Bar Reinforcement for Soil Anchors 1

[Obregon]

Hi everyone. I was just wondering how to explain to a client, in a convincing and irrefutable way, why he should not employ standard reinforcing bars, grade 60, 75, in prestressed ground anchors. I know one of the main issues has to be with relaxation of the tendon, but the question that arises is:

Let's say you are to install a 9 m long ground anchor, in which the bond (fixed) length is 4.5 m and the free length is also 4.5 m, with de anchor having to be prestressed to 100 % of its design capacity, i.e. 30 tons.

In real, objective and numerical terms, what would be the restrictions, disadvantages, difference, of using a 51 mm grade 75 dywidag bar instead of a couple 0.6" diameter prestressing strands?

Best regards.

 

[ATSE]

If prestress is a design requirement, consider A722 (type II) threaded rods, with supplementary bend testing, as an alternate. Proof tested ("cold-stressing") and stress relieved.
Regarding your explanation to the client, note that Williams and Dywidag will tell you not to prestress 75 ksi rods, and also that there is no national code guidance on how to prestress mild steel. Those seem like good enough reasons.
If you are using passive anchors, 75 ksi works fine.

 

[Obregon]

Thank you for your reply

 

[PEinc]

Attached is a selection guide from Williams Form Engineering. The guide does not substantiate what ATSE said.


www.PeirceEngineering.com

 

[Obregon]

Thank you PEinc. As I can observe, according to Williams Form, it is fine to employ a 75 Grade bar and prestress it in a 30 ton - ground anchor.

 

[PEinc]

The Post-Tensioning Institute's (PTI) Recommendations for Prestressed Rock and Soil Anchors calls for ASTM A722 150 ksi threaded bar tendons or ASTM A412 270 ksi strand tendons. However, ASTM A615 60 ksi, 75 ksi, and 80 ksi bars are used for usually lower capacity tieback anchors and for soil nails and micropiles.

The maximum working load and the maximum testing load of the A722 bar and A412 strand tendons are based on percentages of their guaranteed ultimate stress (GUTS) while the lower strength A615 bar tendons are usually based on similar percentages of their yield stress (F_y).

http://www.PeirceEngineering.com

 

[ATSE]

PEinc knows better for sure, so I will defer to him.
However, without digging in too deeply, I will stand by my two statements.
In the past year, when I has asked the technical staff at WF (in Colorado) and Dywidag specifically about using any level of prestress on 75 ksi bars, they said, "Technically you can, but instead please consider 90ksi or A722-150 for any prestress." So they were saying, "yes, we show it, but that doesn't mean it is a good idea"
And if there is national code guidance, I would be happy to see it - please post it.
Low prestress (where relaxation is not a design limit state) would indeed be a useful tool for 75 ksi rods / anchors - I would like to see code guidance; if not, even a published design guide would be a good reference.

 

[PEinc]

ATSE has provided good comments. PTI, FHWA, and AASHTO describe tieback tendons as A722 prestressing bars and A412 prestressing strands. Neither really addresses A615 bars. In my experience, A615 bars are infrequently used (i.e. when the anchor design loads are relatively low) except when the ground anchors are rock bolts or soil nails.

http://www.PeirceEngineering.com

 

[Obregon]

Thank you much to both of you for sharing your extertise, I appreciate it.

 

[ATSE]

Just a friendly reminder to Obregon that double posting (same question, different forums) is bad practice.

 

[Obregon]

I apologize for that. Regards.

Carlos R. Obregón

 

[Obregon]

http://www.sueloanclado.com

 

[Obregon]

Good day to all. I want to share these stress-strain-relaxation graphics for cold drawn vs stress relieved Dywidag bars. As my understanding, the main issue of relaxation-lost of prestressing load, remains as the principal factor to consider when a ground anchor is to be prestressed above, say, 30 tons, and it becomes very convinient to employ a high capacity-prestressing bar like a 150 ksi bar. Comments are very welcome.

 

[Obregon]

Here the other graphic: