Structural consolidation of historic monuments by interlocking cast glass components

Abstract

The search for a transparent, reversible, and reusable consolidation system for monuments led to the ambition to test possible cast glass interlocking brick designs using numerical calculations. The focus of this research is hence the design, simulation and evaluation of a possible cast glass interlocking geometry using Finite Element Analysis (FEA). For this purpose, design criteria are formulated from literature; considering glass, polyurethane (used as interlayer) and interlocking systems. As interlocking systems are determined by their boundary conditions, a case study of a monument is chosen to provide additional design criteria. The goal of the case study is to provide a reversible and reusable restoration and consolidation alternative for the current invalid restorations.
The design criteria obtained then are combined into an initial geometry, whose parameters are varied to test their sensitivity to its shear capacity, using FEA. Christensen’s failure criterion is used to locate prone areas in the geometry, and to evaluate the theoretical moment of failure. This output value combines the three principal stresses into a failure envelope, hence can generate contour plots to envision peak-stress-prone areas. This is important especially for glass structures, as they are prone to peak tensile stresses.
From the results design diagrams are created and applied on a conceptual cast glass interlocking consolidation design for the monument chosen as case study: The Lichtenberg Castle ruin.
The initial design is moreover prototyped to check its interlocking capabilities, residual stresses and deviations introduced by shrinkage.

Being able to evaluate possible geometries using FEA can decrease costs and time when searching for a new interlocking geometry. Prone areas are easily highlighted using the Christensen’s failure criterion output. Hence peak stress sensitive or invalid geometries can be discarded before reaching the prototyping stage, which is time consuming and costly.
The creation of a methodology to predict this behaviour is hence valuable for further research on other cast glass geometries and can moreover be applied in any other field when analysing solid complex geometries.
Another goal is to find a cast glass brick design which not only can consolidate the monument of the case study, but is moreover applicable in other projects or configurations. The brick then is not a one-solution design, but can be reused in other projects.

The geometry hence is varied using Grasshopper plug-in for Rhinoceros. By exporting the geometry using a STEP-file, a solid can be loaded into DIANA FEA, where it can be analysed using their newly implemented output value of the Christensen’s failure criterion.
The geometry of the monument is gained through a 3D laser scan, resulting in a point cloud. The point cloud is adapted using Autodesk Recap, then further processed in Rhinoceros.

The Christensen’s failure criterion output is a proper and fast way to evaluate possible cast glass brick designs. Any compressive stresses on the interlocking brick geometry are beneficial for its shear capacity, as is an increase in interlocking amplitude or brick height. Increasing the amplitude however affects the allowable tolerance negatively, which is also the case for a decrease in brick height. Decreasing the brick height hence results in both negative effects.
The conceptual design for consolidation of the Lichtenberg Castle tower can replace the current interventions with equal or higher capacity, even for all conservative assumptions and simplifications. The design can still be altered less conservative after more experimental results and simulations come available.

The methodology applied can now be further developed and performed on other complex geometry designs. The presented multifunctional cast glass interlocking brick design, and its variations can be further investigated and applied in other projects.