High-Rise Residential Column Retrofit in Seismic Zone

Scenario

A 28-story reinforced concrete residential tower in Istanbul, Turkey — located in seismic zone 1 (high-risk) — required structural retrofitting of interior columns on Levels 5–7 after a post-construction review revealed under-designed axial-moment combinations due to revised occupancy loads and updated Turkish Earthquake Code (TBEC-2018) requirements. Constraints included minimal disruption to occupied units, strict 35 mm minimum concrete cover for corrosion resistance in humid coastal air, and no column enlargement permitted due to tight MEP chases.

Given Data

• Column width: 300 mm
• Column height: 300 mm
• Rebar diameter: 25 mm
• Number of rebars: 8
• Concrete strength: 35 MPa (upgraded from original 25 MPa to improve confinement and ductility)
• Steel yield strength: 420 MPa (S420 ribbed bars per TS EN 10080)
• Axial load: 1420 kN (factored gravity + seismic PΔ effect)
• Moment: 62 kNm (major-axis bending from lateral drift)

Calculation

Using the software’s ACI 318-19-based biaxial interaction model (simplified for uniaxial check with balanced strain assumption):

1. Gross section area: $A_g = 300 \times 300 = 90{,}000\ \text{mm}^2$
2. Reinforcement area: $A_s = 8 \times \frac{\pi}{4} \times 25^2 = 3927\ \text{mm}^2$
3. Nominal axial capacity (pure compression, $\phi = 0.65$):
   $P_n = 0.85 f'_c (A_g - A_s) + f_y A_s = 0.85(35)(90{,}000 - 3927) + 420(3927) = 2{,}584{,}000\ \text{N} = 2584\ \text{kN}$
   → $\phi P_n = 0.65 \times 2584 = 1679\ \text{kN}$
4. Moment capacity at $P_u = 1420\ \text{kN}$ is interpolated from the software’s M–P interaction diagram (generated via strain-compatibility with $\varepsilon_c = 0.003$, $\varepsilon_s = f_y/E_s$ at yield), yielding $\phi M_n = 78.3\ \text{kNm}$.
5. Interaction ratio:
   $\frac{P_u}{\phi P_n} = \frac{1420}{1679} = 0.845$;
   $\frac{M_u}{\phi M_n} = \frac{62}{78.3} = 0.792$
   → Governing interaction ratio = $\max\left(\frac{P_u}{\phi P_n},\ \frac{M_u}{\phi M_n}\right) = 0.845$ (per simplified linear interaction check; software uses more refined Bresler reciprocal method and returns 0.87).

Result and Decision

The software output: axial_capacity = 1679.23 kN, moment_capacity = 78.31 kNm, interaction_ratio = 0.87. Since 0.87 < 1.0, the retrofitted section satisfies ultimate limit state requirements without geometry change. The design team approved the 35 MPa concrete upgrade with existing 8–25 mm bars and increased stirrup spacing confinement (T10@100 mm c/c within plastic hinge zones) — avoiding costly carbon-fiber wrapping or jacketing.

Lesson

Upgrading concrete strength alone — while maintaining rebar layout — can efficiently close interaction ratio gaps in constrained retrofits, provided bond development and cover adequacy are re-verified; here, the 35 MPa mix required adjusted curing and supplementary cementitious materials to maintain workability without increasing water-cement ratio.