Hardwood

The National Forest Inventory shows: The hardwood stock in Switzerland is steadily increasing. Compared to the first survey in 1984 and the second in 1995, the hardwood stock increased by 12%. From 1995 onwards, the hardwood stock increased by a further 10%. In the same period, the proportion of coniferous wood stagnated, decreasing by 1% from 70 to 69% (FLI c/o WSL, 2007). The reason for more hardwood is to be found in the near-natural silviculture applied throughout Switzerland. This type of forest promotes natural forest regeneration. Artificial planting is avoided. In this way, the relatively competitive hardwoods are indirectly promoted, especially at lower altitudes (Central Plateau and Jura). The resulting mixed forests tend to be more stable than forests with a high proportion of spruce or pure conifers. Increasingly frequent storm events cause enormous damage in these forests in particular. Every storm is a step towards more hardwood; reforestation takes place by means of near-natural silviculture. The advancing global climate change also speaks for the cultivation of hardwood. Spruce and silver fir in particular are considered to be less resistant to drought. Climate change is pushing these wood species to higher altitudes. However, as it is highly likely that beech will also come under pressure, this type of wood will also be found at higher altitudes in the longer term. In warm and dry locations, for example, Scots pine will become more competitive. One option in silviculture is the artificial introduction of tree species such as Douglas fir or oak. Both species are more resistant to heat and drought.

Spruce is by far the most common tree species in Switzerland. It will remain the most important type of wood as construction timber in the future, but its dominance will decrease in the medium and long term. In Switzerland, there are pronounced stands of spruce in rural areas such as the Emmental, the Entlebuch and the Jura mountains. The most common deciduous tree species is beech. Over the last few centuries, spruce has been strongly promoted at lower altitudes for economic reasons by means of artificial planting, at the expense of the highly competitive beech. (Removal = utilization + natural mortality due to storms, beetles, avalanches). The situation with softwoods is just the opposite, most strikingly with spruce, which is in high demand

Hardwoods - especially beech and ash - naturally have higher specific strengths than softwoods. This makes it possible to design slimmer, more stable load-bearing structures. neue Holzbau AG has been producing glulam in hardwood for over 10 years. The company has had consistently positive experiences with ash in particular, but also with beech, oak and robinia. Hardwood is considerably more expensive to process than spruce. The procurement of hardwood boards is also more expensive, as they are not (yet) a mass product. The additional work involved causes significant additional costs compared to glulam in spruce. In order to use glulam efficiently, the advantages of hardwood over softwood must be exploited. In addition to the significantly higher strengths (especially in the areas of bending, shear, tensile strength transverse and parallel to the grain, compressive strength transverse and rolling shear strength), these are the massively higher performances in the area of the fasteners. A GSA anchor (fastener with glued-in threaded rod) in ash is 1.5 times more efficient than in spruce, without sacrificing performance in terms of stiffness and ductility. Significantly higher performance with hardwood is also achieved with rod dowel or screw connections.

n'H-BSH made from hardwood
Up to now, hardwood has usually only been used by neue Holzbau AG for local reinforcements in the area of a connection. This means that areas weakened by fasteners can be reinforced locally. The higher performance of the fasteners also means that the cross-section is weakened less. All these measures enable the execution of assembly joints with 100% spruce performance.

For some years now, neue Holzbau AG has also been using the performance of hardwood for entire components. Together with Prof. Ernst Gehri, various in-house test series were carried out in the hardwood sector. The aim of the tests was to create a basis for the design of glulam in ash and beech. The tests covered all relevant work processes in glulam production, from sorting the boards to finger-jointing and surface bonding. In addition, bending and shear tests were carried out on beams in the in-house test center in order to check the performance of the finished product. The entire quality assurance process was geared towards the highly stressed components.

Quality assurance
Compared to GL24 in spruce, which is the most commonly used glulam today, hardwood has twice the performance in almost all areas at a characteristic level! This requires a holistic quality concept, starting with the input raw material board through to the end product load-bearing element including joints. In order to guarantee the strength values listed in the table, neue Holzbau AG has a comprehensive quality assurance system based on three principles:

• Degree and scope adapted to the controls (controls where necessary; as few controls as possible).
• Checking the basic material and each processing step as realistically as possible.
• The specifications and targets (definition of test values) must be easy to check.

