|
Round Pipe vs. Square Shaft -- Helical Screw Foundations and Anchors |
|
Feature |
Round Pipe |
Square Shaft |
| Installation torque related to ultimate bearing capacity |
Less bearing capacity for a given installation torque |
More bearing capacity for a given installation torque |
Shaft twist
|
Not obvious without other means of visual reference |
Visibly obvious —good indicator of torque |
Installation energy (work)
|
More: Larger cross section and increased friction due to round shape |
Less: Smaller cross section and less friction due to square shape |
Total Load Capacity
|
End bearing and skin friction |
End bearing only |
| Soil disturbance |
More: Larger shaft-diameter to helix-diameter ratio, i.e. more disruption and pore pressure
build up |
Less: Smaller shaft-diameter to helix-diameter ratio, i.e. less disruption and pore pressure build up |
| Center-to-center spacing |
Must be spaced farther apart due to increased pore pressure build up |
Can be spaced closer together due to less pore pressure build up |
| Soil penetration |
Larger cross section reduces penetration into soils with higher Standard Penetration Test N values |
For a given torque, will penetrate farther into a given bearing strata
Will penetrate soils with higher Standard Penetration Test N values |
Couplings
|
Multiple bolts are more difficult to connect in the field
Coupling bolts must resist both torque and axial load
Little slack in couplings applicable for load reversal applications |
Single bolt is easy to connect in the field
Torque transfer across square shape, not coupling bolt
Slack in coupling not applicable for load reversal applications (without alterations) |
| Shaft bending resistance |
More: Larger shaft size results in larger section
properties |
Less: Slender shaft results in smaller section
properties |
| Shaft buckling resistance |
Critical buckling load is more for a given soil due to larger shaft size |
Critical buckling load is less for a given soil due to slender shaft (practical concern in only the
softest soils) |
| Lateral shear capacity |
Larger shaft size results in more lateral capacity for a given soil |
Slender shaft results in less lateral capacity for a given soil |
| Column/shaft above grade |
Better: Larger shaft has greater stiffness |
Limited: Slender shaft has less stiffness |
| Corrosion loss potential |
More: Hollow pipe shaft means more surface area (both inside and out) exposed to corrosive
environments |
Less: Solid slender shaft means less surface area exposed to corrosive environments |
| Ground water migration. |
More: Larger cross section results in more disturbed soil acting as a water wick. Hollow shaft allows open conduit to surface water |
Less: Smaller cross section and solid shaft results in less disturbed soil acting as a water wick |
