rock anchors

huangyhg

rock anchors
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!!!
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if the tendons are pretensioned, them much, if not all, of their elastic stretch will have occured.  if you lock off the tendons at a lower load and if the tendons become loaded to the higher load, then the tendons will stretch more.  how much additional elastic stretch can the towers accept wihout presenting a problem?
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.
except some mandatory code indicates otherwise, i would prestress this prestressed connection for only a bit more any service load it is likely to see. shear should be taken by a key or then the additional effect of delivering enough normal force to avoid relative slippage at the anchor's interfaces should be in place. i think this is the usual practice for presstressed bolts and the like, and the reason that comes to my mind supporting this is better fatigue behaviour due to lower standing stresses. with this force in place the interfaces neither should see any aperture; load and security factors are there (and figure in the selection of the rod or anchor) but they are not guesses that the structure will see these forces at such level; just that we want the elements as big just to cover enough adversarial circumstances the code wants us to care for.
the footing would need to be designed to support the vertical load of the tower and the lock-off load from the tiedown anchor.  therefore, you may need to increase the size of the footing.  this is why some engineers try to minimize the lock-off load.  if you use a threadbar tendon, you can lock of at whatever load you need.  if you use strand tendons, you need some minimum lock-off (seating)  load to assure that the strand wedges hold tight if and when the tendons see the full design load.
as far as shear, you didn't mention it so i assume it is not a problem.  also, tiedown anchors are not prestressed bolts.
using 150 ksi threaded bar is not my first choice.  you may come to the same conclusion after you see one of these rock snap during routine handling in low temperatures.
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.

of course rock anchors are not prestressed bolts. you can have them not presstressed if you want, or may be the convenient solution. some even prestress without allowing some free length. the point here is that we are not here (i think) to show who is smarter, but to provide some help.
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.
atse hinted at a question i would ask - why do you need to prestress unless there is a need for "no (read that barely any) movement" permitted under what is most probably transient loading?  why not passive anchors? (e.g., grouted rock bolts - or dowels)  i was on the dew line years ago and we anchored with dywidag thread bars (2 inch) - we did not prestress that i can re  
i agree, bighfff">.  dams are locked-off tightly because they shouldn't have movement.  a tower may be different, as you mentioned.
ishvaagfff">, it's not a question of who is smarter.  it sounds to me like engleprechaunfff"> 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   
i thank you all for the valuable input.  to clear up some things, they are "rock bolts" as the rock is the main foundation system.  we use the bolts to tie 4' to 6' tall piers to the ground.  also "no movement" is and can be very important for these particular types of towers.  they have microwave dishes on them which, depending on the distance of the shot, can make "catching" the signal difficult.  so keeping the dishes in the same spot becomes more crucial the further away form each they become.  thanks again.
atse, why so hesitant about the 150ksi threadbar?  for ground anchors i've never used anything other than 150ksi bar or 270 ksi strand.
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.
high strength bars (or strands) make sense for stressed anchors, but not for passive ones.  deflection is just relative to bar size.
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.