“As all structural engineers familiar with building design know, diaphragms constitute an integral part of the lateral-load resisting system. When the load under consideration is seismic, the diaphragms themselves often constitute a majority of the inertial mass as well as the means of delivering inertial forces to the vertical elements of the seismic load resisting system (SLRS). While building codes have paid careful attention to diaphragm design, this area remains rife with ambiguity and differing opinions. This article will not put an end to such debate, of course, but will serve as one example of a coherent philosophy of diaphragm design for seismic loading.

PART I: ANALYSIS
VERTICAL DISTRIBUTION OF SEISMIC FORCES
In general, buildings are analyzed using either an Equivalent Lateral Force (ELF) analysis, a Modal Response Spectrum (MRS) analysis, or a Nonlinear Response History (NRH) analysis. This article addresses ELF analysis, with some discussion of the techniques used in MRS analysis…”

PART II: LOAD COMBINATIONS
"The mechanics of diaphragms involves both ductile and nonductile limit states. In general, shear yielding of the diaphragm is considered to be more ductile, while failure of a collector or one of its connections is considered less so. In higher seismic design categories (SDC C and above) ASCE 7-05 requires that different approaches be taken for these two classes of element performance: ductile modes are designed to resist forces from the basic load combinations of Section 12.4.2.3, while
nonductile modes are designed using forces from the special seismic load combinations of Section 12.4.3.2. (This is a force corresponding to LRFD design; if a designer chooses to use ASD a 20% increase
in allowable strength is permitted by Section 12.4.3.3.)…”