Diaphragms Are the Horizontal Link in the Lateral Force System
When the wind pushes on the side of a building or an earthquake shakes it horizontally, the lateral load must somehow get from the floors and roof to the vertical lateral force resisting elements (shear walls, moment frames, braced frames). The horizontal structural elements - floor slabs and roof decks - perform this function. They act as horizontal "diaphragms," collecting lateral loads over their entire area and distributing them to the vertical LFRS. Understanding diaphragm behavior is essential for understanding how complete building lateral systems work, and it is tested on the ARE PPD division.
How a Diaphragm Works
Think of a diaphragm as a deep, flat beam lying on its side. The lateral load (wind or seismic) acts as a distributed load on this beam. The beam spans between its supports - the shear walls or frames at the building's ends. The diaphragm develops in-plane shear and moment just like a beam, with the "flanges" being the boundary members (beams and spandrels at the floor edge) and the "web" being the deck or slab.
At the ends, the diaphragm transfers its accumulated load into the shear walls through shear transfer at the connection between diaphragm and wall. This connection is critical - if the diaphragm cannot transfer load to the walls, the walls are useless as lateral elements, even if they are properly designed.
Rigid vs. Flexible Diaphragms
The distribution of lateral load to multiple shear walls or frames depends on the relative stiffness of the diaphragm:
Rigid diaphragm: A concrete slab or concrete topping on steel deck is typically treated as a rigid diaphragm. Load is distributed to shear walls in proportion to their stiffness. Stiffer walls attract more load. Rigid diaphragm analysis requires knowing the stiffness of each wall and the location of the center of stiffness relative to the center of mass - eccentricity between these two points creates torsion.
Flexible diaphragm: Untopped metal deck or wood sheathing with large span-to-depth ratios behaves as a flexible diaphragm. Load is distributed to shear walls in proportion to the tributary area they serve - essentially treating each wall as a simple beam support. Flexible diaphragm analysis is simpler but less accurate for complex building configurations.
Diaphragm Chords and Collectors
Two critical elements that must be designed as part of any diaphragm are chords and collectors:
- Chord: The perimeter member that resists the moment in the diaphragm (analogous to the flange of a beam). Chords develop axial tension and compression. In concrete construction, perimeter reinforcing bars serve as chords. In steel construction, spandrel beams or edge angles function as chords.
- Collector (drag strut): A member that "collects" shear from the diaphragm and drags it into the shear wall over the wall's length. Collectors are needed where the shear wall does not extend the full depth of the diaphragm.
Key Exam Points
- Diaphragm = horizontal structural element that distributes lateral loads to the vertical LFRS.
- Rigid diaphragm: load distributed by wall stiffness. Flexible: load distributed by tributary area.
- Chord: perimeter member resisting diaphragm moment (tension and compression).
- Collector/drag strut: transfers shear from diaphragm into discontinuous shear walls.
- Connection between diaphragm and shear wall must be designed for shear transfer.
Study PPD on AREprep
AREprep’s PPD flashcards cover every concept on this exam with spaced repetition, and the practice exams mirror the real question formats so the actual test feels familiar.