49 CONNECTIONS AND JOINTS IN PRECAST CONCRETE STRUCTURES Ivan, HOLLY 1 * , Iyad ABRAHOIM 1 Address 1 Dept. of Concrete Structures and Bridges, Faculty of Civil Engineering, Slovak University of Technology in Bratislava, Bratislava, Slovakia * Corresponding author: [email protected] Abstract The structural integrity of precast concrete structures mainly depends on the connections between the precast structural elements. The purpose of a connection is to transfer loads, re- strain movement, and/or to provide stability to a component or an entire structure. Therefore, the design of connections is one of the most important aspects in the design of precast concrete structures. All the connections should be designed according to the valid codes. All precasters have developed connection details over the years that suit their particular production and erection preferences. It is common that the structural engineer shows the internal forces and connection locations, and the manufacturer’s engineering department provides the final design and details of the connections. This paper describes basic types of connections and joints used in precast concrete structures Key words ● Concrete, ● Connections, ● Precast elements. 1 INTRODUCTION The most extensive expansion of precast concrete structures in Slovakia was in the second half of the 20th century. The develop- ment of precast skeleton construction in the former Czechoslovak Republic had begun around 1930. The beginnings of prefabrication were mainly related to the development of the intensively evolving industry. A similar situation occurred after the Second World War and was related to the repair of damaged industrial facilities. Af- ter 1960, the building of skeletons for non-residential constructions (hospitals, offices, shops, cultural facilities, etc.) began. In Czecho- slovakia approximately 30 prefabricated skeleton systems were de- veloped (Harvan, 2007). Precast construction (also known as “prefabricated” construction) includes those buildings where the majority of the structural compo- nents are standardized and produced in plants in a location away from the construction and then transported to the site for assembly. These components are manufactured by industrial methods based on mass production in order to build a large number of buildings in a short time at a low cost (Sanghvi, 2015). The most frequently used types of precast concrete elements are columns, beams, hollow-core slabs, pocket foundations, etc. Fig. 1 illustrates some of these units. The use of precast products is regarded in the construction stan- dards as an advantage that helps to reduce some of the coefficients used in design, but there are other specific advantages of precasting that are easily noticed on site. Some of these specific advantages are: – Control of shrinkage: the use of lower water-to-cement ratios in precast concrete may reduce shrinkage. – Reduction of creep: higher strength concretes and proper curing may produce concrete with reduced creep characteristics. – Quality control: the factory production of precast concrete ele- ments is inherently manufactured under the best conditions for the forming and placement of reinforcements and concrete, vi- brations and curing. Dimensional tolerance control is facilitated and more easily corrected. – Timely availability: many standardized, mass-produced el- ements can be furnished to a construction site on very short order. Vol. 28, 2020, No. 1, 49 – 56 DOI: 10.2478/sjce-2020-0007 Slovak Journal of Civil Engineering © 2020 The Author(s). This is an open access article licensed under the Creative Commons Attribution- NonCommercial-NoDerivs License (http://creativecommons.Org/licenses/by-nc-nd/3.0/).
CONNECTIONS AND JOINTS IN PRECAST CONCRETE STRUCTURES
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Ivan, HOLLY1*, Iyad ABRAHOIM1
Address
1 Dept. of Concrete Structures and Bridges, Faculty of Civil Engineering, Slovak University of Technology in Bratislava, Bratislava, Slovakia
* Corresponding author: [email protected]
Abstract
The structural integrity of precast concrete structures mainly depends on the connections between the precast structural elements. The purpose of a connection is to transfer loads, re- strain movement, and/or to provide stability to a component or an entire structure. Therefore, the design of connections is one of the most important aspects in the design of precast concrete structures. All the connections should be designed according to the valid codes. All precasters have developed connection details over the years that suit their particular production and erection preferences. It is common that the structural engineer shows the internal forces and connection locations, and the manufacturer’s engineering department provides the final design and details of the connections. This paper describes basic types of connections and joints used in precast concrete structures
Key words
1 INTRODUCTION
The most extensive expansion of precast concrete structures in Slovakia was in the second half of the 20th century. The develop- ment of precast skeleton construction in the former Czechoslovak Republic had begun around 1930. The beginnings of prefabrication were mainly related to the development of the intensively evolving industry. A similar situation occurred after the Second World War and was related to the repair of damaged industrial facilities. Af- ter 1960, the building of skeletons for non-residential constructions (hospitals, offices, shops, cultural facilities, etc.) began. In Czecho- slovakia approximately 30 prefabricated skeleton systems were de- veloped (Harvan, 2007).
