What structure is designed to stiffen a building against wind and seismic forces?

The correct answer is the shear wall. Shear walls are vertical elements that provide significant lateral resistance to wind and seismic forces. They are typically made of reinforced concrete or masonry and are strategically placed within a building's layout to control the side-to-side movement that can occur during high winds or seismic activity.

These walls function by transferring the lateral loads down to the foundation, thereby maintaining the building's structural integrity and preventing structural failure. The design and placement of shear walls are crucial for ensuring that the building can withstand these forces, making them an integral part of modern building construction in areas prone to high winds or earthquakes.

In contrast, while diaphragms can help distribute loads within a structure, they do not provide the direct lateral resistance that shear walls do. A portal frame can provide support and maintain the overall shape of a structure but is not specifically designed for lateral support against wind or seismic forces. Reinforced ceilings do not play a significant role in resisting lateral forces; rather, they are designed to support vertical loads from the roof and upper floors. Thus, the shear wall is the most effective element for stiffening a building against lateral forces.