ACI - PRC-421.2
Seismic Design of Punching Shear Reinforcement in Flat Plates—Guide
|Publication Date:||1 August 2021|
Seismic design should account for the displacement- induced moment and the accompanying shear forces at flat plate-column connections. This demand may be effectively addressed by changes in dimensions of certain members (for example, shear walls and column sizes), their material strengths, provision of shear reinforcement, or a combination thereof. This guide does not address changes in dimensions and materials of such members but focuses solely on the punching shear design of flat plates with or without shear reinforcement.
This guide is supplemental to ACT 421.1R and focuses on the design of flat plate-column connections with or without shear reinforcement that are subject to earthquake-induced displacement; reinforced concrete flat plates with or without post-tensioning are also discussed in this guide. Stirrups and headed shear stud reinforcement (HSSR) satisfying ASTM A1044 A1044M are the common types of shear reinforcement for flat plates. Shear stud reinforcement is composed of vertical or inclined rods anchored mechanically near the bottom and top surfaces of the slab. Forged heads or welded plates can be used as the anchorage of HSSR; the area of the head or the plate is sufficient to develop the yield strength of the stud, with negligible slip at the anchorage. The design procedure recommended in this guide was developed based on numerical studies (finite element method) and experimental research on reinforced concrete slabs subjected to cyclic drift reversals that simulate seismic effects (Megally 1998). The finite element analyses, supplemental to the experimental research, used software, constitutive relations, and models that were subject to extensive verifications by comparing the results of the analyses with the behavior observed in tests (Megally and Ghali 2000b).
ACI 318 requires that structural integrity reinforcement be provided near the bottom of the slab extending through the columns. The code requirements in ACT 318-14 are addressed (refer to Eq. (4b)). This guide supplements ACT 352.1R and ACT 421.1R, which, respectively, include recommendations such as extending a minimum amount of integrity reinforcement through the column core and providing details of design for shear reinforcement in flat plates. For post-tensioned slabs, the integrity tendons can be in the top of the slab as they approach the column, in which case, the integrity reinforcement should be nonprestressed bars. ACT 352.1R also provides recommendations for the design of flat plate-column connections without slab shear reinforcement subjected to moment transfer in the inelastic response range. The equations in this guide predict punching shear strength and drift capacity, assuming that adequate flexural reinforcement is provided at the flat plate-column connections to prevent local flexural failure (Gayed et al. 2017). Wide flexural cracks in the vicinity of support can extend deep through slab thickness, joining an inclined shear crack inducing failure; this is flexure-induced punching. The minimum amount of top flexural reinforcement above support to resist flexure-induced punching is determined by yield line analysis (Gayed and Ghali 2019). Identifying the punching cause of failure in tests can be difficult when flexural reinforcement is inadequate (Cheng et al. 2010).
Objective-The objective is to provide a design recommendation for flat plate-column connections with sufficient ductility to accommodate the displacement of the selected lateral-force-resist
Remarks-This guide gives recommendations for the design of shear reinforcement for strength and considerations for ductility that supplement the provisions of ACI 318. The term "ductility" is the ratio of displacement at ultimate strength to the displacement at which yielding of the flexural reinforcement develops a yield-line mechanism.
ACT 318 allows the analysis of flat plate-column frames as equivalent plane frames. When the frame is not designated as part of the lateral-force-resist
Chapter 3 defines the story drift that should be considered in design. Chapter 4 recommends a minimum amount of shear reinforcement for certain cases. Chapter 5 describes means of increasing the shear strength of a flat plate-column connection and compares the associated ductilities. Chapter 6 calculates the moment transferred between a slab and column for use in designing the required flexural reinforcement. Chapter 7 and Appendix D discuss relevant provisions of ACI 318 and provide the design procedure and examples for interior, edge, and corner flat plate-column connections. Chapter 8 provides recommendations relevant to post-tensioned flat plates. Appendixes A and B are justifications of recommen- dations in the chapters. Appendix C provides equations for the properties of shear critical sections.