Reliability-based Analysis and Calibration of Eurocode 5 Design Criteria for a Solid Timber Portal Frame

Reliability-based Analysis and Calibration of Eurocode 5 Design Criteria for a Solid Timber Portal Frame

Chapter One

Aim and Objectives

This study aimed to perform reliability-based calibration of partial safety factors for material properties located in Nigeria, considering the Eurocode 5 design criteria of timber portal frame. The specific objectives include:

1. To test some Nigerian timber species in the laboratory according to EN 408 (2004)
2. Assign EN 338 (2009) Strength classes to each of the
3. To perform reliability analysis on a three hinged timber portal frame designed with five timber species, in accordance with the Eurocode 5, in order to asses the effect of uncertainties on the Eurocode 5 design criteria of timber structural systems.
4. Assess the effect of fire and load duration on the Eurocode 5 design criteria of timber
5. Conduct reliability-based calibration of partial safety factors for material properties.

CHAPTER TWO

LITERATURE REVIEW

Preamble

Timber is an efficient building material, not only in regard to its mechanical properties but also because it is a highly sustainable material. Timber is considered a sustainable building material because it is derived from a renewable source and has a low embodied energy (Chanakya, 2009; Porteous and Kermani, 2007; Robert, 2009). Embodied energy reflect the minimal non-renewable energy used in the production of timber and its application in construction. Timber has sound thermal properties, meaning that timber structures rely less on carbon-emitting heating and cooling appliances than building construction of other material. Wood is also durable, since many products, particularly hardwoods have a service life of greater than 50 years, and often require little energy in maintenance (Robert, 2009). Wood can also be recycled, which is important in terms of storing carbon through the life of a product and its transformation. Wood is inexpensive material. Forest is a wood factory which produces wood using only solar energy (Dorina et al, 2012).

Timber is a widely available natural resource in many countries; with proper management, there is a potential for a continuous and sustainable supply of raw timber material in the future. Due to the low energy use and the low level of pollution associated with the manufacturing of timber structures, the environmental impact of timber structures is much smaller than for structures built using other building materials (Porteous and Kermani, 2007). The concept of environmental performance becomes clearer when wood is compared with steel and concrete.

Recent research studies confirmed that houses made from wood present significantly lower risk for the environment. Wood-based buiding materials require less energy to be produced, emit less pollution to the air and water, contribute with lower amount of CO₂  to the atmosphere, are easily disposed of or recycled, and are derived from a renewable resource. It can be easily concluded that the environmental advantages of wood compared to steel and concrete are obvious. Life-cycle analysis results for the steel-framed versus wood-framed home (Dorina et al, 2012) showed that the steel-framed home use 17% more energy; had 26% more global warming potential; had 14% more air emissions; had over 300% more water emission and had about the same level of solid waste production than timber. Analysis results for the concrete-framed home used 165 more energy; had 31% more global warming potential; had 23% more air emission; had roughly the same level of water emission and produced 51% more solid waste.

Timber, is an advantageous building material due to its material properties. Timber is a light material and, compared to its weight, the strength is high; the strength to weight ratio is even higher than for steel (Porteous and Kermani, 2007). However, timber is still not utilized to its full potential in the building and construction sector considering its beneficial properties. Many building owners, architects and structural engineers, do not consider timber as a competitive building material compared to concrete, steel or masonry. Attributes such as high performance in regard to reliability, serviceability and durability are generally not associated with timber as a building material. One of the main reasons for this is that timber is a highly complex material; it actually requires a significant amount of expertise to fully appreciate the potential of timber as a structural building material. In addition to this there are still a number of issues which need to be further researched before timber materials can achieve the same recognition as a high quality building material such as e.g. steel and concrete.

Timber is an abundant, renewable and recyclable material, which has been used by humans for thousands of years. Its use in construction is still widespread, ranging from structural frames to floors, paneling, doors, interior and exterior woodwork, and furniture, among its multiple uses in an average dwelling (FPL 1999). Three polymeric materials make up the wood cells: cellulose, hemi-cellulose and lignin (Kollmann and Cote, 1968). Cellulose makes up the cell walls, and provides the tensile strength of the wood matrix. Hemicellulose is similar to cellulose, and grows around the cellulose fibres. Lignin gives rigidity to the wood, allowing trees to grow upright; it cements the cells together, thus accounting for the compressive and shear strengths of wood (Kollmann and Cote 1968; Moraes 2003). These three polymers form an inhomogeneous and anisotropic material, which exhibits great variability among different species.

