Change of Use to Archive Storage in a 1980s RC Frame
A client wants to convert the first floor of a three-storey 1985 reinforced concrete office building into compact archive storage. The floor is carried by simply supported beams at 4.0 m centres, spanning 6.0 m onto 400 mm square columns. Original drawings show 300 × 600 mm beams with three T20 bottom bars, but the shear link detail is illegible and there are no as-built records. The existing first-floor imposed load was 3.0 kN/m²; the archive will impose 15.0 kN/m². A walkover survey notes hairline flexural cracks up to 0.2 mm at midspan of several beams and isolated diagonal cracks near the supports. Concrete cover is approximately 25 mm. The client wants written approval within one week, before furniture installation, and argues that the building has performed without distress for 35 years. No material testing has been carried out. You are the engineer asked to sign the assessment.
1. Can you approve the proposed archive load on the basis of the original drawings and visual survey? Set out your reasoning and state what you would do first.
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No. The drawings establish geometry but not the resistances that govern the check: link spacing, concrete strength, bar size and corrosion loss. Thirty-five years of satisfactory performance only demonstrates adequacy for the original 3.0 kN/m²; it says nothing about 15.0 kN/m², which is a five-fold increase. The diagonal cracks are particularly relevant because they may indicate existing shear distress. My first actions would be: (1) issue a written interim response stating that approval cannot be given and that no archive loading should be placed; (2) commission a cover meter survey and expose at least three beams to confirm bar size, link spacing and bearing; (3) take cores for compressive strength and carbonation depth; (4) carry out a preliminary check using conservative assumptions. If the preliminary check shows a large shortfall, as it typically would, this justifies the cost of investigation and avoids a false approval.
2. Assume sampling confirms 3T20 bottom bars, T10 two-leg links at 200 mm, fck = 30 MPa and fy = 460 MPa. Check the beam at ultimate limit state for the proposed load and state your conclusion.
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Load take-off: self weight = 0.30 × 0.60 × 24 = 4.3 kN/m. Slab and finishes, say 3.5 kN/m² over 4.0 m = 14.0 kN/m. Archive = 15.0 × 4.0 = 60.0 kN/m. Ultimate load = 1.35(18.3) + 1.5(60) = 24.7 + 90.0 = 114.7 kN/m. M_Ed = 114.7 × 6²/8 = 516 kNm; V_Ed = 114.7 × 6/2 = 344 kN. Capacity with 3T20, fy = 460, fck = 30: A_s = 942 mm², d ≈ 550 mm. x = A_s f_yd/(0.8 b f_cd) with f_yd = 400 MPa, f_cd = 17 MPa gives x ≈ 92 mm, z ≈ 513 mm, M_Rd ≈ 942 × 400 × 513 = 193 kNm. This is far below 516 kNm. Shear: v_Ed = 344,000/(300 × 550) = 2.08 MPa, which exceeds the concrete strut limit and the link capacity. Conclusion: the beam fails both flexure and shear by a wide margin. Approval is not possible.
3. The client cannot reduce the load. Recommend a strengthening scheme and explain the key construction and durability issues.
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Options: (a) add two new columns at the third points, reducing the span to 2.0 m; M falls to about 57 kNm and shear to about 115 kN, both within the existing section; (b) external post-tensioning or bonded CFRP plates; (c) section enlargement. Adding columns is usually the most reliable because it changes the load path and avoids relying on bond and fire performance of adhesives. It requires new pad foundations, checking the slab for point loads and avoiding services. If headroom or layout rules out columns, CFRP flexural strengthening to TR55 or ACI 440.2R must be combined with shear strengthening, because the existing links are already overstressed; the design must include anchorage, substrate preparation, fire protection and a durability review. Whatever scheme is chosen, the works need a temporary load limit, a method statement and post-strengthening verification.