Question: Why the Location of Center of Rigidity Changes for Each Story Even for Symmetric Structures? Answer: The center of rigidity is the center of resistance of a floor or diaphragm against lateral forces. It is the stiffness centroid located within a floor-diaphragm plan. If the lateral forces are applied at this center, the floor diaphragm will experience only the translational displacement (i.e. no rotation). Other levels are free to translate and rotate since the actual behavior is coupled both in plan and along height. The location of center of rigidity is an inherent structural property and has nothing [...]
The Definition of Design Basis Earthquake Level and the Retrofit Need for Structures Designed for Older Codes
Question: I noticed that with the updates of codes it seems that with the passage of time the seismic parameters (response spectral accelerations) are increasing. For example if we compare earlier versions of AASHTO (if we talk about bridges) and the 2013 version, the earthquake changed from 475 years to 1000 years return period earthquake, while the R factor and load factor for earthquake remained same. Wasn't this change in early 2000's a drastic? The earlier structures (bridges for Aashto) required retrofit? Should have been a huge retrofitting activity!For Pakistan we have only PGAs for 475 years in PBC. Practicing [...]
Question: Why in different building codes (UBC 97 or ASCE 7) the results of Response Spectrum Analysis (RSA) should be scaled such that the design base shear should match with the (e.g. 80%, 90% or 100% of the) design base shear obtained from the Equivalent Static Analysis? Answer: The requirement of a base shear scaling for the Response Spectrum Analysis (RSA) procedure is included in the building codes as a means to ensure that the minimum strength of a structure designed using the RSA procedure is similar to the strength that would be required if the structure was designed using [...]
An informative hand-written notes explaining the concepts of End Offsets, Panel Zone and Moment Releases in ETABS.
Prof. Pennung Warnitchai of Asian Institute of Technology (AIT), Thailand explains the phenomenon of resonance amplification of dynamic response of structures using a simplified mode (often referred to as the Boss Model). The resonance is a phenomenon resulting in large amplification of oscilations which occurs when the frequency of a periodically applied force is same as the natural frequency of the system on which it acts. When an harmonic force is applied at a resonant frequency of a system, it will oscillate at a higher amplitude than when the same force is applied at any other frequency. [...]
An interesting two part series of articles authored by Prof. Dr. Graham Powell. Prof. Powell is Professor Emeritus of Structural Engineering at the University of California at Berkeley. Over his career, Professor Powell has developed, or supervised the development of, several computer programs that have been used widely in research and industry. He currently works as a consultant to Computers and Structures Inc., developing and supporting the CSI PERFORM computer program. Professor Powell may be reached at email@example.com.
In current design office practice, a commonly used modelling assumption is that the base of a building structure can be idealised with fixed support conditions, thereby neglecting any effects from soil-structure interaction (SSI). Various recent studies, however, have shown that the explicit consideration of SSI effects in seismic analysis of buildings structures may significantly affect the predicted seismic demands and resulting structural performance. This study addresses some key issues and practices in the area of SSI and its effects on the dynamic response and seismic performance of buildings. It is also intended to demonstrate the significance of considering SSI effects [...]
Question: I am currently working in a Consultancy firm in Islamabad and have few queries regarding torsional Irregularity explained as follows: 1) As per UBC torsional irregularity exists if "Maximum story drift, computed including accidental torsion, at one end of the structure transverse to an axis is more than 1.2 times the average of the story drifts of the two ends of the structure". In our general practice, we compare the ratio of maximum displacement to average displacement with 1.2 to confirm if the torsional irregularity exists or not. But as per the above stated UBC Statement, we should [...]
Once my PhD advisor told me a story. He asked, “You might have seen simply supported beams hundreds of times in textbook illustrations, papers and presentations; Have you ever saw an actual simply supported beam as part of a real structure, with a pin support at one end and roller on the other?” I replied, “No, these support conditions are mere structural idealizations which—in almost all practical situations—are different from what we observe in everyday life”. He said, I have seen many real simply supported beams, with a real pin connection and a real roller support. I have met [...]
Can we build Jacob's Ladder???
Concrete Mix Design - Art of Balancing the Conflicting Requirements Why we need to explore potentials of Indigenous materials in Pakistan Concrete, like all other engineering materials, needs to be designed for properties like strength, durability, workability and cohesion. This extremely versatile material can be designed for strength ranging from 10 Mpa to 200 Mpa and workability ranging from 0 mm slump to 250 mm slump. It’s all characteristics including strength, workability and durability, are in our hands. One can make it flow like a liquid, make it light like foam and dense like a stone. One can predict [...]
Basics of making professional looking graphs in MS Excel Graphical data visualization is the key element of almost every research publication. An impressive data reporting can not only helps in clear deductions and convenient result interpretation but also attracts readers’ attention by conveying message in easy-to-understand way. Recently I came across a document comparing the fundamental periods of 5 buildings (denoted as B1 through B5) computed from actual Eigen-value analysis with the values predicted by an Artificial Neural Network (ANN) model trained on a database of already designed buildings. Here is what the comparison was looking like, when presented [...]
Going beyond the Code!!! An account of Performance-based Design and Seismic Evaluation of Buildings Introduction Once upon a time, there was a poor middle-aged farmer who had two wives (For a moment, please neglect the reason why he went for such an adventure). The first one was of same age as him but the second was relatively younger. When he used to go to the younger one, she start plucking white hair from his beard, trying to make him look younger as her. When he used to go to first one, she start plucking black hair from his beard, [...]
In the aftermath of Nepal Earthquake – The Dilemma of a Structural Engineer Look at this picture! Courtesy: Noelle Soren and David Soren, 2008 While browsing through internet pages and social media, I came across this picture and it somehow got stuck in my mind. These skeletons were found by two archeologists, David Soren and Noelle Soren in the mid-1980s in a Roman house in Kourion, Cyprus. They are believed to be killed during an earthquake that struck Kourion in 365 AD. The picture includes a family of three, showing a mother cradling an infant in her arms as the [...]