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?

 

[SEshore]

The bending capacity doesn't usually control in micropile design. In the micropile industry, we often assume that flush threaded casing has 50% of the theoretical capacity in bending at the threaded joint. I was asked to "prove it" on a project, so I started this thread looking for help.

We don't use external couplers or welded joints because they cost more to install and we don't need the extra capacity.

 

[mrturran]

Hi SEshore,

Ok, but do you need 100% in compression and/or tensile load?

By the way, I´m looking for a thread which is 100% in all three (Bend, tensile, compression). Do you know which types are used?

 

[ishvaaag]

You can get 100% of coupler of cup details IF a closing weld is added to the externally apparent in-built ring crack. Other solution is a generalized weld within the coupler voids, very much alike as with couplers for reinforcement bars, to which, by the way, some of the micropiles used bear some similarity. Otherwise for tensile, for example, you are limited to the full strength at the net area of the threaded part.

 

[SEshore]

There is essentially no reduction in compression. There is a reduction in tension.

If you are looking for full tension you might consider all thread bar as a component of your micropile.

If you need to get the casing to 100% in tension and you want avoid a lot of welding and reinforcing of the joint, you might consider using an external coupler rather than flush threaded casing. With flush threaded casing you have male and female casing ends that thread together. With an external coupler both ends of your casing will have male ends and your couplers will have female ends. The drawback to external couplers for micropiles is that the couplers will have a larger diameter than your casing, so you will likely need to utilize an overreaming drilling system to drill a hole to accomodate the OD of your external couplers.

 

[ishvaaag]

errata:
In the first line of my previous post the second of should be read OR

 

[born2drill]

At a recent ASCE presentation I went to, the topic of welding N80 micropile casing to try and beef up lateral capacity was discussed. The speaker identified this procedure as something to absolutely avoid doing.
Apparently welding has detrimental effects on the high yield steel, and welding will actually end up WEAKENING the steel at the joint.
I don't know if the same holds true for J55 pipe.

 

[mrturran]

born2drill, that´s interesing. We normally work with YP around 460 mpa so I hope it does´nt effect us.

SEshore, Thanks. So the reason why you don´t weld on the construcion site is becouse it´s time consuming?

In Scandinavia we use Symmetrix drilling mostly. What type do you use?

Do you know what type or name of the external couplers that are being used?

 

[mrturran]

Do you have any experiance about the differnce between seamless and welded pipes? I have heard that the seamless is stronger even though it has the same mechanical properties as the welded type.