A new 4-lane Mjoesbridge built in timber? In Norwegian : Mjøsbru

Transcription

1 International Timber Bridge Conference Lillehammer, Norway September 12-15, 2010 A new 4-lane Mjoesbridge built in timber? In Norwegian : Mjøsbru A summary of a workshop study Presented by : Ove Solheim Head of Bridge Section, Eastern Region, NPRA

2 A 2-days work-shop for large timber bridges was held in march 2010 near Lillehammer The purpose of the work-shop was : To sum up the technical status of large timber bridges today, and thereby examine the prospects for building a new 4-lane Mjøsbru in timber. 31 professional people participated: Bridge engineers, architects, glued-timber manufacturers and representatives from local and sentral government. This paper gives a summary of this 2 days work-shop and consentrates on the findings for a New Mjøsbru in timber.

3 Timber bridges in Norway. Norway has a comparatively long tradition in building timber bridges. But they bridged only small rivers and streams. As steel and concrete became cheaper and more available in the later part of the 20th century, these became the sole used materials for bridges. A more concise Code of Practice and difficulties in dokumenting the timber properties, made it difficult to gain acceptance for large timber constructions. OL in Lillehammer in 1994 acted as an accellerator for the use of timber, and new gluing- and connecting-techniques were developed. The first big modern roadbridge in timber in Norway, The Evenstad Bridge, was built in 1996 spanning the river Glomma. Conclusion : Today we have the knowledge and the materials to construct and build timber roadbridges spanning up to 100 m.

4 At present Norway holds the world record for the longest span of road timber bridges and has also the strongest one. A new Mjøsbru in timber will be the longest. Kjoellsaeter Bridge Built in Main span 45m, concrete bridgedeck. Military bridge in Rena Camp. Used by Leopard battle tanks. The strongest timber bridge there is! Tynset Bridge Built in 2001.Main span 70 m, total length 125 m. The worlds longest timber span for ordinary road traffic. Flisa Bridge Built in Main span 70 m. total length 196 m. Built on 150 years old foundations.

5 The members of the work-shop consentrated their efforts on the use of timber in The New Mjøsbru

6 The background for a new Mjøsbru E6 is the main artery road in Norway, and is presently being enlarged from a 2-lane ordinary highway to a 4-lane motorway. Bergen The Mjøsbru E6 The crossing of Lake Mjøsa is for the time being the closure of this roadbuilding activity which started in Oslo. Sweden Oslo E6

7 Skibladner This 150 years old paddle steamer is in daily traffic during the summer season on Lake Mjøsa. The White Swan of Mjøsa is a great turist attraction. This ship determines the free height of the ships channel under the bridge.

8 Skibladner - The existing Mjøsbru Was built in It is the 4th longest bridge in Norway. 21 spans totaling 1421 m. The longest span is 69 m. The superstructure is a concrete box and has 2 vehicle lanes and one pedestrian lane. It is supported by concreted hollow steel pipes embedded in an underwater ridge. The second bridge here will have to be founded in deeper water than the excisting one. View from the west

9 Lillehammer The Mjøsbru The crossing requires 2 extra lanes either as a 2-lane bridge parallell to the existing bridge or a new 4-lane bridge. Gardermoen Airport E6 Lake Mjøsa The existing bridge is neither prepared nor suitable for widening, so a new bridge is needed. Alternatives : Alternatives like a rock tunnel, a floating bridge or a submerged tubetunnel have been discarded in earlier feasability-studies. Annual average dayly traffic (AADT) crossing Mjøsa is Oslo

10 E6 Moelv Why a timber bridge? The counties neighbouring Lake Mjøsa contain the most forrested areas of Norway, and we hope to build the new bridge in a local material. (Alt. 1) Existing Mjøsbru We need to develop the timberindustry further, and a New Mjøsbru in timber will be a strong token for the usability of this material in large constructions. ca m Lake Mjøsa E6 Location If the New Mjøsbru is to be built in timber, it has earlier been concluded that it has to be built at a distanse from the existing concrete bridge, preferably at site 2A or 2B.

11 The main conclusions from the work-shop. It is both possible and realistic to build The New Mjøsbru in timber and with spans up to m. If so, the site for the new bridge should preferably be located 1 km south of the existing bridge, at site 2. A timber bridge is environmentally regarded as the best solution. The work-shop produced 3 different design sketches for a 4-lane 1500 m long timber bridge.

12 Proposed designs for a 1500 m long 4-lane timber bridge Alt. I : The Mjøs-viaduct Underlying truss, ? m. spans, perhaps a larger mid-span 150 m (of steel?). Alt II : The Mjøs-Wave Wave-formed truss by the side of each 2-lane bridge, common foundations, 80 m. spans. Alt III : The Mjøs-Extradosed Underlying truss combined with cable stayed bridges, approx. 80 m. spans.

13 Alt. I : The Mjøs-viaduct

14 Alt. II : The Mjøs-Wave

15 Alt. III : The Mjøs-Extradosed

16 Important themes that were discussed in the Work-shop (Common for all timber alternatives.) Note: The time available allows only for a summery of the conclusions from the work-shop.

