GoStructural.com | JUN 2011

Constructing life
while building
bridges
The trials and evolution of Craig Finley, Jr. 10

Sneek bridges show
potential of timber
Innovative processes used in open truss foot bridges. 15

modi

DURAB

contemporary bridge design

Sneek bridges show
potential of timber
Innovative
processes used inopen truss footbridges.
By Frank Miebach

Sneek, the Netherlands, is home totwo singular bridge constructions thatcarry multiple innovations. Connection
technique, assembly, and a new modified
wood highlight the potential of timber inconstruction.
The Sneek bridges cross the A7motorway, connecting two districts. Eachof them has a weight of approximately
450 tons, a height of 52 feet, a length of
105 feet, and a width of 39 and 26 feet,respectively. The new eye catchers weremanufactured out of a modified wood:

24,367 cubic feet of Accoya wood,
made from FSC-certified radiata pinethat undergoes a non-toxic modification
process called acetylation, making it
more dimensionally stable and highlydurable. The construction is an open truss
bridge with a spatially twisted truss.
The architecture
The architecture firm OAK (OnixAchterbosch Architecture) won a
publicized competition for the projectwith distinctive timber bridge drafts.
Sneek, located in the province of

The twisted construction units feature a cutout from a spiral and/or a helix, requiring direct, twisted gluing for which there are few existing examples.

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2011 June Structural Engineer

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Glued thread rods were used as bending resistant main methods ofjointing. The bars have partial lengths more than 6 feet, 7 inches,
and are drilled into the cross-grained wood of the diagonals.

Friesland (in the north of the country),
has a beautiful historic town center, iswell known for water sports and tourism,and has, among other things, a salient
historical water gate as a landmark. In
the course of a new motorway bypass,which leads circularly around the center,crossing bridges were necessary. In orderto give anyone crossing the bridges thefeeling of passing a gate with certain
reference to the landmark, the planners
devised a daring design: A solid, complex
shell construction made from exposedwood beams.
Based on the cross-section of roofed
bridges, the idea of the framework system
was an intentional creative constant. A
cross-section, triangular arrangement of
the beams made sense for a non-roofed
bridge, but the building geometry to freespace profile ratio was not particularly
favorable. Therefore, a cross-section
curved form, which guaranteed anadequate free space without being toolarge, was created.
The form of a triangular overhead truss
appears in the side view. A frameworklike filling of this triangle stretches
itself with crossing diagonals like a grid
network into the shell geometry.
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Sporting a solid, complex shell construction made from exposed wood beams, a bridge located in
the Netherlands was designed to reference the town of Sneek’s historical water gate landmark.

The daring geometry of the structure
deemed wood the ideal design solution.
The exposed arrangement also set
enormous demands on the material andchallenged the production process. For
both aspects, solutions had to be found.
Material
Apart from the desire for incorporatinga natural material, a life span of 80
years was required. Under normal
circumstances, this would hardly
be achievable for such an exposed,
unprotected construction. This could
only be realized by constructional woodwith high durability and stability.
It was clear that the complex geometry
and large size would require glued
construction units. However, naturally
durable woods such as bongossi (azobe)
or oak can only be used unglued in theexterior. Therefore, a technical innovationin the timber construction was required
using a sustainably sourced softwood,
which is modified in a non-toxic processthat results in Class 1 durability. This
high-technology wood called Accoya,
manufactured by Accsys Technologies,
is made from radiata pine sourced fromNew Zealand.
Accoya wood’s process alters thewood’s chemistry through a process
called acetylation. Ordinary wood
contains molecules called free hydroxylgroups. They easily absorb and giveup water, which causes wood to swelland shrink. Acetylated wood has beentreated with acetic anhydride, whichtransforms the free hydroxyl groups intoa greater number of naturally occurring
acetyl groups that do not bond to water.
Shrinking and swelling is decreased by
75 percent. Acetylation keeps water frombeing in proximity with a food source
and thus an environment prone to fungirot and insect attack.
As with all redwoods, Accoya wood canbe glued easily, so it seemed suitable forthis application; however, it was being
used in this kind of application for thefirst time. Numerous tests had to prove inadvance that acetylating does not impair
the glue procedure negatively. The tests
showed that Accoya can be glued justas well as conventional glued-laminatedtimber when using the specific resorcinolresin.
Construction
Translating the architecturally impressivegeometry into the language of thetechnical mechanics was handled bya well-known firm with considerable
experience in this exacting area: EmilLüning, which has experience withcomplex structures through geodetic
timber-domed structures. Emil Lüning
tackled the job despite the challenges
associated with new methodology.
2011 June Structural Engineer3

