Load Transfer in Residential Structures: From Roof to Foundation

What is Load Path in Structural Design?

Load path refers to the route by which the weight of a structure and live loads are transmitted from their point of application to foundation and finally to the ground.

A load path is typically composed of a series of structural members, including beams, columns, walls, and foundations. An efficient load path design helps to distribute the loads throughout the structure, preventing any one component from being overloaded.

Why is Load Path Important in Structural Design?

The proper design of a load path is critical to the structural integrity of a building. A deliberately designed load path ensures that the weight of the structure is transferred from the roof to the foundation in a safe and efficient manner.

A poorly designed load path can result in a building that is not structurally sound, leading to potential safety hazards and costly repairs. Therefore, understanding the load path and its mechanism is crucial for structural engineers, architects, and building designers.

Types of Load Path

There are two main types of load paths in building design: Gravity Load and Lateral Load.

Gravity Load Transfer Example

Gravity load transfer refers to the process by which the weight of a building is transferred down through the structural components to the ground.

In a residential structure, the weight of the building is supported by the foundation, which is typically made of reinforced concrete.

Consider a two-story house. First, the roof, walls, and floors are transferred to the foundation through load-bearing walls, columns, and beams. These structural components distribute the weight evenly, so no single point of the foundation is overloaded. The foundation transfers the load to the ground, providing the necessary support to prevent the building from collapsing.

Gravity loads can consist of both dead and live loads. Dead loads are permanent loads that are always present in the building, such as the weight of the structure, roofing materials, and fixed equipment. Live loads, on the other hand, are temporary loads that may change over time, such as people, furniture, and snow loads.

In a typical residential structure, the dead load is transferred from the roof to the foundation through the load path discussed earlier. The live load, however, often requires a different load path mechanism as this load can be cyclical in nature and is applied laterally rather than vertically in the case of wind and earthquake load. The live load is usually distributed across the floor joists, which transfer the load to the supporting walls and then down to the foundation. The load path for the live load is designed to prevent any one component from being overloaded, ensuring the structural integrity of the building.

Figure 1: Gravity load transfer in a residential building (Reference)

Figure 2: An example of gravity load transfer in a commercial building (Reference)

Lateral Load Transfer Example

Lateral loads, such as wind and seismic loads, are live loads (temporary imposed loads) that act horizontally on a building.

Lateral loads require a different type of load path than gravity loads. In order to resist lateral loads, buildings typically use shear walls, moment frames, steel bracing, or a combination of them.

Shear walls are designed to resist lateral forces by transferring them to the foundation through the building's floors, walls, and roof.

Moment frames, on the other hand, resist lateral forces by creating a rigid frame that can resist bending forces by fixed connections that transfer load to the footings. Both of these load path mechanisms help to ensure that the building can withstand lateral loads without suffering any structural damage.

Figure 3: An example of lateral load path in residential buildings (Reference)

Figure 4: An example of lateral load path in a commercial building (Reference)

Examples of Load Paths in Common Structures

There are many different types of load paths that can be used in building design. Here are three examples of load paths used in common structures:

Timber Residential Structure Load Path

A residential timber structure typically uses a gravity load path to transfer the weight of the roof to the foundation as residential structures are less susceptible to wind and load path than mid-high rise buildings meaning the dead loads are the critical design actions.

Figure 5: Load path for a timber residential structure (Reference)

The roof load is transferred to the ridge beam, which is then supported by the roof rafters. The rafters are connected to the wall plates, which are in turn supported by the studs. The studs transfer the weight to the sole plate, which rests on the foundation. In this load path, each structural member supports the weight of the components above it, transferring the load to the foundation.

Concrete Commercial Building Load Path

In a concrete commercial building, the load path is usually designed to handle a critical load combination that is specified in design standards that combines gravity loads and lateral loads (earthquake load and wind load) as the actions of wind and earthquake loads are more severe on taller buildings.

' Figure 6: Load path for a high rise building (Reference)

Gravity (vertical) loads are transferred from the roof to the foundation via the slabs to the columns and down into the piles.

However, lateral (horizontal) loads, such as wind or seismic forces, require a different load path mechanism. Lateral loads are transferred to the walls and then down to the foundation through shear walls or moment frames. The foundation then resists the lateral loads by transferring them to the surrounding soil.

Steel Portal Frame Load Path

A steel portal frame is a commonly used load-bearing system in industrial and commercial buildings.

Figure 7: Portal frames have simple load paths (Reference)

This type of structure uses a portal frame to transfer loads from the roof to the foundation. The roof load is transferred to the rafters (secondary beams), which are supported by the purlins (primary beams). The purlins are then bolted to the columns via a welded end plate, which transfers the load to the foundations. The columns can be rigid or braced in a steel portal frame to resist both gravity loads and lateral loads.

Conclusion

In conclusion, the load path is a critical component of building design. It ensures that the weight of the structure is transferred from the roof to the foundation in a safe and efficient manner, preventing any one component from being overloaded.

By designing the proper load path mechanisms, buildings can be designed to resist gravity loads, lateral loads, and other environmental factors, ensuring the safety and longevity of the structure.

With this understanding, the design of residential and commercial structures can be optimized for safety, efficiency, and longevity. Clear Calcs can be used for Load Path Tracking, where a user can link reactions between beams, columns, and foundations to track loads through a structure dynamically, and can even be used to Link Loads and Reactions Between Calculators.

These features can dramatically streamline the design process as the loads applied to subsequent members in the load path will be updated automatically when a parameter is changed in a member that is bearing upon that load path. This can produce a much more structurally efficient building, reducing cost and time during design and construction.