STUDY OF THE STRUCTURAL BEHAVIOUR OF THECONNECTION BETWEEN MULLIONS AND TRANSOMS

IN WOOD-GLASS FACADES

Philip Steinhausen

Department of Civil Engineering, IST, Technical University of Lisbon

1 Abstract

Keywords: Beech, Connection, Facade, Glass, Glulam, Mortise, Mullion, Structure, Tenon, Timber,Transom, Wood

This study analyses the structural behaviour of the connections between mullions and transoms in beechglulam wood-glass facades. The use of beech wood in construction can be a response to possible futurebottlenecks in the supply of construction wood, which might result from the forest conversion. Anotheradvantage is that it is stiffer and stronger than the usually used coniferous wood, in particular when hav-ing in mind the weight of modern three-layered glazing. State-of-the-art CNC milling machines furtheropen up the possibility to link the wooden elements of such a facade through multiple mortise-and-tenonjoints.Based on preliminary data, and an FE model, the author comes to the conclusion that beech wood canin fact be used for these purposes. It was proven that, when subjected to dead loads, the failure of thesestructures occurs perpendicularly to the fibre direction. Tests to determine the transverse tension andtransverse bending strength of beech glulam revealed that, compared to spruce/fir glulam, the resistanceof this still unconventional construction material is considerable. The results of the FE model confirmthese findings, showing the stress peaks in the range of the transverse tension and transverse bendingstrengths of beech glulam. Further, a design concept consisting of two equations which can be used infuture investigations, was formulated.

2 Introduction

This study was conducted at the Karlsruhe Institute of Technology (KIT) with the objective of investigat-ing the load bearing capacity of a beech glulam transom (see Figure 1) with multiple mortise-and-tenonconnections (see Figure 2). The focus was on the strength of the transom to tensions perpendicular tothe grain and the corresponding failure mechanisms that occur due to dead loads (glass loads and ownweight), providing a basis for further investigations.The expanding use of three-layer glazing to improve the energy efficiency of buildings and the resultingincreasing masses of the window panes intensify the risk of failure of the connections between mullionsand transoms. This could make the material wood competitive in comparison to alternative constructionmaterials like steel, even when having high impacts on the supporting structure.So far, connections as used in traditional carpentry are relatively rare in wood-glass facades. Multiplemortise-and-tenon joints themselves are not an innovation but in the last few decades it was more eco-nomical to use industrially produced connectors than to hire specialised manpower with the skills tobuild complex and precise shapes out of the wood. In the last years new technology became available,particularly Computerized Numerically Controlled (CNC) milling machines, which are designed for thattask.

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3 Contextualisation

In recent decades, the importance given to the forest conversion in Europe has grown. Forest conversionconsists in the regeneration of forests with site-adapted tree species, mostly in a semi-natural way becauseof economical reasons. Nowadays approximately 73% of German forests consist of mixed stands, accordingto the Federal Ministry of Food, Agriculture and Consumer Protection (2011b). The trends towards asustainable and nature-oriented forest management led to a strategy of transforming “the coniferousmonocultures into more stable deciduous ecosystems” (de Goede cit. in Nabuurs et al., 2003, p. 26).Moreover, it seems that the limits of sustainable use of coniferous wood are gradually being reached.In central Europe deciduous wood is clearly underutilised and the forest conversion, required due to theclimate change, is well underway, increasing the stocks of non-coniferous trees. These facts result in anincreasing amount of available deciduous wood and a decreasing production of coniferous wood. This waytraditional markets are eliminated and it is therefore necessary and important to develop new productsfor – and adapt existing products to – this natural resource.

“In order to tap into the potential of non-coniferous wood, the timber, pulp and paper indus-tries are called on to develop further innovative and resource-saving areas of use.” (FederalMinistry of Food, Agriculture and Consumer Protection, 2011a, p. 13)

Being the largest segment of deciduous wood in Germany it is especially interesting to investigate thepotentials of beech wood. In this context, one field of use could be the construction industry.

