7.3 Glued connections
7.3.3 Rigid adhesive connections
The search for connection elements with a minimal visual impact has led to intense research in the field of rigid glued connections. Structural silicon is the only adhesive product with a proven track record in glass architecture, however this product is un- suitable for small discrete adhesive confections as it is neither strong nor stiff enough for this application. Epoxies and acrylics, that been used successfully for decades in the aeronautical and automotive industries, are the most promising stiff adhesives for glass construction, however, their performance is largely untested and there are a number of challenges in transferring the technology from other industries to glass construction [315].
Parameters affecting performance
There are many aspects to be considered in the design of a rigid adhesive joint, including the selection of a suitable adhesive, the geometry of the bonded area, the temperature range in which the adhesive must perform and the durability of the adhesive joint.
Some of the first choices to be made in designing an adhesive joint are those relating to the geometry of the adhesive bond. The thickness of the adhesive layer is a primary consideration in this respect. At this stage it is important to distinguish between contact
adhesivesthat require a small adhesive thickness often below 1 mm and gap-filling adhesives that are able to perform at thicknesses in excess of 5 mm. Although annealed glass is a relatively flat product, heat treating the glass causes roller wave distortions and fixing two or more pieces of glass will sometimes require further assembly tolerances. With tolerances in excess of 1 mm it is recommended to use epoxy-based adhesives which have gap filling properties. With lower tolerances it may be possible to use contact adhesives such as acrylic-based adhesives.
UV-curing acrylics have already been used successfully in the furniture industry to assemble pieces made entirely of glass and have been shown to achieve a good joint. Unfortunately, they do not cure effectively in thick layers, because the UV radiation is
not able to reach all the monomers to activate the polymerization. Such adhesives are promising for their use with glass because they are transparent and once cured they are resistant to UV radiation. One of the most interesting applications in architecture to date is the 2001 renovation of the Austrian IBM head office in Vienna: here for the first time a stiff adhesive, instead of structural silicon, is used to structurally bond the façade to the underlying metal structure.
The perimeter shape of the adhesive joint is also an important geometrical con- sideration. One of the disadvantages in using stiff adhesives is their limited capacity to redistribute stress concentrations and to absorb deformation. It is therefore neces- sary to avoid geometrical singularities and sharp edges of the adherents: these lead to stress concentrations[3] and in some cases can be avoided by rounding the edges. If a FE-calculation of the joint is pursued, it is important to include such rounding in the geometrical model, since FE-models could overestimate the stress concentration around points of singularity.
Another aspect to be considered is the temperature range that the connection has to withstand during its service life. If the temperature is above the glass transition temperature of the adhesive Tg, the chain segments of a macromolecule can move, thus
leading to a reduction in stiffness and strength. Below Tgsuch movements are frozen.
The glass transition temperature depends on the chemical composition and on the cross- linking rate of the adhesive. Epoxies often have a Tghigher than acrylics and are therefore
generally more suitable for application at higher temperatures. Both the short term and the long term joint behaviour are influenced by the Tg: the value of the force the joint carries is generally lower when approaching Tgand the viscous component of the deformations increases as the temperature gets closer to the glass transition temperature [287, 290]. When Tgis reached, the change in the fundamental quantities is not abrupt,
but gradual[4]. Moreover, the adhesion forces are lower at high temperatures, so the same product may exhibit a cohesion failure at low temperatures and an adhesion failure at high temperatures[42].
The need for a surface treatment to improve adhesion on glass depends on the kind of adhesive. The glass surface to be bonded can be treated by means of mechanical or chemical processes or with the use of primers, which may provide a more receptive layer to the adhesive. Initial information on certain products as well as on certain treatments is available, but this field needs further investigation[239].
The last important aspect to be taken into account is durability[226]. There is a lack of research on the long term performance of glass adhesives[239], so the following information draws from the experience gained in the field of metal adherents. The influence of service time on adhesives depends on their chemical composition and on their cross-linking rate[185]. It has been found that the mechanical properties of an adhesive joint, which depends on the adhesive layer itself, as well as on the interface adhesive-adherents, may deteriorate upon exposure to its service environment: water, in liquid or vapour form, is the most hostile environment for structural adhesive joints that is commonly encountered. The influence of water on the adhesive is generally reversible, so that any deterioration is recovered upon drying. All polymers absorb greater quantities of water when their temperature is above Tg, so that rubbery materials tend to show greater
water absorption than rigid adhesives[248]. Other parameters affecting environmental durability are temperature, stress rate and distribution in the adhesive layer as well as surface characteristics and pretreatment of the adherents. Accelerated weather tests have
been set up in laboratories to accurately model the complex deterioration process within a reasonable range of time[83, 126, 142, 143, 156–159]. These are obtained by increasing the presence of certain agents above their natural rates: UV radiation, moisture or water and temperature. Specimens may also be immersed in acids or salt solutions. The results obtained should preferably be compared to published information on natural weathering behaviour.
