Hurricane and Tornado Proof Building

Tornadoes, cyclones, and other strong winds damage or destroy many buildings. However, with proper design and construction, the damage to buildings by these forces can be greatly reduced or eliminated. Over time, a variety of methods have been studied and tested (both formally and incidentally by actual storms) that can help a building survive strong winds and storm surge. Local building departments may mandate their use in high velocity hurricane zones, or areas where buildings are likely to have to withstand a hurricane in their lifetime.

Curtain wall is a term used to describe a building façade which does not carry any dead load from the building other than its own dead load. A curtain wall is designed to resist air and water infiltration, wind forces acting on the building, seismic forces, and its own dead load forces.

Storm surge considerations

A common problem for buildings during hurricanes is storm surge. Flooding occurs frequently in coastal areas and waves contain a tremendous amount of energy which can literally batter a building to pieces. Beach front buildings should be able to withstand the ocean rising 20 or more feet with large waves on top of that. They should preferably be built on high ground where possible in order to avoid waves knocking the building down.

If waves can reach the building site, the building should be elevated on steel, concrete, or wooden pilings and/or anchored to solid rock. Whether it is intended or not, the walls on the first floor are often built with sheetrock which can completely deteriorate when wet and/or exposed to lateral forces, leaving structural members in place, and allowing water (or high winds) to pass through. This "gutting" occurs frequently in storm surge areas. If done by design, "sacrificing" the walls of the first floor is not an ideal solution, although it can save the rest of the building from destruction. Of course, building contents left on that level will be lost and considerable damage to the building could still result in costly repairs - see mold, rot, and termite problems below under building materials.

Wind loading considerations

The foundation

A Monolithic dome after Hurricane Dennis in 2005.

Wind acting on the roof surfaces of a building can cause negative pressures that tend to create a lifting force. This is one of the most common ways a building can be destroyed during a storm. Gravity alone may not be sufficient to prevent the roof from lifting, or "peeling" off the rest of the building. Once this occurs, the building is weakened considerably and the rest of the building will likely fail as well. To minimize this, the upper structure should be securely anchored through the walls to the foundation.

Several methods can be used to securely anchor the roof. Traditionally, roof trusses were simply "toenailed" into the top of the walls. These nails provide little to no actual structural advantage; they're mainly used to hold the trusses in place while the rest of the roof is being built. Gravity and friction then ensure the roof stays put. Various products have been developed that can actually anchor the roof to the walls, which should then be anchored to a solid foundation. Metal straps which nail into the wall and wrap over the trusses are one method. Other methods, including temporary straps made of a variety of materials, have also been successfully used and have an advantage in that a building which is not constructed to withstand wind loading can be quickly and temporarily strapped to the ground, even during the approach of a storm.

Earth Sheltering

Earth Sheltered Construction is generally more resistant to strong winds and tornadoes than standard construction. It is for this reason that cellars, and other Earth Sheltered components of other buildings, can provide safe refuge during tornadoes.

Dome homes

The physical geometry of a building affects its aerodynamic properties and how well it can withstand a storm. Geodesic dome roofs or buildings made from wood, steel, or concrete have low drag coefficients and can withstand higher wind forces than a square building of the same square footage.

Building components

Garages, windows, doors, and other openings

These are generally the weak points susceptible to breakdown by wind pressure and blowing debris. Once failure occurs, wind pressure builds up inside the building and in seconds, may lift the roof off a building. Hurricane shutters can also provide effective protection.

Doors

Exterior doors should open outward in hurricane prone areas. An inward opening door can be blown into the house by wind causing potential structural failure. Various companies offer new doors that adhere to the local building codes. Some companies offer retro-fit devices that can be professionally installed. These kits are often just as expensive as a new door. A good source for products include the Miami-Dade building code website[1]. It shows how the products are to be installed to withstand the most punishing of winds. Some of the companies are local and many products were in use prior to Hurricane Andrew, "The Big One".

Windows

It is usually a requirement to install 150 mile per hour tested windows in hurricane prone areas. These windows should have plastic panes, shatter-proof glass or glass with protective membranes (Impact Glass). The panes have to be more firmly attached than normal window panes (possibly even using screws or bolts through the edges of larger panes). See hurricane shutters.