4 State-of-the-art mullion-transom connections

In general there are two different solutions for the connection between mullions and transoms: pluggedconnections (Figure 1a) and slide-in connections (Figure 1b).For plugged connection systems the first mullion is put into place and the corresponding transoms areplugged into the mullion. After that the next mullion is plugged onto the previously connected transomsand the next set of transoms is inserted, etc.. This makes the assembling process difficult. The companySchindler Fenster & Fassaden GmbH suggests a modular concept where all connections are plugged. Inthat case, each pre-fabricated module consists of two half mullions and the corresponding transoms. Whenmounted, the two half mullions that stand side by side are connected through the vertical profiles. Apartfrom being easy to set up, with this solution it would even be possible to screw through the mullions into the grain side of the transoms to secure the transoms in the direction of their own axis. These screwswould be invisible to the user.For slide-in connection systems all the mullions are mounted, and then the corresponding transoms areslid in orthogonally to the transom and mullion axis. This type of connection is convenient because thetransoms do not require any additional, support during the assembly and can be slid into the alreadyset up mullions. In addition, no extra securing means are required for loads parallel to the transom axis.However, special locking means are required for wind loads, resulting in relatively complex connectorconfigurations.Most of the connections between mullions and transoms are made through connection elements like bolts,dowels or screws. In contrast to that, multiple mortise-and-tenon joints, which are plugged connections,can work without these connection elements. Only a safety element to prevent the tenons from beingpulled out of the mortises could be necessary.

5 Preliminary tests

Prior to this study, preliminary tests were conducted with mullion-transom structures made of beechglulam that were connected through mortises and tenons (see Figure 2) and secured in the direction ofthe transom axis with the aid of steel bolts. The average carried load was Q = 11, 04 kN with a deflectionless than 4 mm for all tests.The failure of the multiple mortise-and-tenon joints occurred perpendicularly to the grain, more specifi-cally in two regions: in front the first tenon (hereinafter called zone A) and at the lower edge of the slotthat supports an aluminium profile (hereinafter called zone B). Figures 3a and 3b exemplify these types

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(a) Walter Lang mullion-transom connectionsystem (Z-9.1-688 2008).

(b) RAICO mullion-transom connection sys-tem (Z-9.1-621 2011).

Figure 1: Examples of plugged-in and slide-in connection systems.

of failure. It seems that the failure in zone A occurred as a transverse bending failure that came from thetorsional moment which was originated by the glass load. The failure in zone B was a transverse tensionfailure.

Screws

Glass load Holes for steel bolts

Figure 2: Beech wood transom cross-section with multiple mortise-and-tenon connection and alu-minium profile to support the glass panes.

The results of the preliminary tests can be compared to tests that were made at the KIT with state-of-the-arts connection systems from different manufacturers (see Table 1). The comparison between thetests of the state-of-the-art connection systems and the multiple mortise-and-tenon joint can only bemade within certain limitations because they were not obtained with the same test configurations. Thebeech glulam multiple mortise-and-tenon connection is stiffer than the other connection systems. It canalso be seen that the beech glulam connections had a very high strength.

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(a) Failure of specimen V1-2 in zone A. (b) Failure of specimen V1-1 in zone B.

Figure 3: Failure pattern at the connection cross section of the tested transoms.

Experimental strength (dead loads)

Connection systemTransom size

(h/b/l) in mmQav in

kNδav inmm

Walter Lang 50/260/845 6,85 6,96

Seufert-Niklaus 50/160/845 7,93 5,41

Knapp 50/180/500 8,78 5,54

RAICO 50/260/850 11,43 8,82

Hoffmann 50/150/850 3,32 5,12

Table 1: Experimental loads that were carried by different connection systems.

6 Tests

Having in mind the results and specially the failure modes of the preliminary tests, two different typesof tests were made with beech glulam: four point transverse bending tests (see Figure 4a), to determinethe transverse bending strength and transverse tension tests (see Figure 4b), to determine the transversetensile strength.The four point transverse bending tests were made based on the directives described in DIN EN 408(2012), section 10 and 19 and the transverse tension tests were based on section 16 and 17 of the samestandard. For the transverse tension tests the loads were applied with the aid of screws, instead ofmetal plates. The use of screws implies a stress peak in the region of the screw tip, making this point apredetermined breaking point.

Some samples had to be glued together to get specimens with sufficient length for the tests. Thosespecimens often failed due to problems with the glue joint. This way, the tests series with those specimensare not further referred in this paper.

Within the transverse bending test series V1 it was interesting to observe that the failure always occurredin layer 5, except for the last tested specimen (V1.9.2), which failed in layer 7. For all of these tests thefailure occurred in radial direction, parallel to the medullary rays (see Figure 5), which is the weakestdirection of wood. The results of test series V1, especially the tests with bsp · hsp = 70 · 13 mm2, showthat within the same beam/transom, and even within the same layer, there is a great dispersion of thestrength (from 6, 60 N/mm2 to 13, 59 N/mm2) due to the anisotropic structure of wood.

Within the transverse tension tests, the failure often occurred in the region of the screw tip where thestress peak occurred (see Figure 6). This means that, in those cases, the measured strengths are lowerthan the ones that could have been obtained with an optimized test set-up without stress peaks.

