Static structures (e.g. – buildings) – Processes – Protection
Reexamination Certificate
1991-09-30
2001-04-24
Safavi, Michael (Department: 3635)
Static structures (e.g., buildings)
Processes
Protection
C052S749100, C052S745170, C156S071000, C156S172000, C264S032000, C264S036200
Reexamination Certificate
active
06219991
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to repairing and strengthening internally-reinforced concrete columns of structures and, more particularly, is concerned with a method of externally repairing and strengthing such columns in existing and new structures with a flexible strap of reinforcing material to increase the strength, stiffness and ductility thereof.
2. Definition of Terms
By way of definition, the term “concrete column”, as used herein, is meant to refer to a structural element of a structure, where the structural element is hollow or solid and composed of internally-reinforced concrete primarily subjected to axial force, shear force, and bending moment. The term is synonymous with a bridge pier, pile, pillar, and post. The term also includes regions where beams or floor slabs frame into the column which are known in the art as joints or connections.
The term “structure” is meant to refer to any constructed facility wherein concrete is used, including, but not limited to, buildings, bridges, parking garages, factories, harbors and ports.
The term “repair” is meant to refer to the addition to or alteration of an existing structure for improved structural performance. The term “strengthening” is meant to refer to the addition to or alteration of an existing structure for the purpose of increasing the strength of the structure beyond its original value.
The term “strength” is meant to refer to ability to resist axial forces, shear forces, and bending forces. The term “stiffness” is meant to refer to the resistance to cracking and deformation. The term “ductility” is meant to refer to the ability of the structure to undergo permanent deformation prior to failure.
3. Description of the Problem
As known to those skilled in this art, a typical concrete column is internally reinforced with steel. Basically, the concrete column contains two types of steel reinforcement, as shown in
FIGS. 1 through 6
.
One type is a longitudinal steel reinforcement in the form of individual longitudinal rods or bars
10
which are spaced apart and placed internally along the length of the concrete column. The other type is a lateral steel reinforcement which is placed internally in substantially parallel relation to the exterior surface of the concrete column. The lateral steel reinforcement can be either in the form of individual rectangular hoops or ties
12
, circular ties
14
, or a continuous one-piece spiral rod
16
.
The function of lateral reinforcement is to increase the shear strength of the concrete column and to provide confinement for the concrete
18
and lateral support for the longitudinal bars
10
to prevent them from buckling under large axial loads. Depending on the cross-sectional geometry of the concrete column, other shapes of lateral steel reinforcement can be used.
A variation of the above-described internally-reinforced concrete columns is one known to those skilled in the field as a “composite column”, shown in
FIGS. 7 and 8
. The composite column is composed of a wide-flange steel beam
20
, being H-shaped in cross-section, which is encased in the concrete column.
Concrete columns may require either repair or strengthening or both for various reasons. The repair or strengthening may call for the addition of external longitudinal or lateral steel reinforcement, or both of these. The reasons such repair or strengthening is needed include, but are not limited to, the following:
A. Seismic repair or strengthening
Many structures exist today which are not capable of resisting loads imposed on them during an earthquake. This is partly because when these structures were originally designed, little was known about how to design a structure to resist earthquake loads safely. As a result, many reinforced concrete columns in existing structures have insufficient longitudinal or lateral steel reinforcement or contain poorly-detailed steel reinforcement. Such concrete columns are unsafe in the event of an earthquake and therefore they need to be repaired or strengthened.
B. Gradual deterioration of structure
This deterioration could result from adverse environmental effects such as corrison of steel, salt spray, fire damage, hurricanes, tornadoes and the like. In such cases, the concrete column loses its design strength due to spalling of concrete and corrosion of reinforcement. Therefore, it is desirable to repair or strengthen these columns so that their strength is upgraded to at least that of the original capacity. This is a common problem with concrete columns in many aging structures.
C. Functional changes
In some structures, the introduction of heavier loads requires upgrading of load carrying capacity of concrete columns beyond their original design strength. For example, in order for older bridges to carry today's heavier trucks and traffic volumes, the strength of concrete columns must be increased beyond their original values.
D. Increased shear strength
Concrete columns in some existing structures may lack sufficient lateral steel reinforcement to withstand shear forces. In such cases, additional lateral reinforcement is needed to increase the shear strength of these concrete columns.
E. Increased ductility
In general, concrete columns with sufficient lateral steel reinforcement fail in a ductile manner, that is, they can resist large permanent deformations before they fail. Thus, repair and strengthening in the form of addition of lateral reinforcement may be desirable to increase the ductility of existing concrete columns.
F. Construction errors
Repair or strengthening may be required to correct some construction errors in a fairly new structure where, by mistake, some of the required reinforcement has been omitted or misplaced during the construction.
G. Increased factor of safety
Strengthening of some structures can be performed primarily for increasing the factor of safety against failure.
When repair or strengthening is required, it is necessary to employ the most cost effective technique. In selecting the appropriate repair or strengthening method, such factors as the original repair or strengthening cost and time required, future maintenance cost, expected life of the repaired or strengthened concrete column and the structure, availability of the repairing or strengthening materials, the ratio of the additional strength to cost, etc., should be considered.
For most concrete columns, the primary interest in repair or strengthening lies in providing additional confinement in the form of lateral reinforcement. Since it is not practical to add internal lateral reinforcement to an existing concrete column, some form of external lateral reinforcement is typically utilized.
4. Description of the Prior Art
Up to the present time, several methods known to those skilled in this art have been used to externally repair and strengthen internally-reinforced concrete columns in existing structures. These strengthening methods include, but are not limited to, the following:
1. Steel encasement
This strengthening method, also called steel jacketing, involves the building of a loosely-fitting steel case around an existing reinforced concrete column. The case is constructed of thin steel sheets and fully encloses the concrete column. The gap between the case and the column is then filled with pressurized grouting mortar.
2. Steel straps and angles
In this method of strengthening, steel angles are placed at corners of rectangular concrete columns along the full height of the column. Thin rectangular steel pieces are welded to the angles around the periphery of the column at specified elevations along the height of the column. This will create an encasing cage around the concrete column which will improve its structural response in the event of an earthquake.
3. Steel wire fabric
Welded wire fabrics in the form of orthogonal steel wires are placed around the periphery of the concrete column along the full height. A layer of fresh concrete is then cast on the wires around the column. This increases the cross-sectional
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Hexcel Corporation
Safavi Michael
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