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The 3-Sided Box Diaphragm Design Explained

September 21, 2023 from 1:30pm to 2:30pm Eastern Standard Time

Tampa Convention Center [Lower Level - Room 124-125]

Jim Lintz, P.E., S.E., LEED AP

Engineers have designed tilt-up buildings with shear walls on three sides for many years, with no lateral force resisting system on the fourth side and a light gauge metal deck roof diaphragm. This type of construction can often be seen when single-story additions are built up against existing buildings. The Steel Deck Institute published a methodology for designing these 3-sided box structures in the second edition of the Diaphragm Design Manual back in 1995. Still, this design philosophy remains unfamiliar to many design professionals. This presentation covers this 3-sided box design method using design examples, diagrams, and structural models to clarify the subject. The speaker will discuss relevant load cases, shear diagrams, combining shear from perpendicular loads, chord forces, and diaphragm deflection for 3-sided box structures.


  • Describe the wind load cases that need to be considered for 3-side box diaphragm design.
  • Understand the shear loads to the shear walls, chord forces, and the shear diagrams in the diaphragm from loads in perpendicular directions.
  • Explain how to combine shear forces to ensure the diaphragm will be designed with adequate capacity.
  • Summarize diaphragm deflection and how it can be calculated for a 3-sided box.

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