Transmission And Substation Foundations - Technical Design Manual

A Basic Guideline For Designers APPENDIX C: HELICAL PILES & ANCHORS

square shaft helical piles. It is used to easily determine P2 and P3, which can then be compared to P4 to see which will control the design. The table is broken down by soil type, end condition, and number/diameter of the helix plates. For example, a Type SS175 helical pile in firm soil & fixed end condition with multi-helix plates (3 or more plates) has ASD allowable compression strength of 98.3 kip. However, that exceeds the P4 geotechnical allowable capacity of 52.5 kip. But if the soil is soft with the same fixed end condition, the ASD allowable compression strength is 30.2 kip; which is less than the P4 geotechnical allowable capacity of 52.5 kip. The difference is the depth to fixity, which is 5’-0 in firm soils and 10’-0 in soft soils. Table C-6, reprinted from Section 7 of the TDM lists the P1 (new construction bracket) and P2 (shaft) ASD allowable strengths for Type SS175 helical piles. It is used to easily determine P1 and P2, which can then be compared to P4 to see which will control the design. The table is broken down by concrete strength, soil type and end condition. For example, a Type SS175 helical pile in firm soil & fixed end condition with a new construction cap embedded in 2500 psi concrete has an ASD allowable compression strength of 52.7 kip based on the strength of the cap (P1). The P4 geotechnical allowable capacity of 52.5 kip is basically the same. But if the soil is soft with the same fixed end condition, the ASD allowable compression strength is 30.2 kip based on the shaft strength; which is less than the P4 geotechnical allowable capacity of 52.5 kip. Again, the difference is the depth to fixity, which is 5’-0 in firm soils and 10’-0 in soft soils. Table C-7, repeated from Section 7 of the TDM, lists the P1 (remedial repair bracket) and P2 (shaft) ASD allowable strengths for Type SS175 helical piles. It is used to easily determine P1 and P2, which can then be compared to P4 to see which will control the design. The table is broken down by concrete strength and soil type. Chance Remedial Repair Brackets provide fixed end condition at the bracket-shaft connection.

The strength of the concrete will also factor into the axial compressive strength of helical piles. Higher strength concrete results in higher bearing pressure with both embedded new construction pile caps (P1) and foundation repair brackets (P1). Helical piles can be one-piece foundation elements, but are more commonly produced in sections that are coupled together during installation. Therefore, the strength of the coupling must be considered in the design as part of the shaft (P2). Chance helical pile couplings are designed to meet or exceed the torque correlated geotechnical capacity [Method 2]. They are also designed to meet or exceed the bending strength of the shaft itself. Structurally, the couplings limit both the tension and compression strength. For Chance Type SS helical piles, the coupling bolt is the limiting factor for tension strength. Load direction is an important consideration and strongly affects the shaft type and size required. This was discussed previously under P4. The Application (new construction, foundation repair, earth retention, etc.) also affects the shaft type and size required. For example, it is not practical to use large diameter shaft helical piles for underpinning existing building structures. Section 7 is broken down by specific helical pile product families. Each family sub-section lists the tension and compression strengths in various tables, in addition to specifications and available configurations. For example, the P2 (shaft) strength and P4 (geotechnical) tension capacity for Type SS175 helical piles are shown in Table C-4. The pre-qualified and verified torque correlation factor (Kt) is 10 for Type SS175. The torque rating for SS175 is 10,500 ft-lb. Therefore, per P4 [Method 2], the torque correlated capacity limit for SS175 is 105 kip (see Section 6 TDM). The nominal strength of Type SS175 shaft (P2) is limited to 100 kip by the shear strength of the coupling bolt. Comparing the two, 105

TABLE C-4 - SS175 - P2 Tension Strength and P4 Torque Correlated Capacity Torque Properties Torque Correlation Factor 10 ft-1

33 m-1

P2

P4

Torque Rating

10,500 ft-lb

14,240 N-m

Structural Capacity

Nominal

LRFD Design

Tension Strength

100 kip

445 kN

75 kip

334 kN

Allowable Tension Strength

50 kip

222 kN

Torque Correlated Capacity Capacity Limit Based on Torque Correlation, Tension/ Compression

Ultimate

Allowable

P4

105 kip

467 kN

52.5 kip

234 kN

kip > 100 kip, therefore P2 tension strength controls at max torque. If the installation torque is less than 10,000 ft-lb, then P4 [Method 2] will control. The allowable geotechnical capacity of 52.5 kip is based on a deterministic factor of safety of 2. It is convenient to tabulate axial compression strength in terms of either P2 (shaft) & P3 (helix), or P1 (bracket) & P2 (shaft). Table C-5, from Section 7 of this manual, lists the P2 (shaft) and P3 (helix) ASD allowable strengths for Type SS175

www.hubbell.com/hubbellpowersystems | C-7