Transfer Load

[someengineer]

Hi there,

Does anyone know a good criteria to determining what transfer load (lock off) should be applied to soil anchors?

I understand its a percentage of the design load. Is it a function of the free stress length of an anchor? If anyone knows any guidelines it would help.

Thanks!

 

[someengineer]

It's a retaining wall at rest condition. I've read that the lock off load should be about 95-100% of the design load. Does anyone agree with this?

 

[born2drill]

Anchors can be locked off anywhere within a wide range, usually between say 75% and 110% of the design load. Anchors are generally locked off in order to restrict movement of the structure they are supporting. It's a judgement call by the design engineer where to lock it off, usually based on the anchor's application.

A temporary tieback, on one hand, usually calls for lockoff around 80% DL. This will stop the retaining wall from deflecting excessively, but will allow some movement. Remember that some (albeit minimal) movement is necessary to develop active pressures.

A permanent building tiedown would be on the other extreme, where you might actually want to lock it off at or above the design load, to try to prevent any movement at all of the foundation of your high rise, or tower crane, or wind turbine, or whatever.

I suppose that since the elastic movement of the anchor is a function of the free length, you might want to lock off an anchor with very a long free length at a higher load. I've never really considered it, but thinking about it now that makes some degree of sense.

 

[ishvaaag]

born2drill,

I am interested in the reason of allowing some movement. I understand that some reserve is needed etc always for safety factor, avoiding zip failures, etc.

On the other hand, imagine for a moment a long stick of the kind of those used in fireworks, compressed from both ends, and then braced with two fingers at midheight. The *** small*** bracing force exerted by the two central fingers allows for a bigger buckling load. Now, temporary vertical cuts in the ground may eventually move upon meteorization, settlement or just movements readapting to a new state of equilibrium.

My interest centers on why it has evolved the case where allowing for some movement, i.e., more or less allowing for the formation of active wedges and pressures took precedence on the fact of not allowing any movement at all (no movement on a standing not failing vertical cut, pressure = 0). To prevent initial movement the force is small; to stand active wedge, the active pressures are to be met; for the assumption of not movement ... was it thought that at rest pressures then would have eventually to be met and then the anchors and wall systems woul need be stornger?

I am interested, even knowing of the difficulty in ensuring reliable behaviour (other than by slope stability analysis) in the precise situation at wich what one is more wishing and wanting is precisely that prevention of initial disruption. Any comments?

 

[apsix]

ishvaaag
I would say that your analogy with the buckling column is not valid for soils.
If the force required to resist initial movement was really that small then your typical retaining wall designed to resist active pressure would also be able to resist the initial movement.
However I'm sure there are plenty of retaining walls which see a lot less than the design loads, due to underestimated soil strength.

 

[ishvaaag]

I agree with you apsix in everything with the addtional comment that maybe precisely the need to contain at least the active wedge evolved on that occasionally the vertical cuts were failing, and a theory of just initial prevention of movement was not enough.

Maybe the same kind of difference we can see between some isolated struts and timbers against earth push, just confiding in the cohesive unity in the blocks of earth whilst the works were ongoing (I saw this commonly in trenches till maybe around 10 years ago with the apparition of insertable retention systems), and some system of actual safe retention.

 

[DRC1]

I usually lock off at 100% design load. Typically what will happen is thatsinxe the lock off load is designed for the load to be carried to the next level, say 10 feet down, and at lock off, it is only excavated 2 feet, the soil near the anchor will fail passive ly or at least densfy. The wall element will move into the cut. This andds some soil strength at the anchor location. I typically design one level tie back systems for active load and have not seen excessive deflection. Ishvaag, I understand the reason to induce movement is to move from an at rest (k=.5) to an active (k=.3)(+/-)

 

[someengineer]

thank all of you for your responses,I will take it all into consideration.