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FIELD OF THE INVENTION 
     This invention relates to a curtain wall system for multi-story buildings and, more particularly, to a wall system that is resistant to damage caused by swaying motions of buildings during an earthquake. 
     BACKGROUND OF THE INVENTION 
     Curtain wall systems are exterior wall systems on multi-story buildings that are made of appropriate cladding materials (e.g., glass, aluminum, stone, concrete, etc.) and which carry no superimposed vertical (gravity) loads. Hence, the term “curtain” implies that a curtain wall system is essentially “hung like a curtain” from the primary structural frame of the building. A curtain wall system does not, by itself, help a building stand erect. 
     Although curtain wall systems are normally considered to be “non-structural” parts of a building, such terminology is misleading because curtain walls must have the ability to withstand structural loads imposed by natural phenomena such as earthquakes and severe windstorms. In this context, the term “curtain wall” is a misnomer because non-structural parts of a building can be subjected to structural loads. This invention focuses on a curtain wall system that is highly resistant to the potentially damaging effects of earthquake-induced movements of building frames. 
     Many curtain wall systems are constructed with glass window elements glazed within an assemblage of aluminum framing members. Architectural glass, due to its brittle nature, is inherently vulnerable to curtain wall movements during earthquakes. Research studies have been conducted to investigate the seismic performance of various types of architectural glass elements held within various aluminum curtain wall framing systems using various glazing systems. Among the findings of these studies were the following: (1) architectural glass is vulnerable to damage and fallout under simulated earthquake conditions; (2) horizontal, in-plane racking movements of a curtain wall frame constitute the primary cause of glass damage and glass fallout under simulated earthquake conditions; (3) different types of architectural glass exhibit different degrees of resistance to glass fallout under simulated seismic conditions; and (4) flexural stiffness of aluminum framing members has an influence on the susceptibility of architectural glass to seismic damage (i.e., under simulated seismic conditions, stiffer curtain wall frames are associated with more glass damage and glass fallout than are more flexible frames). 
     Architectural glass is not the only type of curtain wall element that is vulnerable to fracture and fallout under earthquake conditions. Curtain wall systems comprised of any rigid, brittle elements such as stone panels, cementitious panels, etc. are also potentially vulnerable to the damaging effects of earthquake-induced building motions. 
     The primary factors causing earthquake-induced damage of conventional curtain wall systems are: (1) movements of the building&#39;s primary structural frame in response to earthquake ground movements; and (2) the fact that vertical framing members (mullions) in conventional curtain wall systems are connected structurally to more than one floor of the primary structural frame. 
     The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a curtain wall system in which curtain frame panels of each floor are not fixedly connected to curtain wall panels of adjacent floors. 
     Broadly, there is disclosed herein an earthquake-immune exterior wall system for use with a multi-story building structure. The wall system includes a plurality of anchor means for connecting the wall system to the building structure, each anchor means adapted to being fixedly connected to the building structure for a single story of the multi-story building structure. Connecting means are provided for connecting each of a plurality of first elongate members directly to only one of the anchor means so that each first elongate member is fixedly connected to a single story of the multi-story building structure. A plurality of second elongate members are connected between adjacent pairs of first elongate members. The first and second elongate members collectively define panel hanging areas. A plurality of exterior cladding panels are secured to the first and second elongate members at the panel hanging areas to define the exterior wall system of the building structure. 
     It is a feature of the invention that the anchor means comprises steel anchor frames. Each anchor frame is rectangular in configuration and is constructed of tubular steel. The connecting means comprises anchor brackets connecting each first elongate member to upper and lower horizontal members of the anchor frames. 
     It is another feature of the invention that the first elongate members comprise vertical mullions. 
     It is an additional feature of the invention that the second elongate members comprise horizontal mullions. 
     It is yet another feature of the invention to provide flexible means for connecting the first and second elongate members connected to any one story to first and second elongate members connected to the story immediately above the one story. The flexible means comprises a flexible gasket of polymeric material. 
     There is disclosed in accordance with a further aspect of the invention an earthquake-immune curtain wall system for use with a multi-story building structure. The wall system comprises a plurality of anchor means for connecting the wall system to the building structure. Each said anchor means is adapted to being fixedly connected to the building structure for a single story of the multi-story building structure. Connecting means connect each of a plurality of vertical mullions directly to only one of the anchor means so that each vertical mullion is fixedly connected to a single story of the multi-story building structure. A plurality of horizontal mullions are connected between adjacent pairs of vertical mullions. The vertical and horizontal mullions collectively define panel frames for each story. A plurality of exterior cladding panels are secured to the vertical and horizontal mullions at the panel frames to define the exterior curtain wall system of the building structure. 