Sneek bridges characteristics
LengthApproximately 105 feet

Width

Approximately 39 feet and 26 feet

HeightApproximately 52 feet

Material

Open truss Accoya wood bridge with a spatially twisted truss, www.accoya.com

Principal

Province of Friesland, Sneek, Netherlands, www.a7sneek.nl

Architecture Achterbosch, Leeuwarden, Netherlands, www.achterboscharchitectuur.nl
Onix, Groningen, Netherlands, www.onix.nl
Design/Grontmij, De Bilt, Netherlands — coordination/concrete construction
Construction Oranjewoud, Heerenven, Netherlands — steel construction
Emil Lüning, Doetinchem, Netherlands — timber construction
Statistics

Blaß & Eberhardt, Karlsruhe, Germany, www.ing-bue.de

TimberConstruction
CompanySchaffitzel Holzindustrie GmbH, Schwäbisch Hall, Germany, www.schaffitzel.de
Projectmanagement IB Miebach, Cologne, Germany, www.ib-miebach.deIt turned out the methods of laminating and jointing are ofgreat importance because the first bridge handles heavy-dutytransportion, requiring guaranteed trafficability performance of60 tons.
Glued thread rods
Thread rods needed to offer high flux with little visibility. Apartfrom conventional methods, special attention was paid to a newmethod. Glued thread rods (which are certified in Germany only
up to 1.18 inches in diameter) were used with dimensions up
to 1.89 inches as bending resistant main methods of jointing.
The bars have partial lengths more than 6 feet, 7 inches, andare drilled into the cross-grained wood of the diagonals. Since
there is little historical data for these dimensions, numerous
experiments were carried out at SHR, the Netherlands timber
research institute in Wageningen. An epoxy resin that alreadydelivered an optimal performance for timber reconstruction andis frequently used in that application area was used eventually.
Apart from the space requirement by edge distances, the methodof joint connecting causes a high quota of carrier diffusion.
This had to be considered statically. The firm Blaß & Eberhardt
from Karlsruhe, Germany, was commissioned to do verifiablecalculations for the threaded rods for nearly all connections.
Some construction units were penetrated extensively and so
could be calculated at only 50 percent of the cross section.

42011 June Structural Engineer

Tests showed that Accoya can be glued just as well as conventional glued-laminated timber when using the specificresorcinol resin.

Input tension/pre-stressing of the bottom chord
Additionally, the bottom chord had to be coupled with thediagonally upward-running compression arch. This was resolved
with a lengthwise chord, which runs within the bottom chord.
The decking is attached on the underside to the wooden bottom
chords in both constructions. The first bridge was built in 2008
with an orthotropic steel plate; the second bridge, built in 2010,
has a wooden plate with melted asphalt. A prestressed cable intransverse direction also was used here.
An important factor is the possibility of motion between thedecking and supporting formwork, which is avoided throughsuitable connections with elastomer strips and slotted hole
restraints.
Manufacturing
Schaffitzel, located in Schäbisch Hall, Germany, was selected
to manufacture the bridge due to its specialization in timberconstruction. The large body of experience within the company
in laminated block beams was very important for the principal.
The most important factor was the construction of the twoaxled way bent construction units. The first step was to glue theconstruction units’ uniaxial to a block and then to mill out thetwo-axle bending with CNC machines from the full material. It
was necessary to look for alternative solutions in order to findpotential savings. The twisted construction units feature a cutout
from a spiral and/or a helix, requiring direct, twisted gluing forwhich there are few existing examples.
The procedure had to be empiric in this case as well, and sample
construction units were glued. The most urgent question of theowner was whether the geometry could be maintained with thisapproach, as a tolerance of only .08 inches was required. It had
to be clarified how much a twisted construction unit moved
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To prepare for final assembly, workers placed a large tent over the shells and heated it. After this, the bondingof the threaded rods using the prepared holes could take place.

