Design Methods for Tall Building Foundations

This article discusses the challenges of designing foundations for tall buildings. It emphasizes the need to move beyond traditional methods and adopt a more sophisticated approach. The article first highlights some characteristic features of tall buildings and then discusses various foundation options. It then explains the process of foundation design and verification.

Characteristics of Tall Buildings

Several characteristics of tall buildings can significantly influence foundation design.

• Building Weight: The weight of tall buildings increases non-linearly with height. This makes it crucial to carefully consider ultimate bearing capacity (the maximum pressure a foundation can withstand before failure) and settlement (the downward movement of the foundation due to the load).

• Lateral Loads: Tall buildings are subjected to high lateral loads (loads that act horizontally) due to wind and earthquakes. These loads can induce bending moments (a measure of the bending effect on a structural element) and shear forces (forces that act parallel to a surface) in the foundation system.

• Podium Structures: Tall buildings are often surrounded by low-rise podium structures. The difference in load magnitudes between the tower and podium can result in differential settlements.

  
  

Foundation Options

This section describes various foundation options commonly used for tall buildings.

Raft or Mat Foundations

A raft foundation is a thick concrete slab that supports the entire structure. It is a suitable option when the base of the development is founded close to or embedded into competent rock. Raft foundations are feasible for buildings of moderate height. However, for very tall buildings, a raft foundation alone may not be sufficient to handle the large vertical and lateral loads.

• Advantages:

  • Relatively simple to construct.

  • Can accommodate basement structures.

• Limitations:

  • May not be suitable for very tall buildings due to large vertical and lateral loads.

  • Large size can make construction in congested areas challenging.

Piled Foundations

Piled foundations consist of long, slender elements driven or drilled into the ground to support the structure's load. They are used when the soil near the surface is not strong enough to support the building's weight.

• Advantages:

  • Can transfer loads to deeper, more competent soil or rock layers.

  • Can handle large vertical and lateral loads.

• Limitations:

  • Installation can be complex and costly.

  • Requires specialized equipment.

    
    

Piled Raft Foundations

Piled raft foundations combine the benefits of both raft and piled foundations. This system utilizes a raft resting on piles to transfer the building's load to both the soil beneath the raft and deeper soil or rock strata.

• Advantages:

  • It can accommodate large vertical and lateral loads.

  • It can reduce settlements and differential settlements compared to raft foundations.

• Limitations:

  • Design can be complex and requires careful consideration of load sharing between the raft and piles.

  • It may not be suitable for all soil conditions. For example, there may be a reluctance to use piled raft foundations in soft clays because they offer only modest bearing capacity and stiffness for the raft.

    
    

The Design Process

The article outlines a three-stage process for designing high-rise building foundations:

1. Preliminary Design: This stage develops initial foundation concepts and cost estimations.

2. Detailed Design: The selected foundation concept is analyzed in detail, and refinements are made to the layout and specifics of the system. This stage involves collaboration with the structural designer.

3. Final Design: The analysis and parameters used in the design are finalized in this stage.

Key design issues and criteria to be addressed include:

• Ultimate Capacity: The foundation's ability to withstand vertical, lateral, and moment loading combinations without failing.

• Cyclic Loading: The influence of wind, earthquakes, and waves on the foundation's capacity and movements.

• Serviceability: Ensuring that settlements, differential settlements, and rotations are within acceptable limits.

• Ground Movements: Accounting for ground movements caused by construction activities like excavation and pile installation.

• Soil-Structure Interaction: Considering how the foundation and superstructure interact and share loads.

• Durability: Ensuring the long-term integrity of the foundation in potentially corrosive environments.

  
  

Preliminary Design Tools

For preliminary design, engineers can use tools like spreadsheets, MATHCAD sheets, or simplified hand or computer methods. check the app store section for apps that can assist you in this stage.

These tools help to simplify the foundation system analysis. One approach involves simplifying the system into an equivalent pier to examine its overall stability and settlement.

Ultimate Capacity:

• The bearing capacity under vertical loading can be estimated by considering two factors:

  1. The sum of the ultimate capacities of the piles and the net area of the raft.

  2. The capacity of the equivalent pier and the capacity of the raft portions outside the pier.

Settlement:

• The average foundation settlement can be assessed using elastic solutions for a vertically loaded pier. This method requires simplifying the ground profile to a soil layer overlying a stiffer layer.

• The stiffness of a piled raft foundation can be estimated using the approach by Randolph (1994), which considers the pile group stiffness, raft stiffness, and raft-pile interaction factor.

  
  

Detailed and Final Design

For detailed and final design stages, more refined techniques are required. Software programs with advanced capabilities should be employed. These capabilities should include:

• Overall Stability Analysis: The ability to handle non-homogeneous soil profiles, non-linear pile behavior, various loading conditions, and piles with differing characteristics within the same group.

• Serviceability Analysis: The ability to consider pile-pile interaction, raft-pile interaction, raft flexibility, and the stiffness of the supported structure.

  
  

Assessment of Geotechnical Design Parameters

The article emphasizes the importance of accurately assessing geotechnical design parameters. Key parameters that require assessment include:

• Ultimate Skin Friction: The maximum friction force that can be developed between the pile surface and the surrounding soil.

• Ultimate End Bearing Resistance: The maximum resistance the pile tip can withstand before penetrating further into the bearing stratum.

• Ultimate Lateral Pile-Soil Pressure: The maximum pressure the soil can exert laterally on the pile.

• Ultimate Bearing Capacity of the Raft: The maximum pressure the soil beneath the raft can withstand.

Methods to determine these parameters include:

• Empirical Correlations: Relationships based on in situ test data (like SPT and CPT) and unconfined compressive strength for piles in rock.

• Laboratory Testing: Direct shear tests, especially the constant normal stiffness (CNS) test, which simulates pile-soil interface conditions.

• Derivation of Secant Values: Adjusting the small-strain shear modulus to account for the larger strain levels relevant to foundation performance.

  
  

Pile Load Testing

The article highlights the importance of pile load testing to verify design assumptions and refine design parameters. It discusses several common methods of pile testing:

• Static Vertical Load Test: Applying vertical load directly to the pile head. This is considered the most definitive test but can be challenging to implement in practice.

• Static Lateral Load Test: Applying horizontal load to the pile head to assess its lateral stiffness and resistance.

• Dynamic Load Test: Using a hammer or other impact device to apply a dynamic load to the pile head.

• Bi-Directional Load Test: Using a specialized cell to apply load to both the pile base and shaft simultaneously. This test helps determine the load-sharing behavior between the shaft and base.

• Statnamic Test: Applying a controlled, rapid load to the pile head using hydraulic jacks.

Test Interpretation:

• Ultimate Axial Capacity: There are various methods to estimate the ultimate capacity from tests that don't reach complete failure.

• Ultimate Shaft Friction: The distribution of skin friction along the pile shaft can be evaluated from strain gauge readings.

• Load-Settlement Behavior: Pile head stiffness (load/settlement) can be determined from load test data. This information helps validate design assumptions and refine parameters.

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Conclusions

This article presents the importance of a three-stage design process for high-rise building foundations. It underscores the need for collaboration between structural and geotechnical engineers throughout the design process. This article highlights the value of pile load testing, combined with advanced analysis methods, to verify design assumptions and ensure satisfactory foundation performance. If you want to streamline your foundation design process check our catalog of software applications. our team of expert geotechnical engineers can also guide you in the foundation design process. request a free live one-to-one consultation now.