This chapter shall apply to the design of nonprestressed and prestressed diaphragms, including (a) through (d):
(a) Diaphragms that are cast-in-place slabs
(b) Diaphragms that comprise a cast-in-place topping slab on precast elements
(c) Diaphragms that comprise precast elements with end strips formed by either a cast-in-place concrete topping slab or edge beams
(d) Diaphragms of interconnected precast elements without cast-in-place concrete topping
Diaphragms in structures assigned to Seismic Design Category D, E, or F shall also satisfy requirements of 18.12.
Design shall consider (a) through (e):
(a) Diaphragm in-plane forces due to lateral loads acting on the building
(b) Diaphragm transfer forces
(c) Connection forces between the diaphragm and vertical framing or nonstructural elements
(d) Forces resulting from bracing vertical or sloped building elements
(e) Diaphragm out-of-plane forces due to gravity and other loads applied to the diaphragm surface
Design properties for concrete shall be selected to be in accordance with Chapter 19.
Design properties for steel reinforcement shall be selected to be in accordance with Chapter 20.
Diaphragms shall have thickness as required for stability, strength, and stiffness under factored load combinations.
Floor and roof diaphragms shall have a thickness not less than that required for floor and roof elements in other parts of this Code.
Required strength of diaphragms, collectors, and their connections shall be calculated in accordance with the factored load combinations in Chapter 5.
Required strength of diaphragms that are part of floor or roof construction shall include effects of out-of-plane loads simultaneous with other applicable loads.
Any set of reasonable and consistent assumptions for diaphragm stiffness shall be permitted.
Calculation of diaphragm in-plane design moments, shears, and axial forces shall be consistent with requirements of equilibrium and with design boundary conditions. It shall be permitted to calculate design moments, shears, and axial forces in accordance with one of (a) through (e):
(a) A rigid diaphragm model if the diaphragm can be idealized as rigid
(b) A flexible diaphragm model if the diaphragm can be idealized as flexible
(c) A bounding analysis in which the design values are the envelope of values obtained by assuming upper bound and lower bound in-plane stiffnesses for the diaphragm in two or more separate analyses
(d) A finite element model considering diaphragm flexibility
(e) A strut-and-tie model in accordance with 23.2
For each applicable factored load combination, design strengths of diaphragms and connections shall satisfy ϕSn ≥ U. Interaction between load effects shall be considered.
Design strengths shall be in accordance with (a), (b), (c), or (d):
(a) For a diaphragm idealized as a beam whose depth is equal to the full diaphragm depth, with moment resisted by boundary reinforcement concentrated at the diaphragm edges, design strengths shall be in accordance with 12.5.2 through 12.5.4.
(b) For a diaphragm or a diaphragm segment modeled as a strut-and-tie system, design strengths shall be in accordance with 23.3.
(c) For a diaphragm idealized with a finite-element model, design strengths shall be in accordance with Chapter 22. Nonuniform shear distributions shall be considered in design for shear. Collectors in such designs shall be provided to transfer diaphragm shears to the vertical elements of the lateral-force-resisting system.
(d) For a diaphragm designed by alternative methods, such methods shall satisfy the requirements of equilibrium and shall provide design strengths at least equal to required strengths for all elements in the load path.
It shall be permitted to use precompression from prestressed reinforcement to resist diaphragm forces.
If nonprestressed, bonded prestressing reinforcement is designed to resist collector forces, diaphragm shear, or tension due to in-plane moment, the value of steel stress used to calculate resistance shall not exceed the lesser of the specified yield strength and 60,000 psi.
It shall be permitted to design a diaphragm to resist in-plane moment and axial force in accordance with 22.3 and 22.4.
It shall be permitted to resist tension due to moment by (a), (b), (c), or (d), or those methods in combination:
(a) Deformed bars conforming to 20.2.1
(b) Strands or bars conforming to 20.3.1, either prestressed or nonprestressed
(c) Mechanical connectors crossing joints between precast elements
(d) Precompression from prestressed reinforcement
Nonprestressed reinforcement and mechanical connectors resisting tension due to moment shall be located within h/4 of the tension edge of the diaphragm, where h is diaphragm depth measured in the plane of the diaphragm at that location. Where diaphragm depth changes along the span, it shall be permitted to develop reinforcement into adjacent diaphragm segments that are not within the h/4 limit.
This section shall apply to diaphragm in-plane shear strength.
