Browsing by Author "Baran, Eray"

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Citation Count: 45
Behavior of cold-formed steel wall panels under monotonic horizontal loading
(Elsevier Sci Ltd, 2012) Baran, Eray; Alica, Cagatay; Department of Civil Engineering
This paper summarizes the findings of an experimental study on cold-formed steel (CFS) wall panels with oriented strand board (OSB) sheathing. Static lateral load tests were conducted on a total of thirteen full-scale wall panel specimens to investigate the performance of CFS walls utilizing primarily the construction details used in Turkey. Experimental results revealed that the geometry of hold-down attachment used at the base of CFS wall panels to transfer tensile forces has a major effect on the overall behavior of the panels. Extensive damage was observed to occur in the hold-down attachment and in part of the bottom track adjacent to the hold-down, which prevented the wall panels to exhibit the stiffness associated with the main load resisting mechanisms. Tilting of the screws connecting the OSB sheathing to the CFS boundary framing members, as well as the separation of sheathing from the framing members were other commonly observed deformation modes. Existence of diagonal struts inside CFS frame was determined to cause a slight increase in load capacity and initial stiffness of the panels. It is also shown that the measured load capacity of wall panels can be predicted with an acceptable accuracy using design equations available in the literature. (C) 2012 Elsevier Ltd. All rights reserved.
Citation Count: 25
Behavior of steel-concrete partially composite beams with channel type shear connectors
(Elsevier Sci Ltd, 2014) Baran, Eray; Topkaya, Cem; Department of Civil Engineering
This paper summarizes the findings of an experimental study investigating the flexural behavior of partially composite beams incorporating channel type shear connectors. Results from monotonic load testing of four full-scale steel-concrete composite beams and a steel beam are presented. The main effort focused on identifying the variation of strength and stiffness properties of beams with various degrees of partial composite action. Behavior of channel shear connectors in the composite beam specimens is related to those previously obtained from push-out tests of similar connectors. Finally, recommendations of the related AISC Specification on the strength and stiffness of composite beams are used for the assessment of the influence of the degree of partial composite action on flexural behavior. The experimental results revealed that even for beams with relatively low degree of partial composite action, major improvement on moment capacity and stiffness was obtained as compared to the steel specimen. The measured moment capacity of both the partially composite and fully composite beams agreed acceptably with the calculated capacities. The effective moment of inertia and the lower bound moment of inertia as specified by the AISC Specification were observed to overestimate the measured flexural stiffness of beams for all degrees of partial composite action investigated. (C) 2014 Elsevier Ltd. All rights reserved.
Betonarme eğilme elemanlarında öngerme halatlarının çekme donatısı olarak kullanılması
(2011) Arsava, Tuğba; Baran, Eray; Department of Civil Engineering
Bu çalışmada, öngerme halatlarının betonarme yapı elemanları içerisinde eğilme donatısı olarak (öngerilme uygulanmadan) kullanılabilirliği incelenmiştir. Öngerilmeli/önüretimli beton yapı elemanlarının üretimi sırasında artık öngerme halatları oluşmaktadır. Türkiye'de öngerilmeli/önüretimli beton yapı elemanı üretimi konusunda faaliyet gösteren firmalar arasında yapılan bir anketin sonuçlarına göre, oluşan bu artık halat parçaları yalnızca yapı elemanlarında kaldırma kancası olarak kullanılmaktadır. Artık halat parçalarının betonarme yapı elemanlarında eğilme donatısı olarak kullanılmaları durumunda, oldukça yüksek çekme dayanımına sahip ve yurdumuza ithal edilmek sureti ile tedarik edilen bu öngerme halatlarının verimli bir şekilde değerlendirilmesi mümkün olacaktır. Öngerme halatları, normal donatı çeliğine göre farklı gerilme-birim uzama davranışı gösterdiğinden, donatı çeliği içeren betonarme yapı elemanları için günümüz yapı yönetmeliklerinde belirtilen tasarım yaklaşımlarının, yüksek mukavemetli halatlarla donatılandırılmış elemanlar için kullanılabilirliği günümüzde halen araştırılmayı bekleyen bir konu olarak bulunmaktadır. Literatürde varolan bu boşluğu doldurmak amacıyla, analitik