Swiss Bridge Report

Swiss Federal Laboratories for Materials Testing and Research Überlandstrasse 129 CH-8600 Dübendorf Phone +41 (0)1 823 55 11 Fax +41 (0)1 821 62 44

Dübendorf, 23 September 2004 Project Leader:

Anja Fischer, Dipl.Ing. Wood Technology

Head of Laboratory:

Dr. Klaus Richter, Wood Scientist

Note: The test results are valid solely for the tested object. The use of the test report for advertizing purposes, any reference to it or the publication of excerpts require the approval of the EMPA (see Information Sheet). Test reports and supporting documents are retained for 10 years.

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Purbond AG CH-6203 Sempach-Station

Test Report Nr. 434929

Test Assignment: Inspection of 1KPUR / PRF bond lines at selected timber bridges Test object: Timber bridges in Grindelwald / Be (CH), Walde / Ag (CH) and

Gross Bieberau (D)

Reference of client: Mr. F. Stoffel Order dated of: 4. June 2004 Test object received: Test performed: July-August 2004 Number of pages: 21

1. Order

2. Basis Documentation

3. Inspection

4. Conclusion

5. Literature

EMPA, Laboratory: Wood/115 of 21Client: Purbond AG Test Report No. 434 929

1. Order Purbond AG ordered the Wood Laboratory of EMPA in June 2004 to monitor the state of timber joints glued with 1 K PUR adhesives at four selected timber bridges. It was agreed that the order shall consider the following details: • Visual and photographic documentation • Record of the overall performance of the bridges, maintenance • Description of construction details, notice of critical points • Detailed record and measurement of delaminated bonds, if any • Documentation of wood moisture content at critical zones • If necessary sampling of specimen to analyze the bond lines microscopically The following pedestrian and bicycle bridges are the selected objects of the analysis: • One pedestrian and bicycle bridge each in Grindelwald / Be (CH) and Walde / Ag (CH) • Two pedestrian and bicycle bridges in Gross Bieberau / Hessen (D)

2. Documentation The detail drawings and the lay-out plans of the selected bridges have been made available before the inspection as well as, in parts, the documentation of the project works (Walde, Grindelwald). Table 1: Objects of the analysis with specification of building companies and owner

Object Building company Owner

Timber bridge in Walde, Aar-gau (Switzerland)

Hunziker Holzbau Hauptstrasse 5046 Walde

Department for building Aargau

Timber bridge Grindelwald, Bern (Switzerland)

Silvatech, Biel (doesn’t exist anymore)

Wengernalpbahn Association

2 Timber bridges in Gross Bieberau (Germany)

Zang & Bahmer GmbH Justus-von-Liebig-Strasse 2 63128 Dietzenbach

Municipality Gross Bieberau


3. Inspection Object 1: Pedestrian Bridge Walde / AG (CH)

Figure 1: Timber Bridge Walde, Plan of construction The covered bridge spans a small river and is orientated in NE-SW direction. The surrounding area is abundantly covered with vegetation. General Data Year of construction 1993 Span: 10.60 m

Stat. System: Single span beam

Construction type: Covered timber bridge, Crossbeams are hanged up at primary construction

Members Type Wood species (according to project description)

Chem. Preservations (according to project description)

Primary beam: Gluelam (PUR glued, as specified by Purbond AG)

Fir Basilit-CFK- pressure treatment

Secondary beam: Solid Timber Fir

Wind bracing: Staifix-Steel-rod (12mm)

Railing: Gluelam Fir

Road surface: Timber planks

Fir/Oak


Observations

Primary beam The primary beams are made out of gluelam (Photo 1) and are covered with an upper covering of planed oak wood. This cover shield fulfils the function of a railing and ensures, due to its inclination, efficient water draining of the beam. As could be assessed from outside, the bond lines were completely intact and showed no delamination. There were small cracks in the timber lamella at several spots of the beam (Photo 5a and 5b). Secondary beam All cross beam of the secondary construction are made of solid wood graded in strength class II (FK II according to SIA 165), with the exception of the posts that are graded FK I. The longitudinal beams sup-porting the deck planks are made of gluelam (graded FK B, SIA 165) and are penetrated by the steel rods of the wind bracing system. No damage of the constructions caused by high moisture and / or attack of biological organism was detected. Roof The purlins of the roof construction are made of gluelam (graded FK B, SIA 165). All other load bearing timber are made of solid wood graded in strength class II. All wooden elements were in good shape. Surfacing The timber planks are screwed from upside and were mainly free of defects. Some esthetical problems were detected: some few screws were not screwed down fully and stuck out of the surface, and tangen-tially sawn deck lamellas showed signs of splintering (Photo 4). Support The wooden supports (oak) are mounted as recommended what ensures efficient ventilation. They were free of vegetation and dirt and no defects could be detected (Photo 2 and 3). Summary The bridge is an exquisite example for a very good realization of conception of details and good physical measures. In this context, the additional realization of chemical impregnation of all load bearing parts doesn’t seems to be necessary. The sufficiently dimensioned roof protects the load bearing structures a-gainst direct weathering. The two primary beams are weather exposed at their outer side, but showed no signs of delamination.


