Iranian Seismic Code Research Articles

Seismic progressive collapse of MRF–EBF dual steel systems

Designing structures to resist progressive collapse during an earthquake is a key challenge for structural engineers working in seismic zones. This paper introduces a more appropriate brace configuration based on an upper-bound estimation of the probability of seismic progressive collapse in moment-resisting, eccentrically braced frames designed to the third edition of the Iranian seismic code. The code currently only has provisions for the capacity of moment frames, with no detail given for bracing elements. A numerical modelling study of low- to mid-rise residential and office buildings was undertaken. The results show that for five-storey buildings the optimum number of braced bays is 10–20% of the building's perimeter bays, whereas for eight-storey buildings the optimum proportion is 20–30%. Results also show that regular bracing in internal bays is more appropriate for resistance against progressive collapse.

Comparison of Seismic Behavior of Eccentrically Braced Frames with Vertical and Knee Links in Retrofitting the Reinforced-Concrete Buildings with Intermediate Moment-Resisting Frame Systems

Given the fact that some reinforced-concrete (RC) buildings have not met resistance against seismic loads, it is mandatory to adopt a suitable and economically cost-efficient method to retrofit them. One appropriate method is application of steel Eccentrically Braced Frames (EBFs). This article is aimed at evaluation of seismic performance of retrofitted reinforce-concrete buildings by EBFs with single and knee links. For evaluation of modeled buildings, different editions of the Iranian Seismic Code (Standard No. 2800) were reviewed. To this end, three Moment-Resisting Frames (MRF) of four-, eight-, and twelve-story buildings with medium ductility were modeled where they were designed for seismic loads based on Standard 2800, 2nd edition. In order to evaluate buildings under modified seismic loads, models were seismically reloaded based on the Standard 2800, 3rd edition. Reanalysis showed that the stress ratios exceeded 1 in most columns. Therefore, buildings were retrofitted using EBFs with knee and single vertical links and their seismic performance were evaluated using nonlinear static analysis. Results indicate that knee bracing systems are more efficient than bracing systems with vertical link in increasing rigidity and controlling displacements, while they significantly reduce ductility. For example, application of knee bracing system in the twelve-story building can cause an 11% reduction in displacement in comparison to bracing systems with vertical links.

Soil and Subsurface Sediment Microzonation Using with Seismic Refraction Tomography for Site Assessment (Case Study: IKIA Airport, Iran)

The site effects relating to the amplification of ground motion under earthquake loading are strongly influenced by both the subsurface soil condition and the geologic structure. In this study, for site characterization at the Imam Khomeini International Airport (IKIA) area in south of Tehran, in-situ seismic refraction tomography were carried out as a part of site investigations project, in addition geologic setting, borehole drilling, ground waters information and measurements. Based on seismic refraction studies, three layers are separable which with increasing in depth the S and P wave velocity is increased and this indicates increasing in compaction of soil and geologic materials. In the second and third separated layers, the zones with low and high seismic shear wave velocity is approximately equal, and northeast and southwest of the airport site has the low velocities, in addition to containing loose soils, highly weathered stones, and low depth to groundwater. In terms of Poisson’s ratio, the most important and key installations of airport site are located in suitable positions. According to Iranian Seismic Code, most of the lands around the airport are in class 2 and 3. It seems that a fault or a discontinuity is passed from northwest to the southeast of the study area. This site, according to geological, subsurface geophysical, and geotechnical boreholes studies, is high risk-earthquake prone.