çalışmalarla bütünleştirilmiş laboratuvar deneylerinden oluşan bir çalışma yürütülmüştür. Yedi tanesi konvansiyonel donatı çubukları ile altı tanesi de yüksek mukavemetli çelik halatlarla donatılandırılmış toplam 13 adet betonarme kiriş numunesi yükleme deneylerine tabi tutulmuştur. Deneysel çalışmaların ardından, farklı kiriş kesitleri ve malzeme özellikleri kullanılarak rakamsal bir parametrik çalışma yapılmıştır. Deneysel ve rakamsal çalışmalar sonucunda, betonarme eğilme elemanlarında çekme donatısı olarak konvasiyonel donatı çubukları yerine yüksek mukavemetli çelik halatların kullanılması durumunda, elemanın yapısal davranışında ortaya çıkan farklılıklar belirlenmiştir. Betonarme kiriş elemanlarının moment kapasitesi ve minimum sahip olunması gereken süneklik seviyesi ile ilgili Amerikan Beton Enstitüsü (ACI-318) ve TS500 yönetmeliklerinde belirtilen prosedürlerin yüksek mukavemetli halatlar kullanılarak donatılandırılmış betonarme kiriş elemanları için uygulanabilirliği incelenmiştir
Citation Count: 18
Damage in Reinforced-Concrete Buildings during the 2011 Van, Turkey, Earthquakes
(Asce-amer Soc Civil Engineers, 2014) Baran, Eray; Mertol, Halit Cenan; Gunes, Burcu; Civil Engineering; Department of Civil Engineering
Two major earthquakes with magnitudes Mw=7.2 (ML=6.7) and ML=5.6 occurred in eastern Turkey on October 23 and November 19, 2011. The maximum measured peak ground accelerations for the two ground motions were 0.18g and 0.25g, respectively. The earthquakes resulted in various levels of damage to RC moment-resisting frame buildings ranging from minor cracking in brick partition walls to total collapse. This paper summarizes the field observations of the Atilim University Reconnaissance Team carried out in the region a few days after the two main shocks with an emphasis on the performance of RC buildings. A summary of the evolution of the Turkish seismic design code during the last 35 years is given, followed by an explanation of the behavior of RC buildings during the October 23 and November 9 earthquakes. The deformation types that were commonly observed in the heavily damaged or collapsed RC buildings include plastic hinging in columns attributable to stiffer beams, localization of damage in ground-story columns attributable to changes in the stiffness of the lateral load-resisting system caused by brick partition walls, and shear failure of columns caused by discontinuities in the partition walls adjacent to the columns. Poor concrete quality, inadequate development and lap splice length for reinforcement, and inadequate confinement in columns also contributed to the poor seismic behavior.
Citation Count: 27
Experimental and numerical analysis of a bolted connection in steel transmission towers
(Elsevier Sci Ltd, 2016) Baran, Eray; Akis, Toga; Sen, Gokmen; Draisawi, Ammar; Department of Civil Engineering
This paper presents an integrated numerical and experimental study on a bolted splice connection used in main legs of steel lattice transmission towers. At specific locations, where the number of angle sections in built-up cross section of main leg members changes, the complex geometry around the connection region results in eccentricities in the load path and indirect load transfer. Such complex configurations and uncertainties in the load path have led to overdesigned connections with increased number of bolts and redundant connection reinforcing members. The current study was conducted in an attempt to gain a better understanding of the load-flow mechanism at this specific location where the cross section of main leg members changes. The experimental part included tensile load testing of six specimens with different connection details. The main parameters used in the testing program were the number of bolts used in the connection as well as the presence of connection reinforcement angles and tie plate. For all connection configurations studied, the failure occurred due to net section fracture of upper main member angle near leading bolt holes. The calculated load capacity based on the measured material strength closely predicted the measured load capacity of specimens. The experimentally determined response of each connection configuration was better predicted by the FE model that incorporates bolt slip as compared to the model that assumes no slip. The experimental and numerical results also indicate that major differences among the investigated connection details do not cause any appreciable difference in behavior under tensile loading. (C) 2016 Elsevier Ltd. All rights reserved.
