Summary of "Structural engineering considerations for high-rise modular buildings"

This presentation provides an in-depth overview of the structural engineering challenges, solutions, and methodologies involved in high-rise modular construction, focusing on projects such as George Street and College Road in Croydon. The discussion covers modular design principles, structural systems, temporary works, connection details, fire performance, embodied carbon, and dynamic behavior under wind loads, concluding with practical construction considerations and future outlooks.

Main Ideas and Concepts

1. Modular Construction Overview

Volumetric modules are factory-built, including floors, ceilings, internal walls, and finishes.
Modules incorporate structural elements like concrete floors and steel columns.
Off-site fabrication enables simultaneous site work (excavation, foundations, core construction).
Modules are transported and craned into position atop podium slabs and concrete cores.

2. Structural System and Module Types

Modules have vertical load-bearing columns transferring loads to transfer slabs.
Horizontal loads are transferred to a central concrete core.
Two main module support types:
- Corner-supported modules: fewer columns, perimeter spaced apart.
- Wall-supported modules: structural walls provide linear support.
Podium (transfer) slabs provide flexibility, fire resistance, stiffness, and a working platform.
Transfer slabs vary in thickness (600 mm for mid-rise to 2 m for high-rise).

3. Design and Manufacturing Constraints

Factory limitations: module size, transport restrictions (road, bridge height/load limits).
Modularization process:
- Floor plans broken into efficient module layouts.
- Minimize module types and variations.
- Use of various module shapes (rectangular, locked/interpreted ends).
Coordination with architects and manufacturers is critical.

4. Temporary Works and Stability

Concrete cores provide primary lateral stability.
Cores must be completed before module placement.
Temporary tying between cores reduces transient forces during construction.
Wind tunnel testing and stage analysis inform design of temporary ties and permanent connections.
Differential stiffness and movement between cores require careful modeling.

5. Module Connections

Aim for "drop-in-place" connections to simplify on-site work and minimize wet trades.
Connections use steel plates (up to 25 mm thick) with tolerance accommodations (annular rings, hexagonal shapes).
Connections must handle vertical loads, lateral forces, and vibrations.
Connection design considers factory and site tolerances to avoid misfits.

6. Structural Modeling and Analysis

Use of software like ETABS and Tekla for detailed 3D modeling.
Separate but coordinated models by different engineers improve reliability.
Stage construction analysis accounts for core shortening, shrinkage, and module installation sequencing.
Modules modeled as slabs with reduced stiffness; columns modeled individually.
Load cases include lifting, transport, placing, and occupancy.

7. Fire Performance and Finishes

Fire tests conducted on modules show good structural protection.
Fire protection and finishes are factory-installed where possible.
Modules are designed to limit fire and smoke spread.

8. Sustainability and Embodied Carbon

Modular construction reduces waste, improves recycling, and enhances material efficiency.
Factory conditions allow better quality control and less material usage.
Embodied carbon reductions align with future sustainability targets (2030 and beyond).

9. Construction and Site Operations

Smaller, specialized crews perform module installation at height.
Weather-controlled factory conditions improve worker safety and quality.
Cranes are typically installed on top of cores to allow free module placement.
Innovative crane removal strategies (e.g., dairy crane on modules) have been developed.

10. Dynamic Behavior and Wind Effects

Modules contribute stiffness at serviceability limit state (SLS), contrary to initial assumptions.
Damping ratios measured in completed buildings align with Eurocode guidance.
Mass tuned liquid dampers were considered but ultimately not needed on some projects.
Ongoing monitoring of accelerations and natural frequencies informs future designs.

11. Challenges and Future Directions

Differential movement between concrete cores and steel modules requires sliding joints.
Larger cores than traditional buildings are needed due to reduced floor load contribution.
Stability checks include local buckling and punching shear around openings and columns.
Modular construction in seismic zones is less studied but would require increased connection forces.
Maximum building height is currently around 50 stories, limited primarily by core design and dynamic movement rather than modular units.
Potential future solutions include post-tensioning, outriggers, or relying on modules for lateral stability.

Detailed Methodology / Instructions

Modularization Process:
- Break floor plans into workable modules.
- Minimize module types and variations.
- Maximize coverage and factory efficiency.
- Coordinate closely with architects and manufacturers.
Transfer Slab Design:
- Use concrete slabs for stiffness, fire resistance, and flexibility.
- Thickness varies with building height and column spacing.