ACI Prestressed flexural member deflection 1

[ramihabchi]

Hi,
A book by Edward G.Nawy gives limits to tensil stress at service level of 6 square root of f'c.in the ACI code there are 3 classes U T and C.obviously the 3 classes allows to exceed the limit given by Edward except for two way slabs.
Also Edward Nawy gives that the limit could be increased to 12 square root f'c if deflection is computed and is less than that allowable by ACI.
Are the limits given by Edward Nawy used to not calculate deflection?if not what is the purpose of them?and is there a way to avoid deflection calculation of PTmembers?

 

[BAretired]

Are the limits given by Edward Nawy used to not calculate deflection?if not what is the purpose of them?and is there a way to avoid deflection calculation of PTmembers?

That may be the purpose...who knows without reading his book?
The way to avoid deflection calculation is to observe the limits in the code.


BA

 

[Agent666]

The deflection limits only apply to uncracked members. With PT I'm assuming you need to show you have tensile stresses lower than the U limit to be able to use the gross section properties for any deflection checks. Since this is dependent on the level of prestress you can't break it down to a span/depth ratio like a conventionally reinforced slab limit for example.

Basically you should be evaluating the stresses, keeping in mind as well that due to the tendon drape your max stress sometimes isn't where you might intuitively think it is (you'll look at it at multiple stations in an indeterminate structure for example).

The rule you are noting is probably just a limit that's been found to be acceptable in practice, best practice if you like for practical arrangements of prestress.

 

[TehMightyEngineer]

Sounds like the author is just making a recommendation that you keep tensile stresses below 6SQRT(f'c) to avoid cracked section calculations. Antoine Naaman has a similar recommendation in his book:

In short, unless the designer is willing to carry out a cracked section analysis, the author favors keeping the tensile stress in the precomressed tensile zone smaller than or equal to the limit of 6SQRT(f'c) as recommended in prior versions of the code.

His rational for 6SQRT(f'c) instead of the 7.5SQRT(f'c) now in the ACI code is that testing has shown that some partially prestressed concrete beams would have cracking ranging from 5SQRT(f'c) and up, especially in hollow cored slabs and inverted T beams.

If you have an uncracked section than your deflection calculations become relatively trivial and thus it's easier to design as uncracked. If you have a cracked section and do an analysis using effective moment of inertia then you can more effectively utilize the 6 to 12SQRT(f'c) tensile stress range. I suspect this is the intent of your authors comments.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL, CO) Structural Engineer (IL, HI)

 

[cooperDBM]

It seems that the book is written for ACI 318-99 or earlier which set those tensile limits (based on an uncracked section) in service. The higher "limit" recognizes that the section is cracked and that the beam deflection needs to satisfy the code limit. The deflection calculation would be based on the bilinear method.

Newer versions of the code don't define tensile "limits" in service. They classify the section (U, T & C) in order to give specific requirements for each class, or each level of cracking, such as crack and deflection control (ACI 318-14 table R24.5.2.1). The 12 sqrt(f'c) "limit" is now just the point where the section is considered fully cracked or class C. The newer classification gets away from the old distinction between fully and partial prestressed sections.

 

[ramihabchi]

thank you for your answers.
could the balanced load method be used to calculate deflection of prestressed elements?