COVERED BRIDGE OVER THE RIVER AARE NEAR WANGEN

A bridge's life through 456 years in Switzerland
WORLD GUIDE NR.S-06-03 (Wangenbruecke)
REPORT by Konrad P.Meyer-Usteri,
MSc.Swiss Federal Inst. of Technology
February 29, 2008

 

n Switzerland there are in an area of almost the double size of Vermont still about 200 Covered Bridges in use. On more than half of them motor vehicles with reduced loads are admitted.

The oldest bridges in Switzerland (and in most countries of Europe) were built by the old Romans more than 2,000 years ago and even before. Just about 40 years ago digging by excavators for flood protection works exposed oak piles of a timber bridge dated by dendrochronology in 143 B.C.

The oldest covered bridge in Switzerland still in use for pedestrians is the Kapellbruecke in Luzern, first mentioned in a written document in 1333. In August 1993 it burnt down almost completely and was restored in the old way, that means no timber or log is historic but the system of construction is still that of 1333.

Hi,

The Aubrüggli Bridge (S-26-10) have been arsoned April the 14th.

Pictures are from the Werner Minder collection.

Gérald Arbour

The Aubrüggli Bridge was built near Zurich-Schwamendingen in 1810 to cross the Glatt River using an overlapping multiple queen post truss.

The bridge you find today near Wangen a.d.Aare Fig. 1 is much younger: In 1367 a bridge is mentioned in a document of the count of Neuenburg-Nidau for the first time, but we have neither excavations nor pictures and so no idea what it looked like. In 1407 the small town Wangen Fig.2 was conquered by the Bernese, the town of Berne, which was then expanding in all directions. The little town was fortified and last but not least the bailiff was ordered to build a new covered bridge, that was safe to be used by pedestrians, horses and carts or wagons. At the north end even a door for defense was provided.

1480 was a very bad year with three days of uninterrupted precipitations. A famous chronicler, Diebold Schilling, reported in his illustrated book also about the bridge of Wangen, that it could bear up the flood, but repairs became necessary. In the same year many men were killed in a boat hitting against a pier, Schilling reported.

The piers made of timber piles were quite often damaged by floods, ice, rafts and boats. At that time rivers were very important means of transportation as roads were bad all over Europe.

1505 to 1520 again pile driving was necessary for damaged piers. In 1966, before construction of new piers, we took off oak piles out of the river bed, which came from a bridge with spans of about 23 to 27 feet. These are the same spans as used for bridge construction 1,500 years ago at the epoch, when the old Roman Empire constructed roads and bridges also in Switzerland.

But after 1549 a new age arose in timber bridge construction for the covered bridge of Wangen, still now existing, 456 years old, which requires quite a bit of maintenance.

1552 is the date on the masonry stone pier Fig. 3. In its whole size it was visible till 1967 but only in winter, when the river was low. Then the dam for an electric power plant rised the water up to the stone ring on which the struts for the reinforcements of 1934 are fixed. The water level of today is marked by a blue line.

The government had the intention, to let erect stone piers, as those withstand floods, ice pressure and impacts by rafts and boats better. But the river bed is always moving driven by the water flow, and so it let collapse the stone piers in years of flood.

The art of the old Romans to put stone piers on a grid of beams, founded on timber piles, appeared to be unknown to the bridge-builders of Wangen, when they constructed a completely new bridge with only half of the number of piers: 3 timber piers on piles and 2 piers in stone.

They used for the first time kingpost trusses to carry the heavy load of the tiled roof of almost 1,200 lb/foot length weight over spans from 50 up to 56 feet, giving a total maximum dead load of 66,000 lb. or 30 tons for one span equally distributed load. This is quite more than a wagon with horses at that time, which could run with about 5 tons on the 4 big main-beams between.

The same construction system was applied for three bridges in the neighborhood: In the year 1535 Neubruecke just north of Berne, 1568 Aarberg and 1555 Guemmenen. All of them are still in use, the former two for motor traffic up to 10 tons, the last one for pedestrians only. The settlements of accounts of the Bernese Government tell us in a detailed manner all the works that had been done to maintain the bridge usable during the centuries.

