Strength and Deformation Modification Factors of Wood Based Composites for Engineering Design

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, i) to determine whether the strength and deformation modification factors (kmod and kdef ) in Eurocode 5 for formaldehyde based boards are applicable to boards manufactured using new alternative binders such as isocyanate and cement, ii) to evaluate the effect of long-term concentrated loading by a full scale component test and small-scale indicative test, and iii) 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 1 and 5. Numerous results and important findings showed that i) the kc and kd values of isocyanate bonded particleboard (PB) under bending loads were lower than those of formaldehyde based PB, the former being 70-80% the latter; ii) the kc of cement bonded particleboard (CBPB) were similar to that of formaldehyde based particleboards, but with the deflection of the former being about 1/5 the latter, the CBPB test pieces lasted much longer than MUFPB under duration of load tests; iii) stress modes had a significant effect on long term performance: The extrapolated kc values under concentrated load were generally higher than those under bending load and in EC5, depending on the type of materials and joint profiles, the extrapolated kd were very similar between concentrated and bending loads and in EC5, the extrapolated kd values under shear load were higher than those in EC5, however, the kc values varied considerably with the type of oriented strand boards (OSB) and medium density boards (MDF) under shear loading tests. The results clearly showed that there is a need for developing strength and deformation modification factors for new materials and materials under various stress modes for engineering designs. INTRODUCTION EN 1995-1-1:2004 Eurocode 5 (EC5) [1] 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 as described:


INTRODUCTION
EN 1995-1-1:2004 Eurocode 5 (EC5) [1] 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 as described: where, P d is the design value of properties, P k is the characteristic value of properties, k is the modification factor and 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 1 (EC1) [2], the user has to look elsewhere for many of the basic material properties.
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 [1,7]: i) The increase in deflection with time (creep) and ii) The reduced loads, compared to those in short term tests, that can be sustained without failure over a prolonged period (duration of load).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: for serviceability limit states, and The design value of a strength, X d and a resistance (load carrying capacity), R d shall be calculated as: Where, X k and R k are the characteristic values of a strength and load carrying capacity respectively, and 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.
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.This paper presents part of the finding from a complex project, which was: • to determine whether the correct test methodology for wood-based composites is in place (Table 1).
• to evaluate the suitability of the resulting data for use with Eurocodes 1 & 5 (Table 2).
The presentation only focuses on the design modification factors for wood based composites.More information is presented in a series of separate papers [8][9][10][11].

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.

Deformation Modification Factor/Creep Factor (k def )
The k def was derived from creep testing in accordance with EN 1156.The creep test was carried out at 25% of the short-term failure load.The number of repetitions of each test set-up is 10.The duration of each creep test is 6 months.
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.It is dimensionless.The creep factor for a certain period of time t is given by: here, a T is the total deflection in millimetres at time t minutes; a 1 is the deflection in millimetres at 1 minute; a 0 is the deflection in millimetres of the unloaded test piece positioned in the creep test arrangement and (a 1 -a 0 ) is the initial elastic deflection in millimetres as measured after 1 minute of load application.
Thus, each deflection measurement taken during the creep test is converted into a k c value as shown above.The logarithm of the average of the values from the 10 test pieces for each time interval is plotted against the logarithm of time.A linear regression line is fitted through this data, excluding points from the first 10 minutes of the test (Fig. 1).Bending, tension, compression, panel shear, planar shear -using medium size test pieces.
Confirmation on the suitability of EN789 for the determination of characteristic values.

All properties given as requirements in the EN material specifications
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.
For floors and roofs, includes a soft body impact test and a concentrated load test.Also specifies the requirements in relation to EC1 & EC5.
Demonstration that typical constructions can continue to meet the requirements of EC1 and EC5.
EN1156 [17] Test method for determining creep and duration of load in bending.
Size of test pieces, edge sealing, sampling/matching procedures.
Proposal to use small, sealed specimens instead of semi-sized.Recommendations for improved sampling methods.
That is, Where, A and B is constant.
Then estimations of k c for different load classes can made from this presentation format.The load classes include: • Short-term (1 week) • Medium term (6 months) • Long term (10 years) • Permanent (30 years)

Strength Modification Factor/Duration of Load Factor (k mod )
Unless specified, the determination of duration of load factors is carried out on a similar basis to the creep tests ex-cept that higher stress levels, which eventually lead to failure, are employed.A series of tests at stress levels of 55%, 60%, 65%, 70%, 75% and 80% of the short term failure load at 20°C/65%rh were used.At each stress level, 10 tests are carried out and the times to failure recorded.The mean time to failure is calculated and stress level is plotted against the logarithm of time to failure.
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, in the form given as: Where, c is the intercept on the vertical axis; m is the slope; S is the stress level (%); t is the time to failure (minutes) (Fig. 2).
The load duration factor k d is equal to the value S for any given time t.

