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Timber frames reach higher: Waugh Thistleton’s Black & White Building

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Words:
Andrew Pearson

Waugh Thistleton director Andrew Waugh explains why timber was a necessary alternative to steel and concrete for the construction of The Office Group’s Black & White Building in London

The Black & White Building is a major step forward for the development and construction industry. A mainstream, grade-A office, it comprises two wings, five and six storeys tall, linked by central core containing stairs and lifts. The super structure is built entirely from timber.

Why build in timber?

The business case for sustainability has never been more compelling so the ambition to build in timber was there from the beginning. The Office Group came to us because of our expertise in designing in timber.

This building uses four different types of engineered timber: it has a laminated veneer lumber (LVL) frame, cross-laminated timber (CLT) floor slabs and core, glulam curtain walling and tulipwood solar shading. The building’s embodied carbon is only 410kgCO2e/m².

The hybrid LVL frame with CLT slabs has been designed to create open workspaces with no structural partition walls.
The hybrid LVL frame with CLT slabs has been designed to create open workspaces with no structural partition walls. Credit: TOG

LVL is a relatively new material, what makes it appropriate for the structure of this project?

LVL is really fabulous and a very efficient use of the tree. When you plank a trunk, you are making square things out of a round object, so you use about 60% of the tree. Veneers you use 95% of the tree; you effectively put the trunk on a massive spiraliser that spits out sheets of veneer – up to 100m at 3mm thick. You slice that up and layer it.

The LVL for this project is made using beech from a sustainably managed forest. It’s as strong as steel, so we get the kind of internal spans achievable with a steel frame but with a structure that weighs just 20% that of steel. That means columns can be skinnier and beams shallower so we can reduce the height of the building, saving material throughout.

But because the building is lighter, the issue you do have is wind loading, so you’re always thinking about how your structure will respond to shear forces.

How do you assemble an LVL structure?

This is a structurally efficient design.  We tender the timber element of the project early on at Stage 2, so that we can work with a timber contractor and manufacturer to establish the design with them. This timber contractor was Hybrid Structures.

The LVL structural frame is bolted together using a system developed by Waugh Thistleton working with structural and facade engineer Eckersley O’Callaghan, and with structural and civil engineer Engenuiti working alongside the timber contractor. We wanted to fireproof the steel connections by concealing them within the frame in order to utilise the fire-proofing ability of the timber structure. And because the timber components are engineered to be slotted together, the entire structure can be easily disassembled, piece by piece.

Beams are an inverted T-section, held to the rectangular columns by a lateral plate. CLT floor slabs are dropped into the beam recess, which saves the depth of the CLT on every floor. All the gravitational load is taken by the LVL. By sitting within the frame the CLT locks it tight, acting in diaphragm for the shear forces, which, along with the core, provide the stability.

Located in the middle of the two office wings, the building’s core houses a CLT staircase and lift shaft. It is constructed as a solid box entirely from CLT, so is incredibly strong. Additional stability is provided by steel cross-bracing integrated into the lightwell facade, adding enough stiffness to prevent the building twisting excessively.

CLT is used to form the building's lift shaft, which forms part of the central core.
CLT is used to form the building's lift shaft, which forms part of the central core. Credit: TOG

What foundations are used?

Piled foundations support a concrete cassette basement formed of a basement slab and walls supporting the ground floor slab. This gives latitudinal and longitudinal stability because we’re building on top of a buried box. Connections for the LVL columns are embedded in the concrete.

The basement, which contains 41% of the building’s whole life carbon, has space for 94 bicycles. We’ve calculated that each one will have to cycle for over 1000 years in order to repay the carbon embedded in the concrete.

How is the building clad?

We’ve used a timber curtain walling made from softwood glulam which supports the glazing. On the outside vertical, thermally modified tulipwood louvres provide solar shading. An advanced sprinkler system drenches the facade in the event of fire. The cladding is as lightweight as possible to minimise the supporting structure.

Is the construction process different for a mass timber building?

It changes from being a construction site to an assembly site. The building is assembled from 872 pieces of timber, each one designed in BIM. The file then migrates to the factory to create cutting diagrams for the timber.

Assembly is a choreographed performance. Every piece arrives on a truck labelled with where it should go. Logistics meant the rear wing was built first, then the core and then the front.

The frame is assembled halfway up to allow access to the assembly bolts. We backfill it with CLT slabs for stability, and then put up the rest of the frame followed by the remaining CLT.

  • Logistics meant that the rear wing of the office was built first, followed by the CLT core before the front wing is assembled
    Logistics meant that the rear wing of the office was built first, followed by the CLT core before the front wing is assembled Credit: TOG
  • Even the building's internal staircase is built using CLT.
    Even the building's internal staircase is built using CLT. Credit: TOG
  • The LVL timber frame has been engineered to be slotted and bolted together. This will enable the entire structure to be disassembled at the end of its life..
    The LVL timber frame has been engineered to be slotted and bolted together. This will enable the entire structure to be disassembled at the end of its life.. Credit: TOG
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Was building insurance an issue for an all timber building?

The toilet floors are plywood and not CLT because our early conversations with insurance agents revealed that the insurance industry’s concerns were primarily about long term water ingress. Even though leak detection systems are fitted they were worried about dripping taps, leaking pipes and the floor being flooded and losing its structural integrity, so we replaced CLT in the toilets with marine plywood supported on joists, which is on their list of approved substrates.

Did this innovative scheme have any Building Regulation compliance issues?

There were issues around dynamic live floor loads because Building Regulations and BCO guidance are antiquated and assume an intensity of use that no longer exists. Because compliance is not performance based, we had to make the timber floors fit the properties of concrete floors, so we had to use thicker CLT slabs – not for structural reasons but to meet the dynamic criteria.

 

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