Numerical Methods in Civil Engineering Dynamic Response Evaluation of a Super-Tall Tower via Endurance Time Method Amirali Bazmooneh*, Iman Dabbaghchian**, Saeed Behboodi***, Amir Maghsoodi Shaghaghi****, Masoud Sanayei*****, Akbar Esfandiari******. Abstract: ARTICLE INFO In this paper, the performance of a super-tall tower is evaluated through Milad Tower in Article history: Tehran, Iran, as a case study. The structure is a 435 meters tall telecommunication tower and Received: is structurally studied in this paper. For this purpose, linear endurance time (ET) method and October 2020. Revised: time history (TH) analysis are used to compare the results and focus on the structure's dynamic November 2020. properties and behavior. The analyses are performed on a finite element model with SAP2000 Accepted: and Abaqus software. Assumptions in linear modeling are investigated, including shell or solid December 2020. element type and mesh sizing. Furthermore, the model is verified with experimental modal data. Performance-based analysis is performed according to ASCE41; the tower’s behavior and Keywords: Performance-based strength capacity is evaluated for different tower elevations. The scaling method effect on the design, Scaling, Super-tall response of the structure is demonstrated to have a major role. The Endurance Time method as tower, Endurance Time a simplified and alternative analysis tool is exerted to estimate the structure's significant method, Seismic analysis responses and compared to ground motions with different spectrums. Results show that the ET method can adequately estimate the results in comparison with the TH method. D 1. Introduction The finite element method (FEM) can be utilized as a robust tool to suitably model and analyze these structures. Structural modeling and analysis of super-tall buildings At the same time, some problematic aspects need to be have always been crucial among structural engineers and carefully taken into account. The FEM provides different scholars in the field. Even small imperfections in super-tall approaches and element types to simulate the dynamic buildings, which are commonly symbolic structures, bring attributes of a structure's behavior. Choosing a proper up the subject of social and economic impacts and element type and mesh size to capture the most realistic consequences on a city. This matter would lead designers response of the structure at the design and assessment stage to design architecturally magnificent and yet structurally is the art of structural engineering that is usually a full of complicated buildings. twists and turns task, needing much experience. Since the detailed finite element models of tall buildings * Corresponding Author: Ph.D., Civil Engineering Dept., Sharif Univ. of are comprised of a substantial number of degrees of Technology, Tehran, Iran. Email: [email protected] **Ms, School of Civil Engineering, College of Engineering, Univ. of freedom (DOF) and many elements, the analysis phase is Tehran, Tehran, Iran. a time-consuming procedure. Design codes compel ***Ms, School of Civil Engineering, College of Engineering, Univ. of designers to implement time-history analysis on essential Tehran, Tehran, Iran. ****Ms, School of Civil Engineering, College of Engineering, Univ. of structures, like tall buildings, which is even more time Tehran, Tehran, Iran. consuming. The time-history analysis also meets different ***** Professor, Dept. of Civil and Environmental Engineering, Tufts Univ., Medford, USA. uncertainties, including the number of records, record ****** Professor, Marine Engineering Dept., Amirkabir Univ. of selection procedure, and record to record uncertainty. Technology, Tehran, Iran. 53 In general, seismic assessment of existing structures aims evaluated in general within the framework of specific to assess structural and non-structural elements' capacity regulations and instructions. during an earthquake incident to check whether the loads exceed the expected level. Nowadays, codes and As a result, in civil structures, it is difficult to estimate provisions for seismic assessment and rehabilitation of these values accurately. Therefore, the uncertainties of the structures[1–3], mainly focus on performance-based dynamic response can be significantly reduced, if the methods. The evaluation process includes observing the estimation can be done suitably. The typical value of structural response in various hazard levels. However, damping ratio is recommended between 2 to 5% for use in there are challenges in the seismic evaluation of tall seismic analysis [3] and generally structures built with structures that make it rather impossible to apply the concrete material have more inherent damping ratio than available standard methods to the buildings explicitly [4- those with steel material although, in case of towers it is 5]. not conceivable to have a significant damping ratio [16– 18]. The first challenge is estimating the force on the structure. Selecting an accelerometer for the time-history analysis To reduce the computational cost of analyzing these that would appropriately simulate the realistic site superstructures, simplified fast analysis tools are needed location's excitations has been studied in different for numerical examining a wide variety of assumptions for categories of research [6]. The proximity of the structure trial and errors and sensitivity analysis. During recent to the fault, magnitude, and intensity of the seismic loads years [20–24], The Endurance Time (ET) method [19] was are involved in selecting the appropriate accelerometer. introduced as an alternative analysis method than can Also, the impact of velocity component of ground motions effectively reduce computational costs. This method is on the response of the tall structures, which is referred to intended to speed up the analysis phase to a minimum of as the effect of pulse-like records, are shown to be crucial 20 times with regard to the required responses and hazard and sensitive to the ratio of peak ground velocity (PGV) levels. The ET method consists of applying a specially over peak ground acceleration (PGA) of the selected designed artificial accelerograph to the structure and records [7,8]. observe response time histories in a specific and meaningful order. Another challenge is understanding the behavior of the structure. Cantilever towers such as Milad Tower are The ET acceleration function (ETAF) is an intensifying similar to a mega-column that bears all the loads on the excitation whose spectral amplitude increases linearly over structure. The tower's behavior needs to be evaluated at time. As a sample, the spectral acceleration at a period (푆푎) three levels, including material, element, and structure. At of the ETAF section from 0 to 20 seconds is twice that of the material level, due to the limitation of obtaining 푆푎 at 10 seconds. Thus, the maximum value from the information to accurately estimate the characteristics of beginning to the 10 seconds of ET time history is materials, many material models are generated to consider associated to the 푆푎 level of first 10 seconds and the the linear and nonlinear behaviors [9]. At the member maximum value at first 20 seconds is associated with the level, there are uncertainties about considering values like two-fold 푆푎. The following section presents the ET yield stress and modulus of elasticity and also modeling implication on the existing structure in more detail. the correct behavior of material. In cantilever structures, This paper provides different numerical models to evaluate since the structure is determined, and there is no other path the behavior of a case study tower and accuracy of the for loads except the concrete shaft, the internal forces are obtained dynamic properties under seismic loads. Besides, redistributed continuously by cracking the cross-section. the endurance time (ET) method is implemented in the Studies that have examined the seismic behavior of linear structure model, and results are compared to the cantilever towers have confronted challenges as well [10– conventional time-history (TH) method. The linear 15] analysis is adopted for this analogy by remaining focused In technical and engineering problems, especially in on the impact of basic structural parameters on the practical projects, uncertainty is an indisputable part and dynamic behavior, and avoiding complexities and engineering can generally be defined as decision making uncertainties that would arise at each stage of nonlinear under uncertainty. On the other hand, regulations and modeling and analysis. provisions are the result of examining a wide range of The case study is Milad Tower in Tehran, Iran, which is common building structures over several decades to issue thoroughly introduced in the second part of the paper. The the same instructions by taking into account the actual third section presents methodology and assumptions of the conditions and laboratory results. However, the study. This part explains FE modeling and details, regulations have a specific scope of application, and a including element types, damping ratio, mesh sensitivity, structure with a height of more than 300 meters cannot be and post-tensioning considerations, and the model's verification with real data from the tower. Additionally, the Numerical Methods in Civil Engineering, Vol. 5, No. 2, December. 2020 Special Issue on “Recent Achievements in Endurance Time Method” method to calculate the capacities of the tower to obtain meters. The next