In addition to the influence of aging, the effect of each of the previously described described parameters has to be investigated by means of tests which could be carried out on bulk specimens or on the whole joint. Similar tests are carried out for overlapping metal joints[18, 19, 82] so that in certain cases it is possible to adapt the existing tests for glass applications.
In general specific research on the use of glass and stiff adhesives has focused on assembling all-glass systems or on developing mixed structures. The former includes proposals for glass adhesive T-beams composed of two glass panes[277], for a glass cruci- form column composed of three pieces of glass[261] and for a glass shell (Figure 7.21) assembled by means of adhesive butt joints[42]. The latter includes research on com- posite beams made of a wood frame glued onto glass which has a stiffening function [189, 231, 257] and glass-fibre-reinforced plastic profiles glued on glass plates [227]. This innovative research provides a glimpse of the opportunities offered by using stiff adhesives, but it is important to note that there is still a lack of understanding of the basics in glass adhesion. therefore a substantial amount of further research is required in this field for glass adhesion to become an accepted and mainstream form of construction.
Figure 7.21:
Example of a glass shell with butt adhesive joints. (designed and built by Sobek and Blandini, University of Stuttgart, Germany)
Limit state design
According to the limit state analysis, the safety factors used in the design of adhesive joints must take into account the uncertainties associated with the fabrication and analysis of the joint (effects of workmanship, uncertainties concerning the assumed stress distribution in the joint) and the changes in material with time. Suggestions for the values of the safety factors are given in the EUROCOMP Design Code and Handbook[67] (Table 7.22). In this proposal it is assumed that the adhesion to glass is adequate, so that only the behaviour of the adhesive itself is considered. With this premise, the material safety factor is calculated as
Source of adhesive properties γm1
Typical or textbook values 1.5
Values obtained by testing 1.25
Source of adhesive properties γm2
Manual application, no adhesive thickness control 1.5 Manual application, adhesive thickness control 1.25 Established application procedure with repeatable and
controlled process parameters
1.0
Type of loading γm3
Long-term loading 1.5
Short-term loading 1.0
Environmental conditions γm4
Service conditions outside test conditions 2.0 Adhesive properties determined for the service condi-
tions
1.0
Fatigue loading γm5
Adhesive subjected to significant fatigue loading 1.5− 3.0
Loading basically static 1.0
Table 7.22:
Recommended values for partial safety factors to be applied to adhesive properties[67].
γm=Y γmi≥
¨2 for connections designed by testing,
4 for connections subjected to long-term loading. (7.4) For the design of adhesive joints in case of fire it will be necessary to carry out a heat flow analysis and determine the capacity of the joint under the design temperature[218]. However, where fire is a major design consideration, a pure adhesive joint will not be appropriate, unless the adhesive can be effectively insulated.
Wellershoff[333] suggests the following approach for the design of glued connections
τEd τRd = τEk·γF τRk· fT,t γM ≤ 1 (7.5)
whereτEkrepresents the characteristic value of the shear stress,τRkthe characteristic
value of short term shear strength, fT,ta reduction factor which is a function of the material
temperature T and the load duration t (represented schematically in Figure 7.23),γF
is load safety factor (according to national code) andγM is the material safety factor
(according to national code).
Experimental studies have been carried out on the time and temperature dependant behaviour of adhesives which are used in glass construction and diagrams were developed for the reduction factor fT,t[333]. Additionally creep effects under a constant material temperature T may be quantified using Equation (7.3) and Equation (7.5)
fT,III0= fT,I0· IT,I0
IT,III0 = fT,I0· BT· tT,I0α BT· tT,III0α = fT,I0· 0, 1 tT,III0 α (7.6)
Figure 7.23:
Schematic representa- tion of the reduction
factor fT,t. f f T,t T,I0 t [h] 1.0 T [°C] 1 0.1 10 10 10 10 10 10 2 3 4 5 6 20 40 60 80
where IT,I0is the creep resistance at the beginning of deformation region I with tempera- ture T and IT,III0is the creep resistance at the beginning of deformation region III with temperature T . For other stresses in glued connections the method is similar.