Technical insight: structural renovations
Renovation projects differ in many aspects from new construction. They require building within an existing structure, which introduces a number of constraints. For example, installing new large structural elements is challenging, and the often limited load-bearing capacity of existing floors must be taken into account. All of this results in complex phasing. Creativity and thorough problem analysis are therefore essential. In this newsletter, we illustrate our approach through several structural interventions applied during the restoration and adaptive reuse of the former Pathological and Vesalius Institutes into the Vesalius Museum, commissioned by KU Leuven. These buildings were constructed in three phases, dating from 1880, 1908, and 1930, and are characterized by significant heritage value, which means that all interventions are designed to preserve the existing structure as much as possible.
Analysis of the existing structure
As a first step, the existing structure must be thoroughly evaluated, for which several site visits are indispensable. An initial visual inspection can identify the need for further durability and structural assessments. In this case, it was determined that the existing concrete floors were still in excellent condition. The timber joist floors, however, showed signs of moisture and fire damage. Given the age of the buildings, no structural drawings were available. Therefore, a rigorous destructive investigation was carried out to map the reinforcement, floor thicknesses, and beam compositions on every level and in each room. This approach allowed the load-bearing capacity of the structure to be accurately assessed and verified against the project’s functional requirements.
©Studio Roma
Underspanned Beams
In the 1908 building wing, the intermediate floors consist of timber joists supported by steel beams. The existing structure was only capable of carrying the limited self-weight of the floor and a live load of about 200 kg/m², with the steel beams already exceeding current deflection limits. The new design required a live load of 400 kg/m². Fully replacing the floors was not an option, as this would have necessitated extensive façade shoring on public property or a highly phased construction approach. To avoid this, a solution was developed in which new joists were added between the existing ones, and the steel beams were passively underspanned by welding a tension rod beneath them. The floor could thus be strengthened with maximum preservation of the existing structure (only the damaged timber members were replaced) and with minimal disruption to on-site circulation. By attaching L-profiles to the steel beams, the new joists could be installed from below, allowing the original plank flooring above to remain intact.
Strengthening of concrete slabs
The slabs above the ground floor and basement consist of thin concrete slabs reinforced in one direction — a typical characteristic of floors dating from the early applications of reinforced concrete in Belgium. Wherever possible, these were strengthened by applying bonded reinforcement, more specifically by means of mechanically anchored steel plates. The need to use steel plates instead of carbon fibre reinforcement resulted from the fact that the concrete slabs did not provide sufficient strength in fire conditions. The steel plates therefore had to be painted with a fire-resistant coating.
This solution, however, was not applicable everywhere. In the slabs above the ground floor, the existing reinforcement was so insufficient that strengthening with bonded reinforcement was theoretically impossible. These floors, however, had heritage value and could not be demolished. In dialogue with the client, the permissible live load in this area was therefore limited to 200 kg/m². A test load was organized and monitored according to the guidelines of NEN 8700. The test was successfully carried out, demonstrating that this pragmatic approach, within a normative framework, achieved a safe and satisfactory result.
©Sanacon
Structural strengthening of foundations
The walls of the building sections from 1908 were practically un-founded. The walls extended about 1 meter below the foundation slab, resting on a natural stone base approximately 80 cm wide. Given the relatively soft soil beneath this base and the increased live load on the floors, foundation strengthening was required. Additionally, there was a desire to deepen the basement by 0.5 meters for a large part of the building, which naturally reduced the load-bearing capacity of the shallow foundation base. To limit the number of micropiles, a calculation method was developed following the piled raft foundation principle: the micropiles carry part of the total load depending on their stiffness relative to the soil. The soil stress must, of course, remain within the allowable bearing capacity according to current standards. The micropiles were drilled on either side of the wall to be strengthened, with the wall supported on a pile cap.
In the façades, however, this approach was not feasible due to utility lines and public property. In these areas, the micropiles were drilled slightly inclined through the wall. Key considerations included the phasing of the foundation slab, as it provides horizontal support for the horizontally decomposed reaction component. Another important aspect was the need to construct an enlarged grout head at the base of the wall to prevent punching through the masonry.
Preservation of steel dome
The Lenertz Auditorium is distinguished by a glass pyramidal roof dome. The single glazing needed replacement and was to be substituted with modern double glazing. This would increase the load on the structure, while the 95-year-old supporting steel framework was already operating near its maximum capacity. Given the structure’s significant heritage value, the existing steel framework was carefully reanalyzed. The existing profiles were measured using a 3D scan to obtain an accurate survey. The primary Vierendeel beams were found to be sufficiently load-bearing. The smaller steel trusses, composed of L-profiles, were all retained; where structural capacity was insufficient, they were doubled and welded to the gusset plates. By applying minimal reinforcement at only a few locations, the original steel structure could be fully preserved.
Conclusions
Renovation projects require pragmatism, ingenuity, and practical insight to achieve efficient solutions. This calls for the kind of experience that we at Evolv have been cultivating for many years.
