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Volume 27, Issue 4

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October 29, 2007

Buildings Protected By Clear Mylar Film

Written by  Bill Winter
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Important factors to consider when developing a disaster plan for violent weather are high winds and most of all the wind born debris. Most window systems are rated by wind speed alone, but this does not address the affects of wind born debris.

Until recently there were very few ways to protect large buildings from high winds and flying debris. Roll down shutters and plywood are simply not an option for many building designs.

Now there is an exciting alternative, mylar security film fit to existing windows is an economical option for protecting an organization from the hazards that exterior windows and glass doors present.

Security films were first developed in Europe during the early 1980s to protect people and property from flying glass due to terrorism. The first applications were in Northern Ireland to protect people from the Irish Republication Army. Security film is credited with saving hundreds from death and injury from terrorist attacks in Northern Ireland and around the world.

Security films are made from many layers of mylar laminated together with adhesives. It is then coated with a thick mounting adhesive that bonds the film to the glass to provide resistance to impacts and explosive pressures. When bonded together in layers, the film typically has a tensile strength well over 25,000 LBS per square inch.

This optically clear mylar film is installed on the inside of the existing windows. Once installed, the film is then attached to the frame of the window with an attachment system using special adhesives that are flexible.

In addition, the frame of the windows are reinforced into to building to ensure that the frame and the window remain secure. Attaching the film to the frame and securing the window frame to the building are the most important aspects of the installation.

With these high tech films installed properly , if a window is broken there are no dangerous flying shards of glass and the window opening is still secure. This can help keep the forces of nature from entering your facility year round, 24 hours a day.

Immediately following Hurricane Andrew the U.S.Department of Housing and Urban Development initiated a study to assess the damage that occurred. This study also concluded that the failure of windows and doors had the greatest influence on the building's general resistance to damage. ' When the building is not completely enclosed or is penetrated by high winds or wind-driven debris, the building experiences internal pressurization.' The larger the opening the greater the effect.

However, openings as small as one to five percent of the windward facing wall area can create full internal pressurization effects that can double the wind load on some components of a building, according to actual field measurements.

In January of 1993, the Governor of the State of Florida ordered the Department of Community Affairs to create a new disaster plan with recommendations regarding all aspects of disasters.

Recommendation number 27 states that 'small openings, such as broken windows caused many buildings to fail during Hurricane Andrew. These buildings might have survived with little or no damage if the exterior had remained intact.'

Projectiles can be many things, from roof gravel and shingles, tree limbs and other building parts in the area where the building envelope was pierced and blow through occurred. Case in point is the Marriott Towers and the Datran Tower in Kendall, Florida. The Datran Tower had tempered glass and Marriott had annealed or heat-strength glass. The Datran Tower was directly downwind from the Marriott hotel.

Wind speeds during Hurricane Andrew were estimated to be around 120MPH in the area, well within the design assumptions of the buildings. The roof gravel from atop the Marriott peppered the windows in Datran Tower and it experienced almost 100% glass breakage, the Marriott 5 to 6% glass breakage.

The difference in attitude about earthquake and hurricane preparedness is reflected in funding commitments. Seismic programs receive $16 million dollars per year from the National Science Foundation alone, plus $60 million dollars a year through the National Institute of Standards and Technologies, FEMA and U. S. Geological Survey. Wind engineering research funding by the National Science Foundation is $750,000 per year.

Leslie Robertson of the Structural Engineering Consulting Firm Leslie E. Robertson and Associates and Chairman of a National Research Council Panel says 'The nations apparent indifference to the threat of wind hazards is astonishing and perplexing.' Damage from Hurricane Andrew cost more than the sum of all earthquake damage in the U. S. this century. Robertson also points to the retrofit issue as a perfect example of how the nations mind set needs changing.

Capital spent on pre-disaster preparation reduces exposure from many perils i.e. hurricanes, tornadoes, violent thunderstorms and terrorism.

Protecting the building envelope, from internal pressurization and blow through will help insure your company is up and running after the disaster with minimal damage.

Many lessons were learned from the destruction of Hurricane Andrew. The federal government, state governments, and the engineering community have recognized the importance of 'protecting the building envelope' as one of the more important points to consider when designing a plan to protect property from the effects of hurricanes or other violent weather. The failure of windows and doors is identified as the most common 'breach' of that envelope.

Studies indicate that glass failure due to windborne debris was the major cause of structural failures. All buildings are designed as enclosed structures. the function of the building envelope is essential to the building performing consistently with this design assumptions.

It only takes 5 per cent loss of the windward side wall for a building to experience full internal pressurization. Internal pressurization loads building components far beyond the intended design, leading to blow through, meaning the wind comes in one side of the building and exits through the other side, destroying contents..
In the days immediately following Hurricane Andrew stories circulated about it being a monster storm, with winds more than 200 MPH.

Actual wind speeds were difficult to ascertain, since most all the anemometers in the area were destroyed or damaged. Most wind speed estimates developed since the storm have been based on an analysis of structural and non structural damage. These estimates vary considerably, because of different measuring systems, and are still a source of heated debate.

Was Hurricane Andrew a superstorm? The best available data suggest that it was not. Andrew was a compact system with a radius of maximum winds of about twelve miles. Hurricane David, that scraped the Florida coast in 1979, had maximum sustained winds over one hundred miles wide.

Andrew's wind speed at the center of the eye wall slightly exceeded the requirement of the building codes for Dade county, but as you move away from the center, the wind speed quickly decreased. Even when using the most severe wind profile of Andrew, the area where design wind speed was exceeded should have been very limited. The extensive damage of Hurricane Andrew cannot be excused by its intensity.

So what went wrong? In the aftermath of Andrew, the mandate of the wind engineering community has been to 'protect the building envelope.' The vast majority of technical changes evolving in wind design are related to this concept.

All buildings are designed as enclosed structures. The function of the building envelope is essential to the building performing consistent with this design assumption. It only takes five per cent loss of the windward side wall area to have the building change from completely enclosed to partially enclosed.

The resulting internal pressure change is substantial, and will affect the structural system. All current building codes assume that a building is 'closed' and that no breach has occurred in the envelope.

Aerodynamic drag, resulting from high winds passing over the building's exterior, create suction pressures on the other sides. These two forces acting in the same direction (outward) can have an explosive outcome with wind pressures twice as high as before the building envelope was breached. These phenomena, known as the Bernoulli effect, are commonly used to explain the lift mechanism in airplane wings.

Conversely, an opening in the leeward wall or a side wall can result in the air being drawn out of the building.
This negative internal pressure can intensify the inward pressure on the windward wall. Since the wind direction typically changes during the course of a hurricane, openings in the building envelope can produce a variety of forces on the building.

'A survey of glazing system behavior in buildings in the direct aftermath of Hurricane Andrew' was conducted by the Department of Civil Engineering and Graduate Center for Materials Research of the University of Missouri. A report was written by Richard A. Behr and Joseph E. Minor titled 'A Survey of Glazing System Behavior in Multi-Story Buildings During Hurricane Andrew.'

The buildings surveyed spanned a 19 mile distance from Miami on the north to Cutler Ridge on the south. With few exceptions, glazing systems performed poorly and damage to building contents was extensive.

The following specific observations were made: (1) impacts from windborne debris were a dominant cause of glass failure; (2) wall system anchorages sometimes failed; (3) prevention of ' blow-through' after internal pressurization significantly mitigated storm damage to building interiors.

Bill Winter is an operations manager for Secure Window Systems

This article adapted from Vol. 9#2.

Read 2510 times Last modified on October 11, 2012