The article "Direct Strut-and-Tie Model for Prestressed Deep Beams" presents
measurements of the nominal shear strength (in kN) for a sample of 15
prestressed concrete beams. The results are
580 400 428 825 850 875 920 550
575 750 636 360 590 735 950
Assume that on the basis of a very large number of previous measurements of
other beams, the population of shear strengths in known to be approximately
normal, with standard deviation S.D= 180.0 kN.
Find a 99% confidence interval for the mean shear strength.