38 International Journal of Structural Engineering (IJSE) Volume 1, Issue 1, July-December 2019, pp. 38-46, Article ID: IJSE_01_01_004 Available online at http://iaeme.com/Home/issue/IJSE?Volume=1&Issue=1 © IAEME Publication CONNECTIONS IN STRUCTURAL STEEL JOINTS P.R. Kalyana Chakravarthy, R. Janani, S. Durgalakshmi, T. Ilango and S. Sivaganesan Assistant Professor, Vels Institute of Science, Technology and Advanced Studies, Vels University, Tamilnadu, India ABSTRACT Historically, most major structural failures have been due to some form of connection failure. Steel connections have a direct influence on the cost of the framing system. It is a common practice to weld shop attachments and to bolt field attachments. Some of the types of connections are based on: General connections, Geometry and load distribution & Splicing. Under General it’s classified as bolted & welded also according to Load distribution and geometry its moment & shear connections. Shear connections are called simple connections Since they are assumed not to – transfer bending moment, thus allowing end rotation of the member. Seat or hanger connections are the only type of shear connections that connect to the flange of the supported beam. Moment connections are typically designed to also carry the shear component of the load. They provide continuity between the supported and supporting members. Relative rotation between the supporting and supported members is negligible. The flanges of the supported member are attached to either a connection element or directly to the supporting member. Flange plates may also be required for stability of the column during erection. This plate helps align the upper and lower columns. The upper and lower columns may be of different sizes. The flanges and webs of the two columns are field- welded to each other. This type of weld is called a groove weld. In case of large seismic events the design of steel structures must be able to accurately approximate the response of the structure beyond the elastic range. By the means of ductility, similar to the case of structures, connections are divided into ductility classes. In this paper, a review of other literatures is done on the basis of connections and its importance according to structural and seismic point of view. One of the most important aspects that one needs to take into account when designing a steel structure is the dissipative mechanism of the structure as well as the structural properties of the connection. A manual design can only be done for connections in steel structures for better understanding. Key words: Seismic event, Ductility, Shear & Moment connections
CONNECTIONS IN STRUCTURAL STEEL JOINTS
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International Journal of Structural Engineering (IJSE) Volume 1, Issue 1, July-December 2019, pp. 38-46, Article ID: IJSE_01_01_004 Available online at http://iaeme.com/Home/issue/IJSE?Volume=1&Issue=1 © IAEME Publication
CONNECTIONS IN STRUCTURAL STEEL JOINTS
P.R. Kalyana Chakravarthy, R. Janani, S. Durgalakshmi, T. Ilango and S. Sivaganesan Assistant Professor, Vels Institute of Science, Technology and Advanced Studies,
Vels University, Tamilnadu, India
ABSTRACT Historically, most major structural failures have been due to some form of
connection failure. Steel connections have a direct influence on the cost of the framing system. It is a common practice to weld shop attachments and to bolt field
attachments. Some of the types of connections are based on: General connections, Geometry and load distribution & Splicing. Under General it’s classified as bolted &
welded also according to Load distribution and geometry its moment & shear connections.
Shear connections are called simple connections Since they are assumed not to – transfer bending moment, thus allowing end rotation of the member. Seat or hanger connections are the only type of shear connections that connect to the flange of the supported beam. Moment connections are typically designed to also carry the shear component of the load. They provide continuity between the supported and supporting
members. Relative rotation between the supporting and supported members is negligible. The flanges of the supported member are attached to either a connection element or directly to the supporting member. Flange plates may also be required for
stability of the column during erection. This plate helps align the upper and lower columns. The upper and lower columns may be of different sizes. The flanges and
webs of the two columns are field- welded to each other. This type of weld is called a groove weld.
In case of large seismic events the design of steel structures must be able to accurately approximate the response of the structure beyond the elastic range. By the
means of ductility, similar to the case of structures, connections are divided into ductility classes.
In this paper, a review of other literatures is done on the basis of connections and its importance according to structural and seismic point of view. One of the most
important aspects that one needs to take into account when designing a steel structure is the dissipative mechanism of the structure as well as the structural properties of the connection. A manual design can only be done for connections in steel structures for better understanding.
