Using concentric and zipper steel braces by comparison of effect on improved seismic performance level of concrete moment frame structures with moderate ductility
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Recent earthquakes have revealed that, in many cases, the design codes may not guarantee the integrity of all structures for the life safety performance level. Therefore, it seems necessary to study the performance of designed buildings in order to complete, develop and amend the codes. In this paper, the performance level was investigated using the nonlinear static analysis in concrete buildings with moderate ductility and designed according to the latest regulations. Then, as the life safety performance level was not achieved and due to the ease of implementation of steel braces, the concentric chevron brace was used to improve the performance level of these buildings in large bays, and the zipper steel brace was used. The results show that the zipper brace used for strengthening the reinforced concrete structures will increase the performance as well as the stiffness, without the need to strengthen the beams and columns.
KeywordsNonlinear analysis Plastic hinges Performance level Retrofitting Concrete structures Zipper brace
The purpose of the national building codes and regulations considering the seismicity of Iran is to provide the life safety level of the structure in the mild to moderate earthquakes and to prevent the collapse of structure. Recent earthquake related research and experiences show that the national codes and regulations may not guarantee the integrity of all structures for the life safety performance level. However, the purpose of the national building codes and regulations is to provide the life safety level of the structure in earthquakes and to prevent the collapse. The damages in a number of reinforced concrete structures in recent earthquakes such as Sar Pole Zahab earthquake are caused by the unmet goals of the regulations. A number of performance objectives other than life safety, such as controlled damage, cannot be achieved by the existing design regulations for many buildings. For this reason, many researchers believe that the seismic design methods should be revised and, in other words, the performance based design methods should be replaced so as to specify the designed structures with the performance of structural and nonstructural components according to the location and seismicity of the site. In this way, designers can design the structures for the expected damage corresponding to the seismicity of the site [1, 2].
In the structures constructed according to the regulations, the life safety performance is assumed and it is expected to achieve this performance level, while in the performance based design, the damage to the structure is evaluated at certain levels of earthquake hazard in comparison with the established criteria [3, 4]. A significant point to note in the design based on national codes and regulations is that there is no relationship between the strength and the safety based on the expected performance level and the cost of design and implementation of the structure [5, 6, 7].
The study of braced frames has long received the interest of researchers, but the study of reinforced concrete braced frames is relatively recent and does not have much research background. The history of strengthening the concrete moment frames with bracing dates back to the study of Sugano and Fujimura , which performed the experiments on a number of reinforced concrete frames with K and X braces and the similar frames strengthened by masonry and concrete infills. The purpose of these studies was to determine the effect of each system on the increased in-plane strength and ductility of frames . The researchers have found that steel braces have many non-structural advantages over other designs and can be installed with minimal disruption and also do not occupy much space. From the structural point of view, the steel bracing is also very suitable for the lateral strengthening or hardening of multi-story reinforced concrete structures. The bracing system should be designed for the elastic response and be detailed for the ductile behavior. To restrict the non-elastic buckling, the slenderness ratio of braces should be kept low. The use of braces without the buckling (very low slenderness) or with elastic buckling (Very high slenderness) should be taken into account. In the frames with weak columns and strong beams, the combination of steel bracing with beam modification can greatly improve the frame behavior. Wyllie et al.  presented a report on strengthening the concrete building with the steel bracing in the University of California, Berkeley. The method used in this project is the application of braced steel frames inside the reinforced concrete (RC) frames. The report states that the used steel bracing method was the most economical method for strengthening .
Tagawa et al.  in a paper titled ‘‘Experimental Study of New Seismic Strengthening Method for Existing RC Structure’’ described the use of steel frames braced with the K bracing inside the RC frames. The test was carried out on one-story single-bay RC frames on 1:2 scale. Finally, it was found that the ultimate shear strength of the strengthened RC frame can be easily obtained by summing the shear strength of steel elements and existing RC frame based on the conventional computational methods .