Photos

Photo 1: Covered pedestrian bridge, primary beam made of gluelam

Photo 2: Primary beam, abutment

Photo 3: Detail abutment Photo 4: Timber planks

Photo 5a: Detail gluelam primary beam Photo 5b: Small cracks in the timber lamella, but

no bond line delamination


Object 2: Pedestrian bridge Grindelwald / BE (CH)

Figure 2: Timber Bridge Grindelwald, Plan of construction This non covered timber bridge is used as a cross country skiing bridge. The bridge spans the river “Schwarze Lütschine” and is intensively weather exposed. General Data

Year of construction 1994 Span: 23.30 m


Construction type: non covered laminated slab bridge made out of transversely prestressed gluelam beams (QSX-System)

Weight of the bridge:

ca. 11 tons

Members Type Wood species (according to project de-scription)

Chem. Preservations (according to project de-scription)

Primary beam: Gluelam (PRF glued prestressed, as speci-fied by Purbond AG)

Fir Edge beam pressure treated with Xylamon

Railing: Gluelam Fir

Road surface: Mastic asphalt

Abutment Solid timber


Observation

Primary beam The primary beam is made of gluelam beams, which were prestressed perpendicular to the fibre length to a slab by glued in rods. The gluelam lamella have a thickness of 33 mm; total height of the slab measures 53 cm. The slightly elevated prestressed slab made of gluelam elements is top coated with a 2K-Epoxid isolation, roofing felt and mastic asphalt (height ca. 4,5 cm). At their side, the primary beam is protected by a venti-lated timber shelter. Underside, the slab is uncovered. The lamellas of the gluelam are bonded with a PF resin. Bonding of the gluelam to the slab was realized with a 1 KPUR (as specified by Purbond AG). The PUR bond lines of the slab could be only analyzed at the underside of the construction. The analysis of the bond lines detected partly opened bond lines. A more detailed examination revealed that these open bond lines resulted probably from insufficient bonding during fabrication, because no signs of squeezed glue could be detected. The length of the open glue lines measured up to 4 cm, their width 0.1 mm and their depths up to 8 cm (Photo 7). Underside the gluelam elements were considerably overgrown with algae and moss, an indicator for the high moisture impact (Photo 9). The growth was limited to the single lamella, and never exceeds the bond lines. Cladding The lateral cladding of the primary beam is realized as boarding with horizontal lamella. The fixing ele-ments (screws/nails) were extensively corroded. Railing The railing are made of solid larch wood and it had been already replaced in the past. Both the old and the new railing elements showed partly intensive cracks. Surfacing The sealing (mastic asphalt) and the isolation (epoxy resin) had been renovated according to information of the owner. It showed no further damages. Support The supports and the connections were in an excellent condition. The wooden support (oak) is protected against ascending moisture by underlying roofing felt.


Analysis

Moisture Content

Figure 3: Sampling points (Projection) Table 2: Wood moisture content wmc (u) Sample-Nr.

wmcu [%] Depth of sampling [mm]

Remark

1 19.5 10 River zone

19.7 30

2 18.7 10 Embankment

18.9 30

3 21.5 10

21.2 30

4 24 10 Right side beam, mid-dle of beam

27.3 30

20.7 10 Right side beam, lower part

22.8 30

5 16.1 10 Oak Crosstie

The moisture measurements show that all values range in an unproblematic level. The moisture content

Bridge (prestressed gluelam-slab)

embankment

Wooden support (oak)

Sampling point of moi-sture content

N 4

1 Sampling point of wood cores

V

1

2

3

4

5

12

3

4

abutment


tends to increase with the height of the gluelam. It can be seen at the measurements taken at the under-side of the bridge (small distance to the river, Sample Nr. 1) and in a depth of 30 mm (Sample Nr. 4), where the measurements were slightly higher. Coating The lateral beams are painted with a film-forming 3-layer- coating. Coat thickness Layer [µm] Outside (1) 125

