Adding Floors to an Existing Building: What Has to Be Checked First

Why the existing building has to be checked

Every new floor adds weight to the columns and foundation below it. It also makes the building taller and heavier, which increases the forces it must resist in an earthquake. A building designed for four floors has its columns, beams, foundations, and reinforcement sized for four floors.

Capacity is sometimes judged by looking at column sizes. That is not enough. Two columns that look the same can carry very different loads, depending on their reinforcement and the strength of their concrete.

Start with the records

The first step is to collect whatever exists: the approved drawings, the structural drawings and calculations, the soil report, and any notes about future floors. If the original engineer designed the building for a planned extension, the drawings should say so, and that makes the check much simpler.

Many buildings have incomplete records, or were built differently from their drawings. That does not rule out an extension, but it means more has to be measured on site.

Measure what was actually built

An as-built survey records the column grid, the sizes of columns and beams, slab thicknesses, and floor heights, and compares them with the drawings. It also records visible defects: cracks, damp, exposed or corroded reinforcement, and spalling concrete.

Test the materials

The real strength of the concrete and the amount of reinforcement are often different from what was specified. Material testing establishes them:

Rebound hammer

A quick surface test that compares concrete hardness across many locations.

Ultrasonic pulse velocity

Measures how fast sound travels through concrete, which indicates its quality and reveals voids or internal cracking.

Rebar scanning

Locates the reinforcement and estimates its size and cover without breaking the concrete.

Concrete cores

Small cylinders drilled from selected members and crushed in a laboratory. This is the most direct measure of concrete strength. The holes are repaired afterwards.

Non-destructive tests cover many locations quickly. Cores give fewer but more reliable results, and are used to calibrate the other tests.

Check the foundation

The foundation is the hardest part to inspect and the most expensive to strengthen. The check uses the original soil report if it is reliable, or a new soil investigation if not. Where the foundation type or size is unknown, a small exploratory excavation beside a column may be needed to see it.

Analyse the building with real data

With measured dimensions and tested strengths, the structure is modelled with the proposed floors added, under the loads of the Bangladesh National Building Code (BNBC 2020). This detailed engineering assessment shows which members are adequate, which are overstressed, and by how much.

What the answer can be

• Feasible as it is: the building can carry the extra floors without strengthening.
• Feasible with strengthening: some columns, beams, or foundations must be strengthened first, for example by jacketing columns or enlarging footings. This is designed as a retrofitting scheme.
• Feasible for fewer floors: the building can take one extra floor, but not the three that were hoped for.
• Not feasible: strengthening would be impractical or uneconomic, and the honest advice is not to extend.

Each of these answers is useful. Knowing the limit before construction is far cheaper than discovering it afterwards.

Approval for the new floors

Adding floors usually needs a revised or new building plan approval from RAJUK, GAUK, or the Paurashava, depending on where the building is. The authority's rules on height and floor area must also allow the extension. The engineering check and the approval application are best prepared together.

Do not add floors to a building with wide or growing cracks, uneven settlement, or badly corroded reinforcement until it has been assessed. These are signs that the building may already be struggling with its current loads.