Analysis of Catenary Shaped Timber Structures
(2018) VBK920 20171Division of Structural Engineering
- Abstract
- In the design of a building there are many parameters that effect the material efficiency. The choice of structural solution and material are two of the parameters that will be investigated further in this rapport. Suspension structures is one type of structural solution that has high material efficiency, here the bending stiff properties of timber could also be utilized in order to increase the stiffness of the structure. The use of suspension principle on roof structures is however not frequently used as it can give rise to unfavourable load cases which of course causes increased uncertainties. Some reference building has in spite of this been identified and increased understanding about how suspension structures in timber could function... (More)
- In the design of a building there are many parameters that effect the material efficiency. The choice of structural solution and material are two of the parameters that will be investigated further in this rapport. Suspension structures is one type of structural solution that has high material efficiency, here the bending stiff properties of timber could also be utilized in order to increase the stiffness of the structure. The use of suspension principle on roof structures is however not frequently used as it can give rise to unfavourable load cases which of course causes increased uncertainties. Some reference building has in spite of this been identified and increased understanding about how suspension structures in timber could function and what possible weaknesses this type of structure can have.
With some ideas of how a suspension structure in timber could be designed, different load cases were studied, especially those that were expected to be important for design. Here the uneven snow load, wind uplift and dynamic instability from wind can be highlighted as potential problem areas. In order to get a better understanding of the snow drift on a suspension roof, a scale experiment was performed that showed that most of the snow drift is produced with wind oncoming perpendicular to the roofs parabolic shape and in proximity to the separation zones. The test also showed that a double-sided roof slope resulted in a less unfavourable snow load and that approximating the snow load with an butterfly roofs can give an approximation on the unsafe side.
When the loads on the structure were determined, three different factors that affected the load carrying cross section was evaluated. Firstly the support stiffness was studied, here a lower support stiffness meant bigger displacements and bending moments, while its impact on the normal force were negligible. Secondly the member shape was investigated, the conclusion from this test was that a small catenary shape corresponded to an increase in normal force but at the same time resulted in less deflection, bending moment and increased eigenfrequency. Here the result was calculated with both computer aided FE analysis and analytical expressions. A comparison between the result of FE- and analytical calculations, showed a good agreement for calculations of normal force, while the displacement and bending moment showed larger differences between the different calculation methods. Thirdly the effect of slenderness for the cross-section was examined. In this test a slender cross-section means a high bending stiffness which resulted in that a bigger part of the load was resisted through bending. Finally a preliminary design on the catenary "timber-cable" was performed, where the information from the investigations could be applied. Biggest difference compared with the reference building was a reduction of catenary shape, which decreased the deformations and made it possible to utilize both the normal force- and bending moment capacity efficiently. The designed building take advantage of a green roof in order to counteract the large suction forces that can occur for this roof shape. In this way, the green roof result in a more slender load carrying cross-section in combination with having several other benefits. With a span of 70 m the final cross-section was designed with a glulam beam with the dimensions of 250 x 300 mm, with a center to center distance of 800 mm. In the load carrying structure two layers of plywood was also used which mostly contribute to the stability of the building. To be able to compare the the material efficiency of the timbercables, the height of the load carrying elements instead would have used a a truss design. With a truss design the hight of the load carrying element would require several meters which clearly shows the advantage of timber cable concept. (Less) - Popular Abstract (Swedish)
- Bygg kabelkonstruktioner i trä!
I dagens samhälle blir det allt viktigare med hållbara lösningar för att lyckas bromsa den globala uppvärmningen. Stål och betong
står båda för stora klimatutsläpp och har efter industrialiseringen varit det dominerande valet av material för stora byggnader. Ett alternativt, hållbart material som lyfts fram att kunna vara framtidens material är trä. Ett exempel på en nytänkande lösning är att använda sig av
trä på samma sätt som man använder
kablar i en hängbro.
Med formen av en hängande kedja kan byggnadens bärande skelett utformas mycket slankt samtidigt som det ger en mycket intressantare lösning, jämfört med en mer traditionell lösning.
Letar man efter träkabelbyggnader hittar man bl.a. Grandview... (More) - Bygg kabelkonstruktioner i trä!
I dagens samhälle blir det allt viktigare med hållbara lösningar för att lyckas bromsa den globala uppvärmningen. Stål och betong
står båda för stora klimatutsläpp och har efter industrialiseringen varit det dominerande valet av material för stora byggnader. Ett alternativt, hållbart material som lyfts fram att kunna vara framtidens material är trä. Ett exempel på en nytänkande lösning är att använda sig av
trä på samma sätt som man använder
kablar i en hängbro.
Med formen av en hängande kedja kan byggnadens bärande skelett utformas mycket slankt samtidigt som det ger en mycket intressantare lösning, jämfört med en mer traditionell lösning.
Letar man efter träkabelbyggnader hittar man bl.a. Grandview Heights Aquatic Center (Canada) och Nagano Olympic Arena (Japan) som är utmärkta exempel på den här byggtekniken. Genom att titta på dessa gick det att ta reda på vad som är viktigt att tänka på
vid konstruktion av den här typen av byggnad.
Den första utmaning med att använda sig av en takform som liknar en skateboard ramp (se bild), är såklart att det finns risk för snöansamling. Därför gjordes enklare modellförsök med sand för att modellera snö och fläktar för att simulera vind. Dessa försök visade bl.a. att värsta snöfördelning på taket sker i området precis bakom den vindutsatta sidan. Vindriktningen visade sig också avgörande, där vind i samma riktning som takets skateboardform bidrog till vindlä och därmed liten snörörelse. Detta i jämförelse
med vind på byggnadens kortsida som har en mer tunnellik form.