     It is a feature of the invention that each panel frame further includes intermediate horizontal mullions to define plural subframes and an exterior cladding panel is secured at each subframe. 
     This invention relates to a curtain wall system for multi-story buildings that is highly resistant to the damage caused by multidirectional swaying motions in building frames during an earthquake. In a conventional curtain wall system, each story is connected structurally to the stories above and/or below it. Interstory relative movements resulting from earthquake-induced swaying motions of the building frame cause significant load transfer from story to story and cause such a conventional curtain wall system to be susceptible to earthquake damage. Not only does this damage necessitate expensive repairs, but serious threats to life safety are imposed when debris falls from a damaged wall system. In contrast, each story of the newly invented earthquake-immune curtain wall system is structurally isolated (i.e., decoupled) from adjacent stores, which produces the beneficial effects of minimizing wall system damage and the attendant risks of falling debris (in the forms of broken glass, stone, concrete, etc.) during an earthquake. 
     The earthquake-immune curtain wall system achieves structural isolation of each story by employing a newly developed “seismic decoupler joint” between each story and a newly developed structural support system for vertical mullions in the wall system frame. As a result, relative movements between adjacent stories in the building frame transfer no significant forces between adjacent stores in the curtain wall frame. This invention embodies a curtain wall system that is essentially “immune” from the effects of earthquake-induced building frame motions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1C are a schematic depiction of the displacement response of a typical building frame having a conventional curtain wall system to earthquake-induced ground motions; 
     FIGS. 1D-1F are a schematic depiction of the displacement response of a building frame having an earthquake-immune curtain wall system according to the invention to earthquake-induced ground motions; 
     FIG. 2 illustrates a typical framing and anchorage configuration of a conventional curtain wall system; 
     FIG. 3 illustrates a front elevation view, in various stages of assembly, of an earthquake-immune curtain wall system according to the invention; 
     FIG. 4 is a side view of the curtain wall system of FIG. 3; 
     FIG. 5 is a front elevation view of a steel anchor frame for the curtain wall system according to the invention; 
     FIG. 6 is a side elevation view of the steel anchor frame of FIG. 5; 
     FIG. 7 is a front elevation view of a portion of a panel frame of the curtain wall system according to the invention including vision panels and spandrel panels; 
     FIG. 8 is a side view of the panel frame of FIG. 7 also illustrating a seismic decoupler joint; 
     FIG. 9 is a vertical section taken along the line  9 — 9  of FIG. 7 illustrating the seismic decoupler joint according to the invention; 
     FIGS. 10A-10C illustrate front views of the seismic decoupler joint during horizontal, in-plane, interstory movements of a building frame under earthquake conditions; 
     FIGS. 11A-11C are vertical sections depicting positions of the seismic decoupler joint during horizontal, out-of-plane, interstory movements of a building frame under earthquake conditions; 
     FIGS. 12A-12C are vertical sections depicting positions of the seismic decoupler joint during vertical, interstory movements of a building frame under earthquake conditions; 
     FIG. 13 is a front elevation view showing positions of the curtain wall system during in-plane and out-of-plane interstory movements of the building frame during earthquake conditions; 
     FIG. 14 is a vertical section of that shown in FIG. 13; and 
     FIG. 15 is a front elevation view of a steel anchor frame for the curtain wall system according to an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Typical swaying motions of a conventional building frame  27  in response to earthquake-induced ground movements are shown schematically in FIGS. 1A,  1 B and  1 C. Particularly, FIG. 1A illustrates the building frame  27  in a normal, vertical position. FIG. 1B illustrates the building frame  27  in a first mode response. FIG. 1C illustrates the building frame  27  in a second mode response. Specific mode shapes of the building frame are affected by the flexural stiffness of the floor system relative to that of the columns. Regardless of the specific mode shape, interstory drift (the difference in horizontal displacement between adjacent stories in the building frame) is a primary cause of earthquake damage in conventional curtain wall systems. Earthquakes of low to moderate magnitude can cause expensive curtain wall damage and loss of building envelope weather-resistant seals. More severe earthquakes can, in addition to the aforementioned damage and loss of serviceability, impose hazards to life safety if damaged curtain wall fragments fall from the building frame. 
