H 2 S hydrogen sulphide and the effects of moisture and salt. Notched eye-bar material was loaded to failure in various environments. In fact, no evidence emerged for hydrogen embrittlement, and a wet environment in the laboratory tests had no effect on the rate of crack propagation.
However, the life of the steel samples was reduced substantially by hydrogen sulphide, a conclusion that appeared to correlate well with the detection of sulphur in the critical crack and other, sub-critical cracks. Although it is known that a round hole in a flat sample will theoretically produce a stress concentration of about 3, the issue was decided experimentally. It yielded at about 7 MN, and fractured in the shank at a stress of about MPa. By putting strain gauges at various points in the hole of the bar, the stress concentration was calculated to be about 2. Suggest why, in the tensile tests, fracture occurred in the shank rather than at the hole of the eye bar.
What factors contribute to where failure occurs? The shank dimensions are 51 mm by mm, giving a section area of So the section area is about a third greater in the limb compared with the shank, giving a correspondingly lower stress. Nevertheless, the greater stress concentration at the edges of the hole should have ensured failure here rather than in the shank. Surface roughness effects can be critical, so if the pin and eye-bar hole surfaces were smooth and the shank surface was rough, failure in the shank would be preferred.
It is worth emphasising that stress concentrations are of less importance in ductile compared with brittle-type failures.
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After all, a material that usually fails in a ductile manner, such as steel, can yield locally at the root of a notch or the edge of a hole. By contrast, during brittle fracture there is no mechanism for absorbing excessive load by deforming plastically, and the stress at the root of a crack may be extremely high. The experiment established that brittle cracks had not developed in the lower eye bar: if they had, this test bar would have failed at the hole rather than in the shank. An additional possibility was considered. It was known that there was significant movement of the bridge during passage of traffic, because users had noticed it many times when crossing.
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The joints would thus have been subjected to rotary motion around the pin in order to accommodate such vibrations. Could these have caused fatigue crack growth at the bearing surfaces? Contact between a circular and a flat plate creates so-called Hertzian stresses at the contact zone: compressive at the centre, and surrounded by a tensile zone. A similar effect will occur at a circular pin joint, provided there is some clearance between the two parts. In addition, there could be considerable wear caused by corrosion.
Rusting would create particles of Fe 2 O 3. The fact that the rust particles had a larger volume than the metal they replaced would also stimulate wear. The inner surfaces of the eye-bar holes showed deep grooves Figure 40 a , indicating fretting action. Could fretting have initiated critical cracks? To test this hypothesis, pin and collar shapes were machined from eye bar away from the region of the actual failure , fitted together and then rotated so that the pin acted against the collar.
The results are shown in Figure Even with the effects of fretting, the material around the eye still showed a higher fatigue life than the material in the shank.
Suggest how fretting fatigue could occur at a pin joint in the main chains of the Silver Bridge. Indicate the most likely place for such a problem, and compare the actual position of the critical and sub-critical cracks on eye bar , drawing any appropriate conclusions.
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Fretting wear occurs owing to repeated cyclical movement at a joint and was caused in the Silver Bridge pin joints by corrosion producing particles of Fe 2 O 3. The action will have been most severe at the upper joints on the main chains, where the loads were largest. Tension cracks through fatigue could have formed at either side of the contact zone between the edge of the eye bar and the central pin. Although fretting fatigue had been shown in the tests to be a possible failure mode, the mechanism demands that fatigue cracks could grow only very near the points of contact between the eye-bar hole and the pin.
There is no doubt that fretting action on the inner surface of eye-bar joint occurred during its year life.
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