Windows protected by steel or heavy aluminum shutters may be best in some hurricane prone areas.

Building materials

The choice of building materials can affect the ability of a building to withstand high winds. Although it is not always possible to use different materials, if the area is extremely susceptible to high winds, it is good practice to use the most resistant materials available.

Wood

Wood is the most common building material as it is readily available, relatively inexpensive, and has a degree of flexibility which can be beneficial in certain high stress situations. However, termite and dry rot are frequent problems in timber buildings located in areas susceptible to hurricanes, particularly in warm, humid climates. Weakened buildings cannot withstand wind loads as well as intact buildings can. To combat this, certain building codes require the use of pressure-treated wood for all structural elements of the building, which is designed to prevent rot and deterioration.

Also, wood and paper backed sheetrock provide food for black mold which can grow if the inside of the building gets wet during a storm. The mold can then be costly to remove and must be considered as a factor when deciding which building materials to use.

A building constructed with wood can effectively be built to withstand fairly high wind loads. However, flying debris - furniture, trees, parts of other buildings which are common in such a storm - can still damage or destroy a well-designed wood building even if the wind isn't sufficient to do so itself.

Concrete

Reinforced concrete is a strong, dense material that, if used in a building that is designed properly, can withstand the destructive power of very high winds, pounding waves, and even high-speed debris. Concrete used in home construction must be reinfoced with steel (commonly known as "rebar"). While the rebar can rust in wet or humid environments, there are various effective means to retard or prevent rebar corrosion due to moisture.

Examples of Cyclonic Construction Methods

Note! This gallery is of residential construction in Darwin Northern Australia and is provided for general, non specific information only.

See also

External links

Glass curtain wall of the Bauhaus Dessau.

Curtain wall is a term used to describe a building façade which does not carry any dead load from the building other than its own dead load. These loads are transferred to the main building structure through connections at floors or columns of the building. A curtain wall is designed to resist air and water infiltration, wind forces acting on the building, seismic forces, and its own dead load forces.

Curtain walls are typically designed with extruded aluminium members, although the first curtain walls were made of steel. The aluminium frame is typically infilled with glass, which provides an architecturally pleasing building, as well as benefits such as daylighting. However, parameters related to solar gain control, such as thermal comfort and visual comfort are more difficult to control when using highly-glazed curtain walls. Other common infills include: stone veneer, metal panels, louvers, and operable windows or vents.

Curtain walls differ from storefront systems in that they are designed to span multiple floors, and take into consideration design requirements such as: thermal expansion and contraction; building sway and movement; water diversion; and thermal efficiency for cost-effective heating, cooling, and lighting in the building.

History

Medieval curtain wall

Curtain wall is used to describe the set of walls that surround and protect the interior (bailey) of a medieval castle. These walls are often connected by a series of towers or mural towers to add strength and provide for better defense of the ground outside the castle, and are connected like a curtain draped between these posts. Additional provisions and buildings were often enclosed by such a construction, designed to help a garrison last longer during a siege by enemy forces. Examples of curtain walls as part of castles are at Muchalls Castle, Scotland and Dunstanburgh Castle, England, the latter of which is in a ruined state.

Modern curtain wall

A building project in Wuhan China, the difference in progress between the two towers illustrates the relationship between the inner load bearing structure and the exterior glass curtain.

Prior to the middle of the nineteenth century, buildings were constructed with the exterior walls of the building (bearing walls, typically masonry) supporting the load of the entire structure. The development and widespread use of structural steel and later reinforced concrete allowed relatively small columns to support large loads and the exterior walls of buildings were no longer required for structural support. The exterior walls could be non-bearing, and thus much lighter and more open than the masonry bearing walls of the past. This gave way to increased use of glass as an exterior façade, and the modern day curtain wall was born.

