Rock anchors
Rock anchors
(OP)
I am working on a design for a comm. tower for remote cell sites in alaska. Most of these sites make use of 150ksi all thread drilled and embeded into the bedrock. It is the only option to keep these towers down. The problem i am having is with the pretension part. Our factored desgin load to one rock anchor is about 120 kip. We are currently tensioning to 105 kips, however, we are starting to second guess the need to place so much pre-tension on the rock anchors and if so how much do we put on them. The loads have a factor of two due to TIA criteria. Dose anyone know if this large of a tension is needed and where i could go to get documention? Any input would be greatly appreciated. Thank you in advance!!!






RE: Rock anchors
Tiedown anchors for dams are often locked off at 100% to 110% of the tendon design load. This prevents the dam from lifting up when the tiedowns become loaded to their design load. You don't want a dam to lift up off its rock foundation. For your towers, this uplift movement might not be a big problem. Also, deflections (elongation) should not be calculated for factored loads.
RE: Rock anchors
RE: Rock anchors
As far as shear, you didn't mention it so I assume it is not a problem. Also, tiedown anchors are not prestressed bolts.
RE: Rock anchors
Except for high-strength-low alloys, this 150 ksi steel is almost always brittle (low elongation before tensile fracture).
Williams Form offers a 75 ksi threaded rebar (conforms to A615) which works well and is very ductile. I'm sure Dywidag offers something similar.
Keep in mind that the higher your tensile pre-stressing load, the more your creep via your grout (and also creep in your pilecap if you're using one). Over the long term, your prestress will likely dissipate to less than half unless your grouting is extremely robust and your concrete is f'c>5ksi.
Better to stick with mild steel (slightly larger diameter) and not prestress.
RE: Rock anchors
As I have said, I live and practice in Spain. Here since 2005 geotechnical studies are mandatory. 90% percent of them are entirely useless, showing 3 common defects:
1. Not providing probabilistic correct estimates of settlement, but worst case 10 times higher.
2. Repeating as parrots "bulb of pressure" influence blah blah where the compounded integrated effects of loads at their position, whichever the class of foundation shows that except for superestructure rigidity effect, settlements scarcely differ. Not meaningful advertences of local risks, just generalized omission of the technical truth.
3. Consistent with this, recommending k30 values producing astonishing settlements the building never will see. Plus, some seeming to think a ballast modulus it is the sacred approach to designing foundations.
I can live with this, and when I point this to them, even if reluctantly, they admit it. So I admit, yes, prestressed bolts are NOT rock anchors. Yet I would love anyone seeing any of my posts look for the information within, and not if it seems I have outstretched some analogy.
Stay friends.
RE: Rock anchors
RE: Rock anchors
ishvaag, it's not a question of who is smarter. It sounds to me like engleprechaun is describing tiedown anchors rather than prestressed bolts. Maybe I'm right; maybe I'm wrong. However, he does mention these connections as rock anchors. Therefore, my response is appropriatly related to tiedown anchors. BigH's response is also appropriate as it relates to structure movement and elongation of the tension connection member. If the tower footing sits directly on rock, the tiedown connections might be rock bolts, prestressed or not. If the tower sits on soil overlying rock, the tension connections should probably be tiedown anchors with unbonded and bonded lengths. In that case, elastic elongation and lock-off loads are considerations. As for your last response, it sounds to me like you are venting your general dissatisfaction with geotechnical engineers. I'm not sure how your last response relates to this thread.
RE: Rock anchors
RE: Rock anchors
I'm not sure how cold you mean when you say "low temperatures," but I've been crashing and banging those bars around for years in some pretty harsh upstate NY winters and never seen the dramatic failure you described.
RE: Rock anchors
I doubt that it makes much difference whether active or passive anchors are used for these towers. There will be a lot more deflection in the tower itself than in the anchors.
RE: Rock anchors
It seems yo still are not wanting to see if what I said in my post holds something of value to the question, and that is applicable to both presstressed bolts and rock anchors, so maybe it was not as an outstretched analogy as to merit be pinpointed as a separate comment.
Respect venting against, heavens, you may take for sure geotechnical experts here or anywhere are very low in my list; I am old enough to see that people earning their bread working in anything stay in other level that so many (interjective) one meets in his life.