Middle (2) 355

Inside (3) 340

Total 820 ≈ 1 mm Figure 4: Coat layer thickness of the finish Wood species The wood species was determined as Fir (Abies alba). Bond lines The bond lines have an average thickness of ca. 0,2 mm and are of brown colour, indicating a PF resin. Summary This non covered bridge is protected fairly well by an adequate conception as well as by physical and chemical measures (deck overlay, lateral shelter, coating, and chemical preservation to the lateral beams). The thick topcoat (Photo 8) may cause problems when moisture penetrated via cracks can not evaporate rapidly. The wood moisture values show a certain trend: they increase with increasing depth of the sam-pling points and high of the gluelam.

(1)

(2)

(3)

Holz


Photos

Photo 6: Timber Bridge Grindelwald Photo 7: Bond line, a) Squeezed adhesive;

b) thickness gauge (0.1mm) inserted in joint (8 cm)

Photo 8: Detail of lateral beam, Specimen for coat layer thickness measurement and bond line analysis

Photo 9: Bottom view of bridge slab showing overgrowing with algae and moss

a b


Object 3: Pedestrian Bridge Gross Bieberau/ Hessen (DE) / Fischbach

Figure 5: Timber Bridge Gross Bieberau / Fischbach, Plan of construction The non covered bridge is used as a pedestrian and bicycle bridge and is orientated in NS direction. General Data

Year of construction 1998 Span: 8.5 m


Construction type: Non covered timber bridge, Primary beams protected by bitumen sheet and timber deck-ing, Stiffened by steel struts at bottom



Primary beam: Gluelam (PF glued, as specified by Purbond AG)

Larch Pressure treatment with solvent borne preserva-tive and 2 layer surface finish with a low solid oil based stain

Bracing: Diagonal steel struts

Railing: Handrail Gluelam Rods

Larch

Road surface: grooved planks Oak


Observations

Primary beams The two gluelam beams are designed as single span beams. They have a height of 46 cm and a thickness of the lamellae of 25 mm. No bond line delaminations have been detected. All visible cracks and glue lines openings (on the west side) are caused by the rather high impact of moisture changes and direct radiation. The origin of the cracks was in the solid wood and it extended into the wood-adhesive interface. Detailed examination of the interface revealed no adhesive/bond line failure. Bracing The steel stiffening fixed bottom of the bridge showed some lime deposits, but were generally in a good condition. Railing The hand rail is made of larch gluelam and showed intensive cracking but no bond line delamination. The origin of the cracks was in the solid wood and it extended into the wood-adhesive interface. Detailed ex-amination of the interface revealed no adhesive/bond line failure (Photo 12). Road surface The decking is made of grooved natural oak planks that showed impacts of intensive weathering. The fix-ing of the screws is not perfect, because water can accumulate in the drill-holes (Photo 13). Support Generally the supports are designed well. However the distance between the primary beam and the abut-ment in the N-W part is too small. The end grain of the beam is in direct contact to the concrete of the abutment, which leads to an insufficient ventilation (see sampling point, Photo 11). Analysis

Moisture content

Figure 6: Sampling points (Projection)

1

1

Sampling point, wmc

Sampling point wood core

N

1

2

2

43

1

4 3

Primary beams

Distance to end grain of beam to abutment too small


Table 3: Wood moisture content, wmc (u) Sample-

Nr. Wmc u [%]

Depth of meas-urements [mm]

Remark

1 18.7 10

17.7 30

Upper part of footing

2 18.7 10

18.9 30

Lower part

17.7 10

18.1 30

Upper part

3 20 10

19 20

18.4 30

Lower part, footing not well ventilated, accu-mulation of dirt and leaves

17.2 10

50 20

64 30

Upper part, footing not well ventilated, accu-mulation of dirt and leaves

4 16.7 10

16.7 20

16.9 30

abutment

The wmc generally revealed no problematic values, except for Nr. 3. This proofed the consequence of insufficient detailing. Bond lines Table 4: Bond line thickness Sample Nr. Results [µm] Mean [µm] Sdv

1 300 320 330 316.7 15.3 2 200 250 300 250.0 50.0 3 250 270 300 273.3 25.2 4 300 350 250 300.0 50.0

TOTAL 285.0 29.4 The bond lines have a mean thickness of approx. 300 µm (0.3mm) and are of dark colour, indicating a Phenol-formaldehyde-resin. The microscopic examination of the bond line showed neither micro cracks nor delamination.

