In construction, you very rarely pay attention to the challenges of structural design.
The cost, complexity and schedule of construction projects are deeply impacted by the structural elements and their design. It is possible to create alternative structural designs that address budget and complexity issues as well as space, light and habitability.
As the word design suggests, creating supporting structures for buildings is a creative exercise.
The building has external load-bearing brick walls, 3 storeys with suspended timber floor and a middle spiral wall. The width of the structure was 12 meters and the length was 21 meters.
The main challenge is to create the optimal design for the additional floor. The most obvious solution would use very long and large steel beams because of the long span of the structure.
Creating a structural frame in timber, we saved our client money and time and make the installation easier, compared to the equivalent of the steel structure.
With the timber design, we could exploit the added flexibility, distributing part of the weight of the flat roof on the internal walls of the new storey and consequently on the internal walls of all the storeys below.
This would not be possible with a steel structure because the internal walls were not strong enough to bear the load of the steel frame.
To use timber, we designed special connections between the mansard rafters and the flat roof joists, so to make sure that the load was distributed correctly. We transferred half of the load of the timber joists to the rafters and then from the wall plate to the external load-bearing brick walls.
We designed the connections between the joists and the rafters to transfer the vertical load, shear and bending moment from the flat roof to the mansard walls. We also used 2 extra pieces of timber bolted together with M12 bolts connecting the roof joists with the mansard rafters and the latter with the wall plate of the external wall.
To compensate lateral forces, we connected all the timber rafters and joists with plywood sheets, so that the structure was stable and could restrain wind forces.
Using timber, we were adding less weight, as the timber structure would be much lighter than the steel one. A rough estimate is that the total weight of the timber structure was 20% of the equivalent steel one.
The timber structure allowed a more even distribution of the load onto the existing building. We created better load paths with our design concept, also thanks to the higher flexibility of the timber material .
The higher flexibility of the timber material also meant that timber was easier to transport and install, and the operations were less error prone than the ones involving steel. The handling of shorter and lighter joists was less complex and could be done by fewer people at a time.
Plus, timber is less expensive in itself, so our client saved roughly 30% of the cost of the material for the structure.
Finally, we must consider the savings in transportation and installation. We estimate the timber structure saved 50% of the cost compared to the equivalent steel one.
Using the timber structure we completed the project earlier because we built the structure more quickly. Our quotation was more convenient for the client because we had lower contingency rates. We mitigated the risks of a steel structure using a timber design. The savings were even higher because of the lower costs of materials and installation.
Overall, our client saved money in many different ways. We saved them time, cost of material, labour, equipment and finally contingency cost.