Revised zone method R-value calculation for precast concrete sandwich panels containing metal wythe connectors

Abstract

Metal wythe connectors are used in a typical precast concrete sandwich panel to tie concrete wythes together and to keep the panel intact during handling and in service. Connectors interrupt the continuous insulation layer, reducing the effectiveness of the insulation. In current practice, thermal resistance (R-value) of such a panel is calculated from the zone method. However, the zone width parameter W used in the zone method was originally developed for metal-frame structures and an accurate R-value cannot be estimated for precast concrete sandwich panels containing metal wythe connectors. This paper proposes a new zone-width equation for use in the current zone method to compute the R-value of precast concrete sandwich panels containing the metal wythe connectors. The proposed zone width W n was derived from the results of a series of finite element heat-transfer analyses intended to quantify the influence of several key parameters on W n . It was found that the zone method with the proposed zone-width equation can accurately estimate R-value of a precast concrete sandwich panel containing metal wythe connectors. Also the proposed zone-width equation can effectively consider the effects of metal wythe connector sizes and spacing, material conductivities, and panel thicknesses in the zone method of R-value computation.