1 THERMAL INSULATING CONNECTIONS

The connections between the inner and the outer structures often cause the effect of thermal bridges in the external cladding of the building. To break the thermal flow in the steel structures it is necessary to insert an intermediate layer of thermal insulating material between the two bolted end-plates or the end-plate and the column flange. The insulating layer is under the ef-fect of the internal forces transferred by the joint – rotational moment, normal force and shear force (Lange & Göpfert 2005), see Figure 1. The materials which have good thermal insulating characteristics as well as sufficient compression and shear strength are either elastomers/rubber (Nasdala et al. 2007) or technical plastics of the thickness from 5 to 25 mm depending on the actual requirements of the connection.

2 HEAT ENGINEERING

The heat engineering standard is predicting the obligatory values for the energetical costingness of the buildings. To minimize the heat costs it is necessary to break the thermal bridges in the external cladding which are rapidly increasing the value of the overall heat conductivity. In the steel structures the usual thermal bridges are caused by the connections between inner and outer structures, e.g. balconies, loggias, garages, roofs, cantilevers etc., see Figure 2. The thermal in-sulating connections offer a good solution for energy sparing as well as preventing the fungoid

Integration of the new component into the design method for thermal insulating connections

Zuzana Sulcova Faculty of Civil Engineering, CTU in Prague, Prague, Czech republic

Zdenek Sokol Faculty of Civil Engineering, CTU in Prague, Prague, Czech republic

Frantisek Wald Faculty of Civil Engineering, CTU in Prague, Prague, Czech republic

ABSTRACT: The new type of thermal insulating connection has been developed in order to minimize the heat costs of the building. The research is focused on the static design of the bolted steel end-plate connection with intermediate insulating layer which would break the thermal bridges in the external cladding of the building. The static design of the thermal insulat-ing connection is using the component method as in Eurocode [EN 1993-1-8:2005] extended with a new component ’insulating layer in compression’. The materials for the insulating layer are the technical plastics with sufficient compressive and shear strength. A couple of compo-nent tests have been undertaken with the plastic layer Erthacetal H in compression and the cha-racteristics of the new component have been implemented into the component design of the joint to predict the behaviour of the whole connection under the effect of rotational moment and normal force. The connection tests have been undertaken in order to verify the calculated val-ues. Other tests will show the possibility of using the pre-stressed bolts to transfer the shear force via friction in the joint that is depending on the specific material used for the insulating layer. The new solution for the thermal insulating joint seems to be statically functional, easy to design, energy saving and economically efficient.

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growth in critical points of the building. This is another step for improving structural details in ecologically and economically efficient sustainable building (Šulcová et al. 2007).

Figure 1. Thermal insulating connection under the effect of the internal forces.

Figure 2. Critical points of the building where thermal insulating connections are necessary.

Concerning the static requirements of the connection the thickness of the thermal insulating layer can vary only between cca 5 and 25 mm which is not comparable with the thickness of the cladding insulation system. The thermal conductivity of the technical plastics and elastomers is compared to other materials in Table 1. However even with these poor characteristics of the in-termediate layer the structural detail works properly when being a part of the whole structure, see the 3D thermal simulation by TRISCO (Šulcová et al. 2008). The 2D simulation by AREA has shown a big influence of the intermediate insulation on the thermal flow in the connection, for comparison see Figure 3.

Table 1. Thermal conductivity of materials concerned. Material Thermal conductivity [Wm-1K-1] Steel 46 Stainless steel 16 Technical plastics, elastomers 0.2 – 0.3 Polystyrol 0.03 – 0.05

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a) te = 0 mm

b) te = 5 mm

c) te = 10 mm

d) te = 20 mm

Figure 3. 2D thermovision of more or less thermal insulting steel connection (left-hand side interior structure, right-hand side exterior structure).

3 STATIC MODELING

The analytic modeling method is used for static design of the thermal insulating connection. The component method describes the joint as a moment-rotation relation. The main characteris-tics of the joint are the ultimate moment bearing capacity, the initial stiffness and the rotational capacity. Firstly the joint is disintegrated into so-called components which are investigated sep-arately. Secondly the components are put together with respect to their real position in the joint and the characteristics of the joint are calculated from the partial values. The components of the thermal insulating joint are shown on Figure 4. The steel components are already well-described (Sokol et. al 2002, Sokol et al. 2006). The only new component is the 'thermal insu-lating layer in compression'. Once the new component is described (with a couple of experi-ments) the characteristics of the whole joint can be easily derived.

Figure 4. Components of the thermal insulating joint.

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4 COMPONENT TESTS WITH ERTHACETAL H

A set of experiments had been undertaken with a technical plastic material Erthacetal H (for the characteristics see http://www.tribon.cz/). The influence of the thickness of the insulating layer was tested and the values of force-deformation relation, stiffness and the width of the compres-sion area under the beam flange were monitored. The results are to be seen on the following graphs.

The experiments had been done for the thickness of the steel end-plate t1 of 12 mm and 20 mm and for the thickness of the insulating layer t2 of 8 mm, 16 mm and 25 mm, see Figure 5. The Figure 6 shows decreasing stiffness of the investigated component with the increasing thickness of the component.

