Abstract:
Reinforcing system for retrofitting wall to increase the ductility and resistance to shear forces of a gypsum board wall to a level comparable to plywood-sheathed wall. Fiber-reinforced polymer panel is attached to substantially cover surface of gypsum board to protect it against rupture in earthquake. Reinforcement is enhanced by fiber anchors installed along base of wall and door frames. Optional connector strips tie together door and window frames to prevent separation from wall.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation-in-Part of application Ser. No. 11/399,282, filed Apr. 6, 2006 now U.S. Pat. No. 7,574,840, which is itself a C-I-P of application Ser. No. 10/205,294, filed Jul. 24, 2002, which issued Apr. 24, 2007 as U.S. Pat. No. 7,207,149. These previous applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to reinforcing a structure, and more particularly to increasing the shear resistance of an existing structure. 
     BACKGROUND OF THE INVENTION 
     Buildings have traditionally been designed to support their own weight plus that of expected inhabitants and furnishings. Buildings and other structures for supporting weight have long been expected to be very strong under vertical compression. Concrete is a favorite material for weight-bearing structures because it is inexpensive and has exceptional compressive strength. 
     In the mid-1900s, architects began to take lateral forces into account more than they had previously. Wind can exert strong lateral force on tall buildings and long bridges. Smaller structures were still designed without much regard for strong lateral forces, though, until concern for earthquake resistance began growing in the 1970s in the United States, partly due to the massive Anchorage earthquake in 1964. 
     Frame structures consist of a skeleton of elongate wood, metal, or concrete members that are connected together. These elongate members may be connected together by various means. In some cases brackets that join elongate members while resisting twist are used. More typically, framing members are connected with nails or screws that are easily bent and that can allow the framing members to pivot about the connection when under stress from an unusual direction. 
     Once the framework skeleton is complete, a sheathing of some material is applied over the framing to give a smooth surface and to increase the shear resistance of the wall. Such a sheathing material is typically plaster, wood paneling including plywood, or gypsum board, also known as drywall or sheetrock. 
     The stiffness of the sheathing material helps maintain the framework erect under lateral forces such as earthquake or high wind. Building codes take this effect into account and allow designers to include fewer diagonal braces or other shear reinforcements than would be required for unsheathed frame walls. Because various sheathing materials are known to have different shear strength values, there are different code requirements for constructing the frame, depending upon the planned sheathing material. 
     The shear strength values were formerly derived from small scale mechanical tests of the materials themselves. Testing of construction materials has become more realistic and sophisticated in the past few decades. As a result, some of the previously used strength values have been found to be inaccurate. 
     In particular, buildings that use gypsum board for interior walls, also called drywall construction, have been found to have much less resistance to lateral forces than their designers intended. For many years, designers used an erroneously high value for the shear resistance of walls faced with gypsum board. Later research, as well as analysis of buildings damaged by earthquake or wind, has shown that the true shear resistance contribution of gypsum board is only about 10% of what was previously accepted. 
     Many buildings worldwide need to be retrofitted so as to have the desired degree of shear force resistance. A conventional method for strengthening such buildings is to pull out the gypsum board and replace it with plywood that is attached to the building framework. 
     Replacing gypsum board with plywood is an effective method for increasing the resistance to lateral forces, but has disadvantages. The “demolition” step of removing the gypsum is extremely dusty, releasing particles into the atmosphere of the building and generating larger particles that drop to flat surfaces and into crevices. Between disposal of the bulk of the gypsum board and the cleanup of the building, a great deal of solid waste is created. 
     The dust may include gypsum, asbestos, and paper. Because dust in the air is harmful to people, animals, and many machines, the contents of the building have to be wrapped, packed, or removed so they are not contaminated. Residents or workers in the building being retrofitted may be required to absent the building for a day or longer. 
     Both the steps of demolition and of installing plywood are noisy for the entire duration of the work. Even if people and machines in the building can be isolated from the dust by temporary walls, such as of plastic sheeting, it is likely that the noise of the operation would prevent occupants from working or resting in the building during the retrofitting. 
     Simply replacing the drywall sheathing of interior walls with plywood may not be enough to increase the strength of the structure as much as desired. Additional wooden bracing within the walls or use of metal tie straps to connect various components of the structure together may be needed. 
     To withstand lateral forces such as seismic or wind forces, a structure&#39;s components must be strongly connected together. Yet, it has been found that extremely rigid structures do not fare as well in earthquakes or wind as structures with some flexibility. Replacing gypsum board with nailed-in plywood does not significantly improve the ductility of the structure. If additional internal bracing or metal connectors must be installed, the ductility of the structure may be actually reduced, leaving the structure still vulnerable to cracking or rupturing under strong lateral force. A violent failure of one component of a structure often causes a sudden chain reaction failure of other components, possibly trapping or crushing occupants. A ductile structure is more likely to fail in a gradual manner, allowing time for occupants to notice the impending failure and take steps to evacuate. 
