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Probabilistic System Effects in Timber Structures

Hansson, Martin LU (2005) In Report TVBK
Abstract (Swedish)
Popular Abstract in Swedish

Huvudsyftet med den utförda forskningen var att öka förståelsen för hur variabiliteten hos styrkevärden i trä påverkar tillförlitligheten hos en träkonstruktion.



Jämfört med andra byggnadsmaterial som tex stål och betong har trä markant högre variabilitet i styrke- och styvhetsegenskaper både mellan och inom element. I byggnadsnormer tar man generellt inte hänsyn till den låga sannolikheten att hög spänning sammanfaller med låg bärförmåga vilket kan ge upphov till en extra säkerhetsmarginal. På grund av träets inhomogena natur beror bärförmågan hos en träkonstruktion på längden hos elementen samt typ av belastning. För konstruktioner med uttalade momenttoppar som t ex takstolar... (More)
Popular Abstract in Swedish

Huvudsyftet med den utförda forskningen var att öka förståelsen för hur variabiliteten hos styrkevärden i trä påverkar tillförlitligheten hos en träkonstruktion.



Jämfört med andra byggnadsmaterial som tex stål och betong har trä markant högre variabilitet i styrke- och styvhetsegenskaper både mellan och inom element. I byggnadsnormer tar man generellt inte hänsyn till den låga sannolikheten att hög spänning sammanfaller med låg bärförmåga vilket kan ge upphov till en extra säkerhetsmarginal. På grund av träets inhomogena natur beror bärförmågan hos en träkonstruktion på längden hos elementen samt typ av belastning. För konstruktioner med uttalade momenttoppar som t ex takstolar kan man därför förvänta sig en ökning av bärförmågan. Denna ökning kan definieras som en systemeffektfaktor definierad som en multiplikator på det enskilda elementets bärförmåga.



En statistisk modell av Isaksson (1999) användes för att modellera variabiliteten hos bärförmågan mellan och inom träelement av gran (Picea Abies). Två vanliga konstruktionssystem, W-takstol och parallellt träbalksystem, studeras.



Den probabilistiska systemeffekten för W-takstolen beräknas dels med en linjär modell och dels med en icke-linjär kraft-förskjutningsmodell av spikplåtsförbanden. Den metod som används av praktiserande ingenjörer med en ökning av böjstyrkevärdet vid böjmomenttoppar visade sig vara på säkra sidan. Systemeffektfaktorn är mellan 8% och 25% beroende på antaganden. Ett icke-linjärt beteende av spikplåtsförbanden visar sig inte påverka systemeffektfaktorn, åtminstone inte för den studerade takstolstypen.



För ett system med parallella träbalkar föreslås en ny metod för att ta fram systemeffekten, för att ta hänsyn till storleken av systemet. Systemeffektfaktorn är mellan 17% och 27% för denna typ av konstruktion. (Less)
Abstract
The main purpose of the performed research presented in this thesis is to increase the understanding of the effect of variability of strength properties on the reliability of timber structural systems. Compared to other structural materials such as steel and concrete, structural timber has a considerably higher variability of strength both within and between members. The advantage of the low probability that high stresses coincide with low strengths is however not accounted for and may give an extra safety margin. The strength of timber members will, due to their inhomogeneous nature, be dependent on both the length of the member and the type of loading. For structures with pronounced moment peaks such as roof trusses, an important... (More)
The main purpose of the performed research presented in this thesis is to increase the understanding of the effect of variability of strength properties on the reliability of timber structural systems. Compared to other structural materials such as steel and concrete, structural timber has a considerably higher variability of strength both within and between members. The advantage of the low probability that high stresses coincide with low strengths is however not accounted for and may give an extra safety margin. The strength of timber members will, due to their inhomogeneous nature, be dependent on both the length of the member and the type of loading. For structures with pronounced moment peaks such as roof trusses, an important increase in safety could therefore be expected. This can be quantified with a system effect factor defined as a multiplier on the single member strength.



A statistical model by Isaksson (1999) was used to model the strength variability between but also within timber members of spruce (Picea Abies). Two different types of common timber structural systems were studied namely the W-truss and a parallel, sheathed timber system.



The probabilistic system effect was simulated for a roof truss using both a linear elastic model and a non-linear elastic model for the joints. The engineering practice where the bending strength is increased at moment peaks was concluded to be on the safe side. The system effect for this system was found to be in the range 8% to 25% dependent on the assumptions. It was also concluded that the introduction of non-linear behaviour of the joints apparently does not influence the system effect factor, at least not for the studied roof truss.



