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Shear keys are designed and provided in various structures to provide resistance against lateral loads like earthquake loads and sliding forces in various structure such as bridges, retaining walls, basement of residential buildings, precast buildings and culverts, masonry wall in seismic regions, and steel columns.
Shear key can be constructed from concrete as in precast constructions, residential building basement or manufactured from steel such as in the case of steel column base plate, and sometimes steel reinforcement is used to play shear key role for instance between basement wall and footing.
They may play sacrificial roles in structure for example in bridges or prevent movement of different components of structure as in precast structures. To sum up, the purpose of shear keys is to improve the stability of structures.
Purpose of Shear Key
Shear keys provide resistance against lateral forces such as earth and water pressures, earthquakes, and wind.
Shear Key in Various Structures
Different types of structures need the application of shear keys to resist certain forces such as shear forces, lateral forces, and earth pressure. The function and position of shear key in various structures are discussed and presented below:
1. Bridge Structure
Shear keys are placed in the abutments of small to medium-sized bridge structure to provide lateral transverse support to the superstructure under lateral loads. The shear keys in bridge structure play a significant role in resisting seismic loads.
During a strong earthquake, shear keys should function as a sacrificial structural element to prevent the transmission of large seismic forces to the abutment piles. There are two types of shear keys in bridge structure: exterior and interior shear keys. The former has the demerit of the ease of inspection and repair.
2. Retaining Walls
Shear keys are provided at the base of retaining walls to increase its sliding resistance. Practically, it is convenient to place the shear key beneath the stem of the retaining wall because it simplifies the placement of reinforcement that can be extended from the stem directly into the stem.
The shear key should be proportioned such that its width is at least twice its depth to develop the strength of the key in shear instead of flexural, as per ACI 318-17. The shear key is constructed 0.91m from the toe of the footing, with width and depth of 0.508m and 0.381m, respectively.
3. Footing-basement Wall
Shear keys are provided at the joint between footing and basement wall. The shear key is cast as part of the wall extending into the keyway or groove formed in the footing. Different forms of shear key at the joint between footing and basement wall are shown in Fig-3.
Sometimes, vertical reinforcing bars or a combination of shear key and bars are provided as shown in the Fig. An upturned shear key, cast as part of the footing, is sometimes used but usually not in residential work. Reinforcement bars are provided to resist bending, but bars in excess of that needed for bending can withstand shear, that is why bars can be employed as a replacement to the shear key.
4. Steel Column Base and Footing Joint
A special shear key, for instance, a block of I-stub or T-section or steel pad welded onto the bottom of the base plate is used to resist shear force at the base of the column.
5. Connections of Precast Concrete Elements
Shear keys are used in precast constructions to connect two separate precast components and to increase the shear resistivity of the joint surfaces.
6. Masonry Wall
In seismic regions, masonry blocks are equipped with shear keys during manufacturing to resist out-of-plane forces between adjacent blocks during earthquakes.