Fourth International Conference on FRP Composites in Civil Engineering (CICE2008)22-24July 2008, Zurich, Switzerland

1 INTRODUCTION

Being a Hessian Gothic hall church dating from the period of reformation the town church St. Marien represents a monument of national rank. It was built in the second half of the 14 th centu-ry. The timbering above the nave and the choir go back to the year 1640. The static-constructive retrofitting and strengthening of the church was performed in several construction stages bet-ween 2000 and 2005. The church is charactierized by an exposed position on the edge of an ex-tensive plain.

Figure 1: View of the church

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Strengthening of a historical roof structure with CFRP-straps

U.Huster1, R. Broennimann2, A.Winistörfer3

1HAZ, Beratende Ingenieure für das Bauwesen GmbH, Kassel, Germany2EMPA, Electronics / Metrology, Dübendorf, Switzerland3Carbo-Link GmbH, Fehraltorf, Switzerland

ABSTRACT: The Gothic town church St. Marien in Homberg/Efze (Germany) is a monument of national rank. The historic timbering spans about 20 meters and is about 14 meters high. Apart from the usual deterioration caused by moisture, the roof had been overloaded by hori-zontal wind-loads causing considerable permanent deformations. A strengthening of the tra-verse roof stiffening structure was necessary. Among various methods of strengthening a solu-tion with CFRP-straps met all requirements. The straps are made of CFRP-tapes wrapped con-tinuously around metal end fittings several times according to the design requirement. The method was developed at EMPA (Switzerland) and is a non-certified product for construction applications that is mainly used for high performance sailing and industrial applications. The es-sential advantages are the low stiffness, the extremely low thermal expansion, the simple install-ation as well as the monument-conform slender appearance. The application of these non-lamin-ated straps was for the first time performed in Germany in 2005 for the strengthening of histor-ical monuments with a special permission for this particular application.

The upper ending section of the nave is formed by a collar beam roof made of oak. The roof spans about 20 meters and is about 14 meters high. In each 4 th axis the normal rafters are sup-ported by a triple subjacent timbering. Apart from the usual deterioration caused by moisture, the roof had been overloaded and permanently deformed in a considerable way. The beams are deflected by approximately 30 cm. The southern base point lies about 25 cm lower than the northern one. Already in the past struts have been integrated as a provisional traverse roof stiff-ening against the main wind direction but this strengthening has not been sufficiently effective. In an expertise performed in 1999 [1] it was pointed out the insufficient stability of the nave un-der wind-loads. During the analyses for this expertise safety measures have already been instal-led concerning the base points of the axes.

Figure 2: Layout of the roof structure

2 ANALYSIS OF THE ROOF STRUCTURE

The modelling of historical structures differs from the modelling of new structures/constructi-ons. The finding of the model bases on - if possible - self-provided documentations of the infra-structure as well as of the damages in order to be able to estimate the actual load path. The mo-del needs to be of sufficiently high accuracy to guarantee the stability and at the same time to keep the costs of the retrofitting to a minimum. In historical wooden structures connectors of the type carpentors usually do permit to transfer tension loads just to a limited extent - a fact that must be considered for the modelling and for the analysis of the statical results.

Preliminary studies as well as the damage pattern have early proved that the actual roof has got problems with the wind load transmission. In order to be able to improve the estimation of the effects of necessary traverse stiffening, a section of the roof has been reproduced in a simplified three-dimensioned model. In doing so the order of magnitude of wind load distributions from the normal rafters to the strengthened and stiffened axes of the timbering can be estimated. The modelling is performed under the following assumptions:

The historical wooden connections are modelled with freely rotating joints. Compression-loaded joints are directly connected to the respective knots, whereas tension-loaded knots are modelled by soft springs. The stiffness of the springs was varied till the rising tensile forces could be transmitted by the used wooden nails (N<4 kN) and at the same time the extension of the spring was relatively low. The vertical supports on top of the wall were modelled to be stiff, the hori-zontal ones were modelled to be compliant.

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In the unstrengthened but retrofitted roof under self-weight and maximum wind perpendicular to the roof (this corresponds to the main wind direction)”considerable overloading of single knots occurs. This leads to calculated deflections of the rafters of approximately 50 cm for a ba-sic length of about 16.9 m. This corresponds to 1/34. As a result of the statical and local analy-ses one can sum up that stability and serviceability are not sufficient to guarantee a high durabi-lity of the roof. An improvement of the traverse stiffening structure of the roof was necessary.

Requirements of the preservation of historic buildings and monuments:The requirements of the preservation of historic buildings and monuments on strengthening

structures are compatibility of material, gentle interventions that treat the substance with care, reversibility, durability and conservation of the historical design. The structural engineer de-mands stable, durable low-maintenance structures, which are easy to install. The owner himself is looking for an economical and cost-effective structure that preferably also meets aesthetical requirements.

In principle there are 2 different methods available to improve the traverse stiffening:a) Strengthening of selected connectors in order to transfer tensile loadsb) Installation of additional stiffening elements

In the present case it turned out that the first method is out of the question, because nearly all connectors would need to be strengthened. This represents a substantial modification. As re-gards the town church St. Marien a combined concept has been developed:

In the axes of the timbering diagonal tensile trusses work as additional traverse stiffeners in the 2 lower levels. At medium level the axes of the timbering are connected/fixed to the plane of the roof beams in a way that guarantees tensile strength.

