Chance Technical Design Manual

5.1 ATLAS RESISTANCE PIER CAPACITY

Atlas Resistance® piers develop their capacity primarily through end bearing. The current accepted practice is for Atlas Resistance piers to be installed to a preset performance design criterion. The development of a theoretical capacity model is under study. In general, the tip of the Atlas Resistance pier should be embedded in cohesionless soils with Standard Penetration Test (SPT) blow count (N 60 ) values above the 30 to 35 range and in cohesive soils with SPT N 60 values above the 35 to 40 range. Past installation experience indicates that Atlas Resistance piers will provide foundation underpinning support via end bearing when positioned into these SPT N 60 value ranges. See Figures 5-1 and 5-2 for assumed failure patterns under a pile tip in dense sand. The Atlas Resistance pier is a manufactured, two-stage prod uct designed specifically to produce structural support capac ity. The first stage is to drive the pier pipe to a firm bearing stra tum. In the second stage, the lift equipment is combined with a manifold system to lift the structure. The Atlas Resistance pier system procedure provides measured support capacity. Atlas Resistance piers are spaced at adequate centers so that each pier is driven to a suitable stratum and then tested to a force greater than the force required to lift the structure. This proce dure effectively load-tests each pier prior to lift and provides a measured Factor of Safety (FS) on each pier at lift. PERFORMANCE DESIGN CRITERION The following guidelines are intended to serve as a basis for the selection and installation of a proper Atlas Resistance pier. • Pier Working Load: The required working load per pier (P w ) is calculated based on the dead loads and live loads and the pier spacing that was selected considering the ability of the existing foundation to span between the pro posed pier locations.

ASSUMED FAILURE PATTERN UNDER PILE POINT FIGURE 5-1

EQUATION 5-1

P w = xP

where

x = Selected pier spacing P = Line load on footing = DL + LL + SL + W

DL = Dead load LL = Live load SL = Snow load W = Soil load

• Hardware Factor of Safety: Hubbell Power Systems, Inc., recommends a minimum Factor of Safety (FS h ) of 2.0 for structural capacity of the hardware (pier pipe, bracket, etc.). The hardware Factor of Safety may be varied based on engineering judgement. • Hardware Structural Capacity: The required structural minimum ultimate capacity of the hardware (R w ULT ) is calculated from the pier working load and the hardware Factor of Safety.

DESIGN METHODOLOGY

FAILURE PATTERN UNDER PILE POINT IN DENSE SAND FIGURE 5-2

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