Question

You have researched skylights for use in a factory building. The research indicates that skylights can produce design illuminance within the building when the exterior illuminance reaches 4000 footcandles. The factory building measures 160-ft wide x 250-ft long. You require fifty footcandles as design illuminance. The size of the skylights will be 4-ft. wide x 8-ft. long. When new, it is estimated that each skylight will transmit about 50% of the light that hits it when the skylight is new. But the skylights get dirty so because of this, 35% of the admitted light will be further lost due to dirt buildup. Based on this given information, how many skylights will be necessary to accomplish the daylighting mode for this building?  

Answer 1

assume that skylights get dirty after some time. therefore we will consider the worst case, i.e. dirty skylights.

transmittance by skylights=50%[new]

transmittance by skylights[dirty]=32.5%..........................{35% of 50% is 17.5%. Added to previous 50% gives 67.5% stopping of light. 100-67.5=32.5% is transmittance}

now, external luminance is 4000 fc................fc=footcandles

area of skylight=8*4=32 sqft.................................sqft=square feet

incident fc on skylight is proportional to its area, viz 32 fc

thus, fc passing through one skylight= 32.5% of incident fc= 32.5%*32fc= 10.4fc

thus, each dirty skylight passes only 10.4 fc of illuminance.

now, total illuminance required inside factory building= 4000fc.

on dividing 4000 by 10.4, we get 385 {approximately}.

Thus, a total of 385 skylights are required for the purpose to be solved.

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