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You are an expert at summarizing long articles. Proceed to summarize the following text: 
RELATED APPLICATIONS 
     This application claims priority of Provisional Application S/No. 60/335,662 filed on Oct. 23, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of building design; more specifically, it relates to emergency stairwells for multistory building. 
     BACKGROUND OF THE INVENTION 
     Most multistory buildings are provided with emergency stairwells to provide quick evacuation of the building in the event of an emergency such as a fire and as alternative evacuation routes to elevators. 
     In conventional emergency stairwell design, the widths of the stairwell remains constant from the uppermost floors serviced by the emergency stairwell to the lowermost floors serviced by the emergency stairwell. This design is predicated on the assumption that persons entering the emergency stairwell from lower floors will have reached the lowermost egress from the emergency stairwell before persons entering the emergency stairwell from upper floors reach the lower floors. 
     One problem with this assumption is that in high buildings, people get tired and their rate of descent slows down. As persons from upper floors overtake these now, slower moving persons, congestion builds up slowing egress still more. A similar slowdown can occur when more vigorous or able persons overtake less vigorous or able persons. 
     Another problem with conventional emergency stairwells, especially in very high buildings is, other than floor numbering, there is no stimulus that indicates the progress is being made to an eventual egress. Going down floor after floor can become claustrophobic and induce panic in the evacuees. 
     Providing more emergency stairwells does not address these problems, and building uniformly wider emergency staircases, while addressing some of the problems is wasteful of expensive floor space. 
     Therefore there is a need for an improved emergency stairwell that reduces or eliminates buildup of congestion on sections of the stairwell servicing lower floors, provides some more than a textual indication that progress toward an egress is being made and does not consume unacceptable amounts of floor space. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is an emergency stairwell for a building having multiple floors comprising: at least one landing associated with each the floor, each landing increasing in width in at least one horizontal direction from an uppermost landing of an upper floor to a lowermost landing of a lower floor; and at least one set of stairs extending between adjacent pairs of landings. 
     A second aspect of the present invention is an emergency stairwell for a building having multiple floors comprising: a plurality of stairwell sections, each section comprising: a set of landings, one landing of each set of landings associated with one the floor, and at least one set of stairs extending between adjacent pairs of landings, all landings within a stairwell section having the same width in at least one horizontal direction; and each stairwell section and associated landings within that stairwell section increasing in width in at least one horizontal direction from an uppermost stairwell section associated with a group of adjacent upper floors to a lowermost stairwell section associated with a group of adjacent lower floors. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is cross-sectional view of multistory building having an emergency stairwell according to a first embodiment of the present invention; 
     FIG. 2 is top view of a section of the emergency stairwell of FIG. 1; 
     FIG. 3 a top view of a section of an alternative emergency stairwell according to the present invention; 
     FIG. 4A is a top view of a section of an emergency stairwell according to a second embodiment of the present invention; 
     FIG. 4B is a side view of a portion of the emergency stairwell according the second embodiment of the present invention; and 
     FIGS. 4C and 4D illustrate respectively stepped and ramped options for the second embodiment of the present invention; and 
     FIG. 5 is cross-sectional view of multistory building having an emergency stairwell according a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention. 
     FIG. 1 is cross-sectional view of multistory building having an emergency stairwell according to a first embodiment of the present invention. In FIG. 1, a multistory building  100  includes a multiplicity of floors  105 A through  105 L, floor  105 A being the lowest floor, closet to a ground level  110 , and floor  105 L being the highest floor, immediately under a roof  115 . Building  100  also includes an emergency stairwell  120 . One or more fire doors  125  on each floor  105  of building  100  provide access to the emergency stairwell. 
     In one example, fire doors  125  provide access to upper landings  130 . Upper stair sets  135  connect upper landings  130  to lower landings  140 . Lower landings  140  are connected to the upper landings  130  of the immediately lower floor  105  by lower stair sets  145 . 
     Emergency stairwell  120  has two widths, a first width within the plane of the paper and a second width perpendicular to the plane of the paper. Only the first width is illustrated in FIG.  1 . Emergency stairwell  120  has a first width “W 1 ” at the lowest floor in the present example, floor  105 A) and a first width “W 2 ” at the highest floor (in the present example, floor  105 L.) “W 1 ” is greater than “W 2 .” The first (and second) width of emergency stairwell  10  increases by a fixed amount from floor to floor such that the lower of any two adjacent floors is wider than the upper floor. Upper and lower landings  130  and  140  get wider in both first and second widths, while upper and lower stair sets  135  and  145  only get wider only in the second width, progressively from upper to lower floors. The number of steps (and hence the length) in upper and lower stair sets  135  and  145  remains constant from floor to floor as long as the height of each floor is the same. Or more precisely, the ratio of the total horizontal run to total vertical drop of stair sets  135  and  145  remains constant from floor to floor as long as the height of each floor is the same. 
