Steel Mezzanine Substitution in a 1975 Concrete Warehouse
An 8 m × 15 m steel mezzanine is being inserted into a 1975 reinforced-concrete warehouse in Shoubra, Cairo, to create an archive and office. The new floor is at +4.5 m. Steel beams span 8 m at 3.0 m centres, are simply supported on new 200 × 300 mm bearing plates fixed to the existing 400 × 400 mm RC columns, and carry a 150 mm composite slab. The specified beam was UB 457 × 191 × 82 in S355, but the fabricator has delivered S275 sections and says the substitution is harmless. Core tests give fcu = 22 MPa, not the 30 MPa on the 1975 drawings, and the column links are unknown. Service loads are 3.0 kPa superimposed dead and 4.0 kPa live. The reaction on each bearing plate is 180 kN. The building authority requires 60 minutes' fire resistance and the mezzanine is in a protected escape route.
1. Check the delivered S275 beam for bending, shear and deflection at the 8 m span with the stated loads. Is the fabricator correct?
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Loading. Slab 0.15 × 25 = 3.75 kPa, plus 3.0 kPa SDL and 4.0 kPa LL. Service w = 10.75 × 3.0 = 32.3 kN/m. Ultimate w = [1.4(3.75 + 3.0) + 1.6(4.0)] × 3.0 = (9.45 + 6.4) × 3.0 = 47.6 kN/m.
Bending. M = 47.6 × 8²/8 = 381 kNm. For UB 457 × 191 × 82, Wpl,y ≈ 1830 cm³; with S275, Mpl = 1830 × 10³ × 275/10⁶ ≈ 503 kNm. Utilization 381/503 ≈ 0.76, so bending is satisfactory.
Shear. V = 47.6 × 4 = 190 kN. With Av ≈ 460 × 9.9 = 4550 mm² and fy = 275 MPa, Vpl ≈ 4550 × 275/√3 ≈ 722 kN. Shear is not critical.
Deflection. Iy ≈ 37,100 cm⁴. δ = 5wL⁴/(384EI) = 5 × 32,300 × 4096/(384 × 210 × 10⁹ × 3.71 × 10⁻⁴) ≈ 22 mm, i.e. L/360. That passes L/250 but is marginal against L/360 and fails a stricter L/500 limit often imposed for archives.
Conclusion. The fabricator is broadly right on bending and shear, but not on 'harmless': the connection design, bearing plates and fire protection may have been sized for S355, and the deflection limit must be agreed in writing before acceptance.
2. Check bearing on the existing concrete and explain what the low core strength means for the column and the bearing detail.
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Bearing stress. 180 kN over 200 × 300 mm = 0.06 m² gives 3.0 MPa. With fcu = 22 MPa and a design bearing resistance of the order of 0.6 fcu/γc, capacity is roughly 9–13 MPa, so direct crushing is not the problem.
What is the problem. The plate sits on a 400 mm column with an eccentric reaction, so the column must also carry the added axial load and the local moment. The core result of 22 MPa is about 73 per cent of the design value; that does not invalidate the column automatically, but the axial capacity, the slenderness and the confinement at the plate must be recalculated with the measured strength, and the links must be established by a cover-meter survey or a local opening. If the links are sparse, the plate should be enlarged and a confining cage or a steel collar provided around the column at the bearing zone.
Bearing stress should also be checked at the underside of the plate for the concrete's actual strength rather than the original 30 MPa, and a grout pad of adequate strength, not dry pack, should be specified. The 180 kN reaction must be confirmed as a service or factored value; if it is already factored, the bearing check changes.
3. Outline the connection and fire-protection requirements that the substitution and the escape-route location impose.
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Connection. A simply supported beam needs a connection that carries the 190 kN ultimate shear with adequate bolt bearing and block shear, and enough rotational ductility not to transfer unintended moment into the concrete column. Use a flexible end plate or web cleats with grade 8.8 bolts; check bolt group eccentricity, plate thickness and weld throat against the S275 member, since the connection may have been governed by the S355 beam's thinner weld capacity. Anchorage to the concrete must be designed for tension and shear from the eccentric reaction, using chemical anchors with a documented fire rating.
Fire. The mezzanine is in a protected escape route, so 60 minutes is the minimum and the authority may require more. The section factor for a 457 × 191 × 82 partially exposed beam is roughly 130–150 m⁻¹; at that value an intumescent coating needs a substantial thickness and its manufacturer's assessment for the actual section factor and fire regime, or a 25 mm board system. The concrete columns and the underside of the composite slab also need protection. The fire strategy must consider the escape route beneath the mezzanine, and the protection must be inspected and recorded before the floor is occupied.