For many centuries, there has been a strong connection between wood and metals (pardon the pun). In jewellery, tools, weapons, and art, the organic and metallic have been combined to form objects where each material type complemented, contrasted, or enhanced the potential of the other. At times, this association bordered on symbiosis and, in other situations, exposing one to the other resulted in the demise or prolongation of one at the expense of the other. Understanding these complex chemical relationships bordered on the alchemical.
With the emergence of engineered construction, particularly long span lightweight structures, joining relatively slender and light timber structural members using iron and later steel couplings and ties allowed for the creation of a new breed of previously unseen mainly timber buildings featuring far thinner framing members and truss work. Whereas on previous all-timber structures, where the timber section sizes required structural joint sizes that were directly determined by what was required to form strong all-timber pegged joints, often resulting in individual structural members that were well beyond what was needed to withstand structural loads and forces.
When necessarily bulky pegged timber joints were replaced with stronger metal connectors and braces, timber section sizes could be reduced substantially and this had a substantial upstream effect as timber growers were able to grow and sell smaller, younger trees and carpenters were relieved from having to wrestle with squaring large timbers and cutting often complex and timeconsuming joint shapes often fixed using multiple pegs at each joint. With these hybrid timber-steel structures came far finer and adventurous structural forms spanning ever greater distances each driven through a collaboration between carpenters, engineers, metal forgers, and blacksmiths.
What wasn’t readily apparent was that, when in direct contact with each other – especially where structures might be exposed to water or even humidity, chemicals contained within timber would attack most metals causing accelerated metallic corrosion and the eventual risk of structural failure. It was eventually discovered that certain acidic (acetic acid) timber species, especially hardwoods, would damage strong ferrous metals like iron and steel.
In the UK, with some of the more common structural hardwoods such as oak and sweet chestnut (and certainly most imported tropical hardwoods), it became good practise to use high grade stainless steel connectors and ties within hybrid timber-metal structures. With less acidic semi-durable softwoods, like Douglas fir and larch, it was found possible to make use of galvanized steel for metal connectors and ties. To avoid the high costs of high grade stainless steel, it was found that by separating hardwoods like oak and sweet chestnut from galvanized steel using materials such as plastics, rubber, and plastic films would diminish the corrosive effects of exposing hardwoods to galvanized steel. This is the situation with the Downland Gridshell buildings, West Sussex and at Flimwell Park, East Sussex, where galvanized steel is used with both Douglas fir and oak.
In the seminal building, Edward Cullinan Architects’ (now Cullinan Studio), Buro Happold’s, E.A. Chiverton and The Green Oak Carpentry Company’s Downland Gridshell, at the Weald and Downland Living Museum, at Singleton, West Sussex, the extremely lacy French oak gridshell was made possible by the thousands of 4-bolt multi-layered galvanized steel plate couplings locating and clamping the hundreds of 25 x 35 mm gridshell laths together providing the structure’s magical shell action. Because the gridshell would not be exposed to water, the combination of green oak with galvanized steel was deemed acceptable. The stainless steel alternative would have made the gridshell too expensive.
In the case of this building, the galvanized steel connectors enabled the design and construction teams to avoid drilling and cutting into the thin oak laths thereby weakening them. This was done through the layering of galvanized steel plates and the inclusion of four bolts that made it possible to clamp the 4-layered lath gridshell rather than drilling through it thousands of times.
This project takes the combination of timber and galvanized steel in a different direction. Driven in part by structural and market considerations, galvanized steel plays a bigger and more exposed role in this project. Whereas the timber-galvanized steel relationship between the above projects was mainly based upon timber being the major load carrier and galvanized steel the connector transferring loads between timber members, the relationship had shifted to being more equal with each material being applied where most suited for the function. There are places where the relationship is almost animal with timber taking on the role of bone tissue and galvanized steel that of cartilage and tendons.
In other places within the project, galvanized steel takes on an all-inclusive role in external staircases, the bridge, and balconies. In these situations, the strength and durability of galvanized steel offered advantages where exposure to the elements was constant and also where transparency and visibility was paramount – in places where views through and between the buildings into the surrounding woodland especially.
In time, timber will change in colour and take on a silvery grey or even green where mosses gain a surface grip. This will allow for the blending of the timber and galvanized steel.
In further examples, it can be seen that similar technical and aesthetic considerations played a big role regarding the pairing of galvanized steel with timber.
Sutton Hoo, Suffolk by Nissen Richards Studio
A public project where galvanized steel frames, cladding panels, and guarding were combined with dyed concrete and charred larch creating a clear and pleasant composition of organic and non-organic materials.
Aesop Shinbuya, Tokyo by Torafu Architects
Galvanized steel and timber contrast with internally-exposed reinforced concrete and stone inside and outside for this stunning and deceptively simple shop fit-out. Softwood was combined with galvanized steel to create internal fittings and utilities, including sinks. Beautifully-finished galvanized steel cladding was bravely used in places on the front facade of the shopfront.
The Rising Path, Cambridge by Chadwick, Dryer, Clarke Studio
Another public project where the architect very cleverly used galvanized steel as primary structure in columns and as reinforcing and coupling in hybrid timber beams. This project made good use of a radiata pine-based pressure treated (acetic acid) timber product called Accoya.
Crossrail Place by Foster & Partners
This large and very visible project features galvanized steel as the main connecting material to form the building’s enormous softwood timber lamella roof canopy spanning over a publicly-accessible elevated urban garden. As with the Downland Gridshell, these durable metal connectors made this dynamic plant and light-filled building possible.