The first curtain walls were made with steel mullions, and the plate glass was attached to the mullions with asbestos or fiberglass modified glazing compound. Eventually silicone sealants or glazing tape were substituted. Some designs included an outer cap to hold the glass in place and to protect the integrity of the seals. The first curtain wall installed in New York City was this type of construction. Earlier modernist examples are the Bauhaus in Dessau and the Hallidie Building in San Francisco. The 1970’s began the widespread use of aluminum extrusions for mullions. Aluminum offers the unique advantage of being able to be easily extruded into nearly any shape required for design and aesthetic purposes. Today, the design complexity and shapes available are nearly limitless. Custom shapes can be designed and manufactured with relative ease.

Similarly, sealing methods and types have evolved over the years, and as a result, today’s curtain walls are high performance systems which require little maintenance.

The vast majority of curtain walls are installed long pieces (referred to as sticks) between floors vertically and between vertical members horizontally. Framing members may be fabricated in a shop environment, but all installation and glazing is typically performed at the jobsite.

Unitized curtain walls entail factory fabrication and assembly of panels and may include factory glazing. These completed units are hung on the building structure to form the building enclosure. Unitized curtain wall has the advantages of: speed; lower field installation costs; and quality control within an interior climate controlled environment. The economic benefits are typically realized on large projects or in areas of high field labor rates.

A common feature in curtain wall technology, the rainscreen principle theorizes that equilibrium of air pressure between the outside and inside of the "rainscreen" prevents water penetration into the building itself. For example the glass is captured between an inner and an outer gasket in a space called the glazing rebate. The glazing rebate is ventilated to the exterior so that the pressure on the inner and outer sides of the exterior gasket is the same. When the pressure is equal across this gasket water cannot be drawn through joints or defects in the gasket.

Design

Curtain wall systems must be designed to handle all loads imposed on it as well as keep air and water from penetrating the building envelope.

Loads

The loads imposed on the curtain wall are transferred to the building structure through the anchors which attach the mullions to the building. The building structure needs to be designed and account for these loads.

Dead load is defined as the weight of structural elements and the permanent features on the structure. In the case of curtain walls, this load is made up of the weight of the mullions, anchors, and other structural components of the curtain wall, as well as the weight of the infill material. Additional dead loads imposed on the curtain wall, such as sunshades, must be accounted for in the design of the curtain wall components and anchors.

Wind load acting on the building is the result of wind blowing on the building. This wind pressure must be resisted by the curtain wall system since it envelops and protects the building. Wind loads vary greatly throughout the world, with the largest wind loads being near the coast in hurricane-prone regions. Building codes are used to determine the required design wind loads for a specific project location. Often, a wind tunnel study is performed on large or unusually shaped buildings. A scale model of the building and the surrounding vicinity is built and placed in a wind tunnel to determine the wind pressures acting on the structure in question. These studies take into account vortex shedding around corners and the effects of surrounding buildings.

Seismic loads need to be addressed in the design of curtain wall components and anchors. In most situations, the curtain wall is able to naturally withstand seismic and wind induced building sway because of the space provided between the glazing infill and the mullion. In tests, standard curtain wall systems are able to withstand three inches (75 mm) of relative floor movement without glass breakage or water leakage. Anchor design needs to be reviewed, however, since a large floor-to-floor displacement can place high forces on anchors.

Snow loads and live loads are not typically an issue in curtain walls, since curtain walls are designed to be vertical or slightly inclined. If the slope of a wall exceeds 20 degrees or so, these loads may need to be considered.

Thermal loads are induced in a curtain wall system because aluminum has a relatively high coefficient of thermal expansion. This means that over the span of a couple of floors, the curtain wall will expand and contract some distance, relative to its length and the temperature differential. This expansion and contraction is accounted for by cutting horizontal mullions slightly short and allowing a space between the horizontal and vertical mullions. In unitized curtain wall, a gap is left between units, which is sealed from air and water penetration by wiper gaskets. Vertically, anchors carrying wind load only (not dead load) are slotted to account for movement. Incidentally, this slot also accounts for live load deflection and creep in the floor slabs of the building structure.