Citation Count: 2
Experimental Determination of Resistance Characteristics of Support Details Used in Prestressed Concrete Bridge Girders
(Asce-amer Soc Civil Engineers, 2009) Baran, Eray; Rohne, Ryan; French, Catherine E.; Schultz, Arturo E.; Department of Civil Engineering
Static load tests were performed on support details used at the ends of prestressed concrete pedestrian bridge girders to determine the resistance characteristics of girder supports in the direction perpendicular to the longitudinal axis of the girders. The specimens tested represent support details that have also been widely used in prestressed concrete highway bridges in Minnesota and in other states. Two specimens, one representing the free-end detail and one representing the restrained-end detail were subjected to a combination of vertical and lateral loads. The applied loading was intended to simulate the loading conditions to which the girder ends would be subjected in the event of an over-height vehicle collision with the bridge. The tests revealed two types of lateral load resisting mechanisms depending on the type of support detail. The specimen with the free-end detail resisted the lateral loading through sliding friction between the components of the support assembly. Deformation of this specimen was a combination of shear deformation of the bearing pad and sliding of various support components. The restrained-end detail exhibited larger lateral load capacity than the free-end detail due to the resistance provided by the anchor rods that were intended to prevent the lateral movement of the girder ends. Failure of the specimen with restrained-end detail was due to the concrete breakout and bending of the anchor rods.
Citation Count: 63
An experimental study on channel type shear connectors
(Elsevier Sci Ltd, 2012) Baran, Eray; Topkaya, Cem; Department of Civil Engineering
This paper describes an experimental study on European channel shear connectors. While shear studs are widely used in steel-concrete composite elements, the channel connectors are also gaining popularity due to their certain advantages. The channel connectors do not require special equipment and standard welding procedures are adequate for attachment purposes. In addition, this type of connector offers higher amounts of shear resistance due to its high contact area with surrounding concrete. Although first studies date back to 1950s. little work has been done in the past to investigate their behavior. Majority of the work done to date was on channel connectors used in North America with a very limited parameter range. An experimental study consisting of 15 push-out tests was carried out to investigate the behavior of European type channel connectors with various heights and lengths. The ultimate resistance of the connectors obtained from experiments was compared against those predicted by North American steel design specifications. It was observed that the equations presented in American and Canadian specifications are too conservative. Based on a model that represents the typical failure mechanism in push-out tests, a new equation was developed for the ultimate resistance of channel shear connectors. It is shown that the developed equation is capable of predicting the ultimate resistance of channel connectors with reasonable accuracy. (C) 2012 Elsevier Ltd. All rights reserved.
Citation Count: 80
Flexural behavior of lightly and heavily reinforced steel fiber concrete beams
(Elsevier Sci Ltd, 2015) Mertol, Halit Cenan; Baran, Eray; Bello, Hussain Jibril; Civil Engineering; Department of Civil Engineering
Flexural behavior of lightly and heavily reinforced steel fiber concrete beams was investigated. The test series consisted of 20 singly reinforced beams having 180 x 250 x 3500 mm dimensions. The main parameters in the testing program were the type of concrete and the amount of longitudinal reinforcement. Ten different longitudinal reinforcement ratios (with a minimum of 0.2% and a maximum of 2.5%) covering the range from under-reinforced to over-reinforced beam behavior were used in the testing program. Two specimens were cast for each longitudinal reinforcement ratio, one specimen using conventional concrete (CC) and another specimen using steel fiber reinforced concrete (SFRC). Load-deflection behaviors were obtained and evaluated in terms of ultimate load, ultimate deflection, service stiffness, post-peak stiffness, and flexural toughness. The results indicate that the use of SFRC increases the ultimate load and service stiffness of the beams slightly compared to that of CC specimens. As reinforcement ratio increases, the ultimate deflection of SFRC specimens becomes significantly greater than that of CC specimens. For over-reinforced sections, the post-peak stiffness of the SFRC specimens is observed to be significantly lower than that of CC specimens. The flexural toughness of SFRC specimens is greater than that of CC specimens with the difference being significantly larger for over-reinforced sections. Experimental load-deflection relationships were also compared to the load-deflection curves obtained from sectional analyses based on strain compatibility and best fit stress-strain relationships for SFRC in tension and compression. (C) 2015 Elsevier Ltd. All rights reserved.
