Patent Number: 047537728
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates energy dissipating devices such as tension loaded supports which protect the components supported by them from mechanical shock by absorbing and dissipating the shock energy, and has particular application in supports subject to severe seismic loading. 2. Prior Art Existing supports with the capacity to absorb and dissipate significant amounts of seismic shock energy are typically compression devices. Hydraulic snubbers are an example of such devices currently in use. The conventional supports which act in tension, such as tie-rods or pipe hangers are simple, convenient and space saving. Unfortunately, they offer little capacity for absorbing and dissipating energy, since they operate in the elastic range where most of the energy is stored momentarily in elastic strain and is returned to the system. Such devices are normally of such a construction that once the point of plastic deformation is reached, they may fail completely. It is a primary object of the present invention to provide a device which combines all the advantages and conveniences of conventional tensile supports with the ability to absorb and dissipate a significant amount of shock energy. It is another object of the invention to provide such a device which operates in the elastic range during normal operation but plastically deforms to dissipate a large amount of energy when subjected to severe seismic loads. It is still another object of the invention to provide the device of the previous objects having portions which remain elastic to maintain structural support while other portions plastically deform to dissipate significant amounts of energy. It is yet another object of the invention to provide the device of the previous objects which is simple in design and inexpensive to manufacture so that it can be economically replaced after portions have plastically deformed during a severe seismic event. It is an overall object of the invention to provide a device which has a plurality of elements which successively plastically deform to dissipate energy. SUMMARY OF THE INVENTION These and other objects are realized by a device which includes a plurality of successively longer metal straps all connected at their ends to a pair of end members such that at least all the straps but the shortest one, and in the preferred embodiment the shortest one also, are bowed when no tensile load is applied to the end members. The lengths of the straps are selected such that beginning with the shortest, the successively longer straps sequentially reach their yield points and plastically deform to dissipate energy as the tensile load on the end members increases. With this arrangement the longer straps remain in the elastic region to provide support for the component to which the device is connected while the shorter straps one after the other plastically deform to dissipate significant amounts of energy. As used for supporting components which may on occasion be subjected to seismic shocks, the straps are selected so that they all remain in the elastic region during normal loading and so that at least the longest strap remains elastic to maintain support during a seismic event. Of course, once any of the straps have plastically deformed, they must be replaced, however, they are relatively inexpensive and would be suitable for use in areas where significant seismic events were infrequent. The design of the device is very flexible. Through judicious selection of each individual strap, a wide range of load, and energy absorbing and dissipating characteristics can be attained. The variables include the amount of initial bow in the straps, their relative lengths, the materials used and the thickness and width of the individual straps. For instance, where it is desired that each successive strap begin to plastically deform at successively higher tensile loads, the straps may be made of different materials providing the desired characteristics and/or they can be made from stock having successively greater cross-sectional area. Preferably, the increase in cross-sectional area is attained by selecting straps having successively greater width but the same thickness. In one specific embodiment described in detail, the device includes three metal straps all of the same thickness but with the longer straps successively wider than the shorter straps so that they can sustain successively higher tensile loads before plastically deforming. In another embodiment described, the second of three straps is narrower than the first while the third is the widest so that after the first strap ruptures, the second immediately plastically deforms also thereby dissipating a large amount of energy before it fails and the load is transferred to the third strap, which remains elastic. Although the invention has many uses, one application where it is particularly useful is in supporting the piping in a nuclear power plant which must be seismically qualified. In its broadest sense, the invention is directed to a plurality of elongated energy dissipating elements which are joined at each end by end members in an arrangement such that a load applied through the end members results in the elongated elements successively reaching their yield point and plastically deforming to dissipate energy.