     Seismic retrofitting by replacing gypsum board with plywood is expensive and is therefore typically being done on only the highest-risk structures. Costs of the method include loss of productivity and use of the building during the retrofitting, potential cost of temporarily relocating occupants, dust abatement and cleanup, cost of demolition labor and disposal fees for the gypsum, cost of protecting contents of the building, cost of additional bracing and reinforcing, and cost of the plywood itself and its installation. Lastly, after the walls are replaced, paint, trim, wallpaper and other ornamental finishes must be replaced. 
     The need for a less costly method of seismic retrofitting of drywall structures is great. Such a method should provide shear resistance that is at least equivalent to that of plywood. There is a need for such a retrofit method that does not generate large quantities of solid waste and that does not contaminate the structure with harmful dust and particles. 
     There is further a need for a retrofit method that can be performed while people work or live in the building, without undue noise or exposure to harmful materials. Such a retrofit method should preferably make it more likely that any failure of the structure, if it does occur, is gradual instead of sudden and catastrophic so that occupants may escape. 
     SUMMARY OF THE INVENTION 
     The present invention is a system for increasing the shear resistance of gypsum sheathed walls and optionally reinforcing the attachment between multiple structural components. A structure reinforced by the materials and method of the invention is less likely to fail under lateral forces, such as those experienced during an earthquake, hurricane, or explosion. 
     The resistance to shear forces of structures reinforced by the system of the present invention is at least as great as that of structures reinforced by the conventional replacement of gypsum with plywood. However, the apparent ductility of the structure is greater and the total cost is significantly lower. 
     Using the system of the present invention, retrofitting can be performed while people occupy the building, without creation of dust and with much less noise than is made during the conventional procedure. Much less solid waste material is created. 
     The method of the present invention includes covering gypsum wallboard with thin composite sheeting, such as panels of polymer-impregnated textile. Ductile attachment means, preferably fiber anchors as disclosed in U.S. Pat. No. 7,207,149, are installed to connect the covered wall to an adjacent structural component such as a concrete slab or frame member. 
     If connecting straps are needed for additional connection among structural components such as door frames or between floors of multilevel buildings, long strips of composite are applied where needed on the outer surface of the gypsum wallboard. No bracing inside of the wall is necessary and the gypsum board remains in place throughout the structure. 
     This method is fairly quiet and dustless, so therefore does not require relocation or protection of occupants and equipment. The composite panels used contain only small amounts of volatile chemicals, so there is no hazardous or intrusive odor. Because the gypsum wall sheathing remains in place, a large quantity of solid waste is not generated and the retrofitting is completed in less time. 
     The system and method of the present invention provides a lower-cost, safer, and faster alternative to replacement of gypsum board with plywood, yet improves the shear strength of the retrofitted building at least as well as the plywood replacement method. 
     The invention will now be described in more particular detail with respect to the accompanying drawings, in which like reference numerals refer to like parts throughout. In the drawings, not all details of construction of the structures are shown, for the sake of clarity. However, the illustrated structures are intended to represent conventional structures that include framed walls with brittle wall sheathing, such as gypsum board. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front elevation view, cut away, of a first embodiment of the reinforcement system of the present invention reinforcing a conventional frame and gypsum board wall. 
         FIG. 2  is a front elevation view, cut away, of a second embodiment of the reinforcement system of the present invention reinforcing a wall that includes other structural elements, namely door frames. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a front elevation view of reinforcement system  10  of the present invention reinforcing a portion of a structure  100 , such as conventional frame wall  110  that is covered with a sheathing of gypsum board  112 , which is partly cut away. Frame wall  110  includes framing members  115 , vertical and horizontal members, typically of wood. Gypsum board  112  is nailed to framing members  115  to cover framing members  115  and give wall  110  a smooth outer surface  113 . Wall  110  is supported by a foundation, such as concrete slab  130 . 
     Reinforcement system  10  includes a sheet of textile  20 , such as fabric that is woven or knit from fibers with high tensile strength. Textile  20  is stretched over surface  113  of gypsum board  112  and attached by suitable means, such as adhesive. Adhesive may be previously applied to surface  113  before textile  20  is stretched over surface  113 , or textile  20  may be stretched and temporarily attached, such as with staples, then adhesive may be sprayed or rolled on over textile  20  to attach textile  20  to surface  113 . 
