Engineering Design Values of Wood Based Composites

Transcription

1 Engineering Design Values of Wood Based Composites Mizi FAN ) and Vahik ENJILY ) ) Head of Research, Civil Engineering Brunel University, UB8 3PH, UK. ) International Director Building Research Establishment, UK. Summary This paper summarizes some of the findings from a comprehensive study concerning the performance of wood based composites in building construction. The presentation only focuses on the strength and deformation modification factors for engineering design of wood based composites, that is, ) to determine whether the time modification factors (k mod and k def ) in Eurocode 5 for formaldehyde based boards are applicable to boards manufactured using new alternative binders such as isocyanate and cement, ) to evaluate the effect of long-term concentrated loading by a full scale component test and small-scale indicative test, and 3) to examine the effect of long-term shear loading (panel and planar shears) on the performance of wood based composites for structural uses in comparison with long term bending loading in Eurocodes and 5. Numerous results and important findings enable the determination of whether the correct test methodology for woodbased panels is in place and the evaluation of the suitability of the resulting data for use with Eurocodes & 5.. Introduction EN 995--: Eurocode 5 (EC5) [] uses a limit states approach to design (ultimate limit states and serviceability limit states). EC5 includes modification and partial safety factors that are applied to loads and material properties, i.e. the design values of properties shall be proportional to the characteristic values of the properties ( Pd = k. P k / γ M, where P d is the design value of properties, P k is the characteristic value of properties, k is the modification factor and γ M is the partial factor for a material property), but does not include the actual loads and material properties themselves. Whilst guidance on the required loads is given in Eurocode (EC) [], the user has to look elsewhere for many of the basic material properties. Wood-based panels shall use the characteristic values given in the relevant European Standards, i.e. EN369. EN369- [3] includes characteristic values for ) OSB/, OSB/3 and OSB/; ) particleboard, P, P5, P6 and P7; 3) hardboard, HB.HLA; ) medium board, MBH.LA and 5) MDF.LA and MDF.HLS. EN369- [] includes characteristic values for plywood for structural design. When no values are given in European Standards, characteristic strength and stiffness values shall be calculated according to the method given in EN58 and EN789 [5-6]. EC5 requires that the characteristic values are determined from tests for the types of action effect to which the material will be subjected in the structure. The process will not prove effective if the test methods do not yield realistic results. Characteristic values have to be factored, i.e. by a duration of load factor k mod and creep factor, k def, to produce design values for wood based composites due to the nature of timber and timber products when subjected to sustained imposed loads [7]. EC5 includes values for both k mod and k def for a variety of wood-based products including various types of panels. However, these values were derived/agreed by the EC5 drafting committee based upon the limited test data available and entirely bending test data because of the lack of data in some areas.

2 This paper presents part of the finding from a complex project, which was to determine whether the correct test methodology for wood-based panels is in place (Table ) to evaluate the suitability of the resulting data for use with Eurocodes & 5 (Table ) The presentation ONLY focuses on the modification factors for wood based composites. More information is presented in a series of separate papers [8-]. Table Issues concerned with Is the correct test methodology in place? Areas involved Issues involved Aims EN789 Test methods for the determination of characteristic values EN3, EN39, EN3 Small-scale test methods for quality control purposes. EN87, EN95 Performance requirements, specifications and test methods for floors and roofs. EN56 Test method for determining creep and duration of load in bending. Bending, tension, compression, panel shear, planar shear using medium size test pieces. All properties given as requirements in the EN material specifications For floors and roofs, includes a soft body impact test and a concentrated load test. Also specifies the requirements in relation to EC & EC5. Size of test pieces, edge sealing, sampling/matching procedures. Confirmation on the suitability of EN789 for the determination of characteristic values. Confirmation as to whether or not the small-scale methods are suitable for controlling the production level of products with associated characteristic values derived in accordance with EN789. Demonstration that typical constructions can continue to meet the requirements of EC and EC5. Proposal to use small, sealed specimens instead of semi-sized. Recommendations for improved sampling methods. Table Issues concerned with Is the resulting data suitable for use with EC & EC5? Areas involved Issues involved Aims Conversion of creep and duration of load data to k def and k mod values. Consideration as to whether k def and k mod values derived from bending tests can also be used for other properties. EN58 Derivation procedures for characteristic values. The procedures used to analyse and extrapolate the data. The amount of data necessary to provide confidence in the modification factors. How do values vary with property and test method? Are the sampling requirements adequate? Is the calculation procedure suitable? Does the procedure result in adequate values for use with EC/EC5. Background information for the use of EN56. New procedures for assessing creep under other than bending loads. Support for principles in EN58.