Key words: Seismic event, Ductility, Shear & Moment connections
Connections in Structural Steel Joints
39
1. INTRODUCTION The greatest economy for low and medium rise braced multi-story frames will be achieved by
the use of ‘simple construction. Structural steel category of the steel used as a construction material for making structural steel shapes is behavior of a structure with rational principles of mechanics (statics, solid mechanics, dynamics, etc.) and structural analysis to produce a safe and economical structure to serve its intended purposes. Structural steel is one of the most important building materials in modern area. It is used solely or in compaction with other material such as concrete, timber, composites, etc., for a variety of purposes.A braced frame has sufficient structural components to transmit horizontal forces directly to the foundations. These components provide stability to the frame. In a braced frame, the beams are designed as simply supported. The columns carry axial loads and (generally) minimal moments.
Figure 1 Elevated and plan view of web to flange beam-column connection and web to web beam- column connection
Single Angle web cleats:Single angle web cleats are normally only used for small connections or where access desirable from an erector’s point of view because of the tendency of the beam to twist during erection. Care should be taken when using this type of connection in areas where axial tension is high. The bolts connecting the cleat to the column must also be
checked for the moment produced by the product of the end shear force and the distance between the bolts and the center line of the beam.
Flexible end plates:Typical flexible end-plate connections about the major and minor axis of a column. These connections consist of a single plate fillet welded to the end of the beam and site bolted to either a supporting column or beam for fabrication and erection tolerances.
The end-plate is often detailed and avoids the need for temporary bracing. This type of connection derives its flexibility from the use of relatively thin end-plates combined with
large bolt cross-centers.
2. LITERATURE REVIEW Some of the literatures are collected and analyzed to find out the importance of different types
of connections and bracings with respect to each steel structural member and the seismic activity are given below:
Nagao et al (1995) have discussed about the performance based seismic design on beam- column connection in steel structures. Also this was done to reduce the cost of steel
fabrications. Poor welding procedures were observed in the collapsed building. So, an inspection process is very essential.
P.R. Kalyana Chakravarthy, R. Janani, S. Durgalakshmi, T. Ilango and S. Sivaganesan
40
Shin et al (2014) had discussed about the deformation characteristic of H-shape beams with newly T-stub connection for the effect of beam column joints in elastic behavior of the
beam this research applies the model of exponential function based on equations of equilibrium and compatibility equation and the location of bolts.When diameter of bolts is larger and length of span is shorter, capacities of plastic deformation increase. When thickness of T-flanges and T-web is thicker, capacities of plastic deformation increase.
Soltani et al (2011) had discussed about the elementary bolted steel T-stub connections, proportioned according to Eurocode3 (EC3) were studied by means of a 3-dimensional finite
element model using the ANSYS software package.The elasto-plastic response, up to the alternate state was analyzed and aspects relative to stiffness, strength, bolt loads, prying forces
and interaction between flanges were examined. Comparisons of experimental values, computed results and EC3 predictions indicated that the design procedure still needs to be improved, owing to the complex phenomena embodied.
Uang et al (2000) had investigated the cyclic response of the steel RBS moment connections under various loading histories. This paper focus on the plastic rotation, energy ed dissipation, and failure mode of RBS moment connections.No weld fractures were observed
they each reached a plastic rotation of 0.03 radian.Buckling amplitudes for the near fault specimen were lower than those of the standard specimen at comparable drift levels.
Wald et al (2008) described base plate in bending and anchor plate in tension, which are the major components of a base plate connections.The design was done according to the Euro code 3 with a description of how the behavior is influenced by contact between the base plate and the concrete surface. This has been verified by tests and a finite element simulation.
Kaling et al (2015) had studied on semi rigid connections of cold formed steel section with different connection configuration. This was focused to find out the structural
performance of various beam-columns with bolted moment connections on cold formed light gauge steel sections by analytical, FEM analysis and experimental tests.The inclusion of web
angles to beam in the connection didn’t affect the load carrying capacity and connection initial stiffness.
Imran et al (2014) had considered parameters to study were to compare the seismic and wind performances of buildings are base shear, roof displacement. The models were analyzed by equivalent static analysis as per IS 1893:2002.These models were analyzed using ETABS software. Due to bracings at peripheral of structure, the base shear increased up to 40% for zone III and roof displacement value decreased up to 45%. X- Type bracings were effective for highly affected earthquake zones and for different wind speeds.
