High wind pressure can have deleterious effects on all types of buildings. Doors and windows can collapse and roofing and decking can be destroyed in an instant. Roof overhangs that tend to trap air beneath them resulting in high uplift forces are particularly susceptible to damage. Tornadoes with 110-165 mph winds can destroy property in only four seconds. If the connections between the roof and walls are weak, these high wind forces will drive the roof and walls to give way. Once the roof blows off the entire structure can collapse within seconds.
Tall buildings can be affected by the wind and the taller the building is the more sensitive it is to wind loads. Wind is a very complicated phenomenon and consists of an endless variety of flow situations particularly around and on structures. They respond to the effects of wind in several ways. They are known as buffeting, vortex shedding, and galloping. Building design must consider these responses at an early stage for the safety and stability of tall buildings.
Three major wind effects that result around wind-sensitive buildings are:
- Effect on environment-changes in wind flows from a new building can affect the surrounding environment. For skyscrapers in cities, the impact of wind on pedestrians, vehicles, fountains, etc., in the vicinity of the proposed structure must be effectively assessed.
- Effect on façade– the effect of wind pressures on the building’s façade cladding must be assessed. By assessing the design loads on cladding initial costs and maintenance bills can be minimized.
- Effect on structure-the wind load will affect the lateral load on the structural system of the building.
In addition to gravity loads, the structure of tall buildings is to resist lateral loads resulting from wind, earthquake, etc. Wind produces three different types of effects on tall buildings:
- Static– the static effect is a term of analysis independent of time.
- Dynamic– the dynamic response of tall buildings to lateral load, particularly wind, is affected by several factors including structural stiffness, mass, damping, and architectural shape and form. Dynamic analysis is an attempt to take into account how the system responds to the change through the period of time.
- Aerodynamic-when the building is very flexible it interacts with the wind load and affects its response.
As you can see, wind produces many different types of effects on a structure. The response of the load depends on the type of structure. Flexible, slender structures are subjected to wind-induced along and across the direction of the wind and both must be considered.
Wind flow around a building causes forced convection heat transfer from and to the walls and roof, which will result in increased energy consumption.
In order to resist wind, steel columns and beams are clustered in the skyscraper’s core. This allows engineers and architects to create a stiff backbone that has the ability to resist tremendous wind forces. Structural engineers can also reduce the wind effect by choosing and designing efficient structural systems. They create an inner core of the building to be used as an elevator shaft; this design allows lots of open space on each floor. Wind moves all around buildings and when the air hits the building, with nowhere else to go, it is pushed up down and around the sides. The air forced downwards increases wind speed at street level. There can also be an acceleration of wind around the side of the building if it has completely square corners.
When wind reaches the wall of a building, it gets deflected in all directions. Some of the wind is deflected upwards and around the sides of the building, causing no effect at ground level. As the wind strikes the surface of a building, the flow typically splits as it flows over and around the building. At the windward surface, where the wind first impacts the building, the wind stream splits above and around the sides of the building. The airflow actually accelerates around the corners of the building.
When wind hits the wall of a building directly (at a 90-degree angle) none of it bounces back because air behaves like a continuous fluid. It cannot rebound and flow back through itself without interacting with the fluid behind it, so the air will be displaced sideways. There will be a higher pressure in front of the wall. Most buildings can be engineered to withstand hurricane-force winds up to 170 mph.
Most of us have heard of or experienced a tall building swaying in the wind. This occurs because the height of the skyscraper makes them more susceptible to movement. As the strong wind moves around the building, the areas of less pressure on the skyscraper create suction forces that pull at the building and actually cause it to sway.
Another major challenge of wind-induced movement in a building is the human response. People are remarkably sensitive to vibrations and the perception of movement. Even low-level stress or strain-induced vibrations can cause us uneasiness.
As we move into the month of March, take notice of wind patterns in your area and how they affect your home and/or neighborhood. Take precautions to tie down any loose items that you may have outdoors. Remember the force of the wind can be fierce and damaging.