expert meeting on the “design of timber connections in fire
TRANSCRIPT
COST ACTION FP 1404 – WG 2
11-12 February, 2016Lisbon, Portugal
Chair of the Action: Joachim Schmid [email protected]
Vice Chair of the Action: Massimo Fragiacomo [email protected]
Grant Holder: Åsa Rössel as a. r [email protected]
Local organisers: Pedro Palma [email protected]
Helena Cruz [email protected]
Action website: http://www.costfp1404.com
Expert Meeting
Design of Timber Connections in Fire
Editors: Pedro Palma and Robert Jockwer
Contents
Part I – Minutes
1 Participants.................................................................................................................................. 1
2 Opening the meeting............................................................................................................... 1
3 Presentations on 2016.02.11...................................................................................................2
4 Presentations on 2016.02.12................................................................................................... 7
5 Final discussion and closing................................................................................................9
Part II – Presentations
Roy Crielaard, Arup, UKDesigner's perspective on the fire design of timber connections – Example projects and common issues........................................................................................................................ 11
Dhionis Dhima, CSTB, FRFire behaviour of dowelled and bolted timber connections, and three-dimensional nailing plates.................................................................................................................................... 23
Jørgen Munch-Andersen, Træinformation, DKNext generation EC5 rules for connections and possible shortcomings for fire conditions.......................................................................................................................................... 39
Keerthi Ranasinghe, Exova BM TRADA, UKTimber connections in fire: standardisation considerations for 2020................................43
Pedro Palma, ETH Zurich, CHFire design of timber connections – assessment of current design rules and improvement proposals................................................................................................................. 53
Matteo Izzi, University of Trieste, CNR IVALSA Trees and Timber Institute, ITNumerical modelling of joints in timber structures – A state-of-the-art..........................67
Rubén Regueira, University of Santiago de Compostela, ESNumerical simulations of dovetail joints in fire......................................................................73
Daniel Brandon, SP Technical Research Institute of Sweden, SEDowel-type timber connections: improving their fire performance.................................109
Patrick Racher, CUST Univ. Blaise Pascal, FRFire safety of timber connections.............................................................................................. 127
Robert Jockwer, ETH Zurich, CHStudy on the reliability of connections at normal temperature and in fire.....................133
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Part I – Minutes
Minutes of the Expert Meeting on the “Design of Timber Connections in Fire”
The Expert Meeting on the “Design of Timber Connections in Fire”, organisedby Working Group 2 of COST Action FP 1404, took place at the National Laboratoryfor Civil Engineering (LNEC), in Lisbon, Portugal, on 11-12 February 2016.
These minutes were prepared by Pedro Palma and Robert Jockwer.
1 ParticipantsThe meeting was attended by 11 participants from 10 countries, representing
research institutes, universities, and structural engineering companies (Table 1.1).
Table 1.1: List of participants
Name Institution/company Country
Daniel Brandon (DB) SP Technical Research Institute of Sweden SE
Dhionis Dhima (DD) Centre Scientifique et Technique du Bâtiment (CSTB) FR
Helena Cruz (HC) National Laboratory for Civil Engineering (LNEC) PT
Jørgen Munch-Andersen (JMA) Træinformation,Convener of CEN/TC250/SC5/WG5 Connections
DK
Keerthi Ranasinghe (KR) Exova BM TRADA UK
Matteo Izzi (MI) University of Trieste,CNR IVALSA Trees and Timber Institute
IT
Patrick Racher (PR) CUST Univ. Blaise Pascal FR
Pedro Palma (PP) ETH Zurich CH
Robert Jockwer (RJ) ETH Zurich CH
Roy Crielaard (RC) Arup UK
Rubén Regueira (RR) University of Santiago de Compostela ES
2 Opening the meetingThe organisers, PP and HC, open the meeting and welcome the participants. PP
gives general information on how to submit travel reimbursement requests and eligibleexpenses and proposes that the meeting runs as an open forum with presentations(participants can raise questions anytime), which is agreed by all participants. PP statesthe objectives of the meeting:
• assess the current state of knowledge on the structural fire design of timberconnections;
• identify research gaps and ways to address them;• propose improved design methods;• discuss the structure of the connections part in the next EN 1995-1-2.
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3 Presentations on 2016.02.11
3.1 Roy Crielaard (RC), Arup, UK“Designer's perspective on the fire design of timber connections – Example projects and common issues”
Current trends in structural timber
RC presents a few example projects of current trends in structural timber. Open,exposed mass timber frame is particularly suited for office buildings, as it allows forlarge open spaces, and is usually combined with concrete or CLT cores and CLT slabs.Cladding can be used for the CLT but commonly not for the timber frames.Connections with slotted-in steel plates are preferred in practice, as well as bearingconnections (horizontal members supported in compression perpendicular to thegrain).
Issues and challenges faced by designers
RC presents issues/questions he collected from his colleagues:• Exposed timber structural elements will influence fire spread and
development. How much timber can be exposed?• The slip modulus of connections in the fire situation, according to section
4.3.4 of EN 1995-1-3 (Kfi = Ku ηf, with ηf = 0.67 for dowels and bolts andηf = 0.2 for nails and screws) does not work for tall buildings.
▪ PR agrees and mentions that, according to his observations during firetests, it should be Kfi = Kser ηf. Especially in large structures and buildings,where deformations during and/or after fire events may be an issue.
• Are connections, or should they be, expected to carry any load after therequired fire resistance period and how to assess the residual load-carryingcapacity of a connection after a given exposure.
• Fire resistance of connections between CLT floor and wall-elements (e.g.angle brackets).
• Fire behaviour of timber connections loaded in shear, bending, shear andtensions (as most fire resistance tests have been performed on connectionsloaded in tension).
▪ DD says that fire tests on steel-to-timber connections loaded perp. tograin and loaded in bending have been performed in France. PP says thattests on shear beam-to-column connections have been performed inSwitzerland.
• Impact of constructions tolerances, such as gaps between members and slotwidths, in the fire behaviour of connections.
▪ PP says that in the tests he mentioned before, the influence of the gapbetween the beam and the column was assessed and it seems that 10 mmmight be an upper limit, above which the fire resistance is severelyreduced.
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• Performance of various protection materials (timber, gypsum, intumescentpaint, etc.). The need and effectiveness of intumescent paints applied onexposed bolts (nuts and washers) and dowels is questioned. An alternativecommonly used in practice is to protect the fasteners with wooden plugs.
• Impact of elevated temperatures (θ ≈ 50°C) in connections (e.g. how isembedment strength affected?).
▪ JMA states that this temperature might already reduce the strengthconsiderably. PP says that it also depends on the heating rate: in fire theheating rate is very high and, given the low thermal conductivity of wood,only a limited outer layer is influenced by temperature.
• What are the key parameters influencing fire resistance? Fastener type,fastener diameter, side member thickness, applied load?
▪ PP says that member thickness is more important than fastener diameter.PR states that the dowel diameter has only a small influence in thetemperature profiles. Also, failure modes might change (e.g. from blockshear at normal temperature to embedment in fire). KR mentions thatrope effect has to be disregarded in fire.
3.2 Dhionis Dhima (DD), CSTB, FR“Fire behaviour of dowelled and bolted timber connections, and three-dimensional nailing plates”
Experimental results
DD presents the fire resistance tests performed in France since 1999, on timber-to-timber and steel-to-timber dowelled and bolted connections (and the correspondingtests at normal temperature). DD states that the failure mode is longitudinal splittingalong the rows of fasteners and PR agrees. PP mentions that the large deformations aremost likely due embedment failures and splitting might occur only at the very end ofthe tests, as shown by the photos.
Proposed design method
DD presents simulation results that show the load distribution between fasteners.The “1 bolt for every 4 dowels” rule was based on the tests, but the simulations showthat the load carried by the bolt decreases significantly with time, whereas if onlydowels are used the load distribution between fasteners remains mostly constant. DDshows that simulations can follow the time-displacement curves measured during thefire tests, but only up to the steep part of the curve, right before failure.
Based on numerical simulations and experiments on steel-to-timber dowelledconnections, DD presents a design method based in a single empirical formula for allconnections and loading directions:
t fi=c 1+ c 2⋅d + c 3⋅t 1+c 4⋅ln (η)+c 5⋅d⋅ln(η )+ c 6⋅t 1⋅ln(η ) (3.1)
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The results are then adjusted using a coefficient k, which is different for differentconnection typologies and loading directions (1.2 < k < 2.1):
t fi,d=k⋅t fi (3.2)
The method gives safe estimations of the fire resistance, even in situations inwhich EC5 is known to give unsafe results.
Beam-to-beam shear connections with three-dimensional nailing plates
DD shows results of fire resistance tests on beam-to-beam shear connectionswith three-dimensional nailing plates and slotted-in steel plates. Based on the testresults, DD states that the nailing plates reach 15 min of fire resistance for a load ratioof 30% and up to 30 min for a load ratio of 10%. KR mentions that the declared load-carrying capacity of joist hangers is not reliable (issue with EOTA Technical ReportR16), so the load applied during the fire tests should be based on tests performed atnormal temperature and not in the manufacturer's declared value. The exact failuremechanism of three-dimensional plates strongly depends on the test configuration. Inreality the load ratio might be higher than 10-30%. DD states that the slotted-in plateswith a bottom flange exhibit a better fire performance, because after the fasteners failthe member will rest on it. PP mentions that for longer exposures it might have theopposite effect, as it conducts more heat into the connection area.
RC asks about the observed failure modes and DD answers that in fire no blockshear failure has been observed. JMA states that plasticity at elevated temperature mightbe the reason.
RC asks about the differences between the connections reaching fire resistanceslower than 60 min and higher than 70 min. DD answers that the side member thicknessis the reason.
3.3 Jørgen Munch-Andersen (JMA), Træinformation, DK“Next generation EC5 rules for connections and possible shortcomings forfire conditions”
JMA presents the new proposed structure for the section on timber connectionsof EN 1995-1-1.
• 8.1 Introduction: principles, definitions, references (e.g.. EN 14592, P-clauses);• 8.2 Axial capacity of a single fastener;• 8.3 Lateral capacity of a single fastener;• 8.4 Combined axial and lateral capacity of a single fastener;• 8.5 Load distribution in a group of fasteners;• 8.6 Timber strength• 8.7-8.9: Shear connectors, glued-in Rods, punched metal plate fasteners…
JMA says that spacings and edge/end distances to avoid brittle failures arecurrently only diameter-dependent, but they could be also dependent on the thicknessof the timber members (required area or volume of timber per fasteners) or on the
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load-carrying capacity of the fastener (modern fasteners). The participants agree thatchanges in minimum spacings, distances and thicknesses for the design at normaltemperature will require readjusting some design rules for fire, namely the simplifiedrules. PP states that fastener spacing perpendicular to the grain (currently 4·d for boltsand 3·d for dowels) might have to be increased for longer exposures (above 30 min), assimulations and fire tests on connections loaded perpendicularly to the grain show.
Summary of the challenges for the standardisation of connections in general:• strength parameters are not consistent, some are measured, some are
calculated; CE-marking is not consistent, reference testing should berequired;
• transformation of strength parameters to other materials (OSB, LVL, etc.)should be possible;
• spacing requirements have to take modern high capacity fasteners intoaccount.
Some challenges for the fire situation:• definition of penetration depth and zero strength layer for the fastening of
protective panels during fire.
3.4 Keerthi Ranasinghe (KR), Exova BM TRADA, UK“Timber connections in fire: standardisation considerations for 2020”
KR presents a review of the chapters for connections in EN 1995-1-1 (chapter 8)and EN 1995-1-2 (chapter 6). from an end-user's perspective. Current chapters arecomplex to follow and hopefully it will be possible to make them clearer and simpler inthe future. The challenge is to present the current technical content in a betterstructure. RC suggests that, as in the concrete part, tabulated values could be given firstand calculations methods afterwards. DD supports the need for simple and logicalmethods for the engineer (advanced methods, such as finite element simulations, arestill not practical for the design of timber connections). PR agrees thatEN 1995-1-2:2004 might have inconsistencies and lack clarity, but when it waspublished there was no basis to build on, and an urgent need to publish it.
KR asks why, in the simplified rules for unprotected connections, are theincreased dimensions afi calculated with the notional charring rate βn and not the one-dimensional charring rate β0. PR answers that it could be to make the determination ofafi similar to dchar,n in the reduced cross-section method (Equation 4.1 of EN 1995-1-2),only with a kflux instead of a zero-strength layer.
KR mentions that glued-in timber plugs (used to protect fasteners) are alsosupposed to be calculated using afi, and therefore βn and kflux, which seems strange giventhat there are no exposed metal parts anymore. PR mentioned that it could be forsimplification.
KR suggests to keep only one design method in the standard and move additionalor alternative methods to an annex.
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3.5 Pedro Palma (PP), ETH Zurich, CH“Fire design of timber connections – assessment of current design rules and improvement proposals”
PP presents and overview of published fire resistance tests on timberconnections, before and after the publication of EN 1995-1-2:2004.
Simplified rules
PP shows that for timber-to-timber connections the current simplified rules canlead to an unsafe design, namely for high degrees of utilisation in the fire situation (ratiobetween the actions in the fire situation and the load-carrying capacity at normaltemperature Efi/R20°C), and proposes limiting the degree of utilisation to 0.3 for thesimplified rules to be applicable. PP proposes changing the kflux for each connectiontypology, so that the simplified rules always give safe estimates of the fire resistance.
PP states that for steel-to-timber connections with steel plates with unprotectededges in general, the values in Table 6.2 of EN 1995-1-2:2004 appear to have beentaken from the 1994 edition of the Holz Brandschutz Handbuch, which has significantlyincreased these values in the most recent, 2009, edition. For connections withunprotected edges on one or two sides, PP says that the table should not be used forbolted connections, as it gives unsafe results, and that for dowelled connections there isno experimental data to support the values for R60.
Reduced load method
PP states that the thickness of the side member t1 is the most importantparameter regarding fire resistance and proposes changing the current design method,making the k parameters dependent not only on the type of fasteners but also on thethickness of the side member t1:
R fi=e−k⋅t fi⋅R 20°C , with k=k ( t 1)=c 1+ c 2⋅t 1 (3.3)
The proposed coefficients for the k parameters are based on fitting a one-parameter exponential model to test results grouped by thickness. PP states that thediameter only plays a minor role, compared to the thickness of the side member and itdoes not improve the estimates of the proposed model.
DD states that the proposed minimum side member thickness t1 = 28mm fornailed connections is too low. PP answers that it was the minimum thickness used inthe tests performed by J. Norén, in Sweden, and that the maximum period of validity tfi
for the parameters k could also be made dependent on the side member thickness.
PR clarifies a discussion about the taking into account the effective number offasteners to determine the load-carrying capacity in the fire situation: up to a fireresistance of 30 min, there is no need to reduce the number of fasteners, but above thatthere might be.
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4 Presentations on 2016.02.12
4.1 Matteo Izzi (MI), University of Trieste, CNR IVALSA Trees and Timber Institute, IT“Numerical modelling of joints in timber structures – A state-of-the-art”
MI presents different modelling strategies for timber connections in fire.Orthotropic models with plasticity in compression and brittle failures in tension havebeen developed for thermal-structural analyses, but are computationally verydemanding. MI has successfully adapted Carmen Sandhaas' model (Abaqus' UMATsubroutine) to thermal-stress analysis, but the simulations are very unstable, especiallywhen reaching ultimate failure. MI states that there is no need for additional newmodels but the existing ones have to be extended and the complexity of the modelsstrongly depends on the objectives. MI and DB discuss about the numericalimplementation of failure in wood and will continue to work on this topic.
4.2 Rubén Regueira (RR), University of Santiago de Compostela, ES“Numerical simulations of dovetail joints in fire”
RR presents simulations of dovetail joints exposed to fire. Tests show hightemperatures (up to 100 °C) inside the notch but the simulations initially did not. Anadditional heat source has to be simulated in the notch side surfaces to reach thetemperatures observed in the tests. It appears that the gaps in the connections areimportant and have to be evaluated more in detail. PR and DD state they performedsimilar tests and did not observe any charring inside the notch. RR clarifies that at100 °C charring would not be visible, but strength would be reduced. RR says thatfailure criteria for normal temperature, namely Tsai-Wu, is not suited for elevatedtemperatures, as the temperature-reduced strengths are too low and failure is reachedprematurely.
PP says that two fire resistance tests were performed on dovetail connections inSwitzerland; the results showed that the stronger connection at normal temperature(smaller notch) exhibited a lower fire resistance, as the charring reached the notchedsurfaces at an earlier stage. DD added that the same was observed in tests performed inFrance. PR explains the progressive failure of dovetails connection observed in tests,which could differ in normal temperature and in fire.
4.3 Daniel Brandon (DB), SP Technical Research Institute of Sweden, SE“Dowel-type timber connections: improving their fire performance”
DB presents his research on dowelled connections with non-metallic parts – FRPdowels and DVW flitch plates. Test results show that FRP dowels conduct much lessheat than steel dowels and the charring depth along the dowels is similar to that of thesurrounding timber. Numerical simulations were based on finite differences method,using a model of a single fastener (elasto-plastic beam on supported by springs withnon-linear behaviour). Temperatures are obtained from heat transfer simulations andthe properties of the mechanical model are reduced accordingly. The model is able to
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capture the entire experimental displacement-time curve, including the final steep partbefore failure. DB states that reducing the modulus of elasticity after failure causesnumerical problems in FE models, but in his model a secant modulus of elasticity wasused instead, which is numerical much more robust.
DB discusses further aspects of connections in fire, namely the encapsulation ofthe connection is often problematic, e.g. wood plugs to protect the fasteners cause areduction of effective embedment strength. An alternative are the expanded-tubeconnections studied by A. Leijten, in the Netherlands. PR states that widespreadadoption of these connections is limited because self-tapping screws are much easier toapply and also increase, strength and ductility. PP points out that reinforcementsconduct more heat into the connection area and have been shown to reduce the fireresistance. RC asks about the costs of expanded tube fasteners and mentions that FRPdowels have the advantage to use the same construction process as the conventionalmetallic dowels.
4.4 Patrick Racher (PR), CUST Univ. Blaise Pascal, FR“Fire safety of timber connections”
PR presents FE simulations performed in France, in which the non-linearbehaviour was limited to a restricted area around the fasteners, overcoming numericalinstabilities. PR sates that the heating in the connection area is not dependent on thefastener diameter and shows depth-temperature curves for connections with fastenerswith different diameters.