3 . Materials Commercially manufactured boards were used in all tasks relating to the above issues and these all conformed to relevant European product standards (Table 3). Additionally, laboratory boards were manufactured so that all factors of production and furnish were kept constant when studying the performance of new materials. This allowed the strict comparison of performance. Table 3 Panels and corresponding tasks Task Panel/type* Thickness (mm) EN product standard Glue type* T&G profile Sample size Sampling and matching MDI bonded Cement bonded Concentrated load OSB-com MDF-com MDF-com Plywood-com CB OSB-com OSB-com PB-lab PB-lab MDF-com MDF-com MDF-lab MDF-lab EN3-7 EN3-7 EN3-OSB/3 EN6-5LA EN6-5HLS EN636 EN63 EN3-7 EN3-OSB/3 EN3-7 EN3-OSB/3 EN3-7 EN3-7 Fulfil EN3-7 Fulfil EN3-7 EN6-5LA EN6-5LA Fulfil EN6-5LA Fulfil EN6-5LA PF MDI PF Cement PF PF MDI MDI MDI MDI CB 8 EN63 Cement Plywood-com OSB-com MDF-com *com = commercial products lab = laboratory made board EN636-3 EN3-7 EN3-OSB/3 EN6-5LA PB=particleboard OSB=oriented strand board PF PF

4 = melamine urea formaldehyde MDI = methylenene diphenol diisocyanate PF = phenol formaldehyde MDF=medium density fibreboard CBPB=cement bonded particleboard 3. Calculation of Engineering Design Values for Wood Based Composites When wood based panels are subjected to sustained imposed loads, there are two effects that have to be taken into account during design: ) The increase in deflection with time (creep) and ) The reduced loads, compared to those in short term tests, that can be sustained without failure over a prolonged period (duration of load). To account for these effects, EC5 applies a duration of load factor k mod, to the characteristic strength values and a creep factor, k def, to the calculated short-term deflections. That is, the final mean values of modulus of elasticity, E mean,fin, shear modulus G mean,fin and slip modulus, K er,fin shall be calculated as E E mean, fin / Gmean, fin / K ser, fin mean, fin / G mean, fin / K ser, fin = = E E mean mean / G mean + k / G def mean +ψ k / K / K def ser ser for serviceability limit states, and for ultimate limit states. The design value of a strength, X d and a resistance (load carrying capacity), R d shall be calculated as X k / Rk X d / Rd = kmod γ M. Where X k and R k are the characteristic values of a strength and load carrying capacity respectively, and γ M is the partial factor for a material property. It can be seen that the factor k mod effectively reduces the safe design loads that can be applied to a structure and is a function of material type, service class (environment), and load duration. The factor k def increases the calculated deflection and is also a function of material type, service class and load duration. The k def is derived from creep testing in accordance with EN 56 []. The creep factor is defined as the ratio of the increase in deflection with time under load to the initial elastic deflection. The value of the creep factor will therefore change with time under load, level of stressing, and climate. More details can be found in a separate paper [9] The determination of duration of load factors is carried out on a similar basis to the creep tests except that higher stress levels, which eventually lead to failure, are employed. For the determination of duration of load factor, 55%, 6%, 65%, 7%, 75% and 8% of the short term failure load at C/65%rh were used. At each stress level, tests are carried out and the times to failure recorded. Then, the duration of load factor for any given time T, is calculated from a linear regression line through the graph of stress level vs logarithm of time to failure [9].. Strength and Deformation Modification Factors of New Materials The majority of wood-based panels manufactured and used in Europe have formaldehyde based binders. There continues to be pressure to reduce the level of formaldehyde used in panels and this is being addressed both by altering the formulation of these adhesives and by seeking alternative adhesive systems. However, the large volume of data available concerning the behaviour of boards subjected to longer-term loads is based almost entirely on formaldehyde based binders. There is therefore a need to collect corresponding data on boards bonded with the alternative adhesives in