     Interstory drift can cause damage in curtain wall systems because vertical framing members in conventional curtain wall systems are connected structurally to more than one floor of the primary structural frame, as depicted in FIG.  2 . For example, vertical mullions  20  are connected at anchors  22  to the building structure for “Story (i)” and at anchors  24  to the building structure for “Story (i+1)”. Horizontal mullions  26  are connected between adjacent pairs of vertical mullions  20 . Rectangular curtain wall panels or rectangular curtain wall frame units  28 , see FIG. 1A, are connected between each pair of adjacent vertical mullions  20  and horizontal mullions  26 . 
     As illustrated in FIGS. 1B and 1C, such rectangular curtain wall panels or rectangular curtain wall frame units  28  are forcibly distorted into parallelograms  29  as a result of interstory drift when the curtain wall system at a given floor level is connected structurally to adjacent stories of the building frame. This forcible distortion of rectangular shapes into parallelogram shapes can cause frame-to-cladding panel contact, which can result in fracture of brittle cladding elements (e.g., architectural glass panels, stone cladding panels, precast concrete cladding panels, etc.) secured within the curtain wall system. 
     In accordance with the invention, vertical mullions are attached to only one story of the building frame, as depicted in FIGS. 3 and 4. The essence of the invention is to “decouple” (disengage) each story of the curtain wall system from adjacent stories, thereby permitting free movement of each story of the curtain wall system with respect to adjacent stories. By so doing, no significant loads are transferred between adjacent stories of the curtain wall system when the main building frame undergoes swaying motions under earthquake conditions. The result is a curtain wall system that is highly resistant to earthquake conditions. 
     Typical swaying motions of a building frame  27 ′ having an earthquake-immune curtain wall system in response to earthquake-induced ground movements are shown schematically in FIGS. 1D,  1 E and  1 F. Particularly, FIG. 1D illustrates the building frame  27 ′ in a normal, vertical position. FIG. 1E illustrates the building frame  27 ′ in a first mode response. FIG. 1F illustrates the building frame  27 ′ in a second mode response. As illustrated in FIGS. 1E and 1F, rectangular curtain wall panels or rectangular curtain wall frame units  28 ′ remain rectangular even with interstory drift when the curtain wall system at a given floor level is structurally decoupled from adjacent stories of the building frame. This result can be compared to the conventional curtain wall system depicted in corresponding FIGS. 1A-1C. 
     FIGS. 3 and 4 illustrate building structure of a typical multi-story building including vertical columns  30  operatively connected to individual floors  32  and associated spandrel beams  34 . Building structure for three floors or stories identified as “Story (i−1)”, “Story (i)”, and “Story (i+1)”, is illustrated. As is apparent, the building can have any number of floors. A curtain wall system in accordance with the invention is defined by plural curtain wall panel frames, one of which  36  is shown, connected to each story. Plural steel anchor frames  38  are connected to the spandrel beams  34 , as discussed below. The panel frames  36  are connected to the anchor frames  38 . The panel frame includes plural elongate vertical framing members or mullions  40 , three of which are illustrated, connected to the anchor frames  38 . Connected to the vertical mullions  40  are respective lower horizontal mullions  42 , intermediate horizontal mullions  44 , and upper horizontal mullions  46 . The particular sizes of the mullions  40 ,  42 ,  44  and  46  are dependent on the particular building requirement, as well as sizes of cladding panels to be connected therebetween, as discussed below. Also, depending on panel size, the intermediate horizontal mullions  44  may be omitted. In the illustrated embodiment of the invention the mullions are formed of extruded aluminum. Each panel frame  36  is defined by the upper and lower horizontal mullions  46  and  42  and the outermost of the vertical mullions  40 . Any intermediate vertical mullions  40  or the intermediate horizontal mullions  44  divide the panel frame  36  into smaller panel frames or subframes. 
     Referring to FIGS. 5,  6  and  7 , the steel anchor frames  38  are illustrated in greater detail. Each frame  38  is connected to a spandrel beam  34  at each story level in the main building structure using connection bars  48  secured as necessary to the spandrel beam  34 . Each anchor frame  38  is connected to the spandrel beam at two locations to provide stability of the anchor frame  38  against rotation about X, Y, or Z orthogonal axes, as shown in FIG.  5 . Each anchor frame is typically constructed of horizontal and vertical tubular steel members  50  and  52 , respectively, in a rectangular configuration with sufficiently large cross sections to provide adequate strength and bending stiffness to resist design wind loads. Because wind loads are site specific, required cross sections of the anchor frames are determined by structural engineering design for wind loads as appropriate for each specific building site and each location on the building envelope. Alternatively, plural anchor frames  38  could be replaced with a single unit  138  consisting of elongate horizontal members  150  connected with plural spaced vertical members  152 , see FIG.  15 . 
