**ACI 550.5M**-18 Code Requirements for the Design of Precast Concrete Diaphragms for Earthquake Motions (ACI 550.5M-18) and Commentary.

The value of the diaphragm shear overstrength factor is = 1.4R,, and values for R. are specified in 12.10.3 of ASCEI SEI 7-16. The values of the diaphragm design force reduction factor K, are 0.7, 1.0, and 1.4 for the EDO, 13D0, and RDO. respectively. This translates into diaphragm shear overstrength factors , of 1.0. 1.4. and 2.0 (rounded to one decimal place) for the EDO. HDO. and RDO, respectively.

The diaphragm shear oversrength factor fl is applied to the diaphragm design forces and, therefore, requires an increase in the shear strength of the diaphragm relative to its flexural strength. As implied by the foregoing l, values. the level of overstrength required relative to the diaphragm flexural strength varies with the design option. The RDO requires a higher overstrength than the BDO due to the larger anticipated inelastic action. For the EDO. no overstrength is required because the diaphragm design force itself targets elastic behavior in the MCER. The nominal shear strength required for the diaphragm in all three design options is constant, regardless of design option, because the parameter K, in the overstrenglh factor is cancelled oLI by the R, in the denominator of the diaphragm design force expression given in Section 12.10.3 ofASCESEI 7-16. The diaphragm design force at level x, F, equals or F,iR,. where FLT is the inertial horizontal force at level x defined as the product of the mass w. tributary to the diaphragm at level x and the peak horizontal floor acceleration at that level. C. The shear strength required in the design procedure would be c2F,,,. and considering = I 4K,. leads to F,,1 = I .4Fft, which shows that the shear strength required in the design procedure is constant, regardless of design option.

The . values represent upper-bound constant values (for each diaphragm design option) of parametric expressions developed for the required shear overstrength on the basis of detailed parametric studies (Fleischman 2014) performed using the nonlinear response history analysis (NRHA) of Section 16.2 of ASCE!SEI 7-16 and analytical models of precast concrete structures developed and calibrated on hc basis of large-scale physical testing (Fleischman 2014; Zhang et al. 2011). These precast concrete structures were subjected to spectrum-compatible ground motions scaled tO the MCER to determine the required shear overstrength factors.

Precast concrete diaphragms can be designed and detailed for a ductile flexurul response that enables the redistribution of internal forces. However, to achieve ductile flexural response. potentially nonductile shear limit states have to be precluded. To prevent these shear failures, elastic shear response is targeted in the design procedure for both fiexurecontrolled and shear-controlled systems.ACI 550.5M pdf download.