BRE IP3/01 PDF

Introduction

The background and early evolution of wall ties was reviewed in BRE Information Papers IP16/88 and IP17/88. Developments in wall ties since 1988 are outlined in IP11/00, which gives applications and performance levels for the wall tie classifications in DD 140-2:1987. DD 140-2:1987 will be replaced by EN 845-1 (at the time of writing, this is draft standard prEN 845-1:2000). prEN 845-1:2000 specifies the requirements for wall ties in terms of, say, displacement under load when measured according to BS EN 846-5:2000, rather than performance classifications. Both DD 140-1: 1986 and BS EN 846-5:2000 describe methods for testing the tensile and compressive strengths of wall ties. Collectively, these documents deal with the material properties of wall ties and their strength requirements based on static, or quasi-static, tests in tension and compression.

The dynamic stiffness of wall ties is important because, along with the number of wall ties used, it can affect the sound insulation of masonry cavity walls in buildings. This paper describes the measurement procedure to determine the dynamic stiffness of wall ties used in masonry cavity walls.

The sound insulation of separating masonry cavity walls is partly determined by the coupling between their two leaves, which is, in turn, partly determined by the number and the dynamic stiffness of the wall ties used to connect them. All wall ties conforming to BS 1243:1978 and DD 140-2:1987 have a drip so that the likelihood that water will pass from one leaf to another by way of the ties is reduced. These drips mean that, in calculations, wall ties cannot be treated as simple rods whose dynamic stiffness can be calculated if the dynamic Young's modulus of the material from which they are made is known. In addition, a wall tie's dynamic stiffness depends on the width of the cavity in which it is used. For these reasons, it is necessary to determine the dynamic stiffness of wall ties experimentally rather than calculate the value from material properties and dimensions.

A wall tie's dynamic stiffness is determined by measuring the mass–spring–mass resonance frequency of at least six suspended test specimens comprising a wall tie with its ends cast into two nominally identical concrete blocks. From the mass–spring–mass resonance frequencies and the masses of the test specimens, the dynamic stiffness of the wall ties in each of the specimens can be calculated. The wall tie's dynamic stiffness is taken to be the mean value obtained from all the test specimens.