Precast construction (also known as “prefabricated” construction) includes those buildings where the majority of the structural compo- nents are standardized and produced in plants in a location away from the construction and then transported to the site for assembly. These components are manufactured by industrial methods based on mass production in order to build a large number of buildings in a short time at a low cost (Sanghvi, 2015).
The most frequently used types of precast concrete elements are columns, beams, hollow-core slabs, pocket foundations, etc. Fig. 1 illustrates some of these units.
The use of precast products is regarded in the construction stan- dards as an advantage that helps to reduce some of the coefficients used in design, but there are other specific advantages of precasting that are easily noticed on site. Some of these specific advantages are:
– Control of shrinkage: the use of lower water-to-cement ratios in precast concrete may reduce shrinkage.
– Reduction of creep: higher strength concretes and proper curing may produce concrete with reduced creep characteristics.
– Quality control: the factory production of precast concrete ele- ments is inherently manufactured under the best conditions for the forming and placement of reinforcements and concrete, vi- brations and curing. Dimensional tolerance control is facilitated and more easily corrected.
– Timely availability: many standardized, mass-produced el- ements can be furnished to a construction site on very short order.
Vol. 28, 2020, No. 1, 49 – 56
DOI: 10.2478/sjce-2020-0007
© 2020 The Author(s). This is an open access article licensed under the Creative Commons Attribution- NonCommercial-NoDerivs License (http://creativecommons.Org/licenses/by-nc-nd/3.0/).
Slovak Journal of Civil Engineering
CONNECTIONS AND JOINTS IN PRECAST CONCRETE STRUCTURES...50
Vol. 28, 2020, No. 1, 49 – 56
– Reduction in site construction time: actual erection time and site construction time may be efficiently reduced. In favorable circumstances, a floor of a building may be erected and jointed in two days.
– Economy: the mass production of standardized elements reduc- es the cost of forms and manufacturing labor, and site labor can be kept to a minimum.
– Applicable for composite construction: precast members may frequently be combined with cast-in-place concrete to improve the monolithic behavior of the structure.
– Reduced maintenance: the higher quality materials and control used in precast concrete increase durability and reduce the need for maintenance.
2 REQUIREMENTS FOR CONNECTIONS
Strength - connection must resist the forces to which is subjected during its lifetime. Some of these forces such as those caused by dead and live gravity loads, wind, earth and water pressure are obvious. Some are not so apparent and will often be overlooked. These are forces caused by the restraint of changes in volume in the precast components and those required to maintain stability. Joint strength may be categorized by the type of forces that may be induced such as compressive, tensile, flexural and torsion forces.
Ductility - is the ability of the connection to undergo large de- formations without failure. A deformation is measured between the first yield and ultimate failure of the structural materials used in the connection. Ductility in a building frame is usually associated with the moment of resistance with the flexural tension capacity provided by the reinforcing bars, the crushing of the concrete, or the failure of the connectors or steel embedment in the concrete.
Change in volume – combined shortening due to creep, shrinkage and temperature reductions induces tensile stresses in precast com- ponents. The stresses must be accounted for in the connection design by either providing stress relief details in the undertrained joint or by providing additional reinforcing steel to resist the tensile forces in a restrained joint.
Durability – an exposed section in a connection should be period- ically inspected and maintained. Evidence of poor durability is usu- ally exhibited by the corrosion of any exposed steel elements or the cracking and spalling of the concrete. Components that are exposed to weather should have the ir steel components adequately encased in concrete by grout or by painted, galvanized or stainless-steel sections.
Fire resistance - connections which may be weakened by expo- sure to fire should be protected by concrete or grout or enclosed or sprayed with fire resistance materials. The connections should be pro- tected to the same degree as that required for the components and the building frame.
