Chance Technical Design Manual

STEP 10 - DETERMINE THE MAXIMUM HELICAL ANCHOR HEAD LOAD Determine the maximum helical anchor head load that will produce the allowable moments determined in Step 9 using Equation 8-49. Using Table 8-6, determine the facing pressure factor (C F ) for temporary shotcrete facing 4” thick. EQUATION 8-49 T FN, flexure = C F (m v,neg + m v,pos ) 8 (S H /S V ) T FN, flexure = 2.0 (1.30 + 0.57) 8 (5 ft/5 ft) = 29.8 kip

6”

6”

6”

W2.9 x W2.9 Welded Wire Mesh

6”

6”

6”

FACING PRESSURE FACTOR, TABLE 8-6 NOMINAL FACING THICKNESS (in) TEMPORARY FACING C F

PERMANENT FACING C F

12”

4

2.0

1.0

WELDED WIRE MESH FIGURE 8-15

6

1.5

1.0

8

1.0

1.0

EQUATION 8-46

STEP 11 - DETERMINE THE ALLOWABLE PUNCHING SHEAR STRENGTH OF THE FACING The punching shear strength (V N ) is determined using Equation 8-50. EQUATION 8-50 V N = 0.125 √ f’ c ( p )(D’ c )(h c ) V N = 0.125 √ 4( p) (12)(4) = 38 kip where f’ c = 4,000 psi = 4 ksi h c = 4 in D’ c = 8 + 4 = 12 in STEP 12 - DETERMINE CRITICAL HELICAL ANCHOR HEAD LOAD FOR PUNCHING Determine the critical helical anchor head load (T FN ) for punch ing using Equation 8-51. EQUATION 8-51 T FN, punching = V N = 38 kip STEP 13 - CONSTRUCT SOIL SCREW® ANCHOR STRENGTH ENVELOPE Construct the strength envelope at each anchor level as shown in Figure 8-16. At the wall face, the anchor head flex ural strength is less than the anchor head punching strength and therefore controls. There are eight helices per anchor. Each step in strength equals the single-helix bearing capacity for the anchor layer (Step 7). From the last helix (working from right to left) increase the pullout capacity in a stepwise fashion. If the pullout envelope working from the back of the nail does not intersect the flexural limit line, the strength envelope will

DESIGN EXAMPLES

π D 2

π d 2

π

π

EQUATION 8-47

π d 2

EQUATION 8-48

= 1.30k in/in = 1.30k ft/ft

= 566 ft/ft

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