Effect of purlin height on the response performance of smoke detectors in ancient buildings of the Ming and Qing dynasties

The unique design and complex morphological characteristics of roof structures in the "purlin-type" ancient buildings of the Ming and Qing dynasties significantly influenced the spread of smoke. However, the existing fire detection systems used in ancient structures lack sufficient consideration of the impact of roof architecture on the placement of smoke detectors, which makes it challenging to realize early fire detection effectively and accurately. Therefore, further research on the response time of fire detectors considering purlin height is necessary to strengthen fire prevention and control in ancient buildings.

Through the job-site survey of several gable-roofed buildings in the Forbidden City, the existing smoke detectors were found to be primarily installed on the purlins below the ridge or both sides of the ceiling. However, the scientific basis for these installation positions remains unclear. Thus, this study selected typical gable-roofed buildings with a slope of 27.41° constructed during the Ming and Qing Dynasties as the research object. Fire dynamics simulator was utilized for numerical fire simulation to analyze smoke movement in the different fire source positions (center, edge, and corner) and various purlin heights (low: 10-20 cm; middle: 30-40 cm; high: 50-60 cm). The research further explored the influence of smoke detector response performance on each fire position and purlin height by setting up a 9 × 11 smoke detector array and several slices of smoke mass fraction. In this model, as the purlin height increased, the detectors arranged under them maintained their x and y coordinates, while the z coordinate moved down the corresponding height. The position of the detectors on the ceiling remained unchanged.

The findings showed the following: 1) The variation law of the smoke propagation path: In the scenarios of central and edge fires, smoke primarily spread along the horizontal direction of the main ridge. In those of corner fire, when the purlin height was low, the smoke tended to expand upward along the sloping roof; when the purlin height was high, the smoke propagated along the purlin in a "stepped" path. 2) The response time sequence of smoke detectors: For the center and edge fires, the differences in response time of smoke detectors affected by purlin height was approximately 30 s. For the corner fire, when the purlin height was below 20 cm, the detector at the main ridge responded within 60 s, and the variation in the response time of smoke detectors affected by the purlin height can reach up to 45 s. 3) The locations suggested for detector installation: When the purlin height is below 30 cm, the smoke detector should be installed at the center of the main ridge; when the purlin height is above 30 cm, the detectors should be placed on the ceiling near the center of the main ridge on both sides.

These findings provide technical support for the rational placement of fire detectors in similar "purlin style" buildings from the Ming and Qing dynasties to achieve comprehensive and timely early detection and warning of fire prevention in ancient buildings.