NCEES practice test problem 508

[njames024]

508) A construction crew of six can form 48 ft2 of contact area per crew-hour when working at less than 8 ft above grade. At more than 8 ft above grade, the production rate drops 40%. Assuming a crew cost of $372/hr, the labor cost to form walls for a 40 ft long x 90 ft wide x 14 ft tall free standing tank is most nearly:

Solution:

Bottom 8 ft = (40+90)(2)(8)(2) = 4160 ft2 / 48 ft2 / crew-hour = 86.67 crew-hours

Top 6 ft = (40+90)(2)(6)(2) = 3120 ft2 / (48)(0.6) ft2 / crew-hour = 108.33 crew-hours

195 crew hours * $372/crew-hour = $72,540

My question: Why did they multiply the total perimeter [(40+90)(2)] by 2 again?

They did it in both the bottom 8 ft and top 6 ft. I'm sure it's something simple but I can't seem to figure it out. Maybe because it's a free standing tank that has something to do with it? I've never encountered something like this before. Thanks for the help!

Nick

 

[rncole P.E.]

Yes, it's because it's a tank. You have to form both sides of the wall - since they didn't give you a thickness it just assumes equal area inside and out (conservative). Otherwise you'd have a huge pedestal.

 

[njames024]

makes sense. Thanks!