This is analogous to a girder with a deep haunch.
(2) Flexure. Diaphragm flexure is resisted by
Figure 7-51 illustrates the calculation of the chord
members called chords. The chords are often at the
forces at a re-entrant diaphragm corner. These chord
edges of the diaphragm, but may be located
forces need to be developed by an appropriate
elsewhere. The design force is obtained by dividing
connection to the floor or roof framing, or by the
the diaphragm moment by the distance between the
addition of a drag strut, to develop resistance to the
chords. The chords must be designed to resist direct
chord forces within the adjacent diaphragm.
tensile or compressive stresses, both in the members
and in the splices at points of discontinuity. Usually,
chords are easily developed. In a concrete frame,
continuous reinforcing in the edge beam can be used.
(1) General design criteria. The criteria used
In a steel frame building, the spandrel beams can be
to design concrete diaphragms will be ACI 318, as
used as chords if they have adequate capacity and
modified by FEMA 302. Concrete diaphragm webs
will be designed as concrete slabs; the slab may be
otherwise be interrupted by the columns; or special
designed to support vertical loads between the
reinforcing can be placed in the slab. Chords need
framing members, or the slab itself may be supported
not actually be in the plane of the diaphragm as long
by other vertical-load-carrying elements, such as
as the chord forces can be developed between the
precast concrete elements or steel decks. If shear is
diaphragm and the chord. For example, continuous
transferred from the diaphragm web to the framing
chord reinforcing can be placed in walls or spandrels
members through steel deck fastenings, the design
above or below the diaphragm. In masonry walls, the
will conform to the requirements in Paragraph 7-
chord requirements tend to conflict with the control
joint requirements. At bond beams, control joints
will have to be dummy joints so that reinforcement
(2) Span and anchorage requirements. The
can be continuous, and the marginal connections
must be capable of resisting the flexural and shear
Where reinforced concrete
(3) Openings. A diaphragm with openings
slabs are used as diaphragms to transfer lateral forces,
such as cut-out areas for stairs or elevators will be
the clear distance (LV) between framing members or
treated as a plate girder with holes in the web. The
mechanical anchors shall not exceed 38 times the
diaphragm will be reinforced so that forces
total thickness of the slab (t).
developing on the sides of the opening can be
developed back into the body of the diaphragm.
L-and T-shaped buildings.
L- and T-
shaped buildings will have the flange (chord) stresses
developed through or into the heel of the L or T.