What is a proper ways to calculate rebar?

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I want to ask if I should resolve the problem with calculating rebar count the way it shows in Construction Depth Practice Exam by Beth Lin Hartmann (question #5) or I should use my field experience?



Question: 30 ft long precast concrete beam are to be installed. Clear cover on all rebar faces is 3”. What is the total transverse #6 rebar count if spacing is 3 ft o.c. ?



Book Answer: Calculate the quantity #6 transverse bars.

N spaces= (30’-3”/12’-3”/12’)/3’=9.83 spaces

Q bar =whole number of spaces +1 = 9 + 1 = 10 bars



From my field experience the answer should be: 30/3=10 +1 =11 bars

Please, let me know which answer is correct. Thanks.

 

[Structural_Ninja]

You are correct. Nos. of rebars will be always 1 more than the spaces. So if they provide 10 bars, the spacing will be 30-0.5 = 29.5'/9 = 3.28' o.c. which is > 3' and will not work. So I think correct answer is 11 bars with 10 equal spaces. The actual spacing in this case will end up being 2.95' o.c.

 

[John QPE]

Always err on the side of adding more rebar.

So I did 30 ft - 0.5 ft = 29.5 feet (the area with bars)

29.5 ft / 3ft = 9.83 spaces

9.83 + 1 = 10.83 bars or 11 bars