Chance Technical Design Manual

• Maximum Proof Load: The proof load (pier installation force) must be limited to avoid overloading the pier system hardware during installation. Hubbell Power Systems, Inc., recommends that the maximum proof load (R p MAX ) not exceed 1.65 times the published maximum working capacity (R h WORK ) of the pier system (found in Section 7 of this manual) without engineering approval. The maximum working capacity of the pier system is half of the ultimate capacity. EQUATION 5-6 R p MAX ≤ 1.65(R h WORK ) or R p MAX ≤ 1.65(R h ULT )/2 where R h WORK = Pier system published maximum working capacity = (R h ULT )/2 R h ULT = Pier system published ultimate capacity Additional notes: Current practice by Hubbell is to limit the unsupported pier pipe exposure to a maximum of 2 feet at the published work ing loads for the standard pier systems. The soil must have an SPT N 60 value greater than 4. The pier pipe must be sleeved for pier pipe exposures greater than 2 feet and up to 6 feet and/ or through the depths where the SPT N 60 value is 4 or less. The sleeve must extend at least 36 inches beyond the unsupported exposure and/or the area of weak soil. If the anticipated lift is to exceed 4 inches, the Atlas Resistance Continuous Lift pier system should be used. Atlas Resistance piers can be located as close as 12 inches (305 mm) between adjacent piers to develop a “cluster” of load-bearing elements. 5.2 CHANCE HELICAL PILE/ ANCHOR ULTIMATE BEARING CAPACITY The capacity of a helical pile/anchor is dependent on the strength of the soil, the projected area of the helix plate(s), and the depth of the helix plate(s) below grade. The soil strength can be evaluated by various field and lab test techniques. The projected area is controlled by the size and number of helix plates. Helical piles and anchors may be used for a variety of applications for compression loading (helical piles) and tension loading (helical anchors). Helical piles and anchors are generally classified as either shallow or deep depending on the depth of installation of the top helix below the ground surface, usually with respect to the top helix diameter. There are some situations in which the installation may be considered partway between shallow and deep, or intermediate. In this manual, only design procedures for shallow and deep installations will be described. Table 1 gives a summary of the most common design situations involving helical piles and anchors that might be encountered. Note that the use of shallow multi-helix anchors for either com pression or tension loading is not a typical application and is not covered in this manual.

EQUATION 5-2

R w ULT = P w (FS h )

where

P w = Pier working load FS h = Hardware Factor of Safety = 2.0 (may be varied based on engineering judgment)

• Pier System Selection: Select a pier system with a pub lished ultimate capacity (R h ULT ) (found in Section 7 of this manual) equal to or greater than the required minimum ultimate hardware capacity.

EQUATION 5-3

R h ULT ≥ R w ULT

where R w ULT = Minimum ultimate hardware capacity based on pier working load • Maximum Pier Spacing: Check the maximum pier spac ing (x MAX ) based upon the selected pier system hardware capacity. The selected pier spacing must be less than or equal to the maximum pier spacing (x ≤ x MAX ). x MAX = (R h ULT ) / (FS h )P (foundation must be structurally capable of spanning this distance) where R h ULT = Pier system published ultimate capacity FS h = Hardware Factor of Safety P = Line load on footing • Proof Load: Atlas Resistance® piers are installed using a two-stage process as noted above. First, the pier is driven to a firm bearing stratum. The installation resistance force applied during this stage is called the proof load (R p ). The minimum proof load is calculated from the pier working load and the proof load Factor of Safety (FS p ). Hubbell recommends a minimum FS p of 1.5 at installation unless structural lift occurs first. Experience has shown that in most cases the footing and stem wall foundation system that will withstand a given long-term working load will withstand a pier installation force (proof load) of up to 1.5 times that long-term working load. If footing damage oc curs during installation, the free span between piers (L P MAX ) may be excessive. EQUATION 5-5 R p = (FS P )P w = 1.5P w where FS P = Proof load Factor of Safety = 1.5 P w = Pier working load EQUATION 5-4

DESIGN METHODOLOGY

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