Sugano  in a paper titled ‘‘Research and Design for Seismic Retrofit of Existing Buildings in Japan’’ described the methods used to strengthen the RC structures severely damaged by recent destructive earthquakes. In this paper, reviewing the information obtained from existing research, the techniques and methods for the strengthening and the behavior of strengthened structures are investigated. Also, the practical design described by some design examples for the strengthening of existing buildings is proposed before the earthquake. In this paper, different strengthening methods are compared which show the proper behavior of braced RC frames. Different designs of RC frame bracing are also compared. From this comparison, the X bracing increases the strength of structure more than other designs. It should be noted that the bracing method in this study is the use of braced steel frames in concrete frames .
Nateghi  presented a report on the real strengthening in a paper titled “Seismic retrofit of 8-story RC building”. In that research, after examining several strengthening methods, the steel bracing of concrete frame was recognized and used as the most suitable method. The analysis of braced building shows good results from reducing the horizontal displacement. The details of bracing connection to the frame used in this paper were specific to Iran and not found in other papers . Some studies have been conducted on the performance level of reinforced concrete structures. For example, Tasnimi and Bashiri investigated the performance levels and strengthening of reinforced concrete moment frames with moderate ductility by different methods. The results showed that the frames with lower bays and higher seismic risk had poor performance and did not meet the life safety performance level (Tasnimi, Abasali; Bashiri, Ayoub, 2009) .
Godínez-Domínguez and Tena-Colunga  used a nonlinear static method to evaluate the behavior of the dual ductile concrete moment frame and special concentric bracing system. The researchers designed the 4 to 24 story frames by the capacity design method based on the Mexican design regulations. The moment frames were designed for different shares of the base shear (25, 50,75) and the bracing system was designed for the rest of the earthquake force. According to the research, the design method was appropriate and the performance of frames was suitable for the case where the moment frame and braces were individually designed for 50% of the earthquake force. In this context, Godínez-Domínguez and Tena-Colunga  published their findings on the seismic performance of the frames. The result of this study was the high ductility and overstrength capacity and good seismic performance .
Ozcelik et al.  had studied about Comparison of chevron and suspended-zipper braced steel frames and the results appear to indicate that the lateral load capacity and drift demands for both low-rise chevron and suspended-zipper braced frames are very similar; however, the mid-rise chevron braced frame has a better performance compared to the mid-rise suspended-zipper braced frame. Also, in the latest studies, Nezamisavojbolaghi and Karimi investigated the effect of the existence and modeling of infills on the performance level of reinforced concrete structures with moderate moment frame and asymmetric bays. In this research, the results showed that the performance level is affected by not modeling the infill .
Nassani et al.  in a paper titled “Comparative Response Assessment of Steel Frames With Different Bracing Systems Under Seismic Effect’’ described the use of different bracing system in steel frames. The results showed a good improvement in the seismic resistance of frames with the incorporation of bracing. The results revealed that the bracing elements were very effective in diminishing drifts since the reduction of inter-story drifts with respect to unbraced frames were on the average 58%. Also steel braces considerably reduced the global damage index .
The recent research Vahedi et al.  in a titled “seismic Evaluation of a Non ductile Soft-First-Story RC Building Retrofitted with Steel-Braced Frames” described the seismic evaluation in RC building retrofitted with steel-braced frames. The results showed that, in the strength-type retrofit, because of lack of required ductility, premature shear and/or axial failures of RC columns are more likely to occur than those failures in nonretrofitted ones. Also, it was found that the variation of the axial and shear forces in the boundary RC columns and the slenderness ratios of the steel braces are the critical factors influencing the seismic responses of the buildings retrofitted using the steel-braced frames .
