Rooftop Plant Room on a 1960s Concrete Frame

The original 30 MPa should not be used. Core strength is not cube strength: it is affected by drilling damage, moisture, height-to-diameter ratio, and the fact that cores are cut from the actual member rather than cast and cured under standard conditions. A 75 mm core is also small, so it is more variable and more damaged by extraction than a 100 mm core.

The three results give a mean of 22.3 MPa with a range of 21–24 MPa. With only three samples, a characteristic value should be based on a lower-bound or statistical estimate, not the mean. Even after applying a core-to-cube adjustment, the assessed strength is likely to be well below 30 MPa. Using 30 MPa would overstate column capacity and could make an unsafe scheme look acceptable.

Before calculation, require:

1. At least six to nine cores, including columns and beams, with a documented orientation and location plan.
2. Cover-meter survey, carbonation depth, and chloride testing, especially near the 0.4 mm crack.
3. Crack mapping and monitoring to establish whether the crack is dormant or active.
4. A lower-bound strength, plus a load test or further investigation if the calculated utilisation is close to the limit.

Three weeks is not a credible programme for that work.

The average pressure looks modest: 18 t is about 176 kN, and over 6 m × 8 m = 48 m² that is about 3.7 kPa, with the 1.2 t frame adding about 0.25 kPa. That average is not the governing effect.

The plant is supported at discrete points, so the real demand is a set of concentrated loads, possibly eccentric. These must be traced through the spreader frame into the roof beams and then into columns and foundations. The relevant checks include:

1. Local punching shear and bearing at the spreader supports.
2. Shear and bending in the existing roof beams, which were not designed for a heavy point load.
3. Column axial capacity combined with moments induced by eccentricity; 300 mm × 300 mm columns at the assessed concrete strength may be close to their limit even before the plant is added.
4. Foundation pressure and settlement, since the extra 188 kN is permanent.
5. Dynamic actions: fan imbalance and starting loads cause fatigue and vibration, and the roof may transmit them into the building.
6. Wind uplift and seismic actions on the new mass.

The spreader frame helps only if its stiffness matches the supports; otherwise it may attract load to a few points. The 0.4 mm crack also needs explanation before it is loaded further.

The sequence should be investigation, assessment, options, verification, approval.

1. Investigation: complete the core programme, cover and carbonation survey, crack monitoring, and a measured survey of the as-built frame.
2. Assessment: build a structural model with lower-bound material strengths and the new laboratory load of 4.5 kPa. Check the whole frame, not only the roof bay, because the floor load increase and the roof plant both feed the same columns and foundations.
3. Options: if utilisation exceeds unity, choose between reducing plant weight, relocating it over a column line, adding an independent steel frame to the foundations, or strengthening columns by jacketing. Each option must be compared for buildability, fire performance, and disruption.
4. Verification: peer review the calculations, and use a load test only where calculation is inconclusive.
5. Approval: building control and the client sign off before any installation.

Acceptance criteria: factored utilisation at or below 1.0 for all members and connections; punching shear and bearing satisfied; foundation pressure within allowable with settlement assessed; crack stable under monitoring; vibration isolation specified for the fans; and the steel spreader frame protected for the required fire resistance. If any criterion cannot be met, installation should not start.