It is very important to consider the effect of earthquake in proportioning of columns and their reinforcement. The design is based on IS: 13920:1993. This code is applicable to members that have factored axial stress greater than 0.08 $f_{ck}$ under the effect of earthquake loads. Design as flexural member, if the factored axial stress is less than the specified limit.
Strong column – weak beam concept
The main aim of the strong column weak beam strength concept in codes is to reduce the formation of mechanisms. The formation of plastic hinges in beams helps to build the most desired and suitable energy dissipating mechanism for structure. Strong column weak beam means that the flexural strength in column end should be greater than that of beam end. If a beam fails it will only affect that particular story. But if a column fails then the complete structure may fail. It is always preferable that hinges form in beams before they are formed in columns. At the plastic hinge locations, it is very important to provide sufficient confining reinforcement such that the required ductility is achieved.
Detailing of Longitudinal reinforcement
The requirements for longitudinal reinforcement as per IS 13920 are as follows:
- In a frame resisting earthquake forces, at a joint the sum of moment of resistance of the column should be at least 1.1 times the sum of moment of resistance of the beam along each principal plane of the joint. The moment of resistance should be summed in such a way that the column moments oppose the beam moments.
- IS 13920 requires that at least one intermediate bar be provided between the corner bars along each column face. Require intermediate bars to ensure the integrity of the beam-column joint and increase confinement to the column core.
- Seismic moments are the maximum in columns just above and just below the beam. Hence do not change reinforcement in these locations. Since the seismic moments are minimum away from the ends of columns, it is preferable to provide lap slices only in the central half of the columns. The splicing should not extend into the plastic hinge regions.
Detailing of Transverse reinforcement
The requirements for transverse reinforcement as per IS 13920 are as follows:
- A minimum bar diameter is specified for transverse reinforcement in columns to ensure minimum ductility and to prevent local buckling of longitudinal bars. As per IS 13920, the minimum diameter is 8 mm. As per IS 456 it is 6 mm or one-fourth the largest longitudinal bar. For columns with longitudinal bar diameter larger than 25 mm, the minimum diameter of transverse reinforcement is specified as 10 mm.
- The parallel legs of a rectangular hoop should be less than 300 mm c/c. If the length of any side of the hoop exceeds 300 mm, it is necessary to provide a cross-tie. It is important to ensure that the hooks engage the peripheral longitudinal bars.
- Closer spacing of hoops is desirable to ensure better seismic performance.
- Do not assume ties, lap splice in the cover concrete, to make any contribution to strength and stability.
Special confining reinforcement for earthquake design
Ductile response requires that members yield in flexure and shear failure is avoided. Shear failure, especially in columns, is relatively brittle and can lead to rapid loss of lateral strength and axial load carrying capacity. Column shear failure is the most frequently cited cause of concrete building failure and collapse during the past earthquake. The shear strength of concrete reduces considerably in plastic hinge locations. It is often subjected to multiple stress reversals, especially if the axial compressive loads are low. In order to have the desired ductility and rotation capacity, special confining reinforcement has to be provided.
To understand the fundamental concepts of column – check this post.
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