5 order to confirm that the creep and load duration factors given in EC5 are appropriate to these materials.. Creep in isocyanate bonded boards This part of the study was concerned with the testing of isocyanate bonded (PMDI) MDF and particleboard under long-term bending loads, including commercial types as well as boards made under controlled laboratory conditions. As a reference, boards bonded with an adhesive were also included in the test programme. The main conclusions were as follows:- i) For both the laboratory and commercial boards tested, the relative creep of isocyanate particleboard and MDF boards was generally slightly lower than formaldehyde bonded boards tested under C/65% relative humidity conditions (e.g. Fig. ). ii) For duration of load tests, isocyanate bonded boards gave longer times until failure than corresponding formaldehyde boards. However, the variations between individual samples and between institutes were very significant. iii) Although small differences in relative creep were measured for the binders tested, they were within the same order of magnitude. iv) The values of k mod and k def contained in EC5 can be applied to isocyanate bonded MDF and particleboards although this is likely to be conservative. v) For the full exploitation of isocyanate bonded boards to occur, it is necessary for more extensive testing to be undertaken. Reative creep (kc) Time (hrs) LabPB/ LabPB/MDI Reative creep (kc) Time (hrs) ComPB/ ComPB/MDI 35 5 Figure Relative creep of PB and MDIPB. Creep in cement bonded particleboards Two types of cement bonded particleboard (CBPB) were tested in this task to determine the creep and load duration behaviour in bending. The conclusions were:- i) At 5% stress level, the k c of CBPB was very similar to that of bonded particleboard (PB), while the deflection of CBPB was only about 5% of that of PB because of its greater stiffness. In the later stages of the tests, the rate of increase in deflection of CBPB was also much lower than that of PB (e.g. Fig. ). ii) There was a slight difference in creep performance between the two brands of CBPB tested, which were closely related to the short term properties and the sorption behaviour. The higher the MOR the better the creep performance and the better the moisture resistance the better the creep performance.

6 iii) Test pieces subjected to stress levels of 5%, 5% and 55% had not failed after 6 months loading, with some test pieces being under load for months at these levels. iv) The duration of load behaviour at 6% and 75% stress levels was better than that of PB. At 6% stress level, the shortest time until failure for CBPB was over 5 times the longest time to failure for PB under the same conditions. v) The relative creep and deflection of CBPB increased approximately linearly with applied stress levels of 5% to 6% for boards loaded at the same time after manufacture. vi) The creep performance of CBPB is closely related to the storage time between manufacture and test. Both deflection and creep reduce with increasing storage time. The partial evaluation of the creep and duration of load behaviour of isocyanate bonded and cement bonded particleboards suggests that it is safe to use the k mod and k def factors in EC5 with these materials. These factors may be unduly conservative however, but further research would be required to confirm this. The work confirms that the creep and load duration behaviour of boards can be affected by the adhesive type used and it is therefore important that the long term behaviour of boards made with new adhesives is evaluated before their use becomes widespread. In the two particular instances evaluated here any error in the factors in EC5 appears to lie on the side of safety but it cannot be guaranteed that this will always be the case. Actual deflection (mm) ComPB/ ComPB/MDI CBPB Time (hrs) Figure Actual deflection of PB, MDIPB and CBPB 5. Strength and Deformation Modification Factors Under Different Stress modes The values of k mod and k def included in EC5 were derived from the limited test data available which was entirely based on bending creep/dol tests and yet EC5 makes the assumption that these factors are appropriate to all stress types. This could include: bending concentrated loads tension and compression panel and planar shear The volume of data available from bending creep/dol tests is often considered to be too sparse to be able to set accurately the correct values of k mod and k def. Given the different stress forms and