In general tree species can be divided into two major groups, hardwoods and softwoods. Hardwoods are porous, and present greater hardness than softwoods (although some exceptions exist) (FPL, 1999). Timber structures have traditionally been built using heavy timber frames, with the walls being constructed of various materials such as interwoven branches and split logs in the very early versions of these types of structures (as early as 6500 BC), and later using plastered panels and bricks (Foliente, 2000). Other forms of timber structures are palisade-typ

buildings and log-cabin constructions. Roof structures, as integral part of the building, have also traditionally been made in wood, and are still popular to this day especially in Nigeria. Timber roof structures have two basic forms: simple horizontal beams, or more complicated rafters being supported by the frame. Combinations of these two types have developed into hammer-beam roofs, which were used with great skill during the Renaissance, and into trusses, where originally the struts were also under flexion, unlike modern trusses where all the joints are assumed to be pinned (Foliente, 2000).

The coming of the industrial revolution marked the appearance of industrially built planks, boards and nails which spawned the appearance of new construction techniques, especially in the USA, where new framing methods reduced labour costs, increased the flexibility in construction and allowed for prefabrication (Foliente, 2000). These light frame construction methods are nowadays the predominant form of construction in residential and low-rise buildings. Other technological advances developed in this period are the glued laminated timber, first used in Bavaria in 1809 (Pedro, 2008) and improvements in wood trusses (Foliente, 2000). Glued laminated timber in its present form was developed in the twentieth century, as well as another important innovation, plywood (Foliente, 2000).

 Basic Material Properties of Timber

  Background

A base set of material properties as well as some influencing factors are needed for a probabilistic design (Toratti and Ranta Maunus, 2002). Density, modulus of elasticity and bending strength are the reference material properties required to be determined explicitly from laboratory (Yang et al, 2008; Ziwa et al, 2006). The propertries and distribution of other   material   properties   are   linked   with   EN   384   (2004)   and   JCSS   (2006) recommendations. As for the influencing factors, such as moisture content effect, load duration effect, size and system effect, use is made of the deterministic value of such properties.

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CHAPTER THREE

 MATERIALS AND METHODS

 Preamble

 Reliable information regarding the strength and stiffness of timber in Nigeria, is one of the major obstacles to their use in building construction application, based on the requirements of the EN 338 and Eurocode 5. The allocation of a specie to a strength class allows engineers to use the mechanical properties of the strength class in limit state design of timber structures.

One of the objectives of the testing programme in this study was to allocate each of the five timber species selected to appropriate strength classes. The possible strength classes are those contained in EN 338 (2009).

The scientific names of the species selected for test in the study, are Alstonia boonei (Ahun), Triplochiton Scleroxylon (Obeche), Terminalia Ivorensis (Black Afara)_, Terminalia superba (White Afara) and Lophira Alata (Ekki). These timber species are commercially available on the open market of Nigeria, and are widely used especially in roof truss, rail road cross-ties and timber bridge construction.

The number and choice of required tests were determined by the requirements of EN 384 (2004). The programme of test was designed to determine the bending strength, stiffness (Modulus  of elasticity),  density and  moisture  content  of the  five  species.  Testing was conducted  in  accordance  with  EN  408  (2004)  in  the  department  of civil engineering heavy structures laboratory.. The Test data has been used to derive characteristic values for bending strength, modulus of elasticity and density in accordance with BS EN 384 (2004).

CHAPTER FOUR

RESULTS AND DISCUSSION

Experimental Results and Discussion

  Moisture Content Test Results and Discussion

The moisture content test results are presented in Table 4.1. The mean values of the moisture contents for the Alstonia boonei (Ahun), Triplochiton Scleroxylon (Obeche), Terminalia Ivorensis (Black Afara), Terminalia superba (White Afara) and Lophira Alata (Ekki) are 20.59%, 16.83%, 19.26% 20.96% 15.3% respectively, with the corresponding coefficients of variation of 34%, 29%, 24% 23% and 22%.

 

CHAPTER FIVE

CONCLUSION AND RECOMMENDATION

Conclusion

In the present research work, material properties of five timber species of Nigeria, origin were generated. Relability-based calibration of the Eurocode 5 design criteria of a solid timber portal frame was implenented using Genetic algorithms-based first order reliability method (FORM). The following conclusion are made:

1. The equilibrium moisture contents of the timber species tested in this studey were in the range of 15 % to 21%. Terminalia superba has a mean equilibrium  moisture content of 20.96%. This is followed by Alstonia boonei with mean equilibrim moisture content of 20.59%. Terminalia ivorenis has mean equilibrium moisture contents of 19.26%. The mean equilibrium moisture contents for Triplochiton scleroxylon and Lophira alata were 16.83% and 15.30%, respectively. The 18% standard moisture for Nigeria, has fall within the range of 15% to 21% obtained fron the laboratoty
2. Based on the Kolmogorov Smirnov test performed in the study, it was found  that, density of solid timber can best be modelled with normal distribution. The distribution models of best-fit to bending strength and modulus of elasticity of solid structural timber were found to be lognormal
3. Each of the tested timber specie was assigned to appropriate strength classes based on the European solid timber strength classification systems (EN 338, 2009). Alstonia boonei, Triplochiton scleroxylon and Terminalia ivorensis were assigned to solid timber strength class D18. Terminalia superba was assignedto solid timber strength class D24. Finally, Lophira alata was assigned to solid timber strength class D60. The assignment were based on the reference material properties (density, modulus of elasticity and bending strength) of the species.