2.1 Compression Joints
Compressive forces can be transmitted between adjacent precast components by a direct bearing or through an intermediate medium such as in-situ mortar, fine concrete, bearing pads or other bearing elements. Direct contact between the elements should be used when a great degree of accuracy in manufacturing and erection needs to be achieved and when the bearing stresses are small. Cementitious ma- terials such as in-situ mortar, fine concrete or grouting are often used in the joints between load-bearing elements in columns and walls as well as for beam and floor elements. The nominal thickness is about 10 to 30 mm for mortar and grout and 30 to 50 mm for fine concrete. The bedding is usually without any reinforcing bars. The mode of failure is predicated by the crushing of the mortar or splitting of the precast components in contact with it. Although the mortar, grout or fine concrete is in a highly confined state under predominantly plane stress conditions and should achieve a compressive strength higher than fcu, a low design strength is normally used because the edges of the bedding tend to spall off. This will lead to a non-uniform stress distribution. The situation can be exacerbated by poor workmanship, unintentional eccentricity, spurious bending moments, and shear forc- es. Another fact which leads to a reduction of the joint strength is when there is a great difference in the elastic response between the bedded material and the precast concrete, which may result in local- ized contraction, lateral tensile stress, and splitting forces as shown in Fig. 2. This effect may become important when the joints thickness is greater than 50 mm.
The position of the support reaction must be accounted for eccen- tricities due to rotation and tolerances. The rules for this are given in Eurocode 2 (STN EN 1992-1-1, 2015). The basic dimensions of the bearing should be determined such that the stress under the bearing is
Force transfer through compression joint having elastic modules: a) less than the precast b) equal to the precast c) greater than the precast d) greater than the precast, but with reduced breadth
Fig. 2 Vertical transfer of compressive forces
Fig. 1 Typical precast units – columns, beams and a double-T beam
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limited to that of the strength of the bearing material and that of the concrete in the connected components.
2.2 Tensile Joints
The connections with the tensile capacity are often part of the overall tying system that should provide structural integrity and pre- vent progressive collapse. Such a connection should be designed and detailed to have a ductile behavior. Premature brittle failures must be avoided and it should be possible to obtain a rupture of the ductile components of the connections (fib Bulletin, 2008). Tensile forces are transferred between the concrete by means of various types of steel connectors that are anchored to each side of the elements at the joint with a continuity achieved by the overlapping of steel bars, dowel ac- tion, bolting or welding. The tensile capacity of the connection can be determined by either the strength of the steel elements or by the anchorage capacity.
2.3 Shear Joints
Shear forces can be transferred between concrete elements by ad- hesion or friction at a joint interface, a shear-key effect at indented joint faces, the dowel action of transverse steel bars, pins and bolts, etc. Shear keys are generally formed by providing the precast mem- bers with indented joint faces. The shear keys work as mechanical locks, thereby preventing any significant slip along the joint. Shear keys must fulfill certain minimum requirements concerning the length, depth and inclination of the tooth. Such minimum require- ments are given in code and design rules (fib Bulletin, 2008).
A shear transfer by bond between precast and in-situ elements is possible, when the shear stress is low. It is not necessary to deliber- ately roughen the surface texture of precast units beyond the as-cast finish, which may be of a slip-forming, extrusion or tamped finish. Shear transfer by shear friction requires the presence of a permanent normal compressive force. The force may arise from permanent grav- ity loads, by prestressing or be artificially induced by reinforcement bars placed across the joints. Shear keys for the transfer of shear forc- es between elements are obtained by cast in-situ concrete or grout in joints between the elements which surface castellations. Under the action of a shear load, the shear keys act as mechanical locks that prevent significant slips at the interface.
2.4 Flexural and Torsional Joints.
Moment-resisting connections are mainly used to: – Stabilize and increase the stiffness of portal and skeletal frames,
– Reduce the depth of flexural frame members, – Distribute second order moments to beams and slabs and hence
reduce column moments, – Improve resistance to progressive collapse.
Moment-resisting connections are primarily used in column foundations and splices and at beam-column connections. Common methods to achieve moment-resisting connections are the grouting of a projecting reinforcement and the bolting or welding of anchored steel details. Moment-resisting connections should be proportioned such that ductile failures will occur and that the limiting strength of the connection is not governed by shear.
3 TYPE OF CONNECTIONS
3.1 Floor-to-Floor Connections
Fig. 3 and 4 shows a floor-to-floor connection made with a con- crete filling in a continuous joint between the adjacent elements, which is typical of some precast products such as hollow-core slabs. The joint has a proper shape to ensure when it is filled in, i.e., a good interlocking with the transmission of the vertical transverse shear forces. For the transmission of the horizontal longitudinal shear forces, the interface’s shear strength can be improved by providing the adjoining edges with vertical indentations. With reference to the diaphragmatic action, this type of connection ensures the same per- formance to the floor as a monolithic cast-in-situ floor under the con- dition that a continuous peripheral tie is placed against the opening of the joints. For filling it up well, the maximum size of the aggregate of the cast-in-situ concrete should be limited with reference to the joint’s width.
