The notch was set with a constant width of 4 mm (see Figure 11a). Figure 9 shows pictures of these fibers.Īfter the completion cure of concrete and prior to the test, each beam specimen was cut with a notch located at mid-length of the beam. Table 2 and Table 3 give the properties of the steel and sisal fibers. All tests for material and mechanical properties’ characterization were done in the Structural Laboratory and in the Materials Laboratory, both at the Federal University of Cariri (UFCA). The strength of the steel wire was tested before cutting and bending. The steel fiber and sisal fiber, inserted in the concrete matrix, were used with 1% of the concrete volume fraction (V f = 1%). After this process, the fibers were air-dried for 48 h and then manually cut. The fibers were submerged into water at 70 ± 5 ☌ for approximately one hour. Before cutting them into segments of 45 mm, it was necessary to process them to remove impurities. The fibers were received in bundles of long fibers, approximately 1 meter long. The sisal fibers used in this work were extracted from Agave Sisalana plant by a process called decortication. However, the elements incorporating such technology are still on the market. In the context of the civil construction environmental impact, fibers from natural resources, such as sisal fiber, appear as an alternative to synthetic fibers. Table 1 presents briefly some applications of reinforcement with the addition of fiber to the concrete. Since then, this type of concrete reinforcement has gained much interest in the construction industry, and also among researchers. The first structural use of steel fiber-reinforced concrete was in 1971 for the production of collapsible panels for a London Heathrow Airport parking garage. Among the fibers, steel fiber was one of the earliest and it is one of the most effective materials for improving the mechanical properties and impact resistance of concrete. The use of discrete fibers is one effective solution to reinforce the matrix for improving the tensile and flexural performances of the plain concrete. The fibers act as connection bridges, transferring the stresses to another side of the concrete matrix, and minimizing stresses at the ends of the cracks. Thus, fiber additions to concrete is common practice, which seeks to reduce the cracks, improving performance to tensions’ forces. Thus, points A and B ( Figure 7a) can be considered superimposed ( Figure 7b).Ĭonventional concrete still has some deficiencies such as low ductility, plastic shrinkage, and small tension strength. In this case, it is observed that the eccentricity correction E 2 is made using an element that provides the distance d equal to E 2 between the diagonals and the bottom chord. The proposal, developed by, was to correct the eccentricity ( E 2), using a spacer element. It is possible to observe that there are two eccentricities: E 1, horizontal (stamped region) and E 2, vertical (distance between points A and B (see Figure 7)). This situation differs from the nodes idealized in a theoretical truss, and, above all, from the assumptions adopted in most design offices. Consequently, it generates eccentricities (see Figure 7a). This produces the incompatibility of lines passing through the center of gravity of the bars arranged in the diagonals and on the truss edges (see Figure 7). The collapses involving typical connections were a consequence of the stamped tubes overlapped and interconnected with a single bolt. Many space truss accidents involving typical connections are characterized by a progressive collapse triggered by the local collapse of the connection, not to the ultimate buckling of the truss bars. The trusses with encapsulated concrete spacer showed good results, with an increase in collapse load of 36% and failure by buckling bars. The experimental tests showed that the trusses with typical nodes collapsed under reduced load by local failure due to high distortions at the nodes. This reduction is done with a new type of spacer made of encapsulated concrete with steel fiber or sisal fiber.
To increase the truss load capacity, this research presents the results of an experimental program to reduce the eccentricities of the typical nodes. They are inexpensive but present eccentricities that reduce significantly the strength of such space trusses. Connections made with the superposition of flattened tube ends trespassed by one bolt are, generally, known as typical nodes. The load capacity of these structures depends also on the strength of their node connection. In addition to having a low cost, the truss weight is relatively reduced. Space trusses are structural systems, generally made of tubes, used worldwide because of their advantages in covering long-span roofs.
0 kommentar(er)
