method analyses, including in all cases evaluation of the stress conditions around the supports in relation to shear and torsion, as well as flexure. Regardless of analysis method, deviations in physical dimensions of the slab from common practice should be justified on the basis of knowledge of the expected loads, reliability of the calculated stresses, and deformations of the structure. Analysis of a two-way slab system should consider the aspect ratio of each slab panel and the relative stiffness of the slab panels, supporting beams (if any), and supporting columns or walls. Analysis and design of two-way slab systems using the Direct Design Method (DDM) and Equiv- alent Frame Method (EFM) are discussed in 3.2. 3.1.2 Slab stiffness —During the life of a structure, construction loads, ordinary occupancy loads, anticipated overloads, and volume changes can cause cracking of slabs. Excessive cracking exposes concrete to moisture infiltration, which can cause corrosion of reinforcement and deteriora- tion of structural elements. Under sustained gravity loads, cracking can lead to large vertical deflection resulting in damage to nonstructural elements. Cracking reduces stiffness of the slabs, and increases lateral displacement when lateral forces act on the structure. Cracking of slabs should be considered in stiffness assump- tions so drift caused by wind or earthquake is not grossly underestimated. Conservatively, analysis of slab-column connections can use a stiffness reduction factor that provides higher reduction in stiffness to ensure lateral displacement and that the design forces are not underestimated. Refer to 7.2.2 for further discussions. 3.1.3 Total factored static moment —Total factored static moment M o for a span of a rectilinear interior panel may be determined in a strip bounded laterally by the centerline of a panel on each side of the centerline of supports. The absolute sum of positive and average negative factored moments in each direction should not be less than M q o u n = lscript lscript 2 2 8 (3.1.3) In the case of an edge panel, use 0.5 ℓ 2 , where ℓ 2 is the transverse span. Equation (3.1.3) follows directly from Nichols (1914) with the simplifying assumption that the reactions are concentrated along the faces of the support perpendicular to the span considered. In general, the designer will find it expedient to calculate the static moment for two adja- cent half panels, which includes a column strip with a half middle strip along each side. The clear span ℓ n is taken as the distance between columns, capitals, brackets, or walls. The value of ℓ n used in Eq. (3.1.3) should not be less than 0.65 ℓ 1 . Circular or regular polygon-shaped supports should be treated as square supports with the same area (Fig. 3.1.3). 3.2—Analysis methods Both the DDM and EFM are based on analysis of the results of an extensive series of tests ( Jirsa et al. 1963 ) and the well-established performance record of various slab systems. This guide covers these two methods in depth.
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