Chance Technical Design Manual

EXAMPLE The structure has a 6” thick footing along with an 8” tall stem wall that was cast with the footing. It was reported that build ing code required a minimum of two #4 reinforcing bars spaced 3” from the bottom and sides of the concrete. The structure is a single story wood frame building with masonry veneer and a 4” concrete slab. The structural load on the perimeter footing was calculated at 1,020 lb/ft plus 250 lb/ft soil overburden.

For this project, specify the spacing at a maximum 6 feet on center to allow for unexpected defects in the beam or founda tion loading or for possible field adjustments caused by ob structions or utilities. When one wants to reduce the number of piles/piers on a proj ect, it is important to keep in mind that the distances in the tables are for a free span between supports. A supplemental steel footing could be offered to the client, which will effec tively expand the distance between piles/piers while maintain ing the required free span distance. If we consider the example above, depending upon the com plexity of the architecture, the number of piles/ piers could be reduced by perhaps 10% to 15% on the total project by simply installing a 24” long, 3/8” x 6” x 6” supplemental steel beam under the footing.

L s = [F y (d)A s / 1.875(P)] 1/2

Equation 4-1

given

F y = 24,000 psi d = (6” - 3”) + 8” = 11” A S = 2 x 0.1963 = 0.3926 in 2 P = 1020 + 250 = 1270 lb/ft

then L S = [(24,000 x 11 x 0.3926) / (1.875 x 1270)] 1/2 = [43.526] 1/2 L S = 6.6 ft = maximum free span

x = (L S + w p / 12)

Equation 4-2

FS f

given

w p = 24” (supplemental steel beam length) FS f = 1.2 (inspection revealed a well-built foundation)

x = (L S + w p / 12)

Equation 4-2

then

FS f

x = (6.6 + 24 / 12) / 1.2 x = 8.6 / 1.2 = 7.17 ft (pier spacing can be increased to 7 ft on center)

given w p = 10” (width of Atlas® AP-2-UFB-3500.165 pier bracket or Chance® Underpinning helical pile bracket C1500121) FS f = 1.2 (inspection revealed a well-built foundation)

LOAD DETERMINATION

The piles/piers could, if the architecture allows, be spaced on 7-foot centers, while still maintaining the desired 6-foot free span distance. Tables 4-6 through 4-9 will assist the designer and installer to estimate the maximum free span allowable for some common foundation configurations.

then

x = (6.6 + 10 / 12) / 1.2 x = 6.19 ft (specify pier spacing at 6 feet on center)

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