ACI 334.3R-05 Construction of Concrete Shells Using Inflated Forms

ACI 334.3R-05 Construction of Concrete Shells Using Inflated Forms. 3.2.3.1 Fabric stress—Fabric stresses are proportional to the air pressure and the radius of curvature. Structures 160 ft (49 m) in diameter or larger require more precise air pressure controls to provide a stable inflated-form surface and prevent excessive fabric stress. Because a flexible fabric form cannot tolerate a compressive stress, the design should ensure that the desired size and shape is obtained when inflated. When large structures are contemplated, the low air pressure dictated by the allowable fabric stress may not be sufficient to support the shotcrete during construction. Cable nets used on the outside of the inflated form as a secondary restraint of the inflated form can allow increased inflation pressure required to support the shotcrete during construction without over stressing the inflated-form fabric (Jacobs 1996; South 1990). 3.2.3.2 Allowable fabric stress—Most inflated forms are constructed by splicing specially patterned flat pieces that need to inflate to form a smooth compound curve. Whereas the seams are usually as strong as the parent material under short-duration loads, for sustained loads, these seams can fail well below the tensile strength of the fabric. Therefore, inflated-form fabric stresses are limited to 20% of the breaking strength of the fabric. 4.3—Clear spacing between bars The minimum recommended clear distance between parallel reinforcing bars is 2.5 in. (60 mm). When two or more layers of reinforcing bars are used, the first layer should be tied in place followed by shotcrete covering the first layer, then the next layer is tied in place followed by shotcrete; this is continued until all layers are installed and covered by shot- crete. The recommended minimum clear spacing could be reduced when the contractor can demonstrate proper encase- ment of the reinforcing bars by preconstruction tests. 4.4—Splices The dome is a complex three-dimensional structure. The reinforcement within insulated thin shells constructed with inte- rior shotcrete construction should be spliced with mechan- ical splices or contact lap splices. Contact lap splices should only be used when adequate encasement of the bars can be achieved, and also when the splice is oriented so that a plane through the center of the stacked spliced bars is perpendicular to the surface of the shotcrete. Lap splices, where used, should be firmly wired together to add stiffness to the reinforce- ment grid during construction. Lap connections for reinforcing bar sizes of No. 5 (No....