PR states that there are two key issues regarding timber connections in fire: stiffness(very important for tall buildings, as mentioned earlier by RC) and load-carrying capacity.
According to PR, the slip modulus for connections in the fire situation should beKfi = Kser ηf, instead of Kfi = Ku ηf proposed in section 4.3.4 of EN 1995-1-2.
PR discusses two approaches for the load-carrying capacity in fire: reducing theload-carrying capacity of the connection (as in the current reduced load method) orreducing the resistance of a single fastener and then calculating the load-carryingcapacity of the connections (as in EN 1993-1-2). The first approach is well establishedbut has the problem that, in the available experimental data, the load-carrying capacityat normal temperature is not always adequately determined (different parameters aretaken, or not, into account: friction, rope effect, effective number of fasteners) and theempirical formulas might not be accurate. The second approach is similar to what isused in steel structures and a reduced embedment strength (dependent on the requiredfire resistance) would be used in Johansen's failure modes. PR states that adaptedJohansen's model gives good results for fire resistances up to 30 min, but above thatthreshold the thermal behaviour of the steel plate has an increasing influence in the firebehaviour and the fire resistance is overestimated.
PR mentions that there are no fire tests on connections with screws, even thoughthere are becoming ever more popular. PR also mentions that there is no test standardfor fire resistance tests of timber connections.
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4.5 Robert Jockwer (RJ), ETH Zurich, CH“Study on the reliability of connections at normal temperature and in fire”
RJ presents a study on the reliability of timber connections at normal temperatureusing the framework for uncertainty quantification UQLab. Results show that thereliability index reduces with the thickness of side members, as shear splitting andtension perpendicular to the grain failure modes become more relevant. Also,increasing the strength of the dowel can be problematic, as it can change the failuremode towards brittle failures, reducing the reliability index. In fire, as charring reducesthe thickness of the side members, brittle failures may become dominant, thereforereducing the reliability index of the connections. The model should serve as a tool toevaluate the failure behaviour of connections in cold and fire situation. Optimalconfigurations with a preferred failure behaviour can be identified for the cold situationand required member thickness and distances can be specified. If the expected fireresistance and the corresponding failure mode of these configurations in the firesituation can be estimated, safety factors can be adjusted accordingly. RJ asks for testdata to calibrate the model, e.g. the brittle failure modes discussed by PR and DD.
5 Final discussion and closing
5.1 Final discussion
It is mentioned that if fastener spacings and edge/end distances or minimummember thicknesses change in the design at normal temperature, the fire design has tobe adjusted accordingly. JMA says that no immediate changes planned. RC says thatwarning statements could be included in EN 1995-1-1: “(…) this minimumspacing/thickness might not be sufficient for the fire design (…)”.
5.2 Closing
PP thanks HC for hosting the meeting and all the participants for joining,presenting their work, and actively participating in the discussions, especially PR andDD.
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Part II – Presentations
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Numerical modelling of joints in timber structures: A state-of-the-art
COST Action FP1404 – Design of Timber Connections in Fire, LNEC, Lisbon, Portugal.
Matteo Izzi
Ph.D. CandidateUniversity of TriestePiazzale Europa 1, 34127 Trieste, Italy
Research AssistantCNR IVALSA Trees and Timber InstituteVia Biasi 75, 38010 San Michele all’Adige, Italy
February 12, 2016
Motivations
COST Action FP1404 – Design of Timber Connections in Fire, LNEC, Lisbon, Portugal.
The technological advancements of finite elements (FE) software packages have lead toan increasing use of numerical modelling
Numerical simulations have become an important tool to:Verify and extend the experimental resultsLimit the experimental testing to a minimumProvide time-saving and cost-effective solutionsSimulate situations that have not been tested
FE modelling has been successfully applied is several fields of timber engineering:Seismic design (ambient conditions)Heat conduction of timber and bio-based building productsStructural fire engineeringFire safety engineeringCreep/long-term effects
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Introduction
COST Action FP1404 – Design of Timber Connections in Fire, LNEC, Lisbon, Portugal.
The creation of a numerical model requires the careful consideration of several aspects,i.e.:
GeometryBoundary conditions (mechanical supports, external loads)Interactions (fire, hard contact between fasteners and timber)
Little assumptions and simplification
Generalresults
Numericalmodel
Experimentaltest setup
Simple model
”Bermuda Triangle” for numerical modelling
State-of-the-Art: Geometry and boundary conditions
COST Action FP1404 – Design of Timber Connections in Fire, LNEC, Lisbon, Portugal.
