One of Würth's most strategically important logistics centres is located near Riihimäki, around 70 km north of Helsinki. The branch was founded in 1975. From there, Würth supplies a wide range of products to end consumers as well as to 200 branches and shops throughout Finland. Due to continuous growth, the branch has been continuously expanded and modernised throughout its 30-year history. In order to meet this continuing trend and the changing business requirements, the Würth Center in 2021 received a supplementary new building to present its special range of the so-called ASSY screw, which is considered the company's key component in timber construction.
For this purpose, the client wanted the new “Würth Center ASSY” to be an engineered timber construction. As a result, it now combines state-of-the-art timber construction know-how and uses the fastening solutions of the extensive ASSY product family.
Complex geometry for seemingly simple solution
The external appearance of the new hall does not initially suggest an extraordinary building structure. The cuboid with a glass façade on one of its front sides first reveals its surprising wooden structure on the inside. The arched trusses of the roof structure are particularly striking. They have numerous special features that merit closer examination, as does the overall structure with regard to load transfer.
The overall structure consists of beams on two columns in a row, with the beams designed as arched trusses. The arched trusses may appear to the layman to be arch-shaped lattice girders of a special design because of the inserted vertical bars, but they are not. It is true that the initial goal was to construct a truss.
However, after numerous designs for truss variants, the structural engineers from SWG Engineering in Rülzheim abandoned this and looked for a more practical geometry. This eventually resulted in the arch truss with vertical bars. And because the client's representatives in Finland also liked the beam shape, the structural engineers developed the arched truss further, right down to the details. The latter raised numerous questions that had to be answered.
For the supporting structure, the building dimensions were specified as just under 58 m long, 23 m wide and 10 m high. This not only fixed the span for the arch trusses at 22.50 m, but also their truss height at around 2.80 m at the lowest point of the arch.
It also quickly became clear that the arches could not be made of glulam due to the required transverse compressive strengths - especially at the support, where the transverse compressive strength in the chord of the arch would not have been sufficient at an angle to the direction of the grain - but that Kerto was the material of choice. This was also a good solution, as Kerto is a tradition in Finland.
Stair offset connects compression chord and tension arch in support area
The arched truss consists of an upper chord, which functions as a compression chord, and a lower chord in the form of a tension arch, which is designed as a relatively thin chord. An extremely long stair offset of 1.20 m and numerous screws couple the two chords in the area of the supports. Seven vertical bars are inserted at intervals of about 2.80 m between the upper and lower chord.
During load transfer, the forces are transferred into the upper chords and from there transferred via the vertical bars into the arches. To optimise the trusses, the structural engineers calculated the ideal bar spacing of 2.81 m by varying the vertical bar spacing. For the visual appearance, there was a choice of vertical or inclined, i.e. right-angled, arrangement of the bars or the 2 x 45 mm thick Kerto-S panels on the arch. The architects rejected the latter for aesthetic reasons. However, the perpendicular arrangement of the height-variable uprights on the tie arches now resulted in inclined panel edges as contact surfaces on the arches or as connection surfaces on the top chord - the only exception being the highest upright in the centre of the truss.
The inclination now results in a thrust force at each upright point, where the upright axis meets the tangent of the arch, or transverse pressure at the contact surfaces between uprights and arches, which had to be transferred. The engineers solved this with milled-out coupling pieces, also made of Kerto S, which they called "shoes". Screwed onto the arches, the uprights could be connected with a precise fit, which enabled a smooth load distribution of the transverse pressure.
In this way, mainly compressive forces occur in the upper chord - although torques also occur and must be transmitted - and mainly tensile forces in the lower chord. These tensile and compressive forces are balanced via the stair offset in the support area and the differential forces are introduced vertically as a support force into the uprights.
Arch trusses supported on notched columns via screws
The column supports are not transversely pressure-reinforced supports, but instead the arch trusses rest purely mathematically on the screws. For this purpose, they were given a trapezoidal wooden block at the ends of the arch on the underside of the arch into which the "support" screws were screwed. The wooden block itself only acts as a filler or holding material for the screws. These act like small posts that absorb the loads via buckling or indentation from the arch truss and transfer them eccentrically into the steel plates that are placed on the notched columns.
The system lines of the upper and lower chord meet in the middle of this eccentric connection and create a torque. The engineers decided to balance the forces at this point accordingly, as the arch trusses would not have been able to absorb any further forces. That is why the supports were clamped in place. For them however, glulam of strength class GL 30h was used.
