Static structures (e.g. – buildings) – Machine or implement
Reexamination Certificate
1997-10-24
2001-05-29
Chilcot, Richard (Department: 3635)
Static structures (e.g., buildings)
Machine or implement
C052S653100, C052S749100, C052S749100
Reexamination Certificate
active
06237303
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates broadly to load bearing and moment frame connections. More specifically, the present invention relates to connections formed between beams and/or columns, with particular use, but not necessarily exclusive use, in steel frames for buildings, in new construction as well as modification to existing structures.
BACKGROUND
In the construction of modern structures such as buildings and bridges, moment frame steel girders and columns are arranged and fastened together, using known engineering principles and practices to form the skeletal backbone of the structure. The arrangement of the girders, also commonly referred to as beams, and/or columns is carefully designed to ensure that the framework of girders and columns can support the stresses, strains and loads contemplated for the intended use of the bridge, building or other structure. Making appropriate engineering assessments of loads represents application of current design methodology. These assessments are compounded in complexity when considering loads for seismic events, and determining the stresses and strains caused by these loads in structures are compounded in areas where earthquakes occur. It is well known that during an earthquake, the dynamic horizontal and vertical inertia loads and stresses, imposed upon a building, have the greatest impact on the connections of the beams to columns which constitute the earthquake damage resistant frame. Under the high loading and stress conditions from a large earthquake, or from repeated exposure to milder earthquakes, the connections between the beams and columns can fail, possibly resulting in the collapse of the structure and the loss of life.
The girders, or beams, and columns used in the present invention are conventional I-beam, W-shaped sections or wide flange sections. They are typically one piece, uniform steel rolled sections. Each girder and/or column includes two elongated rectangular flanges disposed in parallel and a web disposed centrally between the two facing surfaces of the flanges along the length of the sections. The column is typically longitudinally or vertically aligned in a structural frame. A girder is typically referred to as a beam when it is latitudinally, or horizontally, aligned in the frame of a structure. The girder and/or column is strongest when the load is applied to the outer surface of one of the flanges and toward the web. When a girder is used as a beam, the web extends vertically between an upper and lower flange to allow the upper flange surface to face and directly support the floor or roof above it. The flanges at the end of the beam are welded and/or bolted to the outer surface of a column flange. The steel frame is erected floor by floor. Each piece of structural steel, including each girder and column, is preferably prefabricated in a factory according to predetermined size, shape and strength specifications. Each steel girder and column is then, typically, marked for erection in the structure in the building frame. When the steel girders and columns for a floor are in place, they are braced, checked for alignment and then fixed at the connections using conventional riveting, welding or bolting techniques.
While suitable for use under normal occupational loads and stresses, often these connections have not been able to withstand greater loads and stresses experienced during an earthquake. Even if the connections survive an earthquake, that is, don't fail, changes in the physical properties of the connections in a steel frame may be severe enough to require structural repairs before the building is fit for continued occupation.
SUMMARY OF INVENTION
The general object of the present invention is to provide new and improved beam to column connections that reduce stress and/or strain caused by both static and dynamic loading. The improved connection of the present invention extends the useful life of the steel frames of new buildings, as well as that of steel frames in existing buildings when incorporated into a retrofit modification made to existing buildings.
A further object is to provide an improved beam to column connection in a manner which generally, evenly distributes static or dynamic loading, and stresses, across the connection so as to minimize high stress concentrations along the connection.
Another object of the present invention is to reduce a dynamic loading stress applied between the beam and the column flange connection of a steel frame structure.
Yet another object of the present invention is to reduce the variances in dynamic loading stress across the connection between the column and beam.
It is yet another object of the present invention to reduce the variances in dynamic loading stress across the beam to column connection by incorporation of at least one, and preferably several slots in the column web and/or the beam web near the connection of the beam flanges to the column flange.
It is yet another object of the present invention to reduce the strain rate applied between the beam and column flange of a steel frame structure during dynamic loading.
It is yet another object of the present invention to provide a means by which the plastic hinge point of a beam in a steel frame structure may be displaced along the beam away from the beam to column connection, if this feature may be desired by the design engineer.
Finally, it is an object of the present invention to reduce the stresses and strains across the connection of the column and beam of a steel frame structure during static and dynamic loadings.
The present invention is based upon the discovery that non-linear stress and strain distributions due to static, dynamic or impact loads created across a full penetration weld of upper and lower beam flanges to a column flange in a steel frame structure magnify the stress and strain effects of such loading at the vertical centerline of the column flange. Detailed analytical studies of typical, wide flange beam to column connections to determine stress distribution at the beam/column interface had not been made prior to studies performed as part of the research associated with the present invention. strain rate considerations, rise time of applied loads, stress concentration factors, stress gradients, residual stresses and geometrical details of the connection all contribute to the behavior and strength of these connections. By using high fidelity finite element models and analyses to design full scale experiments of a test specimen, excellent correlation has been established between the analytical and test results of measured stress and strain profiles at the beam/column interface where fractures occurred. Location of the strain gauges on the beam flange at the column face was achieved by proper weld surface preparation. Dynamic load tests confirmed the analytically determined high strain gradients and stress concentration factors. These stress concentrations were found to be 4 to 5 times higher than nominal design assumption values for a typical W 27×94 (690×140) beam to W 14×176 (360×262) column connection with no continuity plates. Stress concentrations were reduced to between 3 and 4 times nominal stress level when conventional continuity plates were added. Incorporation of features of present invention into the connection reduces the high-non-uniform stress that exists with conventional design theory and has been analyzed and tested. The present invention changes the local stiffnesses and rigidities of the connection and reduces the stress concentration factor to about 1.2 at the center of the extreme fiber of the flange welds. Explained in a different way, the condition of stress at a conventional connection of the upper and lower beam flanges at the column flange, the beam flanges exhibit non-linear stress and strain distribution. As part of the present invention it has been discovered that this is principally due to the fact that the column web, running along the vertical centerline of the column flanges provides additional rigidity
Allen Clayton Jay
Partridge James Edward
Richard Ralph Michael
Chilcot Richard
Seismic Structural Design
Small Larkin, LLP
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