TB-Lecture14-Direct-method-for-Concrete-Slabsor-Systems

TB-Lecture14-Direct-method-for-Concrete-Slabsor-Systems -...

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EGN EGN - 5439 The Design of Tall Buildings 5439 The Design of Tall Buildings Lecture #14 Lecture #14 The Design of Reinforced Concrete Slabs The Design of Reinforced Concrete Slabs Via the Direct Method as per ACI 318-05 © L. A. Prieto-Portar - 2008
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Reinforced concrete floor systems provide an economical solution for virtually any span and loading condition.
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Introduction. Selecting the most effective floor system can be vital to achieving overall economy, especially for low- and mid-rise buildings and for buildings subjected to relatively low lateral forces where the cost of the lateral-force-resisting system is minimal. Concrete, reinforcement, and formwork are the three primary expenses in cast-in-place concrete floor construction to consider throughout the design process, but especially during the initial planning stages. Of these three, formwork comprises about 55 percent of the total cost and has the greatest influence on the overall cost of the floor system. The cost of the concrete, including placing and finishing, typically accounts for about 30 percent of the overall cost. The reinforcing steel has the lowest influence on overall cost (15%). To achieve overall economy, designers should satisfy the following three basic principles of formwork economy: 1) Specify readily available standard form sizes. Rarely will custom forms be economical, unless they are required in a quantity that allows for mass production. 2) Repeat sizes and shapes of the concrete members wherever possible. Repetition allows reuse of forms from bay-to-bay and from floor-to-floor. 3) Strive for simple formwork. In cast-in-place concrete construction, economy is rarely achieved by reducing quantities of materials.
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For example, varying the depth of a beam with the loading and span variations would give a moderate savings in materials, but would create substantial additional costs in formwork, resulting in a more expensive structure. The simplest and most cost-effective solution would be providing a constant beam depth and varying the reinforcement along the span. Simple formwork can make construction time shorter, resulting in a building that can be occupied sooner. Additional parameters must be considered when selecting an economical floor system. In general, span lengths, floor loads, and geometry of a floor panel all play a key role in the selection process. Detailed information on how to select economical concrete floor systems for a wide variety of situations can be found in the following Portland Cement Association (PCA) publications: 1) Concrete Floor Systems - Guide to Estimating and Economizing (SP041), and 2) Long-Span Concrete Floor Systems (SP339).
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Preliminary sizing of the slab. Before analyzing the floor system, designers must assume preliminary member sizes.
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