When undertaking the soil-structure interaction modelling the following procedure
is recommended.
The structural model should include:
Raft dimensions (thickness, voids, column/core locations), including the stiffness of core walls, which is significant
Loads (point load - from column, line load - from wall, area loads - from any slabs/
plant rooms)
A grid of spring supports with constant node spacing (generally 20 % of the average column spacing).
Figure 5.4 3D soil structure interaction model.
Foundations 5
The geotechnical model should include:
Ground conditions (strata, stiffness values etc) based on the site investigation data.
The same grid and nodes as for the structural model, at the base of raft.
The soil-structure interaction should then be undertaken using the following procedure:
1. Assign a constant spring under each node in the structural model to achieve overall settlement of approximately 50mm when the model is run.
2. Import each nodal reaction from the structural model into the geotechnical model.
3. Run the geotechnical model to derive the settlement of each node.
4. For each node calculate the spring stiffness (k) using the expression;
k = Pressure (from structural model) Settlement (from geotechnical model)
5. Assign this set of spring values (k) to all the nodes in the structural model to derive a new set of nodal reactions.
6. Repeat steps 2, 3 and 4 to derive a new set of spring values. This process is repeated until the nodal settlement from the structural and geotechnical model matches. This can generally be achieved in around three and five iteration cycles.
Once the settlement in both the software models converges (as established from a comparison of the settlement outputs, see Figure 5.5) the bending moment, shear, bearing pressure and deflection of the raft can be determined.
5.3.2 Piles
When a pile solution is adopted under high-rise buildings, designers usually evaluate the adoption of significant groups of large diameter piles. Although emphasis is often placed on the testing regimes of single piles to validate the overall pile design, there are distinct differences between the performance of a single pile and that of a large pile group.The performance of a single pile is governed by two discrete facets:
Friction generated between the pile shaft and the ground
Strength of the soil/rock underlying the base of the pile.
Figure 5.5 Convergence of settlement from structural and geotechnical software.
5 Foundations
For a pile group, the piles encompass a large volume of soil and the action of piles and soil working in combination will dictate group performance. As with a raft, the base of the pile group is loading a significant area of the underlying strata, and material well below the base of the piles is stressed and will thereby effect pile group settlement.
Some relatively simple procedures have evolved enabling designers to assess pile group performance, however, it is recommended that advice is sought from a geotechnical specialist.
As with raft analysis, the use of 2D finite-element, soil-structure interaction analysis requires the adoption of representative load distribution to replicate the full weight of the building. The piles themselves are represented as equivalent walls. Despite these limitations for straightforward load cases, analysts can make reasonably good predictions.
While 3D FE soil-structure interaction analysis is readily achievable, the number of elements required to present solid piles in a large pile group can be prohibitive. Most analysis packages allow each pile to be represented by a beam, with limits set to represent friction and end-bearing capacity. Complete building loads, substructure and pile cap can be suitably modelled and overall foundation performance predictions are found to be good providing the data used, particularly for the ground, is accurate.
5.3.3 Pile-assisted rafts
While many foundations are either simple rafts or end up as large pile groups, the benefits of both forms can be realised in a pile-assisted raft. This approach affords considerable efficiencies over the adoption of a fully-piled system, while using a raft to avoid excessive settlement.In its elemental form, the pile-assisted raft can be envisaged as a raft with piles
strategically placed beneath highly-loaded columns to reduce the overall load on the raft and limit settlement. However, the full benefit is realised when analysis is able to reasonably predict the portion of load carried by the raft and by each pile.
Analysing pile-assisted rafts is complicated by a three-fold load path, with the building load applied to the top of the raft, transferred in turn to the piles, being stiffer than the ground, and then to the soil beneath the raft as the piles settle. The load in the ground then increases the stress induced on the piles themselves.
While 3D FE soil-structure interaction software has aided comprehension of the complex raft-pile-soil interaction system, inevitable uncertainties related to ground parameters should still be addressed by varying these values as part of the design process.
In summary, pile-assisted rafts offer an economical foundation solution in many situations. To some degree, risks associated with variance in analysis and field performance are offset by the composite approach, while adoption of a raft also potentially offers a reduced factor of safety to guard against unexpected pile failure.
Foundations 5
5.4 Basement design
It is common to construct basements in conjunction with tall buildings, often extending well beyond the footprint of the tower itself. The issues relevant to this situation are broadly similar to that for any basement work, namely: Retaining the surrounding deposits during excavation and post-construction.
Ensuring movement of the ground beyond the basement footprint does not cause damage to surrounding buildings and infrastructure.
Controlling groundwater inflow and uplift pressures during construction.
Providing a waterproof substructure.
Either resisting water uplift pressures or under-draining the basement.
Resisting any long-term heave exerted by the ground.
Elements that can be seen as relatively unique for tall buildings are:
Ensuring slab continuity across the basement to resist lateral soil and water pressures.
Controlling the impact of slab shrinkage and creep ‘pulling in’ the permanent basement walls.
Accommodating the significant load changes occurring at the perimeter of the tower footprint - develop a transition zone, ensuring no abrupt change in raft/slab thickness or reinforcement so as to prevent localised cracking.
Due to the many parameters set out above, it is difficult to give precise guidance on issues relating to basement construction in tall buildings. However, general guidance on the design of concrete basements can be obtained from The Concrete Centre publication:
Concrete Basements, CCIP-044 [3].
6 Buildability
6. Buildability
Early contractor involvement is essential to inform the decision-making process within the design and detailing of tall buildings. With the design team and contractor working in partnership from an early stage, fundamental choices affecting the design can be made, such as core construction methodology, tower cranes, access (hoists and stairs) and screens. In addition, early collaboration affords time to ensure the most advantageous construction sequence can be realised, developed and understood by all parties.
The main drivers for contractors are:
Safety of site operatives and the public
Crane utilisation
Incoming logistics, laydown and storage areas
Vertical movement of operatives and materials
Standardisation of structural elements and components which leads to repeatability of forming.
6.1 Core construction
The formwork system used for core construction needs to be considered integrally with the cranage and logistics solutions for the construction, including the relative position and height of core links to the cranes. If the crane is tied to the core, the two can progress together, whereas if the crane is tied to the slabs, then the core and crane progression will be directly linked to slab progress.Structural engineers and contractors should engage in workshops from an early stage, enabling restrictions on the height of the core beyond the following slabs to be fully understood by all parties. It may be necessary to re-examine the foundation solution to ensure it can cope with the different load case when the core has been jumped in advance of the rest of the building.
Figure 6.1 High-rise construction.
Photo: Laing O’Rourke Plc.
Buildability 6
Good Good Requires making good Requires making good
Concrete required Early age strength for cycle times
Concrete placement Skip or placing boom Skip or placing boom Skip Skip
Wall thickness range (typical)
200-800mm 200-800mm 200- 450mm 200- 450mm
Frame-to-core
Embedded plate or cast-in couplers or pull-out bars
Average cycle time 4 days 4 days 250mm per hour 250mm per hour
Cranage required Very high demand Moderate demand High demand High demand
Can work continue
N/A N/A 24/7 working required
to realise advantages of system
Slipform started each morning then stopped each night to avoid 24/7 working - requires heavily retarded concrete
Table 6.1 Core-forming options and some key attributes.
LEVEL (N+3)
LEVEL (N+2)
LEVEL (N+1)
SET-UP LEVEL (N) Figure 6.2
Schematic of the self-climbing jump-form system
Image: Select Plant Hire Limited & Laing O’Rourke Plc.