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Moritani, F. Y., Martins, C. E. J., and Dias, A. M. P. G. (2021). "A literature review on cold-formed steel-timber composite structures," BioResources 16(4), Page numbers to be added.

Abstract

State-of-the-art steel-timber composite structures (STC), using cold-formed steel (CFS) and cross-laminated timber (CLT), are considered in this review. Literature on this type of construction solution is reviewed to provide an overview of the characteristics and advantages of STC. Previous experimental and numerical studies with STC structures, mainly composite solutions with CFS beams and CLT panels, are discussed to assess the behavior of this structural typology. A comprehensive description of the connection systems performance in different STC structures is also provided. Furthermore, the design and analytical methods currently available are presented. Likewise, details on aspects related to dynamic properties and fire resistance are discussed.


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A Literature Review on Cold-Formed Steel-Timber Composite Structures

Fabiana Y. Moritani,a,* Carlos E. J. Martins,a,b and Alfredo M. P. G. Dias b

State-of-the-art steel-timber composite structures (STC), using cold-formed steel (CFS) and cross-laminated timber (CLT), are considered in this review. Literature on this type of construction solution is reviewed to provide an overview of the characteristics and advantages of STC. Previous experimental and numerical studies with STC structures, mainly composite solutions with CFS beams and CLT panels, are discussed to assess the behavior of this structural typology. A comprehensive description of the connection systems performance in different STC structures is also provided. Furthermore, the design and analytical methods currently available are presented. Likewise, details on aspects related to dynamic properties and fire resistance are discussed.

Keywords: Cold-formed steel; Cross-laminated timber; Composite structures; Connection; Hybrid structures; Wood-based structures

Contact information: a: SerQ – Centro de Inovação e Competências da Floresta, Rua J, Nº 9, Zona Industrial da Sertã, 6100-711 Sertã, Portugal; b: ISISE – Institute for Sustainability and Innovation in Structural Engineering, Departamento de Engenharia Civil, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, Rua Luís Reis Santos, Pólo II da FCTUC, 3030-788 Coimbra, Portugal;

* Corresponding author: fabianamoritani@gmail.com

INTRODUCTION

Composite structures combine the benefits of different materials to overcome their limitations and increase their performance (Ceccotti 2002; Dias 2012; Wacker et al. 2020). Steel-timber composite (STC) for buildings presents many advantages that have proved to be effective as a structural solution, for instance, higher strength to weight ratio, lower environmental impact, and the possibility of recycling and replacing degraded elements (Hassanieh et al. 2017a; Loss et al. 2016a). Composite solutions that consider steel profiles and cross-laminated timber (CLT) panels enable highly industrialized construction; as such, combinations allow a quick connection of the elements with higher precision, using a full-dry system. The STC structures do not require concrete molding or completion on site of precast concrete elements, as is usually required by other composite solutions such as timber-concrete (Loss et al. 2016a). It is also worth noting that structural components may be reversible during the service life of the construction, since STC system designed with fasteners such as screws and bolts allows the deconstruction and potential reuse and/or recycling of construction materials (Hassanieh et al. 2016b; Loss et al. 2016a). If a wooden design is adopted, it results in lower environmental impacts and embodied energy because timber is known as a carbon sink, lowering the carbon footprint of such structures (Asdrubali et al. 2017; Bradford et al. 2017).

The technical possibilities for building with timber have experienced a big advance in recent years. Efforts in developments involving timber and composite are being motivated by environmental concerns. The STC solutions can be considered as a new generation of structural technologies for which the construction system is implemented using modular STC highly-prefabricated elements, and it has considerable potential for construction industry and commercialization (Loss et al. 2016a,b). According to Loss et al. (2016a) three important examples of buildings of timber hybrid tall buildings can be found in Europe that emphasize the wide possibilities for lightweight modern buildings and sustainable solutions; these include structures built in Norway – Treet (Bjertnæs and Malo, 2014); in Austria – Lifecycle Tower (Rhomberg and Bonomo 2011); and in London – Banyan Wharf.

