Estimating Period Lengthening of High-Rise Buildings

Abstract

This thesis investigates the estimation of period lengthening (T˜/T) in structures using model updating, which assesses frequency shifts due to changes in boundary conditions, such as flexible bases. A 2D finite element (FE) model based on Timoshenko beam elements was developed and optimized through Sequential Least Squares Quadratic Programming (SLSQP) to align simulated dynamics with measured natural frequencies and mode shapes of a steel structure.

The research was divided into two phases. Phase one utilized synthetic modal data to validate the model updating process in an error-free environment, showing that the algorithm effectively resolves stiffness-to-mass ratios rather than individual properties. Period lengthening predictions proved more accurate and stable with stiff springs, while soft springs demonstrated higher sensitivity and discrepancy. Phase two applied real measurement data, yielding results consistent with synthetic data but revealing a “plateau” in the cost function, where optimal parameter determination was challenging due to model and measurement uncertainties.

The findings indicate that model updating is a feasible method for estimating period lengthening, especially for flexible foundations. However, results are sensitive to modeling and measurement uncertainties, necessitating careful evaluation to ensure optimization convergence and parameter precision. The study recommends further research to mitigate uncertainties and enhance the method’s applicability in high-rise building design.