By Erin Kelly, P.E.
As you may have already recognized, the load path for lateral loads differs greatly from that of gravity loads, and can, in some cases, be completely independent.
Lateral loads are assumed to be applied initially to the façade of the structure or the exterior walls. The loads applied half-a-floor height above and below a given floor level are then assumed to be transferred to that level, which acts as a horizontal diaphragm. The loads are then transferred from the diaphragm to the vertical elements of the lateral force-resisting system either by tributary area or rigidity, based on the type of floor system. These vertical elements are what we typically refer to the whole system as the moment frames, shear walls, braced frame, etc. The lateral loads are finally resolved into the foundation at the ground level.
In a case where the gravity system consists of slabs and beams supported by interior columns, which carry the loads to the foundation, the gravity and lateral load paths are essentially independent. In other cases, however, such as in conventional wood-frame structures or concrete tilt-up structures, the load paths overlap as the walls act both as lateral and gravity load-resisting elements.
We identify these two types of load paths in the Code as Building Frame and Bearing Wall. Building Frames structures contain a separate load path for gravity and lateral loads. The Bearing Wall structures involve elements that act simultaneously as gravity and lateral load-resisting elements.
Building frame systems are preferred for several reasons, but one major advantage is that they allow for the stiffness of the structure to the maintained through a limited number of elements.
To understand the importance of this, we must consider one of the serviceability functions of the lateral load-resisting system, which is to limit the deflections in the structure. Deflections need to be limited in order to avoid structural damage, plastic deformations, increased load effects due to the P-Delta effect, and, importantly, occupant comfort.
If the entire structure were to be designed to achieve this purpose, the building would need to be extremely stiff, heavy, and expensive. Instead, if we separate the two systems, and make just the lateral system sufficiently stiff, a considerable amount of both labor and materials can be saved.
In the book Why Buildings Stand Up, Mario Salvadori explores this concept as it applied to steel-framed skyscrapers. Salvadori explains, “One must be aware that in steel construction, rigid or moment connections are costly. They require specialized manpower and dangerous work at great heights. Their cost may represent 10% of the entire cost of the structure. But, if the inner core were stiff enough, one could forsake the rigid connections between the beams and columns of the exterior frames and use much cheaper connections, which allow beams and columns to rotate with respect to each other, as if they were hinged. Such hinged, or shear, connections could not be used without a core since the frame would collapse like a house of cards, but they are economical and practical if the core stands up rigidly and the outer hinged frame leans on it. The separation of the two structural functions is now complete.”
You can see through this quote that the separation of the gravity and lateral systems has allowed us to build bigger, taller, and stronger structures while being economical and practical in our design choices. This division is now codified in seismic engineering and plays a key role in how lateral systems are selected and evaluated.