RESULTS AND DISCUSION 1. 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 longerterm 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 order to confirm that the creep and load duration factors given in EC5 are appropriate to these materials.

Creep in Isocyanate Bonded Boards
It was found that for both the laboratory and commercial boards tested, the relative creep of isocyanate particleboard and MDF boards was generally lower than formaldehyde bonded boards tested under 20°C/65% relative humidity conditions (Figs. 3A, B).The k c of laboratory made MDIPB is only about 70% that of MUFPB and that of commercial MDIPB is about 80% that of MUFPB after six month loading.It is apparent that the performance of resin system is closely related to the long term performance of composites made.There is a similar trend for duration of load tests.Isocyanate bonded boards gave longer times until failure than corresponding formaldehyde boards.
It is considered that 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.

Creep in Cement Bonded Particleboards
Commercial cement bonded particleboard (CBPB) was tested to determine the creep and load duration behaviour in bending.It is apparent that at 25% stress level, the k c of CBPB was very similar to that of MUF bonded particleboard (MUFPB), while the deflection of CBPB was only about 25% of that of MUFPB because of its greater stiffness.An example of deflection with loading time is given in Fig. (4).The calculated kc after six month loading is 1.6 for commercial MUFPB, 1.5 for cement bonded particleboard, while the deflection is 5mm for MUFPB and 1.3 mm for cement bonded particleboard.
For duration of load tests, the test pieces subjected to stress levels of 45%, 50% and 55% had not failed after 9 months loading (Fig. 5), with some test pieces being under load for 20 months at these levels.The duration of load behaviour at 60% and 75% stress levels was better than that of MUFPB.At 60% stress level, the shortest time until failure for CBPB was over 5 times the longest time to failure for MUFPB under the same conditions.
The 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.To exploit the potential of and efficiently use new materials, it is necessary to produce correct factors for engineering design.
The work confirms that the creep and duration of load behaviour of boards can be affected by the adhesive type used and it is therefore important that the long term behav- iour 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.

Strength and Dformation 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 and/or duration of load 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 Moreover, the volume of data available from bending creep and duration of load 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 failure modes induced by the various forms of loading and the nonisotropic 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.

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 and duration of load behaviour under concentrated loads.Semi-sized bending tests were carried out in parallel for comparison.
A component-based test method using a 1200mm square test piece and a small-scale method based on a 300mm 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 (25%) and three stress levels (65%, 72% and 80%), for duration of load test.Testing was carried out on both solid and jointed test pieces at a climate of 20°C and 65% relative humidity.
The results are summarised in Figs. ( 6) and (7).It is apparent that the k c values of plywood under concentrated load are higher than those measured under bending and in EC5, especially for long term values.And average kc for longand permanent-term can be three times that of bending or in EC5.The extrapolated kc based on the test data for PB under concentrated load is lower than that under bending load except for small solid tests, however, the values are very similar to those in EC%.
The results for MDF are very similar between concentrated load, bending load and those in EC 5 except for small jointed test which has much higher extrapolated values for long term and permanent loading classes.The kc values for OSB are higher than those in EC5 but the average is similar to those under bending loads.
In general, the kd values are similar between concentrated load and bending load and those in EC5 (Fig. 7).
A comparison of the k mod and k def values from the component and small-scale tests is shown in Tables 4 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 and 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.

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 1, (Figs. 8 and 9).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.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.

CONCLUSIONS AND RECOMMENDATIONS
The conclusions and recommendations can be presented as follows: 1.The creep and duration of load tests were studied and indicative strength and deformation modification factors were developed for the boards manufactured using new alternative binders (isocyanate and cement), and the wood based composites under shear loading (panel and planar shears) and concentrated loading.termine the correct k mod and k def for new materials, and the wood based composites under different stress and environmental conditions for engineering designs.

4 .
Fig. (8).A comparison of kc with kdef in EC5 for MDF and OSB under panel and d planar shear loading (A and B-panel shear, C and Dplanar shear).

Fig. ( 9
Fig. (9).A comparison of mean kd to kmod in EC5 for MDF and OSB under pannel and planar shear loading (A and B-panel shear, C and D-planar shear).

Table 2 . Issues Concerned with "Is the Resulting Data Suitable for Use with EC1 & EC5?"
The procedures used to analyse and extrapolate the data.The amount of data necessary to provide confidence in the modification factors.

Table 3 . Panels and Corresponding Tasks
2. The creep and load duration behaviour of boards could be affected by the adhesive types used.The partial evaluation of the creep and duration of load behaviour of isocy-anate and cement bonded particleboards suggested that it is safe to use the k mod and k def factors in EC5 with these materials.The k c and k d values of isocyanate bonded par- 3. Stress conditions were closely related to the creep and duration of load.The extrapolated k c values under concentrated load were generally higher than those under bending load and in EC5, depending on the type of materials and joint profiles, the extrapolated k d were very similar between concentrated and bending loads and in