Connections in hollow core floors can be required for a wide vari- ety of reasons. Most connection requirements are for localized forces that range from bracing a partition or beam to hanging a ceiling (Ne- gro and Toniolo, 2012).
3.2 Floor-to-Beam Connections
Figs. 5 and 6 show typical details of cast-in-situ connections be- tween floor elements and supporting beams. Proper links protrude from the upper side of the beam and overlap with those protruding from the floor elements. Longitudinal bars are added to improve the mutual anchorage. A concrete casting conglobates the steel links in the joint. This type of connection ensures the transmission of forces without sensible displacements.
Fig. 3 Floor-to-floor connection (Negro and Toniolo, 2012)
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3.3 Beam-to-Column Connections
Fig. 7 and 8 shows the end connection of a beam to a supporting column. In case (a) two dowels protrude from the top of the column and enter the sleeves inserted in the beam. The sleeves are filled with no-shrinkage mortar of an adequate strength to ensure the anchor- age of the dowels by bonding. The anchorage can also be ensured to provide the dowels with a cap fixed at the top with a screwed-in nut. In any case the sleeve should be filled in with mortar to avoid hammering under earthquake conditions. Case (b) refers to the same technology but with only one dowel. The use of two dowels in the transverse direction improves the resistance against overturning mo- ments. Due to the much lower degree of stability against overturning moments, the use of only one dowel is not recommended, especially with reference to uneven load conditions during construction stages.
Other possible solutions for beam-to-column connections are with mechanical couplers, the welding of a beam reinforcement to a steel plate in a column, etc.
3.4 Column-to-Column Connections
Precast columns can be designed as single-storey or multi-storey components. One possible solution of a connection between precast concrete columns is the use of grouted splice bars. In column splice
Fig. 4 Examples of floor-to-floor connections
Fig. 5 Floor-to-beam connection (Buettner and Becker, 1998)
Fig. 6 Floor-to-beam connection Fig. 7 Column-to-beam connections
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greater than the diameter of the projecting bars to ensure the complete encasing of the bar and also to avoid the forming of air pockets. Two types of ducts are typically used in column splices, i.e., proprietary steel sleeves and corrugated metal ducts (fib Bulletin, 2008).
Tab. 1: Possible size of a grout tube for different dowel bar diameters
Dowel size [mm] Grout tube size [mm]
16 40
20 40
25 60
32 60
40 80
3.5 Column-to-Foundation Connections
Fig. 11 shows two possible solutions for the connection of a col- umn to a supporting pocket foundation. For both solutions the col- umn is inserted within the pocket delimited by the four walls of the foundation. It is placed on a pad over the bottom footing slab. After the centering of the column, which is fixed with proper provisional bracing props, the bottom gap to the footing and the peripheral gap to the walls are filled with no-shrinkage mortar. The pocket should be large enough to enable a good compacted filling below and around
connections, the lower concrete column has projecting reinforcement bars that fit into sleeves in the upper element as shown in Fig. 9a and Fig. 10b. The upper element is lowered into position and temporarily braced during grouting. A levelling pad must be provided for a correct position in the vertical direction. The detailing and execution should ensure that there is no concentration of vertical stresses in the com- pleted connection caused by the levelling pad.
Another solution is also possible, i.e., the upper column has pro- jecting bars that fit into sleeves in the lower column, Fig. 9b. and Fig. 10a. The latter solution gives a very simple detailing of the column, since no ducts and no reinforcement splice need to be prepared. How- ever, the holes in the lower column must be protected from dirt and water, which could jeopardize the grouting.
The above connections may be considered as monolithic in a de- sign, provided the length of the anchorage (lap length) is sufficient and the bedding joint and grout sleeves are completely filled in. For both solutions the number of projecting bars is limited because of the limited space to place sleeves with appropriate spacing and a cover to the free edges. These types of connections require good workmanship. The projecting bar should be completely encased by grout, which is impossible to inspect. Accuracy in the erection can be difficult to achieve, and it is also difficult to adjust the connection afterwards. The choice of duct size is influenced by the likely size and location of the column bars that pass up through the column-to-column joint. In an ideal situation, the column bars would be centrally located in the ducts. However, in reality, the column bars can vary from the ideal positions, but should be within certain contractual or specified toler- ances. A duct size should be chosen to accommodate these tolerances as well as a recommended additional 10 mm clearance to the bar to allow the grout to flow between the duct wall and bar. Possible sizes of a grout tube for different dowel bar diameters are shown in Table 1. It is recommended that the inner diameter of a duct is at least 30 mm
Fig. 8 Column-to-beam connections a) beam on a corbel with dowel bars, b) top beam rebar welded with a steel plate casting into a column
Fig. 9 Column-to-column connection Fig. 10 Column-to-column connection a) grout tube on the bottom face of a column, b) grout tube on the top face of a column, c) grout tube on the top face of a column - double-height precast column on site
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the column. In the solution on the left the surfaces of the column and foundation within the joint are smooth. In the solution on the right, they are wrought with indentations or keys so to increase the adherence of the mortar. Pocket with wrought surfaces is on Fig. 12a, bottom side of the connecting column on Fig. 12b.
