Structural columns play a critical role in maintaining the stability and load-bearing capacity of buildings. During earthquakes, columns—especially their top and bottom regions—are highly susceptible to shear forces. These forces can cause severe damage, leading to loss of strength and in many cases, inclination of the column under combined seismic and vertical loads (see Figure 1). To restore the structural integrity of such columns, a well-planned retrofitting process is essential.
This article presents a detailed step-by-step method for retrofitting reinforced concrete columns that have been inclined and damaged at the top due to seismic activity.
Understanding the Damage
Earthquake-induced forces can deform and crack reinforced concrete columns, especially near the joints where beams and columns connect. If not addressed promptly, this damage can compromise the entire structural system. Retrofitting is aimed at restoring the original load-bearing capacity and improving future seismic performance.
Retrofitting Process Overview
The retrofitting of inclined columns involves several stages:
- Estimating the structural loads
- Temporarily relieving loads through jacking
- Removing damaged materials
- Installing new reinforcement
- Reconstructing the damaged area with concrete
- Properly curing the new concrete
Step-by-Step Retrofitting Procedure
1. Jacking Up Deflected Beams
The first step is to relieve some of the load on the damaged column by jacking up the beams it supports. Adjustable jack posts or other mechanical lifting devices are used to raise the beam to its original level. This helps stabilize the structure temporarily and creates a safe working environment (Figure 2).
2. Estimating Load Requirements
Before installation, it’s important to estimate the loads that each jack post will bear. This allows for the correct number of jacks to be used and ensures that the lifting devices are not overloaded. Manufacturers typically provide the lifting capacity of each jack post.
3. Proper Jack Post Placement
Jack posts should be positioned so they effectively lift the beam without obstructing the upcoming repair tasks. Adequate clearance must be maintained for concrete removal, reinforcement installation, and formwork setup.
4. Restoring the Beam Level
Once the jack posts are correctly placed, the beam is carefully lifted to the required level (Figure 3). In cases where lifting proves difficult due to severe deformation, some concrete may be removed, and damaged reinforcement cut to facilitate the process.
5. Removing Damaged Concrete
With the beam elevated, damaged concrete from the top portion of the column and the adjoining part of the beam is removed. This exposes the existing reinforcement and allows for a better joint connection (Figure 4).
6. Installing New Reinforcement
New longitudinal and transverse reinforcement is placed in the column. The new steel bars must be properly spliced with the existing bars, typically requiring a splice length of at least 40 times the bar diameter (40d). Additional stirrups should be added or rearranged as necessary to improve confinement (Figure 5).
7. Formwork Assembly and Concrete Access
Next, formwork is installed around the prepared repair zone. In situations where concrete pouring is difficult, a hole may be created in the floor slab above to allow for easier concrete placement (Figure 6).
8. Concrete Placement and Consolidation
High-quality concrete is poured into the formwork and properly consolidated to eliminate voids. Proper compaction ensures that the new concrete bonds well with the old structure and fully encapsulates the reinforcement.
9. Formwork Removal and Curing
After adequate setting time, the formwork is removed, and the concrete is cured diligently to reach the required strength. Proper curing is vital to prevent cracking and ensure durability.
Adapting the Process for Bottom-Damaged Columns
If the damage is located at the bottom of the column rather than the top, a similar retrofitting approach is followed. However, special attention must be given to extending the column reinforcement into the footing and ensuring correct detailing at the base.
Conclusion
Retrofitting inclined and damaged columns after an earthquake is crucial for structural safety and performance. The outlined procedure ensures that the damaged column regains its original load-carrying capacity and can withstand future seismic events. While each site may require minor adjustments to the process, the steps provided here serve as a reliable foundation for effective column repair.