NON Linear Dynamic Earthquake Analysis of Skyscrapers (Summary)
In "Non linear dynamic Earthquake Analysis of Skyscrapers," published in the
CTBUH 8th World Congress ,
Building a skyscraper in an earthquake prone area (seismic area) is always an issue of concern for construction engineers. In these areas, the earthquake loads are classified into three levels based on their intensities -Frequency level,Design intensity level and Severity level.
Linear analysis is adequate for the first load level but not for the other two because the skyscrapers move to plastic region (An area of the stress-strain graph in which permanent changes to a building begin to occur) when subjected to the other two levels. This is where Lee reinforces the use of Non Linear Dynamic analysis to evaluate the non linear seismic response of the structures to those earthquake loads. It is a practical analysis tool for structural design, and “because of this technique, the skyscraper can be designed with added safety and economy”(1). Without the non-linear dynamic analysis, the engineers can’t estimate the response of the skyscrapers to the design intensity and rare level earthquakes.
Lee states the three obstacles faced by an engineer in applying the nonlinear dynamic analysis.The first being, It is too complex to be solved by older computers.The second is the results might be very sensitive to the properties of the structural non linearities.And the last is the analysis results vary significantly to different earthquake records.
This journal provides a solution on the ABAQUS platform by selecting the appropriate non-linear dynamic analysis procedure, accurate non linear models for the structures, an efficient dynamic equation integration scheme and the appropriate earthquake records. A software package called BEPTA (Building elastic-plastic time history analysis), developed on the ABAQUS platform is used for this.
The Non linear dynamic analysis procedure: Skyscrapers are built one story at a time ,in the computer ,first. The structural elements of each story are activated progressively and the final stress status is obtained at the completion of the construction.
The non-linear model actually presents the non-linearity of the structure which includes the geometric non-linearity, material non-linearity and combination of both. This model presents the nonlinear constitution for steel and concrete.
There are two types of integration schemes of the non linear dynamic equations. One is the unconditional stable implicit scheme to solve structural dynamic problems. The other conditionally stable explicit scheme is used for large scale problems. Emphasis is placed on how the explicit scheme can solve large and complex problems.
Earthquake records are provided by the seismic engineer at the site investigation. Earthquake always comes in three directions (one vertical and two horizontals).Therefore three earthquake records are input into the structure in an analysis run.
The nonlinear dynamic analysis has been used for some prominent projects in china such as the Shanghai World Financial center,Jinta and Guanghou West Tower.
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