Abstract
Owing to the natural defects of wood, structural timber is currently used with conservative safety factors. This study investigates the use of Carbon Fiber Reinforced Polymer (CFRP) for improving the strength of structural timber. Four flexural test stages were carried out, the modes of failure of the three specimens in each stage were studied to determine and strengthen the suspected weak zone which caused the failure. It was found that the tensioned fibers of ordinary timber girders normally fail first especially at the sections containing knots. Using a strip of CFRP to strengthen tensioned fibers is a well known technology, but it will not prevent a premature failure due to horizontal shear. A strength increase of 56% was reached by the adaption of shear connectors in the form of screws. After treating tensile and shearing strengths, still testing specimens showed failure at the compressed fibers. By enhancing the compressed fibers with a steel strip, the prototype girders showed a remarkable increase in flexural strength of about 175% compared with the ordinary wooden girders having the same dimensions.
Highlights
IntroductionStructural timber has some defects that can occur at various stages of its useful life
As a natural material, structural timber has some defects that can occur at various stages of its useful life
In order to deal with this defect, BS 4978 [1] uses visual stress grading which specifies the maximum size of defects acceptable in each of the following different stress grades: General Structural (GS) having a working stress of approximately 40-50 percent of its basic stress or Special Structural (SS) corresponding to a working stress of approximately 65 percent of its basic stress
Summary
Structural timber has some defects that can occur at various stages of its useful life. Schober and Rautenstrauch [4] studied the possibility of strengthening existing timber floors under bending loads. Through their investigations they have tested three different schemes of 3.25 m long beams. It consists of strengthening the bottom, sides, and again the bottom and the two sides with CFRP bonded to the retrofitted beams by epoxy. This procedure increases the strength of the 4.25 m long beams by 67%.
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