All over the world, buildings have become a natural part of the landscape. Over millennia, their designs evolved from basic gravity-resisting structures to the virtually indestructible skyscrapers that make up the skylines of modern cities.
Today, we can count on buildings to shelter us from various weather conditions and natural disasters. Buildings can do all of this thanks to the work of engineers who design them.
From a structural engineer’s perspective, a building is a load managing mechanism. To understand how a building stands up, we should know all the forces, or loads, that act against it. To stay upright, a building must be able to resist gravity loads, as well as loads imposed by the forces of nature.
A structure’s resistance to gravity loads prevents it from collapsing under its weight and that of its occupants. This category of loads includes dead and live loads.
Dead loads are the weight of the structure and the building’s built-in components. Engineers determine these loads by calculating the building’s materials’ weight and quantity.
Because structural elements seldom undergo major changes after completion, dead loads are static. Despite these loads’ permanent nature, engineers apply a safety factor to account for future modifications that may add weight to the structure.
Live loads include anything but the self-weight of the building, such as, occupants, furniture, and equipment. They can be uniformly distributed over a large area, such as a large piece of machinery, or concentrated over a small area. These loads are typically dictated by the building’s occupancy type.
Live loads are often dynamic and can create short impulses that impact the structure. Engineers must assume these loads to be omnipresent in a building and design the structure accordingly.
To resist gravity loads and function as intended, a building’s structure must consist of members that adequately support both the dead and live loads. A typical structure is made up of horizontal and vertical members joined with various connection devices.
The horizontal members of a structure include, but are not limited to, joists, beams, and slabs. These elements support the building’s dead and live loads and can be used in various configurations.
In wood-frame residential construction, joists create the floor structure and rest on the sill plates of underlying exterior and interior wall assemblies. When joists span a long distance, perpendicular beams can be used for added support. These beams can be flush or dropped below the framing.
In concrete and steel buildings, flooring finishes are typically layered on top of reinforced concrete slabs, which are supported by beams.
Slabs, joists, and beams must have good compressive and tensile strength to withstand the loads imposed by occupants, furniture, equipment and their own weight. The structure’s studs and columns pick up these loads at their points of connection and carry the load down to the foundation.
Vertical members carry the weight of the roof and each floor to the foundation below. Depending on the type of building, vertical members can include studs, columns, footings and concrete walls. These elements must have high compressive strength to accommodate the building’s weight and other forces imposed on these members.
In light wood-frame residential construction, studs make up the building’s exterior and interior walls. Exterior stud walls are always load-bearing, as they hold up the ends of joists of each floor plate, as well as the studs and roof rafters or trusses above them. Some interior walls are also load-bearing and support the weight of the elements above them.
In reinforced concrete (RC) and steel-framed buildings, columns are the primary vertical members. They transfer loads from slabs and beams to the foundation. Columns are critical to the structure, as one column’s failure could lead to the building’s progressive collapse.
A building’s foundation, made from reinforced concrete, has the compressive strength necessary to withstand the entire structure’s gravity loads. To provide stability, the foundation itself must rest on undisturbed soil.
Where soil conditions are poor, engineers design foundations with various systems that can reach the bedrock, such as piers, piles, or caissons. These deep foundations also provide stability for exceptionally heavy buildings, hillside homes, and in expansive soils.
Environmental loads are caused by the forces of nature. Unlike other live loads, they are not always influenced by gravity, and their direction is not consistently vertical. Environmental loads include seismic movements, the weight of snow, the pressure of wind, as well as expansion and contraction caused by temperature changes.
When an earthquake shakes the ground beneath a building, loads are imposed horizontally on offset planes. A building’s gravity-resisting structure is incapable of resisting these lateral loads without adequate bracing. Shear walls and moment frames are examples of bracing mechanisms often used to provide lateral reinforcement to structures, thus enhancing their seismic soundness.
Besides having lateral reinforcement, buildings in seismically active regions must be securely anchored to their foundations. This simple measure prevents them from sliding off the foundations in response to ground movements.
Buildings constructed with masonry units, such as brick or concrete block, must have their masonry elements reinforced. Unreinforced Masonry Buildings (UMB), which make up a sizeable chunk of pre-war homes and heritage buildings in California, are notably susceptible to earthquake damage. Mortar, which holds the masonry units together, is not strong enough to resist lateral loads. Past earthquakes have shown that UMBs are prone to crumbling and collapsing when the ground starts to shake, while the debris they project often causes death, injuries, and damage to nearby buildings. Thanks to local governments’ efforts to adopt proactive measures in minimizing seismic vulnerability, most UMBs in California have been retrofitted.
Accumulating snow imposes a load on the roof structure. To prevent the roof’s collapse, its structure must be strong enough to withstand the weight of the snow. In engineering calculations, snow loads are calculated based on historical averages.
The shape of the roof is another critical design factor in snow load management. The flatter the roof, the more snow it accumulates. Steep roofs help keep the snow off, thus sparing the structure from additional loads. That said, steep roofs typically come with greater dead loads that require stronger members.
Wind forces can create several challenging load scenarios, particularly for tall buildings.
The windward side of a building is vulnerable to high wind pressure, while the leeward side experiences suction forces. Because these loads are horizontal, a building’s gravity-resisting structure is not an adequate coping mechanism. Elements of lateral resistance must be included in the structure to manage horizontal wind loads.
Vortex tunneling is another issue experienced by tall buildings. When wind creates suction on one side of a building while applying pressure to another, the building vibrates, and its top floors start to sway. To counteract the movement, engineers can add weight to the top of the structure, or create openings for the wind to blow through.
Most building materials react to temperature changes by expanding when temperatures rise and contracting when they fall. These cycles of expansion and contraction impose loads on a building’s structure and may cause deterioration in some elements. For example, concrete may crack when thermal loads are exerted on it over time.
To absorb movement caused by expansion and contraction, engineers include expansion joints into the building’s structure. These devices add flexibility to an otherwise rigid structure, thus preventing cracks and deterioration.
In construction, engineers play a critical role. They know how to make buildings stay upright. They design structures to withstand gravity loads and resist the forces of nature. On most construction projects, building officials review engineers’ plans and calculations before allowing contractors to commence work.
Design Everest’s team of civil and structural engineers has designed 1,000s of buildings in California over the past 14 years. If you are planning a construction project or would like more information on how structures work, call us at (877) 704-5687 for a FREE consultation.