In this study, the performance level of RC structures with the moderate-ductility moment frame system designed based on the Standard No. 2800, fourth edition  and the Chapter IX of National Building Regulations  is evaluated as the existing structures using the nonlinear static or pushover analysis. Then, due to the failure to achieve the life safety performance level obtained from the pushover analysis to improve the performance level of selected structures using the zipper steel bracing in the 7 m bays which have less been considered in the previous research, the stiffness, ductility, inter story drift and the performance levels are compared with those of chevron braces.
2 Specifications of study models
In this research, the seismic behavior of concrete buildings with moderate ductility strengthened with zipper steel braces based on the performance level is studied and determined, which were loaded according to the provisions of Chapter VI (loads applied to building), designed according to the Chapter XI concrete regulations of Iran and strengthened by “Guideline for Seismic Rehabilitation of Existing Buildings, Journal No. 360, Revision 2013, Iran”. The specifications of concrete are considered as C20 grade with S400 rebar, steel yield stress of 2400 kg/cm2 and failure stress of 4000 kg/cm2. For this purpose, the RC buildings with the moderate ductility, 6 story with 4 bays in the both x and y directions in a region of high seismicity were studied and strengthened. In the both directions, two types of bracing layout were used. The considered buildings with the residential occupancy are located in Mahabad with type III soil and allowable soil stress of 1.2 kg f/cm2. The gravity loading is based on the Iranian Regulations (Chapter VI), and the Iranian Standard No. 2800, fourth edition is used for the lateral loading. The typical elevation of stories is 3.20 m. The 25 cm block joist ceiling is used. The building has exterior (peripheral) walls with the thickness of 20 cm and interior partitions with the thickness of 10 cm. Also, two southern and eastern sides of structure have veneered walls, and the northern and western sides have the non-veneered walls. The chevron and zipper braces are also designed according to the seismic provisions of Chapter X of National Regulations.
The structural analysis and design are done by the limit state method using the ETABS 2015 software .
Dimensions of 6-story model beam and column
50 × 40
55 × 55
50 × 40
55 × 55
45 × 40
50 × 50
45 × 40
50 × 50
40 × 40
45 × 45
40 × 40
45 × 45
Dimensions of 10-story model beam and column
60 × 45
60 × 60
60 × 45
60 × 60
55 × 45
55 × 55
55 × 45
55 × 55
50 × 45
50 × 50
50 × 45
50 × 50
40 × 40
45 × 45
40 × 40
45 × 45
40 × 40
40 × 40
40 × 40
40 × 40
3 Non-linear static analysis and review and improvement of performance level of models
Regarding the results of nonlinear static analysis for improving the performance level of models in the 7 m bays in two axes (1, D), the chevron and zipper steel braces were used for the strengthening. The chevron brace sections were 2UPN80 in the 6-story models and 2UPN100 in the ten story models, and the zipper brace sections were 2UPN80 in the sex story models and IPE100 in the ten story models. Also, the 2UPN100 brace with the IPE100 zipper section was used in the ten story models.
Target displacement parameters and static nonlinear analysis of 6-story structure
With zipper braces
With concentric brace
4 Analysis of results
After adding the zipper braces and chevron braces and performing the static nonlinear analysis, the results of the analysis can be explored. Due to the distribution and formation of plastic hinges, the expected performance level, namely life safety, can be studied in both cases.
Decrease in inter-story drift with zipper brace in sex story model
The bold column shows the reduced values of inter-story drift. For example, in the 19 m level in the Y direction, the inter-story drift was decreased from 0.077 to 0.023.
The steel zipper bracing in concrete structures increases the stiffness and reduces the period of structure. This increase in stiffness and the reduction in period increases the base shear in the structure, but due to the proper structural behavior using a dual system and the good ductility of the zipper steel bracing system, the overall performance level of the models is improved.
By comparing the performance of beams and columns of the above structures, it is observed that in the structure strengthened by the zipper steel bracing, fewer plastic hinges are formed in beams and columns and the structure has a better condition.
The inter-story drift is decreased in all strengthened models.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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