Key Considerations When Strengthening Older Buildings

• Ahmet Toka
• 7 August 2026
• Structural Strengthening
• 4 min read

Strengthening an old building is technically harder than putting up a new one. In a new building everything is planned; in an old one you first have to understand what is there. The drawings are lost, the construction differs from the drawings, the concrete strength is low, the reinforcement layout does not comply with the code — and none of this is visible from behind the walls.

In this article we describe the real difficulties encountered in strengthening old structures, how the investigation process works, which decisions have to be taken early and what to expect from the process.

In an old building the most expensive surprise is the one that appears after the breaking out has started. The budget allocated to the investigation stage comes back several times over during the works.

Ahmet Toka, General Manager

Investigation: Knowing What You Are Strengthening

In old structures the process begins with establishing the existing condition. When this stage is skipped, the strengthening design is built on assumptions and has to change during the works.

1. Document research. The architectural and structural drawings, permit and building inspection records are sought. If they cannot be found, a measured survey is prepared.
2. Measured survey. The existing layout of columns, beams and walls is measured and drawn. In most old buildings a difference emerges between the drawings and the site.
3. Concrete strength. Core samples are taken and subjected to compression testing in a laboratory. Non-destructive methods (Schmidt hammer, ultrasound) support this but are not sufficient on their own.
4. Locating the reinforcement. The diameter, spacing and cover are established with a reinforcement scanner and confirmed by local exposure. Stirrup spacing is the most critical deficiency in old structures.
5. Ground investigation. The bearing capacity and seismic behaviour of the ground beneath the foundations are determined.
6. Damage survey. Existing cracks, corrosion, water damage and previous interventions are recorded.

With this data the structural system is modelled and a performance analysis carried out in accordance with the code. For details of the process see structural strengthening page.

The Five Most Common Problems in Old Structures

• Low concrete strength. Core results are often significantly below what was assumed in the design. This reduces not only the capacity but also the effectiveness of strengthening methods based on anchoring and bonding.
• Insufficient stirrups and confinement. If the stirrups have not been more closely spaced at the ends of columns, the member cannot behave ductilely during an earthquake and fails in a brittle manner. This is the most widespread defect in old structures.
• Short column. Columns whose free length is shortened by ribbon windows or the arrangement of infill walls attract far more earthquake force than expected and are damaged early.
• Soft storey. Leaving the ground floor open as shops or parking reduces its stiffness compared with the floors above. Damage concentrates on this storey.
• Reinforcement corrosion. In areas that have taken water, the reinforcement has rusted and the cover has spalled. Before strengthening, these areas must be made sound with concrete damage repair must be made sound.

Working in an Occupied Building

Most old buildings are in use during strengthening. This influences the choice of method as much as the technical grounds do.

Methods requiring the building to be vacated: Widespread reinforced concrete jacketing and the addition of shear walls. These works require dust, noise and prolonged breaking out; excavation at ground level may also be needed for foundation strengthening.

Methods that can be applied while in use: Carbon fibre and steel strengtheningWith these systems the site work is short, there is no or little enlargement of sections, and the work can be carried out in phases.

The practical approach is to build the design around this constraint from the outset: if there is no lack of stiffness and an increase in capacity at member level is sufficient, a solution is possible with methods that can be applied while the building is in use. If the lack of stiffness is pronounced, vacating is very probably unavoidable, and that should be discussed at the very start.

Decisions That Have to Be Taken Early

1. Target performance level. To what level will the building be brought? This choice directly determines the scope and cost of the work, and changing it later means rewriting the design from scratch.
2. Occupancy scenario. Will the building be vacated, or partly used? The choice of method depends on this.
3. Architectural assumptions. Where can a shear wall go, which columns can be enlarged, which areas must be preserved? These decisions shape the structural design.
4. Services and facade. Will the services be renewed and the facade overhauled during the strengthening? If they are done together, the cost is shared.
5. Foundation intervention. Added shear walls or enlarged columns transfer load to the foundations. Whether foundation strengthening is needed must be established early.
6. Legal process. The relationship with the permit, the resolution of the owners' association and any urban regeneration processes must be planned from the very beginning.

Realistic Expectations

Strengthening does not aim to bring a building to the same level as one built from scratch; it aims to bring it to the target performance level defined in the code. This does not mean there will be no damage during an earthquake. The aim is that the structural system does not collapse, that life is safeguarded and that damage stays within repairable limits.

A second reality is uncertainty of scope. When breaking out begins in old structures, unexpected situations can be encountered: concealed damage, voids not shown on the drawings, weaker concrete than expected. For this reason having a mechanism for changes of scope defined in the contract protects both the client and the contractor.

Thirdly, strengthening concerns only the structural system. Waterproofing, thermal insulation and services problems are not solved automatically within the scope of strengthening; but if they are done at the same time, the cost of scaffolding and disruption is shared. This is a saving overlooked on most projects.