Figure 7: PF Bond line


Shear strength

Table 5: Shear strength Sample

Nr. Dimension K 1) Shear

strength fv corrected [N/mm2]

wmc u

[%]

t [mm] l [mm] A [mm2]

Shear strength

fv

[N/mm2]

Wood fibre perc.

[%]

1 28.98 25.35 734.64 0.91 9.91 8.99 13.24 100 2 27.40 27.10 742.54 0.90 11.38 10.25 13.73 80 3 29.06 26.66 774.74 0.91 10.90 9.90 13.38 100 4 26.83 25.94 695.97 0.90 10.13 9.10 13.24 100

1) correction factor – corrects the shear value of specimen if the dimension t in fibre direction is less than 50 mm Determination of shear strength was based on EN 392 „Glued laminate timber. Shear test glue lines”. The measured shear strength values correspond well with data from literature on shear strength of solid larch wood (Mombächer and Augustin 1988). The failure is rated as wood failure, indicating that the re-maining strength of the bond exceeds the wood strength. Summary The conception of details at this bridge is performed sufficiently. Only the fixing of the decking is subopti-mal. The abutment at the NW side has too little distance to the end grain of the timber beams (wooden support). There is no ventilation gap, and the accumulation of dirt and leaves results in elevated wood moisture concentrations in the timber parts. No delamination of bond lines have been detected. The shear strength of the bonds have been tested based on EN 392.


Photos

Photo 10: General view Fischbach-Bridge

Photo 11: Footing NW area. Accumulation of dirt, no ventilation

Photo 12: Hand railing with cracks in the wood lamellae

Photo 13: Fixing of deck planks; Rainwater and dirt accumulate in the drill-holes

Photo 14: Gluelam beam with cracks in the wood lamellae

Photo 15: Detail; cracks are running in vicinity of the bond line


Object 4: Pedestrian bridge Gross Bieberau / Hessen (D) / Im Briebel

Figure 8: Timber Bridge Gross Bieberau / Im Briebel, Plan of construction The non covered bridge spans a small river in NO-SW direction. There is abundant vegetation in the em-bankment and the area of the footing. General Data

Year of construction: 1997 Span: 15 m


Construction type: Non covered timber bridge, Primary beams protected by bitumen sheet and timber decking, Stiffened by steel struts at bottom



Primary beam: Gluelam (PUR glued, as specified by Purbond AG)

Larch Pressure treatment with solvent borne preserva-tive and 2 layer surface finish with a low solid oil based stain

Bracing: Diagonal steel struts

Railing: Handrail Gluelam Rods Larch

Road surface: grooved planks Oak


Observations

Primary beams The two gluelam beams are designed as single span beams. They have a height of 86 cm and a thickness of the lamellae of 25 mm. No bond line delaminations have been detected (Photo 17). As in the case of bridge No. 4, all visible cracks and glue lines openings (on the west side) are caused by the rather high impact of moisture changes and direct radiation. The origin of the cracks is in the solid wood and it extend into the wood adhesive interface. Detailed examination of the interface revealed no adhesive/bond line failure Bracing The steel stiffening fixed at the underside of the bridge are generally in a good condition. Railing The hand rail is made of larch gluelam and showed intensive cracking but no bond line delamination (Photo 19). The weather impact of the railing is extremely high. The fixing of the railing to the beam is real-ized well with 5 mm washers placed between the members avoiding any dirt accumulation and enabling good ventilation. At one spot the railing was damaged due to mechanical impacts. It was overgrown with moss at some parts as well (Photo 18). Road surface The decking is made of grooved natural oak planks that show impacts of intensive weathering (Photo 20). The fixing of the screws is not perfect, because water can collect in the drilling holes. Support Generally the supports are designed well. But here again, as in the case of bridge No. 3, the distance be-tween the primary beam and one footing (at the southern side) was too small, the end grain of the beam is in direct contact to the concrete of the abutment, and no good ventilation and drying is possible. Analysis

Moisture content

Figure 9: Sampling points (Projection)

11

Sampling points, wmc

Sampling points wood cores

N

3

2

1

4

1

2

4 3

Primary beams

Distance to end grain of beam to footing too small


Table 6: Wood moisture content, wmc Sample-

Nr. Wmc u [%]

Depth of measure-

ments [mm]

