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Jennifer Schuler

November 16, 2019 22:46

Hi, attached is a snapshot from an old NCARB 4.0 Structures practice exam. The answer is 4.2, does anyone know how to solve this?

Thx. 

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  rvgbrusa November 30, 2019 18:00

  Jennifer

  Let see if I can explain it:

  Very complex calculation and it might show on the PPD test:

  step 1: convert 150lb/ft3 to kip/ft3

              150/1000 = 0.15 kip/ft3

  step 2: weight of wall = (width x height x depth) x 0.15 kip/ft3

              (16 x 40 x 1.5) x 0.15 = 144 k

  step 3: weight of footing = (20 x 6 x 2) x 0.15 = 36

  step 4: distance to pivot point A (lower point at bottom right of intersection of the footing) = (20'/2) - 10'

  step 5: SM (stabilizing moment) = DL+ weight of wall + weight of footing x distance

              SM = (240 + 144 + 36) x 10' = 4200 kip-ft

  step 6: OM (overturning moment) = F x D = 24 x 42 = 1008 kip/ft

  step 7: factor of safety FS = SM/OM

                                        FS = 4200/1008 = 4.16 = 4.2 (round up)

  I hope it is right.

  If so, I hope it helps.

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  Susan Arnold March 14, 2020 13:41

  Thanks Ronaldo! I worked through it separately so it's great to see confirmation that you went through a similar process. Basically figuring out the 3 loads and their respective moment arms around the pivot point on lower right corner. The 3 loads being the dead load, the weight of the concrete (acting at same location as the dead load), and the wind load.

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