Citation Count: 18
Flexural Strength Design Criteria for Concrete Beams Reinforced with High-Strength Steel Strands
(Sage Publications inc, 2012) Baran, Eray; Arsava, Tugba; Department of Civil Engineering
This paper summarizes the result of a study investigating the flexural behavior of concrete beams reinforced with high-strength prestressing strands. Thirteen concrete beams reinforced with either conventional reinforcing bars or high-strength strands were fabricated and load tested in the experimental part of the study. No distinct difference was detected between the experimentally obtained cracking patterns of the two groups of beams. For the same reinforcement amount, beams reinforced with high-strength strands exhibited slightly smaller service stiffness than those reinforced with conventional reinforcing bars. A comparison of the measured and numerically predicted response of beam specimens indicated that the cracking load, peak load, and the deformation capacity of concrete beams can be accurately determined by a sectional analysis procedure for both types of reinforcement. The level of ductility present in concrete beams reinforced with high-strength strands is evaluated using the parameter called "displacement deformability ratio." Using the numerically determined maximum reinforcement limits for concrete beams reinforced with high-strength strands, an expression was proposed to be used at the design stage.
Hafif çelik duvar panellerinin yatay yük altında davranışı
(2013) Alıca, Hüseyin Çağatay; Baran, Eray; Department of Civil Engineering
Bu çalışmada hafif çelik profillerden hazırlanan duvar panellerinin yatay yük etkisi altındaki davranışı incelenmiştir. Bu amaçla hazırlanan duvar paneli numuneleri üzerinde statik yükleme deneyleri yapılmıştır. Duvar panellerinin ölçülmüş olan yük kapasiteleri sayısal olarak tahmin edilen kapasitelerle de karşılaştırılmıştır.Çalışmanın deneysel kısmında 21 adet hafif çelik duvar paneli üzerinde monotonik yükleme deneyleri yapılmıştır. Hafif çelik panellerin kaplaması için OSB, fibercement ve betopan paneller kullanılmıştır. Çalışma kapsamında incelenen diğer parametreler; kaplama panelinin kalınlığı, vida aralığı, hafif çelik profil boyutu, panel içerisinde çaprazların bulunup bulunmayışı ve panellerde çift veya tek taraflı kaplama bulunmasıdır. Yükleme deneylerinden elde edilen bilgilerle, bu parametreler altında duvar panellerinin davranışındaki benzerlik ve farklılıklar incelenmiştir. Duvar paneli numunelerinin davranışının büyük ölçüde, çekme ankraj aparatının davranışı tarafından belirlendiği ortaya çıkmıştır. Çalışma kapsamında test edilen numunelerin çoğunluğunda çekme ankraj aparatında oluşan hasardan dolayı duvar panelleri ulaşmaları gereken yük kapasitesi ve rijitlik seviyelerine ulaşamamışlardır. Çalışmada incelenen parametrelerden, duvar panellerinin davranışı üzerinde en büyük etkiye sahip parametrenin kaplama paneli ile dış hafif çelik profiller arasında kullanılan vidaların aralığı olduğu belirlenmiştir. Vida aralığının 30 cm yerine 15 cm kullanılması durumunda, kaplama çeşidi ve kalınlığına bağlı olarak, duvar panellerinin yük kapasitesinde %37 ile %85 arasında artış olmuştur.
Citation Count: 0
Improvement of mechanical performance in different concrete applications through use of steel fibers
(International Committee of the SCMT conferences, 2016) Baran,E.; Mertol,H.C.; Akis,T.; Civil Engineering; Department of Civil Engineering
The use of steel fibers improves the brittle characteristics of concrete and provides superior mechanical performance compared to the conventional concrete. Remarkable increase in tensile strength and flexural toughness is obtained when steel fibers are used in conventional concrete, mainly due to the crack arrest effect of these fibers. The use of waste materials, such as scrap tires as a source of steel fibers, as well as the resulting increase in service life and the savings in the life cycle cost make the steel fiber reinforced concrete (SFRC) a sustainable construction material. From this perspective, SFRC offers a strong potential for a more sustainable and more economical alternative to conventional concrete. This paper presents examples of how the addition of steel fibers improves the mechanical performance in two different concrete applications: (1) bond behavior of prestressing strands and (2) flexural behavior of reinforced concrete beams. The first part of the study aimed at investigating the variation in bonding mechanism of prestressing strands when used in plain concrete and in SFRC. Pullout tests were conducted on 12.7 mm diameter prestressing strands embedded in SFRC blocks with four different fiber concentrations. This way, the applicability of the available transfer length and development length formulas for prestressing strands embedded in SFRC was investigated. In the second part of the study, flexural behavior of SFRC beams with various levels of flexural reinforcement ratio was studied. The aim was to identify the influence of steel fibers on the mechanical response of lightly and relatively heavily reinforced SFRC beams. The response of SFRC beams and the companion plain concrete beams were evaluated based on the moment capacity, deformation capacity, and service stiffness. © 2016 International Committee of the SCMT conferences. All rights reserved.