3. FAILURE MODES IN STEEL MEMBERS Steel moment- resisting frames have become a popular seismic load- resisting system because steel is a well-known ductile material with a high strength- - mass ratio. Damage is expected to to consist of yielding and local buckling but not brittle fracture of connection. Formation of plastic hinges at beam-column joints and column bases is a feature of inelastic behavior for steel moment resisting frame these hinges form through flexural yielding of beams and shear
yielding of penal zones. That is been showed in several experimental works which the connection where applied to cyclic loading, and the failure modes were varied , but mainly
exhibit a ductile behavior depending on several mechanisms characterized by bolt slip, yielding of steel, elongation of bolt holes, etc.
Connections in Structural Steel Joints
41
Beam Behavior: it is expected that beams will undergo large in elastic rotations at targeted plastic hinges locations, at weakened portions of the beams, with reduced beam section
designs, or within the beam span if large gravity moments are present. Failure modes can include excessive local buckling and lateral torsional buckling. Beam- - Column to
Connections: because of the variety in the type of connection configurations, failure mode take different forms like Fracture in or around welds, fracture in highly strained fracture material, fracture at weld access holes, net section fractures at bolt holes, shearing and tensile failure of bolts, bolt bearing and block shear failures.
Joint Panel Zone Behavior: failure modes associated with the direct transfer of forces from the beam flange to the column can include column flange bending, web crippling, and web buckling.
Column Behavior: The intention is to keep inelastic deformations out of most columns to minimize detrimental effects of high axial loads on bending behavior and potential formation of single- story mechanisms shown in figure 2. Therefore, excessive local buckling and lateral torsional buckling are potential failure modes in addition to basic flexural buckling of column.
Column Splices: failure modes at column splices are similar to those for beam- - column to connections. Failure modes are similar to those for beam- - column connections. Failure of to column splices will not only reduces or eliminates bending and tension resistance, it will also reduce or eliminate of the column to transfer shear forces.
Column Bases: failure modes depend on the connection between the column and the foundation. They include anchorage stretching or pull out, fracture in base plates or in
column- - base plate connections, and or excessive local and lateral torsional buckling if to inelastic deformations are concentrated in the region above the base connection.
Structural P-Delta Effects: amplification of internal forces and lateral displacements, known as the P-delta effect, occur when a structure is simultaneously subjected to gravity
loads and lateral side sway. This effect reduces frame lateral tangent stiffness, might cause a negative effective lateral tangent stiffness once a mechanism has formed, and can lead to
collapse.
Figure 1 Formation of a single Story frame mechanism, also called a "Weak Story mechanism"
Side sway Collapse: frame collapse can occur when the effective story shear due to inertial forces and P-delta effects exceeds the story shear resistance.
P.R. Kalyana Chakravarthy, R. Janani, S. Durgalakshmi, T. Ilango and S. Sivaganesan
42
A Braced Frame is a structural system which is designed primarily to resist wind and earthquake forces. Members in a braced frame are designed to work in tension and
compression, similar to a truss. Braced frames are almost always composed of steel members.
Advantages of bracing system: Braced frames are applicable to all kind of structures like bridges, aircrafts, cranes, Buildings and electrical transmission towers, are easy to fabricate and construct, no lots of knowledge or skills are needed and If the bolted connections are used, there is no deformation problem at the connections.
4. COMPARATIVE STUDY The selection of a bracing configuration is dependent on many factors. These include the
height to width proportions of the bay and the size and location of required open areas in the framing elevation. These constraints may supersede structural optimization as design criteria. The introduction of the parameter, e/L, leads to a generalization of the concept of framing system.
Bolted Double Angle cleats: Typical bolted double angle cleat connections about both the major and minor axis of a column. Any simple equilibrium analysis is suitable for the design of this type of connection. The one recommended in this publication assumes that the line of action of shear transfer between the beam and the column is at the face of the column. Using this model the bolt group connecting the cleats to the beam web must be designed for the shear force and the moment produced by the product of the end shear and the eccentricity of the bolt group from the face of the column. The bolts connecting the cleats to the face of the column should be designed for the applied shear only. In practice the cleats to the column are rarely critical and the design is almost always governed by the bolts bearing on to the web of the beam. The rotational capacity of this connection is governed largely by the deformation capacity of the angles and the slip between the connected parts. Most of the rotation of the
connections comes from the deformation of the angles while fastener deformation is very small.
