Experimental Study on the Bending Strength of Prestress Introduced Wooden Beams
Cho, Yo-Seop
Department of Architectural Engineering
Graduate School of Catholic Kwandong University
Supervised by Professor Lee Ung-Kyun
Wood has the advantages of ...
Experimental Study on the Bending Strength of Prestress Introduced Wooden Beams
Cho, Yo-Seop
Department of Architectural Engineering
Graduate School of Catholic Kwandong University
Supervised by Professor Lee Ung-Kyun
Wood has the advantages of high compressive and tensile strength, and a good workability compared to its low specific gravity, however, its strength is lower than those of steel materials and concrete. Due to these material characteristics of timber, the wooden structure has been applied to buildings with short spans and low stories in Korea. On the other hand, researches on structural laminated timbers and structural cross-laminated timber (CLT) materials, which have a higher strength compared to solid timbers, are actively underway overseas, and especially in the United States, Canada, and Europe, high-rise wooden buildings have been planned and constructed using cross-laminated timbers (CLT) and structural laminated timbers. In Korea, researches on multi-stories wooden buildings are underway utilizing CLT and structural laminated timbers in which strength of wood was increased. In addition, many researches have been made on the hybrid wooden structure combining steel plate and timber as a way of producing long span timbers that are used in the large span spatial structures. However, there is a lack of research on the way introducing prestress in timbers.
Therefore, in this study, prestress (compressive force) was introduced to the structural laminated timbers to fabricate wooden beams with increased bending strength. Through the bending strength experiment of the wooden beam, were reviewed the possibility of introducing the prestress, the eccentrical arrangement in the tendons, the change in bending strength according to the shape of cross section to improve the effect of introducing prestress, and the loss in tension according to the time lapse and volume change. Through such experiments, conclusions were obtained as below.
(1) As a result of comparison test of bending strength between the specimens with and without introducing the prestress, it was confirmed that the theoretical strength and the experimental strength were matched within the permissible error range of approximately 5%. From this result, it was concluded that the prestress can be introduced into timber beam.
(2) In the specimen where tendons were eccentrically placed in order to increase the effect of prestress introduction, the theoretical strength and experimental strength were within the error range of approximately 7%, which shows they are matched. From such a result, it was concluded that the effect of introducing prestress into timber beam can be greatly improved by placing tendons eccentrically.
(3) It is assumed that T-shaped cross-section and reinforcement method by inserting the steel sheet to the compression side have a considerable effect to increase the effect of introducing the prestress, although this has not been confirmed as the experimental result due to loss of prestress.
(4) In order to determine the loss in prestress force according to time lapse and volume change, experiments were performed after 7 days, 45 days, and 210 days, respectively. The test specimens after 7 days and 45 days showed that the theoretical strength and the experimental strength fell in the error range of approximately 5~7%, which showed they were matched. However, the specimen after 210 days showed a large error between the theoretical strength and the experimental strength. It is estimated that this resulted from that the moisture (moisture content of about 15%) contained in the wood evaporated during the production of members and shrunk through the process in dry environment at low temperature in winter. Accordingly, it is concluded that the wood should be processed in the environment with nearly 0% of water content, and the prestress should be introduced in the processing plant with low temperature and dry environment.
(5) It is concluded that further researches will be needed on the effect of increasing the bending strength of the prestressed wooden beam according to the shape change in the cross section and the reinforcement on the compression side. Further, many researches are required to find the methods to prevent the loss of prestress force due to time lapse and volume change in dry environment.
Experimental Study on the Bending Strength of Prestress Introduced Wooden Beams
Cho, Yo-Seop
Department of Architectural Engineering
Graduate School of Catholic Kwandong University
Supervised by Professor Lee Ung-Kyun
Wood has the advantages of high compressive and tensile strength, and a good workability compared to its low specific gravity, however, its strength is lower than those of steel materials and concrete. Due to these material characteristics of timber, the wooden structure has been applied to buildings with short spans and low stories in Korea. On the other hand, researches on structural laminated timbers and structural cross-laminated timber (CLT) materials, which have a higher strength compared to solid timbers, are actively underway overseas, and especially in the United States, Canada, and Europe, high-rise wooden buildings have been planned and constructed using cross-laminated timbers (CLT) and structural laminated timbers. In Korea, researches on multi-stories wooden buildings are underway utilizing CLT and structural laminated timbers in which strength of wood was increased. In addition, many researches have been made on the hybrid wooden structure combining steel plate and timber as a way of producing long span timbers that are used in the large span spatial structures. However, there is a lack of research on the way introducing prestress in timbers.
Therefore, in this study, prestress (compressive force) was introduced to the structural laminated timbers to fabricate wooden beams with increased bending strength. Through the bending strength experiment of the wooden beam, were reviewed the possibility of introducing the prestress, the eccentrical arrangement in the tendons, the change in bending strength according to the shape of cross section to improve the effect of introducing prestress, and the loss in tension according to the time lapse and volume change. Through such experiments, conclusions were obtained as below.
(1) As a result of comparison test of bending strength between the specimens with and without introducing the prestress, it was confirmed that the theoretical strength and the experimental strength were matched within the permissible error range of approximately 5%. From this result, it was concluded that the prestress can be introduced into timber beam.
(2) In the specimen where tendons were eccentrically placed in order to increase the effect of prestress introduction, the theoretical strength and experimental strength were within the error range of approximately 7%, which shows they are matched. From such a result, it was concluded that the effect of introducing prestress into timber beam can be greatly improved by placing tendons eccentrically.
(3) It is assumed that T-shaped cross-section and reinforcement method by inserting the steel sheet to the compression side have a considerable effect to increase the effect of introducing the prestress, although this has not been confirmed as the experimental result due to loss of prestress.
(4) In order to determine the loss in prestress force according to time lapse and volume change, experiments were performed after 7 days, 45 days, and 210 days, respectively. The test specimens after 7 days and 45 days showed that the theoretical strength and the experimental strength fell in the error range of approximately 5~7%, which showed they were matched. However, the specimen after 210 days showed a large error between the theoretical strength and the experimental strength. It is estimated that this resulted from that the moisture (moisture content of about 15%) contained in the wood evaporated during the production of members and shrunk through the process in dry environment at low temperature in winter. Accordingly, it is concluded that the wood should be processed in the environment with nearly 0% of water content, and the prestress should be introduced in the processing plant with low temperature and dry environment.
(5) It is concluded that further researches will be needed on the effect of increasing the bending strength of the prestressed wooden beam according to the shape change in the cross section and the reinforcement on the compression side. Further, many researches are required to find the methods to prevent the loss of prestress force due to time lapse and volume change in dry environment.
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