The best of all worlds: a hybrid approach to mass timber

 By Dan Hagan – Technical Director and Modern Methods of Construction Lead, WSP

With modern methods of construction and design for manufacture and assembly, we can use mass timber products alongside concrete and steel in fast, lightweight and low-carbon solutions for commercial buildings.

In the push for net zero, mass timber offers a way to reduce embodied carbon – but using it for sustainability reasons alone can lead to other design issues. For example, to specify a mass timber column at the base of a 20-storey office building, it would need to be circa 6 times greater in plan area than a steel column. By choosing mass timber, we would be reducing the lettable area for our client. 

Using mass timber to cut both carbon and construction time

However, in such a building there is a way we can design with mass timber that would cut both carbon and construction time without any downsides. We can use a hybrid floor plate that has steel columns and beams but replaces a composite deck slab with a cross laminated timber slab manufactured off-site. The timber makes an aesthetically pleasing ceiling for people on the floor below, services can still run as they would in a traditional solution and – with no waiting to fix reinforcement, pour concrete and allow it cure – construction is faster. 

Programmes are also sped up, because the mass timber hybrid floor plate is lighter than the traditional alternatives, meaning the building can have smaller foundations. A multi-storey mass timber founded on piles has up to 85% fewer piles than a traditional alternative. This reduction in substructure material relates directly to a similar saving in costs and programme. Due to the lightweight nature of the solution, it’s an excellent option for retrofit projects that involve adding floors to existing buildings. We’re currently working on a project using lightweight hybrid timber floor plates that enable our client to add three more storeys to a two-storey building in Camden.

Hybrid floor plates go well beyond sustainability

Although the benefits of hybrid floor plates go well beyond sustainability, the carbon savings are impressive: embodied carbon is typically less than half of that for concrete floor plate solutions. In addition, optimising the grid sizes ensures lorries delivering panels to site from the factory are full – reducing the number of deliveries and associated emissions. 

To really see these benefits, let’s have a look at a study of a typical office floor with an area roughly 7000m2:

There’s potential to make further reductions by adjusting the design so that some or all of the beams could be laminated veneer lumber while the columns would remain steel. This incremental approach also enables clients to embrace hybrid structures one step at a time, without having to take a leap into the unknown.

Incorporating mass timber products in a hybrid approach has potential in other types of commercial buildings too. We’re currently investigating how, using modern methods of construction, we could replace carbon-intensive steel frames in long-span distribution centres with mass timber elements. 

As we strive to design buildings for a net zero future, hybrid structures that make use of mass timber offer us the chance to cut carbon while improving building performance and providing value for our clients. They really are the best of all worlds.