     The progressively wider width(s) of emergency stairwell  120  from the upper floors to the lower floors of building  100  works to prevent backup of evacuees in the stairwell on upper floors due to congestion on the stairwell on lower floors by providing increasing area and hence carrying capacity of the stairwell. Further, the progressively wider width(s) of emergency stairwell  120  from the upper floors to the lower floors of building  100  provides visual stimulus that indicates that progress is being made to an eventual egress. 
     FIG. 2 is top view of a section of the emergency stairwell of FIG.  1 . In FIG. 2, emergency stairwell  120 , has a first width “A 1 ” and a second width “A 2 ” in a portion  150  of the emergency stairwell corresponding to an upper floor (for example floor  105 L of FIG. 1) and a first width “A 3 ” and second width “A 4 ” in a portion  155  of the emergency stairwell corresponding to a lower floor (for example, floor  105 K of FIG. 1.) In one example “A 1 ”=“A 2 ,”“A 3 ”=“A 4 ” and “A 1 ”&gt;“A 3 ” by an amount Δ. Upper landing  130  has a first width “A 5 ” and a second width “A 2 .” Lower landing  140  has a first width “A 6 ” and a second width “A 4 .” “A 6 ” is greater than “A 5 ” by amount Δ. Upper stair set  135  has width “A 7 ” and a length “A 8 .” Lower stair set  145  has width “A 9 ” and a length “A 8 .” In one example, “A 9 ”=“A 7 ”+Δ/2. Hence, stairwell  120  increases in first and second widths by an amount Δ from the portion of the stairwell immediately above. 
     FIG. 3 is a top view of a section of an alternative emergency stairwell according to the present invention. One difference between the emergency stairwell of FIG.  2  and that illustrated in FIG. 3 is the number of landings. In FIG.3, an emergency stairwell  160 , has a first width “B 1 ” and a second width “B 2 ” in a portion  165  of the emergency stairwell corresponding to an upper: floor (for example floor  105 L of FIG. 1.) An upper landing  165  has a first width “B 3 ” and a second width “B 4 .” A next lower, first intermediate landing  170  has a first width “B 5 ” and a second width “B 4 .” First intermediate landing  170  is connected to upper landing  165  by first stair set  175 . First stair set  175  has a first width “B 6 ” and a second width “B 4 .” First intermediate landing  170  is also connected to a second intermediate landing  180  by a second stair set  185 . Second stair set  185  has a first width “B 5 ” and a second width “B 6 .” Second intermediate landing  180  has a first width “B 7 ” and a second width “B 8 .” Second intermediate landing  180  resides in a portion  190  of emergency stairwell  160  corresponding to the transition from an upper floor (for example  105 L of FIG. 1) and a lower floor (for example, floor  105 K of FIG. 1.) Second intermediate landing  180  is connected to a third intermediate landing  195  by a third stair set  200 . Third stair set  200  has a first width “B 6 ” and a second width “B 8 .” Third intermediate landing  195  has a first width “B 7 ” and a second width “B 8 .” Third intermediate landing  195  is connected to a lower floor landing (not shown) by a fourth stair set  205 . Fourth stair set  205  has a first width “B 7 ” and a second width “B 6 .” 
     In one example “B 1 ”=“B 2 , “B 3 ”=“B 4 ”=“B 5 ”=“B 6 ,” “B 7 ”=“B 8 ,” =“B 7 ”=“B 5 ”+Δ/2 and “B 8 ”=“B 4 ”=Δ/2. Δ is the incremental increase in size of emergency stairwell  160  from floor to floor progressing from upper to lower floors. 
     FIG. 4A is a top view of a section of an emergency stairwell according to a second embodiment of the present invention. In FIG. 4A, a portion  210  of an emergency stairwell  215  includes a floor landing  220  and a multiplicity of steps  225 A through  225 G between a cone shaped outer wall  230  and a cylindrical inner wall  235 . Inner wall  235  may include a void  240  as illustrated or may be solid. Each step  225 A through  225 G has a width “C 1 ” through “C 7 ” respectively. Since outer wall  230  is cone shaped each step is wider than the immediately upper step by an amount Δ′ but narrower than the immediately lower step by the same amount Δ′ . This is more clearly illustrated in FIG.  4 B. Therefore, “C 2 ”=“C 1 ”+Δ′,“C 3 ”=“C 2 ”+Δ′,“C 4 ”=“C 3 ”+Δ′,“C 6 ”=“C 5 ”+Δ′ and “C 7 ”=“C 6 ”+Δ′. 
