Dowel connections securing roof-diaphragms to perimeter walls in historic masonry buildings and in-field testing for capacity assessment

Abstract

In the seismic retrofit of existing masonry constructions, global interventions are often needed to inhibit the onset of local mechanisms and to engage the whole building box-like structural behaviour. Such interventions are represented by perimeter ties and roof and floor diaphragms. This paper considers the roof diaphragm strengthening solution and investigates the use of stud connections securing the roof thin-folded shell to the perimeter walls. Stud connections serve the dual purpose of collecting and transferring the out-of-plane inertia forces of the masonry walls to the roof diaphragm, as well as transferring the diaphragm reaction forces to the shear walls. Specific detailing of the stud connection and the adoption of an improved lime-mortar overlay on the top of the masonry walls are proposed to improve the connection strength; without such improvements, the connection capacity would be jeopardised by the reduced shear resistance of the masonry wall due to the absence of significant vertical confining action at the roof level. The intervention entirely changes the behaviour of the connection and significantly reduces shear stresses on the masonry wall. The structural behaviour of the connection is analysed and discussed. Emphasis is made on the conceptual design of laboratory and in-field test procedures and testing frames in order to replicate the boundary conditions in real applications. In-situ tests may help during the design of the roof thin-folded shell system and allow for the efficiency assessment of the connections prior to the final intervention, thereby proving the actual feasibility of the retrofit solution.

Appendix: Laboratory test results on stud connections in Specimen A and C

Laboratory test results on stud connections in Specimen A and C are summarised in Table 1 in terms of (Fig. 17):

Table 1 Main results obtained from laboratory tests

Fig. 17

Main properties of the envelope curve and properties of the single load cycle

• Yield Load and net yield displacement (V_y, s_y), excluding the initial recovery of possible clearances;

• Nominal Shear Capacity V₁₀ defined as the shear load at 10 mm net of relative slip;

• Maximum applied load V_max;

• Net Initial Stiffness K defined as the initial slope of the curve in which initial inelastic slip induced by stud-hole clearance recovery is removed. Note that it may substantially differ from the Gross Cyclic Stiffness, defined as the slope of the line connecting two subsequent load reversal points, which reduces due to incremental damage caused by the repeated sliding. The net initial stiffness corresponds to V₁ defined as the shear load at a net relative slip of 1 mm, excluding the initial recovery of clearances anytime V₁ < V_max;