The pre-drilled holes of series V2 were not straight. This flaw had no visible negative impact on thestrength of the specimens when compared to test series V3. In fact, when observed separately, theaverage strength of series V2 is higher: ft,90,V 2 = 7, 07 N/mm2, compared to ft,90,V 3main = 5, 67 N/mm2

for the main series of V3.

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(a) Transverse bending. (b) Transverse tension.

Figure 4: Transverse bending and tension test set-up.

Figure 5: Failure of specimen V1.3.

Figures 7a and 7b, show the force-deflection curves of one test of series V2 and V3, respectively. It can beobserved that the effect of the not straight pre-drilled holes is a pronounced differential deflection of thefront- and backside of the specimen. In fact it can be seen in Figure 7a that the backside (blue line) startswith negative deflections, which lead to compressions. The front side (red line) starts with pronouncedtensions. This behaviour can be explained with the tendency of the screws to become straight, introducinga moment which causes tension on one side and compression on the other. Moreover, it can be seen inboth Figures that the average force deflection curve is approximately linear and that the failure is sudden(brittle). This last observation was made in all other tests and stands for both tension, as well as bendingtests.

As explained before, the force-deflection curves were approximately linear for both the tension and bend-ing tests. Figure 8a emphasizes this fact, showing the results for all the bending tests of series V1. ThisFigure also shows that, as can be expected, the slope varies with the cross-section dimensions (blacklines: bsp = 70 · 36 mm2; red lines: bsp = 30 · 36 mm2;green lines: bsp = 70 · 13 mm2). Nevertheless, ascan be seen on Figure 8b, the slope of the stress-strain curves is very similar, even with a high variationof the maximum strength.

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Figure 6: Failure of specimen V2.2 due to the stress peak at the screw tip.

(a) Force-deflection curve of test V2.3. (b) Force-deflection curve of test V3.1.

Figure 7: Examples of force-deflection diagrams of test series V2 and V3.

(a) Force-deflection curves of tests series V1. (b) Stress-strain curves of test series V1.

Figure 8: Results from test series V1 displayed graphically.

The specimens of test series V7 and V5 were made of five different beams of strength class GL40h. Thefollowing average results were obtained:

Transverse bending test series V7: Transverse tension test series V5:

fm,90,av = 10, 84 N/mm2 ft,90,av = 5, 92 N/mm2

Em,90,av = 1305, 29 N/mm2 Et,90,av = 1254, 25 N/mm2

These results can be compared to the values given in the general technical approval for beech Glulam(Z-9.1-679 2009):ft,90,k = 0.5 N/mm2

E90,mean = 690 N/mm2

E90,05 = 550 N/mm2

It can be seen that the average traverse tension strength is ten times higher than the one that is allowedaccording to the general technical approval. It is also worth a notice that even the lowest transverse

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tension strength ft,90 = 2.20 N/mm2, obtained for test V8.5 which had a considerable imperfection, isfour times higher. The average modulus of elasticity that was measured in the tests is approximatelytwice as high as the one given in Z-9.1-679 (2009).

7 Finite element model

The model built for this investigation was a simulation of a transom with multiple tenons. It was athree-dimensional model because the focus was on a singularity, the connection, where the behaviour cannot be simplified. The glass load was applied to a metal profile on two points, to simulate the effect ofthe two glass supports.The profile had a T -shaped cross-section and was supported by a continuous slot in the transom. Theload was transmitted from the profile to the transom by screws and through contact, leading to a localcompression. Since the interaction between profile and slot was similar to the behaviour of a notchedbeam, transverse tensions occurred. The screws through both flanges of the profile transmitted forcesthrough shear/hole bearing and pulling out. To reduce the processing time of the simulation, the sym-metry of the problem was taken into account. This way, considering the symmetry conditions, only halfof the transom had to modelled. The fact that the transom to be studied was made of glulam wasnot explicitly taken into account in the model, neglecting that in general the glue joint should be moreresistant than the wood. This was a conservative assumption.In the tests the transoms were supported on each side by tenons that were slid into mortises (the slide-indirection is parallel to the z-axis). In x and y direction the tenons were held by the mortises. As asimplification, the mullions, and consequently the mortises, were not modelled and their influence on thebehaviour of the tenons was simplified by fixing two lines of each tenon in x and y direction. The modelignored the effect of the friction between mortises and tenons, putting no restrictions in z direction. Touse the advantage of the symmetry of this model, only half of the transom (400 mm) was modelled andthe nodes that were located on the plane of symmetry were fixed in z direction.In general, it is common practice to support glass panes only at two points/regions (glass supports), tohave a defined static system. This was simulated by applying the force at a point located la = 100 mmfrom the transom supports (tenons). The applied load for the main run was the average load that wasobtained from the preliminary tests with beech glulam transoms was applied (Q = 11040 N).

