Bridge Design Question - Kg Factor

[2457F]

In the 2012 AASHTO LRFD Bridge Design manual on page 4-32 there are several variables required to solve for the longitudinal stiffness factor (Kg). Two of those parameters are A and I.

Just underneath the formula for Kg there is a note that says the following, "The parameters A and I shall be taken as those of the noncomposite beam".

I've seen a design example of a composite section (steel beam and concrete deck) where the A and I where taken just of the steel beam as is per the code, but what if you have a monolithic concrete tee-beam? Do you then just take the A and I of the just the stem of the tee-beam or of the entire member cross section?

Any help would be greatly appreciated.

 

[jayrod12]

Let me preface this by saying I haven't done any bridge design or looked at the AASHTO manual since school. But if you were pouring the stem and tee at the same time (monolithic like you noted) then it wouldn't be a composite section in my mind and you would use the entire cross section in those calculations.

However if the tee portion were being designed as a separate structural entity then I would consider only the stem portion.

 

[2457F]

Thanks. It makes sense to me to treat it as a single unit, not sure why they only consider the beam only in a composite section versus using the entire section (beam and slab).

 

[jayrod12]

I'm inclined to think it's due to the high modulus difference between the two materials in question. The steel would take 90% of the load because its modulus is almost 10 times that of the concrete. But that's just a thought.

 

[2457F]

Thanks again.