     Preferably, textile  20  is a panel of fabric that is pre-impregnated with synthetic resin, such as epoxy, urethane, or other polymers as are well-known in the art. Most preferably, the impregnation step has been performed at another location and most solvents or other volatile components of the resin have already evaporated. The “B-stage” gel that remains in textile  20  thus has low odor and low human toxicity via respiration. The B-stage panel of textile  20  is flexible and easy to cut, drill, or punch, but is not so sticky that it is difficult to handle. 
     This type of textile panel is commonly known as “pre-preg” or “FRP” (fiber-reinforced polymer). The fiber portion of the panel is typically woven or knitted filaments of glass or graphite carbon. A suitable FRP panel typically is tacky enough to adhere lightly to a wall upon contact, then cures at ambient temperature over a period of hours or days to become tightly adhered. Such a panel may also be applied to a ceiling, but may require an additional tack coat of liquid or pasty adhesive to hold it in place during curing. Light mechanical fasteners such as staples may also be used. 
     If textile  20  is not pre-impregnated with resin, various means for attaching textile  20  to wall  110 , as are known in the art, may be used. For example, textile  20  may be run between rollers that apply a suitable adhesive. Alternatively, textile  20  may be stretched over surface  113  then suitable adhesive is applied over textile  20  such as by brush or spray. The adhesive will penetrate textile  20  and adhere to surface  113 . 
     Examples of suitable adhesives include epoxy, polyurethane, latex, and acrylic. It is preferred that the adhesive used should be low in volatile emissions during curing and that any vapors emitted be low toxicity and low odor. 
     For maximum improvement of ductility and strength, textile  20  is attached to wall  110  substantially coplanar, so as to largely cover wall  110 . That is, textile  20  is not attached to wall  110  such as by edges only or by intermittent areas of adhesive. Textile  20  is preferably attached across its entire wall-facing surface to surface  113  of wall  110 . 
     Fiber anchors  30 , as are known in the art and disclosed in U.S. Pat. No. 7,207,149, are installed along one or more edges of wall  110 . Boreholes  31  are drilled into an anchor medium adjacent wall  110 , such as into slab  130 , into the soil supporting structure  100 , or into a frame member  115  of wall  110  or a frame member  115  of an adjacent floor or level of structure  100 . 
     Boreholes  31  are typically drilled into a framing member  115  near the top or bottom of wall  110 . However, a borehole  31  can optionally be drilled through gypsum board  112  and into an underlying framing member  115  in order to install a fiber anchor  30  that is not disposed at an edge of wall  110 . 
     A length of roving  32 , composed of loosely twisted filaments of ductile, strong fiber, is inserted into each borehole  31  with a free end  33  protruding. Free end  33  of roving  32  is splayed out against textile  20  and attached to textile  20  with a suitable adhesive  36 . 
     One preferred method of practicing the invention is to first attach textile  20  to surface  113 , then to attach free end  33  over textile  20  such that free end  33  is attached with adhesive  36  to the outer surface of textile  20 . A second preferred method of practicing the invention is to attach free end  33  directly to surface  113  of wall  110 , then to attach textile  20  with adhesive such that free end  33  is attached to the inner surface of textile  20 . 
     In the exemplary embodiment of the illustrations, a plurality of fiber anchors are shown as arrayed along the sill of wall  110  with boreholes  31  drilled into slab  130 . The combination of textile  20  and fiber anchors  30  provide a strong ductile connection between slab  130  and wall  110 , reinforcing wall  110  against being disconnected from slab  130  by strong lateral force, such as from an earthquake. Perhaps more importantly, textile  20  increases the ductility of surface  113  of wall  110 , making gypsum board  112  unlikely to rupture catastrophically. 
     To reinforce connection among floors of a structure, anchors  30  may be installed such that borehole  31  is drilled into a frame member  115  of an adjacent floor. For example, borehole  31  may be drilled upwardly into a support member  115  of the floor above. In this case, free end  33  would extend downward and be splayed against an upper portion of surface  113  of wall  110 . 
     It is also within the scope of the present invention to drill borehole  31  through an adjacent frame member  115 , such as a joist or beam, and insert roving  32  through borehole  31  such that a free end  33  protrudes from each end of borehole  31 . A first free end  31  is splayed and attached to a first wall, ceiling, or floor; and a second free end  31  is splayed and attached to a second wall, ceiling, or floor. 
     From observing the effects of actual strong earthquakes and simulated earthquake tests on conventional structures  100 , non-reinforced gypsum board  112  has been found to respond in a brittle manner, cracking and rupturing away from framing members  115 . Once gypsum board  112  ruptures, it contributes no strength to wall  100 , allowing framing members  115  to bend their connections, typically nails, screws, or brackets, so as to allow wall  100  to collapse. This type of failure in one section of wall  110  may lead to further failures in other sections of structure  110 . 