The model schematization may vary depending on the problem under analysis:
2D modelling space with shell elements, e.g. for:Heat transfer analyses within the cross sectionThermal-structural analyses of timber floors/beams
3D modelling space with shell elements, e.g. for:Structural/thermal-structural analyses of timber floors/beams
3D modelling space with solid elements, e.g. for:Thermal-structural analyses of timber studs/floors/beams/connections
The use of 3D solid modelling is encouraged, although the computational effort requiredto carry out the simulations is higher
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State-of-the-Art: Material models for timber
COST Action FP1404 – Design of Timber Connections in Fire, LNEC, Lisbon, Portugal.
Mechanical behaviour of real timber:Orthotropic materialDifferent failure mechanisms, depending on applied loads and internal stresses
Mechanical behaviour of simulated timber, several approaches:Elastic orthotropic material with isotropic plasticity (unique yield strength for bothtension and compression)“equivalent” elastic isotropic material with Continuum Damage Mechanics plasticity(different yield strength in tension and compression)Advanced numerical modelling using a UMAT (User MATerial) subroutines
Basic information to characterize the material models are:Density*, conductivity* and specific heat*
Mechanical behaviour*
Thermal expansion coefficient (* defined as temperature-dependent parameters)
The mechanical and physical properties are not always easy to obtain, and their valuemay depend on the approach used to measure them (stationary/transient heat flux)
State-of-the-Art: Other limitations
COST Action FP1404 – Design of Timber Connections in Fire, LNEC, Lisbon, Portugal.
Aside to the above mentioned considerations, there are other limitations related to thetechnology and the software packages used to carry out the simulations:
The computational effort is very high, leading to very time-consuming analyses
The amount of data saved by each simulations is very big, and it’s interpretation isnot always “user friendly”
Not all the software packages allow to perform coupled thermal-structural analyses(simultaneous application of fire and external loads). Therefore the simulation iscarried out in two separate steps:
1st step: heat transfer analysis simulating the heat conduction process withinthe cross section
2nd step: thermal-structural analysis, in which the temperature distributionresulting from the 1st step is used as an input for the mechanical simulation
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Advance numerical modelling: UMAT Sandhaas
COST Action FP1404 – Design of Timber Connections in Fire, LNEC, Lisbon, Portugal.
Features:Elasto-plastic orthotropic materialDeveloped for 3D modelling using solid elementsEight different failure mechanismsElasto-plastic in compression, brittle in tension
Add-ons:Strength and stiffness reduction due to temperature (EN 1995-1-2) for coupledthermal-structural analyses
Limitations:Origin-oriented approachTime-consuming analysesConvergence issues
Sandhaas C (2012) Mechanical Behaviour Of Timber Joints With Slotted-In Steel Plates. Ph.D. Thesis. Delft Universityof Technology, Delft, The Netherlands.
Sandhaas C, Van de Kuilen J-WG, Blaß HJ (2012) Constitutive Model For Wood Based On Continuum DamageMechanic. World Conference on Timber Engineering (WCTE), Auckland, New Zealand.
Advance numerical modelling: Future perspectives (1 of 2)
COST Action FP1404 – Design of Timber Connections in Fire, LNEC, Lisbon, Portugal.
Development of a new UMAT based on Tsai-Wu failure criterion
Features:Simple strength criterion for anisotropic materialsTakes into account the difference in strengths due to positive and negative stressesCan be specialized to account for:
Different material symmetriesMulti-dimensional spaceMulti-axial stresses
Possible use:More or less everywhere
Current challenges for implementation:Plastic fluxes
Tsai SW, Wu EM (1971) A General Theory of Strength for Anisotropic Materials. Journal of Composite Materials 5(1):58-80, doi: 10.1177/002199837100500106.
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Advance numerical modelling: Future perspectives (2 of 2)
COST Action FP1404 – Design of Timber Connections in Fire, LNEC, Lisbon, Portugal.
Numerical modelling of timber connections using UEL (User ELements)
Features:Elasto-plastic beam in a non-linear medium that acts only in compressionCalibrated using quantities with whom practicing engineers are familiarAdapts to any input history (either force or displacement)Develops pinching as gaps are formed
Possible use:Steel-to-timber jointsTimber-to-timber (2/3 members) jointsSlotted-in steel plates joints
Current challenges for implementation:Introducing the effect of fire
Rinaldin G, Amadio C, Fragiacomo M (2013) A component approach for the hysteretic behaviour of connections incross-laminated wooden structures. Earthquake Engineering & Structural Dynamics, 42(13): 2023-2042, doi:10.1002/eqe.2310.
Numerical modelling of joints in timber structures: A state-of-the-art
COST Action FP1404 – Design of Timber Connections in Fire, LNEC, Lisbon, Portugal.
