Code Sections and Analysis - The Analysis of Irregular Shaped Structures - page 6

12.3.1.2 Rigid diaphragm condition
Diaphragms of concrete slabs or concrete filled metal deck with span to depth
ratios of 3 or less in structures that have no horizontal irregularities are permitted
to be idealized as rigid.
12.3.1.3 Calculated flexible diaphragm condition
Diaphragms not satisfying the conditions of sections 12.3.1.1 or 12.3.1.2 are permit-
ted to be idealized as flexible where the computed maximum in-plane deflection of
the diaphragm under lateral load is more than two times the average story drift of
adjoining vertical elements of the seismic force resisting system of the associated
story under equivalent tributary lateral load, as shown in ASCE 7-05 Fig. 12.3-1.
12.10 Diaphragm chords and collectors
12.10.1 Diaphragm design
Diaphragms shall be designed for both shear and bending stresses resulting from
design forces. At diaphragm discontinuities, such as openings or re-entrant cor-
ners, the design shall assure that the dissipation or transfer of edge (chord) forces
combined with other forces in the diaphragm is within the shear and tension
capacity of the diaphragm.
12.10.2 Collector elements
Collector elements shall be provided that are capable of transferring the seismic
forces originating in other portions of the structure to the elements providing
resistance to those forces (all seismic design categories).
12.10.2.1 Collectors in Seismic Design Categories C through F
In structures assigned to Seismic Design Categories C through F, collector ele-
ments (see ASCE 7-05 Fig. 12.10-1), splices, and their connections to resisting ele-
ments shall resist the load combinations with over-strength of Section 12.4.3.2.
Exception: In structures or portions thereof braced entirely by light frame shear
walls, collector elements, splices, and connections to resisting elements need only
be designed to resist forces in accordance with 12.10.1.1.

1.4 AF&PA-SDPWS 2008 Sections Referencing Wind and Seismic 3

2.2 Terminology
Boundary Element : Diaphragm and shear wall boundary members to which
sheathing transfers forces. Boundary elements include chords and collectors at
diaphragm and shear wall perimeters, interior openings, discontinuities, and
re-entrant corners.
Diaphragm Boundary : A location where shear is transferred into or out of the
diaphragm sheathing. Transfer is either to a boundary element or to another
force-resisting element.
Collector : A diaphragm or shear wall element parallel and in line with the applied
force that collects and transfers diaphragm shear forces to the vertical elements of
the lateral-force-resisting system and/or distributes forces within the diaphragm.
Chord : A diaphragm boundary element perpendicular to the applied load that
resists axial stress due to the induced moment.
Diaphragm : A roof, floor, or other membrane bracing system acting to transmit
lateral forces to the vertical resisting elements. When the term “diaphragm” is
used, it shall include horizontal bracing systems.