     Referring to FIG. 5, each steel anchor frame  38  has two anchor brackets  54  at locations that provide for pin supports via bolted connections to each vertical mullion  40 . Each anchor bracket  54  is centrally located at the opposite horizontal tubular steel members  50 . Vertical mullions  40  are connected to the steel anchor frames  38 , as shown at  55  in FIG.  4 . As a result, each vertical mullion  40  has a simply supported portion  56  between the anchor brackets  54  and a cantilever portion  58  above the uppermost anchor bracket  54 . 
     The lower, intermediate, and upper horizontal mullions  42 ,  44  and  46  are secured mechanically to vertical mullions  40  supported in the steel anchor frames  38  as shown in FIG.  7 . With the aluminum curtain wall framing thus in place, vision panels  60  and spandrel panels  62  of any appropriate construction are secured′to the curtain wall frame  36  by an appropriate glazing system or perimeter anchorage technique. For the purposes of illustration in this example, a combination of structural silicone glazing and dry glazing gaskets is employed to secure vision panels  60  and spandrel panels  62  to the curtain wall frame  36 . Again, it should be noted that the selection of cladding material and the selection of glazing system is at the discretion of the designer and is not an intrinsic part of the earthquake-immune curtain wall system. 
     Connections between the horizontal mullion  42 ,  44  and  46  and the vertical mullions  40  are the same as those in conventional curtain wall systems. Required cross sections of all vertical and horizontal mullions are determined by structural engineering design for site-specific wind loads. Unlike the conventional curtain wall system illustrated in FIG. 2 the vertical mullions  40  according to the invention are not secured by mechanical attachment to adjacent stories (i.e., Story (i+1) and/or Story (i−1)). This structural decoupling is accomplished by means of continuous seismic decoupler joints  64  along the top surface of the upper horizontal mullion  46  and the bottom surface of the lower horizontal mullion  42  as shown in FIG.  8 . By means of this configuration, relative movements of adjacent stories in the main building frame (such as those caused by earthquakes) transfer no significant loads from story to story. It should also be noted that, for maximum seismic resistance, the earthquake-immune curtain wall system should not be connected directly to interior ceiling elements, and that the ceiling of Story (i) should be attached to the underside of the floor structure of Story (i+1). 
     The interior facing side of the steel anchor frames  38  can also serve as a convenient and stable surface upon which interior architectural coverings  39  can be affixed in the spandrel area of Story (i), as shown in FIG.  8 . 
     A vertical section of the seismic decoupler joint  64  is shown in FIG.  9 . The decoupler joint uses a pair of continuous, flexible gaskets  66  made of polymeric material that accommodates in-plane, out-of-plane, and vertical movements between adjacent stories of the main building frame under earthquake conditions. 
     Each gasket  66  is made of an elongate, extruded flexible material that my span the entire width of a floor. In cross section, each gasket includes a central portion  68  connected between locking end portions  70 . The central portion  68  is originally flat. When installed, the central portion is rolled into position and assumes a U-shape, as illustrated in FIG.  9 . The locking end portions  70  are force-fit into channels  72  provided in the lower horizontal mullions  42  and upper horizontal mullions  46 , as shown. The channels  72 , in cross section, include teeth  74  for lockably engaging corresponding notches  76  in each locking end portion  70 . As shown, a flexible gasket is placed at both the front and rear of adjacent lower horizontal mullions  42  and upper horizontal mullions  46 . As a result, the central portions  68  extend inwardly between the lower horizontal mullion  42  of Story (i+1) and the adjacent upper horizontal mullion  46  of Story (i). 
     The seismic decoupler joint  64  also includes a rotation-accommodating face cap  78  that accommodates movement by means of a face cap hinge  80  and the use of a bead  82  of glazing sealant, e.g., structural silicone or other appropriate material, that has high deformation capability. This bead  82  of glazing sealant is located along the lower edge of the cladding panel, such as the spandrel panel  62 , as shown in FIG.  9 . When the face cap hinge  80  rotates counterclockwise, the sealant  82  is compressed, as shown in FIG.  11 B. If the face cap hinge  80  were to be rotated clockwise, then the sealant  82  would be stretched. However, as will be described later, the glazing sealant bead  82  adjacent to the rotating face cap hinge  80  will see only compression (and not tension) as a result of horizontal, out-of-plane, relative movements between adjacent stories of the main building frame under earthquake-induced motions. 
