Options for Modeling Bored Piles

Technical Article

RFEM and RSTAB offer different options to model bored piles. One option is to display bored piles as single-valued supports or hinged columns. Another option is the realistic modeling while taking into account the soil by means of applying a member elastic foundation. The two following examples will describe it in detail. However, pile base resistance, skin friction and soil layers are not considered in this technical article.

First Example: Concrete Slab on Nine Bored Piles

A concrete slab of 10 x 10 meters with a thickness of 180 mm is supported by nine bored piles, of which four bored piles (nodal support 5) including the middle bored pile only assume forces in Z-direction. Two of the remaining bored piles have each an inclination of 10° about the X-axis (nodal support 6), another two have each an inclination of 10° about the Y-axis (nodal support 7) and one bored pile has an inclination of 10° about the X- and Y-axis (nodal support 8), so that horizontal forces can be absorbed in the corresponding directions and the system is stable.

Figure 01 - Ground Plan of Concrete Slab on Nine Bored Piles

The system is loaded by a surface load of 4.5 kN/m² and two line loads, each with 1.0 kN/m.

In the first modeling option, the bored piles are displayed with straight and inclined, single-valued, rigid supports, while in the second option, the bored piles are modeled as beams with hinged connection. These two options only differ in the strain stiffness of the beams.

Figure 02 - Modeling Option 1 and 2 Without Member Elastic Foundation

The third modeling option corresponds to the second one - extended by a member elastic foundation in local y- and z-direction of each member, which simulate the elasticity of the corresponding subsoil.

Figure 03 - Modeling Option 3 with Member Elastic Foundation

For entering spring constants, see the following article Member Elastic Foundations 1: Translations and the FAQ How do I specify elastic member foundations?.

The results show the differences of the three options. As already mentioned above, there is basically no difference between options 1 and 2 with regard to support forces, so that the results also correspond to each other. With the given loading, tension and also compression forces occur in the bored piles which leads to the fact that the load is distributed relatively unevenly with high amounts at the same time. With regard to the maximum support force, choose these two options to be on the safe side. Figure 04 shows the support forces of the options 1 and 2 in one row in the visibility mode.

Figure 04 - Support Forces of Options 1 and 2, in One Row in the Visibility Mode

It is recommended to enter a member elastic foundation to reduce the support forces and distribute them more evenly. The resulting shear forces and bending moments are relatively low so that only the occurring normal forces and support forces will be considered here. Figure 05 shows the support forces of the options 3 in one row in the visibility mode.

Figure 05 - Support Forces of Option 3 with Member Eleastic Foundation, in One Row in Visibility Mode

2. Example: Steel Structure on Three Bored Piles

A steel structure, consisting of two steel beams rigidly connected with each other, each 3 m long, as well as of a 1 m long bracket, is loaded by three single loads in all three directions at the end of the bracket. Three bored piles with inclinations of 10° each about the X-axis and Y-axis are the supports.

Two modeling options are considered here: Option 1 without member elastic foundation, option 2 with member elastic foundation.

Figure 06 - Isometry Option 2

If you consider the structural system in both XZ plane and YZ plane, you will quickly notice that the structure is instable due to the effective lines of the forces of the bored piles intersecting at one common point.

Therefore, you have to apply a member elastic foundation here.

Summary

When modeling bored pile structures, the two options  with or without member elastic foundation provide different results. The option without member elastic foundation can be considered unfavorable with regard to the normal forces of the piles, whereas the option with member elastic foundation is usually more economically. In case of insufficiently stiffened structures, you have to use member eleastic foundation, provided that any other horizontal stiffening is not present.

Keywords

Bored pile Member elastic foundation Hinged column Stiffening

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