When diaphragm action is considered in the design of post-frame buildings, the endwalls may require supplemental reinforcement to replace lost strength and stiffness, due to placement of large doors and openings in the endwalls. The diaphragm design standard, ASAE EP484, does not provide guidelines concerning such endwall reinforcement. Flat steel strapping 1s proposed for use as supplemental reinforcement to replace this lost strength and stiffness. An analysis procedure was developed, with the intention of serving as input to endwall diaphragm design. Using PPSA 3.00 and calibrated endwall data, a computer structural analog was developed that “mimics” the deflection of an actual solid endwall. Sections of the structural analog were removed to represent a centered door opening. Fictitious members representing steel-strap bracing were added to the remaining endwall diaphragm to restore the endwall deflection to that of the same endwall with no door openings. Large corner post uplift values were observed for tall endwalls. For purposes of endwall analysis, it is necessary to distribute corner post uplift forces to the other posts along the adjacent endwall and sidewall. Along with the tabulated results in this research, this analysis procedure provides building designers a tool that will enable them to analyze virtually any endwall-door combination. All that is required is PPSA 3.00 software (or equivalent), a personal computer, and endwall stiffness data. / Master of Science