11 A post-tensioned prestressed concrete roof deck is to be designed using 8-ft width single-T beams spanning 70 ft between simple supports. Service loads include live load of 704 plf, superimposed dead load of 80 plf, and estimated self-weight of 600 plf. Normal weight concrete is used with specified compressive strength of 5000 psi and strength at time of transfer of 3500 psi. The prestressing force will be provided using 1/2-in. diameter Grade 270 low relaxation strands with fpu=270 ksi and fpy=243 ksi and maximum eccentricity at the midspan. The prestress residual ratio can be assumed to be 0.85. If a section is selected as shown below, and the designer provided the beam with sixteen strands, the beam is: M (2 points) 96 3.0 1.5 3 1. = 68917 in A. = 570 in2 C = 9.99 in y = 26.01 in S. = 6899 in Sy = 2650 in 3 72 = 120.9 ir? e = 22.51 in WD fpi is assumed to be 0.7 fou 36" 594 plf ok The maximum eccentricity has to be reduced The minimum eccentricity has to be increased Not satisfactory and larger section should be used The number of strands has to be reduced Satisfactory and can be used A constant eccentricity has to be used