A system for earthquake retrofit of existing building structures uses a knee brace connected on one end to an existing stud and to an existing floor joist on the other end. An additional stud is provided for reinforcement to prevent the existing stud from failing. The capacity of all other components of the system exceed the capacity of the connections between the knee brace and the existing stud and existing floor joist, which connections act as a fuse in the event of an earthquake and fail first, thus preventing failure of other components and improving collapse capacity of the building structure.

FIELD OF THE INVENTION 
     The present invention pertains to a system for earthquake retrofit of existing building structures that makes wood-frame buildings more robust and helps to improve their earthquake resisting capacity. 
     BACKGROUND OF THE INVENTION 
     Many wood-frame buildings built in the early 20 th  Century have ground floors used as garages with large openings on the front wall and almost no interior walls. These buildings are broadly referred to as “soft-story”. They have bearing walls typically comprised of studs spaced 16″ on center. The studs are designed to carry vertical load, but they are not relied on to resist lateral loads generated by the building mass during an earthquake. As such, these building structures are susceptible to ground floor collapse during an earthquake. The earthquake retrofits of the prior art are expensive, complicated and require displacement of occupants during retrofit. What is needed is an inexpensive earthquake retrofit that can be performed by persons of average skill, with minimum invasion into the existing spaces and allowing current occupants to remain in the building during construction. 
     SUMMARY OF THE INVENTION 
     The present invention satisfies this need. The system for earthquake retrofit of existing building structures according to the present invention uses existing walls and floor joists to form a frame with particular connections that if used at a ground floor significantly improve lateral resistance of the building to earthquakes. A knee brace is placed and connected on one end substantially in the middle of an existing stud and is connected to the corresponding existing floor joist on the other end. The resulting knee brace assembly forms a frame and significantly increases vertical load and adds lateral load on the stud, thus an additional stud is provided for reinforcement to prevent the existing stud from failing. The capacity of all other components of the system, such as joist and other than knee brace connections, exceed the capacity of the connections between the knee brace and the existing stud and existing floor joist by at least 30%. Therefore, these connections between the knee brace and the existing stud and existing floor joist act as a fuse in the event of an earthquake and fail first, thus preventing failure of other components of the assembly and defining capacity of the system improving collapse capacity of the building structure. 
     A plurality of these systems, knee-brace frames, is installed at strategic location in the building and at stud spacing intervals, the number depends on the desired degree of protection and physical space limitations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         FIG. 1  shows an elevation view of a system according to the preferred embodiment of this invention. 
         FIG. 2  shows an elevation view of a knee brace to existing floor joist connection in a system according to the preferred embodiment of this invention. 
         FIG. 3  shows an elevation view of a knee brace to existing stud connection in a system according to the preferred embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This invention will be better understood with the reference to  FIG. 1  through  FIG. 3 . The same numerals indicate the same elements in all drawing figures. 
     Viewing, simultaneously,  FIG. 1  through  FIG. 3 , numeral  10  indicates a support structure. A pair of identical support structures  10  is disposed in a mirror image arrangement, as particularly shown in  FIG. 3 . 
     Numeral  20  indicates an existing stud. An existing building structure has a plurality of existing studs  20 , which are typically spaced 16″ on center. Each existing stud  20  comprises a bottom end indicated by numeral  20   a  and a top end indicated by numeral  20   b . Bottom end  20   a  is affixed to an existing bottom plate indicated by numeral  30 . Top end  20   b  is affixed to an existing first top plate indicated by numeral  40 . Existing first top plate  40  is affixed to an existing second top plate indicated by numeral  50 . Existing second top plate  50  is affixed to an existing floor joist indicated by numeral  60 . Existing bottom plate  30  is affixed to an existing foundation indicated by numeral  70 . 
     Numeral  80  indicates a new knee brace. New knee brace  80  comprises a first end indicated by numeral  80   a  and a second end indicated by numeral  80   b.    
     Numeral  90  indicates a new stud. New stud  90  is sized identically to existing stud  20 . New stud  90  is disposed next to existing stud  20  between existing first top plate  40  and existing bottom plate  30 . New stud  90  and existing stud  20  sandwich first end  80   a  substantially in the middle of existing stud  80  (i.e., the distance from bottom end  20   a  to first end  80   a  is substantially equal to the distance from first end  80   a  to top end  20   b ). This is done to maximize the vertical load carrying capacity. 
