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Pilecap Design per Canadian A23.3 2019

Workflow

  1. Either:
    • Imports a superstructure model from a STAAD.Pro superstructure model that has been successfully analyzed (either by import from STAAD Foundation Advanced or by export from STAAD.Pro). This brings in the support reactions and column/pedestal data from that model into STAAD Foundation Advanced. The load cases / combinations for which the reactions need to be imported need to be selected during this step.

    or

    • Start with a blank model in STAAD Foundation Advanced, define the locations of column supports (if any), assign the column sizes and load cases, and then specify the loads acting on the foundation through those columns.
  2. Create a pilecap job and include in that job the supports where a pilecap needs to be designed. Choose the code as Canadian (A23.3-2019). Select the load combinations (categorized into Service and Ultimate) for which the pilecaps should be analyzed and designed.
  3. Only one column is permitted on a pilecap.
  4. Generate load combinations if needed.

    Load combinations from the superstructure model can also be imported through Step (1)

  5. In the Pile Layout pages, specify the pile capacities under serviceability state in axial compression, shear and uplift, along with parameters such as minimum c/c spacing between piles and minimum edge distance from c/l of outmost piles to edge of pilecap.
  6. Find all possible pile arrangements that meet the above criteria for all the service load cases/combinations contained in the job.
  7. From the list of arrangements proposed by the program, select one of the arrangements that you find acceptable. The program will determine a pilecap whose plan dimensions are suitable for that arrangement.
  8. Specify the parameters for the concrete design of the pilecap. Launch the design. The program will determine the adequacy of the specified pilecap thickness in flexure, oneway shear and punching along the cross section planes it deems weakest for the selected pile arrangement. These checks are performed for all the ultimate (strength) load cases contained in the job.
  9. If the pilecap thickness if found to be inadequate for any of the concrete checks, meaning, if a failure will occur unless the higher thickness is used, that information will be conveyed in the program's calculation sheet along with the recommended thickness. In this situation, the user needs to increase the pile thickness to the recommended value (or greater) and repeat steps 5 through 7.
  10. The details of the pile arrangement, pilecap selection, various concrete design checks, and associated critical load cases are reported in the calculation sheet for each support where a pile arrangement is selected for the thickness that the program deems necessary for a safe design.

Generation of load combinations

Load combinations can be generated to the 2005 edition of the NBCC code within the STAAD Foundation Advanced environment provided that the column reaction loads for primary load cases, categorized as Dead, Live, Wind, Seismic, etc., have been specified or have been imported from the STAAD.Pro model. Alternatively, the combinations can be specified in the STAAD.Pro superstructure model and, after the analysis of that model, the support reactions for those combination cases can be imported into STAAD Foundation Advanced for the service and ultimate checks. If they are imported, it is the user’s responsibility to categorize them into service and ultimate.

Transfer of loads from column/pedestal to the pilecap

Column support reactions from the STAAD.Pro superstructure model are converted to loads on the pilecap by reversing the signs of the support reactions. The sign convention for the resulting loads is the same as that of JOINT LOADS in STAAD.Pro. STAAD Foundation Advanced uses the same global coordinate system as STAAD.Pro’s Y-Up system.

The loads derived from the above process are assumed to act at the top of the pilecap in the absence of a pedestal, and at the top of the pedestal if a pedestal is present. The lateral loads (FX and FZ) are multiplied by the thickness of the pilecap (or pilecap + pedestal) and added to the moments derived from the column reactions, and the resulting moments are then used for calculating the pile reactions.

Calculation of pile reactions

Pile reactions are calculated using the Bolt Theory. Refer to Pile Cap Theory.

If the forces and moments at the bottom of the pilecap are:

  • P = vertical load derived from the column reaction force FY, selfweight of the pilecap and pedestal, weight of soil above the pilecap, and, weight due to surcharge pressure.
  • Mx = Moments obtained from the column reaction term MX + FZ × d
  • Mz = Moments obtained from the column reaction term MZ - FX × d
  • FX and FZ are the lateral (horizontal) forces obtained from the column reactions
  • d = thickness of the pilecap
  • n = number of piles in the pile arrangement
  • Shear on each pile = F = F x 2 + F z 2 n
  • Axial load on each pile = P n     ± ± M z . X 1 ∑ k = 0 n X k 2 +   ± M x . Z 1 ∑ k = 0 n Z k 2

where X1 and Z1 represent the distances along the global X and Z directions of the pile from the center of the column. The column center is assumed to be the origin of the local axis system of the pilecap.

