Prestress evaluation in prestressed concrete plate-like structures

Condition assessment and capacity evaluation of existing structures using their vibration responses has been subjected to extensive research for many years. Prestressed concrete structures have been one of the main focuses of those studies. In the case of prestressed concrete structures, effective prestress force is the most important parameter for their best performance and yet currently there is no effective method in identifying the prestressing force in an existing prestressed concrete structure. Effect of prestress is different for different types of structural elements and has to be treated accordingly for its accurate quantification. This paper presents a new approach to evaluate the effective prestress force of plate-like structures with simply supported boundary conditions using their vibration responses. The proposed method quantifies the prestress effect with a reasonable good accuracy, even with noisy measurements using both periodic and impulsive excitations. Prestress estimation can be done using collected data from as less as two measurement locations

Effects of differential axial shortening on outrigger systems in high rise buildings with concrete filled steel tube columns

Concrete Filled Steel Tube (CFST) columns are popular in high rise buildings due to their superior strength, seismic and fire resistance capacities and construction simplicity. Structural framing systems in high rise buildings are commonly coupled with reinforced concrete outrigger and belt systems to facilitate lateral load resistance. When axial shortenings of vertical elements occur due to time dependent phenomena of creep, shrinkage and elastic deformations, the horizontal stiff elements balance the shortening differentials in the vertical elements and cause load redistributing among them dynamically. This can result in high transfer stresses induced in the stiff outrigger and belt systems which need to be considered in design or mitigated during construction. To plan mitigation strategies such as the time to connect the shear core to the structural frame to effectively reduce time dependent transfer stresses, it is necessary to quantify current and future differential axial shortenings. This paper first quantifies the differential axial shortening (DAS) between the shear core and columns, considering effects of construction sequence, time dependent material properties and reinforcement and then quantifies the transfer stresses built up in outrigger and belt systems in CFST high rise buildings. This information will be useful in mitigating the adverse effects of these high transfer stresses

Health monitoring of buildings during construction and service stages using vibration characteristics

Columns and walls in buildings are subjected to a number of load increments during the construction and service stages. The combination of these load increments and poor quality construction can cause defects in these structural components. In addition, defects can also occur due to accidental or deliberate actions by users of the building during construction and service stages. Such defects should be detected early so that remedial measures can be taken to improve life time serviceability and performance of the building. This paper uses micro and macro model upgrading methods during construction and service stages of a building based on the mass and stiffness changes to develop a comprehensive procedure for locating and detecting defects in columns and walls of buildings. Capabilities of the procedure are illustrated through examples

Quantifying in-plane deformation of plate elements using vibration characteristics

Plate elements are used in many engineering applications. In-plane loads and deformations have significant influence on the vibration characteristics of plate elements. Numerous methods have been developed to quantify the effects of in-plane loads and deformations of individual plate elements with different boundary conditions based on their natural frequencies. However, these developments cannot be applied to the plate elements in a structural system as the natural frequency is a global parameter for the entire structure. This highlights the need for a method to quantify in-plane deformations of plate elements in structural framing systems. Motivated by this gap in knowledge, this research has developed a comprehensive vibration based procedure to quantify in-plane deformation of plate elements in a structural framing system. This procedure with its unique capabilities to capture the influence of load migration, boundary conditions and different tributary areas is presented herein and illustrated through examples

A numerical method to quantify the axial shortening of vertical elements in concrete buildings

Differential axial shortening, distortion and deformation in high rise buildings is a serious concern. They are caused by three time dependent modes of volume change; “shrinkage”, “creep” and “elastic shortening” that takes place in every concrete element during and after construction. Vertical concrete components in a high rise building are sized and designed based on their strength demand to carry gravity and lateral loads. Therefore, columns and walls are sized, shaped and reinforced differently with varying concrete grades and volume to surface area ratios. These structural components may be subjected to the detrimental effects of differential axial shortening that escalates with increasing the height of buildings. This can have an adverse impact on other structural and non-structural elements. Limited procedures are available to quantify axial shortening, and the results obtained from them differ because each procedure is based on various assumptions and limited to few parameters. All these prompt to a need to develop an accurate numerical procedure to quantify the axial shortening of concrete buildings taking into account the important time varying functions of (i) construction sequence (ii) Young’s Modulus and (iii) creep and shrinkage models associated with reinforced concrete. General assumptions are refined to minimize variability of creep and shrinkage parameters to improve accuracy of the results. Finite element techniques are used in the procedure that employs time history analysis along with compression only elements to simulate staged construction behaviour. This paper presents such a procedure and illustrates it through an example.\ud \ud Keywords: Differential Axial Shortening, Concrete Buildings, Creep and Shrinkage, Construction Sequence, Finite Element Method.\u

Differential axial shortening of concrete structures

Non linear deformation of concrete has an adverse impact on high-rise buildings with complex\ud geometries due to differential axial shortening. These adverse effects are caused by time dependent\ud behaviour resulting in volume change known as “shrinkage”, “creep” and “elastic” deformation. These\ud three phenomena govern the behaviour and performance of all concrete elements during and after\ud construction. Reinforcement content, variable concrete modulus, volume to surface area ratio of\ud elements, environmental conditions and construction quality influence the performance of concrete\ud elements and differential axial shortening will occur in all structural systems. Their detrimental effects\ud escalate with increasing height and non vertical load paths resulting from geometric complexity. The\ud magnitude of these effects have a significant impact on building envelopes, building services,\ud secondary systems and the life time serviceability and performance\ud Analytical and test procedures available to quantify the magnitude of these effects are limited to a\ud very few parameters and are not adequately rigorous to capture the complexity of true time\ud dependent material response.\ud With these in mind, a research project has been undertaken to develop an accurate numerical\ud procedure to quantify the differential axial shortening of structural elements. The procedure has been\ud successfully applied to quantify the differential axial shortening of a high rise building and the\ud important capabilities available in the procedure have been discussed. Additionally, a new practical\ud concept based on the variation of vibration characteristic of structure during and after the construction\ud will be developed and used to quantify the axial shortening and to assess its performance.\ud Keywords: Axial shortening, Concrete buildings, Creep, Shrinkage, Elastic deformation, Vibration\ud characteristic of structur