7 failure modes induced by the various forms of loading and the non-isotropic nature of wood-based panels, it is unreasonable to expect that the same k mod and k def values would be appropriate in all cases. Of the alternative loading conditions, shear and concentrated loads gave more cause for concern. Therefore, creep under concentrated loads and shear loads is presented below as an example to demonstrate the differences. 5. Concentrated loads The study of creep under concentrated loads included the development of a component-based test and small scale indicative test methods for measuring the creep/dol behaviour under concentrated loads. Semi-sized bending tests were carried out in parallel for comparison. The objective was to develop the test procedures and to carry out indicative testing to establish whether or not the k mod and k def factors in EC5 are appropriate to concentrated loads. A component-based test method using a mm square test piece and a small-scale method based on a 3mm square test piece were successfully developed. Both of these methods allow testing of solid and jointed panels. Having developed the methods, testing was carried out on particleboard, OSB, MDF and plywood. One stress level was used for creep (5%) and three stress levels (65%, 7% and 8%), for DOL. Although this is not sufficient for a full evaluation of DOL in accordance with EN 56, it was considered to be sufficient to indicate whether or not the EC5 values are appropriate. Testing was carried out on both solid and jointed test pieces at a climate of C and 65% relative humidity. Plywood kc Particleboard kc small solid comp solid small Jointed comp Jointed Bending test EC MDF kc OSB kc Figure 3 Comparison of k c values from concentrated load tests, bending tests and EC5 The results are summarised in Figures 3 and. It is apparent that the k c values under concentrated load are similar to those measured under bending, but there are differences between the materials. In relation to k d values, there is a suggestion that the values may be higher than the k mod values given in EC5, and if this is the case it means that EC5 may be conservative for concentrated loads.

8 Plywood kd Particleboard kd.9.8 small solid comp solid small Jointed comp Jointed Bending test EC MDF kd OSB kd Figure Comparison of k d values from concentrated load tests, bending tests and EC5 A comparison of the k mod and k def values from the component and small-scale is also shown in Tables and 5. It can be concluded that the component test developed is effective and that the small-scale test gives a good indication of likely results under the component test. Whilst the level of testing has not been sufficient to define k mod & k def for concentrated loads, there does not appear to be a safety problem with the use of the current EC5 values with concentrated loads. There is a suggestion that the k mod values in EC5 may be too low for concentrated loads, but further testing would be required to confirm this. Table Comparison of calculated k c for component and small scale test Creep factor k c (k def ) Test type Duration Material S EC5 S EC5 S EC5 S EC5 Compone Plywood.7 SC.6 SC.33 SC.79 SC nt test PB. Ben. Bendin.9 Bendin. Bendin MDF.7 ding.65 g test.6 g test.98 g test OSB.8 test Smallscale Plywood test PB.3 *.76.5*.75.*.37.5* MDF OSB.56 **.8.5**.8. **.57.5 ** Key : S = solid, Short = week, Med = 6 months, Long = years, Perm = 3 years, * for P7 value, ** for OSB3

9 Table 5 Comparison of calculated k d for component and small scale tests Load duration factor k d (k mod ) Test type Duration Material S EC5 S EC5 S EC5 S EC5 Compone Plywood.75 SC.69 SC.63 SC.6 SC nt test PB.79 Bendin.68 Bendin.57 Bendin.53 Bendin MDF.7 g test.63 g test.5 g test.8 g test OSB Smallscale Plywood test PB.77.9*.65.7*.5.5*.5. * MDF OSB.7.9**.6.7 **.58.5**.56.** Key : S = solid, Short = week, Med = 6 months, Long = years, Perm = 3 years, * for P7 value, ** for OSB3 5. Shear loads The estimation from the experimental results under long term shear loading has also been compared to creep factors given in Eurocode 5 for the relevant panel types, service class, Figures and 5. Creep results showed that the values from experimental results are different from the values laid down in Eurocode 5. In general, for MDF under panel shear load: k c < k def ; for OSB under panel shear load: k c > k def ; for MDF under planar shear load: k c > k def ; for OSB under planar shear load: k c > k def. It should be noted that the values in EC5 are for bending creep. It has proved that they are not suitable for shear creep, and this means that new k c has to be developed for inclusion in Eurocode 5 for shear creep loading. Duration of load of test showed that the values from experimental results are different from the values laid down in Eurocode 5. In general, for all the materials tested under both panel and planar shear loading, k d < k mod for the short term and medium term, while k d > k mod for the long term and permanent term. Again, it should be noted that the values in EC5 are arisen from bending creep tests. The results have proved that they are not suitable for shear creep loading and new characteristic values should be developed for shear loading design. A comparison of relative creep data from bending, panel and planar shear tests suggests that there is a need for separate k def values in Eurocode 5 for bending, panel shear and planar shear stresses for each panel type. 3 MDF MDF MDF3 EC5-mdf 8 6 OSB OSB OSB3 EC5-osb A B

