Reinforcing Bars for Ground Anchors 2

[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.

 

[oldestguy]

Of course there is no such thing as corrosion potential on that site. Right?

 

[Obregon]

No. Corrosion is not an issue in this case.

 

[dik]

AS long as you can achieve the anchorage and there is no corrosion, there should be no difference. Corrosion can take many forms and as long as these are addressed. I've often used Dywidag threadbars for anchorage... other than the cost of the 'nuts' they are quite economical. If embedded in soil, I like HDG... may not be possible with strand.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ingenuity]

If you use grade 75 Dywidag bar and stress to 80% of MUTS that will be 60 ksi and a strain of 0.0020 (i.e. 60/29000). So over a free length of 4,500 mm that is 9 mm of elongation before any prestress losses.

Allow for some losses...and you have little remaining prestress over time!!!

Similar reason why Eugene Freyssinet 'discovered' that you cannot use regular grade rebar as prestressing reinforcement in concrete elements, and expect to have any remaining prestress after X years.

I would use grade 150 Dywidag PT bar - gain all the benefits of prestressing grade steel, with a robust product that works great in permanent ground anchors. Less net prestress losses compared to a grade 75 equal, and easier to corrosion protect and handle on a project site compared to 7-wire strand.

From a cost perspective, Grade 150 ksi Dywidag is more expensive than 7-wire strand, but NOT 2 x cost of grade 75 bar (for twice the bar capacity).

Grade 75 Dywidag would work in non-stressed (passive) anchors.

 

[dik]

Thanks...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[HTURKAK]

I agree with your comment a pink star for this explanation.. but not always true.. The regular grade rebar still used for pretensionning of PC elements and post tensionning of 'massive elements' such as pier caps. Regarding the use of ground anchors, the following picture ( snap from VSL Ground anchor systems ) is good guide for selection of regular grade rebar vs strand rebars.

One may look to the full doc. attached :

 

[Ingenuity]

The regular grade rebar still used for pretensionning of PC elements and post tensionning of 'massive elements' such as pier caps.

Are you referring to grade 75 ‘regular rebar’ stressed and working as prestressing reinforcement for such elements?

 

[HTURKAK]

No... the round bar .. to differentiate from strand . In my region, S 950/1050 (MPa) , (equivalent to grade 150 ) is used for prestressing . However, the use of S 42 ( grade 60) is a common practice for passive anchorage, soil nailing systems.

 

[Obregon]

I thank you all of you for your interesting insights. Best regards.

 

[Obregon]

http://www.sueloanclado.com

 

[Ingenuity]

No... the round bar .. to differentiate from strand . In my region, S 950/1050 (MPa) , (equivalent to grade 150 ) is used for prestressing . However, the use of S 42 ( grade 60) is a common practice for passive anchorage, soil nailing systems.

Thanks for clariying.

When you stated "regular grade rebar still used for pretensionning" I thought you were referring to 'regular grade' as in grade 60 or 75 rebar, but your reference to 'regular' is to high tensile round/threaded (grade 150) as compared to 7-wire strand. Thanks.

 

[Obregon]

HTURKAK STATED "...but not always true. The regular grade rebar still used for pretensionning of PC elements and post tensionning of 'massive elements'"

I believe he is actually referring, in his own words, to non-high capacity rebar ("REinforcing Bar), i.e. regular grade (60, 75) bars. Nothing wrong with rectifying, although employing a 75 grade bar is, in the opinion of manufacturers, still fine for prestressed ground anchors of a low working capacity, i.e. less than 25 ton. Best regards.

 

[HTURKAK]

Please do not interpreted my post as an attack, but your original post was ( 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,)

You did not mention the type and purpose of prestressed ground anchors ..


The type of anchor should be decided acc. to tolerance of the supported system to the anticipated deformations of soil with relaxation of the prestressing ..

You may choose regular grade (60, 75) REBAR for passive anchors or soil nailing system which these are IMO ,essentially slope protection measurement rather than retaining..

If anchorage system is for soil retaining of a deep excavation at a city center, you should prefer prestressed strand system.
For rock anchorage ( the loss of prestress will be limited ) , you may choose prestressed anchor bar with grade 150..

If the anchor system is for bouyancy ( against floating ) , IMO, prestressed micropiles
with grade 150 is better..

If your question is based on a specific case, please describe the specific problem or better a new pot at geotechnical engineereing group to get better responds..

 

[Obregon]

Good morning. I'm sorry for having upsetting you dear HTURKAK; I didn't mean it. By the way, soil nailing is a passive earth retention technique, and not really a slope protection method as you've mentioned.

Regarding the prestressing of grade-75 bars that you first suggested as reinforcement for ground anchors, having changed your mind later, these (the G75 Bars) can and are used safely as prestressing elements for ground anchors with up to 25 ton working load capacity. I hope you can feel better now. Best regards.

Carlos R. Obregón

http://www.sueloanclado.com

 

[Obregon]

Viva México!

 

[r13]

At a tunnel project, we used grade 60 bars exclusively for rock stabilization, the bars were pretensioned to 50% of yield.

 

[HTURKAK]

-Good morning.. ( when you wrote your last post , it was night in my time zone..) You did not disappoint me.. You may classify soil nailing a passive earth retention technique, but i said retaining not retention..

- The discussion with Mr. Ingenuity (Structural) was for the use of round bars instead of strands for prestressing of concrete elements. I agree also with the use of grade 60, 75 for passive anchorage systems , soil nailing .. you may look to the posts..

- Mr r13 explains his past experience for the use of grade 60 bars exclusively for rock stabilization with pretensionning to 50% of yield. I do not know any code specified that.. IMO, at least , it is not good idea to drill and grout a lot of hole in order to use grade 60, 75 instead of grade 150...

Good Luck..

 

[Obregon]

Good morning HTURKAK. Here is a Selection Guide, by Williams Form, for rock and soil anchors for illustration purposes:

 

[Obregon]

Williams Form´s people are part of the PTI DC-35 Prestressed Rock and Soil Anchor Committee, i.e. this is a serious source of information. Again, Grade 75 bars can be employed as prestressing reinforcement for Ground Anchors; please believe me and believe the PTI experts buddy!

By the way, Soil Nailing is not only a Passive Earth Retention Technique, but also a Passive Earth Retaining Technique. There you go.

Carlos R. Obregón

http://www.sueloanclado.com

Viva México!