39. What is the MAXIMUM allowable span for a 2 x 8 #2 spruce-pine-fir rafter at 16 inches oc? The roof live load is 20psf and dead load is 10psf. Assume the ceiling joist are not attached to the rafters.
Answer: A
The maximum allowable span for a 2 x 8 #2 spruce-pine-fir rafter at 16 inches oc is 13 feet 9 inches.
This span is determined based on the specified roof live load of 20 psf and dead load of 10 psf, ensuring the structural integrity of the rafter within these loading conditions.
A) 13 feet 9 inches.
This option correctly represents the maximum allowable span for the specified rafter size and loading conditions. According to building codes and engineering principles, a 2 x 8 #2 spruce-pine-fir rafter can safely span 13 feet 9 inches when subjected to the given live and dead loads.
B) 18 feet 2 inches.
This option is incorrect because it exceeds the maximum allowable span for a 2 x 8 #2 spruce-pine-fir rafter under the specified loading conditions. Such a span would not provide adequate structural support and could lead to failure or excessive deflection.
C) 19 feet 5 inches.
This choice is also incorrect as it significantly surpasses the maximum span limit for the rafter in question. Relying on this span could endanger the structural performance of the roof, compromising safety and stability.
D) 21 feet.
This option is incorrect since it is well beyond the maximum allowable span for the rafter type and loading situation. A span of 21 feet would not be feasible, resulting in potential structural issues that could arise from insufficient support.
Conclusion
The correct answer, 13 feet 9 inches, is definitively the maximum allowable span for a 2 x 8 #2 spruce-pine-fir rafter under the specified loads, ensuring safety and structural integrity. All other options fail to meet the necessary criteria for load-bearing, thus highlighting the importance of adhering to established building standards.