Engineering
Assessing Seismic Resilience of Modular Steel Buildings with Moment-Resisting Frames
Quick fact
In modular steel buildings, the floor diaphragms are not continuous—each module has its own floor slab, leaving a gap at the seams. This interruption in the diaphragm can significantly reduce the building's lateral stiffness and change its seismic load path, which is a key reason why seismic resilience assessment must treat them as a distinct structural system.
Why this is interesting
Modular steel buildings are assembled like giant LEGO blocks from prefabricated boxes, but when an earthquake strikes, their behavior can be radically different from a conventional steel frame. Why would a structure that is so regular in appearance behave so unpredictably?
Read the full explanation
Understanding Assessing Seismic Resilience of Modular Steel Buildings with Moment-Resisting Frames
Imagine a multi-story building made of stacked shipping containers. Each container is a complete box, and they are welded or bolted together at their corners. Inside, the floor of each box is a rigid concrete slab. But between boxes, there is a crack—the slab is not continuous. This is analogous to modular steel buildings. When an earthquake hits, the ground shakes, and the building sways. In a conventional steel frame with continuous floor slabs, the slabs act as diaphragms that distribute lateral forces to the frame. In a modular building, these diaphragms are broken at the module boundaries. As a result, the load path is not smooth: the lateral force must travel from one module's floor to the next through the inter-module connections. These connections, often simple shear tabs or bolted plates, must transfer the force. This creates a 'soft' area that can amplify the dynamic response of the building, especially at the floor levels. To assess seismic resilience, engineers first model the building with these intermittent diaphragms and flexible connections, then run dynamic analyses to see how it responds to ground motions.
A deeper explanation
The core mechanism behind the unique seismic behavior of modular steel buildings lies in the discontinuity of the load path. In a conventional moment-resisting frame, the beams and columns are rigidly connected, and the floor slabs are continuous, providing a stiff horizontal plane that distributes lateral loads to the vertical frames. In a modular building, each module contains its own moment frame, but the transfer of forces between modules happens at discrete points—the connections. These connections typically have lower stiffness than the continuous frame, making them akin to 'flexible links' in the vertical load path. This results in reduced overall lateral stiffness compared to a monolithic structure of the same geometry. The reduced stiffness lengthens the building's natural period, potentially increasing its sensitivity to long-period ground motions from distant earthquakes. Conversely, the flexibility can also reduce the base shear in some cases—it's a delicate balance. To accurately assess resilience, engineers must perform a three-dimensional nonlinear response history analysis that explicitly models the inter-module connections and the discontinuous floor diaphragms. This analysis reveals the true distribution of drift demands and plastic hinge formation. The key to resilience is ensuring that the connections have sufficient ductility and that the frames develop their full plastic moment capacity without premature failure. Without this careful assessment, a modular building might perform poorly in an earthquake—not because the steel is weak, but because the assembly method concentrates damage at the connections.