Masonry infill shear curve - inf_shear

This is the masonry infill shear model, developed and initially programmed by Crisafulli [1997] and implemented in SeismoStruct by Blandon [2005], to be used (almost exclusively) in association with the infill panel element.

It is recalled that the shear strength results as the combination of two mechanisms, namely, bond strength and the friction resistance between the mortar joints and the bricks. The shear strength can thus be expressed as the sum of the initial shear bond strength and the product of coefficient of friction by the absolute value of the normal compressive force in the direction perpendicular to the bed joints. This approach to estimate shear resistance is pragmatically adopted by design codes, independently of the failure mechanism (shear friction failure, diagonal tension failure, compression failure) being developed in the infill panel.

The values of and can either be evaluated by direct shear tests or be obtained from design specifications. However, whilst the former may lead to an overestimation of the values [Wan and Yi, 1988; Riddington and Ghazali, 1988], the latter tend to be over-conservative, so care should be taken in their use; Mann and Muller [1982], for instance, proposed expressions for reducing the usually overestimated values from shear tests.

Four parameters need to be defined in order to fully characterise this response curve:

Shear bond strength -
An ample sample of shear bond strengths has been measured by different researchers. Hendry [1990] presented experimental results obtained from tests using diverse materials and to varied from 0.3 to 0.6MPa. Paulay and Priestley [1992] indicated that typical values range from 0.1 to 1.5MPa, while Shrive [1991] limits this range between 0.1 to 0.7MPa. Similar values were reported by Stockl and Hofmann [1988] and by Atkinson et al. [1989] for a wide range of materials. From these and other research initiatives, several empirical expressions have been proposed, dependent on different parameters, but their use must be cautious considering the numerous variables that affect the shear bond strength. By default, SeismoStruct assumes a relatively conservative value of 0.3 MPa for this parameter.

Friction coefficient -
Due to lack of a clear and undisputed agreement on the factors that affect the coefficient of friction, relatively contradictory results have been reported in the past, with different researchers [e.g. Sahlin, 1971; Stockl and Hofmann, 1988; Atkinson et al., 1989; Hendry, 1990; Paulay and Priestley, 1992] thus presenting values of that range from 0.1 to 1.2. Of such past work, we may note here that Atkinson et al. [1989] suggested the value of 0.7 as a reliable lower bound estimate for this parameter (stemming from experimental results where it was observed that varied between 0.7 and 0.85 for a wide variety of materials), and which is adopted as a default in SeismoStruct (it is noted that the much lower "default" value of 0.3 suggested by Paulay and Priestley [1992] is intended for design applications, rather than assessment).

Maximum shear strength -
This is the largest shear stress that may be mobilised by the infill panel and, as stated above, it will depend on the failure mechanism (shear friction failure, diagonal tension failure, compression failure) developed in the latter. In the absence of additional and more precise information, users may be pragmatically assume this value to be equal to the sum of and the product of by the normal compressive strength of the masonry units. A value of 0.6 MPa is assumed as default in SeismoStruct, that is, to the 0.3 MPa coming from the default shear bond strength, an additional 0.3 MPa friction-induced shear resistance is assumed.

Reduction shear factor -
This empirical parameter represents the ratio between the maximum shear stress and the average stress in the panel, and may range between 1.4 and 1.65 Crisafulli [1997]. The default value of 1.5 is likely not to require any change from the user.