     Reinforcement system  10  of the present invention increases the ductility of wall  110  and connects wall  110  to the foundation, such as slab  130  or a lower floor (not shown) of structure  100 , in a strong ductile manner. Even under strong lateral forces, such as from a major earthquake, reinforced structure  100  maintains connection among all components such as framing members  115 , gypsum board  112 , and slab  130 . As long as all the components of structure  100  remain connected, they act cooperatively to maintain structure  100  in a non-collapsed state, even if some lesser damage such as breaking of windows occurs. It has been found in laboratory testing that reinforced gypsum board  112  may crack, but because it is supported against rupture by textile  20 , gypsum board  112  remains attached to framing members  115  and does not fully break or collapse. 
       FIG. 2  is a front elevation view of a second embodiment of reinforcement system  10  of the present invention reinforcing a wall  110  that includes other structural elements, namely door frames  120 . Wall  110  is sheathed by gypsum board  112  and is reinforced with textile  20 , shown partly cut away, and fiber anchors  30  of the type previously discussed. 
     Some structures  100  may need further reinforcement among individual components, such as connecting door frames  120  to reinforced wall  110  to prevent them from separating from wall  110  and toppling to one side or the other of wall  100  under strong lateral forces. In the past, people were advised to take refuge in a doorway during a strong earthquake and many people still do this. Thus, it is especially desirable that door frames  120  not separate from wall  110 , possibly injuring a person trying to shelter in the door opening. 
     In conventional structures  100  that experience strong lateral forces, especially forces that change direction such as earthquakes, certain structural components such as door frames  120  may sway with a different frequency than the sway frequency of the rest of wall  110  or structure  100 . The unsynchronized swaying may cause gypsum board  112  around door frames  120  to crack or rupture, allowing door frames  120  to separate from wall  110  and possibly topple away from wall  110 . 
     To further reinforce structural components that are not strongly connected to the rest of structure  100 , such as door frames  120 , long “drag” or “collector” strips  40  of textile  20  connect a plurality of door frames  120 . As shown in  FIG. 2 , each door frame has two vertical collector strips  40  attached generally over or in proximity to the vertical members of the door frame  120 . A long horizontal collector strip  40  is attached above door frames  120 ; horizontal strip  40  is attached with a suitable adhesive to surface  113  and to the vertical collector strips  40 . The adhesive used to attach connector strips  40  may be the same as used to attach textile  20 , but different adhesive may also be used. Collector strips  40  may alternatively be additionally attached to framing members  115  with mechanical fasteners, such as screws (not shown) to further increase the strength of the structure. 
     Drag, or collector, strips  40  provide strong ductile connection among door frames  120  and connect door frames  120  to other structural components, such as wall  110 . In the event of a major earthquake or other strong lateral force, such as from a hurricane or explosion, door frames  120  will sway in unison with framing members  115  and reinforced gypsum board  112  instead of breaking away from them. 
     In like manner, collector strips  40  may be employed to reinforce the connection among many structural components, including but not limited to doors, windows, tilt-up walls, chimneys, and balconies. Collector strips  40  are not always required, but may be optionally employed to meet the requirements of a given application. 
     Collector strips  40  are optionally used to create a load path among floors or other portions of a structure  100 . A slot may be cut, such as through a ceiling or floor, to allow a collector strip  40  to be passed through. Collector strip  40  is then attached by suitable adhesive to surfaces  113  of walls  110  on different floors of structure  100 , or to framing members  115  or other components of structure  100 , as appropriate. For creating a load path through structure  110 , collector strip may be oriented vertically, horizontally, or at an angle. 
     Another preferred use of collector strips  40  is to buttress the attachment of fiber anchors  30  to surface  113 , as seen in the middle portion of  FIG. 2 . An elongate collector strip  40  about 12 wide may be placed over a plurality of anchors  30 , whether anchors  30  are disposed along the bottom or the top of wall  110 . 
     System  10  of the present invention is described herein as being useful for reinforcing walls that are covered, or sheathed, with gypsum board  112 , often known as drywall or sheetrock. While there are very many gypsum board walls urgently in need of reinforcement, there are also other types of structural components that can be reinforced using system  10 . For example, reinforcement system  10  may be used to strengthen walls that are sheathed with plywood, if a very strong and ductile wall is required. Reinforcement system  10  is most simply applied to planar surfaces, such as wall  110  described herein, but may be employed to connect walls that are at an angle to each other, including both “inside” and “outside” right angles. 
     Although particular embodiments of the invention have been illustrated and described, various changes may be made in the form, composition, construction, and arrangement of the parts herein without sacrificing any of its advantages. Therefore, it is to be understood that all matter herein is to be interpreted as illustrative and not in any limiting sense, and it is intended to cover in the appended claims such modifications as come within the true spirit and scope of the invention.