Fin plates: A more recent development, which follows both Australian and American practice, has been the introduction of the fin plate connection. This type of connection is
primarily used to transfer beam end reactions and is economical to fabricate and simple to erect. There is clearance between the ends of the supported beam and the supporting beam or column, and minor axes of a column. These connections comprise a single plate with either pre-punched or pre-drilled holes that is shop welded to the supporting column flange or web.
Bracing systems are generally three types namely Moment resisting frames (MRFs),frames are rectilinear assemblages of beams and columns, with the beams rigidly
connected to the columns, Concentric braced system (CBFs),effective and economic lateral load resisting systems to withstand earthquake loading in seismic regions around the world, Eccentric braced frame (EBFs), reduce overall material requirements and result in a frame which is still under moderate loads yet ductile at extreme overloads, an essential feature in seismic design.
The seismic response of steel structures is mainly influenced by the behavior of connections. Lots of studies by different authors were done but couldn’t come up with any
sure conclusions on what type of bracings to be used.Some of the comparative studies of various authors are discussed below:
Jagadeesh et al (2016) assessed the seismic response of steel structure with concentric bracing system. Two configurations were utilized; vertical irregular model (VIRM) and
Connections in Structural Steel Joints
43
vertical irregular model with mega bracing (VIRM_MB). The steel frame models were of five bays with fifteen stories for with and without bracing system. The study was accompanied with the usage of ETABS. The comparison was done by comparing a number of parameters; story drift, story displacements and base shear. The performed inelastic analysis stated that
mega bracing was more efficient to resist earthquakes due to the reduction in story drifts, story displacement of the irregular vertical structure was 77.64% and the maximum base shear was 23.42% than VIRM without bracing frame.
Anitha et al (2015) had studied about the seismic effect of different types of steel bracings and a comparison of knee braced steel frame with other types of bracings had been done. The modeling under non-linear time history analysis and non-linear static analysis was performed using finite element software, ANSYS 14.5. Dynamic loading of 10kN was given for non- linear static analysis and El-Centro earthquake data for time history analysis was assigned.
The knee bracing can be used for suitable seismic retrofitting method. In non-linear static analysis, double knee bracings showed good performance during seismic activity as the
ultimate loading was very high and had more lateral stiffness. In time history analysis, the maximum displacement observed was 90.5% more than without bracing and 50% more than eccentric bracing system.
Dhiman et al (2015) had evaluated the response of braced and unbraced structure subjected to seismic loads. Different types of bracings were used such as cross braced, chevron braced, K-braced and diagonal braced for dead and live loads, seismic load in X and Z direction respectively. The displacement of the structure decreased after the application of bracing system and maximum reduction in lateral displacement occurred after applying cross bracing system.It also reduced bending moments and shear forces in columns.
Ghomi et al (2008) had discussed on the new idea and application of EGS theory for improvement of behavior of X-Bracing systems. This theory can increase the lateral stiffness of buildings and decrease lateral displacements.The P- and the stability Δ effects were reduced was improved. The ductility and the stiffness of the system as well as its energy absorption capacity increased significantly. A general comparison on EBFS and Knee bracings is studied:
In EBFs, some of the bracing members are arranged so that their ends do not meet concentrically on a main member, but are separated to meet eccentrically.
Figure 3 Eccentric Link Elements
The eccentric link element between the ends of the braces is designed as a weak but ductile link which yields before any of the other frame members. It therefore provides a
dependable source of ductility and, by using capacity design principles, it can prevent the shear in the structure from reaching the level at which buckling occurs in any of the members.
P.R. Kalyana Chakravarthy, R. Janani, S. Durgalakshmi, T. Ilango and S. Sivaganesan
44
The link element is relatively short and so the elastic response of the frame is similar to that of the equivalent CBF. The arrangement thus combines the advantageous stiffness of CBFs in its elastic response, while providing much greater ductility and avoiding problems of buckling and irreversible yielding which affect CBFs in their post-yield phase.
The yielding element here is the ‘knee brace’ shown in fig. 4, which remains elastic and
stiff during moderate earthquakes, but yields to provide ductility and protection from buckling in extreme events.Unlike the link in the EBF, the 5 knee brace does not form part of the main structural frame, and could be removed and replaced if it is damaged in an earthquake.
