Fig 1.
Proposed methodology for “progressive structural capacity loss assessment”.
Fig 2.
3D Analytical model of the considered building, showing the propagation and projection of diagonal struts from 5th floor to 7th floor, as well as the ties at the top.
Central diagonal strut at each side of the structure consisted of 13 meters of length, while the two, side diagonal struts, at each corner of the structure, comprised of 10.8 meters of length. Each strut possessed a typical cross-section of 800mm X 800mm, and a yielding capacity of 25548 KN (the yielding capacity was evaluated using actually expected material strengths).
Fig 3.
Elevation view showing the specific locations of strut(s) and tie(s).
To produce the inherent redundancy in the structure, steel ties were provided at the roof deck level to ensure that the structure remains standing and no loss of life occurs even when any or all of the struts lose their capacity up to significant level. The ties were modelled as multi-linear plastic link elements as link elements can be utilized to model special behavior (CSI, 2011). The ties conceptually tolerated the load only when the struts were supposed to fail, and they were essentially modelled to remain in tension when the struts lose their respective capacities. The F-D relationships of steel ties, the link elements, were primarily dependent upon the material used for their fabrication; Fig 4 shows the F-D relationship evaluated for ties.
Fig 4.
Force-Deformation Relationship of Steel Ties, located at the roof deck level.
As described; the procedure for progressive structural capacity loss assessment is uniquely based on incremental reduction of structural strength, thus, a scenario based Progressive Structural Capacity Loss Assessment (PSCLA) was performed. The nonlinearity in the model was introduced by means of concentrated hinges at certain critical elements.
Table 1.
Modelling parameters for diagonal struts and link elements.
Fig 5.
a) F-D relationships of replacing link element, 8.14 m in length b) F-D relationships of replacing link element, 4.96 m in length (spring elements F-D relationships for 13m long strut).
Fig 6.
a) F-D relationships of replacing link element, 5.96 m in length b) F-D relationships of replacing link element, 4.9 m in length (spring elements F-D relationships for 10.8 m long strut).
Table 2.
Forces in struts and ties.
Fig 7.
3D Analytical model after replacing the struts with link elements.
Fig 8.
3D Analytical model after replacing the struts with link elements.
Table 3.
Forces in replacing link elements and Ties– 4th scenario– 50% capacity reduced.
Table 4.
Forces in replacing link elements and ties– 4th scenario– 75% capacity reduced.