Remark

1 18.2 10

18.5 20

18.6 30


16.6 10

17.4 20

18.6 30

Lower part of footing

2 16.1 10

17.3 20

19.1 30

Upper part

15.0 10

15.2 20

15.3 30

Lower part

3 18.5 10

21.5 20

26.1 30


16.2 10

16.2 20

16.1 30

Lower part of footing

4 17.8 10

19.6 20

21.9 30

Upper part

15.0 10

15.1 20

15.1 30

Lower part

The wmc generally revealed no problematic values, except for Nr. 3. This proofed the consequence of insufficient detailing. Bond lines Table 7: Bond line thickness Sample Nr. Results [µm] Mean [µm] Sdv

1 140 180 150 156.7 20.8 2 150 140 100 130.0 26.5 3 70 100 100 90.0 17.3 4 170 190 200 186.7 15.3

TOTAL 140.8 41.0 The bond lines have a mean thickness of approx. 140 µm and are of light colour, indicating a PUR adhe-sive. The microscopic examination revealed that the bond lines are intact, they showed whether micro cracks nor delamination.


Figure 10: PUR-Bond line

Shear strength

Table 8: Shear strength

Sample Nr.

Dimension K 1) Shear strength

fv

[N/mm2]

Shear strength fv corrected [N/mm2]

wmc u

[%]

Wood fibre perc.

[%]

t [mm] l [mm] A [mm2] 1 26.93 26.70 719.03 0.90 8.52 7.66 13.28 802 28.93 25.51 738.00 0.91 Twisted test specimen 13.82 -3 21.09 27.61 582.29 0.87 11.45 9.99 13.28 604 28.54 26.70 762.02 0.91 9.93 8.99 13.60 20

1) correction factor – corrects the shear value of specimen with length is fibre direction of less than 50 mm Determination of shear strength was based on EN 392 „Glued laminate timber. Shear test glue lines”. The measured shear strength values correspond well with data from literature on shear strength of solid larch wood (Mombächer and Augustin 1988). The failure is rated as wood failure, indicating that the re-maining strength of the bond exceeds the wood strength. Sample No. 4 shows a rather low wood fibre percentage. Own scientific research on the interface phenomena of PUR-wood bonds revealed that there is no good correlation of high shear strength values and fibre failure percentage, as is with bonds of wood and polycondensation adhesives (Schirle et al. 2002). Summary The conception of details at this bridge is performed sufficiently. Only the fixing of the decking is improv-able. The abutment at the NW side has too little distance to the end grain of the timber beams. There is no ventilation gap, and the accumulation of dirt and leaves results in elevated wood moisture concentrations in the timber parts. No delamination of bond lines have been detected. The shear strength of the bonds have been tested based on EN 392.


Photos

Photo 16: Bridge „Im Briebel“

Photo 17: Detail view of gluelam beam

Photo 18: Mechanical damage of the railing Photo 19: Hand railing with cracks in the wood

Photo 20: Suboptimal support with first signs of deterioration


4. Conclusion The four timber bridges inspected in this study have been built in the years 1993 to 1998. They represent both covered and non-covered timber bridges. In general all four bridges are in a good condition. They provide an excellent example that the recommended combination of conception details and physical measures is a requirement for durable and functional primary bridge elements. Where the principle meas-ures had not been respected (e.g. insufficient distance between primary beams and abutment at bridge no. 3+4) higher wood moisture levels were measured, presenting the precondition of future biological deterio-ration. The detailed inspection of the bond lines of gluelam beams and prestressed gluelam slab revealed neither for the PF- nor for the 1-k PUR bonded elements any sign of glue line delamination. Even in the case of extensive weathering, as for the bridges 3 and 4, some detected cracks had their origin in wood failure. Glue line thickness was measured at bridges 3 and 4. The PF-bond lines were a slightly thicker (0.3mm) than the PUR bond lines (0.14 mm). Exemplary shear strength measurements on core samples taken at bridges 3 and 4 supported the good bond quality. At one of three specimens taken from PUR gluelam beams, the fibre failure percentage was reduced, although the numerical shear strength value ranged above the requirements. The reason for this weak correlation between shear strength and fibre failure percentage at PUR bond lines, as previously detected in earlier studies, remains unclear.

5. Literatur Mombächer R, Augustin H (1988) Holz-Lexikon Nachschlagewerk für die Holz- und Forstwirtschaft. DRW-

Verlag, Stuttgart. Schirle MA, Kuenniger T, Fischer A, Richter K (2002) Charakterisierung und Optimierung der Holzverkle-

bung mit 1 Komponenten Polyurethan (1K-PUR) Klebstoffen. KTI-Abschlussbericht 4126.1, EMPA Duebendorf, Abteilung Holz.

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