     Numeral  100  indicates a first new connection means. First new connection means  100  affixes first end  80   a  to existing stud  20  and to new stud  90 . First new connection means  100  is configured to withstand a first force. First new connection means  100  is designed to 2 to 2.5 times the horizontal load on the frame and is based on seismic demand calculations for specific locations, the angle of new knee brace  80  and capacity of the connection times 1.3. 
     Numeral  110  indicates a second new connection means. Second new connection means  110  affixes second end  80   b  to the existing floor joist  60 . Second new connection means  110  is configured to withstand a first force. 
     First new connection means  100  and second new connection means  110  are configured in such a way that the distance between top end  20   b  and first end  80   a  is substantially two times the distance between top end  20   b  and second end  80   b.    
     Numeral  120  indicates a new spacer. New spacer  120  is sandwiched between existing stud  20  and new stud  90 . 
     Numeral  130  indicates a new stud to joist connection means. New stud to joist connection means  130  affixes existing stud  20  to existing floor joist  60 . New stud to joist connection means  130  is configured to permit rotation of existing floor joist  60  with respect to existing first top plate  40 , existing second top plate  50  and existing stud  20 . At the same time, new stud to joist connection means  130  is configured to prevent vertical separation of existing joist  60  from existing first top plate  40 , existing second top plate  50  and existing stud  20  (i.e. it resists vertical uplift force yet the structural elements are free to rotate). The nails connecting existing joist  60  to the top plate prevent horizontal movement of the joist relative to the stud. 
     New stud to joist connection means  130  is configured to withstand a second force. 
     Numeral  140  indicates a new stud to bottom plate connection means. New stud to bottom plate connection means  140  is configured to permit rotation of existing bottom plate  30  with respect to existing stud  20 . 
     At the same time, new stud to bottom plate connection means  140  is configured to prevent separation of existing bottom plate  30  from existing stud  20 . 
     New stud to bottom plate connection means  140  is configured to withstand a second force. 
     The second force is at least 30% higher than the first force. It has been determined through experiment that the capacity of all other components of the system is based on the second force being at least 30% higher than the first force, it all works best to allow first new connection means  100  and second new connection means  110  to act as a fuse and fail first in the event of an earthquake, thus saving the building structure from developing less desirable non-ductile (or brittle) collapse mechanism. 
     Numeral  150  indicates a new block means. New block means  150  is disposed between new knee braces  80  in identical support structures  10  disposed in a mirror image arrangement. New block means  150  is bracing new knee braces  80 , placed side by side on opposite side of the adjacent existing joist  60 , against each other. 
     In the preferred embodiment described with reference to  FIG. 1-3 , first new connection means  100  and second new connection means  110  comprise six nails, said six nails being 0.131″×3.25″ nails. In the case of first new connection means  100 , three of said nails are applied from the side of new stud  90  and three of said nails are applied from the side of existing stud  20 . The nails are spaced as a minimum such as to meet industry accepted requirements for preventing wood splitting. In the case of second new connection means  110 , all six nails are applied from the side of new knee brace  80  following the same requirements. 
     Further, in the preferred embodiment described with reference to  FIG. 1-3 , new stud to joist connection means  130  comprises a hurricane tie, such as Simpson H2-A product. Also, new knee braces  80 , new block means  150 , new stud  90  and new spacer are  120  are each shown as formed of a 2×4 wood members. 
     At least one pair of identical support structures  10  must be used for earthquake retrofit. Usually, several pairs of identical support structures  10  are installed, depending on the level of protection desired. Obviously, a larger number of pairs of identical support structures  10  installed provides greater earthquake resistance. 
     In an alternative embodiment, combinations of singular support structures  10 , rather than identical pairs thereof, are installed. The above description applies, with equal force, to singular support structures  10 , except new block means  150  is not provided. A minimum of two support structures  10  must be used, but they do not need to be used in pairs. 
     While the present invention has been described and defined by reference to the preferred embodiment of the invention, such reference does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alteration, and equivalents in form and function, as would occur to those ordinarily skilled and knowledgeable in the pertinent arts. The depicted and described preferred embodiment of the invention is exemplary only, and is not exhaustive of the scope of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.