First of all, the construction is given a degree of control based on its utilization:
KK1: Component with low static utilization
KK2: Engineering structures in general
KK3: Sophisticated engineering structures

The three control classes differ in the degree and scope of the tests to be carried out. The following checks are carried out for the manufacture of glulam:

1. Tensile properties of the boards (tensile strength and tensile modulus of elasticity)
   Random check of the 5% fractile value by means of test loading.
2. Tensile strength of finger joints Random check of the 5% fractile value for breakage.
3. Shear strength of the surface bonding Random check for breakage.

Projects or components with strength GL48 (hardwood) are assigned to control class 3 in the new Holzbau AG (demanding engineering structures).

The following are required:

• Board tensile tests: 1 of 25 boards.
• Tensile finger joint tests: 4 samples per shift.
• Shear tests: 1 section per glue layer

The decisive factor here is that both the boards and the finger joints are tested directly for tension. Tensile testing systems have not yet become established in many companies as they are somewhat more complex. The finger-jointing in particular is subject to enormous demands, which cannot be adequately checked without a tensile testing system and therefore cannot be justified.

Farm building in Lauenen
This pilot project in hall construction was carried out for the first time using 1:1 high-quality beech glulam (GL48). Beech was used for the roof construction of the agricultural farm building in Lauenen BE. The FOEN (Federal Office for the Environment, Wood Action Plan, Bern) helped to co-finance the project.

Client: Christian von Siebenthal, Lauenen
Timber construction: Bach + Perreten Holzbau AG, Gstaad
Supporting structure planning; delivery of main supporting structure: neue Holzbau AG, Lungern

The farm building spans 15.50 m with a truss spacing of approx. 4 m. The building is located at 1350 m.a.s.l. + 200 m (allowance according to SIA 261), which results in a roof load at ground level of approx. 13.5 kN/m2.

A conventional load-bearing structure in spruce CLT was already available. Based on the existing geometry of the building, the aim was to optimize the shape and the system.

The shape outlined above results in the same internal dimensions of the hall. The lower construction height of the beech beams enables a steeper shape with a correspondingly favorable geometry. This reduces the corner moment by around 5 to 8%. In addition, the tension rod has a greater inclination; thanks to an enlarged lever arm, the decisive shear force is reduced by 15 to 20%. The statically optimized system also made it possible to design the tie bar with a constant cross-section. This simplified production considerably. In addition, the cut fibers present in the spruce variant were eliminated.

Beech findings
Beech has low durability when exposed to moisture. The shrinkage and swelling behavior of this wood is also extremely high. Even slight fluctuations in moisture result in enormous stresses. For this reason, lamella dimensions must be kept as small as possible. Other types of wood generally allow lamella dimensions of 40 mm and widths of up to 240 mm. Beech can only be used permanently with lamella thicknesses

Without financial support, the Lauenen farm building could not have been built in beech. The costs for the truss construction in spruce glulam were around 20,000 Swiss francs, in beech glulam over 30,000 Swiss francs.

Technically, glulam beech can be produced in strength classes GL40 and GL48. Beech can probably be used even more efficiently, for example as laminated veneer lumber.

Roofing of parking garage in Arosa
In 2009, the municipality of Arosa built a two-storey underground parking garage with an attached wooden roof in Innerarosa. The structure covers a building used for mountain railroads and the ski school on both sides. The parking building is located on the main access road to Arosa. Winter sports enthusiasts and hikers can reach the new Innerarosa-Tschuggen gondola lift directly from the car park via a covered conveyor belt.

Client: Municipality of Arosa
Architecture: Lutz & Buss Architects, Zurich masKarade, Montreuil
Structural engineering: Walt Galmarini AG, Zurich
Timber construction: Brunner Erben AG, Zurich
Delivery of main supporting structure: neue Holzbau AG, Lungern

The timber building extends over a width of 42 m and lengths of 37 and 32 m. The entire width is covered only at the front. On the mountain side, the roof remains open over a length of approx. 24 m between the buildings and thus provides a view of the Weisshorn summit from the road. The first five beam axes are also at an angle to each other.