Accidental explosions and terrorist threats have brought on increased concern for the fragility of a curtain wall system in relation to blast loads. The bombing of the Alfred P. Murrah Federal Building in Oklahoma City, Oklahoma, has spawned much of the current research and mandates in regards to building response to blast loads. Currently, all new federal buildings in the U.S., and all U.S. embassies built on foreign soil, must have some provision for resistance to bomb blasts.

Since the curtain wall is at the exterior of the building, it becomes the first line of defense in a bomb attack. As such, blast resistant curtain walls must be designed to withstand such forces without compromising the interior of the building to protect its occupants. Since blast loads are very high loads with short durations, the curtain wall response should be analyzed in a dynamic load analysis, with full-scale mock-up testing performed prior to design completion and installation.

Blast resistant glazing consists of laminated glass, which is meant to break but not separate from the mullions. Similar technology is used in hurricane-prone areas for the protection from wind-borne debris.

Infiltration

Air infiltration is the air which passes through the curtain wall from the exterior to the interior of the building. The air is infiltrated through the gaskets, through imperfect joinery between the horizontal and vertical mullions, through weep holes, and through imperfect sealing. The American Architectural Manufacturers Association (AAMA) is the governing body in the U.S. which sets the acceptable levels of air infiltration through a curtain wall. This limit is expressed (in America) in cubic feet per minute per square foot of wall area at a given test pressure. (Currently, most standards cite less than 0.6 CFM/sq ft as acceptable).

Water penetration is defined as any water passing from the exterior of the building through to the interior of the curtain wall system. Sometimes, depending on the building specifications, a small amount of controlled water on the interior is deemed acceptable. To test the ability of a curtain wall to withstand water penetration, a water rack is placed in front a mock-up of the wall with a positive air pressure applied to the wall. This represents a wind driven heavy rain on the wall. Field tests are also performed on installed curtain walls, in which a water hose is sprayed on the wall for a specified time.

Deflection

One of the disadvantages of using aluminum for mullions is that its modulus of elasticity is about one-third that of steel. This translates to three times more deflection in an aluminum mullion compared to the same steel section under a given a load. Building specifications set deflection limits for perpendicular (wind-induced) and in-plane (dead load-induced) deflections. It is important to note that these deflection limits are not imposed due to strength capacities of the mullions. Rather, they are designed to limit deflection of the glass (which may break under excessive deflection), and to ensure that the glass does not come out of its pocket in the mullion. Deflection limits are also necessary to control movement at the interior of the curtain wall. Building construction may be such that there is a wall located near the mullion, and excessive deflection can cause the mullion to contact the wall and cause damage. Also, if deflection of a wall is quite noticeable, public perception may raise undue concern that the wall is not strong enough.

Deflection limits are typically expressed as the distance between anchor points divided by a constant number. A deflection limit of L/175 is common in curtain wall specifications, based on experience with deflection limits that are unlikely to cause damage to the glass help by the mullion. Say a given curtain wall is anchored at 12 foot (144 in) floor heights. The allowable deflection would then be 144/175 = 0.823 inches, which means the wall is allowed to deflect inward or outward a maximum of 0.823 inches at the maximum wind pressure.

Deflection in mullions is controlled by different shapes and depths of curtain wall members. The depth of a given curtain wall system is usually controlled by the area moment of inertia required to keep deflection limits under the specification. Another way to limit deflections in a given section is to add steel reinforcement to the inside tube of the mullion. Since steel deflects at 1/3 the rate of aluminum, the steel will resist much of the load at a lower cost or smaller depth.

Stress

Contrary to popular belief, stress is not related to deflection; it is a separate criterion in curtain wall design and analysis. For example, the advantage of some curtain wall designs is the ability to span more than one floor (commonly known as twin-span or multi-span, as opposed to single or simple span). Multiple floor spans significantly reduce the required area moment of inertia for a mullion. The stresses in the mullion, however, are significantly increased in a multiple span, giving way for a higher required section modulus (S, expressed in cubic inches) in the mullion.

As mentioned above, the deflection of aluminum is three times greater than an equivalent steel shape under the same load. However, the allowable stress in that same aluminum member may be roughly equivalent to or higher than its steel counterpart. This means that aluminum mullions can be as strong as or stronger than steel members.