Other possible solutions for column-to-foundation connections are:
– the connection of a column to the foundation obtained by the anchorage of the reinforcing longitudinal bars protruding from the base of the column within the corrugated sleeves inserted in the foundation and filled with no-shrinkage mortar (Fig. 13a). Due to their size (80 to 100 mm in diameter), the sleeves jut out of the column profile in a wider dimension of the foundation el- ement so that the longitudinal bars can enter without deviating from their straight peripheral position in the column.
– the connection of a column to the foundation obtained through steel sockets inserted in the column base and bolted to the foun- dation (Fig. 13b). The sockets are anchored to the column by means of pairs of bars welded to them and spliced to the current longitudinal reinforcement by lapping. Other transverse links can be welded to the sockets to avoid their detaching laterally.
– the connection of a column to the foundation obtained through a steel plate (flange) attached at the column base and bolted to the foundation (Fig.…
Address
1 Dept. of Concrete Structures and Bridges, Faculty of Civil Engineering, Slovak University of Technology in Bratislava, Bratislava, Slovakia
* Corresponding author: [email protected]
Abstract
The structural integrity of precast concrete structures mainly depends on the connections between the precast structural elements. The purpose of a connection is to transfer loads, re- strain movement, and/or to provide stability to a component or an entire structure. Therefore, the design of connections is one of the most important aspects in the design of precast concrete structures. All the connections should be designed according to the valid codes. All precasters have developed connection details over the years that suit their particular production and erection preferences. It is common that the structural engineer shows the internal forces and connection locations, and the manufacturer’s engineering department provides the final design and details of the connections. This paper describes basic types of connections and joints used in precast concrete structures
Key words
1 INTRODUCTION
The most extensive expansion of precast concrete structures in Slovakia was in the second half of the 20th century. The develop- ment of precast skeleton construction in the former Czechoslovak Republic had begun around 1930. The beginnings of prefabrication were mainly related to the development of the intensively evolving industry. A similar situation occurred after the Second World War and was related to the repair of damaged industrial facilities. Af- ter 1960, the building of skeletons for non-residential constructions (hospitals, offices, shops, cultural facilities, etc.) began. In Czecho- slovakia approximately 30 prefabricated skeleton systems were de- veloped (Harvan, 2007).
Precast construction (also known as “prefabricated” construction) includes those buildings where the majority of the structural compo- nents are standardized and produced in plants in a location away from the construction and then transported to the site for assembly. These components are manufactured by industrial methods based on mass production in order to build a large number of buildings in a short time at a low cost (Sanghvi, 2015).
The most frequently used types of precast concrete elements are columns, beams, hollow-core slabs, pocket foundations, etc. Fig. 1 illustrates some of these units.
The use of precast products is regarded in the construction stan- dards as an advantage that helps to reduce some of the coefficients used in design, but there are other specific advantages of precasting that are easily noticed on site. Some of these specific advantages are:
– Control of shrinkage: the use of lower water-to-cement ratios in precast concrete may reduce shrinkage.
– Reduction of creep: higher strength concretes and proper curing may produce concrete with reduced creep characteristics.
– Quality control: the factory production of precast concrete ele- ments is inherently manufactured under the best conditions for the forming and placement of reinforcements and concrete, vi- brations and curing. Dimensional tolerance control is facilitated and more easily corrected.
– Timely availability: many standardized, mass-produced el- ements can be furnished to a construction site on very short order.
Vol. 28, 2020, No. 1, 49 – 56
DOI: 10.2478/sjce-2020-0007
© 2020 The Author(s). This is an open access article licensed under the Creative Commons Attribution- NonCommercial-NoDerivs License (http://creativecommons.Org/licenses/by-nc-nd/3.0/).