Citation Count: 0
Influence of the proportion of FRP to steel reinforcement on the strength and ductility of hybrid reinforced concrete beams
(Taylor & Francis Ltd, 2023) Kartal, Saruhan; Kalkan, Ilker; Mertol, Halit Cenan; Baran, Eray; Civil Engineering; Department of Civil Engineering
The present study pertains to the influence of variation of FRP (Fiber Reinforced Polymer) proportion in tension reinforcement on the flexural behavior of RC beams with FRP and steel reinforcing bars. A total of 25 beams, including FRP-, steel- and hybrid FRP-steel reinforced ones, were tested to failure under four-point bending. Two types of FRP bars, GFRP (Glass Fiber Reinforced Polymer) and BFRP (Basalt Fiber Reinforced Polymer), were used and both over- and under-reinforced beams were tested. The beams in each group were designed to have close flexural capacities to fully reveal the effect of FRP proportion in the tension zone on beam ductility for a fixed bending capacity. A new analytical model was developed for estimating the bending capacities of beams. Different deformation and curvature ductility definitions were adopted and an energy-based definition, revealing the expected tendency in beam ductility, was determined. The test results revealed that the presence of even a single FRP bar in the tension zone results in reductions up to 40% in beam ductility as compared to the beam with full steel reinforcement. Each additional replacement of a steel bar with FRP was found to cause a further decrease up to 20% in beam ductility.
Kompozit önüretimli beton boşluklu döşeme elemanlarının eğilme davranışı
(2014) Haruna, Sadi İbrahım; Baran, Eray; Department of Civil Engineering
Bu çalışmada, üzerine yerinde dökme beton kaplama yerleştirilmiş öngerilmeli önüretimli beton boşluklu döşeme elemanlarının eğilme davranışları deneysel olarak incelenmiştir. Çalışma kapsamında toplam beş adet tam ölçekli numune üzerinde yükleme deneyleri yapılmıştır. Numuneler geniş ve dar olmak üzere iki gruba ayrılmış olup bu gruplardan birer numune beton kaplama ile, birer numune de kaplamasız olarak test edilmiştir. İlave olarak, bir numune de beton kaplama ile boşluklu döşeme elemanı arasındaki yatay kaymaları önlemek amacıyla çelik aparatlar kullanılarak test edilmiştir. Sonradan eklenen kaplama betonunun, boşluklu döşeme elemanlarının çatlama yükünü, maksimum yük kapasitesini ve eğilme rijitliğini arttırdığı gözlenmiştir. Kaplama betonu ile test edilen numunelerde belirli yük seviyesine ulaşıldığında kaplama betonu ile boşluklu döşeme elemanı arasında ani bir kayma oluşmuş ve kompozit davranış kaybedilmiştir. Kompozit davranışın kaybedilmesi sonucunda numunelerin yük taşıma yeteneklerinde ani bir düşüş olmuştur. Kompozit numuneler için hesaplanan arayüz kayma dayanımı değerlerinin ACI ve AASHTO Yönetmeliklerinde verilen değerlerden daha düşük olduğu görülmüştür. Bu çalışma kapsamında ulaşılan sonuçlar, boşluklu döşeme elemanları ile bu elemanların üzerinde herhangi bir pürüzlendirme yapılmadan yerleştirilen kaplama betonundan oluşan döşeme sistemlerinde, tam kompozit davranış oluşması için gerekli arayüz kayma dayanımının oluşmadığı sonucunu ortaya koymaktadır. Anahtar kelimeler: Öngerilmeli beton, boşluklu döşeme elemanları, kompozit davranış, arayüz kayma dayanımı, yerinde dökme beton kaplama.
