5.2 Structural function of pile caps
5.2.3 A structural element subjected to a wide range of load cases
Pile caps are prone to be subjected to a wide range of load cases due to several variable loads acting on the structure like wind, snow or earthquake actions.
Therefore, pile caps must be designed to carry several load combinations.
Assuming that every pile has the same stiffness and that the pile cap is loaded by a single column, four types of design situations can be distinguished depending on the ratio between the vertical load and the moment applied at the column (M/N ratio). For each of these four cases, a simplified representation of a possible distribution of forces inside the pile caps is illustrated below. Note that the drawings refer to 2-D models like in beams; in pile caps, the choice of a strut-and-tie model can be more complicated.
(a) No moment resulting in a constant compressive normal stress in the column, equal compression in each pile, Figure 5.3.
Figure 5.3 Strut-and-tie model for a case with no moment in the column
(b) Small moment resulting in variable normal compressive stresses in the column and variable compressive forces in the piles, Figure 5.4.
Linear stress profile in column
(a)
Figure 5.4 Strut-and-tie model for a case with small moment in the column
(c) Average moment resulting in both compressive and tensile normal stresses in the column but only compressive forces in the piles, Figure 5.5.
Figure 5.5 Strut-and-tie model for a case with moderate moment in the column (d) High moment resulting in both compressive and tensile stresses in the column and a combination of piles loaded in compression and in tension, Figure 5.6.
Linear stress profile in column
(c)
Linear stress profile in column
(b)
Figure 5.6 Strut-and-tie model for a case with high moment in the column
In practice at Skanska it is common to use some simplifications on the safe side for these different cases. For instance, in case (b) the most loaded pile is on the left and the left part of the pile cap is more critical than the right one. Assuming that no dominant direction is chosen for the moment induced by external loading, each quadrant of the pile cap should be designed for the most critical case. Case (b) can be treated by applying the load of the most loaded pile to the four piles, which means that case (b) can be treated with an equivalent strut-and-tie model similar to case (a).
The same kind of procedure can be applied for case (c). Indeed, it is possible to say that one of the quadrants (represented by the left part of the beam in Figure 5.5) is exposed to the highest compressive stresses. Therefore each of the quadrants should be designed to carry this possible load case and it is possible to refer to case (a) with increased load for all the piles. However, in case (c), some tension is found below the column and this should be considered in the design. Thus, if the designer considers that the magnitude of the tension is too high for concrete and minimum reinforcement alone to carry, then some extra reinforcement should be provided. For example a vertical stirrup like the one shown in Figure 5.5 is an acceptable solution. Of course, the same procedure applies for each direction and reinforcement should be provided equally in the four quadrants below the column.
In the particular case (d) it is not possible to refer to case (a) because of the tension in some of the piles. Therefore, the designer should consider an additional strut-and-tie model.
For the model developed in this thesis work, it is possible to calculate the needed reinforcement arrangement for a wide range of load cases with moments applied at the columns by referring to an equivalent strut-and-tie model similar to the one in Figure 5.3. However, when some piles work in tension, a different strut-and-tie model has to be provided; an example is given in Figure 5.6. In this case, if, for example, no dominant direction for the moment is given, each quadrant of the pile caps has to be designed for the case where maximum compression is found in the pile (the strut-and- tie model in the left part of Figure 5.6 could be used for example) and for the case where maximum tension is found in the pile (the strut-and-tie model in the right part of Figure 5.6 could used).
Linear stress profile in column
(d)
Pile caps are subjected to a wide range of load combinations. However, a limited number of strut-and-tie models are necessary in order to handle them.