huangyhg

centroid to bridge deck
this should be simple, but i'm having some troubles. looking through the fhwa lrfd steel bridge design example
i only took a quick look at the example so i can't be positive with my response. the centroid of the deck would include the concrete haunch and the slab. this combination would have a lower centroid than the slab alone (which is what you're calculating).
they may also have included the transformed area of the longitudinal rebar in with the deck. the centroid of this steel is a bit lower that the centroid of the concrete slab. if this is the case then it would also show up in the area (i didn't check).
good luck with the exam.
sorry, since the properties are in steel units the rebar area wouldn't be transformed.

they do consider the steel in the negative moment region, but it it separated from the deck in the table. i wondered about the haunch too, until i read this excerpt before the section properties table:
"for this design example, the slab haunch is 3.5 inches throughout the length of the bridge. that is, the bottom of the slab is located 3.5 inches above the top of the web. for this design example, this distance is used in computing the location of the centroid of the slab. however, the area of the haunch is not considered in the section properties.
some states and agencies assume that the slab haunch is zero when computing the section properties."
i had a closer look and noted that the haunch thickness is measured from the top of the web (see tip - slab haunch) on the previous page. you had measured the haunch from the top of the top flange. that would give you the d of 62.375, ignoring the rebar and haunch area.
doh! thanks for the heads up. i knew it was something simple!!

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