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Figure 9. Asymmetric behavior emphasized under asymmetric loading (Truss 1 and SLE).

Masonry Structures Behavior And Design

Figure 9. Asymmetric behavior emphasized under asymmetric loading (Truss 1 and SLE).

Figure 10. Distortion of the tie beam under asymmetric loading (Truss 1 and SLS).

Figure 11. Behavior of the king-post/tie beam connection: (a) in plane bending of the bolt (b) out-of-plane movement prevented.

Figure 10. Distortion of the tie beam under asymmetric loading (Truss 1 and SLS).

beam connection, the asymmetric response of the overall truss causes distortion in the tie beam (Figure 10). This distortion induces important bending stresses in the bolt of that connection (Figure 11a).

The connection between the king-post and the tie beam presents rotational stiffness in the plane of the truss and in the normal plane. Therefore, this connections prevent also the out-of-plane movement (Figure 11b) improving consequently the stability of the substructure composed by the rafters, the king-post and the struts.

As expected, under asymmetric loading the relative displacement between the king-post and the tie beam, measured in the connection between both elements, is nearly zero. In addition, the horizontal movement recorded in the unrestraint support is reduced.

For the case of the Ultimate Limit State (ULS), different behaviors were observed for both tested trusses. While Truss 2 was able to perform all tests (symmetric and two asymmetric), in Truss 1 only the first cycle of the symmetric tests was achieved. When the maximum load corresponding to this limit state was reached, considerable out-of-plane movements were visible, Figure 12. To prevent the global failure of the truss it was decided to stop the test and to strength Truss 1 before proceeding with the experimental program.

In the case of Truss 2, the tests undertaken for SLS were repeated for the load level corresponding to the

Figure 12. Out-of-plane movement observed in the truss 1 (unstrengthened) under ULS at the end of 1st cycle.

ULS. The main conclusion about the overall behavior of the truss pointed out for the SLS can be extended to ULS. Therefore, it can be concluded that, apart from the instability observed in the case of Truss 1, the safety of the trusses under the Ultimate Limit States was verified. No local collapse or failure in the timber members occurred. The bracing forces transmitted by the purlins and the covering structure should prevent the instability observed in the case of Truss 1.

4.3 Strengthening

The next phase in the experimental program was to strength the trusses based in the response obtained in first phase and to repeat the tests. Strengthening of Truss 1 aimed essentially to repair the out plane movement observed and to prevent this instability making possible to perform the carrying tests corresponding to the ULS. The out plane deformations were removed and a UPN profile was bolted to the king-post. The objective was to prevent the out plane movements, increasing the stiffness of the connection in this direction, keeping the tie beam suspended to the king post,

Increase Displacement
Figure 13. Strengthening of Truss 1.

Figure 11a. The timber elements did not presented any local failure or damage however, the connections between the rafters and the tie beam were weakened. In particular, the left connection, over the support with horizontal displacement, the depth step was insufficient and the timber beyond the step was fissured. Then, it was decided to strength this part of the tie beam (beyond the step) with screws (12 screws M6-200) aiming to increase its shear resistance. Moreover, using the existing holes, an internal bolt of 20 mm diameter was introduced at midjoint and normal to the rafter in each rafter/tie beam connection, Figure 12b.

In the case of Truss 2, only the connections between the rafters and the tie beam were strengthened. The stiffness ofthe king-post/tie beam connections demonstrated to be adequate to prevent the out-of-plane instability. Rather, the timber beyond the step demonstrated some signs of deterioration and the depth of the steps in the rafters/tie beam connections looked insufficient. In this last case, the same strengthening technique used before in Truss 1 was applied: one internal bolt tightening the connection and screws beyond the step to improve the shear resistance. However, and as an improvement based in the failure modes observed in Truss 1, a binding strip was used to confine the timber, Figure 13.

4.4 Efficiency evaluation of the strengthening

The series of tests corresponding to the Service Limit States (SLS) were repeated after strengthening the trusses. The main objective was to analyze the

Figure 14. Strengthening of Truss 2. Screws and binding strip applied in the frontal part of the step in the rafter/tie beam connection.

influence of the joint stiffness, in this case, the stiffness of the rafter/tie beam connection, in the overall behavior of the truss in particular under asymmetric loads. However, the main goal of the strengthening was to increase the truss resistance: to make Truss 1 able to perform all load procedures corresponding to ULS, and to assess the failure load of Truss 2.

Load capacity of Truss 1 was improved by the strengthening, but only the symmetric test corresponding to the ULS was accomplished. At the end of this test, the local damages in the rafters/tie beam connections not permitted the execution of the asymmetric tests. It was decided to perform a test until failure. It is important to point out that the ultimate load achieved in the failure test is lower than the load level attained in the ULS test. Despite the strengthening measures undertaken to improve it, the failure was caused by lack of shear resistance of the tie beam beyond the step. Theoretically, the screws should increase the shear resistance but, because of the fissures already existing before placing the screws, the timber part beyond the step did not work as a rigid body. The screws expanded the fissures and the rigid body became separate in small pieces, Figure 14a. However, it is important to point out that this behavior was also accentuated by the previous shear failure of the rear step, Figure 14b. With the horizontal movement of the rafter in the connection rafter/tie beam, the joint king-post/struts was dismantled, Figure 14c, and important bending stresses are introduced in the internal bolt, Figure 14d.

In the case of Truss 2, it is difficult to conclude if the strengthening was efficient in the increase of the load capacity of the truss as no failure was obtained in the previous tests performed in unstrengthened conditions. The strengthened truss was subjected to the full loading history foreseen in the test procedure (SLS and ULS, under symmetric and asymmetric loads). After that, a loading procedure until failure was followed. This procedure is similar to the previous one but now, the load increment did not stop at the value of 80kN (maximum load corresponding to ULS).

Figure 15. Failure damages of Truss 1.

Figure 16. for Truss 2.

Shear failure of the rafter/tie beam connection

Figure 16. for Truss 2.

Shear failure of the rafter/tie beam connection

Figure 17. Double step connections between the rafters and the tie beam of Truss 2.

The maximum load value achieved during the tests at failure was ^100 kN.

Again, the failure of the truss was obtained by shear failure in the timber beyond the step in the connection rafter/tie-beam. First, the shear failure of the rear step happened and then the shear failure of the timber beyond the frontal step took place, Figure 15. The geometry decided which connections rafter/tie beam failed. The failure happens in the double step connection in which the rear step presented a faulty geometry. The rear step must be deeper than the frontal. If not, the surface resisting to shear stresses is insufficient. The double step connections of the right side (over the fixed support) failed as a consequence of a first shear failure in the rear step.

Once the rear step fails, the stresses concentrated in the frontal step and the failure is reached, Figure 16.

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