Combinations of timber with steel can also be made in other ways, for instance, timber panels and steel beams (Kyvelou et al. 2015; Loss et al. 2016b; Hassanieh et al. 2017a; Loss and Davison 2017); timber-concrete composite slabs with steel profile (Kuhlmann and Schänzlin 2008); steel frame with timber wall panels (Loss et al. 2016a); and timber slabs with steel beams and column (Xu et al. 2014; Keipour et al. 2018b; Nouri and Valipour 2019; Keipour et al. 2018a; Ataei et al. 2019b; Nouri et al. 2019). At the structural element level, steel sheets can be used to reinforce timber beams (Hassanieh et al. 2016b; Ataei et al. 2019b), and beam-beam connections can be completed using steel sheets and screws, or through steel beams as the mainframe in an STC floor.

The CFS structural elements provide lightness, flexibility, and extra potential for bringing material recycling to the composite solution. On the other hand, such characteristics tend to increase the potential for instability issues of the CFS profile. This problem can be reduced through bracing, which can be provided by the timber member (e.g., particle boards, plywood panels, laminated veneer lumber, or cross-laminated timber), through an appropriate detailing. In recent years, the use of CFS beams supporting timber panels has gained market share as a flooring system in industrial and commercial buildings (Vella et al. 2020). This composite solution requires an efficient connection system to guarantee the composite action, allowing a more efficient use of the materials, and decreasing the execution time. According to Loss and Davison (2017), the association of the CFS beams and CLT can also satisfy requirements in terms of prefabrication, modularity, sustainability, on-site installation, and relative manufacturing costs.

This paper aims to provide a review of previous research studies on STC structural solutions, specifically the development of composite structures with CFS and CLT. Studies on properties of STC, connection systems, design and analytical methods of composite structures, short- and long-term behavior, and numerical analysis are presented and discussed.

RELATED WORKS ON STEEL-TIMBER COMPOSITE (STC)

Loss et al. (2014, 2016a,b) developed an extended study concerning the construction system with prefabricated steel-timber components in highly industrialized modular construction, combining CLT panels with different steel profiles (hot-rolled steel, cold-formed steel, and welded steel). Connection solutions were investigated to reduce time consumption and costs of construction.

According to Loss et al. (2016b) the impact in the building market and the structural efficiency of the STC structures is remarkable when compared to other composite systems such as timber-concrete or steel-concrete.

Figure 1 shows three composite structural solutions designed considering similar capacity, in free span (6 m), height of the cross-section (252 mm), design loads, and deflection in the serviceability limit state (l/250). Structural performances were estimated without the composite action (k = 0) and with full-composite action (k = ∞). From the comparison of composite slabs with steel-concrete, timber-concrete and steel-timber, it can be seen that the STC system with CLT panels and steel profiles is equivalent to the other systems, in terms of the bending stiffness and design load carrying capacity. The self-weight of STC slab was found to be four times lower than steel-concrete slab. The capacity-to-self-weight ratio of STC section was 17.02 (k = 0) and 36.21 (k = ∞), while the capacity-to-self-weight ratio of steel-concrete section was 3.02 (k = 0) and 8.40 (k = ∞) and for timber-concrete was 5.40 (k = 0) and 18.80 (k = ∞). In general, steel-timber systems offer advantages of each material creating very slim and light floor components and, therefore, leading to lightweight constructions, which provides cutting-down the forces acting on the foundations and reducing the seismic effects on the structure.

Fig. 1. Composite slab systems (Adapted from Loss et al. 2016b)

Loss and Davison (2017) performed an experimental campaign and numerical modeling to access the flexural behavior of a floor system composed of prefabricated ultralight modular components. The proposed solution was implemented to support the design loads and serviceability conditions for the main floor of a residential building comprised of CLT panel and two custom-made CFS beams joined with mechanical connectors and an epoxy-based resin (Fig. 2).

