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Practical Advice for Foundation Design – Micropiles for Structural Support - page 10 / 25

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GSP 131 Contemporary Issues in Foundation Engineering

Regarding casing and bar, the area of the former is typically much larger than the latter. Thus the majority of the load (in the cased length) will be supported by the casing, requiring that the allowable yield stress of the bar be further reduced for strain compatibility between the pile elements.

In summary, strain compatibility requirements dictate that a smaller yield stress be used for the steel reinforcing bar and casing in the calculations. For compression loadings, this yield stress value (i.e., Fy-steel in Equation 1) should be the lesser of: (1) the yield stress of the reinforcing bar; (2) the yield stress of the steel casing; or (3) 600 MPa. A higher compression ultimate capacity could be utilized for the cased portion if documentation were to be provided to show that the grout within the casing could sustain strains larger than 0.003. Research is currently being sponsored by ADSC on this question.

An alternative method for computing allowable loads utilizing the transformed section of the micropile could be used. Such a method is beyond the scope of this paper, but includes estimating allowable strains for each of the component materials.

5.2.2 Combined Axial Compression and Bending of Cased Length. In some cases, a

micropile will be subject to both axial compression load and bending stresses.

combined stress evaluation is not

evaluation is conducted, based on AASHTO (2002).

Such

performed

for

the

uncased

length

since

micropiles

are

designed

A an so

that bending stresses are negligible below bar would have little moment capacity.

the

cased

length

since

grout

and

a

central

5.2.3 Allowable Tension of Cased Length. The allowable tension load for the cased length is given as

Ptallowable =0.55 Fysteel

× (Abar

  • +

    Aca sin g

)

(Equation 4)

Note: Acasing must consider the reduced steel area at threads. A higher yield stress (compared to the minimum of the steel bar or casing previously discussed) may be utilized if the strains due to the working loads are shown to not cause permanent deformations in the threaded casing joints. This information should be based on laboratory testing of casing joints provided by the casing manufacturer. For tension loading without bending, it may be conservatively assumed that the reinforcing bar

alone carries the tension load.

5.2.4 Axial Compression and Tension of Uncased Length. compression load for the uncased length of a micropile is given as:

The allowable

Pcallowable

[ g r o u t ' c f = 4 . 0

× Agrout

+0.47×Fybar

× Abar

]

(Equation 5)

The allowable tension load for the uncased length of a micropile is given as:

10

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