Chance Technical Design Manual

DESIGN RECOMMENDATIONS The result of the analysis provides the following design specifications: • Underpinning product: Atlas Resistance® modified 2-piece pier AP-2-UF-3500.165 [14’-0] • Pier spacing: 8.6’ on center • Installation proof load: 64,332 lb ± (unless lift of the struc ture occurs first) • Working load is anticipated to be 42,900 lb ± (4,987 lb/ ft x 8.6 ft) • Anticipated pier depths: 15 ft ±

EQUATION 8-5

R p = (FS P )P w

= (1.5)(8.6 x 4987) = 64,332 lb

where

FS p = Proof load Factor of Safety 1 = 1.5 R p = Installation force based on weight of structure to achieve proof load verification R h MAX = Maximum installation force based on hardware ultimate capacity 2 = (R h ULT /2) (1.65) = (86,000/2) (1.65) = 70,950 lb R w MIN < R h MAX = OK, where R w MIN = R p 1 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 of up to 1.5 times that long term working load. If footing damage occurs during installation, the free span

DESIGN EXAMPLES

(L P MAX ) may be excessive. 2 It is recommended that R

h MAX not exceed (R h ULT / 2) x (1.65) during

installation without engineering approval.

DESIGN EXAMPLE 2: ATLAS RESISTANCE PIERS WITH INTEGRATED TIEBACK

UNDERPINNING SYSTEM SELECTION The availability of a dense stratum with “N” values greater than 40 bpf allows the use of the Atlas Resistance pier. The addi tional lateral loading can be designed for using a helical tieback anchor and the integrated Atlas Resistance pier bracket. Based on the design compression loading (P) of 5 kip per lineal ft and the allowable pier spacing (x) of 8’ the required minimum design capacity of the Atlas Resistance pier (P w MIN ) is (x) x (P) = 8.0 x 5.0 or 40 kip. The AP-2-UF-3500.165 system could be used since it has a maximum working (design) capacity of 42.5 kip. However, due to the possibility of scour and subsequent lack of soil support the modified pier with a working capacity of 45.5 kip is rec ommended (AP-2-UF-3500.165M) with at least three modified pier sections to increase the rotational stiffness of the bracket. HELICAL TIEBACK DESIGN AND INSTALLATION With a maximum spacing of 8’ and an estimated design lateral line load of 2 klf, the horizontal design load (DL h ) at the tie back anchor location is 16 kip. The tieback anchors are typically installed between 15° to 25° from horizontal. An installation angle of 20° was chosen after determining that there are no underground structures/conduits that may interfere with the tieback installation. The tieback must be designed with a mini mum embedment depth of 5D (distance from the last helical plate to the ground surface) where D = diameter of the helical

PROJECT INFORMATION An existing three-story commercial building located within a hurricane prone region requires foundation retrofitting for potential scour activity and lateral load forces from hurricane force winds. The structure sits on a shallow foundation system consisting of a 4’ high 10” thick stem wall and a 4’ wide 12” thick spread footing with three #5 reinforcement bars (Grade 60). The structural Engineer of Record has requested a new founda tion system capable of withstanding 2 kip per lineal foot design lateral forces and temporary scour depths to 1’ below the exist ing spread footing. The estimated design compression loading is 5 kip per lineal ft for the existing structure. The structural engineer has determined that the existing foundation system can handle underpinning support spans of 8’ or less. GEOTECHNICAL INVESTIGATION A geotechnical investigation was performed to determine the soil types and strengths at the project location. The soil bor ings advanced near the project location show medium dense silty sand with SPT “N” values ranging from 15 to 25 bpf to a depth of 20 ft below ground surface (bgs). This medium dense silty sand layer is underlain by dense sand and weathered limestone bedrock with SPT “N” values greater than 40 bpf. Groundwater was observed at 18’ bgs during the investigation.

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