Investigation of nonlinear seismic response of high-rise RC wall structures using modal decomposition technique | |
| Author | Mehmood, Tahir |
| Call Number | AIT Diss no.ST-15-01 |
| Subject(s) | Tall buildings--Earthquake effects Earthquake resistant construction |
| Note | A dissertation submitted in partial fulfillment of the requirements for the Degree of Doctor of Engineering in Structural Engineering, School of Engineering and Technology |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. ST-15-01 |
| Abstract | Reinforced Concrete (RC) walls are commonly used in high-rise structures due to their significant lateral stiffness and strength against the lateral loads such as wind and earthquake. The seismic response of the high-rise RC wall structures is very complicated as several vibration modes other than the fundamental mode normally contribute significantly to the response—this is commonly known as ―higher mode effects‖. Among various numerical analysis procedures for evaluating seismic performance of buildings and structures, the Nonlinear Response History Analysis (NLRHA) procedure has been widely considered and accepted as the most reliable and accurate one. However, the procedure is computationally very expensive, and it does not provide much physical insight into the complex inelastic responses of the structure. Another well-known analysis procedure is the Nonlinear Static Procedure (NSP). This is a simplified procedure which accounts only for the seismic responses of the fundamental vibration mode of the structure. Hence, the NSP procedure is not suitable for evaluating the structures where many vibration modes may participate significantly in their seismic responses. In this study, a simplified but accurate procedure is studied. It is called the Uncoupled Modal Response History Analysis (UMRHA) procedure. In this procedure, the nonlinear response of each vibration mode is first computed, and they are later on combined into the total response of the structure. The effectiveness of this procedure has never been assessed systematically for different configurations of RC walls in high-rise structures. Moreover, the reliability of the UMRHA procedure for high-rise buildings with varying heights under different types of seismic hazard scenarios needs to be evaluated. In the this study, reliability and accuracy of the UMRHA procedure is thoroughly examined by using four actual case study buildings. The gravity-load-carrying system of the case study buildings consists of RC slab-column frames, while the lateral-load-resisting system is composed of RC walls or cores. The responses of these four tall buildings are computed by this simplified UMRHA procedure and compared with those obtained from the NLRHA procedure. The comparison shows that the UMRHA procedure is able to accurately compute story shears, story overturning moments, floor accelerations and inter-story drifts of these tall buildings with heights varying from 20 to 44 stories. The required computational effort is also extremely low compared to that of the NLRHA procedure. Since the UMRHA procedure computes the response of each individual vibration mode, it provides more understanding and insight into the complex nonlinear seismic responses of these tall buildings. The modal decomposition results indicate that due to higher modes of vibration, high-rise RC wall structures can develop plastic hinges at unintended locations. Modal responses helped to identify the critical locations which need careful detailing for ductile responses. Furthermore, the UMRHA procedure is extended to calculate component responses. The proposed procedure to obtain component response behavior can be easily used to evaluate the performance of critical members with an acceptable degree of accuracy. Another important issue related to the nonlinear seismic response prediction of the high-rise RC wall structures is the numerical modeling of RC walls. Most of the previous studies were conducted using simple lumped plasticity beam-column models or fiber models for RC walls. The use of the fiber modeling approach for RC walls, where inelastic behavior is represented at the material level, has become very common in engineering practices. The conventional fiber modeling approach takes into account the axial-flexure interaction while ignoring the iv axial-flexure-shear interaction. Furthermore, shear behavior is modeled either as linear elastic with unlimited shear strength or as nonlinear with code-based limiting shear strength value. However, both of these cases are not realistic. Code-based empirical equations do not provide an accurate estimation of the shear strength of RC walls. Recent studies have found that code-based empirical equations significantly underestimate the true shear strength of the RC walls, especially under the axial load effects and lower shear span ratios.The axial load and shear span ratio fluctuate depending upon the structural configuration, location, geometry of RC walls and characteristics of ground motion used for analysis; consequently, shear strength as well as shear stiffness is not constant throughout the time history. Moreover, under significant flexural yielding, shear stiffness and strength decrease due to opening of cracks and decrease in aggregate interlocking, which can lead to ―flexure-shear‖ type of failure in RC walls. As the flexure and shear responses are treated as independent of each other in the conventional fiber modeling approach; therefore, flexure-shear type of failure cannot be simulated. Such computer modeling assumption can affect the prediction of nonlinear seismic response of high-rise RC wall structures. Therefore, it is essential to employ an appropriate computer model for the investigation of nonlinear seismic response of high-rise RC wall structures. To identify an appropriate numerical modeling approach for RC walls, various analytical models are used to predict the nonlinear quasi-static response of three RC walls tested in the current study. Experimental results demonstrate that the shear strength and stiffness of RC walls are significantly affected by the axial-flexure-shear interaction. Based on the findings of analytical study, a Finite Element (FE) model, based on Modified Compression Field Theory (MCFT), is found to be capable of simulating the axial-flexure-shear interaction observed in the experimental tests with significant accuracy. This FE model is then used for the detailed investigation of a high-rise RC core wall case study building. A commonly used fiber computer model is also used to compare the results with the axial-flexure-shear interaction FE model. Results show a significant decrease in the nonlinear seismic shear demands obtained from the FE model as compared to the conventional fiber modeling approach, due to the shear yielding of RC core wall at the base. The detailed modal decomposition results of the FE model help to identify the cause of shear yielding at the base. Based on the improved understanding, the case study building is optimally re-designed and the shear yielding at the base of the RC core wall is avoided in an economical way. |
| Year | 2015 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. ST-15-01 |
| Type | Dissertation |
| School | School of Engineering and Technology |
| Department | Department of Civil and Infrastucture Engineering (DCIE) |
| Academic Program/FoS | Structural Engineering (STE) /Former Name = Structural Engineering and Construction (ST) |
| Chairperson(s) | Pennung Warnitchai; |
| Examination Committee(s) | Anwar, Naveed ;Punchet Thammarak;Lam, Nelson ; |
| Scholarship Donor(s) | Higher Education Commission, Pakistan ;AIT Fellowship; |
| Degree | Thesis (Ph. D.) - Asian Institute of Technology, 2015 |