PCA Beam output verification (T beam design)

[Satishpkd]

Hello all,

I am using a PCA_Beam program to design the 40’-0” Long beam resting on column at one end and another end is supported by shear wall. I am designing this beam as T-beam and trying to verify the program output. Here is my question:

The only trouble I have is verification of deflection; when it calculates the initial deflection in output file program writes the value of Ig (i.e. gross moment of inertia) which is giving me trouble.

When I define slab thickness equal to zero than it simply take the beam properties and calculates the I based on (bd3/12) formula. Now, when I assign slab thickness equal to 6”, the program gives the value completely different. I have done manual calculations for I value based on T-Beam Moment of Inertia formula which is not matching with the program output.
If someone knows how to verify the PCA beam moment of inertia calculations for T-beam, please advice.

Thank you in advance for your help.

 

[asixth]

The I value it should be using in the deflections calculations should be more towards I_cracked than I_gross. Depending on how heavily the beam is reinforced this may only be 10% to 60% of I_gross. Taking into consideration the slab to act as a flange (which you are entitled to do) it will considerably stiffen the beam and hence reduce your deflections.

I believe ACI318 uses the Bischoff forumla for the calculation of I_effective, I_efficitive is slightly greater than I_cracked to account for tension stiffening. I however ignore any contribution and use I_cr solely when it comes to deflection calculations.

 

[rowingengineer]

would be a good idea to get in contact with the software developers, and figure out if the program is doing what you want it too.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[Satishpkd]

Hello asixth,

Thanks for your reply. you are right that for deflection calculation we rely on Icrack. But, this program calculates the Ig, Icrack and Ieffective and display it in output file.

Now, as Icrack calculation depends upon the Ig, and to verify that it is calculating the moment of inertia correctly i am looking for details of Ig.

as the basic assumption for T-beam design is that neutral axis must fall in the web,i don't see that part in output file and therefore want to double check that deflection and design output is realistic.

Please advise your thoughts on this.

Thank you.

 

[asixth]

The I_gross of a solid T-section can be found in any mechanics textbook (I recommend Roarks formula formula for stress and strain).

Now, as Icrack calculation depends upon the Ig

This statement isn't true, when calculating the I_cracked it depends on the reinforcement and depth of the neutral axis of the cracked section. Any good concrete textbook will run through the calculations for I_cracked (I recommend MacGregor or Nillson). The calculations will need to be adjusted for a T-section as opposed to a rectangular section but the modifications shouldn't be too difficult.

I have the calculations programmed in a spreadsheet for rectangular sections but I haven't updated it to include T-sections.