دانلود رایگان مقاله چگونه ستون فلزی بلند و باریک توسط عیوب و تولید بند سختی تحت تاثیر قرار می گیرند

Abstract

Finite-element programs can be used for designing columns and their bracing systems. It is well known, however, that the output obtained from such programs is highly dependent upon the input (such as imperfections and stiffness properties). In the present study, the effects of imperfections on the predicted strength and stiffness requirements of steel columns and of their bracing systems are investigated. Two different systems are analyzed: 1) a braced non-sway column and 2) a braced sway column. It was found that a poor choice of the shape of the initial imperfections can provide unrealistic results in terms of both the buckling load on the columns and the predicted reactions of the bracings. It was also found that superimposing different imperfection shapes can contribute to obtaining realistic and trustworthy results. Furthermore, it was shown that the shapes of the initial imperfections that lead to the lowest buckling load and those that result in the strongest bracing forces, are generally not the same.

4. Conclusions

The effects of the imperfection shapes on the strength and stiffness requirements of two different column systems and their bracings were investigated. An energy solution to the initially straight and elastic column was derived for both cases and was used successfully for defining the imperfection shapes and the elastic limits of the column. The first column system investigated was that of a sway-prevented column having one intermediate bracing (System A), the second one being a sway-permitted column having one bracing at the top and one at mid-length (System B). The general conclusions of the study, which were found to hold for both systems, are as follows: 1. A poor choice of an imperfection shape can lead to unrealistic results when nonlinear FE analyses on slender columns are performed. For the systems analyzed in the present study, an imperfection choice predicting an elastic capacity twice as high as the Euler load for the buckling between the restraints was presented. At least one solution to this problem was to superimpose different imperfection shapes. 2. A higher degree of stiffness of the bracings was found to be required in order to force the imperfect column to buckle between the restraints than was determined to be the case for initially straight elastic column. This was not possible to detect in the FE-modeling that was conducted when only an imperfection mode having pivot points at the bracing points was employed, i.e. an imperfection mode associated with the highest possible buckling mode (Euler buckling between the restraints). In order to solve this problem, the imperfection mode in question would need to be complemented by at least one secondary imperfection shape of another mode. 3. The approach of classifying one imperfection shape as being the major one and assuming one or more other shapes to be secondary turned out to yield reliable results in most cases. However, it was also demonstrated that there could be combinations of different two imperfection shapes that yielded unrealistic results. Accordingly, use of a combination of at least three different imperfection shapes is recommended. 4. An imperfection shape that generates a large initial displacement between the bracings led to higher bending moments in the column at loading than an imperfection shape for which the maximum initial displacement was at the bracing points. This means that an imperfection shape of this type is needed in order to obtain a conservative estimate of the column strength required. 5. An imperfection shape generating a large initial displacement at the bracing points was found to predict a higher level of forces in the bracings at loading. Accordingly, such an imperfection shape should be used so as to obtain a conservative estimate of the required bracing strength and stiffness. 6. Points 4 and 5 suggest that at least two different sets of combinations (each consisting of at least 3 sub-shapes according to paragraph 3 above) of imperfection shapes should be used in design so as to obtain safe results both in terms of the strength of the column and the stiffness and strength requirements of the bracings.