Study on the Seismic Response Parameters of Steel Medium-Height Buildings with Framed-Tube Skeleton under Near-Fault Records

In this research, the performance abilities of tube type lateral load resistant framed systems are studied in order to assess the dynamic response properties of mid-rise steel structures under effects of far and near-field ground motions. For this purpose, three 10 story structural models with separated framed tube based skeletons were selected and designed. The structural models have been designed according to the Iranian seismic code 2800 (4th edition). The main criterion which was considered to select powerful ground motions for performing nonlinear time history analyses is the existence of energized coherent velocity pulses as well as high amplitude acceleration spikes in the time history of each earthquake record. Assessment of the analytical results should emphasize the importance of both lateral displacement and drift parameters which must be taken into account during the structural design phase. Furthermore, it was concluded that the maximum drift demand is more than 0.035 and the upper level of rotation of rigid connecting zones was obtained more than 5 percentages of a radian. As a general note, it was concluded that the seismic response parameters of mid-rise steel framed tube structures are intensively influenced by those strong earthquake records which contain powerful forward directivity effects and are able to emerge long period pulses in their time histories.

Seismic Risk Analysis of Steel-MRFs by Means of Fragility Curves in High Seismic Zones

In this study a steel moment resisting frame in a high seismic prone area in Iran has been remodeled based on different versions of Iranian Seismic Code (Standard No. 2800 1988, 1999, 2005) to estimate the seismic fragility and mean annual loss for the modeled buildings. At the first stage a probabilistic seismic hazard analysis (PSHA) has been performed to extract hazard curve in CRISIS platform. (Akkar and Bommer 2007) earthquake prediction model for the Middle East has been used in the hazard analysis. In the next stage, hundreds of nonlinear incremental dynamic analyses by means of lumped-parameter based structural models have been simulated and performed to extract the fragility curves by using OpenSees. Consequently by combining the hazard curves, judgmental event trees and seismic fragility curves for the extensive damage mode (EDM), the required annual loss curves are extracted.

Characteristics of near-source ground motions from the 2012 Varzaghan–Ahar double earthquakes, Northwest of Iran

Wavelet-based analyses were used in this study to interpret the near-source ground-motion characteristics of records obtained during the 2012 Varzaghan–Ahar double earthquakes in the East-Azerbaijan province of Iran. For this purpose, the large pulse of ground motions was extracted by applying continuous wavelet transforms, and then, the residual motions were achieved by subtraction of extracted pulse from the original motions. Analysis of ground motions illustrated that the records obtained at Varzaghan station could be classified as pulse-like motions with pulse index around 0.9. As well, the acceleration response spectra for horizontal directions were compared with the design response spectrum of Iranian seismic design code (Standard No. 2800). The comparisons showed a relatively large amplification in spectral values due to near-source pulse effects.

Seismic Performance of RC Frames Irregular in Elevation Designed Based on Iranian Seismic Code

Article history: Received: 6 November 2012 Accepted: 7 May 2013 Setback in elevation of a structure is a special irregularity with considerable effect on its seismic performance. This paper addresses multistory Reinforced Concrete (RC) frame buildings, regular and irregular in elevation. Several multistory Reinforced Concrete Moment Resisting Frames (RCMRFs) with different types of setbacks, as well as the regular frames in elevation, are designed according to the provisions of the Iranian national building code and Iranian seismic code for the high ductility class. Inelastic dynamic time-history analysis is performed on all frames subjected to ten input motions. The assessment of the seismic performance is done based on both global and local criteria. Results show that when setback occurs in elevation, the requirements of the life safety level are not satisfied. It is also shown that the elements near the setback experience the maximum damage. Therefore it is necessary to strengthen these elements by appropriate method to satisfy the life safety level of the frames.

Investigating the effect of bond slip on the seismic response of RC structures

It is reasonable to assume that reinforced concrete (RC) structures enter the nonlinear range of response during a severe ground motion. Numerical analysis to predict the behaviour therefore must allow for the presence of nonlinear deformations if an accurate estimate of seismic response is aimed. Among the factors contributing to inelastic deformations, the influence of the degradation of the bond slip phenomenon is important. Any rebar slip generates an additional rotation at the end regions of structural members which are not accounted for in a conventional analysis. Although these deformations could affect the seismic response of RC structures considerably, they are often neglected due to the unavailability of suitable models. In this paper, the seismic response of two types of RC structures, designed according to the Iranian concrete code (ABA) and the Iranian seismic code (2800), are evaluated using nonlinear dynamic and static analyses. The investigation is performed using nonlinear dynamic and static pushover analysis considering the deformations due to anchorage slip. The nonlinear analysis results confirm that bond slip significantly influences the seismic behavior of RC structure leading to an increase of lateral deformations by up to 30% depending on the height of building. The outcomes also identify important parameters affecting the extent of this influence.