Figure 4 Knee Braced Frame
The general principlethat the diagonal elements of bracings should be placed in such a way that the load deflection characteristics of the structure are the same for both positive and –
negative phases of the loading cycle.
In more rigid systems, the role of anchor bolts is crucial.A study on base plates with different conditions proved the importance of anchor bolts and its arrangement as shown in fig. 5. According to the experimental studies,
In circumstances where the anchor bolts are connected to the system through an added top plate, such plates are the most vulnerable elements in the system. Besides, such configuration
has an adverse effect on the rigidity as well as the strength of the system. Apparently, increasing the thickness of such plates increases the rigidity and the strength. With moderate sized added top plates cannot be considered as rigid.
Figure 5 Base Plates at Different Locations
Connections in Structural Steel Joints
45
In the design of a fin plate connection it is important to identify the appropriate line of action for the shear. There are two possibilities: either the shear acts at the face of the column or it acts along the centre of the bolt group connecting the fin plate to the beam web. For this reason both critical sections should be checked for a minimum moment taken as the product of the vertical shear and the distance between the face of the column (or beam web) and the
centre of the bolt group. Both critical sections are then checked for the…
CONNECTIONS IN STRUCTURAL STEEL JOINTS
P.R. Kalyana Chakravarthy, R. Janani, S. Durgalakshmi, T. Ilango and S. Sivaganesan Assistant Professor, Vels Institute of Science, Technology and Advanced Studies,
Vels University, Tamilnadu, India
ABSTRACT Historically, most major structural failures have been due to some form of
connection failure. Steel connections have a direct influence on the cost of the framing system. It is a common practice to weld shop attachments and to bolt field
attachments. Some of the types of connections are based on: General connections, Geometry and load distribution & Splicing. Under General it’s classified as bolted &
welded also according to Load distribution and geometry its moment & shear connections.
Shear connections are called simple connections Since they are assumed not to – transfer bending moment, thus allowing end rotation of the member. Seat or hanger connections are the only type of shear connections that connect to the flange of the supported beam. Moment connections are typically designed to also carry the shear component of the load. They provide continuity between the supported and supporting
members. Relative rotation between the supporting and supported members is negligible. The flanges of the supported member are attached to either a connection element or directly to the supporting member. Flange plates may also be required for
stability of the column during erection. This plate helps align the upper and lower columns. The upper and lower columns may be of different sizes. The flanges and
webs of the two columns are field- welded to each other. This type of weld is called a groove weld.
In case of large seismic events the design of steel structures must be able to accurately approximate the response of the structure beyond the elastic range. By the
means of ductility, similar to the case of structures, connections are divided into ductility classes.
In this paper, a review of other literatures is done on the basis of connections and its importance according to structural and seismic point of view. One of the most
important aspects that one needs to take into account when designing a steel structure is the dissipative mechanism of the structure as well as the structural properties of the connection. A manual design can only be done for connections in steel structures for better understanding.
Key words: Seismic event, Ductility, Shear & Moment connections
Connections in Structural Steel Joints
39
1. INTRODUCTION The greatest economy for low and medium rise braced multi-story frames will be achieved by
the use of ‘simple construction. Structural steel category of the steel used as a construction material for making structural steel shapes is behavior of a structure with rational principles of mechanics (statics, solid mechanics, dynamics, etc.) and structural analysis to produce a safe and economical structure to serve its intended purposes. Structural steel is one of the most important building materials in modern area. It is used solely or in compaction with other material such as concrete, timber, composites, etc., for a variety of purposes.A braced frame has sufficient structural components to transmit horizontal forces directly to the foundations. These components provide stability to the frame. In a braced frame, the beams are designed as simply supported. The columns carry axial loads and (generally) minimal moments.
Figure 1 Elevated and plan view of web to flange beam-column connection and web to web beam- column connection
Single Angle web cleats:Single angle web cleats are normally only used for small connections or where access desirable from an erector’s point of view because of the tendency of the beam to twist during erection. Care should be taken when using this type of connection in areas where axial tension is high. The bolts connecting the cleat to the column must also be
checked for the moment produced by the product of the end shear force and the distance between the bolts and the center line of the beam.
