Client
KF Aerospace
Design Architect
Meiklejohn Architects
General Contractor
Sawchuk Developments
Structural Engineer
StructureCraft
Our Service
Engineer-Build
Clearspan
70 ft (spiral)
Location
Kelowna, BC, Canada
Completion
2022

KF Aerospace Spiral Stair

Client
KF Aerospace
Design Architect
Meiklejohn Architects
General Contractor
Sawchuk Developments
Structural Engineer
StructureCraft
Our Service
Engineer-Build
Clearspan
70 ft (spiral)
Location
Kelowna, BC, Canada
Completion
2022

A Helical Stair

The KF Aerospace Centre for Excellence is a true aerospace museum, showcasing historical planes and other industry memoirs while maintaining real aspects of aerospace design throughout the structure. Barry Lapointe, founder of KF Aerospace, wishes for the space to "use wood wherever possible" and "feel like a plane". With this spiral stair, we strive to achieve that vision.

Using a special application of timber concrete composite (TCC), the structure is comprised of doubly curved CLT with a concrete topping throughout the full spiral. Addition of the concrete provides the required mass to control vibrations in the 70ft long free span. Creating composite action between the concrete and the CLT significant increases the overall stiffness of the stair, removing the requirement for any support columns and maintaining a highly aesthetic structure that will serve as a welcoming showcase for the Centre.



Engineering & Design

Timber composite concrete (TCC) is a unique hybrid structural system which is gaining more widespread adoption in longer span mass timber structures. Cross Laminated Timber (CLT) is commonly used as a planar element for floors or walls to carry out bending out-of-plane and shear-diaphragm forces in the plane of the element.

For KF Aerospace, our structural engineers proposed a unique design concept: could we connect the two levels of a museum space with a free-standing spiral CLT stair. This posed significant structural challenges; the spiral shape requires the CLT to both bend and warp, and creates forces in a combination of strong- and weak- axis bending as well as torsion.

Bending lumber and gluing it together (GLT) is an established method around one single axis, but bending in addition to twisting the CLT without cut-offs requires highly complex fabrication, and our engineering and fabrication team iterated using small-scale prototypes on a multitude of parameters such as thickness of the lamination, bending radius, the rate of twist and the lumber species.

In addition to the added complexity of manufacturing, the stair spans an unsupported 70ft in a spiral fashion and is susceptible to dynamic excitation, a response due to footfall which tends to govern the design of stairs. The concrete topping was used to limit these dynamic effects and stiffen the overall cross-section through TCC-action, but also added significant further complexity to the design.

Testing & Fabrication

Since this stair is highly unique, bending tests of laminations were conducted in our shop to justify a viable product from manufacturing to service. A mock-up jig was created to determine the prospect of bending and twisting Hem-Fir lumber of various thicknesses.

Once the lamination thickness was determined, our fabrication team created a small test portion of the stair to further explore our options with the innovative design concept. We had to approach the creation of this stair not only from an engineering perspective but also a constructability (including fabrication and erection) standpoint.

As further testing occurred in the shop, our internal engineering team was busy using our findings to create the most feasible solution to the design.

Our engineers worked to overcome significant challenges such as predicting the radial and torsional spring-back on the full section based on measured values from small-scale mockup specimens, and understanding the shear flow in a timber-concrete composite section which was composite for both strong- and weak-axis bending, which is very atypical for TCC applications.

Once a final plan for assembly and erection was achieved, fabrication of both halves of the stair went quickly, and the full structure will be erected later this spring. A special tribute must go to our ingenious fabrication team of skilled carpenters for carrying such an innovative concept to fruition.