Desafíos históricos del desarrollo social en Ecuador

The scenario of the elevated anchorage of the PT tendons was based on the relevant scenario of AISC Steel design Guide 1 (Fisher and Kloiber, 2006), where preloaded anchor bolts are suggested to be anchored on an elevated steel plate as a means of providing increased moment resistance with enhanced anchor bolt strain capacity to exposed steel column bases. Later, Yamanishi et al., 2012 adopted a similar elevated steel plate in their proposed steel tubular column base (refer to Chapter 2) in order to independently tune its strength and rotational stiffness to different (target) levels, by adjusting the level of the elevated plate and modifying the characteristics of unbonded

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PT anchor bolts. Capitalizing on the findings of the above works, the elevated steel plate adopted in this study for the upper anchorage of the PT tendons, referred to as anchor stand (Figure 3.3), aims at achieving four intends which pertain both to the base columns and the novel base connections. The following paragraphs elaborate on these four intends and how the anchor stand is expected to achieve them.

First, as it was shown in past research (Yamanishi et al., 2012 - Section 2.5.2.3), placing the anchor stand in different levels, the strength and rotational stiffness of the novel column base can be independently tuned to different (target) levels. On the other hand, the geometry of the anchor stand can also affect the strength and the stiffness of the novel column base. That is because by modifying the length and the width of the anchor stand, the novel column base offers flexibility in the plan view arrangement of the PT tendons outside the perimeter of the column section and thus flexibility in the selection of their lever arms. The lever arms of the PT tendons are defined as the horizontal distances of the centres of the tendons from the centre of rotation (COR), as seen in Figure 3.13. In turn, and as it will be shown below (Section 3.5), enabling the capability for selecting appropriate lever arms for the tendons, the anchor stand offers an additional means for the independent tuning of the rotational stiffness and strength of the novel column base. Capitalizing on the above strength and stiffness control mechanisms and selecting appropriate length (as reported above) and material strength for the tendons, the novel column base can also achieve self-centering capability for different target connection performances, i.e., a self-centering control mechanism. Thus, the anchor stand serves as a basic means for providing the novel column base with a strength, stiffness and self-centering control mechanism.

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Second, allowing the anchorage of the upper ends of the tendons on the top of the anchor stand and not on the top of the PT columns, as it is the case in the majority of the existing PT column bases with self-centering and damage-free characteristics (refer to Chapter 2), the base columns avoid receiving the large PT forces from the tendons and thus avoid axial shortening and axial capacity exhaustion, as it was described above in Section 3.3.4. In this manner, the anchor stand affects the strength of the base columns.

Third, by adjusting the level of the anchor stand, the stiffness of the base columns can also be tailored. The presumption of the validity of this mechanism was based on the work of Yamanishi et al. (2012), where their column base which employed an anchor stand with a function similar to that of the novel column base, could adjust its stiffness by appropriately modifying the level of the anchor stand. Subsequently, this study proved that the aforementioned presumption can be safely extended to the novel column base (Section 5.6.4). In fact, this is reasonable since the level of the anchor stand determines the ratio of the CFT height, LCFT, over the total height of the base column, hbase, which in turn affects the total stiffness of the latter. That is because the total stiffness of a base column is a function of the rotational stiffness of the novel column base and the flexural stiffness of the column. Hence, given the fact that the latter two stiffnesses depend on the height of their corresponding components, the total stiffness of a base column depends on the LCFT/hbase ratio, and thus on the level of the CFT for a given hbase.

Fourth, by being the interconnecting link between the CFT and the column, the anchor stand allows the connection of the novel column base with columns of different cross- sections and materials. The above demonstrates the crucial role of the anchor stand

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towards the applicability of the novel column base in various types of earthquake- resistant systems.

Towards attaining the above intends, the anchor stand must avoid inelastic bending and plastifications due to the aforementioned large PT forces. To this end, the anchor stand is strengthened with steel stiffeners – named anchor stand stiffeners – as seen in Figure 3.3 and Figure 3.6. By avoiding inelastic deformations in the anchor stand, unfavourable pinching hysteretic behaviour (Baber and Noori, 1985; Mostaghel, 1999; Mostaghel and Byrd, 2000; Zeynalian, Ronagh and Dux, 2012) and strength and stiffness degradation of the column base is also avoided. Thus, the novel column base attains a stable and repeatable hysteretic behaviour which is desirable for seismic- resistant structures. That is because a behaviour as such it has been proved to reduce the financial losses associated with earthquakes, and thus to increase structural, economic and social resilience to major earthquakes (Rodgers et al., 2016).

Overall, the anchor stand plays a very important roles on the attainment of a strength, stiffness and self-centering control mechanism for the novel column base, while, on the other hand, can affect the stiffness and strength of the base columns and thereby the overall structural response of the building. The above demonstrate the way the anchor stand contributes in the attainment of the relevant resilience-based performance objectives of Section 1.4.