The resulting stresses of the FE modeling are shown in Figure 9a and Figure 9b. As can be seen theresults are in the range of the results obtained from the transverse bending and tension tests: σx,max =11, 13 N/mm2 can be compared to the obtained fm,90 = 10, 84 N/mm2 and σy,max = 13, 76 N/mm2 toft,90 = 5, 92 N/mm2. The stresses in zone A and B (as defined in Figure 3) are the highest transversetensions in x and y direction, as can be seen in Figures 9a and 9b respectively. This can be interpretedas a realistic result since it is confirmed by the failure modes of the preliminary tests. One reason forthe difference in the values might a non-ideal element size and the fact that the the model is based onthe linear elastic theory, which is not an ideal explanation for singularities. Another possibility is thatthe stressed volume of the tests is much higher than the one that would be admitted in practice. Thisway, and admitting that the size effect has to be taken into account for the occurred types of failure,the 11.04 kN that were applied to the FE model would have occurred for higher localized stress peaks.A third possibility could be that the average maximum load of the preliminary tests happened after apartial failure of the transom. This would mean that the initial failure would have happened for lowerloads and that the stresses that this lower load would cause in the FE model would be lower too.

8 Design concept

According to the preliminary tests and the main run of the FE model, zones A and B seem to be theones that limit the strength of the whole transom. With the objective of formulating a design conceptthat enables the prediction of the stresses in zones A and B, the FE model was run to show the variationof the stresses when varying three parameters: the applied load (Q), the distance of the load applicationpoint to the support (la) and the slot depth (bn). The following expressions were deduced:

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(a) Stresses in x direction (σx) in the connection cross-section.

(b) Stresses in y direction (σy) in the connection cross-section.

Figure 9: Stress distributions in the connection cross-section.

Zone A (Transverse bending)

σm,90 =25 ·Q · (2 · bst + bt − bn)

8 · h2· kA (1)

Zone B (Transverse tension)

σt,90 =Q

la· kB (2)

In those expressions kA = 1/(dz) and kB = 1/(dx · cz). To adapt the calculated values to the onesobtained in the modelling, values of kA = 0, 012 mm-1 and kB = 0, 15 mm-1 were chosen.

The following graphs (Figures 10a, 10b and 10c) show the results of the different runs that were madewith the FE model. In the graphs σmax are the maximum stresses at zone A or B obtained from theFE model and σcalc are the stresses obtained through Equations 1 and 2. In the graphs of Figure 10,the solid lines show the stresses that were obtained from the FE model while the dashed lines show thestresses that the previously deduced expressions predict for kA = 0, 012 and kB = 0, 15. The blue linesrefer to zone A and the red lines refer to zone B.

It is noted that when la = 25 mm, the maximum σx occurs in zone B instead of zone A. Also, the themaximum σy occures in zone C (below the first tenon) for la = 150 mm and la = 175 mm . For la = 200mm the the maximum σy occurs again at the inner edge of the slot but in z direction it occurs at theload application point.

It is mentioned that when bn = 10 mm, the maximum σx occurs in zone B instead of zone A. This leadsto the conclusion that the slot should not be to deep.

It also has to be mentioned that the obtained expressions only apply to the given geometry and therange of the variations. The values that were given to kA and kB would result in dz ≈ 83, 33 mm anddz · cz ≈ 6, 67 mm. It could be that these values occur due to other geometrical factors that were nottaken into account.

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(a) Variation of the stresses in zones A and B whenvarying Q.

(b) Variation of the stresses in zones A and B whenvarying la.

(c) Variation of the stresses in zones A and B whenvarying bn.

Figure 10: Variation of the stresses in zones A and B.

9 Discussion of the research results

It was shown that beech wood is a material that could avoid bottlenecks in the supply of constructionwood, which might result from the forest conversion. In fact, beech wood could even open new marketsfor wood since it is stronger and stiffer than the usually used coniferous wood. In this context it couldbe a solution for the challenge that the increasing use of three layered glazing posed to the design ofwood-glass facades. The availability of CNC milling machines opens up the possibility to establish theconnection between mullions and transoms through multiple mortise-and-tenon joints instead of relyingon other more or less complex specifically designed connection means. If the milling process is relativelyeconomical, a decrease of the global cost of the structure can be possible due to the reduction, or evenelimination, of additional connection means.

