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. 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. 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. 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. 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. Accumulating snow imposes a load on the roof structure. 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.
This shifting soil destabilized the foundation stones, resulting inthe sinking and leaning that occurs to this day. Some places are just better equipped to support the weight and foundation of an entire building. The city of Manhattan is supported by shallow, strong bedrock. Unlike Pisa, towers and skyscrapers stand tall more easily because of the solid foundation.
One of the most common ways to approach this dilemma is by simply building a better foundation. The most commonly used material for the job is concrete.
Of course, it takes quite a bit of concrete to support a skyscraper. The concrete foundation supporting Salesforce Tower, a foot tall building in San Francisco, is feet thick and spread over almost an entire acre.
Pouring the million-pound concrete foundation took nearly 18 hours! To save money, engineers are always trying to use as little material as possible while keeping the building safe. But designing the exoskeleton is a problem that architects and engineers tackle together—how to make a structure that is both cost-efficient and beautiful.
The basic engineering principle is simple. Exoskeletons are typically made up of triangles, which are the most structurally stable two dimensional shape. Because Chengdu is typically cloudy, the architects wanted the tower to face many different directions to reflect more natural light.
In addition to supporting the weight of the building, the 3-D exoskeleton also makes the building look better on the interior compared to a flat 2-D exoskeleton.
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