The ultimate loads were not reached and, therefore, the load carrying capacity (Fc) was assumed to a deflection limit in terms of accepted damage and potential local breakages in the steel beams (up to the deflection of l/40 and l/60). The tested systems showed a considerable structural efficiency with effective stiffness close to 0.76 and high capacity-to-weight ratio ηF (ηF = Fc/Wp, where Fc is the load carrying capacity and Wp is the self-weight of specimens) up to 80.6, while the capacity-to-weight ratio of STC by Loss et al. (2016b) was 36.21 with full-composite action. The STC solutions help to overcome the limits of the timber elements in terms of serviceability under service loads. While the CFS beams achieved inelastic deformations at high loading levels (four of five times higher than the design loads) and connections undergo ductile behavior, the CLT remained at an elastic level and substantially intact after the tests. In the specimens, pulled-out screws were detected, as well as local buckling in the flanges of the mid-span section, and deformation at the restraints. Besides, these floor systems remained in equilibrium, since the instability CFS beams was prevented by the connection with CLT, which provides a very high ductility even for large flexural deformation. Two types of restraints were compared. It was shown that the adoption of fixed restraints at the ends of the CFS beams could increase the stiffness by 40% and the strength of the floors by 37%. According to Kyvelou et al. (2018), such results were previously achieved by Xu and Tangorra (2007) and Lawson et al. (2006).

Fig. 2. Two types of prefabricated floor developed with CFS and CLT components (republished from Loss and Davison (2017) with permission from Elsevier)

Hassanieh (2017) developed research on the short- and long-term behavior of STC structures for large-scale construction, with CLT and LVL (laminated veneer lumber) panels connected to steel beams by mechanical fasteners and adhesives. Different types of bonding systems in STC structures were evaluated, and improved solutions were proposed. The load-slip behavior, load carrying capacity, and failure modes were evaluated for the steel-CLT composite joints of three different types of connections (i.e. high-strength bolts, coach screws, and combination of glued with coach screws). The influence of reinforcement by using steel nail plates was verified by Hassanieh et al. (2016c), through push-out tests. In Fig. 3 the failure modes in steel-CLT connections are presented, namely the crushing zone and densification of grains (Fig. 3ac-c) and the failure Mode II with plastic hinge in coach screws (Fig. 3d-f). The reinforcement of the CLT panels using nail plates increased the initial stiffness and strength, but it had no influence on the failure mode of the screw connection, since a slight decrease in the ductility was observed. In general, screwed connections presented ductile behavior, whereas the combination of adhesives and screws had higher load carrying capacity and stiffness compared with the conventional connections. In the opposite way, this connection presented an undesirable brittle failure associated with the failure of the adhesive and the splitting of the CLT panel from the steel beam (Hassanieh et al. 2016c). The performance of a composite floor comprising steel beams and CLT panels connected by a high-strength bolted shear connection embedded in a grout pocket (BGP) and conventional shear connectors (coach screw, dog screw e bolts) was assessed by Hassanieh et al. (2017a). Connections with BGP showed higher initial stiffness, pre-peak stiffness, and peak load capacities compared to conventional connectors (i.e. coach screw and dog screw); however, the width of the grouted pockets had little or no influence on the pre-peak load-slip behavior of the joints.

Fig. 3. Failure modes in steel-CLT with screw connectors (a-c) Crushing zone and densification of grains, (d-f) Plastic hinge of the coach screws (republished from Hassanieh et al. (2017a) with permission from Elsevier)

Chiniforush et al. (2019b) assessed the long-term behavior of STC connections using four different types of connectors (i.e. coach screws, dog screws, post-tensioned high strength bolts, and high strength bolts embedded in grout pocket). A proposed rheological model was used to predict the slip and thus to calculate the creep coefficient of the STC structure over a service life of 50 years. The model was calibrated from the experimental results that considered the effect of the stiffness change due to the variation of the moisture content, temperature, creep, mechanic-sorption, and inelastic shrinkage. The results showed that an increase in the load level of 30% of the ultimate load capacity led to up to 70% reduction in the slip modulus of the coach screws, dog screws, and post-tensioned bolts connectors. However, the reduction in the bolts in grout pockets connectors was limited to 40%.

An extensive experimental and numerical study on shear interaction of the hybrid system comprising a CFS beam and wood-based panel was carried out by Kyvelou et al. (2015) and then Kyvelou et al. (2017b). The spacing of the fasteners and the application of adhesives at the interface between the CFS beam and floorboards had a significant effect on the bending moment capacity of the flooring system. The best performance achieved up to 68% of composite action, which turned into a 100% increase in the bending moment capacity and a 42% increase in effective stiffness.

