Antonio
Nanni and Pei-Chang Huang
University of Missouri-Rolla
In recent decades, precast prestressed concrete structures have become more and more prevalent in the construction industries. The use of precast concrete in particular has been shown to be technically advantageous, economically competitive and esthetically superior because of the reduction of cross-sectional dimension and consequent weight savings, enlargement of span length, cracking and deflection control, and larger shear force resistance.
The use of precast concrete can improve the quality of the final products, decrease construction time and assist the progress of construction in adverse weather conditions. Unlike a cast-in-place reinforced concrete structure that is by nature monolithic and continuous, a precast concrete structure is composed of individual prefabricated members that are connected by different types of connections. The type of connections used determines the behavior of a precast structure under load.
The concept of dapped-end beams is extensively used in buildings and parking structures as it provides better lateral stability and reduces the floor height. The design of dapped-end connections is an important consideration in a precast concrete structure even though its analysis is complex. The unusual shape of the dapped-end beam develops a severe stress concentration at the re-entrant corner. Furthermore, in addition to the calculated forces from external loads, dapped-ends are also sensitive to horizontal tension forces arising from restraint of shrinkage or creep shortening of members. Therefore, if suitable reinforcement is not provided close to the re-entrant corner, the diagonal tension crack may grow rapidly and failure may occur with little or no warning. On the basis of the above observations, reinforcing schemes and associated methods of design, which combine simplicity of application with economy of fabrication and which provide the margin of safety required by present building codes, have to be investigated.
This thesis reports on the strengthening and performance of dapped ends that were initially constructed without the required steel reinforcement. The research program focused on precast prestressed concrete double tee members with thin stems. One dapped-end of each member was reinforced with mild steel according to the Prestressed Concrete Institute design method and the other end, intentionally deficient, was strengthened with carbon FRP sheets. Two different configurations were tested and compared to attain a better understanding of the dapped-end behavior and the novel upgrading method of concrete reinforcement with externally bonded FRP composites. A 0o/90o wrapping technique was used. The failure mode resulted from peeling of the carbon FRP sheet. In order to attain fiber rupture rather than peeling, an end-anchor was added. The system involves cutting a groove into concrete, applying the sheet to the concrete, and anchoring the sheet in the groove. It was demonstrated that the number of plies (stiffness) of FRP reinforcement and the application of anchor increase ultimate capacity and that the failure by fiber rupture is achieved. Algorithms are provided to estimate the capacity of the dapped-end.
University of Missouri-Rolla
In recent decades, precast prestressed concrete structures have become more and more prevalent in the construction industries. The use of precast concrete in particular has been shown to be technically advantageous, economically competitive and esthetically superior because of the reduction of cross-sectional dimension and consequent weight savings, enlargement of span length, cracking and deflection control, and larger shear force resistance.
The use of precast concrete can improve the quality of the final products, decrease construction time and assist the progress of construction in adverse weather conditions. Unlike a cast-in-place reinforced concrete structure that is by nature monolithic and continuous, a precast concrete structure is composed of individual prefabricated members that are connected by different types of connections. The type of connections used determines the behavior of a precast structure under load.
The concept of dapped-end beams is extensively used in buildings and parking structures as it provides better lateral stability and reduces the floor height. The design of dapped-end connections is an important consideration in a precast concrete structure even though its analysis is complex. The unusual shape of the dapped-end beam develops a severe stress concentration at the re-entrant corner. Furthermore, in addition to the calculated forces from external loads, dapped-ends are also sensitive to horizontal tension forces arising from restraint of shrinkage or creep shortening of members. Therefore, if suitable reinforcement is not provided close to the re-entrant corner, the diagonal tension crack may grow rapidly and failure may occur with little or no warning. On the basis of the above observations, reinforcing schemes and associated methods of design, which combine simplicity of application with economy of fabrication and which provide the margin of safety required by present building codes, have to be investigated.
This thesis reports on the strengthening and performance of dapped ends that were initially constructed without the required steel reinforcement. The research program focused on precast prestressed concrete double tee members with thin stems. One dapped-end of each member was reinforced with mild steel according to the Prestressed Concrete Institute design method and the other end, intentionally deficient, was strengthened with carbon FRP sheets. Two different configurations were tested and compared to attain a better understanding of the dapped-end behavior and the novel upgrading method of concrete reinforcement with externally bonded FRP composites. A 0o/90o wrapping technique was used. The failure mode resulted from peeling of the carbon FRP sheet. In order to attain fiber rupture rather than peeling, an end-anchor was added. The system involves cutting a groove into concrete, applying the sheet to the concrete, and anchoring the sheet in the groove. It was demonstrated that the number of plies (stiffness) of FRP reinforcement and the application of anchor increase ultimate capacity and that the failure by fiber rupture is achieved. Algorithms are provided to estimate the capacity of the dapped-end.
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