The program is equipped with a library of pile arrangements. For each service load case, it checks each arrangement to ensure that,

Shear "F" as calculated above should be less than the shear capacity of each pile.

Axial load on the pile as calculated above should be less than the axial capacity of each pile. If axial load is upwards (tensile), then it is checked against pile uplift capacity.

If any of the capacities are exceeded, that arrangement is discarded.

The successful arrangements are then displayed in a list from which the user can choose one of those arrangements for which to perform the concrete design.

Examples illustrating this method are available in the program’s Verification Examples manual.

Pilecap Size

The pilecap size is defined using two terms – PCL, and PCW. These are the overall plan dimensions of the pilecap along the local X and Z axes.

Minimum Thickness of the Pilecap

As per section 15.8.3 of the Canadian A23.3-2019 code, the minimum required thickness of the pilecap is calculated as Pile in pile cap + 300 mm

where "Pile in pilecap" is a value that the user is required to specify in the concrete design parameters page of the program. It represents the embedment of the piles in the pilecap.
Parent topic: Pile Cap Theory

Concrete Design Checks

Concrete design of the pilecap is performed for four conditions:

  1. Oneway shear in the vicinity where the column or pedestal meets the pilecap
  2. Punching shear (also known as twoway shear) due to the column or pedestal punching through the pilecap in the downward direction
  3. Flexure in the vicinity where the column or pedestal meets the pilecap
  4. Corner pile actions
  5. Bearing

The basic principle in concrete design is that the capacity of the section should be greater than or equal to demand. Demand represents the forces or moments corresponding to the term being checked (flexure, oneway shear, two-way shear, etc.), while, capacity represents the resistance that can be mobilized from the concrete and steel contained in the pilecap.

Oneway shear

Checks for one-way shear are performed at a cross section (perpendicular to the plane of the pilecap) located at a distance dv from the face of the column (or pedestal if a pedestal is present) as required by section 11.3.2 of A23.3-2019.

Demand Vf:

This is the shear force acting at the section of the pilecap where the check is performed. It is computed from the summation of the algebraic values of the axial force in the piles, selfweight of the footing, soil weight, and force due to surcharge.

Shown below are figures depicting the locations on the pilecap of the oneway shear planes where the demand and capacity are calculated. The check is performed for 2 planes – local XY, and local YZ.

The position of the pile with respect to the oneway shear line has a bearing on the amount of force that it contributes to the total oneway shear force. For this, the guidelines of section 15.5.3 of A23.3-2019 are used.





Capacity Vr:

STAAD Foundation Advanced does not compute shear reinforcement in pilecaps. Thus, the capacity Vr at the critical section has to be provided entirely by the shear strength of concrete alone.

The factored shear resistance of concrete, Vc, is calculated using equation 11.6 shown in section 11.3.4 of the Canadian code A23.3-2019, with a cap applied as per equation 11.5 of section 11.3.3. In the implementation of these equations, the nominal maximum size of coarse aggregate "ag" is set to 20 mm (see section 3.2 of the code).

The shear resistance which is in the form of a stress is converted to a force by multiplying it by the shear area which is equal to:

Shear area = Length of the oneway shear line multiplied by "dv"

Punching shear

Demand:

The punching shear force is calculated at a distance of deff/2 away from the face of the column (or pedestal if one is present) as recommended in section 13.3.3.1 of A23.3-2019

where deff = effective depth of the pilecap.

As in the case of oneway shear, there is a likelihood that the punching shear boundary may pass through the cross section of one or more piles. In that event, the guidelines of section 15.5.3 of A23.3-2019 are used to determine how much reaction that pile contributes to the punching shear force.