4. Failure of column-rafter connection was found to be predominant for timber portal frame. It is therefore the weakest list requiring the greatest attention during deign. Also, the safety indices for the premominant mode of failure was found to be almost equivalent to the system safety index. Example, for a particulay portal frame made with Lophira alata timber specie, designed to meet a predefined target safety index (Tables 4.41 to 43 ). The safety index of the predominant mode of failure was 3.86, and the system safety index was found to be 3.86. In conclusion therefore, the system reliability of a three-hinged solid timber portal frame is the reliabilty of its predominant mode offailure.
5. Based on the reliability-based calibration results obtained in this study, it was found that, material safety factor of solid timber is not unique as considered in the deterministic design code. The material safety factor if affected by the required safety For example in Table 4.44, the calibrated material safety factor was 1.26 at target safety index of 2.0. This safety factor changed to 1.6, when the taeget safety index was increased to 4.0.

6. The calibrated material safety factor was also found to be affected by the ratio of variable to total load, as observed in Table 4.45. At load ratio of 0.2 (Heavy frame) the calibrated material safety factor was 1.64. At load ratio of 0.8 (light frame), the calibrated safety factor was found to be55.
7. Coefficient of variation of material properties was found to have the most pronounced effect on material safety factor. As observed in 4.55, for a frame designed to meet a target safety index of 3.8, the required material safety factor is zero when the material coefficient of variation is zero. As the coefficient of variation of material properties was increased to 30%, required material safety factor changed to 2.5. Regression models were developed to take into account of the effects of changes in the required target reliabity and the coefficient of variation of materialproperties.
8. The developed uncertainty sensitive material safety factors models can be used for the design of timber structures, but could be more appropriate for the frame designed with any of the five timber species. This is because the research work was based on the material properties of the five timber species.

Recommendations

Based on the results obtained in this study, the following recommendations are made:

1. The strength classification of timber presented in this study was limited to only five timber species. There is the need to consider the classification of other timber species. This is pre-requisite to their use for design of timber structures using the Eurocode
2. Since the material properties of timber are highly uncertain, as attested by the laboratory experimebnts presented in this study, there is the need to integrate reliability-based designed criteria in the design of timber structures in order to fully accommodate the
3. It was fully established in this study that system reliability of the timber portal frame agreed with the reliability of the critical mode of failure of the frame. It is therefore recommended that the system reliability of timber portal frame be taken as the reliability of the frame critical mode of
4. The material properties generated in this study were adjusted to 18% moisture content, to agree with the NCP 2 recommendations. It is recommended that a separate code equivalent to EN 338 be developed for Nigeria, based on 18% moisture content, as against the 12% moisture cotent baseline used in the current EN
5. The timber species tested in this study were first adjusted to 18% before being classified in accordance with EN 338. The strength classes generated could be used for the design of timber structures with Eurocode
6. The material properties for loading used in this study, were obtained from data reported in the international literature. There is the need for a field study to come up with data on the statistics of loading that are peculiar to
7. The partial safety factors recommend for timber structures in the current  Eurocode 5 is determistic and conservative. It is recommended that,  the mathetical models of material safety factors developed in this study be used when designing timber structures in The partial safety factors were derived from reliability method and are uncertainty sensitive.

REFERENCES

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• Afolayan, J. O. (2004). Cost effective vibration criteria for wooden floors. Asian Journal of Civil Engineering (Building and Housing) 5(1-2) pp 57-67.
• Afolayan, J. O. (2005) Probability-based design of glued thin-webbed timber beams. Asian Journal of Civil Engineering (Building and Housing). 4(1-2) pp 75- 84.
• Afolayan, J. O. and Abdulkareen, Y. O. (2005). Effective material utilization in timber industry: Problem of glued thin-webbed beams. Asian Journal of Civil Engineering (Building and Housing) 6(1-2) pp 55-65.
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• Alade, G.A. and E.B. Lucas E. B. (1982). Timber connector: a major contributor to structural failure in wooden components in Nigeria. Paper presented at the 36th annual meeting of the Forest Products Research Society, mechanical fastening session, New Orleans, U.S.A., June 24, 1982. 22 pp.

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