I simply thought useful to pinpoint something that happens here, that, by the way, it is a fault still not committed by some geotechnical engineer, simply because maybe there's still no one with such title here(the career has just been created and there may be still no graduates); these things are made here usually by geologists maybe signed as well by an ICCP, that is, a Civil Engineer of here, Ingeniero de Caminos, Canales y Puertos. I must say that the ICCP's are for sure the more competent of the pair without doubt. They come from responsibiliy, not theory.
And remember, I am an architect. Here we were trained to do our structures, and those that so want, so do. This may come to surprise to some engineers, because the practice elsewhere is or may be different. Yet no one should be surprised, architects have been in charge of firmitas since Hammurabi through Vitrubius, and even today we could, on the statements of standing laws from the nineteen thirties, if we were ready to fight for, project a main bridge as long it is in an urban environment. We would be not only the architect of the bridge, but the engineer of the bridge. Maybe that Calatrava is both an architect an an engineer is not casual. Surprising? A publication of Dragados y Construcciones, then the main firm doing civil works in Spain expounds over a hundred of bridges of the nineteenth and early twentieth century in Spain. Almost all were the design and sign of an architect.
I have never had been in fear of liberty. So I don't think illustrative anecdotary of personal and local experiences may damage to anyone, the information appearing where it appears. Very contrarily, I think anecdotary is what makes stories interesting, you note through it that what told is true.
Take my comment above. Negatively, you may think is what you have said is. Positively, anyone reading such thing may reflect and think, hey, this guy is saying something important: the users of geotechnical studies want information meaningful to them. And so in his mind rests a note on that indicating overestimated settlements in an order of magnitude and going home to sleep well, because the worst that can happen has been covered in such excess, is unethical, rendering the information useless. I am just asking more precision; exactly the same you demanded from me.
RE: Rock anchors
I am sorry but I cannot understand what you are saying. I will make no further comment.
RE: Rock anchors
I don't know what was the trigger, but as an architect practicing in structural, I commend your sophistication in engineering as a whole. However, you may need to spend some time on geotechnical theories as well to make yourself a complete all-around, if you lack of confidence on the fact telling practitioners.
RE: Rock anchors
I think, to paraphrase a quoate that Focht3 (when he was active on the forums) like to put forth, is that soil is a different animal than steel and concrete which has fairly consistent properties and homogeneously applied within the material (unless, like concrete it is/was a major construction problem.
ishvaaag did indicate that geotechnical engineers did not provide probabilistic correct estimates of settlement - sometimes 10x different. He has a point - 10x is too obviously too high (for settlement but not for coefficient of permeability). Still, one must remember, statistically, is that the coefficient of variation of soil properties (std-dev/average) typically ranges from 30 to 50 percent and can be as high as 100%. The average of a soil property can be bracketed by in the order of 2 to 2.5 standard deviations - and yet these may be exceeded. (see Baecher and Christian "Reliability and Statistics in Geotechnical Engineering", page 140) The inherent variation in measuring soil properties is high - depends on the type of test, the bias of the operators, the disturbance of the soils and the like. This is all taken into account when a geotech writes a report and provides reccomendations (and, don't forget, variability due to poor to good construction practices). I remember my mentors telling me - if you do a consolidation settlement estimate and are within 30 percent of the real value - "hell, you've had a good day." So, while ishvaaag rightly criticizes geotechnical estimates when viewed on the macro-level and with 100% hindsight, it appears that we all must step back and look, then, on the micro-level. It is a tough business - once the foundations are in there is very little you can do about reinforcing them. With structures, you can brace, remove and replace, etc.
It is interesting that Spain permits architects to design bridges (in urban settings). Ishvaag - your view are appreciated - that is why this site, with engineers from so many different countries, cultures and the like, being able to provide their views.
RE: Rock anchors
RE: Rock anchors
RE: Rock anchors
And yes, I understand all this if anywhere should be in another post, yet it shouldn't harm here, neither; as Sy Barry's The Phantom said, "Looks do not kill".
To end, we can't manage the use of settlements 10 times those reasonably expected. All doing structural analysis know that imposing vertical displacements in the 1 foot range to framed structures is going to make something ugly to the structural design. Our clients can't afford that; we can't afford to say so to our clients if not warranted. And almost all know these values are not usually seen even with the most basic foundations. Imposed settlements has been to my knowledge traditionally adscribed to the probabilistic hypotheses, that is, just like earthquakes, as something one needs to consider as something happening but to deal with at the service level as an eventuality (this distinction surely was made at the time of first factored load codes, since before all was service level). Of course, settlements are to occur and earthquakes may not; I even could agree in changing the traditional view; yet I think we should keep pace with reality.