Seismic Retrofitting of Steel Frames with Buckling Restrained Braces

Buckling-restrained braced frames (BRBFs) are used as primary lateral load resisting systems for buildings in high seismic areas. The main characteristics of buckling-restrained braces (BRBs) are enhanced energy dissipation potential, excellent ductility and nearly symmetrical hysteretic response in tension and compression. This paper presents an analytical study aimed to assess the feasibility of using BRBs as a retrofit scheme for existing steel frames. For that purpose, the seismic response of four two-dimensional frame models representative of typical steel frames was analyzed prior to and after including BRBs as a retrofit strategy. Comprehensive nonlinear static and time-history analysis were carried out for analyzing the frames. A set of seven code-compliant natural earthquake records was selected and employed to perform nonlinear time-history analysis. Frames were analyzed and designed based on 2800 Iranian Seismic Code. The evaluation was based on comparing seismic displacement demands such as target roof displacements, maximum displacements and inter-storey drifts. The results showed that the effectiveness of braced frames can vary significantly with ground motions, frame rises and retrofitting systems. It perceived that the seismic response of frames coincides well with the given design objectives.

Response Modification Factors of Steel Moment Frames Equipped with Completely Closed Ribbed Bracing System (CC-RBS)

In this paper completely closed RBS (CC-RBS) is proposed as a function of a new ribbed bracing system (RBS). RBS is an innovative system which has capacity of performing variable stiffness that can be used for controlling structural deformations and frequency shifting to compensate seismic energy. The overstrength, ductility, and response modification factors of steel moment frames equipped with CC-RBS was evaluated by performing pushover analysis of model structures with various stories and compared with those which Iranian seismic code of practice, Standard No. 2800 is supposed for steel moment frame equipped with ordinary brace. The results were compared with those from nonlinear incremental dynamic analyses (IDA). According to the analysis results, the response modification factors of model structures computed from static pushover analysis (SPO) were generally smaller than the values given by incremental dynamic analysis, but both SPA and IDA lead to response modification factors larger than the value given in Iranian code, standard 2800, for steel moment frame equipped with ordinary brace.

Seismic damage assessment of reinforced concrete frames with shear wall according to load pattern

Studies have shown that most structures subjected to strong ground motions fall into an inelastic state. Therefore, it is necessary to study the inelastic behaviour of structures undergoing such earthquakes. In this study, three reinforced concrete (RC) frames with shear wall are considered. The preliminary designs of these frames are based on equivalent static forces in accordance with six patterns of loading distribution (namely Uniform Building Code (UBC), National Earthquake Hazards Reduction Program (NEHRP), Iranian seismic code, modified rectangular, first mode and the first three modes). The aim of this study is to investigate the distribution of damage in accordance with these six load patterns. IDARC 2D software has been used to calculate maximum drift, hysteretic energy and structural damage index when subjected to severe earthquakes. The results show that the current seismic design based on strength principles, even considering uniform strength distribution, does not lead to a uniform distribution of hysteretic energy, drift and damage index in storey height. However, modified rectangular and the first three mode patterns show more uniform distribution of damage than the others.

Assessment of Using of Knee Bracing in Retrofitting of Existing Steel Structures with Special Moment Resistant Frame System (Technical Note)

In this paper, non-linear behaviors of structures with special moment resistant frame were studied and possibility of using knee bracing for retrofitting of those structures was examined. For this aim, the structures that have been designed according to the second edition of Iranian seismic code (2800), were selected. Those models have 3, 5 and 7 stories. To evaluating and retrofitting of models, non-linear static analysis and capacity spectra (ATC-40) method were applied and providing of the expected performance levels in FEMA356 was studied. Results showed that the response of structures that designed according to the previous edition of code (second edition) is not compatible with the criteria of third edition. The knee bracing system was used to seismic retrofit of those structures. It would be concluded that the using of knee bracing increases lateral stiffness and improves their energy dissipating capacity. Furthermore, the results showed that the shorter knees have the highest stiffness and ductility in the structures.