For a diaphragm that is entirely cast-in-place, Vn shall be calculated by Eq. (12.5.3.3).
 | (12.5.3.3) |
where Acv is the gross area of concrete bounded by diaphragm web thickness and depth, reduced by void areas if present; the value of 
used to calculate Vn shall not exceed 100 psi; and ρt is distributed reinforcement oriented parallel to the in-plane shear.
used to calculate Vn shall not exceed 100 psi; and ρt is distributed reinforcement oriented parallel to the in-plane shear.For a diaphragm that is entirely cast-in-place, cross-sectional dimensions shall be selected to satisfy Eq. (12.5.3.4).
 | (12.5.3.4) |
where the value of 
used to calculate Vn shall not exceed 100 psi.
used to calculate Vn shall not exceed 100 psi.For diaphragms that are cast-in-place concrete topping slabs on precast elements, (a) and (b) shall be satisfied:
(a) Vn shall be calculated in accordance with Eq. (12.5.3.3), and cross-sectional dimensions shall be selected to satisfy Eq. (12.5.3.4). Acv shall be calculated using the thickness of the topping slab for noncomposite topping slab diaphragms and the combined thickness of cast-in-place and precast elements for composite topping slab diaphragms. For composite topping slab diaphragms, the value of f'c in Eq. (12.5.3.3) and (12.5.3.4) shall not exceed the lesser of f'c for the precast members and f'c for the topping slab.
(b) Vn shall not exceed the value calculated in accordance with the shear-friction provisions of 22.9 considering the thickness of the topping slab above joints between precast elements in noncomposite and composite topping slab diaphragms and the reinforcement crossing the joints between the precast members.
For diaphragms that are interconnected precast elements without a concrete topping, and for diaphragms that are precast elements with end strips formed by either a cast-in-place concrete topping slab or edge beams, it shall be permitted to design for shear in accordance with (a), (b), or both.
(a) The nominal strength of grouted joints shall not exceed 80 psi. Reinforcement shall be designed to resist shear through shear-friction in accordance with 22.9. Shear-friction reinforcement shall be in addition to reinforcement designed to resist tension due to moment and axial force.
For any diaphragm, where shear is transferred from the diaphragm to a collector, or from the diaphragm or collector to a vertical element of the lateral-force-resisting system, (a) or (b) shall apply:
(a) Where shear is transferred through concrete, the shear-friction provisions of 22.9 shall be satisfied.
(b) Where shear is transferred through mechanical connectors or dowels, effects of uplift and rotation of the vertical element of the lateral-force-resisting system shall be considered.
Collectors shall extend from the vertical elements of the lateral-force-resisting system across all or part of the diaphragm depth as required to transfer shear from the diaphragm to the vertical element. It shall be permitted to discontinue a collector along lengths of vertical elements of the lateral-force-resisting system where transfer of design collector forces is not required.
Collectors shall be designed as tension members, compression members, or both, in accordance with 22.4.
Where a collector is designed to transfer forces to a vertical element, collector reinforcement shall extend along the vertical element at least the greater of (a) and (b):
(a) The length required to develop the reinforcement in tension
Reinforcement to resist shrinkage and temperature stresses shall be in accordance with 24.4.
Except for slabs-on-ground, diaphragms that are part of floor or roof construction shall satisfy reinforcement limits for one-way slabs in accordance with 7.6 or two-way slabs in accordance with 8.6, as applicable.
Reinforcement designed to resist diaphragm in-plane forces shall be in addition to reinforcement designed to resist other load effects, except reinforcement designed to resist shrinkage and temperature effects shall be permitted to also resist diaphragm in-plane forces
Development lengths of deformed and prestressed reinforcement shall be in accordance with 25.4, unless longer lengths are required by Chapter 18.
Splices of deformed reinforcement shall be in accordance with 25.5.
Maximum spacing s of deformed reinforcement shall be the lesser of five times the diaphragm thickness and 18 in.
Except for slabs-on-ground, diaphragms that are part of floor or roof construction shall satisfy reinforcement detailing of one-way slabs in accordance with 7.7 or two-way slabs in accordance with 8.7, as applicable.
Calculated tensile or compressive force in reinforcement at each section of the diaphragm or collector shall be developed on each side of that section.
Reinforcement provided to resist tension shall extend beyond the point at which it is no longer required to resist tension at least ℓd, except at diaphragm edges and at expansion joints.