Scia User Contest 2005 - page 95

Connections
Special elements in the structure are the nodes
that connect the round
wood beams. These
nodes have large eccentricities (the heart lines of
beams that are jointed in a node cross, rather
then that they cut in the node). To prevent insta-
bility in these joints, a moment resisting connec-
tion couples the nodes in the outer structure to
the floor-columns structure that surround the
stairs.
By having the round wood beams cross in the
joints, and making a smart choice for the heart
line of the connections, the axial and lateral cou-
pling could be carried out on a standard
way.
This approach made it possible to use identical
parts for all round wood connections.
The detailing of the coupling blocks in the nodes
is such that it is possible to disassemble each
round wood beam in the structure, and have it
replaced easily. This aspect of the nodes is impor-
tant, as the wood in the tower is not treated with
preservatives.
During the drying of round wood, cracks devel-
op along the length of the stem. The cracks are
caused by differences in radial and tangential
shrinkage of wood. This phenomenon is charac-
teristically for round wooden poles in which the
pith is still in the section.
When making bolted or
dowelled joints in such beams, the cracks lead to
problems. Therefore another jointing principle
was developed. In the EC research project men-
tioned earlier, extensive research
was done to
the ‘block shear joint’. This joint is less sensitive to
the cracks in the round
wood. Based on this
jointing technique the (patented) construction
node was developed that was used in the tower.
The principle of the jointing technique is also
applicable for other structural elements, like
trusses.
ESA-Prima Win
In ESA-Prima Win a complete model of the tower
was built. The eccentricities in the joints of the
round wood beams are of key importance on the
structural behaviour of the tower. Therefore the
eccentricities are all in the calculation model.
Self weight and loading of the stairs are essential
loading components of the structure. To design
the stairs and have overall clarity in the calcula-
tion note, the stairs are part of the model. Special
modelling
was used to correctly predict the
forces on the (concrete) foundation.
The eccentricities in the joints and other connec-
tion detailing cause bending
moments in the
round wood beams. The ‘block shear joint’ con-
nections are designed essentially for axial loads,
and have only limited bending moment capacity.
The extreme slenderness of the beams, large ini-
tial curvature of the round
wood, high design
tension and compression forces and the bending
moments that result from the eccentricities in the
connections,
made it
necessary to perform
design calculations in which the influence of the
initial curvature is accounted for. In the ESA-
Prima
Win
model the round
wood elements
were given initial curvatures, corresponding to
the measurements we made during the selection
of the stems. The 2nd order frame calculation
was used to do the analysis in ESA-Prima Win.
Calculations of the non-linear combinations with
initial curvature
were carried out using the
‘Timoshenko’ algorithm.
Light weight structures like this tower are sensi-
tive to dynamic response of horizontal loading
by
wind and/or persons. Notably in this case,
were also the eccentricity in the nodes has nega-
tive influence. To analyse this aspect of the struc-
ture, the horizontal deformations of the structure
were studied carefully, and hand calculations
were made to verify correct dynamic behaviour.
We clearly lacked the ESA-Prima Win eigenfre-
quency module here, but unfortunately the engi-
neering budget was insufficient to purchase the
module for this project.
ESA-Prima Win 3.50 modules used
Base
3D frame
Dynamic document
2nd order frame with initial curvature
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