Bracing via roof panel and intersections in longitudinal wall plane
Self-stiffening rib or box elements serve as the roof covering. They span 6 m from arch truss to arch truss and are similar in design to the Kerto-Ripa elements. Designed as a roof panel, they provide the longitudinal stiffening of the hall together with intersections in the longitudinal wall level. The transverse bracing is provided by the clamped supports. PUR-insulated sheet metal sandwich elements were used for the building envelope.
Concept and design of a multi-part arched truss
The chords of the arch trusses consist of two parts. The top chord consists of 2 x 2 Kerto-S beams (w x h: 15 cm x 45 cm) with a length of 11.25 m. Laid with a clearance of 20 cm, the top chords reach a total width of 50 cm. The notched uprights, which act as "spacers" and at the same time form the support for the Kerto-S-beams, engage in this intermediate space.
In Germany, Kerto has a general technical approval (abZ) (Z9.1-847), respectively a technical approval for curved components (Z-9.1-291) and additionally a technical approval that regulates block-gluing (Z-9.1-100). The Finnish counterpart has a VTT certificate (VTT-C-184/03). The documents thus describe the application of the wood-based material in detail, the German ones even the multi-part application including block-gluing, even in curved form.
On this basis, the production of tension arches from Kerto-S up to a defined thickness was an approved process. Accordingly, the lower chord of the arch truss is composed of 2 x 25 cm wide and 16.8 cm high partial cross-sections.
The 50 cm truss width was on the one hand the decisive measure for the component load-bearing capacity, but on the other hand also for the required area of the stair offset. Conversely, this meant that the overall cross-section of the upper chord could have been smaller, thereby opening up the possibility of constructing it in two parts - and even spaced apart.
The filler timbers in the top chord serve on the one hand as connecting pieces - especially at the ridge point, where they couple the 11.25 m long beams coming from both sides. However, in the tension chord it needs them as statically relevant timber, as the entire offset with a 1.20 m length and 50 cm offset depth is required as a connection area for force transmission. The fillers therefore provide the necessary connection surfaces here, but were also led a little further out of the gusset into the splaying beam area.
On the one hand, the arched truss spandrels or bearing areas received a correspondingly large number of friction-locked screws in order to transfer the transverse forces between the three structural components, i.e. the filler timber and the two beams of the top chord. However on the other hand, they also transfer the compressive force from the three-part overall package into the bottom chord via the stair offset.
Challenge: Production of a block-glued arch truss
The bonding of the components - especially that of the tension arch, i.e. the chord - also meant a great challenge for production. Here, the planners searched for a long time until they found someone who dared to produce the block bonding of the curved components for the bottom chord. In addition, due to the depth of 50 cm, they could not be produced in one piece. As a result, the bottom chords also consist of two parts - contrary to what was stated in the structural analysis, where they were calculated as one part. The two 25 cm wide and 16.8 cm high arches are positioned next to each other. They are fixed via the "kerto shoes" of the uprights as well as the uprights themselves. But they are also fixed at the ends of the arches, where they are held immovably via the stair offset and the screw connections.
Another challenge was the joining of the 50 cm deep stair offsets and the precision required to ensure that all five components that meet at this point fit together exactly.
During the joining process, it was also important to allow the constant radius of the bottom chord arches to taper tangentially towards the support. This allowed for straight surfaces into which a level stair offset could be easily cut.
Completely prefabricated arch trusses connected to columns on site
All eight arched trusses were completely pre-assembled in the factory, packed in film and transported to the construction site. There they were hooked into the already erected supports and connected.
The two-part supports (w x h: 2 x 16.50 cm x 54 cm) are made of glulam and clamped into the foundations via special steel parts. Similar to the top chords, they received filler timbers at the head and foot points so that solid timber cross-sections were available for the connections.
The live loads to be carried result not only from wind loads, but above all from snow loads. Snow has a so-called "medium load duration" (kmod: 0,8), which is to be regarded as quite a high load (sk: 2,75 kN/m2). As snow can occur as an asymmetric load, it was necessary to consider this in the structural calculations with regard to the deformation sensitivity of the arch trusses or the overall construction.
Fire protection only required for F30
As escape from a ground-level hall can be quick in the event of a fire, only an F30 assessment was carried out in terms of fire protection, which turned out to be quite manageable. A sprinkler system is nevertheless part of the fire protection concept.
Advancing screws for timber construction and timber construction itself
The hall is intended as a sales room with a training option and for meetings. This also includes training courses for engineers or structural engineers, who, in addition to classic timber house construction, are now to further promote timber engineering in Finland in the spirit of the Würth Group.
Dipl.-Ing. (FH) Susanne Jacob-Freitag, Karlsruhe
Pictures: Würth Oy
