How Shearwalls Resist Lateral Forces In A Building

Shearwalls resist lateral loads by transferring the lateral loads induced on a building by wind and earthquakes from the roof, through the height of a building, to the foundation system. The primary function of a shearwall is to resist the shear loading, or lateral sliding loads, for each story of a building and transfer the loads to the story or foundation below.

Introduction:

Shearwalls are a critical part of a building’s Lateral Force Resisting System (LFRS). Shearwalls are the primary structural element that resist wind and seismic loads the building is subjected to. These vertical elements are designed to resist lateral forces that act in the plane (parallel to) the wall. By redistributing these forces, shearwalls provide lateral stability to the structure. This article explores the principles and mechanisms behind how shearwalls resist lateral forces in a structure.

Concept of Shearwalls:

Shearwalls are vertical elements that provide lateral resistance to horizontal forces, primarily those induced by wind and seismic loads. They are typically made of plywood attached to studs, reinforced concrete, reinforced masonry, or structural steel and strategically placed throughout the building to form a continuous load path from the roof to the foundation system. The primary purpose of a shearwall is their ability to transfer lateral forces to the foundation, prevent excessive deformations, and maintain structural stability of the building.

Shear Wall Components and Types:

Shearwalls can take various forms, including coupled shear walls, core shear walls, and panel shear walls. In wood-framed buildings, panel shearwalls are almost exclusively used as they are easily built by nailing plywood to wood studs. Regardless of the type, the fundamental components of every shearwall includes boundary elements, which connect the shearwall to the rest of the structure. The panel, or core, is the primary resisting element that absorbs and redistributes lateral forces from the top of the wall to the bottom, where the loads are then transferred to the structural element below.

In a wood-framed shearwall, the boundary elements consist of the double top plate, studs at each end of the designated shearwall, and a bottom plate. Most commonly, the panel is structural plywood, or occasionally sheetrock, that is applied to the framing.

Shearwall Construction:

Shearwalls are commonly constructed of reinforced concrete and reinforced masonry blocks, however the most common type of shearwall construction is plywood sheathing nailed to wood studs. Wood-framed shearwalls are all around us and we see them in our houses, strip malls, low-rise office buildings, apartment buildings, and more.

Wood-framed shearwalls consist of several structural elements. At the top of the wood-framed shearwall is a connection from the floor or roof above to the top of the shearwall, the double top plate which has edge nailing from the plywood sheathing, the plywood sheathing and studs, a bottom plate which connects to the floor system or foundation below, and holdown anchors at each end of the shearwall. Each of these elements are crucial to the performance of the wood-framed shearwall.

Lateral Force Resistance:

The primary mechanism by which shearwalls resist lateral forces is through their capacity to carry shear loads. Shear loads can also be thought of as sliding loads. Overall, loads to a shearwall cause it to slide along the length of the wall and the same forces act internally in the wall and the wall’s ability to resist those sliding forces are what makes it a shearwall.

When lateral forces act on a structure, shearwalls transfer these forces along the height of the wall from the top to the bottom. The boundary elements at the top and bottom of the shearwall distribute these forces into the adjacent structural elements.

Diaphragm Action:

In addition to their flexural behavior, shearwalls also contribute to the overall diaphragm action of a building. The floors and roof are horizontal diaphragms that distribute lateral forces to the shearwalls when designed and connected properly. Together, the floor and roof diaphragms along with the shearwalls make up the Lateral Force Resisting System for the structure to resist lateral loads.

Importance of Aspect Ratio:

The aspect ratio of a shearwall, defined as the ratio of its height to its width, significantly influences its performance. The longer a shearwall is, the higher its aspect ratio. For example, looking at two shearwalls that are each 9 feet tall and one has a length of 4 feet while the other has a length of 12 feet. The aspect ratio of the 4 foot wall is 9/4 = 2.25 while the 12 foot long shearwall has an aspect ratio of 9/12 = 0.75.

Shearwalls with lower aspect ratios generally provide better resistance to lateral forces, as they are longer. Shearwalls with a high aspect ratio may lead to an instability in the structure, and proper engineering design and detailing are necessary to account for the shorter shearwalls with higher aspect ratios.

While there are some exceptions that an engineer can account for, the maximum aspect ratio for a wood-framed shearwall is 3.5:1. This means that the minimum length of a wood-framed shearwall is the wall height divided by 3.5.

Distribution of Shear Forces:

Shear forces induced by lateral loads are distributed through the roof and floor diaphragms and by the shearwalls to the foundation of the building. The foundation then transfers these forces into the ground, providing stability for the entire structure. A properly designed foundation is critical to prevent differential settlements and ensure uniform distribution of forces from the building to the ground that supports the building.

Performance Under Wind and Seismic Loading:

Shearwalls are particularly essential in the design of the Lateral Force Resisting System, where lateral forces can be dynamic and often unpredictable. The ductility of materials used to construct a building’s shearwalls allows them to absorb and dissipate energy during high wind and seismic events. Good engineering design and detailing, along with good construction are important for a building’s shearwalls to successfully resist lateral loads.

Importance of Proper Design and Detailing:

Good engineering design and detailing are essential for a shearwall to be effective in resisting lateral forces. Engineers consider factors such as material properties, loading conditions, as well as code prescribed wind and seismic loads when designing shearwalls. Adequate detailing is crucial to achieve the desired level of performance.

Conclusion:

Shearwalls are integral elements in the structural design of buildings by providing resistance to lateral forces induced by wind, earthquakes, or other lateral loads. Through their flexural behavior and ability to distribute lateral forces, diaphragm action, and effective distribution of forces, shearwalls are critical maintaining the stability and safety of structures. As technology and engineering practices continue to advance, the understanding and implementation of shearwalls will evolve, contributing to more resilient and sustainable structures in the face of lateral forces.