The CFS elements had some drawbacks, namely those regarding compressive stresses, which governs the buckling behavior of the elements. Awaludin et al. (2015) assessed the mechanical performance of a CFS member stiffened by timber lamellae. CFS profiles of double Z-, C- and double C-sections and 14 mm thick planks of the Swietenia mahagoni species were considered, with 12% moisture content and a specific gravity of 0.69. The load-carrying capacity and the buckling failure mode of the cold-formed steel structure and the CFS-timber composite structure were determined and compared. The composite section showed an increase in the buckling load capacity comparing with the non-composite section. However, at the critical load, the gain varied from 1.4 for longer spans to 6.7 for shorter spans.

The performance of a roof made of structural truss elements with a span of 25 m composed of CFS profiles reinforced with timber lamellas was assessed by Irawati et al. (2017). The analysis of the compressive capacity of CFS-laminated timber was conducted using a full composite action assumption. The CFS-timber composite structures obtained satisfactory results, as they supported the maximum external load. Besides, the deflection of the composite structure (1.74 cm) was lower than the maximum allowable deflection (12.50 cm). CFS profiles achieved the critical buckling load until the external load, so unreinforced CFS profiles as the main compressive elements may not be recommended.

Chiniforush et al. (2018) studied the life-cycle energy of a floors and shear walls system adopting STC elements by accounting for energy use in material extraction and processing. The results indicated a decrease in the embodied energy at the expense of only a slight increase in the operating energy. Furthermore, a considerable life cycle energy saving was found in STC floors adoption when compared with the same building designed with a concrete structure.

Izumi et al. (2016) developed a non-loaded combustion test and numerical modeling of the different types of cross-sections comprising the association CFS and glulam in the beam element. According to the authors, this association increased the structural and fire performance of the composite beam. Akotuah et al. (2015) studied the effect of shear connection geometry of a hybrid steel-timber under fire by full-scale tests. The results showed that the fire resistance of the connection depends on the load ratio, the connection type, and the relative exposure of the steel-timber section. Due to the high conductivity of the steel, the wood charred at the connection regions, and this effect initiated brittle failure and propagation of the crack, which led to the ultimate failure of the assembly. The CFS profiles at high temperatures have their load-carrying capacity affected due to the reduction of the yield strength and the modulus of elasticity, as well as the appearance of additional stresses by expansion restriction and secondary stresses due to deformations caused by the temperature gradient.

According to the facts previously presented, the STC structures combine the performance of each material (steel and timber) to overcome their drawbacks. The CFS elements are susceptible to buckling effects; however, the modulus of elasticity is significantly higher than the modulus of elasticity of the CLT. Life cycle assessments and energy of wooden composite systems shows lower environmental impacts and higher savings in embodied energy and carbon. The experimental studies of composite structures comprised of CFS and CLT showed considerable structural efficiency, reflected in the effective stiffness and enhancement of the load-carrying capacity, but the behavior was strongly dependent on the performance of the components of the connection system. The studies on the behavior and enhancement of each connection system tested are explained in the next section. Since the CFS profiles under high temperatures presents a decrease in their mechanical properties, the CFS-CLT composite solution should be also investigated in order to guarantee the performance under fire, mainly in the CFS elements. Table 1 provides a summary of the advantages and disadvantages of each STC system (CFS-CLT, steel-CLT, steel-LVL, and CFS-wood based panel).

Table 1. Advantages and Disadvantages of each STC System

CONNECTION SYSTEMS

Studies have been carried out to obtain a more efficient and low-cost connection system. The following tables summarize the results of steel-timber connection types present in the literature, namely the average values of ultimate load capacity (Fu) and the service connection stiffness (ks). It is worth mentioning that for each material composition and design of the connection system, results were obtained inherent to each study, generating a great variability of results for the same properties of load carrying capacity and stiffness.

Awaludin et al. (2016) evaluated the connection of CFS and timber lamellas using two bolts and two different side plates (plywood and steel). The connection system using steel side plates was able to accommodate the strength increase of a composite member, as this connection system had an ultimate load-carrying capacity four times higher and initial joint slip modulus 36% higher than the CFS connections without plates. The composite connection with plywood plates did not provide a significant increase compared to the CFS connections without plates.