Matteo Izzi
Ph.D. CandidateUniversity of TriestePiazzale Europa 1, 34127 Trieste, Italy
Research AssistantCNR IVALSA Trees and Timber InstituteVia Biasi 75, 38010 San Michele all’Adige, Italy
E-mail: [email protected]
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MMS Axis (Mechanics, Materials & Structures) - Patrick RACHER – COST FP1404, Lisboa 1
Patrick RacherDhionis Dhima
Fire safety of timberconnections
MMS Axis (Mechanics, Materials & Structures) - Patrick RACHER – COST FP1404, Lisboa 2
3D thermo-mechanical FEMJoint scale
t1(tfi)
F
θx
θyθz
F i, tfi
Local stiffness variation
Plasticity beneath fasteners
Cinematic and static criteria in fire depend on :
K0(θ)
K90(θ) Kw(θ)
K0(θ)
K90(θ) Kw(θ)Structural scale
General consideration
Rjoint,tfi
Kser,tfi
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MMS Axis (Mechanics, Materials & Structures) - Patrick RACHER – COST FP1404, Lisboa 3
Slip modulus of mechanical Fasteners
Joist hangers
33,0KK serfi =
Timber to timber and steel to timber joints
67,0KK serfi =
25
20
15
10
5
0
0 10 20 30 40 50 Temps (mn)
Glissement(mm)
Test N° 7tfi=27mn
Test N° 8tfi=22mn
Test N° 4tfi=38mn
25
20
15
10
5
0
0 10 20 30 40 50 Temps (mn)
Glissement(mm)
Test N° 7tfi=27mn
Test N° 8tfi=22mn
Test N° 4tfi=38mn
Slip (mm)
Time (mn)
MMS Axis (Mechanics, Materials & Structures) - Patrick RACHER – COST FP1404, Lisboa 4
Tb3
Tb2Tb1
2x10T1 ,T2 ,T3
2x15
TintTb3 Tb2 Tb1
10 10T1, T2
20
50 100 150 200 250 300 350 θ (°C)
20
40
60
80
100
Profondeur(mm) Broches 20mm Boulons 20mm
Broches 12mm Boulons 12mmBrochesprotégées
50 100 150 200 250 300 350 θ (°C)
20
40
60
80
100
Profondeur(mm) Broches 20mm Boulons 20mm
Broches 12mm Boulons 12mmBrochesprotégées
FastenersDowels
DowelsProtectedDowels
Bolts
BoltsDepth
Fasteners heating
128
MMS Axis (Mechanics, Materials & Structures) - Patrick RACHER – COST FP1404, Lisboa 5
Fire resistance
Reduction factor (on material properties and fastener resistance)
Load ratio (capacity of the connection )
Which approach ?
0
0,2
0,4
0,6
0,8
1
0 100 200 300
k (%)fh
θ (°C)
EN 1995-1.2Model x
Embedding strength
ηd
MMS Axis (Mechanics, Materials & Structures) - Patrick RACHER – COST FP1404, Lisboa 6
Behaviour of steel plate
- tfi < 30 mn : Johansen ~ FEM
- tfi > 30 mn : Johansen > FEM
0
10
20
30
40
50 100 150 200 250t1 (mm)
Rj (kN)
Johansen
0
10
20
30
40
50
50 100 150 200 250t1 (mm)
Rj (kN)
Simulation
Cold30 mn60 mn90 mn
Reduction factor
129
MMS Axis (Mechanics, Materials & Structures) - Patrick RACHER – COST FP1404, Lisboa 7
Steel to timber fastener design
Reduction factor
Timber behaviour
- t1 = 50 mm
30 < tfi < 60 mn : Plate behaviour
- t1 = 125 or 200 mm
0,0
0,2
0,4
0,6
0,8
1,0
0 15 30 45 60 75 90
kRj
(%)
tfi (mn)
t1 = 50 mmt1 = 125 mmt1 = 200 mm
( ) 11Rjfi1, RtkR ⋅=
MMS Axis (Mechanics, Materials & Structures) - Patrick RACHER – COST FP1404, Lisboa 8
Timber to timberfastener design
( ) 11Rjfi1, RtkR ⋅=Mode 2
Mode 1
3
2
FEM
Reduction factor
130
MMS Axis (Mechanics, Materials & Structures) - Patrick RACHER – COST FP1404, Lisboa 9
Load ratio
0,0
0,1
0,2
0,3
0,4
0,5
0 15 30 45 60 75 90
ηexp
0,4
0,3
0,2
0,1
0tfi (mn)
Bois sur bois
Bois-Métalη = 0,58e-0,020tfi
η = 0,80e-0,027tfi
ηd
MMS Axis (Mechanics, Materials & Structures) - Patrick RACHER – COST FP1404, Lisboa 10
135mm
1
4
Fi/F (%)
t (mn)
0
2
4
6
8
10
0 10 20 30 40 50
g (mm)
t (mn)
t1=60mm t1=135mm
Bolt
ts
F
t1
Dowels
1 2 3 4
60mm 1
4
neff (tfi)
Extensions to connections
Failure criteria
131
MMS Axis (Mechanics, Materials & Structures) - Patrick RACHER – COST FP1404, Lisboa 11
Pagode de Fogong-Shanxi (1056)
TIMBER, The 3rd millenium material
Thank youfor your attention ...
