Skin Friction Resistance Calculation

For cohesive soils, skin friction is calculated based on API eq 6.4.2-1

f = α c

Where:

α = a dimensionless factor

c = undrained shear strength of the soil at the point in question

For cohesionless soils, skin friction is calculated based on API eq 6.4.3-1

f = K p_o tan(δ)

Where:

K=coefficient of lateral earth pressure (ratio of horizontal to vertical normal effective stress)

p_o =effective overburden pressure lb/ft² (kPa)at the point in question

End Bearing Resistance Calculation

End bearing is calculated from geometry of pier.

Q_p=q A_p

Where:

q = unit end bearing capacity,lb/(ft² )(kPa)

A_p = gross end area of pier, ft² (m²)

For cohesive soils, unit end bearing capacity is calculated based on API eq 6.4.2-3

q = 9 c

Where:

c = undrained shear strength of the soil at the point in question

For cohesionless soils, unit end bearing capacity is calculated based on API eq 6.4.3-2

q = p_o N_q

Where:

p_o =effective overburden pressure lb/ft² (kPa)at the point in question

N_q = dimensionless bearing capacity factor

Effect of water table

The water table depth is used to calculate buoyant forces on the pier. Buoyant forces are deducted from total axial capacity of the pier

Self weight of the pier and factors of safety

Pier self weight is taken into consideration while calculating the axial capacity.

The end bearing safety factor is applied to tip resistance and skin friction safety factor is applied to side resistance. Total axial capacity is calculated as,

Total Axial Capacity = (End bearing resistance – Pier self weight)/(End bearing Safety factor) + (Skin friction resistance – Buoyant force )/(Skin Friction Safety factor)

The program also offers % of skin friction and end bearing contributing to axial capacity through user input.