3 MATERIALS OF DIAGONAL TENSILE MEMBERS

3.1 Tensile members made of stainless steelThe diagonal tension ties should be designed as pure tensile members due to the fact that com-pressive struts - as a result of the large buckling length - would have needed considerable cross-sectional dimensions. Usually round tensile members made of stainless steel are used for the structure. Cylindrical tensile members with a diameter of 20 mm were tested. As a result of their stiffness EA ≈ 60.000 kN the structure attracts considerable wind-loads in the compliant roof structures. Consequently to that the historic wooden connections close to the load introduction points would have been overloaded. In addition to that the load case temperature decrease ∆ ϑ = - 30°C causes due to the necessary symmetric position of the diagonal tension ties in the tim-bering considerable loads. The tensile members would have to be installed using spring-ele-ments. Due to the huge spans of the tensile elements additional mid span anchorages would be necessary.

3.2 CFRP StrapsAs an alternative to the above mentioned steel tensile member CFRP-Straps developed at

EMPA for shear strengthening purposes of concrete have been evaluated. The CFRP strap shown in Figure3 comprises a number of unidirectionally reinforced layers, formed from a sin-gle, continuous, thermoplastic tape of about 0.13 mm thickness. The tape is wound around the two pins and only the end of the outermost layer is fusion bonded to the next outermost layer to form a closed loop.

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The non-laminated strap element enables the individual layers to move relative to each other which allows an equalisation of forces in the layers as the strap is tensioned [3]. The stress con-centrations are reduced since the new structural form is more compliant than the laminated equivalent. Control of the initial tensioning process reduces interlaminar shear stresses so that a more uniform strain distribution in all layers can be achieved. The approach allows greater flex-ibility in terms of the geometry of the tendon, and it can be manufactured on site. Moreover, the concept is going to be less expensive because there is no consolidation process required.

Figure 3: Conceptual design of non-laminated pin loaded straps.

Pin loaded straps based on the original Empa development are used extensively as external sup-port cables on crawler cranes made by Liebherr as shown in Figure 4a. Furthermore the design is employed regularly for rigging applications in high performance sailing boats in the Ameri-ca's cup to support the mast as shown in Figure 4b.

Figure 4: Applications of CFRP pin loaded straps.

3.3 Advantages of the CFRP-strapsThe CFRP strap is a tape measuring approximately 12 mm x 0.13 mm. The number of layers de-pends on the design requirement. In the town church St. Marien straps with 12 or 10 layers are used. Considering E = 138.000 N/mm² und A = 37,4 mm² (12 layers) this leads to a tensile stiff-ness of EA = 5,1 MN. At the same time the thermal expansion is nearly 0. In comparison to the use of cylindrical stainless steel bars this means considerable advantages regarding the compati-bility with the historical structure. The subtle appearance/design also brings about that the visual impression of the historical timbering is not negatively effected (Figure 5). Connectors that use standard tensile systems have the favourable effect that the straps are at any time tensionable with enough tolerance. Due to the compliance of the cable and the low weight of the straps a simple installation is possible. The connection between the tensile systems and the historical wooden structure is to a large extent reversible and minimises the interventions in the historical design.

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Figure 5: Installed CFRP cables

4 PERMISSION IN INDIVIDUAL CASES

In Germany the straps are non-certified and unlicensed products for construction applications and so far have not been used in civil engineering till now. In 1995 in the Church of our lady Meißen laminated CFRP straps were applied – in that special case an official permission had not been required whereas for the strengthening of the town church of Homberg a special permissi-on of the Hessian ministery for economic affairs, transport and regional development was ne-cessary although representatives of the building law disagreed. Legal proceedings were to be avoided. The permission was based on expert reports and imposed various conditions such as an annual assessment of the new construction. The affirmative notification is – as explicitly stated in the notification -not transferable to other objects.

5 LONG TERM MONITORING

Under maximum wind load the numerical analysis resulted in a load of the straps in the lower section of 59kN. The wind pressure for southerly wind was set to 1.1 kN/m² corresponding to a wind speed of 150 km/h. The corresponding load per rafter pair was 23.4 kN. The straps are used up to 79% of the load carrying capacity. The elongation of four strap pairs is monitored continuously since November 2005. The data is recorded by a data logger every two and a halve minute. The data is segmented in data blocks of three hours and median value for each sensor is evaluated. In Fig. 6 the median elongations as well as the temperature are shown for a period of two years. The eight elongation sensors are all in band of -4 to 0 mm and follow the annual climate changes. The two elongations of a sensor pair have a good agreement.

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Figure 6: Deformation and temperature measurements.

6 CONCLUSIONS

The installation of the straps has met the different requirements not only of the structural en-gineer and the owner but also the requirements of the preservation of historic buildings and mo-numents and finally those of the executing company. If the long-term monitoring does not show any disadvantages and if the process of asking official permission can be avoided then a wide application for the retrofitting and strengthenig of historical structures will be possible with the described method. The non-linear relatioship between the measured wind speed peaks and cor-responding elongations indicates the small influence of the strengthening due to climatic chan-ges but shows its effects against imposed wind loads.

7 REFERENCES

[1] HAZ, Haberland+Archinal+Zimmermann, Beratende Ingenieure für das Bauwesen: Gut-achten zum baulichen Zustand der Stadtkirche St, Marien in Homberg/Efze, 1999, unveröffent-licht

[2] Gutachterliche Stellungnahme zum Einsatz von Bändern aus kohlefaserverstärkten Kunst-stoffen (CFK) für die Ertüchtigung der Queraussteifung der Stadtkirche St. Marien Homberg/Efze, angefertigt am 17. april 2004 durch die Eidgenössiche Materialprüfungs- und Forschungsanstalt EMPA Dübendorf, Überlandstr. 129, CH-8600 Dübendorf

[3] Development of non-laminated advanced composite straps for civil engineering applicati-ons, Ph.D. Thesis, Winistoerfer A, University of Warwick, May 20 1999.

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