Seismic Design of Very High Importance Buildings Regulations Using Standard 2800: Critical Review (Technical Note)

The performance of buildings designated as very high importance buildings (such as hospital buildings) according to Iranian Seismic Code (Standard 2800) is that; these buildings maintain their operational level in major seismic ground motions (design level earthquake) without major structural damage. This paper discuses the design requirements to reach the purposes in this code and shows that the code regulations can not provide the enough strength for the buildings to operate acceptable during sever earthquakes. To evaluate the code requirements, some R/C moment resistance frames with difference in stories numbers (designated according to standard 2800) was selected and their strength was evaluated based on Instruction for Seismic Rehabilitation of Existing Buildings (No. 360), by pushover analysis. The results indicate that the buildings designated as very high importance according to standard 2800, do not remain operationally after sever earthquake.

Ground motion studies for microzonation in Iran

An important step in effectively reducing seismic risk and the vulnerability of a city located in an earthquake prone area is to conduct a ground motion microzonation study for the desired return period. The International Institute of Earthquake Engineering and Seismology (IIEES) initiated a number of seismic microzonation projects for Iran. This paper presents the steps followed by IIEES in ground motion microzonation. IIEES performs both probabilistic and deterministic seismic hazard analysis. IIEES uses his own fault map for seismotectonic studies and develops modulus and damping curves for the soils in the study area. The experience of ground motion microzonation shows that in almost all cases, the estimated 475-year peak ground acceleration (PGA) values are higher than the PGA proposed by the Iranian seismic code. Although ground motion microzonation in Iran has some shortcomings, IIEES is making new improvement. This includes development in deterministic seismic hazard analysis, two-dimensional and three-dimensional modelling of basin and topographical effects, using microtremor measurements to find shear-wave velocity profiles in high-density urban areas and providing maps for spectral acceleration in the study area.

Evaluation of Steel Moment-Resisting Frames Performance in Tall Buildings in Near Fault Areas

Evaluation of strong ground motions and effect of them on performance of structures are the important subjects that nowadays are considered in structural and earthquake engineering. It is necessary to evaluate of the near-field earthquake characteristics and effect of them on response of buildings. In this paper, the performance of steel Moment-Resisting Frames (MRFs) in tall buildings in near fault areas has been studied. Six steel MRFs 15- and 20-story with three different ductility; ordinary, intermediate and special moment-resisting frames have been designed according to the Iranian seismic code 2800 (3th Edition) and Iranian national building code-steel structures (Part 10). To assign the performance parameters of frames and to present the effects of near-field earthquakes on steel MRFs, pushover and non-linear dynamic time history analyses have been performed and conducted for chosen structural models. The ensemble of used earthquakes contains both far-field and near-field earthquake records. It is worth mentioning that the selected earthquake records contain some strong ground motions from the 1978 Tabas, 1979 Imperial Valley, 1992 Landers and 1994 Northridge Earthquakes. According to the analytical results, concerning near-field earthquake records which emerge velocity pulses in their time history, the maximum values of story displacement, story drift, roof displacement and base shear are relatively higher than those which are obtained from near-field records that do not display velocity pulse and also those ones obtained from far-field strong ground motions. Based on using near-field earthquake records specially those ones that display velocity pulses, the calculated members stress and deformation resultants are higher than those ones that are recommended by the Iranian Seismic Code. Also, by more ductility in steel special moment-resisting frames the structural damping in non-linear domains can be considerably higher than the value of this parameter in other structural systems. This subject should be confirmed by comparing the calculated values of seismic base shear in analyzed models. On the other hand, in special moment-resisting frames, under the effect of near-field earthquake records which emerge velocity pulses, the maximum values of storey displacement and storey drift is much higher than allowable values which are recommended by the Iranian seismic code. Hence, it is advised to increase the capacity of member sections and connections, in order to achieve the code-defined limited values of displacement and drift demands in steel special moment-resisting frames.