     The cladding panels  60  and  62  are otherwise sealed in the curtain wall frame  36  using, for example, setting blocks  84 , backer rods  86 , glazing tape  88 , and glazing gasket  90 , as is conventional. 
     Detailed depictions of how the seismic decoupler joint accommodates in-plane, out-of-plane, and vertical interstory movements are shown in the drawing figures, as described below. The continuous, flexible gaskets  66  within the seismic decoupler joint  64  also provide thermal insulation and a weather seal between adjacent stories of the building. 
     FIGS. 10A,  10 B and  10 C illustrate a front view of operation of a segment of the seismic decoupler joint  64  in the following positions: (1) in its normal position (FIG.  10 A); (2) when Story (i) moves horizontally in-plane to the right relative to Story (i+1) (FIG.  10 B); and (3) when Story (i) moves horizontally in-plane to the left relative to Story (i+1) (FIG.  10 C). Horizontal, in-plane interstory movements are accommodated by the seismic decoupler joint  64 , located between each story, which prevents the transfer of any significant loads between stories of an earthquake-immune curtain wall system. 
     FIGS. 11A,  11 B and  11 C illustrate a vertical section of the seismic decoupler joint  64  in the following positions: (1) in its normal position (FIG.  11 A); (2) when Story (i) moves horizontally out-of-plane outward (i.e., outward from the building face) relative to Story (i+1) (FIG.  11 B); and (3) when Story (i) moves horizontally out-of-plane inward relative to Story (i+1) (FIG.  11 C). Horizontal, out-of-plane, interstory movements are accommodated without stressing the continuous flexible gasket  66  in the seismic decoupler joint  64 —provided that the magnitude of the relative movement is less than approximately the total length of each individual strip of gasket  66  in the seismic decoupler joint  64 , or approximately twice the length “L” in FIG.  11 A. Out-of-plane movements in excess of approximately the length 2L would stretch the flexible gaskets  66  (and possibly tear them), but there would still be no significant amount of interstory load transfer in the curtain wall system. It is also shown in FIGS. 11B and 11C that the sealant bead  82  at the bottom of the Story (i+1) cladding panel is compressed, but is not stretched, as a result of horizontal, out-of-plane, interstory movements. 
     FIGS. 12A,  12 B and  12 C illustrate a vertical section of the seismic decoupler joint  64  in the following positions: (1) in its normal position (FIG.  12 A); (2) when Story (i) moves vertically upward relative to Story (i+1) (FIG.  12 B); and (3) when Story (i) moves vertically downward relative to Story (i+1) (FIG.  12 C). Vertical interstory relative movements are accommodated without vertical interstory load transfer, provided that the relative vertical movement does not exceed the vertical gap built into the seismic decoupler joint  64 , or the distance “H” in FIG.  12 A. 
     FIG. 13 contains a front view and FIG. 14 a vertical section of the earthquake-immune curtain wall system during simultaneous in-plane and out-of-plane interstory movements. (The movements are drawn to an exaggerated scale for clarity and emphasis.) It can be observed that, within the geometric limits designed into a specific version of the earthquake-immune curtain wall system, simultaneous in-plane, out-of-plane, and vertical interstory movements can be accommodated by the system without significant interstory load transfer. 
     In summary, the seismic decoupler joint  64 : (1) accommodates interstory movements in all directions; (2) transfers no significant loads between adjacent stories; and (3) provides an effective thermal insulation and weather seal between adjacent stories in an earthquake-immune curtain wall system.

Summary:
A curtain wall system for multi-story buildings that is highly resistant to the damage caused by multidirectional swaying motions in building frames during an earthquake. In a conventional curtain wall system, each story is connected structurally to the stories above and/or below it. Earthquake-induced swaying motions of the building frame cause significant load transfers from story to story and cause such a conventional curtain wall system to be susceptible to earthquake damage. Not only does this damage necessitate expensive repairs, but serious threats to life safety are imposed when debris falls from a damaged wall system. In contrast, each story of the earthquake-immune curtain wall system is structurally isolated (i.e., decoupled) from adjacent stories, which produces the beneficial effects of minimizing wall system damage and the attendant risks of falling debris (in the forms of broken glass, stone, concrete, etc.) during an earthquake.