RCD – Quiz #1

• Variability in structure allows for adaptation to function in an economic manner, providing the architect and design engineer with a wide variety of esthetically satisfying structural solutions.
• Concrete is a stonelike material obtained by permitting a carefully proportioned mixture of cement, sand and gravel or other coarse aggregate, and water to harden in forms of the shape and dimensions of the desired structure.
• The bulk of the material consists of fine and coarse aggregate.
• Cement and water interact chemically to bind the aggregate particles into a solid mass.
• Additional water, over and above that needed for this chemical reaction, is necessary to give the mixture the workability that enables it to fill the forms and surround the embedded reinforcing steel prior to hardening.
• Concretes with a wide range of properties can be obtained by appropriate adjustment of the proportions of the constituent materials.
• Special cements (such as high early strength cements), special aggregates (such as various lightweight or heavyweight aggregates), admixtures (such as plasticizers, air-entraining agents, silica fume, and fly ash), and special curing methods (such as steam-curing) permit an even wider variety of properties to be obtained.
• These properties depend to a very substantial degree on the proportions of the mixture, on the thoroughness with which the various constituents are intermixed, and on the conditions of humidity and temperature in which the mixture is maintained from the moment it is placed in the forms until it is fully hardened.
• The process of controlling conditions after placement is known as curing.
• To protect against the unintentional production of substandard concrete, a high degree of skillful control and supervision is necessary throughout the process, from the proportioning by weight of the individual components, through mixing and placing, until the completion of curing.
• The factors that make concrete a universal building material are so pronounced that it has been used, in more primitive kinds and ways than at present, for thousands of years, starting with lime mortars from 12,000 to 6000 BCE in Crete, Cyprus, Greece, and the Middle East.
• The facility with which, while plastic, it can be deposited and made to fill forms or molds of almost any practical shape is one of the factors that make concrete a universal building material.
• Its high fire and weather resistance is an evident advantage.
• Most of the constituent materials, with the exception of cement and additives, are usually available at low cost locally or at small distances from the construction site.
• Its compressive strength, like that of natural stones, is high, which makes it suitable for members primarily subject to compression, such as columns and arches.
• On the other hand, again as in natural stones, it is a relatively brittle material whose tensile strength is low compared with its compressive strength.
• This prevents its economical use as the sole building material in structural members that are subject to tension either entirely (such as in tie-rods) or over part of their cross sections (such as in beams or other flexural members).
• The figures that follow show some of the principal structural forms of reinforced concrete.
• 1.4, frequently used for more heavily loaded buildings such as warehouses, is similar to the flat plate floor, but makes use of increased slab thickness in the vicinity of the columns, as well as flared column tops, to reduce stresses and increase strength in the support region.
• Pertinent design methods for many of them are discussed later in this volume.
• The cylindrical shell of Fig 1.6 is also relatively easy to form because it has only a single curvature; it is similar to the folded plate in its structural behavior and range of spans and loads.
• Doubly curved shell surfaces may be generated by simple mathematical curves such as circular arcs, parabolas, and hyperbolas, or they may be composed of complex combinations of shapes.
• Floor support systems for buildings include the monolithic slab-and-beam floor, the one-way joist system, and the flat plate floor, without beams or girders.
• The complex dome of Fig 1.7, which provides shelter for performing arts events, consists essentially of a circular dome but includes monolithic, upwardly curved edge surfaces to provide stiffening and strengthening in that critical region.
• The folded plate roof of Fig 1.5 is simple to form because it is composed of flat surfaces; such roofs have been employed for spans of 200 ft and more.
• The Bennett Bay Centennial Bridge, a four-span continuous, segmentally cast-in-place box girder structure, is shown in Fig 1.9.
• Shells of this type were once quite popular in the United States and remain popular in other parts of the world.
• It has the interesting property that the doubly curved surface contains two systems of straight-line generators, permitting straight-form lumber to be used.
• Bridge design has provided the opportunity for some of the most challenging and creative applications of structural engineering.
• The award-winning Napoleon Bonaparte Broward Bridge, shown in Fig 1.8, is a six-lane, cable-stayed structure that spans St John’s River at Dame Point, Jacksonville, Florida.
• The choice among these and other systems for floors and roofs depends upon functional requirements, loads, spans, and permissible member depths, as well as on cost and esthetic factors.
• Special attention was given to esthetics in the design of the Natchez Trace Parkway Bridge, a two-span arch structure using hollow precast concrete elements, which carries a two-lane highway 155 ft above the valley floor.
• Where long clear spans are required for roofs, concrete shells permit use of extremely thin surfaces, often thinner, relatively, than an eggshell.
• The durability and sustainability of concrete structures is evident in the 10,000 seat multipurpose Dakota Dome, originally constructed in 1979 as an inflatable roof dome, which was entirely retained and a steel roof installed in 2001.
• The hyperbolic paraboloid shape, defined by a concave downward parabola moving along a concave upward parabolic path, has been widely used.
• Concrete structures may be designed to provide a wide array of surface textures, colors, and structural forms.
• Concrete is a stonelike material obtained by permitting a carefully proportioned mixture of cement, sand and gravel or other coarse aggregate, and water to harden in forms of the shape and dimensions of the desired structure.
• The bulk of the material consists of fine and coarse aggregate.
• Cement and water interact chemically to bind the aggregate particles into a solid mass.
• Additional water, over and above that needed for this chemical reaction, is necessary to give the mixture the workability that enables it to fill the forms and surround the embedded reinforcing steel prior to hardening.