Slovak Journal of Civil Engineering
CONNECTIONS AND JOINTS IN PRECAST CONCRETE STRUCTURES...50
Vol. 28, 2020, No. 1, 49 – 56
– Reduction in site construction time: actual erection time and site construction time may be efficiently reduced. In favorable circumstances, a floor of a building may be erected and jointed in two days.
– Economy: the mass production of standardized elements reduc- es the cost of forms and manufacturing labor, and site labor can be kept to a minimum.
– Applicable for composite construction: precast members may frequently be combined with cast-in-place concrete to improve the monolithic behavior of the structure.
– Reduced maintenance: the higher quality materials and control used in precast concrete increase durability and reduce the need for maintenance.
2 REQUIREMENTS FOR CONNECTIONS
Strength - connection must resist the forces to which is subjected during its lifetime. Some of these forces such as those caused by dead and live gravity loads, wind, earth and water pressure are obvious. Some are not so apparent and will often be overlooked. These are forces caused by the restraint of changes in volume in the precast components and those required to maintain stability. Joint strength may be categorized by the type of forces that may be induced such as compressive, tensile, flexural and torsion forces.
Ductility - is the ability of the connection to undergo large de- formations without failure. A deformation is measured between the first yield and ultimate failure of the structural materials used in the connection. Ductility in a building frame is usually associated with the moment of resistance with the flexural tension capacity provided by the reinforcing bars, the crushing of the concrete, or the failure of the connectors or steel embedment in the concrete.
Change in volume – combined shortening due to creep, shrinkage and temperature reductions induces tensile stresses in precast com- ponents. The stresses must be accounted for in the connection design by either providing stress relief details in the undertrained joint or by providing additional reinforcing steel to resist the tensile forces in a restrained joint.
Durability – an exposed section in a connection should be period- ically inspected and maintained. Evidence of poor durability is usu- ally exhibited by the corrosion of any exposed steel elements or the cracking and spalling of the concrete. Components that are exposed to weather should have the ir steel components adequately encased in concrete by grout or by painted, galvanized or stainless-steel sections.
Fire resistance - connections which may be weakened by expo- sure to fire should be protected by concrete or grout or enclosed or sprayed with fire resistance materials. The connections should be pro- tected to the same degree as that required for the components and the building frame.
2.1 Compression Joints
Compressive forces can be transmitted between adjacent precast components by a direct bearing or through an intermediate medium such as in-situ mortar, fine concrete, bearing pads or other bearing elements. Direct contact between the elements should be used when a great degree of accuracy in manufacturing and erection needs to be achieved and when the bearing stresses are small. Cementitious ma- terials such as in-situ mortar, fine concrete or grouting are often used in the joints between load-bearing elements in columns and walls as well as for beam and floor elements. The nominal thickness is about 10 to 30 mm for mortar and grout and 30 to 50 mm for fine concrete. The bedding is usually without any reinforcing bars. The mode of failure is predicated by the crushing of the mortar or splitting of the precast components in contact with it. Although the mortar, grout or fine concrete is in a highly confined state under predominantly plane stress conditions and should achieve a compressive strength higher than fcu, a low design strength is normally used because the edges of the bedding tend to spall off. This will lead to a non-uniform stress distribution. The situation can be exacerbated by poor workmanship, unintentional eccentricity, spurious bending moments, and shear forc- es. Another fact which leads to a reduction of the joint strength is when there is a great difference in the elastic response between the bedded material and the precast concrete, which may result in local- ized contraction, lateral tensile stress, and splitting forces as shown in Fig. 2. This effect may become important when the joints thickness is greater than 50 mm.
The position of the support reaction must be accounted for eccen- tricities due to rotation and tolerances. The rules for this are given in Eurocode 2 (STN EN 1992-1-1, 2015). The basic dimensions of the bearing should be determined such that the stress under the bearing is
Force transfer through compression joint having elastic modules: a) less than the precast b) equal to the precast c) greater than the precast d) greater than the precast, but with reduced breadth
Fig. 2 Vertical transfer of compressive forces
Fig. 1 Typical precast units – columns, beams and a double-T beam
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Vol. 28, 2020, No. 1, 49 – 56
limited to that of the strength of the bearing material and that of the concrete in the connected components.
2.2 Tensile Joints
The connections with the tensile capacity are often part of the overall tying system that should provide structural integrity and pre- vent progressive collapse. Such a connection should be designed and detailed to have a ductile behavior. Premature brittle failures must be avoided and it should be possible to obtain a rupture of the ductile components of the connections (fib Bulletin, 2008). Tensile forces are transferred between the concrete by means of various types of steel connectors that are anchored to each side of the elements at the joint with a continuity achieved by the overlapping of steel bars, dowel ac- tion, bolting or welding. The tensile capacity of the connection can be determined by either the strength of the steel elements or by the anchorage capacity.
