There are several methods have been developed since early 70’s when T. T. Lie developed a simplistic method to design heavy timber beams exposed on three sides for fire resistance at NRCC .
I have performed several design examples for calculating endurance time for beams exposed to three sides for various sizes based on the equations provided in technical literature provided by AF&PA  and below are my calculated results*. The sizes which has endurance time of 1 hour or more are highlighted in green.
1. “Calculating Fire Resistance of Exposed Wood Members” By AF&PA – Technical Report 10.
2. “Design for Code Acceptance” by AF&PA.
*”These results shall not be used directly for any design without performing a design and analysis by a structural engineer and confirmed by a professional engineer. This blog is for educational purposes only and are based upon calculations done based on formulas provided in technical literature by AFPA ”