Section 27-579. Distribution of horizontal loads

The following provisions
  shall apply to superstructure framing  only,  and  shall  not  apply  to
  structures wherein horizontal loads are transmitted to the foundation by
  stay-cables,  arches,  non-rectangular frames, or by frames, trusses, or
  shear walls not oriented in vertical planes.
    (a) Distribution of horizontal loads to vertical frames,  trusses  and
  shear  walls. Horizontal loads on the superstructure shall be assumed to
  be distributed to vertical frames, trusses, and shear walls by floor and
  roof systems acting as horizontal  diaphragms.  The  proportion  of  the
  total horizontal load to be resisted by any given vertical frame, truss,
  or  shear  wall  shall  be determined on the basis of relative rigidity,
  considering the eccentricity of the applied load  with  respect  to  the
  center  of  resistance  of  the  frames,  trusses,  or  shear walls. For
  vertical trusses, web deformations shall be considered in evaluating the
  rigidity.
    (b) Distribution of horizontal  loads  within  rigid  frames  of  tier
  buildings.
    (1)  ASSUMPTIONS.  The  distribution  of horizontal loads within rigid
  frames of tier buildings may be determined on the basis of a  recognized
  method  of  elastic analysis or, subject to limitations in paragraph two
  of this subdivision, may be predicated on one or more of  the  following
  simplifying assumptions:
    a.  Points  of inflection in beams or columns are at their midspan and
  midheight, respectively. The story shear is distributed to  the  columns
  in proportion to their stiffnesses.
    b.  The  change  in  length  of  columns  due  to axial effects of the
  horizontal loads may be neglected.
    c. Vertical column loads due to horizontal forces  are  taken  by  the
  exterior  columns  only, or are resisted by the columns in proportion to
  the column distances from the neutral axis of the bent.
    (2) LIMITATIONS.
    a. For buildings over three hundred feet  in  height,  the  change  in
  length of the columns, due to the effects of the horizontal loads, shall
  be evaluated or the framing proportioned to produce regular movements of
  the  successive joints at each floor so that warping of the floor system
  may be neglected.
    b. Simplifying assumptions used in design shall be subject to approval
  by  the  commissioner  for  any   of   the   following   conditions   or
  circumstances:
    1.  For  buildings  over three hundred feet in height or for buildings
  with a height-width ratio greater than five.
    2. At two-story entrances or intermediate floors.
    3. Where offsets in the building occur.
    4. Where transfer columns occur.
    5. In any similar circumstances of irregularities  or  discontinuities
  in the framing.
    (c)  Distribution  of load in self-relieving construction. The framing
  of self-relieving construction may be  proportioned  on  the  assumption
  that  connections  are  fully  rigid in resisting moments due to lateral
  load and that any larger moments due to the gravity loads or  due  to  a
  combination of gravity and lateral loads will be relieved by deformation
  of the connection material, provided that:
    (1)  The  fasteners  shall  be  capable  of developing the full moment
  capacity of the connection at the allowable unit stress  established  in
  subchapter ten of this chapter.
    (2)  The  connection  shall  be capable of resisting the moment due to
  lateral load, and the shear due to lateral load plus vertical load,  all

  at  the  allowable  unit  stresses established in subchapter ten of this
  chapter.
    (3)   The   framing  and  the  building  are  within  the  limitations
  established in subparagraph b of paragraph two  of  subdivision  (b)  of
  this section.
    (4)   The  connections  shall  be  detailed  to  permit  the  required
  deformations without fracture, and their capacity to so  function  shall
  be verified by test or other means.
    (d)   Structural  walls  and  partitions.  Walls  and  partitions,  if
  specifically designed to resist the applied forces, may be considered as
  contributing to the resistance or rigidity of the structure with  regard
  to horizontal loads.