Reliability Pointers for Modal Parameter Identification of Precast Concrete Terraces
8. CONCLUDING COMMENTS
A comprehensive account of experimental and numerical modal analysis for a family of reinforced concrete grandstand terraces, supported on a rectangular hollow section (RHS) frame was presented in this study. Based on the findings, the following conclusions can be drawn:
1. Overall, and after studying the first six modes of vibration it was found that
experimentally obtained natural frequencies were in good agreement with the ones predicted by a continually updated finite element model.
2. Experimental procedures may not be adequate to provide a complete account of modal analysis unless the equipment used is in abundance, of good quality, high resolution and standards and therefore extremely expensive and unsustainable for research work alone. Hence, some limitations are inevitable as certain modes that tend to be more complex than others, may not be depicted accurately by the experimental analysis alone. A rigorous finite element model should help to
overcome this problem by capturing the resulting mode shapes with confidence and accuracy, especially at higher modes of vibration.
3. Initial and short term results suggest that the amount of reinforcement has little effect on the dynamic properties of the “uncracked” reinforced concrete terrace units.
However, interim but carefully studied results from the finite element analysis hinted towards the possibility that an increase in the amount of reinforcement is likely to force the structure into a different mode of vibration, hence altering the previously obtained properties.
4. The dynamic properties of the terrace units were found to be very sensitive to two parameters, the physical properties of the main material (concrete) and the conditions of the supports of the structure, as expected. The degree of sensitivity is discussed in some detail in Sections 5 and 6. Essentially, it was found that a gradual improvement of the predicted natural frequency values was evident by progressively improving the representation of boundary conditions. Best correlation was achieved when the inserted between the units neoprene pads were modelled by simulating their stiffness characteristics with the ANSYS dedicated stiffness matrix element.
5. Also, it was found (section 5.1) that the subsequent adoption of Block Lanczos solution technique, over the initial choice of the subspace method, reduced the CPU- time significantly and made a relatively modest contribution to the accuracy of the results. It is therefore recommend for similar type of numerical analysis work.
6. Verification of the experimental and numerical results may not always be possible, or may sometimes be impractical due to complex mathematical calculations and a plethora of uncertainty factors arising mainly from the constants involved.
9 AKNOWLEDGEMENTS
The author would like to thank the following organisations for their help:
Bison Concrete Products Ltd, The British Gaskets Group, Howgate & Lane Ltd, Al Ashwaq Rubber products Ind.
Corresponding author, Senior Lecturer, email: [email protected] 120
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