Reinforcement of cementitious walls to resist seismic forces

Details Reinforcement of cementitious walls to resist seismic forces Reinforcement of cementitious walls to resist seismic forces

1999-09-14

2001-05-29

Friedman, Carl D. (Department: 3635)

Static structures (e.g., buildings)

Intersection of wall to floor, ceiling, roof, or another wall

Flexible barrier covering: shaped or edge-attached

C052S167400, C052S236600, C052S745050, C156S071000, C428S296100

Reexamination Certificate

active

06237292

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to the construction of buildings and, more particularly, to the reinforcement of cementitious building walls to resist damage produced by seismically induced forces.
There are thousands of buildings located in the earthquake-prone regions of the world whose cementitious walls are susceptible to seismically induced damage. (As used herein unless otherwise indicated, “cementitious” walls include cement or concrete walls made of either masonry or poured construction, which do or do not contain internal steel reinforcing structure.) During an earthquake, the ground upon which the building rests moves laterally and/or vertically. These ground motions are transmitted through the building foundation and thence into the building walls as force responses. The walls may be cracked as a result of the ground motions or, if the motions are sufficiently severe, the walls may fail completely and collapse.
The ground motions produce force response components in the building wall that lie in the plane of the building wall or out of the plane of the building wall. The nature of the ground motions and force responses at any particular location that might result from an earthquake cannot be predicted with complete certainty. However, in many cases the predominant mode of ground motion and the resulting forces on the walls can be estimated. That is, an engineer who analyzes a building and the geological fault structure in its vicinity can often predict that a wall would likely be subject to particular force components that are out-of-plane, vertical in-plane, and horizontal in-plane, where the “plane” refers to the plane of the wall. The present invention is directed toward reinforcement of cementitious building walls to resist damage induced by such in-plane forces and out-of-plane forces, also termed “flexural” response.
It has been known to externally reinforce building walls to resist damage induced by seismic movements. Studies by the inventors have determined that different modes of seismic forces require different types of external reinforcement for optimal damage resistance, and have further shown that some of the most common modes of external reinforcement may have little beneficial effect in resisting cracking and/or failing of the building walls in many cases. There is, therefore, a need for improved approaches to external reinforcement of cementitious building walls to resist damage caused by various types of force and motion components, which are optimized for particular force responses in the building walls. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides an approach to the external reinforcement of cementitious building walls against damage induced by seismic movements, which is tailored to the types of movements and forces expected. The approach is readily applied to both new construction and to the retrofit of existing construction. The technique of the present invention is relatively inexpensive to utilize, as compared with redesign or reworking of the interior structure of the wall. Reinforcement is achieved using materials that are well known in building construction or are known in other industries and can be readily applied to the case of building construction.
In accordance with the invention, a method is provided for externally reinforcing a vertically extending cementitious wall having a base adjacent to a horizontal building structure which extends laterally therefrom, against damage induced by out-of-plane seismic forces. The cementitious wall has a first side and a second side. A base strip of fiber composite material overlies the first side of the cementitious wall at its base. The base strip comprises vertically oriented fibers in a curable matrix. The method further includes providing a right-angle tie having a horizontal leg and a vertical leg, positioning the right-angle tie with the vertical leg contacting the base strip and the horizontal leg resting on the horizontal building structure, and applying an overlay layer of fiber composite material overlying the first side of the wall and the vertical leg of the right-angle tie. The vertical leg is captured between the base strip and a portion of the overlay layer of fiber composite material. The overlay layer of fiber composite material comprises vertically oriented fibers in a curable matrix. Lastly, the horizontal leg of the right-angle tie is fixed to the horizontal building structure.
The composite material used for the base strip and the overlay layer is preferably unidirectional graphite or carbon fibers embedded in a curable matrix, such as an epoxy matrix. Desirably, the wall is reinforced with the same approach on the second side of the wall. The right-angle tie preferably comprises a plurality of discrete right-angle ties arranged in a side-by-side fashion along the base of the wall. The right-angle ties preferably have protruding ears on at least the vertical leg to improve load transfer from the composite overlay.
At locations where there are horizontal building structures, such as a foundation, floor, or roof, extending horizontally from the wall, the loads in the composite overlay may either be passed through the wall structure or tied into it. Wall loads in the composite overlay are typically tied to foundations and roofs, but may be either tied to or passed through floor structures.
In the approach for passing the loads through the horizontal building structure without transferring them into the horizontal building structure, a floor opening is formed through the horizontal building structure at a location adjacent to the cementitious wall. An above-floor base strip of fiber composite material is applied overlying the first side of the cementitious wall immediately above the horizontal building structure, and a below-floor base strip of fiber composite material overlying the first side of the cementitious wall immediately below the horizontal building structure. Both base strips comprise vertically oriented fibers in a curable matrix. The method further includes providing a flat tie, having an above-floor leg and a below-floor leg, and positioning the flat tie through the floor opening with the abovefloor leg contacting and overlying the above-floor base strip and the below-floor leg contacting and overlying the below-floor base strip. The overlay layer of fiber composite material is applied overlying the first side of the wall and the flat tie, so that the flat tie is captured between the base strips and a portion of the overlay layer of fiber composite material. The overlay layer of fiber composite material comprises vertically oriented fibers in a curable matrix.
In another approach for tieing the loads of the composite material overlay into the horizontal building structure, the overlay layer of fiber composite material is applied having a first segment overlying the first side of the wall and a second segment overlying a portion of the horizontal building structure adjacent to the wall. The method includes providing a right-angle tie having a horizontal leg and a vertical leg, positioning the right-angle tie with the vertical leg contacting the first segment and the horizontal leg contacting the second segment, and fixing the horizontal leg of the right-angle tie to the horizontal building structure, through the material of the second segment.
The approach of the invention is also used to externally reinforce a wall against in-plane seismic forces. In accordance with this aspect of the invention, a method for reinforcing a cementitious wall comprises the steps of providing a vertically extending cementitious wall having a first side and a second side, and applying an overlay layer of fiber composite material to the first side of the wall. The overlay layer of fiber composite material comprises horizontally oriented fibers in a curable matrix, there being substantially no non-horizontal external fiber reinforcement. A similar approach may be used on the se

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