According to Vella et al. (2020), the inclined screws decrease the risk of the two composite members separating under shear stress due to the component of force within the connector that acts perpendicular to the shear plane. The authors carried out an experimental study on connections between CFS and wood-based panels (particleboard and plywood) formed with inclined screws. The screwed connections were considered with or without wings, inclined 0° and 45° from a normal to the steel-timber interface that were installed parallel to each other. Table 2 showed that the non-winged screws inclined at 45° had a higher load-carrying capacity and initial slip modulus for both plywood and particleboard. The enhancement of the peak load-carrying compared to the reference specimen was about 30%, while the enhancement of the slip modulus was up to 140%. There was no significant effect of the steel thickness on the connection stiffness or peak load capacity unless the failure was governed by thread withdrawal.

Loss et al. (2016a) tested different connection solutions for steel-timber hybrid prefabricated systems. Specific connections for the composite section and other types used to fasten the CLT panels all together and to connect these to the main steel frame were tested. The authors recommended that the I-A-1 and I-A-3 connectors should be used to join CLT panels with the steel frames and I-B-7 connectors were used to build steel-timber composite systems such as prefabricated floor elements. These connections allowed a quick joining of components on-site and provided an enhanced inelastic deformation capacity. The experimental results are listed in Table 3. According to Loss et al. (2016a), the I-B-7 was the best solution to build STC floor elements that ensured a higher load-bearing capacity and high slip modulus (372.4 kN/mm), as well as improved ductility capacity.

Hassanieh et al. (2016c) reported that the use of nail plates to reinforce the CLT slightly reduced the ductility of screws but did not modify the failure mode of the joints with screw connectors. The effect of the grout pocket in bolted connections was evident, showing a slip modulus five times higher compared with the same connector without grout pocket. The load-slip responses of screw connectors exhibited close to elastic-perfectly plastic and relatively ductile behavior, while the load-slip responses of bolted connectors showed some hardening characteristics and brittle failure mode. In the glued steel-CLT composite joints, a non-sag gel type epoxy resin was used with 16 mm coach screw to provide full composite action that exhibited significantly higher strength and stiffness compared to the joints with only mechanical connectors.

Table 2. Mean Values of the Mechanical Properties of Connections Systems with CFS and Timber

Hassanieh et al. (2017a) presented the push-out tests on specimens with coach screws, dog screws, and bolts embedded in the grout pocket (BGP); the resulting failure modes in steel-CLT are shown in Fig. 3. The BGP is an STC connection that allows minimizing overhead works while preserving the possibility for prefabrication and deconstruction of STC floors and beams. The experimental results are presented in Table 3. Since the modulus of elasticity and compression strength of the cementitious grout are higher than mechanical properties of spruce wood (CLT), the BGP had higher strength and stiffness compared to screwed connection. The peak load capacity of the BGP was up to 2.4 times higher and the slip modulus was up to 10 times higher compared to coach screws and up to 4 times if compared with dog screws. Despite the similar peak load capacities achieved for both coach and dog screw connections, the slip modulus of dog screws connection was significantly higher (up to 4.4 times), probably due to steel grade considered.

Table 3. Mean Values of the Mechanical Properties of Connections Systems with Steel Profile and CLT

Yang et al. (2020) presented an experimental study on an STC connection system composed of steel H-section and glulam using bolt connectors and SDS (Self-Drilling Screw), which is one type of self-tapping screw but typically has denser steel thread. The static push-out results showed the influence of the type, diameter, and spacing of shear connectors, and thickness of glulam in composite mechanical properties. The connector varied as bolt (6 mm and 8 mm) and 5.5 mm SDS; the specimens only varied in bolt spacing, i.e. 100 mm, 150 mm, and 200 mm; and the glulam flange thickness was varied as 30 mm, 40 mm, 50 mm, and 60 mm, but all other parameters were kept the same. The ultimate load carrying capacity was influenced by the bolt diameter, spacing, and glulam flange thickness. Table 4 shows that the highest ultimate load (Fu = 69.28 kN) was recorded for glulam flange thickness of 50 mm and 8 mm nominal diameter of the connector. The SDS connectors were more ductile and produced stiffer connection when compared to the bolted connectors for all considered STC specimens, which can be confirmed by the higher values of the slip modulus (ks0.4 = 36.51 kN/mm) and (ks0.6 = 27.92 kN/mm). The lowest ultimate load (Fu = 36.41 kN) and slip modulus (ks0.4 = 10.74 kN/mm and ks0.6 = 10.62 kN/mm) were obtained for smallest glulam flange thickness (30 mm) and 6 mm nominal diameter of the connector. According to the results, the ultimate load of connections was directly proportional to the bolt diameter but was inversely proportional to the bolt spacing.

