Static structures (e.g. – buildings) – Means compensating earth-transmitted force
Reexamination Certificate
1999-12-03
2002-07-02
Stephan, Beth A. (Department: 3635)
Static structures (e.g., buildings)
Means compensating earth-transmitted force
C052S690000, C052S655100
Reexamination Certificate
active
06412237
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to architectural framed structures and methods in general, and more particularly to ones having a coupled girder system capable of dissipating seismic energies.
BACKGROUND OF THE INVENTION
Framed structures have been used for centuries in building construction. Most typically, they are in the form of a rigid frame, utilizing rigid couplings between columns and girders. Buildings in seismically active locations are likely to be subject to oscillatory (i.e., repeated back and forth) lateral forces or lateral shaking during an earthquake. The rigid frame forms a moment frame that provides a building with resistance to lateral forces by the stiffness of the columns and girders and the rigid connection between them.
FIG. 1
illustrates a cross-sectional view of a modern steel I-beam that is used to form columns and girders of a framed structure. The head and foot sections of the “I” is known as the “flanges”, and connecting them in between is the body section known as the “web”. A steel I-beam comes in various dimensions as classified by the American Institute of Steel Construction. For example a W24 I-beam would be a wide flange rolled steel with a nominal depth of 24 inches from head to foot.
FIG. 2
illustrates a conventional framed structure formed by a lattice of columns and girders. One modern example of such a structure is the Special Moment Resisting Frame (“SMRF”). For simplicity only two columns and two girders are shown, even though in general the lattice formed is a three-dimensional one. In a framed structure, a girder is coupled to a column at an angle, typically at right angle, to form a rigid moment connection.
In the early part of the twentieth century, the I-beams were joined together using rivets that connected both the web as well as the flanges of the girder to the column. Angles or bent plates were used to transfer the forces from the girder flange to the column flanges. Later, with the advent of welding technology, the girder-to-column rigid moment connections were made utilizing the technique of welding the girder flange to the column flange. The girder webs were traditionally bolted using high strength bolts.
When a force is applied to a rigid material such as steel, there is a “stress” on the material which results in a displacement or “strain”. The characteristics of steel ante such that the stress-strain relation is initially in a linear or (“elastic”) regime where the strain is proportional to the stress. Furthermore, the process is reversible in that the strain is reduced in proportion with the stress by retracing the linear relation. Since, energy is given by integrating the applied force over the displacement, it is equivalent to the area under the stress-strain curve. In the elastic regime, as force is applied, energy is stored in the rigid material, but when the force is removed, the energy stored as strain energy is translated into kinetic energy (i.e., movement of the frame). Thus, there is no dissipation or removal of energy from the material.
On the other hand, when the stress exceeds a certain value for a given material where the resulting strain is beyond a certain point called the “yield” point, the material enters into a regime where it starts to yield inelastically. Here, the stress-strain relation begins to deviate from a linear relation. More importantly, in the inelastic regime the process is irreversible in that the stress-strain curve is not retraced as the stress is subsequently decreased. Thus, in the inelastic regime the energy stored in the material is dissipated as heat instead of kinetic energy during the yielding of the material. Since the heat escapes to the outside environment, this energy is then permanently lost from the material which allows the motions to die out. This phenomenon is called “damping”.
FIGS. 3A-3C
illustrates schematically the behavior of a simple conventional framed structure in response to lateral forces.
FIG. 3A
is a schematic representation of a simple conventional framed structure formed by a girder supported by two columns.
FIG. 3B
illustrates schematically the deformation to the conventional framed structure of
FIG. 3A
in response to a force from left to right. The moment frame rotates clockwise resulting in a deformation of the girder which tries to maintain at right angle at the joints to the columns. The stress and strain in the girder is flexural in nature without any net load along the long axis of the girder (axial load). At the right end of the girder, the stress in the top half portion is compressive and the stress in the bottom halfportion is tensile. At the left end of the girder, the reverse is the case. Similarly,
FIG. 3C
illustrates schematically the deformation to the conventional framed structure of
FIG. 3A
in response to a force from right to left.
When the drift is below a predetermined value for a given structure, the strain is in the elastic regime and the energy stored is returned when the stress is removed in the form of reverse movement of the frame (i.e., kinetic energy).
However, when the drift exceeds the predetermined value, the girder begins to yield inelastically and energy is dissipated as heat while the material changes character by becoming hardened. After repeated cycles of post-yield stresses the material is ultimately susceptible to rupture. In major earthquakes (DBE), it has been observed that there are only two or three cycles of the highest magnitude which will likely push a structure to go into inelastic yielding.
In the case of the welded joints, it was assumed, based on a limited number of tests, that the welded connection will be stronger than the parent metal. In the event of large earthquakes, the steel in the girder will yield inelastically and thus absorb energy and provide damping to the structure. The 1990 Northridge Earthquake in Southern Calif., USA and a short time later the Kobe Earthquake in Japan have showed the assumed ductility (i.e., ability for the frame to continue displacing after the girder column joint had reached the yield point in steel and thus absorb energy) was not achieved in a large number of joints. This led to further research and new connection joints were developed. However, even after a good deal of testing, only a few type of joints have been tested and confirmed to have sufficient ductility that is required to absorb energy from the earthquakes by the inelastic rotation of the joint. During the inelastic rotation of the joint, the girder behaves in a flexural manner by bending slightly near the joints. One type of joint that is now considered desirable is one where the girder incorporates a weaker spot at each end near but slightly away from the joint. This weak spot is known as Reduced Beam Section (“RBS”), also known as “dog bone”. The incorporation of RBS in the girder allows better control of the yielding of the girder away from the region of material that may have been modified by the formation of a joint and its welding.
Two seismic design criteria have been established by the structural engineering profession. The Design Basis Earthquake (“DBE”) is defined in statistical terms as an earthquake event that has less than 10% likelihood of being exceeded in the economic life of the structure, deemed for most civil structures as 50 years. On the other hand, the Maximum Capable Earthquake (“MCE”) is defined as an event that has less than 10% likelihood of being exceeded in 100 years. Beyond these two design criteria, there are no defined expectations for most structures except the overall goal is to prevent collapse.
It has been established in building practices that a building should be sufficiently stiff not to suffer more that 0.3% to 0.6% drift elastically. In terms of the moment frame, it approximately translates to 0.2% rotation in the elastic regime. This will prevent the building from straying uncomfortably under wind loads and to recover after mild to moderate earthquakes.
Moreover, the columns and girders of a moment frame should be proportioned such that in
Skjerven Morrill LLP
Stephan Beth A.
Structural Design Engineers
LandOfFree
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