Thermal criteria

Relative to other building components, aluminum has a high heat transfer coefficient, meaning that aluminum is a very good conductor of heat. This translates into high heat loss through aluminum curtain wall mullions. There are several ways to compensate for this heat loss, the most common way being the addition of thermal breaks. Thermal breaks are barriers between exterior metal and interior metal, usually made of polyvinyl chloride (PVC). These breaks provide a significant decrease in the thermal conductivity of the curtain wall. However, since the thermal break interrupts the aluminum mullion, the overall moment of inertia of the mullion is reduced and must be accounted for in the structural analysis of the system.

Thermal conductivity of the curtain wall system is important because of heat loss through the wall, which affects the heating and cooling costs of the building. On a poorly performing curtain wall, condensation may form on the interior of the mullions. This could cause damage to adjacent interior trim and walls.

Infills

Infill refers to the large panels that are inserted into the curtain wall between mullions. Infills are typically glass but may be made up of nearly any exterior building element.

Regardless of the material, infills are typically referred to as glazing, and the installer of the infill is referred to as a glazier.

Glass

The French hothouse at the Jardin des Plantes, built by Charles Rohault de Fleury from 1834 to 1836, is an early example of metal and glass curtain wall architecture.

By far the most common glazing type, glass can be of an almost infinite combination of color, thickness, and opacity. For commercial construction, the two most common thicknesses are 1/4 inch (6 mm) monolithic and 1 inch (25 mm) insulating glass. Presently, 1/4 inch glass is typically used only in spandrel areas, while insulating glass is used for the rest of the building (sometimes spandrel glass is specified as insulating glass as well). The 1 inch insulation glass is typically made up of two 1/4-inch lites of glass with a 1/2 inch (12 mm) airspace. The air inside is usually atmospheric air, but some inert gases, such as argon, may be used to offer better thermal transmittance values. In residential construction, thicknesses commonly used are 1/8 inch (3 mm) monolithic and 5/8 inch (16 mm) insulating glass. Larger thicknesses are typically employed for buildings or areas with higher thermal, relative humidity, or sound transmission requirements, such as laboratory areas or recording studios.

Glass may be used which is transparent, translucent, or opaque, or in varying degrees thereof. Transparent glass usually refers to vision glass in a curtain wall. Spandrel or vision glass may also contain translucent glass, which could be for security or aesthetic purposes. Opaque glass is used in areas to hide a column or spandrel beam or shear wall behind the curtain wall. Another method of hiding spandrel areas is through shadow box construction (providing a dark enclosed space behind the transparent or translucent glass). Shadow box construction creates a perception of depth behind the glass that is sometimes desired.

Stone veneer

Thin blocks (3 to 4 inches (75-100 mm)) of stone can be inset within a curtain wall system to provide architectural flavor. The type of stone used is limited only by the strength of the stone and the ability to manufacture it in the proper shape and size. Common stone types used are: Arriscraft(calcium silicate);granite; marble; travertine; and limestone. The stone may come in several different finishes, which adds many more options for architects and building owners.

Panels

Metal panels can take various forms including aluminum plate; thin composite panels consisting of two thin aluminum sheets sandwiching a thin plastic interlayer; and panels consisting of metal sheets bonded to rigid insulation, with or without an inner metal sheet to create a sandwich panel. Other opaque panel materials include FRP (fiber-reinforced plastic) and stainless steel.

Louvers

A louver is provided in an area where mechanical equipment located inside the building requires ventilation or fresh air to operate. They can also serve as a means of allowing outside air to filter into the building to take advantage of favorable climatic conditions and minimize the usage of energy-consuming HVAC systems. Curtain wall systems can be adapted to accept most types of louver systems to maintain the same architectural sightlines and style while providing the necessary functionality.f

Windows and vents

Most curtain wall glazing is fixed, meaning there is no access to the exterior of the building except through doors. However, windows or vents can be glazed into the curtain wall system as well, to provide required ventilation or operable windows. Nearly any window type can be made to fit into a curtain wall system.

