A reliable and economical method to join prefabricated members is the most serious problem faced in the precast industry. Connection are locations of high-stress concentrations and are weak points in the structural system; they have to withstand high forces and displacement when subjected to lateral loads transmitting joints. “Joints” can be defined as the point of action of forces that takes place at the interface between two or more structural elements. While “connection” can be defined as an assembly of joints that transmit forces and moments at the interface of structural members.
In general, joints can be classified into three main systems:
a) Typical joints system -dry joints (angles, plates, channel bars, anchors, fasteners, bolts, dowels bars).
b) Emulative joints systems -wet joints (rebar splices and cast-in-situ concrete).
c) Mechanical joints system- dry joints (bolted flanges or other steel fittings).
Most of the damage and collapse of precast structures were mainly due to the failure of the connections. Considering the performance of these connections, system, as it provide excellent performance against lateral loads. These types of connections tend to behave monolithically, provide continuity and add to the structural integrity of the system.
Details of Connections
The design of connections should be carried out to ensure that the connections are able to resist the applied structural action and meet the functional requirements.
Column to Foundation Connections
The socketed connection will provide a fixed base connection to the precast column, which is particularly useful in low rise precast building. The cantilever action of the column provides the lateral stability for the building, thus reducing the amount of horizontal and vertical bracing required. in this connection, the overturning moments in the column will be resisted through proper embedment of the column into the concrete socket.
Bolted or Based Plate Connections
This is a bolted connection between a column and footing and is designed to resist tension, compression, and shear. It is also a good practice to set the base plate overall dimensions slightly less than column dimensions to ensure proper placement in the formwork. Non-shrink grout should be used to avoid any voids underneath the plate. It is highly recommended to use closed ties around the anchor bolts.
Mechanical Splice Connections
Mechanical splice connection is a typical moment connection to resist overturning moments and to resist cantilever moment. These types of connections are used where,(1)Columns that are part of the column-beam framing system that need to be fixed at the bottom, (2) Moment resistant column splices and (3) Cantilever columns and wall panels.
This type of connection can be treated in par with in situ connections since the bedding joint and grout sleeves are completely filled with grout. In this connection, the dowel bars can be projected from the base or can be projected downward fro the column.
Beams to Column Connections
The efficiency of precast concrete structures in resisting gravity or lateral loads relies on the behavior of joining the system, including beam-column connections. The connection configuration and response including strength, rotations.siffness and ductility affect building frames in several ways.
In the case of the dry connection between beam and column, they can be widely classified into two types. In type I the vertical members are continuous and in type 2, the vertical members are discontinuous. Type 1 can be further classified into (1)hidden connection and(2) visible connections. Type 2 can be grouped as (a) Beam ends resting on top of columns and (b) Continuous beam anchored at the head of the beams.
This connection is generally designed as simply supported connections and have to carry enormous shear force. The beam connection may have to accommodate one column and one column ane /two/ three/ beams.