     FIG. 4B is a side view of a portion of the emergency stairwell according the second embodiment of the present invention. In FIG. 4B, emergency stairwell  215  is shown passing through an upper floor  245  and a lower floor  255 . Access to stairwell  215  from upper floor  245  is through fire door  250  onto landing  220 . Access to stairwell  215  from lower floor  255  is through fire door  260  onto a floor landing  265 . Floor landing  220  on floor  245  is “C 1 ” wide, while step  225 D is “C 4 ” wide and floor landing  265  is “C 8 ” wide, where “C 8 ”=“C 7 ”+Δ′. 
     FIGS. 4C and 4D illustrate respectively stepped and ramped options for the second embodiment of the present invention. FIG. 4C is a cross section through a portion of several steps,  225 A through  225 C of emergency stairwell  215 . Optionally, steps may be replaced with a ramp as illustrated in FIG.  4 D. FIG. 4D is a cross section through a portion of ramp  270 , which replaces steps,  225 A through  225 C illustrated in FIG. 4C of emergency stairwell  215 . 
     FIG. 5 is cross-sectional view of multistory building having an emergency stairwell according a third embodiment of the present invention. In FIG. 5, a multistory building  3100  includes a multiplicity of floors  305 A through  305 L, floor  305 A being the lowest floor, closet to a ground level  310 , and floor  305 L being the highest floor, immediately under a roof  315 . Building  300  also includes an emergency stairwell  320 . One or more fire doors  325  on each floor  305  of building  300  provide access to the emergency stairwell. 
     In one example, fire doors  325  provide access to upper landings  330 . Upper stair sets  335  connect upper landings  330  to lower landings  340 . Lower landings  340  are connected to the upper landings  330  of the immediately lower floor  305  by lower stair sets  345 . 
     Emergency stairwell  320  has two widths, a first width within the plane of the paper and a second width perpendicular to the plane of the paper. Only the first width is illustrated in FIG.  51 . Emergency stairwell  320  has a first section  350 A having a width “W 3 ” comprised of the three lowest floors (in the present example, floors  305 A,  305 B and  305 C), a second section  350 B having a width “W 4 ” comprised of the next three higher (in the present example, floors  305 D,  305 E and  305 F), a third section  350 C having a width “W 5 ” comprised of the next three higher (in the present example, floors  305 G,  305 H and  305 I) and a fourth section  350 D having a width “W 6 ” comprised of the highest three floors (in the present example, floors  305 J,  305 K and  305 L.) The number of floors  305  within in each section  350  of emergency stairwell  320  may be a number other than three, for example from two half the number of floors in the building or more. The number of floors  305  within each section  350  of emergency stairwell  320  need not be the same. The first (and second) widths of emergency stairwell  320  increases by a fixed amount for example Δ″ from section to section such that the lower of any two adjacent sections is wider than the upper section. Thus “W 5 ”=“W 6 ”+Δ″,“W 4 ”=“W 5 ”+Δ″ and “W 3 ”=“W 3 ”+Δ″. Upper and lower landings  330  and  340  get wider in both first and second widths, while upper and lower stair sets  335  and  345  only get wider only in the second width, progressively from upper to lower sections. The number of steps (and hence the length) in upper and lower stair sets  335  and  345  remains constant from floor to floor as long as the height of each floor is the same. 
     The values for all widths “W 1 ” through “W 6 ”, “A 1 ” through “A 9 ”, “B 1 ” through “B 6 ”, “C 1 ” through “C 7 ” and “D 1 ” through “D 2 ”and all delta&#39;s Δ, Δ′ and Δ″ are primarily functions of the number of occupants of each floor and the number of floors in the building. 
     The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. For example, more than one emergency stairwell according to the present invention may be present within the same building. Further, a single emergency stairwell of the present invention need not run through all floors of the building, but only through a contiguous subset of the floors. Still further, the various embodiments of the emergency stairwell of the present invention herein described, may be used in combination with one another within the same building. Finally, one or more emergency stairwells according to the present invention may be used in combination with one or more emergency stairwells of conventional design. 
     The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.

Summary:
An emergency stairwell for a building having multiple floors comprising: at least one landing associated with each the floor, each landing increasing in width in at least one horizontal direction from an uppermost landing of an upper floor to a lowermost landing of a lower floor; and at least one set of stairs extending between adjacent pairs of landings.