INTRODUCTION

Thumbs up for timber frame construction

Metsä Wood structural engineer Jussi Björman was involved in the project from the very beginning, as the architect’s vision affected the structural design – and vice versa. Likewise, material and production efficiency was carefully planned, and resulted in exceptionally environmentally friendly design, with no waste material generated.

A particular challenge for Björman was the unusual elliptical form of the structure, and the detailed building process itself. Issues from load bearing structures and bracing, to connections and fire safety of the timber frame had to be considered.

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Load bearing

Values guide Metsä Group's operations 
 

Lighter, yet super-strong

Using prefabricated timber elements was the only sensible way to go, as there are more than 5,000 different wood elements in 84 frames in our Colosseum design. Metsä Wood's Kerto LVL  was selected for the timber columns.

     

Thanks to the load bearing capabilities of Kerto, the timber columns need be only 0,5 metre wide – compared with the original 1,5 metre wide stone ones. Each column is made of seven re-glued 72 millimetre thick by 500 millimetre high Kerto-S panels.

Using Metsä Wood’s own Finnwood tool we were able to calculate that the columns could bear a load of 1,7 Meganewtons (about 170 tonnes) – the weight of a couple of blue whales. We could easily have added a couple of extra floors, as the maximum load for a column like this is 800 tonnes.

The beams will be arch-shaped and made of re-glued Kerto-Q. The excellent strength-to-weight ratio of Kerto enables long spans in the design. Kerto allows spans up to 25 metres, but due to the original design the spans in the new design were only 12.9 and 6.7 metres. The beams will be installed on both sides of each column to reduce bending stress.

The Colosseum’s elevator shafts additionally support the bike-wheel shaped steel truss roof, and the terraces will be supported by beams with triangle shaped top edges. The floors and terraces will be made with Kerto-Ripa timber elements, which are prefabricated and structurally glued composite slabs.

Bracing

3D model of the timber structure for wooden Colosseum 
 

Engineered to support an empire

There will be 5 timber frame floors above ground and 2 subterranean floors for parking and storage. The structure is strengthened with 12 elevator shafts and, in subterranean levels, with bracing walls. Horizontal loads are transferred to the vertical bracing structures through the use of floor elements. The terraces inclined beams likewise provide additional support for the beams.

While it was not a key requirement in this design project, many Kerto buildings are constructed to withstand earthquakes, due to their lightness and the flexibility of their connections.

     

Connections

Kerto-Q is a load-bearing, bracing and dimensionally stable LVL panel
Product: Kerto-Q

​Kerto-Q is a load-bearing, bracing and dimensionally stable LVL panel used in the most demanding structures. It can be used in Kerto-Ripa elements that enable stiff and high-quality floor and roof structures. The elements can be equipped with holes for HVAC equipment, and the load-bearing capacity is not reduced significantly.

Main beam and column connections
Main beam and column connections

​Inclined tension screw connections are used to join consoles supporting the arch-sharped Kerto-Q floor beams. When load is placed on the connection tension is transferred through the screws and placed directly onto the load-bearing column. The compression force is relayed via direct contact through the console piece to the column, and is further supported by kinetic friction.

 


Kerto-S LVL beams
Product: Kerto-S

​Kerto-S combines excellent technical performance with ease of use. Its essential qualities include strength, dimensional precision and stability. It is the ideal choice for beams whenever long spans are required.

Floor connections
Floor connections

​Floor elements are held in place by large, 10mm timber screws which go through the flooring and anchor to the supporting beams below. The screws are simple and safe to put in place as they go directly into the horizontal floor from above.



 

Kerto-Q
Product: Kerto-Q

​Kerto-Q is a load-bearing, bracing and dimensionally stable LVL panel used in the most demanding structures. It can be used in Kerto-Ripa elements that enable stiff and high-quality floor and roof structures. The elements can be equipped with holes for HVAC equipment, and the load-bearing capacity is not reduced significantly.

Inclined terrace beam connections
Inclined terrace beam connections

​Timber connectors straddle the columns and beams and function as torsional restraint supports for the beams. Lateral screw connections between columns and beams are used to hold terrace beams firmly in place atop the Kerto-S columns.

Fire resistance

Fire safety of a wooden structure 
 

Safe, even when Rome burns

The ignition temperature of Kerto is about 270 °C when exposed to flame. Spontaneous ignition does not occur until a temperature of over 400 °C is reached. In the event of a fire, Kerto performs more predictably than many non-combustible materials, and has a notional charring rate of 0,7 mm per minute. When burned, the surface of Kerto is charred, which protects the product, insulates it, and slows down the burning process. The fire resistance design of Kerto products is conducted according to the EN 1995-1-2 standard and its national annexes.
     

Conclusion

Engineering a more economical empire

The light structures, load bearing capabilities and modular solutions would allow the cost-efficient construction of amphitheatres all over the Roman Empire – saving the Caesars lots of money and time.

     

SEE NEXT

The Construction phase

See how the construction of the wooden Colosseum is planned and the design finalised.