During a fire event, columns and beams may be exposed to fire on more than one side, which results in the corners charing faster. The unidirectional models used to estimate the char thickness would exclude the effect of multiple side fire exposure on a wood member. To account for this effect, corner rounding needs to be considered in the structural fire design, where the radius of the corner (r) is equal to the estimated char layer thickness as shown in the figure below.
This corner rounding effect is especially important when designing beam hanger connections. In the figures below, the blue section represents the wood that will not be affected in case of a fire. This wood will be protected by the chared layers. The red line represents the maximum height at which the connector can be installed. In order to achieve proper fire design, the connector must be installed below the red line and in the blue section. The blue section is measured using the char layer thickness and the corner rounding effect.