Flexible end plates:Typical flexible end-plate connections about the major and minor axis of a column. These connections consist of a single plate fillet welded to the end of the beam and site bolted to either a supporting column or beam for fabrication and erection tolerances.
The end-plate is often detailed and avoids the need for temporary bracing. This type of connection derives its flexibility from the use of relatively thin end-plates combined with
large bolt cross-centers.
2. LITERATURE REVIEW Some of the literatures are collected and analyzed to find out the importance of different types
of connections and bracings with respect to each steel structural member and the seismic activity are given below:
Nagao et al (1995) have discussed about the performance based seismic design on beam- column connection in steel structures. Also this was done to reduce the cost of steel
fabrications. Poor welding procedures were observed in the collapsed building. So, an inspection process is very essential.
P.R. Kalyana Chakravarthy, R. Janani, S. Durgalakshmi, T. Ilango and S. Sivaganesan
40
Shin et al (2014) had discussed about the deformation characteristic of H-shape beams with newly T-stub connection for the effect of beam column joints in elastic behavior of the
beam this research applies the model of exponential function based on equations of equilibrium and compatibility equation and the location of bolts.When diameter of bolts is larger and length of span is shorter, capacities of plastic deformation increase. When thickness of T-flanges and T-web is thicker, capacities of plastic deformation increase.
Soltani et al (2011) had discussed about the elementary bolted steel T-stub connections, proportioned according to Eurocode3 (EC3) were studied by means of a 3-dimensional finite
element model using the ANSYS software package.The elasto-plastic response, up to the alternate state was analyzed and aspects relative to stiffness, strength, bolt loads, prying forces
and interaction between flanges were examined. Comparisons of experimental values, computed results and EC3 predictions indicated that the design procedure still needs to be improved, owing to the complex phenomena embodied.
Uang et al (2000) had investigated the cyclic response of the steel RBS moment connections under various loading histories. This paper focus on the plastic rotation, energy ed dissipation, and failure mode of RBS moment connections.No weld fractures were observed
they each reached a plastic rotation of 0.03 radian.Buckling amplitudes for the near fault specimen were lower than those of the standard specimen at comparable drift levels.
Wald et al (2008) described base plate in bending and anchor plate in tension, which are the major components of a base plate connections.The design was done according to the Euro code 3 with a description of how the behavior is influenced by contact between the base plate and the concrete surface. This has been verified by tests and a finite element simulation.
Kaling et al (2015) had studied on semi rigid connections of cold formed steel section with different connection configuration. This was focused to find out the structural
performance of various beam-columns with bolted moment connections on cold formed light gauge steel sections by analytical, FEM analysis and experimental tests.The inclusion of web
angles to beam in the connection didn’t affect the load carrying capacity and connection initial stiffness.
Imran et al (2014) had considered parameters to study were to compare the seismic and wind performances of buildings are base shear, roof displacement. The models were analyzed by equivalent static analysis as per IS 1893:2002.These models were analyzed using ETABS software. Due to bracings at peripheral of structure, the base shear increased up to 40% for zone III and roof displacement value decreased up to 45%. X- Type bracings were effective for highly affected earthquake zones and for different wind speeds.
3. FAILURE MODES IN STEEL MEMBERS Steel moment- resisting frames have become a popular seismic load- resisting system because steel is a well-known ductile material with a high strength- - mass ratio. Damage is expected to to consist of yielding and local buckling but not brittle fracture of connection. Formation of plastic hinges at beam-column joints and column bases is a feature of inelastic behavior for steel moment resisting frame these hinges form through flexural yielding of beams and shear
yielding of penal zones. That is been showed in several experimental works which the connection where applied to cyclic loading, and the failure modes were varied , but mainly
exhibit a ductile behavior depending on several mechanisms characterized by bolt slip, yielding of steel, elongation of bolt holes, etc.
Connections in Structural Steel Joints
41
Beam Behavior: it is expected that beams will undergo large in elastic rotations at targeted plastic hinges locations, at weakened portions of the beams, with reduced beam section
designs, or within the beam span if large gravity moments are present. Failure modes can include excessive local buckling and lateral torsional buckling. Beam- - Column to
Connections: because of the variety in the type of connection configurations, failure mode take different forms like Fracture in or around welds, fracture in highly strained fracture material, fracture at weld access holes, net section fractures at bolt holes, shearing and tensile failure of bolts, bolt bearing and block shear failures.
