Larch observation tower
Tentech bv
Postbus 619
2600 AP Delft
the Netherlands
Tel.: +31 15 2784034
Fax: +31 15 2784004
Contact:
M. Peter de Vries
Email:
Website:
Tentech B.V. is a Design and Engineering
company of lightweight structures
with a
large experience in
membrane structures.
Tentech
was established in 1997 and has
now a team of 7 people.
Our work is distributed over different area’s
of lightweight structures:
• Tensile membrane structures
• ETFE cushion roofs
• Rental tent structures
• Spatial (roundwood) structures
Technical data project
Physical location
: Stiltegebied
Holmers
Halkenbroek,
Drenthe
Site owner
: Staats Bosbeheer
Architect, Engineer
: ir. Peter de Vries
Engineering office
: Tentech BV,
Delft
Total
weight of the structure
: 16.5 tons
Height of the structure
: 14 m
Largest diameter
: 7 m
Smallest diameter
: 5 m
Building costs
: 137.000 Euro
Building period
: 09/2004 - 11/2004
Introduction: architectural design
In the centre of the nature reserve Holmers-
Halkenbroek, situated in the province of
Drenthe in the
Netherlands,
Tentech
designed and engineered a public accessi-
ble wooden observation tower. The tower is
constructed of slender Larch beams and has
a spatial structure based on the geometry of
a hyperboloid. In clear language the shape
is a diabolic.
A light steel stairway leads the
visitor to the top level platform, approx.
14
m. above the ground.
While the
wide
base of the tower makes sense from a struc-
tural point of view, the large area of the
upper floor is clearly beneficial for the users
of the structure. From the
waist of the
tower, the shape ‘reaches out’ to the sur-
roundings. The orientation of the fence on
the observation platform once again stress-
es this. The tower is situated in a silence area
and can only be reached by foot or bicycle.
Structural system of the tower
There are two cooperating structural sys-
tems: a) the vertical columns and platform
floors that support the stairs and b) the
hyperboloid diagonal structure at the out-
side of the tower. The floor-column system
has a pure vertical bearing constitution,
while the outer diagonal structure bears it’s
own
weight, some of the floor loading of
the top platform and all horizontal forces
(lateral and torsional, caused by both wind
and horizontal load by persons) on the
structure.
The used structural concept made it possi-
ble to build the tower
with very slender
beams. The diameter of the poles at the
ground level is only 140 mm,
with a length
of approx. 4
meter,
while in the storeys
above the diameter is only 120 mm.
Small diameter round wood
The small diameter beams are produced of
wood harvested in forest thinnings. Tradi-
tionally in structural applications
where
round
wood from thinnings is used, the
stems are debarked. This means the beams
are slightly tapered.
Milling the stems cylin-
drical has the disadvantage of loosing a
large (up to 50%) part of the section.
On the
other hand however,
milling has significant
advantages
with respect to the design of
connections. Furthermore, using cylindrical
sections the integration of the round wood
stems
with other, standard, building ele-
ments is
much
more straight forward. An
other advantage of milling is the fact that all
sapwood is removed, and stems consisting
of only heartwood remain. This
makes it
possible to use the Larch beams unpre-
served. For the tower all stems were milled
cylindrical.
Construction
wood from thinnings is not
available via the normal commercial routes.
The material requires special selection tech-
niques to ensure high enough strength.
Using the results of the EC research project
“Small diameter Roundwood in Construc-
tions (1996-1999)”, is was possible to select
wood
with sufficient strength characteris-
tics.
Using ring width,
maximum knot diam-
eter and maximum initial curvature as selec-
tion criteria, the necessary 140 beams were
selected from a batch of 180 poles. The EC5
strength class of the round wood was C24
and all beams used have initial curvatures
less than or equal to 1/200.
92
Company
Project
Larch observation tower
SCIA User Contest 2005 / Civil Engineering
2
Categorie
