ICC Commercial Mechanical Practice Exam

If a 2x12 nominal ceiling joist is 12 feet long, what is the maximum allowable diameter of bored holes in inches to accommodate an air duct?

1. 3.75 inches

2. 4.75 inches

3. 5.75 inches

4. 6.75 inches

The correct answer is: 3.75 inches

The maximum allowable diameter of bored holes in structural members like ceiling joists is determined by the need to maintain the structural integrity of the wood. According to building codes and engineering guidelines, bored holes should not exceed certain percentages of the depth of the joist, particularly when it comes to maintaining load-carrying capacity. For a 2x12 joist, the actual dimensions are typically around 1.5 inches by 11.25 inches. The depth in this case is 11.25 inches. The code usually allows a bore hole to be up to 1/3 the depth for holes that are bored through the length of the member. Calculating 1/3 of the depth of the joist gives approximately 3.75 inches (11.25 inches / 3 = 3.75 inches). This means that a bored hole up to this diameter would generally not compromise the strength of the joist significantly. Therefore, the maximum allowable diameter of bored holes to accommodate ductwork would be 3.75 inches, making that the correct choice. The other options exceed this limit and could potentially weaken the structural integrity of the joist, which is why they are not appropriate in this context.