Table 4. Mean Values of the Mechanical Properties of Connections Systems with Steel and Glulam

In general, the proposed connection types were tested to assess the enhancement achieved compared to common type mechanical fasteners for STC structures. The experimental results showed that the connection systems were efficient and could provide higher performance. However, the material behavior can significantly affect the mechanical performance, as can the diameter, spacing, and orientation of the connector. The composite structures with steel profile and CLT had higher values of both slip modulus and ultimate load capacity. In addition, the composite solutions comprised of CFS beams reinforced by timber lamellas or assembled with wood-based panels had lower values of the mechanical properties than structures composed with steel profile and timber (e.g. CLT and LVL). It is expected that the mechanical performance of the connection system CFS-CLT composite solutions had higher performance than the solution presented by Vella et al. (2020), which was composed by CFS and wood-based panel. Connection systems proposed by Awaludin et al. (2016), Loss et al. (2016a), Hassanieh et al. (2016c), and Hassanieh et al. (2017a) should be tested in CFS-CLT composite solution in order to analyze the improvement technologies that can achieve in mechanical properties.

DESIGN AND ANALYSIS ANALYTICAL MODELS FOR COMPOSITE STRUCTURES

Within composite structures, there are standards for the design of steel-concrete composite structures (CEN 2004) and timber-concrete composite bridges (Dias et al. 2021). On the other hand, design and standard recommendations for steel-timber composite structures are not well established (Kyvelou et al. 2017a).

The γ-method is derived from the approximation of the Möhler (1956) method for composite beams with flexible connection and is proposed in Annex B of Eurocode 5, Part 1-1 (CEN 2004a). This method is considered for all materials comprising that the composite structures remain within the linear-elastic range. Loss and Davison (2017) used the γ-method to design the STC floor solution with semi-rigid connection, in which the CFS beams, CLT panel, and connections remain within the linear-elastic range and that the ultimate load was more conservative compared to the experimental bearing capacities of the systems. The design equations are presented for the attained degree of shear connection and determination of the effective bending stiffness and the normal stress.

Hassanieh et al. (2016a) proposed an empirical model to characterize the load-slip behavior of steel-LVL composite slabs with screw, bolt connectors, and reinforcing nail plates. The model was inspired by the Ramberg-Osgood model (1943) that has been widely used in the analysis of steel structures. In Hassanieh et al. (2017a), the authors applied this model for the load-slip behavior, stiffness, and strength of steel-CLT connections with dog screw and bolt connectors incorporated in the grout pocket. The proposed shear load versus slip function has seven parameters, which were estimated by non-linear regression. According to the authors, the analytical load-slip curves correlated very well with experimental results (R² = 0.97) and can be incorporated into component-based finite element models, non-linear analysis, and design of hybrid STC lap joints (Hassanieh et al. 2016a).

Chybiński and Polus (2019) developed a theoretical analysis of the aluminium-timber composite (ATC) structures, through push-out tests of screwed connection to obtain the slip modulus and the peak load capacity. The theoretical analysis of the ATC beams was evaluated from the method of elastic and plastic resistance to bending. The method from the plastic resistance to bending consisted of determining the position of the plastic axis that considered a tensile force in the aluminium beam section that was equal to the compression force in the timber slab. The analytical estimation from the plastic model was about 7% lower than the mean bending strength from the tests. However, the theoretical model did not take into account the sudden tensile fracture and the slip that influence in the stiffness and the strength of the composite beam that may be significant.

After several experimental and numerical tests on composite flooring systems with CFS and wood-based floorboard, Kyvelou et al. (2017a) proposed a method for its design, considering the theoretical bases of CFS from Eurocode 3, Part 1-3 (CEN 2006) and steel-concrete composite structures according to the Eurocode 4 (CEN 2004). The bending moment capacity of the composite beam was calculated from the attained degree of partial shear connection, which ranged between that of the bare steel beam and the equivalent composite beam with full-composite action. The comparisons with the results of 12 experimental tests and 80 numerical models demonstrated that the proposed design method gives accurate predictions of the bending moment capacity and flexural stiffness.