An alternative connection in steel towers for wind turbines

Abstract: The increasing prices of fossil fuels during the last decades have sparked
increasing interest and investments in wind power plants to expand their
share of electricity production and improve windmill technology with new and
more economical inventions and sustainable solutions. A RFCS (research fund
for coal and steel) research project called HISTWIN has the aim to improve
the competitiveness of the windmill towers made of steel. The towers for
high power wind generators often consist of several cylindrical slightly
tapered steel segments connected by bolted flanges. This is a traditional
and well established solution. One part of the project is to examine if a
friction connection can be used instead of the traditional flange
connection, because the friction connection will introduce two mprovements:
it is expected to have higher fatigue class, leading to less steel
consumption and the tower will be faster to assemble.
The proposed friction connection is a lap connection bolted with tension
control bolts, TC-bolts. One plate of the connection has normal sized holes
while the other plate has long open slots. This solution is expected to
reduce the total cost of the tower with at least 10%.
The main objective of this report is to establish structural characteristics
of static behavior and the ks factor for this type of connection according
to Eurocode and to investigate the performance of TC-bolts in the
connection. The ks factor is a factor that reduces the slip resistance for
friction connections in EN 1993 - Part 1-8 regarding size of holes used in
the connection. There are no specific rules for long open slots in EN 1993 -
Part 1-8 which are used in the proposed connection.
Another aim of this report is to evaluate different ways to control the
preload. The objective is to evaluate properties of TC-bolts and its
competitiveness. The ks factor is evaluated from four groups of specimens.
The tested specimens have two types of surfaces and have three different
configurations of bolt placement in the specimens. Design of the tower
connections are done with the experimentally obtained ks factor for a lap
connection and comparison is made with an ordinary flange connection.
The results in this report shows that the use of TC-bolts is a very
competitive method to control the preload with respect to speed of
installation, and that the bolts perform well under various conditions and
treatments.
The experiments done in this report shows that the proposed connection has a
ks factor equal to 0,75 as the connections are prevented from in-plane
rotation, this can be compared with the ks factor for long slotted holes
which is equal to 0,63.
Economical calculations show that if a lap connection with long open slots
is used instead of an ordinary flange connection, the cost will be reduced
by more than 85 % per connection in a wind tower. These calculations are
done with respect to price of flanges and bolts provided by HISTWIN
partners.