# Composite section properties for bridge question



## buening (Oct 19, 2008)

I've been tearing through LRFD bridge design examples like a madman lately, and this simple thing has been bugging me. On page 3-17 of the Steel Girder Design of the FHWA LRFD design example (Design Example), they go through and calculate the moment of inertia for the girder only, 3n, and n. I cannot figure out how they got the "Centroid, d" for the slab in either of the 3n or n.

The bottom flange thickness in the positive moment region is 0.875", web height is 54", top flange thickness is 0.75", haunch is 3.5", and effective slab thickness is 8.0"(according to page 3-7). I believe the centroid to the deck should be all of those added together (using half effective slab thickness). Doing this, d = 0.875 + 54 + 0.75 + 3.5 + 8.0/2 = 63.125" In the table, they show 62.375". Any idea what they are doing differently, or what I may be doing wrong? I thought maybe they assumed the top of the deck was cracked and used the depth to the top bars. The clear distance from the top of the deck is 2.5". Using that depth, I get a d = 61.875 using an effective slab depth of 5.5. In order to get the 62.375" from their table, their effective slab depth has to be 6.5". I haven't thrown out the idea of it being an error, but I have the latest version using the new 4th edition of AASHTO and it shows the same centroid depth....not that the error couldn't have gone unnoticed.


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## buening (Oct 19, 2008)

To further my confusion, at the bottom of the same section properties table they calculate the Ytopslab. They show it to be 24.992 for the Composite 3n. Subtracting the Ytopgdr from the Ytopslab, you get 10.75". This includes the haunch, so subtracting the 3.5" haunch gives the distance of 7.25" from the haunch to the top of the slab. Consistency is a beautiful thing :smileyballs:


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## buening (Oct 20, 2008)

I figured it out. The 3.5" haunch thickness is measured from the top of the web, whereas I assumed it was 3.5" above the top flange. It must be the old school way of embedding the top flange into the haunch (not used much nowadays).


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