Figure 5. Scheme of component tests.

Figure 6. Force-deformation relation for different thicknesses of the component.

4.1 Width of compression area The width of the compression area had been measured using the copy-paper, see results on Fig-ures 7 and 8. It shows a good accuracy when compared with the calculated values using the analytic relation for the effective width:

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225,2 21

tttb fbeff ++= (1)

However it is also possible with a good agreement to use the modified relation for the offset width c of the compression area around the column flange known from design of the steel col-umn bases:

45,13)

2( 21 ∗

+=e

y

f

fttc (2)

where fe is the experimentally stated elasticity limit of the insulating material Erthacetal H.

Figure 7. The compression area measured using a copy-paper during the experiments.

Figure 8. Width of the compression area compared with the calculated values.

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4.2 Component stiffness The stiffness of the insulating component is the lower the thicker the component is, which is similar to the behaviour of the steel plate component in compression and can be described with the following rule

E

E

t

Attfkk eeffc

22115 *);(== (3)

where

20012,0);( 21

21t

tttf ++= (4)

and Ee is the experimentally stated Young's modulus of elasticity of the insulating material Er-thacetal H. For comparison see Figure 9.

Figure 9. Component stiffness compared with the mathematical approximation.

Figure 10. Limit of component elasticity compared to calculated approximation.

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4.3 Limit of elasticity The force at the limit of elasticity of the insulating layer is also the lower the higher the thick-ness of the component is, see Figure 10. It can be described as

eeffy fAttfF *);( 21= (5)

where

1,040

3);( 12

21 −+=t

tttf (6)

4.4 Creep The creep effect of the insulating layer is increasing with the thickness of the layer. A couple of long-term tests are being undertaken to measure the amount of creep and its influence on the loss of the stress in pre-stressed bolts during the connection lifetime. In case of large creep be-haviour of the insulating material the shear force in the connection cannot be transferred via friction and the connection needs to be upgraded with a special shear bracket. Creep could also cause an enormous rotation in the structure.

4.5 Material tests A couple of material tests had to be done with the insulating material to work out the Young's modulus of elasticity Ee and the elasticity limit fe mentioned above in (3) and (5). The experi-mental values for Erthacetal H had been measured on the cubes sized 30 x 30 x 25 mm and the relations mentioned above are fit to these values. It is obvious that the material characteristics measured on different experimental cubes would need some change in the approximation rules (4) and (6).

5 CONNECTION TESTS WITH THE THERMAL INSULATING JOINT

The whole connection tests have been undertaken, see Figure 11, to compare the real connec-tion behaviour with the calculated values. The moment-rotation relation of the connection has been measured as well as the width of the compression area. The results are being worked up to verify the component method as a design method for the thermal insulating connection.

Figure 11. Scheme of the connection tests.

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6 SUMMARY

The new component 'insulating layer in compression' made of the insulating plastic material Er-thacetal H has been described. The results have been implemented in the design calculation of the connection using the component method to describe the whole thermal insulating connec-tion. The connection tests as well as some long-term creep tests and material tests are being un-dertaken to confirm the suitability of the component method for the design of this type of con-nection.

The goal of this research is to introduce the new type of thermal insulating connection with a complete design solution which should be simple, functional, reasonable priced and economi-cally efficient. It should spare a large amount of energy in ordinary family houses as well as in big business buildings.

7 ACKNOWLEDGEMENT

This project is supported by grant COST C25 OC09065 Component method for the connections without thermal bridges. This support is gratefully appreciated.

REFERENCES

Lange J. & Göpfert T. 2005. The Behaviour of Semi-Rigid Beam-to-Beam Joints with Thermal Separa-tion. In 3rd International Symposium on Steel Structures – ISSS’05, Seoul, Korea: 399-408. Seoul.

Nasdala L.,, Hohn B. & Rühl R. 2007. Design of end-plate connections with elastomeric intermediate layer. In Journal of Constructional Steel Research 63(4): 494-504. Oxford: Elsevier Ltd.

Sokol Z., Wald F. & Delabre V. & Muzeau J. & Švarc M. 2002. Design of End Plate Joints Subject to Moment and Normal force, In Eurosteel 2002, Vol. 2: 1219-1228. Coimbra: Universidade de Coimbra.

Sokol Z., Wald F. & Chlouba J. 2006. Prediction of End Plate Joints Subject to Moment and Normal Forces. In Proceedings of the International Conference in Metal Structures 1: 235-240. London: Tay-lor & Francis.

Šulcová Z., Sokol Z. & Wald F. 2007. Steel end-plate connection with thermal insulating layer, In Sustai-nability of Constructions – Integrated Approach to Life-time Structural Engineering: 3.109-3.114. Lisbon: Multicomp Lda.

Šulcová Z., Sokol Z. & Wald F. 2007. Structural connections with thermal separation, In Proceedings of CESB 07 Prague International Conference, Vol. 2: 672-677. Prague: Czech Sustainable Building So-ciety and CBS Servis.

Šulcová Z., Sokol Z. & Wald F. & Rabenseifer R. 2008. Component method for connections with thermal separation, In Eurosteel 2008, Vol. A: 621-626. Graz.

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