ACI 318.2M-19 Building Code Requirements for Concrete Thin Shells (ACI 318.2-19)

ACI 318.2M-19 Building Code Requirements for Concrete Thin Shells (ACI 318.2-19). R2.4—Stability R2.4.1 Thin shells, like other structures that experi- ence in-plane membrane compressive forces, are subject to buckling when the applied load reaches a critical value. The surface geometry of shells makes calculating buckling loads complex. If one of the principal membrane forces is tensile, the shell is less likely to buckle than if both principal membrane forces are compressive. The membrane forces that develop in a shell depend on its initial shape and the manner in which the shell is supported and loaded. In some types of shells, post-buckling behavior should be considered in determining safety against instability (IASS Working Group No. 5 1979). Investigation of thin shells for stability should consider the efect of: 1) anticipated deviation of the geometry of the shell surface as-built from the idealized geometry; 2) local variations in curvature; 3) large defections; 4) creep and shrinkage of concrete; 5) inelastic properties of materials; 6) cracking of concrete; 7) location, amount, and orientation of reinforcement; and 8) possible deformation of supporting elements. Measures successfully used to improve resistance to buckling include providing two mats of reinforcement, one near each outer surface of the shell; a local increase of shell curvatures; the use of ribbed shells, and the use of concrete with high tensile strength and low creep. A procedure for determining critical buckling loads of shells is given in the IASS recommendations (IASS Working Group No. 5 1979). Some recommendations for buckling design of domes used in industrial applications are given in ACI 372R and ACI SP-67. R3.1—Minimum thickness R3.1.1 Thin shell sections and reinforcement are required to be proportioned to satisfy the strength and serviceability provisions of Chapters 22 and 24 of ACI 318-19 and to resist internal forces obtained from a numerical or experimental analysis, or a combination thereof. Reinforcement sufcient to minimize cracking under service load conditions should be provided. The thickness of the shell is often controlled by the required reinforcement and the construction constraints (Gupta 1986), shell stability, or by the minimum cover requirements. R3.2 If a principal tensile stress produces membrane cracking in the shell, experiments indicate the attainable compres- sive strength in the direction parallel to the cracks is reduced (Gupta 1984; Vecchio and Collins 1986). R3.3—Stress limits in prestressed shells R4.2.1.4 Experimental analysis of elastic models (Sabnis et al. 1983) has been used as a...