17 Important themes. Technical solutions - Types of foundation At site 2 the lake is deeper than at the existing site 1, espesially in the middle of the lake. The depths here are registered to be up to 60-70m. Foundations will be a challange, but they are technical feasible. The bridge will rest on fundations which are supported by piles to solid bed rock or imbedded in frictonal earth. A timber bridge will have a lower total weight and thereby smaller foundations. - Distribution and size of span In the outset the distribution af spans is independant of total length of the bridge. It is important however that the size and distribution of the spans finds a rythm of what is visually and technically feasable. A bridge adjacent to the existing one at site 1 must have similar spans i.e. 69 m x 21 spans. At site 2 the bridge designers will have more or less full freedom of size and distribution of spans.

18 Important themes continues. - Superstructure The superstructure of a 4-lane timber bridge have two principal solutions : A single 4-lane bridge Requires the principal supporting trusses to be under the bridge deck, and can be combined with towers and wires to become an exciting cable-stayed bridge. Two 2-lane parallell bridges. Can have commen foundations. Will be free to use any type of truss support, either under the deck or side supported. - Bridge deck For a timber bridge there is two relevant bridge-deck solutions : Transverse poststressed timber deck. Advantage : only one fifth in weigth compared to a concrete deck. A floating concrete deck. More suiteable for a single 4-lane bridge. Advantage : reduced need for transverse steel supports.

19 Important themes continues. Design -Esthetics This topic produced a lively debate among the participants of the work-shop. Some stressed the importance of design as the bridge was visible from a long distance. Others that this long timber brigde in itself was large enough without implementing spesiell designs. Some voiced the opinion that the bridge must have a horisontal curve to avoid uniformity, while others meant that this was best taken care of by a strong vertical curve. Everybody however agreed that the design should incorporate some strong curved or ached form, perhaps as a wave floating upon the lake. As the bridge will be 1500 m long, it is no goal in itself to have a nondescript and transparent construction. Here it is an opportunity to design a forceful and heavy timber-construction. - Environmental aspects A timber bridge will be a strong climate winner. Forresting and the use of wood products represent an important part of the struggle against global warming, and should be one of the reasons for building a timber bridge. This bridge will have a total climate effect of more than tons of CO 2.

20 Important themes continues. Logistics A timber bridge can be prefabricated i large parts. The degree of prefabrication depends upon the choice of alternative. The amount of glued timber is too big for one producer to handle in a reasonable time period whilst ordinary production of standard elements goes on. It will require cooperation among producers. The construction time for this large timber bridge is difficult to estimate. The glued timber in itself will require up to 2 years production time.

21 Important themes continues. Service life of a timber bridge - Durability Our road bridges must have at least 100 years of service life. The timber bridge is no exception, and it will require a great effort in the detailing. The experience in detailing from the recent modern timber bridges is a good base to build on. The durability of a timber construction depends mainly upon: the continued use of creosote as an impregnating material. the use of copperlining to protect exposed surfaces. good detailing practices for preventing moisture-traps - Maintenance Roadsalt has a positive effect upon timber contrary to the negative effect it has upon steel and concrete. The steel connectors between the timber members must be of the best material with respect to corrosion. Timber elements between connectors ought to be replaceable if necessary. To ensure easy and continues inspection one should consider building a pedestrian path beneath the deck.

22 Important themes continues. Cost and competetive prices The New Mjøsbru will be financed by vehicle tolling. The work shop concluded that if one also put a reasonable importance and weight on esthetics and environmental quality, a timber bridge will be competitive to a steel or concrete bridge. The cost of the foundations of a timber bridge will be the same, independant of which static design is chosen. Type of road-deck is of importance. A concrete deck weighs 4 times as much as a timber deck of similar thickness. The weigt/strength ratio of timber compared to that of concrete ought to result in less costly foundations. It was estimated a cost of 20mill. kr/foundation for Alternative II (The Mjøswave). Roughly calculated cost for the different timber alternatives : Alt. I The Mjøsviaduct : MNok 950 (M 110) Alt. II The Mjøswave : MNok 1100 (M 125) Alt. III Extradosed Mjøsbridge : MNok 1200 (M 135)

23 Additional themes - Innovation Norway is today among the leading nations w.r.t. large timber bridges. This is a result of a collective willingness for continous innovation between the authorities, different professional spheres and the timber industry. This important innovation will continue if a Mjøsbru of timber is chosen. - Lighthouse effect!? A new bridge crossing Lake Mjøsa can be seen from long distances. It can become an attraction and a logo for the Mjøs-counties like the New Holmenkollen and the Opera is for Oslo. It is in itself not sufficient that the bridge is built of timber. The bridge must have a distinctive form and character that is simple and beautiful. And it must be so esthetic distinct that spectators embrases it as THE BRIDGE among all the other bridges!

24 Thank you for your attention! Axle load of 6/32 tons allowed Timber weight is nearly1/4th of concrete Axle load of 10/50 tons allowed