Elsevier

Engineering Structures

Volume 32, Issue 10, October 2010, Pages 3225-3234
Engineering Structures

Modeling of timber beams strengthened with various CFRP composites

https://doi.org/10.1016/j.engstruct.2010.06.011Get rights and content

Abstract

This paper presents a modeling approach to predict the behavior of timber beams strengthened with carbon fiber reinforced polymer (CFRP) composites. A three-dimensional finite element analysis (FEA) model is formulated, based on the orthotropic constitutive characteristics of timber species. The model provides the load–displacement relationship, strain development, stress concentration, and failure modes of the CFRP-strengthened timber beams and those responses are compared to the experimental data. The validated models are used for a parametric study to further examine the effect of various CFRP properties on the behavior of five timber species: Douglas Fir, Yellow Birch, Sitka Spruce, Yellow Poplar, and Northern White Cedar that cover most of the engineering properties available in practice. The strengthened beams show improved load-carrying capacity and energy absorption capacity when compared to unstrengthened counterparts. An optimal CFRP-reinforcement ratio is found beyond which no strength increase is achieved. Even though the elastic modulus of the CFRP composites influences the failure mode of the strengthened beams, it may not significantly affect the strength-increase of the beams because the properties of timber species are a dominant factor influencing the failure of the beams, rather than the CFRP properties.

Introduction

Wood is a widely used structural material for lightweight buildings and short-span bridges. Glued laminated (glulam) timber and sawn lumber are commonly used for structural engineering applications. Deterioration of existing timber structures may result from increased service loads, ageing, and biological attack [1], [2], [3], [4]. Biological deterioration of timber elements due to the diffusion of enzymes, for instance, is a phenomenon analogous to, though different in mechanism from, corrosion-induced deterioration of steel or concrete elements. Rather than costly replacement of deteriorated timber elements, repair or strengthening of the elements may be recommended [3], [4], [5]. Conventional strengthening/reinforcing methods for timber structures use steel plates or bars, aluminum plates, or simply timber patches [6], [7], [8], [9]. Such methods, however, may increase dead loads, transportation expenses, and installation costs. Conventional repair methods generally require mechanical connection (e.g., bolts and nails) which may not be effective in deteriorated timber. Steel components are susceptible to corrosion and aluminum plates may buckle when thermal loads are applied [10]. The application of fiber reinforced polymer (FRP) composites is a promising solution to upgrade/strengthen timber systems, particularly where increased load-carrying capacity or stiffness is desired. FRP composites provide favorable strength and stiffness-to-weight ratios; are non-corrosive; and offer excellent tailorability, reduced long-term maintenance costs, and prompt installation on site [11], [12], [13]. Although FRP composites are intensively used for strengthening concrete structures [14] and more recently have been introduced as viable methods of retrofitting steel structures [15], relatively little information is available for timber applications such as the interaction between FRP composites and damaged timber element, the effect of various types of wooden species on the performance of FRP-strengthening systems, and the failure mode of strengthened members. This paper presents a modeling approach to predict the flexural behavior of timber beams strengthened with carbon FRP (CFRP) composite materials to address the identified research gap mentioned above. The developed model is further used for a parametric study of CFRP-strengthening for timber applications, including various CFRP properties and wood species.

Section snippets

Strengthening of timbers with FRP composites

FRP composites may be bonded on the tensile soffit of timber beams to enhance load-carrying capacity and stiffness. Such strengthening work is conducted with externally bonded FRP strips/sheets or near-surface mounted (NSM) FRP bars [4], [16], [17]. The NSM application is a relatively new strengthening technique in which a narrow groove is cut into the member and FRP composites are inserted with a bonding agent [18], [19]. Prestressed FRP laminates have also been shown to improve the

Experimental program

This section provides a summary of the experimental work on which the modeling effort is based; further details are available in Kim et al. [38]. The experimental program included six Douglas Fir beams having dimensions of 38 mm wide × 138 mm deep × 2690 mm long. The beams were recovered from a 38-year old Quonset and used to evaluate the effects of CFRP-strengthening. The moisture content of the beams was not measured prior to testing, given that the members have been inside the Quonset for

Numerical modeling approach

To predict the behavior of timber beams strengthened with CFRP composites, a three-dimensional computational model was developed using the general-purpose FEA program ANSYS. The following summarizes detailed modeling approaches.

Load–deflection response

Fig. 3 shows the load–displacement relationship at midspan of selected beam specimens. The load-carrying capacity and corresponding deflection of all timber beams are available in Table 1. The predicted response of the model agreed well with that of the experiment. An average error of 8.2% in the ultimate load was obtained, as shown in Table 1. The response of the beams was essentially linear until failure occurred (Fig. 3). It should, however, be noted that limited damage propagation was

Parametric study

A parametric study was conducted to further examine the response of timber beams of various species and CFRP having different properties, based on the FEA modeling approach (i.e., the beams marked with ‘FEA’ in Table 1). To provide a consistent evaluation of the effect of specific parameters, the beam dimensions used for this study were those of Beam DF2 unless otherwise stated. The beams were simply supported and subjected to four point bending (Fig. 1(b)).

Summary and conclusions

This paper has presented a modeling approach to predict the flexure of timber beams strengthened with CFRP composites, including a comparison to experimental behavior. The model included orthogonal properties of the wood species and was able to simulate two distinct failure modes of the CFRP-strengthened timber beams, namely, the grain fracture at a flexure-critical region (near midspan) and at the end of the bonded CFRP composites where stress concentrations occurred. A parametric study was

Acknowledgement

The writers would like to acknowledge Mr. Mozahid Hossain at North Dakota State University for the experimental contribution.

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