Meeting COST E 24 special taskforce, Zurich, 6./ M i n u t e s

Transcription

1 Meeting COST E 24 speial taskfore, Zurih, 6./ M i n u t e s Attendane: H.J. Larsen J.D. Sorensen A. Jorissen T. Toratti M.H. Faber J. Köhler Content: Opening, Conlusions from the CIB W18 meeting The draft version of the Timber PMC was presented at the CIB W18 meeting in Edinburgh. The main reation of the CIB W18 members was positive. However, only little speifi disussion took plae. A tehnial issue whih was ritiized at the CIB meeting was the utilization of a log-normal distribution for modeling the stohasti harateristi of timber strength properties. It was agreed that the properties of graded timber an only be modeled taking into aount the ommon grading pratie at present and in future. The new European grading ode EN has to be taken into aount. It was disussed if P. Glos and F. Rouger, the main authors of the grading ode should be invited for one of the next meetings. Contat P. Glos and F. Rouger and the COST administration. (Larsen, Faber, Köhler) 1

2 Content: Stress strain relationship The following stress strain relationship was proposed by J.D. Sorensen aording to Blass (1988): σ Eε + k ( ε / ε ) 7 1 = (1) 7 k1( ε / ε ) 1+ k2ε + f, y where k 1 = ε f, y ( 1 f / f ) 7 6, y (2) k E 7 = (3) 6ε 2 f Typial values for the parameters are: f / f 0.8 = %, y ε u ε ε 3 It was disussed that this model is originally from Glos. The proper referene has to be verified. At the reent CIB W18 meeting Glos (Paper ) used a different formulation for the stress strain relationship of beeh lamellas. The main differene is that aording to the urrently presented model the MOE for tension and ompression is different. It was agreed to ontat Glos and also Gehri to verify a model. For a statistial investigation, it would be neessary to obtain data, probably from Glos or Gehri. It was agreed that this model would fit into the PMC in hapter 5. Contat Glos and Gehri regarding literature and data (Köhler) Quantifiation of model parameters (Sorensen, Köhler) 2

3 Content: Servieability T.Toratti presented a refined version of the paper presented in Florene. It was pointed out that the onsiderations are for small stress levels, namely around 20 MPa, in the fiber diretion and for European softwoods. It was disussed that this stress range should be better expressed as a fration, e.g. 35%, of the assumed short term strength. The model is an additive formulation equivalent to a serial arrangement of Kelvin elements; i.e. different kinds of strain are summarized and there are depending on time and some of them are also depending on the gradient of the moisture. It is shown in T. Torattis paper that the satter of the test results an be quantified to a COV of 10%. It an be also observed that the predited time deformation urve lies within the test observations whereby the deformation is tending to be underestimated by the model (Figure 1 in the paper). It was pointed out that it is a problem to define a moisture load. The moisture onditions have to be standardized, whih would be also important for DOL alulations under varying limate. The air veloity has a big influene. A. Jorissen remarked that the RH in the Netherlands is between 75% and 90% outside and 50% and 80% inside between summer and winter. Fast moisture variation would be important for onnetions beause of the fast diffusion of moisture in fiber diretion. In the most pratial ases the stress level is lower than 35%. T. Toratti underlined that it is hard to find a reasonable simplifiation for a ode format. It seems that there is only a hoie between a very simple format (k def similar to the EC5) and the sophistiated one presented by T. Toratti. It was proposed to produe a matrix of k def values for different load and limate onditions. Therefore moisture load standardizations from Thelandersson an be used as well as existing load models whih are mostly already programmed. The program for the reep estimations exist and is written in PASCAL. The performane of the program has to be verified. It ould be possible that some Kelvin elements are not signifiant for ertain load moisture ombinations. The model ould be simplified for these ombinations. End deformations are a good indiator to alibrate the model. The model parameters should be quantified as random variables. The elasti ompliane is an important material harateristi of the model. Again it was pointed out that there may be a different behavior of onnetions in regard to long term deformations. Ario Ceotti may have some experiene in using the model. The model unertainties involved have to be quantified. E.g. how are the doubling times estimated? Test data has to be reviewed to quantify the unertainties. 3

4 It was agreed that the model is good for alibration but to ompliate for design. The model should be part of the model ode a simplified alternative in form of a matrix of k def values should be also given in the ode. Produing a matrix for different load and limate situations. Therfore the following is needed - quantify moisture loads, ontat Thelandersson (Toratti) - quantify load sequenes (Sorensen) - ontat A.Ceotti regarding experiene and data (Toratti) - aquiring data, quantify model parameters (Toratti) - doing alulations and sensitivity analysis (Toratti, Sorensen) 4

