The precast concrete walls gain higher in-plane stiffness providing higher stability for the precast buildings. They gain larger in-plane stiffness due to the combination of flexural and shear stiffness. Based on the geometry, the precast walls can be either short or slender.
When considering slender walls for construction, it is necessary to check them for global in-plane buckling. These are due to the flexibility in foundations or other second order effects.
The buckling due to out of plane stiffness is avoided. But the check for the same have to be carried out for at least single-story height.
The slender precast concrete walls dominate in their flexural stiffness and they are sometimes called shear walls. This is because they have higher ability to resist the horizontal loads.
Structural Actions of Precast Walls
A proper interaction between the precast floors and the stabilizing shear wall is very much necessary to promote overall structural behavior and stabilization of the structure. Proper interaction will be facilitated only if the connection between the floors and the precast walls are properly connected.
The interaction of wall elements helps in bringing diaphragm action enabling the transfer to shear and tensile joints across the joints. This highlights the importance of proper joint design.
The strut and the tie models are used to understand the equilibrium system of the whole arrangement. The joint design is carried out based on the desired structural behavior.
The precast concrete walls are subjected to higher axial loads and stresses when compared to precast floor diaphragms. An important property of shear walls is the stiffness. Any formation of crack in the wall will affect the stiffness of the wall structure.
Hence, design have to be carried out in such a way that no tensile stresses are allowed in the horizontal joints of the wall or to check that only lesser tensile stress will occur. Having more vertical loads on the walls is the most economical solution to reduce the tensile stresses on the walls.
Fig.1. Behavior of Shear wall under the action of horizontal loads and vertical loads.
The elements are best recommended to be connected to the foundation, to the adjacent floor diaphragms as well as to each other in order to facilitate good interaction.
The walls employed in the precast shafts can be designed like an individual shear wall or they can be connected along the vertical joints. This will hence form a closed or an open cross section. This will make the shaft act as a single and a stabilized system.
Incorporation of interaction of walls in the system requires connections along the vertical joints. This is to resist the shear forces.
Fig.2: Stabilizing Units with Interacting Precast Wall Units
Connection Between the Precast Wall Units
Most common choice of connection is welding and concrete filled joints. Another connection method is by the interlock elements. These interlocking connections tend to provide higher shear capacity.
There is deformation of connection in the vertical joint when these are loaded in shear. This is mainly dependent on the stress versus slip characteristics. This deformation will hence affect the whole structural response.
Based on how effective the connections are to resist the shear deformation, the wall element interaction can be divided into partial or full.
Fig.3: Interlocking of Elements for Connection between the Walls
There are two connections for precast concrete wall:
- Cast in situ concrete joints
- Welded connection
Cast-in-Situ Concrete Joints
The cast in situ concrete connection have transverse reinforcement which provides continuous shear transfer along the joint. The concrete filled can be either plain or castellated.
There can be well distributed or concentrated transverse reinforcement in the horizontal joints. These are more stiffer and stronger compared to welded connections when their difficulty to employ at corners is neglected.
The welded connection undergoes intermittent transfer of shear force. They come into action once they are mounted together as a stabilizing system. These connection demand fire resistance, higher durability, and proper finish.
Painting or grouting will help as protection for welded connections. These are less stiff compared to concrete or interlocking connections.
Use of standing multi-storey wall elements or laying of story height wall elements help in establishing the facade elements. These elements can carry the vertical loads or can behave as a non bearing cladding.
Both the mentioned types can be either designed as shear wall or not as shear walls.
To act as shear wall, the connection has to be joined such a way to transfer the shear force. If no transferring has to be carried out, just a non-structural filling must be carried out as a means for climatic protection.
Fig.3: Alternative arrangements for the Facade elements (a) Arrangement of standing multi-storey wall elements (b) Laying of single story wall elements (c) Examples
When a structure has, both stabilizing and unstabilizing walls, the appropriate study of stiffness of the building have to be studied. Attraction of load by a unstabilizing walls will bring unfavorable outputs and structural behavior.