The response of precast structure and panels

This section describes the typical response of a precast structure in detail, and parameters essential for analysing demand and capacity on the fastening system are defined. Because the silicone sealant could, in certain cases, appreciably influence the response of panels, the typical response of the precast structure is presented for both cases, without and with silicone. In the following sections, the typical response mechanism is explained, and the most significant observations are shown for

three characteristic examples (structures m60H5, m60H7 and m60H9) for the case of one accelerogram (ground motion record number 14, see Appendix A).

Here, the parameter column drift along the single panel is explained. As will be demonstrated later, it gives information about the response of the fastening system and is used to define the fastening system’s demand and capacity.

In literature, the term drift is used to describe both the displacement and rotation of the element.

Because of this inconsistency in the use and definition of the term, the terminology used in the dissertation must be clarified. Therefore, drift is the lateral displacement of one level relative to the level above or below, whereas drift ratio is the drift divided by the height between the considered levels.

As shown in Figure 6.30 and Equation 6.9, the drift of the column along the single panel (Δdcol) is defined as the difference in absolute column displacements at the top and bottom levels of the panel (|dcol,top - dcol,bottom|). Because the movement of the panels and connections is predominantly translational, this is the same as the absolute value of the difference in slips at the top and bottom connections (|dslip,top - dslip,bottom|).

∆𝑑_{𝑐𝑜𝑙,𝑝} = |𝑑_{𝑐𝑜𝑙,𝑡𝑜𝑝}− 𝑑_{𝑐𝑜𝑙,𝑏𝑜𝑡𝑡𝑜𝑚}| = |𝑑_{𝑠𝑙𝑖𝑝,𝑡𝑜𝑝}− 𝑑𝑠𝑙𝑖𝑝,𝑏𝑜𝑡𝑡𝑜𝑚| (6.9)

Figure 6.30: Column drift along the single panel Slika 6.30: Pomik stebra na nivoju panela

In parametric study, maximum drifts were monitored (an example is shown in Figure 6.31). The connections failed at a certain column drift along a single panel. This drift at failure is the maximum

difference in slips at the top and bottom connections of the panel and presents the capacity of the complete fastening system (see also Sections 6.3.1 and 6.3.2).

Figure 6.31: Maximum column drift along a single panel Slika 6.31: Največji pomik stebra na nivoju panela

6.3.1 Interaction of the connections and demand on the fastening system

The parametric study results show that the top and bottom connections interact with each other and should be treated together as a complete fastening system. The demand on the complete fastening system can be expressed in terms of the column drift along the single panel. As observed during experiments, the top connection is the weakest component of the fastening system. After contact, its strength and stiffness are lower, and thus panel failure always occurs at the top connect ion.

A summary of the response is given in the following:

- At low seismic excitations, the panel behaves as if it was pinned at the top and sliding occurs only at the bottom connection.

- With increasing demand, sliding also occurs at the top connection. From that point on, the drift demand is taken over by both connections that move simultaneously.

- When the gap in one of the connections is depleted (either at the top or bottom connection), the stiffness of that connection increases and high lateral forces occur (demand on the connection).

Δdcol=|dslip,top-dslip,bottom|

- When the column and panel are in contact at one of the connections, the slip increases in the other connection until the gaps in both connections are depleted. There is a strong tendency of the fastening system to slide until the gaps in both connections are closed.

- After the gaps are depleted in both connections, the slips increase faster at the top connections because of their smaller stiffness compared to the bottom connections.

- Failure of the fastening system and panel occurs when the resistance of the top connection is reached.

Because column drift increases along column height, the panel at the top of the structure was most exposed to failure. However, if the bottom panel was fixed to the foundation, all the demand on bottom fastenings was taken only by the top connection. In that case, the first failure of the fastening system often occurred at the bottom panel. This will be further explained within the parametric study in Section 6.4.3.

6.3.2 Capacity of the fastening system

Because the demand on the fastening system is expressed with column drift along a single panel, the capacity of the fastenings should also be expressed this way. At this point, it is necessary to differentiate the capacity of the top connections in terms of connection slips and the capacity of the complete fastening system in terms of column drift along a single panel. In this section, they are denoted as displacement capacity of the top connection and drift capacity of the fastening system.

The first is the capacity of the top connection that is always the same and is known from experiment;

it is the sliding capacity of the connections plus approximately 3.5 cm after the gap in the connection is depleted. At this slip, the resistance of the top connection is reached (approx. 55 kN).

As explained earlier, the column drift along the single panel is the same as the difference in slips at the top and bottom connection. Thus, the drift capacity of the fastening system is the column drift along the single panel at which the connections fail.

The most important parameter that affects the drift capacity of the analysed system is construction imperfections. When connections are positioned eccentrically, there was no available sliding capacity, and failure of the fastening system occurred earlier. The influence of different eccentrical positions (LL and LR) is discussed in Section 6.4.2.

The drift capacity of the fastening system is the highest if connections are positioned centrally with no interaction between adjoining panels. When silicone sealant is applied, the drift capacity of the