'Prestressed' Concrete Deep Beam

[KimWT]

Hi!

My questions are about R/C deep beam with 'pre-tentioning.'

ACI318-08 says:
"The longitudinal reinforcement ...should be anchored by embedment, hooks, or welding to special devices."

1. How can I anchor pre-tensioning strands at the end of the deep beam? (Not post-tensioning strands)
2. ACI also says "Bent-up bars are not recommended." What does this article mean?
Why not bent-up bars are not recommended though books are allowed?
3. I have seen many papers, articles and examples about Strut-Tie Model with non-prestressed concrete beams.
If you know any paper or article about the deep beam design using pre-tensioning (not about bursting force at the end region),
please let me know.

I am looking forward to precious comments.

Thanks!

WT

 

[mike20793]

I'll give this one a shot:

1. The strands are developed through bonding with the concrete. A form is made, the strands are stressed, then the concrete is poured. Once the concrete cures, the strands are cut.
2. I'm not sure, Maybe the bent bars have to do with the compression strut forming at the end of the beam; you're potentially trying to anchor into concrete that could potentially be in tension (because of the prestress force).
3. If you find a paper, let me know. We have been searching for some good examples to no avail. The strut and tie concept is the same, you just have to account for the effect of the prestress force in the free body diagrams, which gets tricky when you have harped/debonded tendons. Straight tendons might not be so bad.

 

[Ingenuity]

Go to ACI's

http://www.concrete.org

website and search for this paper from the Structural Journal:

Strut-Tie Shear Design of Pretensioned Concrete

Author(s): Julio A. Ramirez

Publication: Structural Journal

Volume: 91

Issue: 5

Appears on pages(s): 572-578

Keywords: beams (supports); detailing; models; prestressed concrete; shear properties; ties (reinforcement); trusses; web reinforcement; Structural Research

Date: 9/1/1994

Abstract:
This paper presents guidelines for the strut-tie design of pretensioned concrete members. The design of a pretensioned single-tee with draped strands is illustrated using a full member strut-tie approach. The results of the full member strut-tie design are compared with those of current ACI section-by-section design recommendations.

 

[Ingenuity]

...and this one too, with emphasis on pretensioned deep beam:

Title: Strut-Tie Approach in Pretensioned Deep Beams

Author(s): A. Alshegeir and J. A. Ramirez

Publication: Structural Journal

Volume: 89

Issue: 3

Appears on pages(s): 296-304

Keywords: concretes; deep beams; detailing; prestressed concrete beams; shear properties; strut-tie model; Design

Date: 5/1/1992

Abstract:
Presents an evaluation of the strength and behavior of prestressed concrete deep beams using the strut-tie method. Strut-tie systems reflecting actual support and loading conditions are developed for three pretensioned deep beams tested to failure. The strut-tie approach is used to illustrate the effects of prestressing, concrete compressive strength, and reinforcement detailing on the behavior and strength of these members.

 

[rapt]

I do not know of any papers, but a simply supported deep beam require the tension tie to develop past the compression strut at the support point.

Bent up bars cannot achieve this as they do not intersect the compression strut at the bottom at the support.

Pre-tensioned strands cannot achieve this unless the strands extend past the support/intersection point. Pre-tensioned strands provide no tension at their ends and for a short distance from the end (about 100mm) and then gradually build up tension over a length of about 1m at stressing time (the transmission length). No more stress is assumed to build up in the strands over this length under load. Then under load the development length increases to about 2-3m to build up the full ultimate capacity of the strands.

So unless the pre-tensioned strands extend well past the junction of the compression strut and the tension tie, it is impossible to develop sufficient force in the tension tie.