Retrofit of Historic Masonry School Building with RC Jacketing

Scenario

A 1928 unreinforced masonry (URM) elementary school in Lisbon, Portugal, is being seismically upgraded. Due to heritage preservation rules, exterior walls and floor plans cannot be altered. The retrofit strategy involves adding reinforced concrete jackets to existing brick columns — requiring minimal added footprint. Critical constraint: jacket thickness cannot exceed 75 mm per face (i.e., max 150 mm total width increase), limiting effective column depth. Also, existing foundations limit allowable axial load increase to ≤15% — thus new jacket must resist nearly all lateral moment while sharing axial load.

Given Data

• Axial Load (shared with existing column): 1,240 kN (factored, post-jacket load path redistribution)
• Bending Moment (entirely resisted by jacket): 310 kN·m (due to infill frame action and base shear transfer)
• Concrete Strength: 25 MPa (self-consolidating, low-heat mix compatible with bond to aged brick)
• Steel Yield Strength: 450 MPa (lower-strength bars selected for improved bond and ductility compatibility with historic substrate)
• Column Width (existing + jacket): 350 mm (original 200 mm brick + 75 mm jacket each side)
• Column Depth (existing + jacket): 450 mm (original 300 mm brick + 75 mm jacket each side)

Calculation

Using the Reinforced Concrete Column Design Software:

1. Inputs: axial_load=1240, moment=310, concrete_strength=25, steel_yield_strength=450, column_width=350, column_depth=450.
2. Software performs confined concrete analysis (using Mander model for jacketed section) and computes required steel area considering reduced effective depth due to asymmetrical jacket geometry and bond-slip assumptions.
3. Iterative check confirms that 350×450 mm jacket requires required_rebar_area = 3,920 mm² — achievable with six 28 mm bars (As = 3,695 mm²) plus four 16 mm corner bars (As = 804 mm²), totaling 4,499 mm² (>3,920 mm²). Minimum cover (30 mm) and bar spacing (≥40 mm) comply with EN 1992-1-1.
4. Output confirms column_size = 350 mm x 450 mm is viable — no enlargement beyond heritage limits.

Result and Decision

The 350×450 mm RC jacket with 10 total bars (6×28 mm + 4×16 mm) and C25/30 concrete was approved by the Portuguese DGPC (Directorate-General for Cultural Heritage). Installation used mechanical anchors and polymer-modified grout for brick–concrete interface bonding. Post-installation load testing validated predicted stiffness and moment capacity within ±6%.

Lesson

In retrofit applications, yield strength selection is as critical as geometry: 450 MPa steel provided optimal balance of bond performance, ductility, and constructability on aged substrates — whereas 500 MPa would have risked brittle interface failure despite higher strength.