The National Gallery

Investigating The National Gallery

GBG Group (UK) had the privilege of working at The National Gallery located on Trafalgar Square in central London during the tail end of 2025. As part of planned renovation work to the existing Wilkins Building, GBG were appointed to conduct localised structural surveys to concrete beams and floor slabs, along with two trial pits to confirm existing foundation arrangement.

Whilst GBG have unparallelled experience of working in Heritage Buildings and Museums, this was the first project where the team were asked not to use percussive tools during our investigation. Due to the fragility of paintings located above our survey areas GBG had to take great consideration in to how the work would be undertaken without use of the traditional methods of breakers and drills.

To add additional complexity, much of the structure was of a non-standard construction and containing Kahn-System reinforcement. This was a US patented design for a slab system with a main diagonal bar with “wings” attached to the side, bent upwards to provide shear resistance. This meant that our usual inspection method, of exposing the intersection of two reinforcement, was not appropriate and that we instead needed to chase an entire “wing” to confirm dimensions and condition of the system.



The museums vibration team were in constant contact with the GBG site team ensuring the works did not create excessive vibrations and, following a few tests of intrusive work, we were able to advance using a core rig to expose the reinforcement. Exposing reinforcement with a coring rig, without damaging the reinforcement, is extremely delicate and required surgical precision from both the intrusive team and NDT team. Due to the risk of damage to the reinforcement, NDT interpretation had to be as accurate as possible to confirm the location of a curved “wing” as well as ensuring the coring rig stopped before cutting into the reinforcement.

The accompanying images demonstrate how much care and attention was taken with the breakouts; incrementally taking 50-75mm diameter cores out of the primary beams to expose the reinforcement.

In addition to the reinforcement exposures a number of concrete cores were taken for laboratory testing and the slab reinforcement, also constructed via the Kahn System, was exposed at the slab soffit for assessment.

The planned changes at the museum required confirming the arrangement of foundations below a store room by digging of a trial pit. The trial pits were advanced using concrete coring to the slab, which is a low vibration method, and then digging with hand tools we were able to confirm the structural arrangement of the foundations.

The final area of investigation was located below a particularly high-risk gallery, as the paintings were hung in a specific manner and were thus more susceptible to vibration through the structure. Unfortunately, while coring is a low vibration technique, the single drill hole required to secure a coring rig into position was causing excessive vibrations. We successfully completed cores for compressive strength testing to the slab deck and soffit, however anything involving exposure of reinforcement had to be postponed until that particular room closes in mid-2026.

The project was an overall success, despite the challenges faced onsite, and allowed the engineers to advance with their plans while minimising as much design risk as possible. Feedback from the Project Management team was also very complimentary stating “as a general note we were very happy with both you and your team’s performance and would be more than happy to recommend you on subsequent survey works through projects of a similar nature.”

GBG will return in 2026 to complete the survey!