Incremental dynamic analysis of steel braced frames designed based on the first, second and third editions of the Iranian seismic code (standard no. 2800)

AbstractIncremental dynamic analysis (IDA) is an emerging method in structural analysis which allows evaluation of seismic capacity and demand of structures through a series of nonlinear dynamic analyses using multiple scaled ground motion records.Seismic behaviour of concentrically braced frames designed based on the first, second and third revisions of the Iranian seismic code, standard no. 2800, has been evaluated through IDA in the present paper. Besides, a brief comparison is made between seismic behaviour of these frames, frames with different heights and different bracing types. Seismic capacity and limit states of such structures have been reviewed through the paper.The IDA results imply that frames designed with the first edition are seriously vulnerable and fail before reaching the acceleration levels predicted in the code. On the other hand, frames designed with the second and third editions, although behaving better, need partial reinforcement in some cases.Other results of this study show that chevron braced frames behave seismically better than X‐braced ones. Copyright © 2009 John Wiley & Sons, Ltd.

A simplified method to determine seismic responses of reinforced concrete moment resisting building frames under influence of soil–structure interaction

As the Iranian seismic code does not address the soil–structure interaction (SSI) explicitly, the effects of SSI on RC-MRFs are studied using the direct method. Four types of structures on three types of soils, with and without the soil interaction, are modeled and subjected to different earthquake records. The results led to a criterion indicating that considering SSI in seismic design, for buildings higher than three and seven stories on soil with V s <175 m/s and 175< V s <375 m/s, respectively, is essential. A simplified procedure has been presented, on the basis that lateral displacement increments could be applied to the fixed-base models using simple factors.

Assessment of Reinforced Concrete Buildings with Shear Wall Based on Iranian Seismic Code (Third Edition)

Assessment of Reinforced Concrete Buildings with Shear Wall Based on Iranian Seismic Code (Third Edition)

On the December 26, 2003, southeastern Iran earthquake in Bam region

In this paper, the December 26, 2003, earthquake in the historical city of Bam is studied. The seismological aspects are considered and the earthquake source is introduced. The strong ground motion is studied and its spectra are compared to the design spectra of UBC-97 and the Iranian seismic code, as well as previous earthquakes in Iran. Finally, the structural behavior of buildings and lifeline systems in this region is studied and damages are outlined.

INFLUENCE OF IRAN'S EARTHQUAKE GROUND MOTION PARAMETERS ON DESIGN SPECTRA USING DETERMINISTIC AND PROBABILISTIC APPROACHES

This paper summarises the results of comprehensive statistical study of response and design spectra in which a total of 106 Iran's horizontal acceleration components recorded during various earthquakes was considered. Special emphasis is given to the influence of effective duration of strong motion, soil condition and damping ratio of structure on elastic response and design spectra. The study included both deterministic and probabilistic approaches- The records with peak ground accelerations greater than 0.05 g from 23 stations in 47 seismic events are divided into four soil categories in accordance with Iranian seismic code. For each soil group, mean and mean plus one standard deviation spectral ordinates were computed for: effective duration of strong motion and damping ratio of structures and also a design spectra for different soil conditions has been constructed by deterministic and probabilistic approaches and results have been compared with code provisions (i.e. NEHRP, UBC, EC8 and seismic code of Iran). The influence of effective duration of strong motion has been studied by definition based on energy of accelerogram. The paper also presents an evaluation of the damping ratios on the response spectra. Results and conclusions from this study can be directly used for the evaluation of the present Iranian seismic code in order to construct new design spectra and also for the evaluation of seismic design provisions.