The first four girder axes are statically interesting. The girders stand on only three supports, resulting in spans of up to 19.70 m. The angle of the axes would cause large constraining forces in a rigid ridge connection. The beams are therefore connected by a joint. The building is located at over 1800 m above sea level, and the characteristic snow load is over 11 kN/m2. The shape of the beams corresponds to the static load. This leads to optimum utilization in every section. Although the building is wide, it is not high. Architecturally, a filigree timber construction was desired.

In order to achieve the desired cross-sections, various options were examined in advance of the submission. Initially, the competition concept envisaged the use of block-glued laminated veneer lumber. In the end, a better hardwood solution in terms of both quality and strength was put out to tender.

A spruce construction with the same dimensions would not be able to absorb the same bending moments or shear forces. By using ash wood, it is possible to produce lamellas with a characteristic tensile strength of 40 N/mm2 (the highest class of spruce is T26 with a characteristic tensile strength of 26 N/mm2). The shear strength of ash can also be increased by a factor of 1.5 compared to spruce. Ash also has the advantage of being easy to combine with spruce.

The load on the first axes is so high that no cut fibers are allowed in the compression zone. As the beams have a variable height, the compression zone was glued over the support using a general finger joint. This is bonded to the tension chord after precise dressing.

The connection of the timber construction to the concrete supports should not be underestimated. At this point, loads of up to Vd=1200 kN are transferred almost invisibly into a concrete column or steel column with a cross-section of 200/300 mm. The loads are transferred via spread GS anchors (threaded rods glued into the timber) to the support plate, which is also embedded in the timber. A screw connection between the support plate and the head plate forms the connection with the support

Findings Ash
In contrast to beech, ash is much easier to work with. It can also be easily combined with spruce. Projects such as the Arosa parking garage have shown that ash already enables competitive solutions when special requirements are placed on the timber construction (such as small cross-sectional dimensions with high loads). Hardwood extends the glulam classes with higher performance. This opens up additional areas of application that were not possible in timber engineering and have been replaced by other building materials such as steel or concrete.

I am convinced that hardwood has the potential to be a natural high-performance building material in timber engineering. Today, high-quality materials still have a decisive disadvantage compared to mass-produced goods: the cost. This applies not only to hardwood, but also to glulam GL36, for example. Mass-produced goods are so cheap that a component in GL24 is cheaper than a component with the same performance in GL48, despite consuming almost twice as many m3. The costs of the hardwood influence the design. The engineer must find solutions that use as little material as possible. Only in this way does hardwood stand a real chance against softwoods. The aim should be to utilize all the advantages of hardwood; in addition to the high strength, the performance of the fasteners is also significantly higher.

Bridge minimum
For an architectural conference, neue Holzbau AG designed a footbridge with optimum use of hardwood. The aim of the work was to impress the experts with a filigree "deciduous" timber construction. The result is the "Minimum" footbridge:

The footbridge spans just over 10 m and is dimensioned with the load capacity of a pedestrian bridge. The shape of the fish belly allowed the cross-sections to be absolutely minimized. The top chord, cross girders and struts are made of ash. Compared to spruce, ash has twice the tensile and compressive strength. Due to the higher density, fasteners such as GS anchors or fully threaded screws can be used much more efficiently. Laminated veneer lumber (LVL) or plywood achieve even higher performance. In the case of laminated veneer lumber, veneers are glued together in parallel, in the case of plywood with cross layers to form a beam or panel. Laminated veneer lumber is used for stress in one direction (bottom chord built in FSH ash). Plywood is required for stresses in several directions (support nodes in beech plywood).

All three cross-sections are designed for the same tensile strength. The glulam GL24 cross-section corresponds to the usual dimensions used. Around 80% of all glulam products from neue Holzbau AG are designed in this strength class. Glulam GL36 corresponds to the highest strength class used today (spruce). The comparative cross-section of the beam on ash glulam shows the enormous potential of ash glulam in the "Minimum" web.