In reinforced concrete columns, vertical reinforcement bars often need to be adjusted at certain stories to align with the smaller column section above. This adjustment is achieved through offset bent longitudinal reinforcement, which involves bending the vertical bars at a particular level so that they fit within the smaller cross-section of the upper column, as illustrated in Fig. 1, A. This technique ensures continuity of reinforcement while accommodating changes in column dimensions between stories.
Technical Specifications and Requirements
When bending vertical reinforcement bars, it is critical to control the slope of the bent portion. The maximum slope of the bent bars should not exceed 1 in 6. This limitation helps avoid excessive stress concentrations and ensures that the bars can effectively carry axial and bending forces.
Additionally, extra ties (or lateral reinforcement) must be provided in the area close to the bent bars. These ties serve to resist the forces generated due to the change in bar direction, preventing concrete cracking and ensuring confinement around the bent reinforcement.
Alternative to Offset Bent Reinforcement
Offset bending is suitable only when the horizontal offset between the faces of the columns above and below is less than 75 mm. If the offset is 75 mm or more, offset bending is not recommended. Instead, the vertical bars of the column below are terminated at the floor slab, and splicing with separate dowels is used to connect with the bars of the column above, as shown in Fig. 1, B. This method provides a more reliable connection when significant offsets exist.
Detailed Requirements According to ACI 318-14
The American Concrete Institute (ACI) 318-14 code provides guidelines to ensure safe and effective detailing of offset bent bars:
- The maximum slope of the inclined portion of an offset bent bar must not exceed 1 in 6.
- The portions of the bar above and below the bend must be parallel to the column axis to maintain load transfer efficiency.
- Additional lateral ties, spirals, or elements of the floor construction must be provided close to the bend to counteract the transverse forces generated by the change in bar direction.
- These horizontal supports must be designed to resist 1.5 times the horizontal component of the force in the inclined portion of the bars.
- When lateral ties or spirals are used, they should be placed no more than 150 mm away from the points of bend.
- Bars must be bent before placement in the formwork to avoid damage or improper positioning.
Use of Dowels for Splicing
When the offset between the column faces exceeds 75 mm, or when construction delays or joining of separate structural units occur, dowels are used to splice vertical bars.
- The dowel bars must match the size and grade of the main bars being spliced.
- They must have sufficient length to ensure proper anchorage and transfer of stresses.
- Similar to offset bent bars, additional ties must be provided near the splice ends to confine the concrete in the highly stressed regions and maintain structural integrity.
Conclusion
Proper detailing of offset bent longitudinal reinforcement is essential for maintaining the continuity and strength of vertical reinforcement in columns with varying cross-sections. By adhering to slope limits, providing adequate lateral support, and choosing the correct connection method—whether offset bending or dowel splicing—engineers can ensure the safety and durability of the structure. These practices, guided by standards such as ACI 318-14, help prevent cracking and failure, maintaining the integrity of reinforced concrete columns throughout the building’s height.