Joint Panel Zone Behavior: failure modes associated with the direct transfer of forces from the beam flange to the column can include column flange bending, web crippling, and web buckling.
Column Behavior: The intention is to keep inelastic deformations out of most columns to minimize detrimental effects of high axial loads on bending behavior and potential formation of single- story mechanisms shown in figure 2. Therefore, excessive local buckling and lateral torsional buckling are potential failure modes in addition to basic flexural buckling of column.
Column Splices: failure modes at column splices are similar to those for beam- - column to connections. Failure modes are similar to those for beam- - column connections. Failure of to column splices will not only reduces or eliminates bending and tension resistance, it will also reduce or eliminate of the column to transfer shear forces.
Column Bases: failure modes depend on the connection between the column and the foundation. They include anchorage stretching or pull out, fracture in base plates or in
column- - base plate connections, and or excessive local and lateral torsional buckling if to inelastic deformations are concentrated in the region above the base connection.
Structural P-Delta Effects: amplification of internal forces and lateral displacements, known as the P-delta effect, occur when a structure is simultaneously subjected to gravity
loads and lateral side sway. This effect reduces frame lateral tangent stiffness, might cause a negative effective lateral tangent stiffness once a mechanism has formed, and can lead to
collapse.
Figure 1 Formation of a single Story frame mechanism, also called a "Weak Story mechanism"
Side sway Collapse: frame collapse can occur when the effective story shear due to inertial forces and P-delta effects exceeds the story shear resistance.
P.R. Kalyana Chakravarthy, R. Janani, S. Durgalakshmi, T. Ilango and S. Sivaganesan
42
A Braced Frame is a structural system which is designed primarily to resist wind and earthquake forces. Members in a braced frame are designed to work in tension and
compression, similar to a truss. Braced frames are almost always composed of steel members.
Advantages of bracing system: Braced frames are applicable to all kind of structures like bridges, aircrafts, cranes, Buildings and electrical transmission towers, are easy to fabricate and construct, no lots of knowledge or skills are needed and If the bolted connections are used, there is no deformation problem at the connections.
4. COMPARATIVE STUDY The selection of a bracing configuration is dependent on many factors. These include the
height to width proportions of the bay and the size and location of required open areas in the framing elevation. These constraints may supersede structural optimization as design criteria. The introduction of the parameter, e/L, leads to a generalization of the concept of framing system.
Bolted Double Angle cleats: Typical bolted double angle cleat connections about both the major and minor axis of a column. Any simple equilibrium analysis is suitable for the design of this type of connection. The one recommended in this publication assumes that the line of action of shear transfer between the beam and the column is at the face of the column. Using this model the bolt group connecting the cleats to the beam web must be designed for the shear force and the moment produced by the product of the end shear and the eccentricity of the bolt group from the face of the column. The bolts connecting the cleats to the face of the column should be designed for the applied shear only. In practice the cleats to the column are rarely critical and the design is almost always governed by the bolts bearing on to the web of the beam. The rotational capacity of this connection is governed largely by the deformation capacity of the angles and the slip between the connected parts. Most of the rotation of the
connections comes from the deformation of the angles while fastener deformation is very small.
Fin plates: A more recent development, which follows both Australian and American practice, has been the introduction of the fin plate connection. This type of connection is
primarily used to transfer beam end reactions and is economical to fabricate and simple to erect. There is clearance between the ends of the supported beam and the supporting beam or column, and minor axes of a column. These connections comprise a single plate with either pre-punched or pre-drilled holes that is shop welded to the supporting column flange or web.
Bracing systems are generally three types namely Moment resisting frames (MRFs),frames are rectilinear assemblages of beams and columns, with the beams rigidly
connected to the columns, Concentric braced system (CBFs),effective and economic lateral load resisting systems to withstand earthquake loading in seismic regions around the world, Eccentric braced frame (EBFs), reduce overall material requirements and result in a frame which is still under moderate loads yet ductile at extreme overloads, an essential feature in seismic design.
