Modeling Piles/Piers in FEM Software

When we use finite element software for analyzing deep foundations then it becomes important to model piles with accurate axial stiffness so that the load can distribute properly based on pile stiffnesses.

Some people suggests to calculate pile stiffness by AE/L formula. By doing this method the problem is that as you increase the length of the pile, the stiffness will reduce which is not accurate.

The best thing is to ask Geotech Engineer for the allowable loads and the allowable axial displacement on working loads. Most of the times this allowable displacement is ranged from ¼” to ½” which can be confirmed from geotech report.

Take your allowable load which will vary based on the depth of pile and divide it by the axial displacement, this will be the required pile stiffness.



k = Pile axial stiffness (kips/inch )

P = Allowable load (kips)

Δ = Allowable vertical displacement (inches)

Punching Shear Ratio in SAFE

SAFE: Punching shear results are displayed as a ratio of maximum calculated shear with respect to capacity. A ratio above 1.0 would indicate capacity was exceeded somewhere along the critical section. The ratio is displayed for each column and each point load. A notation of N/C means the value was not calculated by the program. From my experience i have found that SAFE punching shear calculation are conservative, therefore i always check manually if SAFE displays ratio more than 1.0.

Area of Reinforcement in SAFE

SAFE reports Area of reinforcement in unit lentgh2/length (mm2/m, cm2/m, and in2/ft).
For calculating Number of bars@spacing, do the following:

Select Bar Diameter, multiply its area with strip width & divide Strip width by required Area of steel.

Assume we get strip Area of Reinf. from SAFE, As = 323 mm2 /m
suppose T10 mm diameter bar, Ab = 78.5 mm2
Check the strip width from SAFE, Assume, Ws = 1 m
Spacing can be calculated in this way, S = Ab x Ws/As

Therefore, S =  78.5×1/323 = 0.243 m

Provide T10@225mm Spacing,
As = 78.5×1/0.225 = 348.88 mm2/m

Assigning Pier and Spandrel Labeling in ETABS

Wall piers are typically assigned in the model to the wall over height of floor (as in Figure d below).

Often, it is more appropriate to assign the pier to the height of wall adjacent to opening (Figure a or c), resulting in better calculations of shear/moments from the area object directly above/below the opening.
Unlike wall pier elements, a single wall spandrel element can include area
objects from two adjacent story levels. For Spandrel labeling you can follow the following figure