Construction, RC Frame, Structural
Cast-In Steel Supports Inside Circular Columns for a Moving BMU Cradle System
Nov
The Structural Challenge: A Moving Machine, a Curved Canopy, and Circular Columns
The Landmark project required a travelling BMU that could run on rails around an oval canopy line. The machine and its cradles impose large vertical and horizontal forces as they move, slew, brake and park. Design loads from the BMU supplier ran into hundreds of kilonewtons, including dynamic effects.
Architecturally, the perimeter is formed by circular concrete columns and a slender canopy. There was no appetite for heavy external brackets or visible transfer structures hanging below the canopy. The structure had to:
- Anchor the gantry frame and rail system securely.
- Transfer high local shear and bending forces into the main building frame.
- Maintain the clean architectural profile of the canopy edge.
- Remain buildable within the tight column reinforcement cages.
Embedding cast-in steel within the circular columns became the key move.
Design Philosophy: Cast-In Elements as Invisible Mega-Brackets
The design report sets out a clear philosophy: treat the gantry frame and cast-in connections as a robust but conservative system, using load data from the BMU supplier and applying additional safety margins for uncertainty and dynamic behaviour.
The core ideas were:
- Use the concrete columns as the primary support.
Steel plates and stiffeners are cast into the circular columns, turning each column face into a powerful fixing zone for the gantry truss. - Separate functions of top and bottom rails.
The top rail picks up most of the horizontal slewing forces and global stability, while both rails share the vertical wheel loads from the moving carriage. - Conservative load models.
A 3D STAAD-Pro model was used to derive reactions for different load cases: near support, mid-span and slewing conditions. The worst-case reactions dictated plate sizes, welds and the capacity checks on the steel sections. - Compatibility with the canopy frame.
The gantry frame carrying the rails is structurally independent of the canopy steelwork. This avoids overloading the canopy while still positioning the rails exactly where the BMU needs to run.
How the Cast-In Steel Works Inside the Circular Columns
If you cut through one of the circular columns between Levels 6 and 8, you would see more than rebar. The drawings show radial steel plates, tension plates and stiffeners cast directly into the concrete section, carefully threaded between vertical and hoop reinforcement.
Key elements include:
- Cast-In Plates – Thick S275 plates are set out on the column face at the gantry level, with full-penetration welds to the steel truss members. These plates are sized to carry high shear forces (up to around 1680 kN in some cases) and strong bending moments from the gantry frame.
- Tension Plates and Rebar Anchors – Where the gantry tries to pull away from the column, inclined tension plates connected to large diameter bars develop the force deep into the concrete core. Tension capacity checks are based on steel strength and concrete cone failure, using Hilti design data.
- Compression Bearing Zones – In compression, the loads are spread into the circular column via bearing on the plate and local reinforcement around the interface. The layout of plates around the column perimeter ensures that no single location is overstressed.
- Full-Penetration Welds – All connections between truss members and cast-in plates are specified as full-penetration butt welds, ensuring the welds have at least the same capacity as the steel sections they join.
In effect, each column becomes an invisible “socket” for the gantry frame, capable of resisting large horizontal and vertical actions without compromising the circular architectural form.
The Steel Gantry and Cradle Frame System
The gantry itself is a truss frame running around the building, aligning with the canopy perimeter. Top and bottom chords are universal beams and columns in S275 steel, designed to BS 5950 using published tables for shear, bending and web buckling resistance.
The BMU machine and its cradle sit on a shunting carriage that runs along the rails fixed to the gantry. As the cradle moves horizontally along the façade:
- The vertical wheel loads are transferred through the rails, into the gantry chords, and then into each cast-in plate and column.
- The horizontal slewing and braking forces are absorbed by the rail fixings and the truss action of the gantry, and then resolved back into the circular columns.
Because the gantry is fixed at multiple column locations, the load from the moving machine is shared along the building perimeter instead of hammering a single bracket or canopy beam.
Anchoring into the 7M Mezzanine Slab
At locations where the BMU is parked within the plant room, the rail system connects directly into a reinforced concrete mezzanine slab at Level 7M, rather than through steel trusses. Here the report details the design of U-bolt groups and reinforcement to pick up a vertical load of 840 kN plus horizontal forces.
Hilti HIT-RE 500 adhesive anchors are used to develop the reinforcement bars into the slab, with design checked for combined tension and shear, edge distances and spacing in accordance with Hilti technical literature.
The result is a continuous structural concept: where there is no room for trusses, the slab itself becomes the supporting beam, still tied back safely into the main building frame.
Construction, Quality and Coordination
Designing cast-in elements inside heavily reinforced circular columns requires tight coordination between structural engineer, contractor and steel fabricator:
- Setting-out and tolerances – Plate locations are coordinated with column reinforcement drawings so that bars can be threaded around plates without congestion.
- Concrete placement – The geometry of the plates and stiffeners must still allow proper compaction of concrete and cover to reinforcement.
- Welding and inspection – Full-penetration welds between gantry members and cast-in plates are specified with appropriate inspection regimes to ensure they achieve the design capacity.
Because everything is hidden inside the column, getting these steps right during construction is critical; once concrete is poured, there is no easy way back.
Why This Solution Works So Well
For owners, facilities managers and architects, this cast-in support system offers several clear advantages:
- Clean Architecture
The main structure sits inside the column, so the façade and canopy lines remain uncluttered by heavy external brackets or visible transfer frames. - Robust Structural Behaviour
Loads from the moving BMU cradle are taken straight into the building’s primary concrete frame, not through secondary canopy steelwork that was never designed for such forces. - Safe, Reliable Canopy Cleaning
The BMU can travel horizontally along the building side, reaching the canopy and façade safely for regular cleaning and maintenance, without temporary access systems or disruption at street level. - Future-Proofing
With generous design allowances and conservative load assumptions, the system is engineered for long-term performance under repeated dynamic actions from the cleaning machine.
Conclusion
This project shows how intelligent structural engineering can solve a very practical maintenance problem without compromising architecture. By embedding cast-in steel supports inside circular concrete columns, the design team created a powerful, invisible frame to carry a gantry and cradle system that keeps the building canopy clean and safe.
Instead of treating façade access as an afterthought, the structural design integrates the BMU into the building itself—proving once again that the best engineering solutions are often the ones you never see.