The factored shear stress vf due to punching is calculated using equation 13.9 in section 13.3.5.5 of A23.3-2019. This method produces a corner stress whose value receives contribution from

  1. the vertical compressive force from the column or pedestal
  2. unbalanced moments about the 2 principal bending directions of the column

If the column or pedestal transfers a biaxial moment to the pilecap, the maximum stress will occur at a single corner, and is referred to as the maximum corner stress in punching.

Capacity:

The factored shear stress resistance of concrete (capacity) is calculated as the minimum of the values obtained from equations 13.5, 13.6 and 13.7 in section 13.3.4.1 of A23.3-2019. That stress is compared against the maximum corner stress in punching as described above under "Demand".

If demand exceeds capacity for the specified thickness, the program calculates the overall depth needed to ensure that capacity exceeds demand, and treats the design as failed. The required thickness is also shown in the calculation report in this situation.

Flexure

Flexure design is performed at the face of the pedestal, or, at the face of the column if a pedestal is not present.

Demand:

This is the bending moment acting at the section of the pilecap where the check is performed. It is computed from the summation of the algebraic values of moments caused by the axial force in the piles, selfweight of the footing, soil weight, and force due to surcharge. It is calculated in 2 planes – local XY, and local YZ, as shown in the following figures.

Capacity:

At the section that is being designed for bending, the flexural capacity is computed for a singly-reinforced section. The flexural strength contributed by concrete is calculated for the condition that the maximum strain at the extreme compression fibre is 0.0035 as suggested in section 10.1.3 of A23.3-2019. The tensile force in reinforcing steel is based on the user-specified yield strength fy with an upper limit of 500 MPa, as described in section 8.5.1. of A23.3-2019.

The width of the section of the pilecap "bw" is calculated at the section where the flexure design is performed. It will be equal to the full width of the pilecap for rectangular shaped caps, while its value for a 3-pile arrangement will be restricted by the location of the critical section.

Based on the pilecap thickness, pile in pilecap value, clear cover, and preferred bar diameter, the program calculates the effective depth "deff". Then based on that deff and grade of concrete, the maximum moment capacity of the section is calculated. If the factored moment (demand) exceeds the moment capacity, the thickness required to meet that demand is determined, else, the specified thickness is used in calculating the area of steel reinforcement required. This is performed for each ultimate load case, and the highest thickness and largest amount of reinforcement from these checks is then reported in the calculation sheet as critical. This is performed for the 2 primary planes of bending – local XY and local YZ.

If demand exceeds capacity for the specified thickness, the overall depth needed to ensure that capacity exceeds demand is shown in the calculation report, and the design is deemed to have failed.

Minimum flexural steel is based on sections 10.5.1.1, 10.5.1.2 and 7.8.1 of A23.3.

For 3-pile arrangements, the two locations (on either side of the pedestal) are checked independently for bending in the local XY plane because the shape of the pilecap results in two different widths for those two locations.

Corner Pile Actions

The portion of the pilecap in the vicinity of the piles that are closest to the corners of the pilecap is subjected to the following additional checks.

  1. Corner pile punching
  2. Oneway shear

The above diagram shows the plane for the 2 checks. For the punching check, if clear distance from the edge of the pile to the edge of the pilecap exceeds deff/2, a circular profile is used for the punching shear boundary. Else, the profile shown above is used.

The demand and capacity values for these two checks are similar to these checks described earlier.

Bearing of Column/Pedestal on the Pilecap

The program also performs a bearing check in which the maximum compressive force on the bearing area from amongst all the ultimate load cases is compared against the bearing capacity of concrete which is calculated as per section 10.8.1 of A23.3-2019.

Output from the Program

The program output in the calculation report consists of the following: The load case(s) that are critical for each of these checks are also shown.

  1. Pilecap dimensions
  2. Service load cases responsible for the maximum pile reactions for compression, tension and shear
  3. Detailed output for design for flexure for the 2 principal planes
  4. Detailed output for design for oneway shear for the 2 principal planes
  5. Detailed output for design for punching shear
  6. Detailed output for design for bearing
  7. Detailed output for design for corner pile checks.
  8. If the user specified thickness is not sufficient, meaning a higher thickness is required, that information will be reported in the form of an error message.