advanced building skins 14 | 15 June 2012 - lamp.tugraz.at - Graz ...

5 Connecting Methods
Advanced Building Skins
One of the core issues of prefabrication is the connection of the segments. In a loadbearing shell made
of prefabricated elements, the joint primarily has to carry membrane forces, but also bending moments
and transverse forces. Just a few examples using joining technologies that can fulfil the requirements
were realized.
a) Wet joints - Subsequent cast of the joints with overlapping reinforcement [10].
b) Use of semi-precast parts, which are casted with a second external concrete layer. Prominent
example is the “Palazetto dello Sport” in Rome by Pier Luigi Nervi [11].
c) Carry of the internal forces by high precision, subsequent finished “dry joints” in combination
with reversible fasteners, eventually used for prestressing. Realized example is the
“Wildbrücke” in Völkermarkt [12].
5.1 Methodology
The method that is brought into focus is the “dry joint”. Idea of this technique is to maximize the
surface contact and thereby the static friction. The segments can be connected by reversible fasteners.
This technique requires high precision grinding of the contact faces.
According to these requirements the tools and the grinding material were developed in close
cooperation with Rappold Winterthur Technology GmbH. They are available for first tests. The
specification was defined as following:
a) Material: Steel fibre reinforced UHPC
b) Precision: 2/10 mm measured at the outline
c) Minimize forces in the tool
The process scheme is known from the typical milling process and is divided in two stages. In the first
step the tool roughs down the concrete. In the second step the abrasive disc cuts the steel fibre
reinforcement. The first tests attest the strategy fulfilling all demands. On-going research focuses on
questions of speed and the rate of removal.
Figure 12: First attempt of wet machining Figure 13: Grinded contact face
A transfer of this strategy for the application on double curved elements is aim of the next steps.
Arising questions to solve are:
Get knowledge about the tool material interaction using thin elements made from UHPC
Questions of measurement systems to align the elements and to verify the results of machining
- 8 -