Infrastructure Sustainability: Differential Axial Shortening of Concrete Structures

Infrastructure Sustainability: Differential Axial Shortening of Concrete Structures

Praveen Moragaspitiya (Queensland University of Technology, Australia), David Thambiratnam (Queensland University of Technology, Australia), Nimal Perera (Queensland University of Technology, Australia) and Tommy Chan (Queensland University of Technology, Australia)
DOI: 10.4018/978-1-61692-022-7.ch014
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High density development has been seen as a contribution to sustainable development. However, a number of engineering issues play a crucial role in the sustainable construction of high rise buildings. Non linear deformation of concrete has an adverse impact on high-rise buildings with complex geometries, due to differential axial shortening. These adverse effects are caused by time dependent behaviour resulting in volume change known as ‘shrinkage’, ‘creep’ and ‘elastic’ deformation. These three phenomena govern the behaviour and performance of all concrete elements, during and after construction. Reinforcement content, variable concrete modulus, volume to surface area ratio of the elements, environmental conditions, and construction quality and sequence influence on the performance of concrete elements and differential axial shortening will occur in all structural systems. Its detrimental effects escalate with increasing height and non vertical load paths resulting from geometric complexity. The magnitude of these effects has a significant impact on building envelopes, building services, secondary systems, and lifetime serviceability and performance. Analytical and test procedures available to quantify the magnitude of these effects are limited to a very few parameters and are not adequately rigorous to capture the complexity of true time dependent material response. With this in mind, a research project has been undertaken to develop an accurate numerical procedure to quantify the differential axial shortening of structural elements. The procedure has been successfully applied to quantify the differential axial shortening of a high rise building, and the important capabilities available in the procedure have been discussed. A new practical concept, based on the variation of vibration characteristic of structure during and after construction and used to quantify the axial shortening and assess the performance of structure, is presented.
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Many developers currently economize on land area by constructing high rise buildings. Reinforced concrete is commonly used for gravity load bearing frames of high-rise buildings. Relatively simple concrete shear walls are often replaced by steel brace walls and moment resisting frames for lateral load resistance. Unlike structural steel, concrete does not require fireproofing. It has an inherent soundproofing quality and its natural resistance to prevent fire, smoke and mechanical penetration can enhance safety of the building. Thus, reinforced concrete currently plays a significant role in high rise building constructions (Neville, 2005; Fintal & Fazlur, 1971).

Concrete has three significant modes of volume change after being placed. Shrinkage, as the name implies, causes the concrete volume to decrease as the water within it dissipates and the chemical process of concrete causes hardening to occur. Elastic shortening occurs immediately the hardened concrete is loaded and is a function of the applied stress, the length of the concrete element and the modulus of elasticity. Creep is a long-term effect that causes the concrete to deform under exposure to sustained loading. These three phenomena occur in every concrete structure (Neville, 2005).

During construction of high rise buildings, the vertical elements are subjected to a number of load increments. Each load increment causes elastic shortening of columns and shear walls. This will also lead to long term creep and shrinkage shortenings of such elements. The design of some structural members such as cores is governed by the combined gravity and lateral load requirements, whereas some columns are designed for gravity loads with different tributary areas. This means that adjacent structural members may have different percentages of reinforcement which carry different axial loads and have different volume to surface ratios. As a result, differential axial shortening, which is the relative movement between adjacent vertical members, occurs in these structural members. Although the differential axial shortening of vertical elements will not significantly affect these elements, horizontal elements such as beams and slabs could be seriously affected. The slabs may not be truly horizontal after some time. Beams could be subjected to higher bending moments and shear forces when connected to perimeter columns and inner shear walls. These factors could have certain structural effects and could also affect the non-structural elements such as finishes, claddings, pipe lines, and elevator functioning (Fintal & Fazlur, 1971).

The problems occurring due to the differential axial shortening were first observed and reported when increasing the height of buildings. Several analytical and test procedures have been introduced to quantify the magnitude of the differential axial shortening. However, they are limited to a very few parameters and are not adequately rigorous to capture the complexity of true time dependent material response. Moreover, measuring the axial shortenings of the concrete elements during and after the construction period using gauges such as vibrating wire gauges, external mechanical gauges, and electronic strain gauges to verify the pre-estimated amount obtained from the analytical procedures is another method. However, installing these gauges on the concrete elements during the construction, and protecting them during and after the construction, are not economical or convenient tasks. As a result, this method is being excluded from present usage.

With this in mind, from a practical design point of view, a simple and accurate numerical method to predict the long-term shortening of reinforced concrete structures and a practical concept to quantify the axial shortening using ambient measurements are necessary, especially for tall buildings with complex geometries and mechanisms.

Advanced Finite Element Methods are becoming more accessible in the design offices nowadays since they can be used to accurately capture the behaviour of structure under different load conditions. At present, these methods are successfully implemented at the design stage of high rise buildings (Smith & Loull, 1991; Baker et al, 2007). The numerical method presented in this paper is based on Advanced Finite Element Method and popular creep and shrinkage models of the GL2000 method (Gardner, 2004). The GL2000 method has been developed recently, and creep, shrinkage and elastic deformation models of this method are commonly used as they are very accurate and convenient for application to design procedures (Goel et al, 2007).

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