Chance Technical Design Manual

DESIGN EXAMPLE 1: ATLAS RESISTANCE PIERS TYPE OF STRUCTURE The structure is a two-story, 20’ x 40’ frame residence with full brick veneer siding located in the Midwest. The house sits on 8” thick by 8’ high cast concrete basement walls with steel rein forced concrete footings 1’-8” wide by 1’ thick. The roof is com position shingles over 1/2” plywood decking and felt underlay ment. There is six feet of peaty clay soil overburden present. PRELIMINARY INVESTIGATION Settlement of up to 2-1/2” is evident in portions of the structure. Checking with local building officials reveals no special con trolling codes for underpinning existing structures that must be observed. Preliminary geotechnical information indicates the footing is situated in peaty clay type soil with Standard Penetration Test (SPT) blow count (N) values of six and higher. This soil extends to a depth of 15 feet where a dense glacial till exists. It is determined that the glacial till layer will serve as an adequate bearing stratum for the Atlas Resistance® piers. PRELIMINARY ESTIMATE OF TOTAL LOAD ON FOOTING EQUATION 8-1 P = DL + LL + SL + W = (1,890 + 667 + 120 + 2,310) = 4,987 lb/ft where P = Total live load on perimeter footing DL = Dead load (1,890 lb/ft) LL = Live load (667 lb/ft) SL = Snow load = S K (w)L / 2(w + L) where L and w are the building dimensions = 18 lb/ft 2 x (800 / 120) ft = 120 lb/ft S K = Snow load requirement factor = 18 lb/ft 2 (for this example) W = Soil load = W b1 + W b2 = (330 + 1,980) lb/ft = 2,310 lb/ft W b1 = Soil load directly above footing (see Table 4-5) W b2 = Soil load from soil wedge (see Table 4-5) ATLAS RESISTANCE PIER SELECTION While the Atlas Resistance continuous lift pier could be used for this application, the small lift required makes it unneces sary. The Atlas Resistance predrilled pier is not a good choice here due to the absence of a hard, impenetrable layer above the intended bearing stratum. Therefore, the Atlas Resistance 2-piece standard pier is selected for strength and economy. The more expensive Atlas Resistance plate pier system could (See Tables 4-2, 4-4, and 4-5 in Section 4 for DL, LL, and W.)

also be attached to the concrete basement wall and used for this application. Since there are suitable soils with N counts above four, there is no need to sleeve the pier pipe for added stiffness. PIER SPACING Using the information obtained about the stem wall and foot ing to be supported and applying sound engineering judg ment, the nominal pier spacing based on the foundation sys tem’s ability to span between piers is estimated at about eight feet. This results in a nominal working pier load (P w ) of: EQUATION 8-2 P w = xP = 8 ft x 4,987 lb/ft = 39,896 lb where P w = Pier working load x = Selected pier spacing = 8 ft P = Line load on footing = 4,987 lb/ft FACTOR OF SAFETY Hubbell recommends a minimum Factor of Safety (FS) for the mechanical strength of the hardware of 2.0. EQUATION 8-3 R w ULT = P w (FS h ) = (39,896 lb) x 2 = 79,792 lb where P w = Pier working load FS h = Hardware Factor of Safety = 2.0 (may be varied based on engineering judgment) R w ULT = Minimum ultimate hardware strength requirement based on structural weight Select a pier system with an adequate minimum ultimate strength rating: EQUATION 8-4 x MAX = (R h ULT ) / (FS h )P = 86,000 lb / (2 x 4,987) = 8.6 ft (Wall and footing are judged able to span this distance) where FS h = Hardware Factor of Safety R h ULT = 86,000 lb - Choose AP-2-UFVL3500.165 [14'-0] modified 2-piece pier system x MAX = Maximum pier spacing based on hardware capacity PROOF LOAD Hubbell recommends a minimum Factor of Safety of 1.5 at in stallation unless structural lift occurs first.

DESIGN EXAMPLES

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