In 1934 the legal truck load was 16 tons and the bridge again was in bad shape. A project was made with steal beams but still protected by a roof to keep the appearence of a timber bridge. A bridge in iron-concrete was too expensive. An enterprise specialised in timber work proposed a solution with reinforcement of the 6 beams in each span, every beam 19x21 incches wide.

1552 is the date on the masonry stone pier Fig. 3. In its whole size it was visible till 1967 but only in winter, when the river was low. Then the dam for an electric power plant rised the water up to the stone ring on which the struts for the reinforcements of 1934 are fixed. The water level of today is marked by a blue line.

Struts from both ends reduce the spans by 40 percent and beams of 9"x16" are bolted on the lower sides of the existing beams. The total length of the covered bridge is 295 feet over 5 spans, the width for motor vehicles and pedestrians measures 15 ½ feet, the height was 12 ft. The bridge deck was made by floor beams 4"x8" perpendicular to the main beams and on top came a layer of 2 ½ " oak planks parallel to the longitudinal axis of the bridge. The three bents in timber construction stood on wooden piles of 18" diameter, driven 20 ft. into the river ground. That secured each pier a bearing capacity of 100 tons. 5 of the total of 30 main-beams were exchanged. The importance of the covered bridge of Wangen for motor traffic at that time shows the fact, that it was maintained open for motor vehicles all the time during construction work.

In 1966 the project of an electric power plant downstream the Aare created a completely new situation. The tailback of the Aare caused by the dam should cover most parts of the timber bents. Consequently the Electric Power Company had to finance new piers.

A pile-head plate in reinforced concrete measuring 43x8 ft., 3 ½ ft. high has 3 concrete piles in steel tubes upstream and 3 piles downstream.

The idea arised again, to build a reinforced concrete deck and put it under a roof on timber trusses. Not less than 8 projects were elaborated. Fortunately these projects met with harsh opposition of the inhabitants of Wangen, the council of Wangen and even of the Agency of Transportation. Finally the project engineer wanted to replace the stone pier of 1552 with a concrete plate on 6 concrete piles. So they had destroyed the only part of the bridge of the 16th century, as all the timber beams, planks etc. have been changed during the long bridge life of 456 years.

Fig. 5 shows one of the mighty vertical posts in oak, which most probably dates also from 1549-52. Dendrochronology has not yet been applied since now. The floor was completely replaced and 3 main-beams exchanged. They were rotten inside, what we could only find out by borings. The stone pier is slightly inclined downstreams. Mortar injections and a sheet piling around it have preserved the masonry pier of 1552. The cost was only half of the cost of a reinforced concrete construction! And amazing too: After all the work was made a load test with a 35 t truck of the army (in 1967 the legal load was 28 t) proved an adequate bearing capacity of the 415 years old construction with the highly reasonable reinforcement of 1934. During the last decades the timber floors were exposed to the water that the motor vehicles carried in by the tires. In Switzerland you find moisture even in summertime. That requires periodical replacement of bridge decks.

2005/06 there were indications about considerable movements of the river bed although the flow is slow in the tailback except when a flood occurs. Depth measurements and diver observations showed a critical state of the piles Fig. 4. The ground had lowered in 35 years since the construction of the new piers up to 16 ft. A carpet of stones of 1,000 to 2,600 lb. weight each one was put on the river bed to prevent further sinking.

2006/0 the roof was considered to have missed a general renewel for at least the last fifty years. Broken tiles and rotten timber were carefully detected and exchanged. The tiles could be found in the historic, appropriate form. The bridge deck is in the original way once again, the water from the tires runs through; a floor consisting of glued, laminated boards with an isolation and a bituminous layer on top had given an almost absolute protection for the main-beams. But it was considered too expensive. As a relief road with a new bridge was opened in 1978 trucks were banned from crossing the little town Wangen, the bus disapeared and for passenger cars a one lane width of 10 ft. and a height of 8 ft. today is sufficient.

The state of conservation of the Aare bridge of Wangen is one of the best in Switzerland and it is under the protection of the Swiss Confederation.

CH 3065 Bolligen, February 29, 2008 Konrad P.Meyer-Usteri