2.3 Shear Joints
Shear forces can be transferred between concrete elements by ad- hesion or friction at a joint interface, a shear-key effect at indented joint faces, the dowel action of transverse steel bars, pins and bolts, etc. Shear keys are generally formed by providing the precast mem- bers with indented joint faces. The shear keys work as mechanical locks, thereby preventing any significant slip along the joint. Shear keys must fulfill certain minimum requirements concerning the length, depth and inclination of the tooth. Such minimum require- ments are given in code and design rules (fib Bulletin, 2008).
A shear transfer by bond between precast and in-situ elements is possible, when the shear stress is low. It is not necessary to deliber- ately roughen the surface texture of precast units beyond the as-cast finish, which may be of a slip-forming, extrusion or tamped finish. Shear transfer by shear friction requires the presence of a permanent normal compressive force. The force may arise from permanent grav- ity loads, by prestressing or be artificially induced by reinforcement bars placed across the joints. Shear keys for the transfer of shear forc- es between elements are obtained by cast in-situ concrete or grout in joints between the elements which surface castellations. Under the action of a shear load, the shear keys act as mechanical locks that prevent significant slips at the interface.
2.4 Flexural and Torsional Joints.
Moment-resisting connections are mainly used to: – Stabilize and increase the stiffness of portal and skeletal frames,
– Reduce the depth of flexural frame members, – Distribute second order moments to beams and slabs and hence
reduce column moments, – Improve resistance to progressive collapse.
Moment-resisting connections are primarily used in column foundations and splices and at beam-column connections. Common methods to achieve moment-resisting connections are the grouting of a projecting reinforcement and the bolting or welding of anchored steel details. Moment-resisting connections should be proportioned such that ductile failures will occur and that the limiting strength of the connection is not governed by shear.
3 TYPE OF CONNECTIONS
3.1 Floor-to-Floor Connections
Fig. 3 and 4 shows a floor-to-floor connection made with a con- crete filling in a continuous joint between the adjacent elements, which is typical of some precast products such as hollow-core slabs. The joint has a proper shape to ensure when it is filled in, i.e., a good interlocking with the transmission of the vertical transverse shear forces. For the transmission of the horizontal longitudinal shear forces, the interface’s shear strength can be improved by providing the adjoining edges with vertical indentations. With reference to the diaphragmatic action, this type of connection ensures the same per- formance to the floor as a monolithic cast-in-situ floor under the con- dition that a continuous peripheral tie is placed against the opening of the joints. For filling it up well, the maximum size of the aggregate of the cast-in-situ concrete should be limited with reference to the joint’s width.
Connections in hollow core floors can be required for a wide vari- ety of reasons. Most connection requirements are for localized forces that range from bracing a partition or beam to hanging a ceiling (Ne- gro and Toniolo, 2012).
3.2 Floor-to-Beam Connections
Figs. 5 and 6 show typical details of cast-in-situ connections be- tween floor elements and supporting beams. Proper links protrude from the upper side of the beam and overlap with those protruding from the floor elements. Longitudinal bars are added to improve the mutual anchorage. A concrete casting conglobates the steel links in the joint. This type of connection ensures the transmission of forces without sensible displacements.
Fig. 3 Floor-to-floor connection (Negro and Toniolo, 2012)
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3.3 Beam-to-Column Connections
Fig. 7 and 8 shows the end connection of a beam to a supporting column. In case (a) two dowels protrude from the top of the column and enter the sleeves inserted in the beam. The sleeves are filled with no-shrinkage mortar of an adequate strength to ensure the anchor- age of the dowels by bonding. The anchorage can also be ensured to provide the dowels with a cap fixed at the top with a screwed-in nut. In any case the sleeve should be filled in with mortar to avoid hammering under earthquake conditions. Case (b) refers to the same technology but with only one dowel. The use of two dowels in the transverse direction improves the resistance against overturning mo- ments. Due to the much lower degree of stability against overturning moments, the use of only one dowel is not recommended, especially with reference to uneven load conditions during construction stages.
Other possible solutions for beam-to-column connections are with mechanical couplers, the welding of a beam reinforcement to a steel plate in a column, etc.