A sheathed parallel timber beam structure was simulated and the system effect was determined. The failure load for the system was calculated for the weakest T-section in the system as well as the system failure load. At 5th percentile (characteristic) level the ratio between the failure load of the system and the weakest T-section in the system was calculated. The ratio was found to be in the range 1.19 to 1.30 depending on the studied parameters. The COV of the system strength was found to be around 10% compared to the COV of the bending strength of single beams of around 20%. A new method to evaluate the system reliability was suggested which reduces the effect of the system size. The system effect factor that can be introduced in deterministic calculation is the ratio between the load factor of the single element and the load factor that is applied on the system. The system effect factor was found in the range 1.17 to 1.27 for the input data studied. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Associate Professor Dalsgaard Sørensen, John, Aalborg University, Denmark
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Byggnadsteknik, Building construction, Anläggningsteknik, Construction technology, first order reliability method, Monte Carlo simulation, system effect, sheathed parallel member system, structural timber, roof truss
in
Report TVBK
pages
93 pages
publisher
Division of Structural Engineering Lund Institute of Technology Lund University
defense location
Lecture hall V:B, V-building, John Ericssons väg 1, Lund, Lund Institute of Technology
defense date
2005-06-03 10:15
external identifiers
  • other:ISRN:LUTVDG/TVBK-1030/05(93p)
ISSN
0349-4969
language
English
LU publication?
yes
id
be897f11-dcb3-4b51-aeaf-951f33e1bbfd (old id 544968)
date added to LUP
2007-09-07 13:46:55
date last changed
2016-09-19 08:44:59
@phdthesis{be897f11-dcb3-4b51-aeaf-951f33e1bbfd,
  abstract     = {The main purpose of the performed research presented in this thesis is to increase the understanding of the effect of variability of strength properties on the reliability of timber structural systems. Compared to other structural materials such as steel and concrete, structural timber has a considerably higher variability of strength both within and between members. The advantage of the low probability that high stresses coincide with low strengths is however not accounted for and may give an extra safety margin. The strength of timber members will, due to their inhomogeneous nature, be dependent on both the length of the member and the type of loading. For structures with pronounced moment peaks such as roof trusses, an important increase in safety could therefore be expected. This can be quantified with a system effect factor defined as a multiplier on the single member strength.<br/><br>
<br/><br>
A statistical model by Isaksson (1999) was used to model the strength variability between but also within timber members of spruce (Picea Abies). Two different types of common timber structural systems were studied namely the W-truss and a parallel, sheathed timber system.<br/><br>
<br/><br>
The probabilistic system effect was simulated for a roof truss using both a linear elastic model and a non-linear elastic model for the joints. The engineering practice where the bending strength is increased at moment peaks was concluded to be on the safe side. The system effect for this system was found to be in the range 8% to 25% dependent on the assumptions. It was also concluded that the introduction of non-linear behaviour of the joints apparently does not influence the system effect factor, at least not for the studied roof truss.<br/><br>
<br/><br>
A sheathed parallel timber beam structure was simulated and the system effect was determined. The failure load for the system was calculated for the weakest T-section in the system as well as the system failure load. At 5th percentile (characteristic) level the ratio between the failure load of the system and the weakest T-section in the system was calculated. The ratio was found to be in the range 1.19 to 1.30 depending on the studied parameters. The COV of the system strength was found to be around 10% compared to the COV of the bending strength of single beams of around 20%. A new method to evaluate the system reliability was suggested which reduces the effect of the system size. The system effect factor that can be introduced in deterministic calculation is the ratio between the load factor of the single element and the load factor that is applied on the system. The system effect factor was found in the range 1.17 to 1.27 for the input data studied.},
  author       = {Hansson, Martin},
  issn         = {0349-4969},
  keyword      = {Byggnadsteknik,Building construction,Anläggningsteknik,Construction technology,first order reliability method,Monte Carlo simulation,system effect,sheathed parallel member system,structural timber,roof truss},
  language     = {eng},
  pages        = {93},
  publisher    = {Division of Structural Engineering Lund Institute of Technology Lund University},
  school       = {Lund University},
  series       = {Report TVBK},
  title        = {Probabilistic System Effects in Timber Structures},
  year         = {2005},
}