Investgation of Effects of Imperfection and Their Significance in Analysis Results
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Date
2007-02
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Addis Ababa University
Abstract
Structural systems are prone to global as well as member imperfections
in geometry. These imperfections are accounted for when considering
structural stability and strength requirements of both sway and non-sway
frames.
In the first part of this thesis, stability of imperfect systems is examined.
With the intention of understanding the ideas necessary to tackle more
practical aspects of instability, stability of equilibrium are first introduced by
analysis of the behavior of some simple mechanical systems. Existence of
imperfection cause the maximum load to be attained to fall below the critical
value and the larger the imperfection, the larger is this reduction. The
bifurcation point defines the form of instability of a structural system.
For simple mechanical systems, the behavior of the imperfect system is
governed by the magnitude and the sign of the imperfection and also the
critical load of the system without imperfection. In systems with more than one
degree of freedom, it is the fundamental mode and load which governs the
elastic portion of the behavior frames possessing imperfection. The pattern of
imperfection of form and loading, considered as a whole determines the mode
of buckling.
To evaluate strength requirements of imperfect structural systems, the
imperfection effects are quantified in terms of an initial sway imperfection at
the base of the column and then converted to equivalent horizontal forces.
Analysis is carried out which includes these equivalent horizontal forces and
the maximum bending moment in columns and beams for frames with varying
number of bays and number of stories are examined. For a fixed number of
stories, the effect on maximum bending moment in beams decreases with
increasing number of bays. The effect on columns is not as large as that on
beams but is still significant for high rise buildings. For fixed number of bays
the percentage increase rises with the story height.
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