Som en sorts kontroll jämfördes också modellförsökets snöfördelning med närmaste snölast enligt gällande byggregler. Jämförelsen visade likheter med största snöansamling i takets lägsta punkt vilket ökar förtroendet för modellresultatet. Andra utmatningen med takformen är vindlasten som uppstår. Det uppstår nämligen stora sugkrafter vilket i kombination med träs låga egenvikt kan orsaka att byggnaden sätts i gungning t.ex. vid hårda vindar. En enkel lösning blev därför användandet av ett grönt
tak (sedumtak) vilket ökar spänningen i träkablarna och i likhet med ett spänt rep ökas även takets stabilitet. Det går också att
jämföra detta resultat med tidigare studier på detta område och konstatera att det går att utforma en träkabelbyggnad som kan motstå även den tuffaste stormen.
För att visa potentialen i ”träkabel” konceptet dimensionerades en byggnad med ett avstånd på hela 70 m mellan de bärande väggarna.
Detta ger en stor frihet i rums utformning och beräkningarna resulterade i en bärande träkabel med en höjd på 300 mm och en bredd på 250 mm. För att kunna relatera detta till andra konstruktionslösningar, kan man jämföra om samma byggnadsdel hade utformas med byggteknik som användes för Eiffeltornet. Denna byggteknik kallas för fackverk och
skulle inneburit en höjd på de bärande elementen på flera meter.
För att sammanfatta ger träkabelkonceptet fördelar genom en elegant och hållbar byggnad samtidigt som takformens nackdelar med snölast och vindlast är överkomliga även om man har ett tak som ser ut som en skateboard ramp. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/student-papers/record/8931984
- author
- Persson, Kristoffer LU
- supervisor
- organization
- alternative title
- Analys av hängkonstruktion i trä
- course
- VBK920 20171
- year
- 2018
- type
- H3 - Professional qualifications (4 Years - )
- subject
- keywords
- Timber construction, Caternary structures, Tensile timber roof, Stress ribbon
- report number
- TVBK-5262
- ISSN
- 0349-4969
- language
- English
- additional info
- Examiner: Eva Frühwald Hansson
- id
- 8931984
- date added to LUP
- 2018-01-12 14:35:32
- date last changed
- 2018-01-12 14:35:32
@misc{8931984,
abstract = {{In the design of a building there are many parameters that effect the material efficiency. The choice of structural solution and material are two of the parameters that will be investigated further in this rapport. Suspension structures is one type of structural solution that has high material efficiency, here the bending stiff properties of timber could also be utilized in order to increase the stiffness of the structure. The use of suspension principle on roof structures is however not frequently used as it can give rise to unfavourable load cases which of course causes increased uncertainties. Some reference building has in spite of this been identified and increased understanding about how suspension structures in timber could function and what possible weaknesses this type of structure can have.
With some ideas of how a suspension structure in timber could be designed, different load cases were studied, especially those that were expected to be important for design. Here the uneven snow load, wind uplift and dynamic instability from wind can be highlighted as potential problem areas. In order to get a better understanding of the snow drift on a suspension roof, a scale experiment was performed that showed that most of the snow drift is produced with wind oncoming perpendicular to the roofs parabolic shape and in proximity to the separation zones. The test also showed that a double-sided roof slope resulted in a less unfavourable snow load and that approximating the snow load with an butterfly roofs can give an approximation on the unsafe side.
When the loads on the structure were determined, three different factors that affected the load carrying cross section was evaluated. Firstly the support stiffness was studied, here a lower support stiffness meant bigger displacements and bending moments, while its impact on the normal force were negligible. Secondly the member shape was investigated, the conclusion from this test was that a small catenary shape corresponded to an increase in normal force but at the same time resulted in less deflection, bending moment and increased eigenfrequency. Here the result was calculated with both computer aided FE analysis and analytical expressions. A comparison between the result of FE- and analytical calculations, showed a good agreement for calculations of normal force, while the displacement and bending moment showed larger differences between the different calculation methods. Thirdly the effect of slenderness for the cross-section was examined. In this test a slender cross-section means a high bending stiffness which resulted in that a bigger part of the load was resisted through bending. Finally a preliminary design on the catenary "timber-cable" was performed, where the information from the investigations could be applied. Biggest difference compared with the reference building was a reduction of catenary shape, which decreased the deformations and made it possible to utilize both the normal force- and bending moment capacity efficiently. The designed building take advantage of a green roof in order to counteract the large suction forces that can occur for this roof shape. In this way, the green roof result in a more slender load carrying cross-section in combination with having several other benefits. With a span of 70 m the final cross-section was designed with a glulam beam with the dimensions of 250 x 300 mm, with a center to center distance of 800 mm. In the load carrying structure two layers of plywood was also used which mostly contribute to the stability of the building. To be able to compare the the material efficiency of the timbercables, the height of the load carrying elements instead would have used a a truss design. With a truss design the hight of the load carrying element would require several meters which clearly shows the advantage of timber cable concept.}},
author = {{Persson, Kristoffer}},
issn = {{0349-4969}},
language = {{eng}},
note = {{Student Paper}},
title = {{Analysis of Catenary Shaped Timber Structures}},
year = {{2018}},
}