Anchor pull testing... am I crazy or is this wrong?

[atrizzy]

I'm working on a project for which a client has:

1- asked me to design a structure supported with rock anchors in tension
2- asked me to define an anchor test load equal to the ULS load effect on the anchors
3- provided an anchor pull testing procedure for which the final load increment is 1.33 x ULS load effect

Am I crazy, or does that not de facto mean that my actual ULS design load (if I wish to pass the load test) should be 1.33 x ULS loads provided?

In other words, if designed for given ULS and tested to 1.33 ULS, isn't it bound to fail?

 

[Lomarandil]

Your structure should be designed for the appropriate loads, which will result in an ultimate level (ULS) reaction at the anchors.

The client then wants the anchors themselves to be sized for and tested to 1.33x the expected ULS reaction.

This is common in the realm of soil and rock anchors, especially in cases where not every anchor is tested (at least partially due to inherent variability in anchor performance since the substrate is not as well controlled as typical engineering materials, sometimes to aid the process of prestressing anchors when that is desired to prevent load reversals).

----
The name is a long story -- just call me Lo.

 

[Settingsun]

Or is a case of designing for the uls load (not 1.33x) but eating into your capacity margin during a controlled test that doesn't present danger to the public?

What is the capacity reduction factor used in the anchor design?

 

[atrizzy]

Thanks Lo. We're actually testing every rock anchor, but the approach you laid out makes sense.
I'll confirm that the intent is to oversize the rock anchors themselves.

 

[SwinnyGG]

What is the capacity reduction factor used in the anchor design?

Usually very large.

Here's an example from Hilti- granted this is a concrete anchor not a rock anchor- but I would expect the calculations to be similar with, further appropriate reductions due to variation in the base material.

https://www.hilti.com/medias/sys_ma...ncrete_Approval_document_ASSET_DOC_LOC_24.pdf

 

[EngineeringEric]

I do think typical values have a larger reduction since they are not going to be tested. If you are testing everyone why do you need a FS of 5?

I think this is similar to visual graded timber has a larger reduction than machine stress rated.... same wood (ignoring knots and quality) but the testing reduces the standard deviation and allows a smaller reduction.

 

[atrizzy]

RE geotech SF, I'm not sure yet as we're in the preliminary stages. I haven't received instruction from the geotech as far as the design parameters.

I just spotted that odd looking client requirement, especially since all anchors require testing.

 

[MotorCity]

To me, it never made sense to specify a maximum test load. I am always curious to see the failure or breaking load. That really gives you an idea of how much of a "fudge factor" you have. And its cool to break stuff.

 

[Lomarandil]

You don't really want to put a stressing crew at risk by failing an anchor in the field. Even with the proper safety precautions (never, ever, ever stand in line with the jack)-- it's still a lot of energy wanting to go somewhere, and several of the potential failure modes can be non-ductile.

In a lab, sure. But a lab environment won't give you much useful information on field performance.

----
The name is a long story -- just call me Lo.

 

[Ingenuity]

It has been way more than a decade since I designed, installed and tested rock anchors, so take this with a grain of salt.

There may be a misunderstanding with nomenclature between you and your client re the "1.33 x ULS".

PTI's Recommendation for Prestressed Rock and Soil Anchors has a bunch of info on design and testing. My experience was with the 2^nd Edition - I think the latest is the 5^th Edition. [Link]

It discusses Proof and Performance Testing, and for both test types the test loads increment to 1.33 x P, where P is the design load of the anchor (P = anticipated final max. effective load in the tendon, allowing for time-dependent losses).

The max test load will not exceed 80% of minimum UTS of strand/bar - you do not wish to yield or fracture the tendon.

Also, when you undertake Proof and Performance testing it is an UNconfined test, so it does NOT test the rock mass pull-out - only the rock/grout interface, the grout/tendon interface and the tendon section.

 

[EireChch]

I am with Ingenuity on this one, i dont know the US codes but I always understood testing of anchors, piles etc to be your design load x (some factor). You are correct in saying that you should not be testing past your ULS load.

An example from our pile test spec below which is in essence the design load times 1.5.

For static pile load tests, test piles shall be loaded to the DVL (Design Verification Load) + 50% using the extended proof load test procedure in SHW Clause 1609.34 (where DVL in this case is qual to the SWL (Specified Working Load) as shown in Table 7.

 

[Lomarandil]

Correct, you should not try to (and theoretically would not be able to!) test past the UTS (Ultimate Tensile Strength) of the anchor.

I think MotorCity accidentally put us on a tangent.

----
The name is a long story -- just call me Lo.

 

[PEinc]

Ground anchors are usually tested to no more than 1.33 x the UNFACTORED SERVICE LOAD of the anchor. You can't test an anchor beyond its ultimate structural or geotechnical capacity. The maximum anchor test load should be no more than 80% of the guaranteed ultimate strength (GUTS) of the high strength tendon (GR150 for threaded bars or GR270 for strands) or not more that 80% of the yield strength of a GR 60, 75, or 80 threaded bar. GUTS and yield strength are not the same or equal. Per PTI, the maximum allowable working load of a tendon is 60% of the GUTS or yield strength depending on steel grade. DOT's and railroads will lower the allowable work load percentage because, apparently, their projects are "much more important" than private projects. Strangely, DOT's usually want the anchored walls designed using LRFD which gives factored anchor design loads. However, the anchor testing is usually specified to be to 120% or 133% of the unfactored anchor load. Therefore, you need to design the anchored wall twice - once using LRFD, once again using ASD to get the unfactored anchor DL for testing and any required wall deflections. IMHO, LRFD for anchored walls is just busy work!

Added comment: Every ground anchor, soil or rock, should be tested. The days of testing only a few anchors to a much higher percentage of the DL ended back in the early 1970's.

http://www.PeirceEngineering.com

 

[atrizzy]

Ingenuity, PEinc, I completely agree which is why I felt this was funny.

If anything, the test load should be 1.33 x SLS, not ULS loads.

 

[PEinc]

atrizzy, for the record, please define SLS and ULS. I don't want to assume what you mean. I've been designing and installing ground anchors since 1975 and have never seen SLS or ULS.

http://www.PeirceEngineering.com

 

[Ingenuity]

ULS = Ultimate Limit State
SLS = Service Limit State

It is like the metric system - Everyone but the USA use it

 

[PEinc]

Thanks, Ingenuity.

http://www.PeirceEngineering.com