Önüretimli beton kiriş-kolon bağlantıları için yapısal çelik elemanların kullanıldığı alternatif bağlantıların geliştirilmesi
(2013) Kurtoğlu, Metin; Baran, Eray; Baran, Mehmet; Department of Civil Engineering
Bu çalışma, önüretimli beton yapı endüstrisinde kullanılmak üzere, moment aktarabilen ve çelik profil elemanların kullanıldığı alternatif hibrit kiriş-kolon bağlantı detaylarının geliştirilmesini içermektedir. Bu amaçla, on dört adet hibrit önüretimli beton kiriş-kolon bağlantı numunesi ile iki adet birdöküm (monolitik) beton kiriş-kolon bağlantı numunesi depremi benzeştiren tersinir tekrarlı yükleme altında test edilmiştir. Test sonuçları analiz edilerek deney elemanlarının dayanım, rijitlik, enerji sönümleme kapasitesi ve süneklik özellikleri irdelenmiştir. Kiriş elemanları içerisinde bırakılan bağlantı aparatlarının ankraj edilme yöntemi, bağlantı çubuklarında kullanılan öngerme miktarı ve kiriş elemanı içerisindeki donatının düzenlenme şekli gibi parametrelerin bağlantı bölgesinin davranışını ne şekilde etkilediği belirlenmiştir. Deneysel çalışmalara ek olarak, önerilen bağlantı detaylarının kullanıldığı önüretimli beton yapı sistemlerinin yatay yük etkisi altındaki performansları analitik olarak da incelenmiştir. Bu kapsamda, üç ve beş katlı düzlemsel betonarme çerçevelerin sonlu elemanlar modelleri hazırlanmış ve modeller üzerinde doğrusal olmayan statik itme (pushover) analizleri yapılmıştır. Tamamlanmış olan deneysel ve analitik çalışmalar, önerilen hibrit önüretimli kiriş-kolon bağlantı yönteminin birdöküm kiriş-kolon bağlantılarına özdeş performans gösterme potansiyelini ortaya çıkarmıştır.
Citation Count: 3
Performance of a Prestressed Concrete Pedestrian Bridge System under Equivalent Static Lateral Impact Loads
(Asce-amer Soc Civil Engineers, 2013) Baran, Eray; Schultz, Arturo E.; French, Catherine E.; Department of Civil Engineering
The resistance of prestressed concrete through-girder (PCTG) pedestrian bridges to lateral loads was studied in response to the increasing number of vehicular impacts in the United States. This research was motivated by the lack of reported studies analyzing the behavior of such bridges to lateral impact loads, as well as their potential vulnerability in comparison with bridges that are better able to redistribute and transfer locally applied impact loads through alternate load paths. Pedestrian bridges are of lighter construction than highway bridges and they do not have the high degree of redundancy, making them more vulnerable to collapse in the event of vehicular impact. Results from static lateral load analyses using three-dimensional, geometrically nonlinear, full-scale finite element (FE) models of a typical bridge system and bridge subassemblages were used to evaluate the characteristics of the system. The FE models were calibrated with experimental test data on typical subassemblages and connection details for PCTG bridges. Results of the experimental part of the program have already been published elsewhere. This paper summarizes the observations obtained from nonlinear static FE analyses of a PCTG pedestrian bridge system subjected to lateral impact loads. The analyses indicated that the location of impact, the type of connector, and the flexibility of the end support details affected bridge performance. Improved connection details are suggested for enhanced PCTG pedestrian bridge performance.
Citation Count: 50
Pull-out behavior of prestressing strands in steel fiber reinforced concrete
(Elsevier Sci Ltd, 2012) Baran, Eray; Akis, Tolga; Yesilmen, Seda; Civil Engineering; Department of Civil Engineering
This paper presents the results of an experimental study investigating the effects of steel fibers on the mechanical properties of concrete and the enhancement of bond strength of prestressing strands in steel fiber reinforced concrete (SFRC). The first part of the experimental program consisted of compression, tension and flexural tests on SFRC. Two types of steel fibers with 30 mm and 60 mm fiber lengths were used with five different fiber contents. The second part of the study consisted of simple pull-out tests on 12.7 mm and 15.2 mm diameter seven-wire untensioned prestressing strands embedded in concrete blocks. The pull-out tests were conducted with two different fiber lengths and five different fiber contents for each strand diameter. The steel fibers were observed to improve the pull-out resistance of strands by controlling the crack growth inside concrete blocks. (C) 2011 Elsevier Ltd. All rights reserved.