Sliding Door Security Bars: The Dowel Diameter at Which a Timber Bar Stops Buckling Under a Forcing Load

A length of timber dropped into the track of a sliding patio door is among the cheapest security devices sold, and among the least specified: dowels are bought by eye, in diameters that vary by a factor of two. This study computes the buckling load of a 0.9 m pine dowel from the Euler relation with a stated elastic modulus of 9 GPa, and compares it with a stated forcing load of 2,600 N at which the door's rollers and track begin to deform. A 25 mm dowel buckles at 2103 N, below the forcing load, so the door's own hardware is not the weakest element; the bar is. The buckling load reaches 2,600 N at a diameter of 26.4 mm, and a 32 mm dowel carries 5645 N at a stated price of USD 12.96. Because buckling load grows with the fourth power of diameter, small increases in thickness are disproportionately effective, and the rule is to fit a bar of at least 30 mm diameter, cut to a close fit in the track so that it is loaded axially rather than bent. SIMULATED, NOT MEASURED.