Question

A beam having b=12in, d=21.5in and h=24in is reinforced with three No.11 (No.36) bars. Material strengths are fy=60,000 psi and fc'=4,000 psi. It is used on a 28 ft simple span to carry a total service load of 2430 lb/ft. For this memeber the sustainted loads include self-weight of the beam plus additional superimposed dead load of 510 lb/ft, plus 400 lb/ft representing that part of the live load that acts more or less continuously, such as furniture, equipment, and time-average occupancy load. The remaining 1220 lb/ft live load consists of short-duration loads, such as the brief peak load in the corridors of an office building at the end of a workday.

(a) Find the increment of deflection under sustained loads due to creep

(b) Find the additional deflection increment due to the intermittent part of the live load.

In your calculations, you may assume that the peak load is applied almost immediately after the building is placed in service, then reapplied intermittently. Compare with ACI Code limits from Table 7.2. Assume that, for this long-span floor beam, construction details are provided that will avoid damage to supported elements due to defelections. If ACI Code limitations are not met, what changes would you recommend to improve the design?

Answer 1