5 DOL An alternative model (Barrett & Foshi) was presented by J.D. Sorensen, whih allows a time independent limit state formulation for ultimate failure in a long term perspetive. The known partiularities in regard to DOL tests, model fitting and the involved unertainties have been disussed. It was agreed that it is important to speify a model unertainty whih takes into aount these speial features. Data has to be reviewed in this ontext. It was agreed to ontat Foshi and Rosowsky to obtain more data. In regard to the moisture load interation little experimental and theoretial evidene exist. It was referred to an ongoing researh projet haired by Preben Hoffmeyer about this subjet. DOL - Tension perpendiular to the grain It was pointed out that the situation is different for tension perp. to the grain. Substantial work has been done by Alpo Ranta Maunus and Simon Aiher. DOL - Connetions The situation is different for Connetions. Van de Kuilen did his thesis on this subjet and may ontribute. Aquire North Amerian data (Larsen) Review data, quantify model parameters (Sorensen, Köhler) Distribute tasks aording Tension perpendiular to the grain and Connetions (Larsen, Faber) 5

6 Glulam H.J. Larsen presented a review about the modeling of Glulam properties. It was mainly referred to the Karlsruhe model and to the work of Eri Serrano. A very simple model based on CEN in form of f = Af + B glulam, m, k lam, t, k whih relates the harateristi values of the glulam bending strength and the lamination tension strength was also disussed. It was pointed out that this simple model is not suffiient to model the different statistial harateristis of both properties. On the other hand the more sophistiated Karlsruhe model whih is based on the finite element method shows inonsistenies with test observations. E.g. a strong depth effet an be predited using the Karlsruhe model, but an not be observed in tests. It was agreed that the definition of failure riteria is always the ruial part of these stress based models. H.J. Larsen presented some findings from Eri Serranos work. As a dominant failure mehanism the peeling of of the undermost lamination was pointed out. It an be onluded that the bending performane of a glulam beam mainly depends on the property of the undermost lamination if its thikness is greater than 12mm (whih in pratie is always the ase). It was disussed if it s possible to propose a simple model whih takes this feature into aount. The ritial strains for the undermost lamination and the longitudinal variation of this property would be of interest for this model. It was disussed that it would be hard to find data about the ritial strain for finger joints. Test Data would be neessary. Gehri should have data. A. Jorrissen has data and will ontribute. Kjell Solli from Norway has data and should be asked. There should be also some data in Austria. A CIB paper was mentioned where the volume effet of glulam is investigated. It was written by Norwegians but the orret referene was not present. Proposal of a simple model (Larsen, Köhler) Test data aquisition : Gehri, Jorissen, Solli, Austria (Larsen) 6

7 Topi : Connetions The proposal from A. Jorissen was presented. The question aroused if these formulations also inlude plug shear, and if not how it ould be inluded. The parameters of the model should be expressed as random variables. (ould be quantified by revision of test data) Frederi Rouger and André Jorissen should be asked for Data. It was disussed how to inlude other types of onnetions. The properties of punhed metal plates an be speified but they are produer dependent. Models exist for ring or shear plates and toothed plates. A huge data base exists in Karlsruhe. Further a tehnial disussion aording the regression methods used in the CIB paper of Leijten, Köhler and Jorissen took plae. J.D. Sorensen argued that there is something wrong in the regression parameter estimation due to an unrealisti big satter of one of the regression parameters. Further, an error was pointed in Equation (4) of the paper. The general proedure to predit the mean value by given mean value of the density was also mistrusted. Addendum: Instantly after the meeting some reassessment of the used method took plae. The estimated regression parameters where proofed to be orret. It was found that the error in Equation (4) was a writing error; the equation used in the alulation sheet was proofed to be orret. By using the more diret method proposed by Sorensen minor hanges, e.g. in the estimation of the 5%-fratile line an be observed (the observed hanges are in the range of +/- 10%). Task: test data (Jorissen) revise regression (Köhler) reliability analysis for different onnetion geometries (Jorissen, Köhler, Leijten) 7

8 Task: Quantifiation of model parameters It was disussed that it is neessary to have a simple relationship between the statistial properties of the load bearing apaity of test speimen and the load bearing apaity of referene volumes. E.g. for the bending strength: a lear relation between weak setions and the properties of test speimen. For the bending strength model it is also important to get some information of the longitudinal distribution of weak setions. It would be also interesting to know how important this is for different design situations. It is important to speify a definition of a weak setion. E.g. KAR (different for different speies?) propose a framework to relate test data to referene volume properties (Köhler, Sorensen) It was onluded that it is a general task to produe a data base in the very near future. (All, editing: Köhler) 8

9 Data Request: Topi Task Data required Soures (as mentioned in our meeting) Grading Stress-strain urve Servieability Desribe the stohasti properties of graded timber material Quantify model parameters, proposal for perpendiular to the grain Quantify model parameters Bending strength, MOE bending, density. Regression with indiators Tension and ompression tests Long term test - defletion Glos, Gehri Toratti Speify moisture load model DOL Quantify model parameters Duration of load test data Foshi, Rosowsky, Hoffmeyer Glulam Propose a simple model Lamella properties, finger joint properties, properties of glulam omponents Connetions Quantify model parameters Material properties, properties of onetions Bending Quantify model parameters for Longithudinal distribution of weak strength model different speies setions (Isaksson) Definition of weak setion Strength of weak setion Gehri, Jorissen, Solli, Austrians Rouger, Jorissen Isaksson 9