Uplift on Rock Foundation
Uplift on Rock Foundation
(OP)
Does anyone know where I could get some design information for the design of tension bores in rock. I have a customer that would like to use a 6" drilled bore in rock, place rebar in the bore, and fill it full of grout for tension anchoring for uplift, rather than just use the weight of the footing. I have seen this done before, but am not sure how to determine the actual uplift capacity of the bore. Any help would be appreciated. Also, the customer owns a limestone quarry, and the foundations are for conveyors. No geotechnical was hired by the owner, and I am using an allowable of 10000 psf for the bearing. The quarry floor stone appears very solid. Thanks in advance.





RE: Uplift on Rock Foundation
RE: Uplift on Rock Foundation
I will check these out.
RE: Uplift on Rock Foundation
RE: Uplift on Rock Foundation
Ask the owner hire a geotechnical consultant to help you address these issues - at least make a formal written request. Good posterior protection either way.
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: Uplift on Rock Foundation
DL = 4000#
LL = 5960#
WL = +/-10800#
EQ = ?? (determining this morning)
I have already decided to use a 30" x 30" pier, 2' tall. This still keeps bearing @ 3000psf, which I seriously doubt will cause a problem on rock. I threw out the 10,000psf because an old wise engineer once told me you could get that on about any type of rock confortably. The rock shows no visable joints, and the floor seems smooth and in good condition. I also contacted the owner yesterday evening to see if he had a concrete contractor hired for the job yet. He said no, but he had someone in mind. I am going to contact the contractor today and see if he has a certain type of grout that he would like to use, then I will contact the manufacturer and see if I can get the bonding strength to use in my calcs. Thanks for all your input, I appreciate your help as always.
RE: Uplift on Rock Foundation
RE: Uplift on Rock Foundation
I think you probably mean design vs capacity ie. the anchors are usually tested to 1.3DL (design Load). The bars are sizes so the design load is typically 60% GUTS (ultimate).
Grade 150 steel is used for post tension work and can be tested to 80% of Guts. The rebar grade 75 can be tested to 90% of yield, and could be a better choice for passive anchors, providing there is a concern about loading the foundation.
cheers
RE: Uplift on Rock Foundation
RE: Uplift on Rock Foundation
Just a quick question about the proposed anchors and your personal philosophy. Should the anchors be prestressed, or the nuts "finger tightened plus 1/4 turn"? I don't see a need for tightening to 100% design load; but would 10% be prudent or desirable? Some other level?
I realize this question is fraught with unknowns. But it is quite different from designing tie-backs for a retaining wall. I don't have a firm opinion on this one - wondering what your thoughts are -
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: Uplift on Rock Foundation
1. Bond of steel bar to grout - usually not a problem
2. Bond of concrete to the rock
3. Shear failure of the rock in tension - Army Corps has the figures - it's been discussed - say use 30deg from vertical as cone from tip of anchor. Neglect, say first metre of rock - it is not that expensive to drill rock - in the fact that 1 or 2 m extra won't break the bank.
Next - do you use passive system or active. Passive means that the tension load is created from "rest" condition. Can have relatively "significant" movement in order to mobilize strengh. Active means that you would put some prestress on the rock - less movement of system to pull out to gain design load.
It is most likely that passive system is sufficient unless, of course, small uplift movements cannot be tolerated. This way, you can put in bar and embed in foundation. For higher loads, I would probably consider some prestress as in some passive systems, there might be a bit of "play" in the initial load take-up. For his light loads, I'd say passive.