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EX4.4 - CEE 415 Example 4.4 Design a doubly reinforced...

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CEE 415 1 Example 4.4 – Design a doubly reinforced section for negative bending moment in the indicated girder at the face of the first interior column of the floor plan shown on the last page. We cannot use the ACI moment coefficients for this case because of the concentrated loads applied to this girder from the floor beams. For an initial calculation of the weight of the girder (web), assume it has a depth of 26 in. and a width of 14 in. Calculations indicate that the reduced live load for this design section is 30 psf. Assume f ´ c = 4000 psi and f y = 60 ksi. From RISA analysis, M u = 610 k-ft = 7320 k-in. Start design process with 1 3 0.0189 0.019 c y f f ρ β = Then, ω = ρ (f y /f ´ c ) = 0.285 and, R = ω f c ’ (1 - 0.59 ω ) = 0.948 ksi 0.95 ksi Now, assuming φ = 0.9, the required value of b d 2 = (M u / φ )/R = 8560 in. 3 Select b 0.6 d, so d [8560/0.60] 0.33 = 24.3 in. (round to 23.5 in.) ( h = 26 in.) Then, with d = 23.5 in, b 8560/(23.5) 2 = 15.4 in. (round to 16 in.) No need to adjust M u .
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  • Spring '11
  • Wight
  • negative bending moment, doubly reinforced section, psf Slab thickness, Start design process, tension zone steel

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