In simple language, allowing a plastic hinge to form without additional material degradation or structural instability will limit the load a structure experiences. Such structures will see more deflection but if the hinge areas are properly detailed and confined it will save a structural from certain collapse.
Also a properly placed hinge will limit damage to other members. For example if a hinge is allowed form at the bottom of a column near the footing, the footing will experience much less load and therefore damage than if a more robust column was used.
The hinge provides the same function as a fuse in a electrical cicuit by not allowing too much current through. If too much current tries to pass the fuse it will burn out resulting in a gap or open circuit. That way the entire circuit is not damaged.
Regards,
Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
Thank you very much for the response. However, I like to ask a few more questions on this subject.
1. What do you mean by "properly placed hinge"? Is this the point of expected maximum moment? Or maximim shear? How do you select the location of plastic hinge?
2. You're saying that the hinge is like a fuse in an elecric circuit. In other words, the hinge acts like an internal energy dissipation device. But how do we know how much energy can be dissipated at the plastic hinge location? What if, after the energy dissipation the demand is more than the capacity(plastic moment capacity) at the hinge location? What will be the result? More deflection or structural failure?
3. What is exactly the mechanism by which the hinge dissipates energy? According to the principle of conservation of energy, the energy is not lost but it is transformed to another form of energy? Which one? Heat? And then how heat affects the structure?
1. The plastic hinge term is mostly used for seismic design, when the elements cannot be reasonably sized to resist elastic forces or tolerate imposed displacements without yielding. Typical load combination is D+EQ/R where R>1 for elements where plastic hinges allowed.
2. Plastic hinges form only when flexural capacity is exceeded (ductile failure) Exceeding shear capacity leads to collapse (brittle failure)
3. Designing bridges plastic hinges allowed in columns, for buildings in beam (at connection with column).
I worked on a bridge project where we chose to put the plastic hinge in the pier columns because this would be easier to repair than the capbeam or foundation. We designed the column for the required group loads and calculated it's plastic moment capacity. Next we factored the Mp and used that to design the cap and foundation.
The subject of the plastic hinges acting as a "fuses" in the structures was covered in the previous posts.
There some other applications of the same principle, such as plastic hinges (Freyssinet's hinges) used in the concrete structures. In this case, a joint is formed in the structure, where the average stress in the concrete is x1.5 of the crushing strength, resulting in permanent plasticizing of the concrete in the zone, and allowing for rotation. The rotation angles are typically limited to 2.5 - 3 degrees. Some of the common applications are hinges in the concrete arches and hinges at the top and base of the columns (piers) in the multibay continuous flyovers.
This might interest u, This is an extract from a relevant article......
A plastic hinge is a type of energy dampening device allowing plastic rotation of an otherwise rigid column connection.
Plastic hinges are an extension of the ductile design concept in building seismically resistant structures. Energy is dissipated through the plastic deformation of
specific zones at the end of a member without collapsing the rest of the structure.
In reinforced concrete columns, the detailed plastic hinge consists of a weakened portion of the column near the top and bottom where the longitudinal reinforcement is
decreased, allowing yielding in this zone before the rest of the column is damaged. These specially weakened steel bars are termed fuse-bars since they are designed to yield and thus protect the rest of the column during repeated ground motion. Fuse-bars are attached so as to be easily replaced, restoring the column to its original condition. The practicality and effectiveness of this method was demonstrated in the work of Cheng and Mander
(1997).
1. Usually the hinge is expected to be at the point of maximum moment for case of seismic demands on columns. This may be at the bottom or top or both depending on the support for a bridge (frame or hammerhead). When I said place, I mean that we ensure a hing forms in the column rather than the beam or foundation. If properly detailed and confined a hinge (local yielding) will displace more and if stability is ensured for large displacements, the strucure will simply dissapate more energy. The usual standard of comparison for this is the ductility ratio, which is a measure of the actual yield (displacement or rotation) over the estimated elastic yield (displacement or rotation). Most older bridge columns have ductility ratio of 2-3 but retrofitted columns and new columns with proper details can achieve ductility ratios of 5-8.
2. I think I answered most of this in the last part of response to 1 above.
3. Damage, friction, heat. There is not a significant amount of heat generated such as to damage the components of the concrete frame.
Regards,
Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
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