A pre-tensioned beam is to carry a superimposed dead load of 600plf and service live load of…

1. A pre-tensioned beam is to carry a superimposed dead load of 600plf and service live load of 1,200 plf on a 55 ft simple span. A symmetrical I-section with b=0.5hwill be used. Flange thickness hr0.211 and web width bw=0.4h. The member will be prestressed using Grade 270, ‘A in diameter strands. Time dependent losses are estimated at 20% pf Pi. Normal density concrete will be used with fr5,000 psi and fice=3,000psi. a. Using straight strands, find the required concrete dimensions, prestress force, and eccentricity. Select an appropriate number of strands. b. Revise the design in part (a) using strands harped at the third points of the span, with eccentricity reduced to zero at the beam ends. c. Briefly comment on your results. In both cases make all necessary checks. The prestressed beam is designated as a Class U member (Re: ACI Table 24.5.2.1). Finally, determine nominal moment capacity, Mn, based on the ACI approximate formula for fps.

2. The T-beam shown below is to be used on an 80-ft roof span and must carry superimposed dead load of 20 psf and service live load of 50 psf. Using stress limitations in the unloaded and full service load stages, as imposed by the ACI Code, and assuming 22% loss of Pi, determine the best combination of prestress force and eccentricity at midspan. Strands will be harped at midspan. Maintain at least 3 in from the prestressing steel centroid to the bottom face of the concrete to insure adequate cover. Select the appropriate amount of 1/2 in diameter, Grade 270 strands. Finally, determine nominal moment capacity, Mn, based on the ACI approximate formula for fps. For the section shown, Ao= 782 in2, Ic=169,000 cr12.8 in, 02=35.2 in, wo=815 plf. Use eiG = 5 ksi and f’d = 3.5 ksi. Assume Class U designation.

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