Belfast Truss Anatomy 101
As promised, we’ve been working on some drawings to better explain the structure that makes up Hangar Three. First up on the list, the Belfast Truss and the roof construction.
Made entirely of short lengths of timber deemed as scrap at the time of building, the Belfast Trusses make up almost the entirety of the roof structure of the hangar, creating the wide span and plenty of height that is needed for the aircraft inside. The outer skin of the building is made up of masonry columns supported by buttresses, on top of which the wooden trusses rest. In all there are 17 individual trusses in each bay of the hangar, making 34 in total. Whilst they all look identical, given that they were built 100 years ago, we imagine there was a lot of reliance on “carpenter’s eye” — some have more lattice than others, most purlins aren’t quite evenly spaced, and each truss foot is more than likely a little bit different to how we expect. The Schrodinger’s Cat of the truss, we won’t be sure of what’s inside until we open them up.
The base of each truss is the bottom chord. This is made up of a number of smaller pieces of wood laminated together to give it is thickness and length, creating a strong framework for the rest of the truss to grow from. These rest on top of the masonry column structure, with the addition of a knee brace for extra support. From the ends of the bottom chord the top chord, made up of 2 identical arched pieces, creates the domed roof shape of the building. Again, these are made up of smaller pieces of wood connected together to achieve the length necessary. These 2 pieces of top chord sandwich a lattice of thinner pieces of wood, slotting into a groove in the bottom chord, connecting the 2 chords together. This is what creates the compressive strength that allows the truss to bear the weight of the roof structure above it. At the foot of each truss, a small amount of outer sheathing board is in place to protect and aid the joint between top and bottom chord, also acting in conjunction with the lattice to strengthen the truss foot.
Each of the 17 trusses in each bay are then connected together by a number of secondary structures. Firstly, wind bracing runs perpendicular to the trusses, connecting them together and holding them in their upright position. This works against any strong wind that blows through the hangar when both end doors are open, preventing them from straying from their positions on the masonry support columns. Secondly, there is a network of purlins and small rafters that creates a grid above the trusses, upon which the roof cladding and bitumen felt skin sits. This is what makes the building watertight — or in the case of Hangar Three, what we need to fix to stop the building leaking.
With this complex system of small parts all connected together, it’s easy to see how a failure in one area can have an impact on the others — the term “domino effect” is very appropriate here. With the purlins nailed into the trusses, the rafters nailed into the purlins, and the cladding skin nailed onto the rafters, it’s easy to understand that if one truss were to drop by even a small amount, the roof structure above it would drop down too. This creates stress in the bitumen felt that keeps the water out and eventually, it tears. The ingress of water into the timber of the structure is our biggest challenge — it creates somewhat of a vicious downward cycle. The more a truss drops, the more water it allows in, which in turn damages the truss further, causing it to deteriorate and drop lower.
These drawings, along with drawings of the damage identified on each individual truss, will go into the conservation package that we submit when we seek the consent of the relevant authorities. Hopefully this gives a better idea of the way Hangar Three is built and the work we have ahead of us on the project — the trusses are one part of a large, complex puzzle of a building. As we complete drawings for our conservation package they will be available to view on our website and you can also keep up to date with what’s going on on our Instagram — @thehangarthreeproject.