backward when the gluing presses areopened (resetting). A defined measure
showed up with the test pieces, so thata sufficient resetting could be considered
and the maximum tolerance could bemaintained in the manufacturing process.
The drilling of the 8.8-foot-long holes
also posed additional challenges. Since
the close drilling raster allowed only
small deviations, high precision wasnecessary, but without CNC support.
The tilted drillings were completed in
the technically most demanding curved
units with templates and auxiliaryconstructions.
As a pre-assembly assistance for the twoshells, a curved assembling table was built
for each part with the same radius as thefinal shell. The glued construction units
were laid out and tied successively on thetable, with individual parts weighing as
much as 20 tons. After this pre-assembly,
the threaded rods were glued in the
middle vertical carrier (king handle).
Due to the large size, the remaining
construction units could only be glued tothe others at the final assembly site.
Assembling
For the final assembly, all prefabricatedparts were delivered to an assemblingarea near the final bridge location. The
parts were laid-out on the assemblingwww.gostructural.com

Two self-propelled modular transporters (SPMT) vehicles were used to move the 400-ton bridge 2,624 feet,
8 inches from the assemble area to the final location.

tables. After all threaded rods wereadded, the workers placed a large tentover the shells and heated it. After this,the bonding of the threaded rods using
the prepared holes could take place.
In the same way, special plans werenecessary for the two ridge steel platesof the first bridge — they were 3.94
inches thick, more than 16 feet, 4 inches
long, and 5 feet, 3 inches high — andthus could only be inserted onsite. Aftercompletion of the two timber shells, thejoining of the three large elements took
place: The two timber shells were taken
up laterally on the decking and attachedfrom the underside.
The final assembly of the bridge took
place as a complete unit with nearly 400
tons of weight that had to be moved 2,624
feet, 8 inches from the assembly area
to the final location. For that purpose,two self-propelled modular transporter(SPMT) vehicles, each with 10 guidable
single axles, were used.
After reaching the final location over themotorway, the construction was pushed
up to the final height of approximately 16
feet, 5 inches by presses and brought inaccurately.

Conclusion
The impressive impact of the Accoya
wood bridge is possible through its
solidity and uniqueness of design byusing timber. During the production
phases, several challenges wereencountered, which were close to thelimit of what is technically feasible.
Thus, such a construction project also
must be considered from a criticalperspective. The technical achievementof the timber construction company
Schaffitzel deserves special mentionhere — particularly as the proof of highprecision without CNC assistance wasdemonstrated.
The enormous motivation by all parties
involved brought the project to perfectcompletion, and will probably be ahistorical moment in the building oftimber bridges.

Frank Miebach, Dipl. Ing. (FH), is a graduatedengineer and co-founder of Schaffitzel + Miebach Faszination
Brücken, a firm specializing in the design, planning, andconstruction of timber bridges. He can be contacted atpresseservice@schaffitzel-miebach.com or
www.schaffitzel-miebach.com.
2011 June Structural Engineer5modified wood by Accsys Technologies

www.accoya.com

DURABLE,STABLE &SUSTAINABLE

WOOD WITHOUTCOMPROMISEACCOYA® and the Trimarque Device areregistered trademarks owned by Titan Wood
Limited, a wholly owned subsidiary of Accsys
Technologies PLC, and may not be used or
reproduced without written permission.

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non-toxic, Accoya® is a revolution in wood technology:- Guaranteed for 50 years above ground- Guaranteed for 25 years below ground- Exceptional dimensional stability- Outstanding coating performance for low maintenance- Consistent quality throughout- Available in FSC and PEFC certified- Cradle to Cradle Gold certified