ACI 447R-18 Design Guide for Twisting Moments in Slabs

ACI 447R-18 Design Guide for Twisting Moments in Slabs. 3.7—Traditional slab design methods Traditionally, slab design is performed using design forces determined by simplifed analysis methods that idealize the slab as a set of intersecting strips (efectively supports and wide beams) in two perpendicular directions. The required reinforcement is calculated using the strip bending moments and standard beam bending design approaches. These methods have two primary faws. The frst is that equilibrium is not fully satisfed because there is no consis- tency between the load paths of the two sets of strips. The second faw is that deformation compatibility between both parallel and perpendicular strips is ignored. Despite these faws, slabs designed using these methods have generally performed well. For slabs with supports arranged in a rectangular grid it is often shown, using yield- line theory or lower-bound methods, that the traditional strip methods provide adequate capacity when reinforcement is distributed appropriately. (Burgoyne 2004; Kennedy and Goodchild 2003). For those slabs with supports not arranged in a rectangular grid, engineering judgment is necessary to determine if the traditional methods will produce a safe design. 3.8—Finite element analysis (FEA)-based slab design resultants To complete slab design, an engineer should determine the quantity of reinforcement required for each design cross section. Although design cross section locations and lengths are often guided by code rules, in general, sections are needed at peak stress locations and their lengths based on the extent of the slab that can be assumed to act as a unit in resisting internal forces. The width of each section should be chosen so that the moment distribution along the section is reasonably uniform, does not change sign, and can be resisted by uniformly distributed reinforcements. When using FEA to support slab design, the engineer should convert the slab analysis element results to resultant forces and moments acting on these sections. At this stage, the engineer should frst transform the results of FEA into a coordinate system orthogonal to the section, and then inte- grate/sum all forces and moments acting on the section to determine the design moment. Consideration of twisting moments is the most difcult aspect of this conversion. ACI 318 does not explicitly address twisting moments, nor does the commentary provide guidance on their consid- eration. CHAPTER 4—AVAILABLE DESIGN METHODS This chapter discusses various options to consider for twisting moments. Chapters 5 through 7 evaluate some of these options...