3.4 Column-to-Column Connections
Precast columns can be designed as single-storey or multi-storey components. One possible solution of a connection between precast concrete columns is the use of grouted splice bars. In column splice
Fig. 4 Examples of floor-to-floor connections
Fig. 5 Floor-to-beam connection (Buettner and Becker, 1998)
Fig. 6 Floor-to-beam connection Fig. 7 Column-to-beam connections
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greater than the diameter of the projecting bars to ensure the complete encasing of the bar and also to avoid the forming of air pockets. Two types of ducts are typically used in column splices, i.e., proprietary steel sleeves and corrugated metal ducts (fib Bulletin, 2008).
Tab. 1: Possible size of a grout tube for different dowel bar diameters
Dowel size [mm] Grout tube size [mm]
16 40
20 40
25 60
32 60
40 80
3.5 Column-to-Foundation Connections
Fig. 11 shows two possible solutions for the connection of a col- umn to a supporting pocket foundation. For both solutions the col- umn is inserted within the pocket delimited by the four walls of the foundation. It is placed on a pad over the bottom footing slab. After the centering of the column, which is fixed with proper provisional bracing props, the bottom gap to the footing and the peripheral gap to the walls are filled with no-shrinkage mortar. The pocket should be large enough to enable a good compacted filling below and around
connections, the lower concrete column has projecting reinforcement bars that fit into sleeves in the upper element as shown in Fig. 9a and Fig. 10b. The upper element is lowered into position and temporarily braced during grouting. A levelling pad must be provided for a correct position in the vertical direction. The detailing and execution should ensure that there is no concentration of vertical stresses in the com- pleted connection caused by the levelling pad.
Another solution is also possible, i.e., the upper column has pro- jecting bars that fit into sleeves in the lower column, Fig. 9b. and Fig. 10a. The latter solution gives a very simple detailing of the column, since no ducts and no reinforcement splice need to be prepared. How- ever, the holes in the lower column must be protected from dirt and water, which could jeopardize the grouting.
The above connections may be considered as monolithic in a de- sign, provided the length of the anchorage (lap length) is sufficient and the bedding joint and grout sleeves are completely filled in. For both solutions the number of projecting bars is limited because of the limited space to place sleeves with appropriate spacing and a cover to the free edges. These types of connections require good workmanship. The projecting bar should be completely encased by grout, which is impossible to inspect. Accuracy in the erection can be difficult to achieve, and it is also difficult to adjust the connection afterwards. The choice of duct size is influenced by the likely size and location of the column bars that pass up through the column-to-column joint. In an ideal situation, the column bars would be centrally located in the ducts. However, in reality, the column bars can vary from the ideal positions, but should be within certain contractual or specified toler- ances. A duct size should be chosen to accommodate these tolerances as well as a recommended additional 10 mm clearance to the bar to allow the grout to flow between the duct wall and bar. Possible sizes of a grout tube for different dowel bar diameters are shown in Table 1. It is recommended that the inner diameter of a duct is at least 30 mm
Fig. 8 Column-to-beam connections a) beam on a corbel with dowel bars, b) top beam rebar welded with a steel plate casting into a column
Fig. 9 Column-to-column connection Fig. 10 Column-to-column connection a) grout tube on the bottom face of a column, b) grout tube on the top face of a column, c) grout tube on the top face of a column - double-height precast column on site
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the column. In the solution on the left the surfaces of the column and foundation within the joint are smooth. In the solution on the right, they are wrought with indentations or keys so to increase the adherence of the mortar. Pocket with wrought surfaces is on Fig. 12a, bottom side of the connecting column on Fig. 12b.
Other possible solutions for column-to-foundation connections are:
– the connection of a column to the foundation obtained by the anchorage of the reinforcing longitudinal bars protruding from the base of the column within the corrugated sleeves inserted in the foundation and filled with no-shrinkage mortar (Fig. 13a). Due to their size (80 to 100 mm in diameter), the sleeves jut out of the column profile in a wider dimension of the foundation el- ement so that the longitudinal bars can enter without deviating from their straight peripheral position in the column.
– the connection of a column to the foundation obtained through steel sockets inserted in the column base and bolted to the foun- dation (Fig. 13b). The sockets are anchored to the column by means of pairs of bars welded to them and spliced to the current longitudinal reinforcement by lapping. Other transverse links can be welded to the sockets to avoid their detaching laterally.
– the connection of a column to the foundation obtained through a steel plate (flange) attached at the column base and bolted to the foundation (Fig.…
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