WHERE WE ASSUME THE POSITION OF NEUTRAL AXIS IN CASE OF T-BEAM and WHY

[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.

 

[Hariharan]

You would normally not want to have tension in concrete. So
start with neutral axis at the top or bottom of the beam
depending on whether you are checking the unloaded or loaded
condition. Mind you, for a single beam, several stages are to be
checked before you finalise the design

Any design text book will give you the procedure for design,
where to start and how to iterate

Hariharan

 

[Polecat]

If you are using ASD stress method, then the neutral axis will be defined by the geometry of the T-section and the transformed area of the steel. From there, you would get the upper and lower section modulus and determine the eccentricity of the steel pattern. From that point on, it's just P/A +/- Pe/S +/- M/S to determine the stresses.

However, if you are using the LRFD method and want to compute the nominal strength of the section (ultimate failure mode), then I would refer you to the PCI Design Handbook which contains several design aids that make life much easier than trying to assume a neutral axis and going thru a long iteration process (unless, of course, you have access to some very sophisticated software).

http://www.spiraleng.com

If you wish to visit my website, we can discuss these or other issues further.

 

[ishvaaag]

Sophisticated may not be but free is one sheet that I have for free download that optimizes the design of Ts of complex form to specified dimension limits for simple spans. You can download it from the Collaboratory for Mathcad 2000 Pro.

 

[ishvaaag]

Yet the question maybe related to what happens in a prestressed column: when you compress it, the compressive stress is increased, and some of the tensile stress in the prestressing steel, lost. So it is clear that once the transfer has taken place (and assuming grouted tendon or adhered) the whole section, steels included needs be taken for the computation of the center of gravity of the shape, to serve in the elastic calculation of the member.

Before prestress and its grouting, only the passive rebar and concrete section is usually taken unto account for finding center of gravity of the section, the acting prestress being if wanted substituted bu equivalent forces to act unto such pre-existing shape.

Postensioning in a non grouted duct is akin to this, but the integrity of the section can't be assumed. SO for this case again, only passive rebar plus concrete would give the center of gravity, the prestress being substituted by its equivalent forces...

what brings us to the conclusion of that to consider the prestressing steel part of the section to the effect of the determination of mechanical properties to be used in elastic analyisis of the section and member, it is extant bonding what determines its inclusion.

The non bonded cases, to be treated as forces acting on what bonded.