The seismic response of steel structures is mainly influenced by the behavior of connections. Lots of studies by different authors were done but couldn’t come up with any
sure conclusions on what type of bracings to be used.Some of the comparative studies of various authors are discussed below:
Jagadeesh et al (2016) assessed the seismic response of steel structure with concentric bracing system. Two configurations were utilized; vertical irregular model (VIRM) and
Connections in Structural Steel Joints
43
vertical irregular model with mega bracing (VIRM_MB). The steel frame models were of five bays with fifteen stories for with and without bracing system. The study was accompanied with the usage of ETABS. The comparison was done by comparing a number of parameters; story drift, story displacements and base shear. The performed inelastic analysis stated that
mega bracing was more efficient to resist earthquakes due to the reduction in story drifts, story displacement of the irregular vertical structure was 77.64% and the maximum base shear was 23.42% than VIRM without bracing frame.
Anitha et al (2015) had studied about the seismic effect of different types of steel bracings and a comparison of knee braced steel frame with other types of bracings had been done. The modeling under non-linear time history analysis and non-linear static analysis was performed using finite element software, ANSYS 14.5. Dynamic loading of 10kN was given for non- linear static analysis and El-Centro earthquake data for time history analysis was assigned.
The knee bracing can be used for suitable seismic retrofitting method. In non-linear static analysis, double knee bracings showed good performance during seismic activity as the
ultimate loading was very high and had more lateral stiffness. In time history analysis, the maximum displacement observed was 90.5% more than without bracing and 50% more than eccentric bracing system.
Dhiman et al (2015) had evaluated the response of braced and unbraced structure subjected to seismic loads. Different types of bracings were used such as cross braced, chevron braced, K-braced and diagonal braced for dead and live loads, seismic load in X and Z direction respectively. The displacement of the structure decreased after the application of bracing system and maximum reduction in lateral displacement occurred after applying cross bracing system.It also reduced bending moments and shear forces in columns.
Ghomi et al (2008) had discussed on the new idea and application of EGS theory for improvement of behavior of X-Bracing systems. This theory can increase the lateral stiffness of buildings and decrease lateral displacements.The P- and the stability Δ effects were reduced was improved. The ductility and the stiffness of the system as well as its energy absorption capacity increased significantly. A general comparison on EBFS and Knee bracings is studied:
In EBFs, some of the bracing members are arranged so that their ends do not meet concentrically on a main member, but are separated to meet eccentrically.
Figure 3 Eccentric Link Elements
The eccentric link element between the ends of the braces is designed as a weak but ductile link which yields before any of the other frame members. It therefore provides a
dependable source of ductility and, by using capacity design principles, it can prevent the shear in the structure from reaching the level at which buckling occurs in any of the members.
P.R. Kalyana Chakravarthy, R. Janani, S. Durgalakshmi, T. Ilango and S. Sivaganesan
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The link element is relatively short and so the elastic response of the frame is similar to that of the equivalent CBF. The arrangement thus combines the advantageous stiffness of CBFs in its elastic response, while providing much greater ductility and avoiding problems of buckling and irreversible yielding which affect CBFs in their post-yield phase.
The yielding element here is the ‘knee brace’ shown in fig. 4, which remains elastic and
stiff during moderate earthquakes, but yields to provide ductility and protection from buckling in extreme events.Unlike the link in the EBF, the 5 knee brace does not form part of the main structural frame, and could be removed and replaced if it is damaged in an earthquake.
Figure 4 Knee Braced Frame
The general principlethat the diagonal elements of bracings should be placed in such a way that the load deflection characteristics of the structure are the same for both positive and –
negative phases of the loading cycle.
In more rigid systems, the role of anchor bolts is crucial.A study on base plates with different conditions proved the importance of anchor bolts and its arrangement as shown in fig. 5. According to the experimental studies,
In circumstances where the anchor bolts are connected to the system through an added top plate, such plates are the most vulnerable elements in the system. Besides, such configuration
has an adverse effect on the rigidity as well as the strength of the system. Apparently, increasing the thickness of such plates increases the rigidity and the strength. With moderate sized added top plates cannot be considered as rigid.
Figure 5 Base Plates at Different Locations
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In the design of a fin plate connection it is important to identify the appropriate line of action for the shear. There are two possibilities: either the shear acts at the face of the column or it acts along the centre of the bolt group connecting the fin plate to the beam web. For this reason both critical sections should be checked for a minimum moment taken as the product of the vertical shear and the distance between the face of the column (or beam web) and the
centre of the bolt group. Both critical sections are then checked for the…
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