BEHAVIOUR OF POST-TENSIONED BRICKWORK CANTILEVER FIN WALLS

Yehia Anis DAOU

This Thesis describes an investigation of the behaviour of post-tensioned fin walls subjected to bending moment and shear force. A series of experiments was carried out on 0.293 scale model post-tensioned fin walls is described. Different lateral loading systems arranged to give different distributions of bending moments and shear forces simulating earth pressure with various degrees of accuracy were used. Prestress was applied using an unbonded tendon which was placed inside a cavity in the web, and anchored at the ends of the wall. The wall tests, as well as being designed to simulate the general behaviour of prestressed masonry fin walls subjected to earth pressure, were also intended (i) to provide information on the influence of prestress level and shear span / depth ratio, (ii) to study the effect of change in the percentage steel area and (iii) to categorise the mode of failure. Coincident with these tests, a series of material tests was carried out to evaluate the properties of the materials used, mainly the brickwork and the steel tendons. Further brickwork prism tests, carried out to investigate the effect of bed joint orientation relative to the compressive stress on the brickwork strength, are also reported. The modes of failure for post-tensioned fin walls obtained from the test walls are described. The influence of level of prestress, shear span / depth ratio, percentage steel area and the bed joint orientation relative to the principal stresses on the behaviour of prestressed brickwork walls are discussed.

In addition to the experimental work, the development of a non-linear analysis and an iterative computer program to simulate the behaviour of prestressed walls and to predict the behaviour throughout the whole loading history, in particular, at the serviceability and ultimate limit states is also described. The non-linear behaviour of both the brickwork and the steel tendons were considered. The anisotropic nature of brickwork was also taken into account by developing a failure criterion under biaxial stresses from prism tests with the mortar joints oriented at various angles relative to the applied stress. Allowance is also made for cracking at mortar joints and the relative movement between the tendon and brickwork. The results of the wall tests are compared with the analytical predictions and good agreement is shown. Simplified approximate equations to determine the deflection, crack width and the force in the steel tendons are also suggested.

The main conclusions of the work are that, prestressed brickwork masonry walls exhibit three phase behaviour. These phases correspond to the initial uncracked section (elastic stage), cracked section with the steel stress still in the elastic stage (second stage) and cracked section with either flexural or shear failure. The ultimate strength and modes of failure are highly influenced by the level of prestress, percentage steel area and shear span / depth ratio. Variables such as steel strain / adjacent brickwork strain ratio, relative movement between the steel tendon and brickwork and the anisotropic nature of brickwork are also shown to have a significant effect on the ultimate strength. When these variables were taken into account in the analysis, it was found that the ultimate strength could be adequately predicted.