18.1 Diaphragms, Collectors, Struts, and Composite Slabs

A diaphragm is the horizontal part of the lateral load path. Wind pressure or seismic inertia enters the roof or floor, travels through the diaphragm, and reaches shear walls, braced frames, or moment frames. The force does not end at the diaphragm edge. For July 2026, use the April 2024 PE Civil: Structural specification, ASCE 7-16, the AISC Steel Construction Manual 15th edition, ACI 318-14, and NDS 2018/SDPWS 2015 using ASD only for wood. Do not import April 2027 editions.

Classify the Diaphragm From Behavior

A flexible diaphragm distributes lateral force mainly by tributary geometry to vertical resisting lines. A rigid diaphragm enforces approximately compatible in-plane translation and rotation, so vertical elements attract force according to relative stiffness and plan location; torsion from eccentricity matters. A semirigid diaphragm is modeled with finite in-plane stiffness and can redistribute force between these ideals.

Material names do not establish classification. An untopped steel deck may be flexible in one building, while a concrete slab with large openings or long narrow geometry may require semirigid modeling in another. Use the ASCE 7-16 classification criteria and structural model required by the problem. Also distinguish diaphragm classification from diaphragm strength: a “rigid” diaphragm can still have inadequate shear, chord, collector, or connection capacity.

The Complete Set of Diaphragm Actions

Treat the diaphragm as a deep horizontal beam when that analogy is appropriate:

• Shear flows through deck, slab, sheathing, seams, fasteners, reinforcement, and boundary connections.
• Chords at opposite diaphragm boundaries form a tension-compression couple resisting in-plane moment. Chord splices and anchorage must develop the force.
• Collectors or drag struts gather distributed diaphragm shear and deliver concentrated axial force to a shorter wall or frame line. They can be beams, slab reinforcement, steel members, or other detailed elements.
• Connections transfer force at deck seams, slab joints, chords, collectors, and vertical-system interfaces.
• Vertical elements and foundations carry shear and overturning through base connections, footings or pile caps, and soil.

A collector is not interchangeable with a chord. A chord primarily resists diaphragm bending, while a collector primarily moves axial force between the diaphragm and a vertical element. One member may serve both roles only if designed and detailed for the combined demands.

Worked Diaphragm Analogy

A rectangular diaphragm spans 80 ft between two parallel shear-wall lines. The problem idealizes it as a simply supported horizontal beam carrying uniform factored lateral load w = 4.0 klf over the 80 ft span. The effective distance between chord force resultants is 36 ft. Find each wall reaction, maximum diaphragm moment, and chord force at midspan.

Wall reactions:

R = wL/2 = (4.0)(80)/2 = 160 kips

Maximum in-plane moment:

M_max = wL^2/8 = (4.0)(80^2)/8 = 3,200 kip-ft

Chord couple:

F_chord = M_max/d = 3,200/36 = 88.9 kips

One boundary chord is in tension and the opposite is in compression for the stated loading; reversal exchanges roles. Chord splices must transfer 88.9 kips on a compatible design basis. Each wall interface and any collector delivering force into that wall must transfer the 160 kip reaction. Then wall shear, overturning, anchors, and foundation resistance continue the path to ground.

The beam analogy does not automatically give local shear flow near openings, torsional distribution, amplified seismic collector force, or connection capacity. Those follow the controlling ASCE 7-16 and material provisions. Preserve units: diaphragm shear demand may be total kips, force per unit length, or connection force, and capacities must use the same basis.

Openings and Discontinuities

A large opening interrupts the direct shear field and can cut a chord or collector. Draw load arrows around all four sides. Local boundary reinforcement or members develop tension and compression, while corner forces can concentrate at reentrant corners. A narrow remaining strip may be too flexible or weak to carry the assumed flow. Staggered walls, setbacks, transfer levels, and a frame that stops below the diaphragm create similar collector demands.

Do not distribute the original shear over only the remaining net width without checking the actual load path and analysis model. Openings can also change the center of diaphragm stiffness and increase torsion. Connections around the opening must develop the calculated forces, not merely the deck or slab field strength.

Composite Deck and Slab Behavior

Steel deck can serve as concrete formwork during placement, as reinforcement or a composite slab component after curing when designed for that role, and as part of a diaphragm through its seams and attachments. These are different stages. Wet concrete loads act before final composite stiffness and strength exist.

Composite beam action requires longitudinal shear transfer, commonly through shear connectors between steel beam and concrete slab. That transfer does not by itself connect the diaphragm to a shear wall or braced frame. Separate collectors, embed plates, reinforcing bars, deck attachments, welds, bolts, or anchors may be required. At precast or cast-in-place interfaces, ACI provisions govern shear transfer and reinforcement continuity.

Exam Workflow

1. Identify lateral force at each level and diaphragm classification.
2. Distribute force to vertical elements, including torsion when required.
3. Calculate diaphragm shear and in-plane moment.
4. Design chords, collectors, struts, openings, splices, and connections.
5. Use stage-appropriate composite properties.
6. Trace every reaction through the vertical system, anchorage, foundation, and ground.

If any arrow stops at a slab edge, deck flute, beam, or wall top without a transfer detail, the load path is unfinished.