Bending Capacity of Micropile Splice 1

[SEshore]

We are being asked to design micropiles to IBC 2006 code.

IBC 2006 Sect. 1808.2.7 requires that pile splices develop 50% of the pile capacity in bending.

We'd like to drill the low headroom micropiles utilizing male to female threaded casing. Does anyone know of a resource or method of calculation to determine the bending capacity of a threaded casing splice?

 

[born2drill]

The ADSC has a paper in their library on load tests performed on 7" micropile casing, but the section on bending wasn't very helpful because they didn't fail the unthreaded pipe section, so it's impossible to know how much threading reduced the capacity.

I've seen designs for soldier beams using threaded micropile pipe where the bending capacity was reduced anywhere from 35% to 65%, but I've never seen any data to back up those assumptions.

 

[oldestguy]

When questions like this have come up for me on repair of deteriorated piling on rail road bridges, we ran tests in the lab to determine the actual bending resistance. Sure it is time consuming and takes some physical work, but you can't beat this for proving or not proving the strength is there.

 

[SEshore]

I agree that nothing beats an actual test.

One would think that others have tested the bending capcity of threaded casing and it might be possible to interpolate the data to the various casing sizes, so long as the material properties are known.

 

[Erdbau]

Just to take a step back, are you looking to use the casing for primary compression reinforcement, reinforcement for bending at the top, for confinement of the grout column for seismic, or for some or all of the above?

 

[SEshore]

The casing is the primary reinforcing for the micropile (no center bar).

The micropiles will be loaded in compression. The only bending would be a result of accidental eccentricity. I am told that the "building official" will not allow an exception to the above mentioned code requirement.

For this particular project, even though the magnitude of any potential bending may be negligible, we are required to provide a rational justification that the threaded casing is capable of developing 50% of the piles bending capacity at the splice.

 

[Erdbau]

Abstract from DFI:

Proceedings of the 32nd Annual Conference on Deep Foundations, 2007, Colorado Springs, CO, USA, (DFI)

Bending Strength of Threaded Micropile Connections
Steven R. Musselman; J. H. Long; N. Carroll; S. Farr

The bending strength and flexural stiffness of a micropile at its threaded connection is less than its intact strength and may fail due to thread slippage or by fracture through the pin (reduced cross-sectional area of male threads in the threaded zone of the casing). The mode of failure depends on the reduction in cross-section necessary for machining the threads, and details of the threaded connection such as the length of threaded connection, size and shape of threads, and the thread density (threads per inch). Based on data from bending tests conducted on 9-5/8 inch diameter micropile casing, a method has been developed to predict the ultimate moment capacity and initial bending stiffness of round threaded micropile connections. Two modes of failure were observed from four-point bending tests conducted at the University of Illinois on 9-5/8 inch casing with round threads. Threaded micropile casing connections with an effective thread length of 1.35 inches typically failed by slippage at the threads (jump out), while connections using an effective thread length of 1.89 inches failed by fracture of the pin. These two failure modes were described by W.O. Clinedinst in his report to the American Petroleum Institute on tension tests of threaded pipe casing. Clinedinst developed an empirical formula to determine the tensile strength of threaded connections. A method to predict moment capacity for threaded micropile connections by modeling the micropile cross-section and applying Clinedinst’s equation to determine the max tensile stress along the threaded connection while satisfying force equilibrium is proposed. The predicted moment capacity for threaded connections using this approach is within ten percent of measured for both modes of failure observed from the bending tests.

That might be worth a look.

 

[SEshore]

Thanks for the suggestion. I'll check it out in addition to the one born2drill suggested.

 

[dirtydude]

You may also want to check FHWA-SA-97-070 Micropile Design and Construction Guidelines Implementation Manual. Most micropile threaded joints, if designed properly, should develop the full capacity of the pipe in compression.

 

[SEshore]

If there is something in FHWA-SA-97-070, it eludes me.

I'm trying to quantify the capacity in bending, I'm not worried about compression.

Today the mailman delivered the paper that born2drill suggested. They tested 7"x0.5" casing in bending. It failed by thread jump. This particular casing failed at around 57% of the theoretical capacity of an unspliced casing section. But since this is just one casing size with one thread pattern it might not represent typical performance.

 

[ishvaaag]

Notionally for enough threaded length both sides of the joint can attain to be fully loaded, but the critical point is at the root of whichever the side, since there you have an open crack and a diminished section. So, as long as the threaded part passes enough stresses to feed the bending strength of the joint at the root, it is of no use to make the threaded part longer. For the exposed root this could be solved by external welding, but you always will have the internal for which no corrective action can be put in place. Hence, except for additional inner rebar otherwise not counted elsewhere, the only way to make pipe micropiles joints of the same bending strength than the shaft is by thicker externally reinforced joints.

 

[SEshore]

I have just recieved the paper suggested by Erdbau. It is exactly what I was looking for.

I would recommend to others that are attempting to predict the bending capacity of flush threaded casing.

Thanks for the suggestion Erdbau!

 

[mrturran]

I´m looking for different types of threads used for micropiles. Which ones are used? Is there a way to get them? Is it all API?

Thanks!

 

[ishvaaag]

A number of the providers are delivering special details with coupler or alike in intent in order to get full moment capacity at the extension joints.

http://www.dsicanada.ca/uploads/media/DSI-USA_DYWIDAG_Driven_Ductile_Iron_Pile_us.pdf

The special details lessen the uncertainties of preparing joints at the worksite, in more thant using the full capacity of the overall section.

 

[Erdbau]

I'm not sure these ductile iron driven piles are being discussed here. Drilled minipiles are typically API 5CT steel casing. I've never read the spec, but there is probably a section on threads.

Mrturran, do you want the casing itself or the thread spec? You can get casing from several steel manufacturers. It typically comes in two grades J55 and N80. The number is the yield stress of the steel. I have no idea what the letter means.

 

[mrturran]

Erdbau,

Im looking for the threads only. Is there a standard used by pilers or do you take oil pipe threads or similar?

/Mrturran

 

[Erdbau]

The spec might have something about the threads.

 

[mrturran]

Yes, but it would be great to know which one of them.

 

[Erdbau]

As noted in an earlier post, API 5CT.

 

[mrturran]

About % bending capacity. Why don´t pilers weld on stronger sleevs like the do on drill pipes?

I can´t see the point if you can only use 50 % of the pipes capacity.