ASSUMING POSITION OF NETURAL AXIS IN DESIGNING THE PRE STRESS T-BEAM

[nedians]

I want to know when ever we start the design of a post-tension or pre tension T-beam where we assume the position of neutral axis , in the flange or in the web and why ??
Is there any other way of starting the design beside assuming the position of netural axis.
I know from my statics and mechanics of materials courses. Wherever the neutral axis is located, the compression on one side must be exactly balanced by the tension on the other. If you assume a n.a. location and sum the forces, it should be obvious whether you've got the right location or it needs to be adjusted.
But where should we assume in case of pre stress T-beams and why ? in one book i saw that it was written that in case of pre stress it should alway be in the flange but i don't know the reason

 

[Qshake]

It is not necessary that the na fall in the flange, in fact, many cross-sections don't have a flange to make that distinction.

 

[Qshake]

Where was I...Oh yea, depending on the initial and final conditions, its quite possible that at some point in the loading you won't even have a tensile stress to worry about. However, I am sure that the whole reason behind the na falling in near the bottom (or top, depending on the support conditions) is that concrete is allowed only a small tensile stress. In order to achieve that it will be necessary to have a proportionately larger compressive stress at the opposite end. In doing so, the na will naturally favor the end of the smaller stress. Assuming, of course, that the stress distribution is linear.

For more information on the formulas necessary to calculate the section properties transformed or not...see Nawy's text or Mitchell and Collins.