KF Aerospace Spiral Stair
Engineer-Build
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 strove 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 significantly increases the overall stiffness of the stair, removing the requirement for any support columns and maintaining a highly aesthetic structure that serves 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.
A final plan for assembly was achieved, and then fabrication went quickly. The CLT slab is made up of three layers of timber: two running the length of the stair in dual curvature, with a middle layer sandwiched between and running perpendicular to the outer layers. Once the final slab was sanded and finished, notches were cut into the top face to allow proper bonding when the concrete slab was poured on site.
A special tribute must go to our ingenious fabrication team of skilled carpenters for carrying such an innovative concept to fruition.
Installation
Transportation
In order to transport the CLT segments from the shop in Abbotsford to the site in Kelowna, a customized jig was designed and built in-house. It featured stud walls with curved top plates to mimic the curvature of the stair and ensure that the CLT was bearing flush without crushing the fibers of the wood.
Preparation on Site
The site preparation included an array of 8 stud walls up to 7m tall that serve as a shoring structure while the stair was under construction as well as serving as a scaffold to provide comfortable access along the height of the stair throughout all installation stages. The stud walls were designed and prefabricated in our Abbotsford facility and then set out and braced on site to be in the precise locations that were needed to install the CLT panels accurately.
Erection
Challenged by the tight space that was available on site, the stair was installed with different approaches for each of the CLT halves.
The installation of the lower CLT half was constrained by the 2nd floor above the final location, which did not allow a crane or telehandler to lift the 1-ton panel. Instead a rigging system was designed to be attached onto the existing beam structure and then connected to the CLT via adjustable chain hoists that would be reeled in manually. This lifted the CLT off the transportation jig and into its final position.
Before the second half was lifted into place, the location of critical connection points with the first half were surveyed to guarantee an exact fit at the splice location. When the second half was finally lifted into place with a telehandler, the accurate work of our carpenters in the shop and on site paid off as all three connections at the bottom, middle, and top lined up perfectly. The panels were fastened securely with fully-threaded screws and perforated steel plates to connect both halves. Afterwards, the preparation work for the Timber-Concrete-Composite system began, which included placement of formwork, rebar and additional fasteners that tied concrete and timber together once concrete was poured.
Additional components of the structure include glulam stair treads and customized metal perforated railings to create a beautiful structure for visitors to interact with.