Stability Analysis of Steel Structures Codal Provisions - is:800-2007 and Aisc:360-05

Abstract

Stability is the fundamental safety criterion for steel structure. Structural or geometrical instability occurs due to compressive load and is usually known as buckling. The buckling load is the maximum load at which the compression member becomes unstable. There are many analysis methods of varying degree of detail and preciseness available at structural engineer’s disposal. Higher end analysis types are meant to consider more realistic (near to real life conditions) aspects and less ‘ideal’ assumptions; but obviously they involve more time consuming and complex procedure. The decision to select an appropriate analysis type is always of paramount importance and therefore has to be made judiciously. Stability requirements and related codal provisions provide here very useful guideline for selection of proper analysis method. New codal provisions also make the design aspect ‘simpler’ by eliminating use of certain amplification factors used by earlier code (e.g. Effective length factor K) when refined analysis method is employed. Nonlinearity e.g. geometrical imperfections, material imperfections, residual stresses etc. affects the structural stability significantly and is therefore needed to be considered either at analysis stage or at design stage. To consider nonlinearity at design stage, codes have introduced factor called “effective length factor” - K, in addition to the moment multiplication factor used during member strength check. The effective length factor is just a mathematical adjustment to enable application of Euler theory to say frames and to consider the above nonlinearity effects. Evaluation of ‘K’ factor has always been an intricate task for Structural engineers. Hence, it’s a relieving aspect of new advancement of Stability Design highlighted in this work that by explicitly considering the above listed aspects the ‘K’ factor can be set as 1. The interesting history behind the birth of the ‘K- factor’ up to it’s cessation has be traced in this work. In the present study, two eminent steel design codes IS:800-2007 and AISC:360-2005 are studied with special focus on provisions meant for ensuring structural stability and related analysis requirements. It is interesting to note that both the codes have come up with new but similar stability provisions superseding respective earlier edition of the codes. The new provisions are based on recent research work in this field and they provide good insight into structural behaviour and it’s failure pattern under buckling case as well as various practical factors affecting the buckling phenomenon. To consider nonlinear effect at analysis time and design using K=1, AISC:360-2005 has presented a new versatile method called “Direct Analysis Method”. Other two methods second order analysis and first order analysis have a limited use whereas direct analysis is applicable to all types of structure. The direct Analysis method is of particular relevance for the Structural Engineers as it is described as ‘The Future of Stability Analysis’ by AISC specification committee chairman Mr. Shankar Nair and it is the main/mandatory method of the contemporary 2010 edition of AISC:360 published in 2011. The New edition of Indian standard IS:800-2007 presents various methods of analysis of steel structure with regard to stability. There are three approaches permitted: (1) First order analysis and moment amplification during design, this has limited application though, (2) Second order elastic analysis, (3) Advanced structural analysis. A notable observation on IS:800-2007 code is the need of further explanation for implementation of each of these methods. Piperack structure is an important and the most common structure in the industrial plants. Hence, a typical piperack structure is selected to study and demonstrate impact of new provisions for stability analysis. STAAD Pro being the most popular software package of field is used for analysis and design. The piperack structure selected as Case Study problem is solved with various methods prescribed by Codes AISC:360-2005 and IS:800-2007. The results are compared to illustrate the variation. For academic purpose the case-study is also solved as per IS:800-1984 provisions and compared with results from 2007 edition. The present work is aimed at exploring a relatively complex phenomena of Structural Engineering : ‘Stability analysis of steel structure’ with special focus on relevant codal provisions. With example of case study, a ‘real-life’ structure has been solved to demonstrate the application of all the background theories explained in the body of work. In practical perspective, this work deciphers the latest codal provisions related to structural stability and further validate and justify it’s application by exemplifying through Case-Studies.