Fire safety

Effects of fire having "leapfrogged" up a building in Brasilia 27. December 2005, destroying glass panes on the way up, sending shards down. The framework is still in place here because it is made of steel. Aluminium melts at 660°C. Building fires reach ca. 1000°C very rapidly.

Combustible Polystyrene insulation in point contact with sheet metal backban. Incomplete firestop in the perimeter slab edge, made of rockwool without topcaulking.

Firestopping at the "perimeter slab edge", which is a gap between the floor and the backpan of the curtain wall is essential to slow the passage of fire and combustion gases between floors. Spandrel areas must have non-combustible insulation at the interior face of the curtain wall. Some building codes require the mullion to be wrapped in heat-retarding insulation near the ceiling to prevent the mullions from melting and spreading the fire to the floor above. It is important to note that the firestop at the perimeter slab edge is considered a continuation of the fire-resistance rating of the floor slab. The curtain wall itself, however, is not ordinarily required to have a rating. This causes a quandary as Compartmentalization (fire protection) is typically based upon closed compartments to avoid fire and smoke migrations beyond each engaged compartment. A curtain wall by its very nature prevents the completion of the compartment (or envelope). The use of fire sprinklers has been shown to mitigate this matter. As such, unless the building is sprinklered, fire may still travel up the curtain wall, if the glass on the exposed floor is shattered due to fire influence, causing flames to lick up the outside of the building. Falling glass can endanger pedestrians, firefighters and firehoses below. An example of this is the First Interstate Bank Fire in Los Angeles, California. The fire here leapfrogged up the tower by shattering the glass and then consuming the aluminium skeleton holding the glass. Aluminium's melting temperature is 660°C, whereas building fires can reach 1,100°C. The melting point of aluminium is typically reached within minutes of the start of a fire. Firestops for such building joints can be qualified to UL 2079 -- Tests for Fire Resistance of Building Joint Systems. Sprinklering of each floor has a profoundly positive effect on the fire safety of buildings with curtain walls. In the case of the aforementioned fire, it was specifically the activation of the newly installed sprinkler system, which halted the advance of the fire and allowed effective suppression. Had this not occurred, the tower would have collapsed onto fire crews and into an adjacent building, while on fire. Exceptionally sound cementitious spray fireproofing also helped to delay and ultimately to avoid the possible collapse of the building, due to having the structural steel skeleton of the building reach the critical temperature, as the post-mortem fire investigation report indicated. This fire proved the positive collective effect of both active fire protection (sprinklers) and passive fire protection (fireproofing).

Fireman knock-out glazing panels are often required for venting and emergency access from the exterior. Knock-out panels are generally fully tempered glass to allow full fracturing of the panel into small pieces and relatively safe removal from the opening.

Maintenance and repair

Curtain walls and perimeter sealants require maintenance to maximize service life. Perimeter sealants, properly designed and installed, have a typical service life of 10 to 15 years. Removal and replacement of perimeter sealants require meticulous surface preparation and proper detailing.

Aluminum frames are generally painted or anodized. Factory applied fluoropolymer thermoset coatings have good resistance to environmental degradation and require only periodic cleaning. Recoating with an air-dry fluoropolymer coating is possible but requires special surface preparation and is not as durable as the baked-on original coating.

Anodized aluminum frames cannot be "re-anodized" in place, but can be cleaned and protected by proprietary clear coatings to improve appearance and durability.

Exposed glazing seals and gaskets require inspection and maintenance to minimize water penetration, and to limit exposure of frame seals and insulating glass seals to wetting.

Contents

Storm surge considerations

Wind loading considerations

The foundation

Earth Sheltering

Dome homes

Building components

Garages, windows, doors, and other openings

Doors

Windows

Building materials

Wood

Concrete

Examples of Cyclonic Construction Methods

See also

External links

Contents

History

Medieval curtain wall

Modern curtain wall

Design

Loads

Infiltration

Deflection

Stress

Thermal criteria

Infills

Glass

Stone veneer

Panels

Louvers

Windows and vents

Fire safety

Maintenance and repair

See also

References

External links