The results of tests that were made prior to this study show that the use of beech glulam mullion-transomstructures with multiple mortise-and-tenon joints can be an alternative to state of the art wood-glassfacades. These tests also reveal that, when subjected to dead loads, the failure of these structures occursdue to tension stresses that occur perpendicularly to the fibre direction (transverse tension stresses).With those results in mind there were made test to determine the transverse tension and transversebending strength of beech glulam. The results revealed that, compared to spruce/fir glulam, considerabletransverse tension and transverse bending loads can be carried by beech glulam. In fact, the transversetension and transverse bending strengths of spruce/fir are often ignored for safety reasons. It seems thatthis would not be necessary for beech, especially when used in wood-glass facades, due to the high visualrequirements that reduce the presence of imperfections (cracks, knotholes, etc.).

The FE model showed that there were stress peaks at the zones that failed in the preliminary tests. Itcould also be seen that the stress peaks were in the range of the transverse tension and transverse bendingstrengths that were obtained in the preliminary tests. The difference in the values could be explained bya non-ideal size of the finite elements and the fact that the FE model is based on the linear elastic theory.

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Another reason could be the size effect associated to the problem of the need to determine the stressedvolume in the affected singularities. It was observed that the variation of different parameters causedvariations of the stresses at the two prior identified failure zones. These variations had clear tendenciesthat helped with the formulation a design concept for the connection.

The design concept consists of two equations (Equation 1 and 2) that were deduced through the equi-librium of the forces in the transom. However, for each of the equations a constant had to be definedto adapt the calculated values to the ones obtained from the FE model. The values that were admittedfor the constants might be explained by geometrical parameters that were not taken into account in thisstudy. Another approach to the problem through fracture mechanics would result in similar expressionsto the ones obtained but in an improved explanation for the admitted values since it gives a better ex-planation for the fracture behaviour. Nevertheless the equations serve to estimate the stresses at zonesA and B for the given geometry and the range of the variations.

In further studies, the effects of the wind loads on the transom should be studied. With those results itwould be interesting to study the configuration and distribution of the tenons in the connection sectionto optimize the resistance to the different acting loads.

For an extended practical use of beech wood in the construction industry it would be necessary to amplifythe number of transverse tension and bending tests to obtain more representative values. These testscould also serve to give a more precise look at the variation of the strength for varying stressed volumes(size effect).

Since the failure occurs due to transverse tension and wood is weakest perpendicular to the fibres, one canthink about a means to reinforce that direction, in particular in the zones where the stress peaks occur.One option could be the use of longer screws to secure the aluminium profiles. If those screws would benearly as long as the transom is wide, and had an unthreaded shank, the axial forces in the screws wouldbe transmitted to the less stressed part of the transom. The treadles shank would allow the profile andthe front part of the transom to be pulled against the back side of the beam, resulting in compression ofthe front side. This way the front part could be considered as being pre-stressed. This would have to bedone for both the top and bottom screws to prevent possible errors during the assembling process. Thusit would also be necessary to pay attention to the peak compression at the bottom of the transom.

10 References

10.1 Bibliography

Federal Ministry of Food, Agriculture and Consumer Protection (Nov. 2011a). Forest Strategy 2020.Sustainable Forest Management: An Opportunity and a Challenge for Society. Governmental Report.Bonn (Germany): Federal Republic of Germany.

Federal Ministry of Food, Agriculture and Consumer Protection (Mar. 2011b). German Forests. Natureand Economic Factor. Governmental Report. Berlin (Germany): Federal Republic of Germany.

Nabuurs, G. J., M. J. Schelhaas, A. Ouwehand, A. Pussinen, J. van Brusselen, E. Pesonen, and A. Schuck(2003). Future wood supply from European forests. Implications for the pulp and paper industry.

10.2 Cited Standards and General Technical Approvals

DIN EN 408 (2012). Norm DIN EN 408. Timber structures – Structural timber and glued laminated tim-ber – Determination of some physical and mechanical properties. Brussels, Belgium: Comite Europeende Normalisation (CEN).

Z-9.1-621 (May 2011). Allgemeine bauaufsichtliche Zulassung Z-9.1-621. RAICO Pfosten-Riegel-Verbinderfur Holzfassaden. Berlin (Germany): Deutsches Institut fur Bautechnik.

Z-9.1-679 (Oct. 2009). Allgemeine bauaufsichtliche Zulassung Z-9.1-679. BS-Holz aus Buche und BS-HolzBuche-Hybridtrager. Berlin (Germany): Deutsches Institut fur Bautechnik.

Z-9.1-688 (Nov. 2008). Allgemeine bauaufsichtliche Zulassung Z-9.1-688. Lang Posten-Riegel-VerbindungenHolz-Glass-Fassaden. Berlin (Germany): Deutsches Institut fur Bautechnik.

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