10 6 8 6 MDF MDF MDF3 EC5-mdf 5 3 OSB OSB OSB3 OSB EC5-osb C Figure A comparison of k c with k def in EC5 for MDF and OSB under panel and planar shear loading (A and B-panel shear, C and D-planar shear) D..8 MDF EC5..8 OSB EC A B..8.6 MDF MDF EC OSB OSB EC C Figure 5 A comparison of mean k d to k mod in EC5 for MDF and OSB under panel and planar shear loading (A and B-panel shear, C and D-planar shear) D

11 6. Conclusions and Recommendations The conclusions and recommendations can be presented as follows: SUBJECT CONCLUSIONS RECOMMENDATIONS Suitability of EN56 test methods for determining creep (CR) and duration of load factors (DOL). Creep and duration of load testing under different stress and environmental conditions. Conversion of creep and duration of load test data. Creep and duration of load behaviour in panels with alternative adhesive types. Variable climates had a significant effect on the results of EN56 testing. Edge sealing of small test pieces gave similar behaviour to that of semi-sized test pieces, which are more representative of structural use. The high degree of variability in CR/DOL test results was also, at least in part, due to the adoption of sidematching in the setting of the stress levels. More accurate methods of setting the stress level for creep tests can reduce the variability of results. Stress and environmental conditions were closely related to the CR and DOL. The values of k def and k mod in EC5 were agreed based on the data available, with a high degree of interpretation being required. These values may not be suitable for some of the stress modes and exposure conditions of structural design. The component test developed was effective and that the small-scale test gave a good indication of likely results under the component test, whilst the level of testing has not been sufficient to define k mod & k def for concentrated loads. The partial evaluation of the creep and duration of load behaviour of isocyanate and cement bonded particleboards suggested that it is safe to use the k mod and k def factors in EC5 with these materials. It is confirmed that the creep and load duration behaviour of boards can be affected by the adhesive type used. ENV56 should be modified to specify edge sealing when small test pieces are tested in a variable climate. The sampling procedures should be reconsidered. Further testing should be carried out to determine the correct k mod & k def for different stress and environmental conditions. k def and k mod values agreed by the CEN committee should be the only ones permitted to be used in conjunction with EC5. The k mod and k def factors in EC5 factors may be unduly conservative with isocyanate and cement bonded particleboards, but further research should be carried out to confirm this. It is important that the long term behaviour of boards made with alternative adhesive types is evaluated before they are widely used.

12 7. References [] EN995--: Eurocode 5 - Design of timber structures. Part -: General rules and rules for buildings, [] EN99--: Eurocode -Actions on Structures, [3] EN369-: Wood based panels: Characteristic values for structural design, [] EN369-: Wood based panels: Characteristic values for structural design, [5] EN789: Timber structures: Test methods Determination of mechanical properties of woodbased panels, 996 [6] EN58: Wood-based panels Determination of characteristic values of mechanical properties and density, 995 [7] Fan M., Non-destructive systems for stress determination for long term (time dependent) creep loading. J of Wood Science, 6 [8] Fan, M., Bonfield, P., Wood based Composites-Material parameters and specifications. Proceedings 8 th European Panel Products Symposium, [9] Fan, M., Effect of size on rheological behaviour of plant based composite materials. ASCE J of Engineering Mechanics, 6 [] BRE: An integrated study of the long-term performance of wood based panels in floor and roof construction: Final report for EC project, 998 [] BRE: Enhancing the efficiency and competitiveness of wood-based panels in construction: Final report for EC project, 3 [] ENV 56: Wood based panels-determination of duration of load and creep factors, 997