In the third edition of the PPI practice exam problem 21 states that the PNA for a bridge deck is 6.35" below top of concrete, in the solution, the author uses the full height of the slab for the compression area. Why is that done? Should 6.35" of the slab be used? Or better yet, shouldn't 0.85*PNA be used to approximate the depth of the Whitney stress block?
PPI Practice Exam Buildings Question
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If you sum moments about the section, given the location of the PNA and using the T-C force = 2665 kips, you will arrive at Mp = 5826 k-ft, which is what the correct answer is. You could solve it that way as well.
But the portion of the Mp equation in App. D6 for the slab compression is multiplied by Y^2 has the 2*ts term in the denominator. If you do the algebra you will find that it resolves out to the compression force of the slab above the PNA multiplied by the distance of that force to the PNA, which is correct as well.
But the portion of the Mp equation in App. D6 for the slab compression is multiplied by Y^2 has the 2*ts term in the denominator. If you do the algebra you will find that it resolves out to the compression force of the slab above the PNA multiplied by the distance of that force to the PNA, which is correct as well.
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