Text: Lauri Lepikonmäki, Planning manager, Sweco

Hybrid construction makes it possible to lengthen spans and build higher. In a hybrid building, several materials are combined to create the load-bearing frame or other essential components, such as exterior walls, roofs, or balconies. These can be simple combinations of materials or more complicated composite structures. Note that hybrid structures differ from standard wooden multi-storey buildings, even though the latter also often use steel joints and plasterboard.

Read the article in Finnish here.

Extensive expertise and collaboration

Hybrid construction requires considerable expertise. Designers must be familiar with the behaviour of different materials, and their calculations and reviews need to be precise. In high-rise construction in particular, differences in settlement, deformation, weight, and stiffness can escalate rapidly. Novel solutions are therefore best developed as a collaboration between experts in sound, moisture, fire, and architectural design.

Current wood products are the result of innovative material development, but the efficient, industrial use of these new structural systems and solutions requires that the entire construction chain understands their general principles. Only then can we truly leverage their full potential

Flexible use of structural solutions

Joints can significantly reduce the overall stiffness of wooden buildings. For this reason, taller wooden buildings are typically braced with concrete or steel. Ascent MKE, the world’s tallest wooden building, uses concrete shafts, and Sweco’s Mjøstårnet, the tallest building with wooden bracing, has a concrete intermediate floor under the roof to improve vibration behaviour in the wind. Multi-section dowel joints add further stability.

In hall-like structures, hybrid buildings are nothing new, and the most suitable structural solutions are selected according to the needed span lengths and other goals. Wooden roof elements are doing well in competition thanks to their quick installation and reasonable prices.

In modular construction, material combinations are already commonplace. When lightness, insulation, and environmental friendliness are a must, one of the best options is to combine wooden structures with other materials. Wooden structures are also used to shorten installation time, provide a fresh look, or to leverage extensive prefabrication.

For example, wooden balconies are popping up in apartment buildings that are otherwise made from concrete. Any balconies not in fire class P1 must be approved by the authorities, but a site-specific fire engineering report on the balconies has sufficed so far. Wooden balcony modules are quick to install, and they create a certain appeal in otherwise solemn concrete architecture.

Wet rooms and building technology use a variety of technical solutions, and wet room elements in wood, concrete, and steel are already on the market. Sydänpuu (Heartwood) by Lakea is a hybrid system where self-bearing concrete wet room modules and separate volumetric elements are assembled into one building at the construction site. The first apartment building built with the Lakea Sydänpuu concept was completed in 2018.

Composites and composite structures in hybrid construction

Composite structures are the core of hybrid construction. They combine the best properties of each material in the same component. In composite structures, stiffness varies according to connection rigidity, with maximum stiffness achieved when the composite materials are rigidly attached to each other. Some displacement typically occurs in the joints, however, decreasing the stiffness somewhat. Minimum stiffness is when the structural components are entirely separate.

Wooden intermediate floors use a variety of joint structures to lengthen spans and decrease structural thickness. In Finland, intermediate floors that are CLT-only typically have 4 5 m spans, compared to about 7 m for composite structures. Composites are common in office buildings and similar complexes where structural thickness is a significant issue.

In fact, CLT-concrete composite slabs have already established themselves as a go-to intermediate floor structure in Finland, including at the Helsinki Upper Secondary School of Natural Sciences, and WithSecure’s new headquarters. In structural calculations for composites, the drying shrinkage of concrete is critical as it causes a large part of the final deflection. This final deflection typically defines the slab’s size limits. Slabs with a thick bottom deck are typically the most efficient structurally. As slab floors are often low-frequency, they also need to be tested for vibrations.

In Finland, the generally accepted guideline for the vibration testing of low-frequency floors is guideline NCCI 1 of the national appendix for steel structures. Vibration as a phenomenon does not depend on the material, and in my opinion, neither should the related requirements. In the upcoming Eurocode, the instructions for low-frequency floor testing are provided in the section on wooden structures.

The prefabrication industry already manufactures wood-concrete composite structures, including beam-slab and composite-slab elements. In fact, final deflection is easier to reduce in factory conditions with pre-raising and support during casting.

Rapid product development in Finland

In Finland, research on steel-wood composite structures is currently conducted at Tampere University, for example, where the Metal and Lightweight Structures research group is experimenting with CLT-steel composite slabs at normal and elevated temperatures (fires).

Among other results, their study found that the effective width of CLT in the direction of the steel beam can be greater than in steel-concrete composite structures, the contact force of CLT differs significantly from steel-concrete composite structures, and simulation results for wood charring near the steel beam corresponded well with actual combustion tests.

Hybritut is another project advancing hybrid construction in Finland, as is Peikko with its groundbreaking work in connecting Delta beams to wooden structures and developing the related joints.

Wooden construction and the general use of natural materials are gaining popularity. Investments in meticulous research are necessary if buildings are to remain healthy and safe in the future.

Sustainable bio-based materials – an EU Commission initiative

The current European Parliament hopes to further the green transition of the built environment, and wood construction has a vital role to play.

Hybrid structures can provide building solutions that are both sustainable and cutting-edgeNew European Bauhaus (NEB) is the EU Commission’s initiative for facilitating the EU Green Deal. To reach its goals of sustainability, beauty, and collaboration (inclusiveness), NEB promotes the green transition through sustainable solutions and the use of bio-based materials in the built environment.

W4B (Wood4Bauhaus) is an official partner of NEB and, in its role as the alliance for the European wood products industry, promotes good practices in the innovation of wood products and structural systems.