• Document: Construction Loading in High-Rise Buildings
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8 Construction Loading in High-Rise Buildings S.K. Ghosh, Ph.D., P.E.* 8.1 Introduction ........................................................................8-1 8.2 Construction Loads.............................................................8-1 Design Considerations • Field Verification • Refined Analysis 8.3 Properties of Concrete at Early Ages ...............................8-19 Compressive Strength • Tensile Strength and Bond Strength • Punching Shear Strength • Modulus of Elasticity • Shrinkage of Unreinforced (Plain) Concrete • Creep of Unreinforced (Plain) Concrete • Effects of Drying on Flexural Cracking 8.4 Strength Consequences of Construction Loads..............8-37 Safety Analysis • Refined Safety Analysis 8.5 Serviceability Consequences of Construction Loads......8-47 Causes of Excessive Deflections • Components of Long-Term Deflection • Experimental Investigation • Control of Slab Deflections 8.6 Codes and Standards.........................................................8-55 References .....................................................................................8-55 8.1 Introduction Structural formwork and its support system must be given careful consideration in two different respects: (1) loads that are applied to the formwork and its supports, and (2) loads that the formwork and its supports apply to the structure. The first is the subject of Chapter 7 on formwork and falsework; the second is the subject of this chapter. 8.2 Construction Loads 8.2.1 Design Considerations In the construction of multistory buildings with reinforced concrete floor slabs, a step-by-step sequence of operations is employed. The sequence is comprised of the steps of setting up shoring on the most recently poured floor, forming the next floor, setting up reinforcement, and concreting the slab. Because the floor below the one being concreted is usually only a few days old, it is common practice to leave * President, S.K. Ghosh Associates, Inc., Palatine, Illinois, and Fellow of the American Concrete Institute; specializes in structural design, including wind- and earthquake-resistant design. 8-1 © 2008 by Taylor & Francis Group, LLC 8-2 Concrete Construction Engineering Handbook T Days 2T Days 3T Days 3T + n Days i i –1 i–2 i–3 i–4 4T Days Basic Operations Place Floor i Remove Shores Under x days Floor i – 2 x + n Days FIGURE 8.1 Construction sequence using three levels of shores. formwork supports in place between that floor and a small number of floors below it. A typical con- struction cycle using three levels of shores is illustrated in Figure 8.1. In discussing construction loads, it is convenient to express them as a factor times the sum of the self-weight of the floor and the dead load of the formwork. The term floor loading ratio is used for this factor. Progressive slab deflections that had been causing problems in Sweden in the early 1950s led to the first study by Nielsen (1952) of a rational approach to stripping formwork for floors. Nielsen gave a detailed analysis of the distribution of load between a system of connected shores and floor slabs. The met

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