POST-TENSIONED AS SMRF SYSTEM-MAXIMUM PRECOMPRESSION(P/A)

[eljineer]

As per code:

(a) The average prestress fpc calculated for an area equal to the least cross-sectional dimension of the beam multiplied by the perpendicular cross-sectional dimension shall not exceed the lesser of 500 psi (3.5Mpa) and fc′/10.

Some saying that this is not mandated by the code and one of the concern in too much prestress is the effect of support restrain, but I still not get a good reference to prove this. I hope someone can enlighten me regarding to this problem.

Best regards.

 

[rapt]

A clause number might make it easier for people! There are a lot of (a)'s in the code!

It may also be an indirect method of controlling ductility. 3.5MPa would normally result in an over reinforced section.

 

[eljineer]

Hi rapt,

Sorry for forgetting to write the exact code.

The Code is ACI 318-14:

Section 18.6.3.5 Unless used in a special moment frame as permitted
by 18.9.2.3, prestressing shall satisfy (a) through (d):
(a) The average prestress fpc calculated for an area equal to
the least cross-sectional dimension of the beam multiplied
by the perpendicular cross-sectional dimension shall not
exceed the lesser of 500 psi and fc′/10

I have been researching about the maximum precompression on the post-tensioning system a long ago and it's hard to get a reference or any guidelines until I found out that this section only released at the ACI 2008 Edition with the reference of:

21.15. Joint ACI-ASCE Committee 423, “Recommendations for
Concrete Members Prestressed with Unbonded Tendons (ACI
423.3R-05),” American Concrete Institute, Farmington Hills, MI,
2005, 21 pp.


My question is that the section stated above is only applying for the unbonded post-tensioning system? Because on my case, I have bonded post-tensioned members and part of smrf system which have a high average prestress to satisfied the tensile stress limit prescribed on the ACI.

Sorry, I'm a newbie in post-tensioning and as a young engineer, I'm trying to study and seeking some tips from the experts like you for my development.

Best regards and have a good day,

eljin

 

[rapt]

18.6.3.5 applies to bonded and unbonded PT.

For bonded PT, it requires that the tendons be unbonded over the length of the plastic hinge zones.

I would assume that this arrangement is impractical to achieve, and ACI does not allow fully bonded tendons in this situation.

So you cannot use bonded PT in this situation according to ACI.

 

[eljineer]

rapt,

On my point of view, previous and recent ACI 318 code do not explicitly provide restrictions or prohibitions to the use of Bonded Tendons for Special Moment Resisting Frames (SMRF).

Also on their commentary said that "these provisions were developed, in part, based on observations of building performance in earthquakes(ACI 423.3)" which is for Un-bonded tendon application.

So Im trying to find out behind this limitation like thinking outside the box or the code.

 

[rapt]

Clause 18.6.,3.5 is pretty specific.

(a) has the P/A limitation you mentioned in your first posting.

and (b) requires that bonded tendons have the length of the tendon over the plastic hinge zone be unbonded.

I do not know how more specific they can be.