132
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Table 1: Tentative target reliability indices �� (and associated target failure rates) related to one year reference period and ultimate limit states
1 2 3 4
Relative cost of safety
measure
Minor consequences
of failure
Moderate
consequences of
failure
Large
consequences of
failure
Large (A) �=3.1 (pF�10-3) �=3.3 (pF � 5 10-4) �=3.7 (pF � 10-4)
Normal (B) �=3.7 (pF�10-4) �=4.2 (pF � 10-5) �=4.4 (pF � 5 10-6)
Small (C) �=4.2 (pF�10-5) �=4.4 (pF � 5 10-6) �=4.7 (pF � 10-6)
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−
g = z · − −
x2
x1
g(x1,x2)=0
gX1 X2
12 ��j
j��R
α1�R
α2�R
u1
u2
�R
g(u1,u2)=0
β
−
P (g ≤ 0) = P (z · − − ≤ 0)
β
(P ≈ 10−3
) (P ≈ 5 · 10−4
) (P ≈ 10−4
)(P ≈ 10−4
) (P ≈ 10−5
) (P ≈ 5 · 10−6
)(P ≈ 10−5
) (P ≈ 5 · 10−6
) (P ≈ 10−6
)
135
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α =Q
G
Definition of markers: Span Weight of roof or ceiling
short heavy large intermediate short or large light
Snow load 1) Imposed load 2)
Altitude [kN/m2] Category [kN/m2] 0 m 0.76 A 1.5
100 m 0.91 B1/D1 2 500 m 2.53 B2/C1 3
1000 m 3.73 C2 4 2000 m 31.76 C3-C5/D2 5
1) according to DIN EN 1991-1-3/NA 2) according to DIN EN 1991-1-1/NA
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
A B1/D1 B2/C1 C2 C3-C5/D2
Load
ratio
α [-
]
Category of imposed load (DIN EN 1991-1-1/NA)
Ceiling beams
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 500 1000 2000
Load
ratio
α [-
]
Site altitude above sea level [m]
Rafters
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 500 1000 2000
Load
ratio
α [-
]
Site altitude above sea level [m]
Roof trusses of halls
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R1,1
2= fh,1t1d
R1,2 = fh,2t2d
R1,3
2= fh,3t3d
t1
t3
t2
d
% !"
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R2,12
2= −d
2
t1fh,1fh,2fh,1 + fh,2
+
+
√(d
2
t1fh,1fh,2fh,1 + fh,2
)2
+
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)dfh,1fh,2
2fh,1 + fh,2
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R3,12
2=
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−
Rconnection = min {2R1,i, R2,i +R2,j , R3,i +R3,j , R2,i +R3,j}
Mode 1 Mode 2 Mode 3
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0 20 40 60 80 100
F v,R
k [N
]
penetration length t2
with rope effectwithout rope effect
t
R
− fh
fh = AρBdCε
ABCε
15 20 25 30 35 40 45 50 55������� � ����� �
15
20
25
30
35
40
45
50
55
� ����
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h
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1− μtilendft,90
lend
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))��
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κ2 =κ3GI,c
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σv
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σv
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E90E0
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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2�������� � � �
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200
400
600
800
1000
1200
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300 350 400 450 500 550 6000
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
Density ρ [ kg/m3 ]
Gf,I
[N/mm
]
LarsenRiberholtGustafssonJockwer
∗
CoV
G 0.3 20%
∗
141
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0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Gf,II [ N/mm ]
Cumulativeden
sity
[-]
Method 1Method 2
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0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
fv [ N/mm2 ]
Cumulativeden
sity
[-]
I-Beam-SpecimenEN 408 SpecimenWeibulLognormal
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0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
ft,90 [ N/mm2 ]
Cumulativedistribution[-]
V =0.57 dm3
V =10 dm3
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h
t1
t3
t2
dlend
dt1,2t2lend 10dh 10d
−
0 10 20 30 40 50 60 70 80 90 100���� ����
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1
2
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4
5
6
7
8
9
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1
1.5
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−
lend
ht1
t3
t2
dlend
⇒ t1 t2 lend h
−
0 5 10 15 20 25 30 35 40 45 50�� � ���
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0 5 10 15 20 25 30 35 40 45 50 55 60� ���
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8
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