Tendon Elongation in foundation ground anchors

[farqy]

Hi all,

We have installed and proof loaded a test ground anchor for a tower pre-cast foundation.

I am trying to get a handle on how much of the extension recorded (24mm) is due to elongation of the tendon?

The bar is a MacAlloy 1030 40mm. 3m un-bonded, 8.5m bonded. Loaded up to 530kN. Bar secant modulus of elasticity = ~170MPa

Any thoughts anyone.

Thanks in advance.

 

[PEinc]

PL/AE = elastic elongation.

P = test load
L = unbonded length of tendon plus jacking length = total length from bottom of unbonded length to top of testing jack. Actual L could be more if/as load transfers down into the bond length.
A = crtoss section area of tendon
E = Modulus of Elasticity of tendon

http://www.PeirceEngineering.com

 

[paddingtongreen]

These questions always bother me. If we follow the load down the tendon, we reach the beginning of the bonded length. What happens here? At what rate does the tendon transfer load into the concrete? It must be over a very short distance, or some weird changes in the shape of the tendon and concrete are indicated.

The interface intrigues me, suppose that the entire load is transferred in say 10 units of length, one tenth goes into the first unit and nine tenths of the load continues down the tendon, but for it to continue, there must be strain, strain that should break the bond in the first unit. I don't see how these things don't just "unzip".

Okay, I know they don't but...

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

Extension of the unbonded length would only account for about 1/3 of the OP's recorded amount. The balance has to be due to extension within the "fixed" length, movement of the "fixed" part, or inaccuracy of measurement.

I think the OP knows this, but is asking for opinions on acceptance/failure criteria.

 

[farqy]

Thanks for the replies. Very useful.

Only really looking for a back-of-the-fag-packet ball park assessment, so PEinc's reply is spot on.

Cheers

 

[dcarr82775]

Since the tension load in the bar has to be zero at the bottom, calculate the elongation using the full free length and half the bond length. This assumes the load transfer to the soil is uniform over the full bonded length.

 

[GeoPaveTraffic]

paddingtongreen

You raise a point that, unfortunately, many engineers choose not to think about. The short answer is that the bond between the anchor and soil and/or rock is not inelastic. As load is transfered to the soil/rock strain occurs both at the bond and within the soil/rock. That strain accounts for some portion of the load being developed by the anchor.

Additionally, the amount of load at any given point in the bonded length of the anchor may change with time. This is very true of anchors in clay soils. The only way to know how much load the anchor is developing at any point is by placing strain gauges on the anchor bar at different depths and that only gives an approximate value.