Protecting YourFacility AgainstMajor Windstorms

This brochure is made available for informational purposes only in support of the insurance relationship between FM Global and its clients. This information does not change or supplement policy terms or conditions. The liability of FM Global is limited to that contained in its insurance policies.

Over the many years that FM Globalhas analyzed windstorm damage to buildings, including damage

caused by severe windstorms in thepast decade or so, one fact is clear:Buildings designed, built and main-tained according to FM Global’s recommended guidelines have with-stood high winds with minimal dam-age. Significant loss has been limitedmostly to buildings that did not meetFM Global standards. Another factor:FM Global guidelines are likely toexceed the requirements of local building codes.

By Any Name,A Formidable HazardNo peril can match that of a severewindstorm in its ability to cause widespread devastation. Such stormsinclude hurricanes, typhoons andcyclones—depending on the part of the world they affect—and their fiercewinds have been among the most significant contributors to FM Globalclient losses, for an annual average of US$210 million gross in the last 25 years. Ten percent of all damage sustained by FM Global clients during this period was related to wind.

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Approximately 50 percent of theselosses resulted from severe hurricanesand tropical storms, including the foursignificant hurricanes that struckFlorida, USA, in 2004—an industryrecord-breaking hurricane season. Withproper knowledge and preparation,however, you can minimize extensivewind damage and the risk it representsto your business—even in the mostsevere storms.

Understanding WindBefore you can secure your facilityagainst the impact of windstorms,you would do well to arm yourself with information about wind in general. The following sections offer some background.

Speed and ForceAs wind speed increases, the wind’sforce accelerates rapidly. For example,a constant breeze of 20 mph (32 kph)exerts a pressure of 1 psf (48 Pa).

When wind speed is increased fourtimes to 80 mph (129 kph), pressureincreases 16 times to about 16 psf (766 Pa). The stronger winds from a hurricane, combined with gustingeffects and the extra force at the roofperimeter and corners, cause pressureto quickly multiply.

Remember that wind speeds are notnecessarily uniform over a given area.If you think you’ve survived a majorhurricane, find out the top wind speedat your facility. The diameter of thedamaging winds of a hurricane,typhoon or cyclone is typically 50 to100 miles (100 to 200 km), with thestorm area from 100 to 300 miles wide(200 to 500 km). Within these areas,the wind speeds vary significantly, withthe most intense and damaging windsusually within 30 miles (48 km) of theeye and concentrated at an area knownas the Radius of Maximum Winds(RMW). As a result, the range reportedat a local weather station may be differ-

ent from the speed at your facility, so ifa Category 4 hurricane strikes yourarea, your facility may experience onlyCategory 1 winds.

DirectionTropical cyclones spin around a centrallow-pressure core, with the direction of circulation governed by the rotationof the earth. Storms in the NorthernHemisphere spin counterclockwise,and storms in the Southern Hemispherespin clockwise. In the NorthernHemisphere, the winds on the easternside of the storm are typicallystrongest, whereas the western side of the storm presents the strongestwinds in the Southern Hemisphere.

IntensityIt is important to know which basis is being applied in reports on the intensity of a windstorm. Wind speedcan be measured by various averagingtimes, such as the three-second gustand 60-second mean. The comparison

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3-Second Gust Wind Speed

187 mph (301 kph)

158 mph (254 kph)

133 mph (214 kph)

115 mph (185 kph)

88 mph (142 kph)

Saffir-Simpson Scale

Improvements Needed

Catastrophic

Extreme

Extensive

Moderate

Minimal

60-Second Mean Wind Speed

156 mph (251 kph)

131 mph (211 kph)

111 mph (179 kph)

96 mph (154 kph)

74 mph (119 kph)

CAT 5

CAT 4

CAT 3

CAT 2

CAT 1

FM Global Recommendations Followed

Extreme

Extensive

Moderate

Minimal

Negligible

in the Saffir-Simpson HurricaneIntensity Chart at left shows the relationship between these two measurements. For example, a windspeed at the threshold of a Category 3hurricane is 133 mph (214 kph) by the three-second gust wind speed compared with 111 mph (179 kph) by the 60-second mean wind speed.

Loss Prevention GuidelinesVulnerable LocationsBuildings located next to open terrain,where the wind can blow unimpeded—such as near fields, large bodies ofwater, parking lots and airport run-ways—are likely to be damaged when the wind strikes with full force.FM Global testing and modeling haveshown that wind also can acceleratearound hills, berms and escarpments.

Recommendations:The location of a facility is dictated primarily by availability, costs andbusiness strategy. Where possible,choose a built-up area, away from flat, unobstructed terrain. FM Globalengineers can help you factor in groundroughness as you determine how muchwind resistance you need for a new or renovated building.

Vulnerable OccupanciesFM Global has found that, at some facilities, rain entering the building—not wind striking it—accounted for most of the damage.Rain pouring in through the roof or broken windows will damage products, metallic surfaces prone

to rust, electronic instrumentation and chemicals. Items stored directlyon the floor are especially vulnerable.

Recommendations:Place extra emphasis on a properlydesigned and maintained building,especially the roof, according to FM Global Property Loss PreventionData Sheets and the Approval Guide,a publication of FM Approvals. Storeproducts off the floor on pallets, andkeep ample waterproof tarpaulins onhand to cover vulnerable equipment,material in process and finished goods.

Design and Installation DeficienciesMost design and installation deficiencies are in the roof, leading to problems with flashing, roof covering, insulation and even the deck itself. Such deficiencies allowrainwater to enter buildings anddestroy equipment, furnishings andinterior finish. Frequent damage has been the result of unreinforcedmasonry parapet walls and decorativefacades; these structures can blow over and destroy the rooftop or anything else they strike.

Recommendations:Follow recommended design andinstallation guidelines in the datasheets, select products from theApproval Guide, and ask your FM Global engineer to review construction plans. Avoid unreinforcedparapets and decorative facades. Also, consider replacing roofs that are nearing the end of their service life or have known deficiencies.

During construction projects, closelymonitor contractors to ensure productselection and installation follow youroriginal specifications.

Glass Windows and Other Openings Glass wall panels and plate-glass windows are vulnerable to breakage by wind-borne debris and the directforce of wind. Tree branches, outdoorfurniture, building materials, gravel androof tiles can become missiles that willbreak glass. If a large opening results,wind and soaking rain will cause sig-nificant damage in the building. Incoastal areas, wind-driven saltwaterspray may cause additional damage tosensitive equipment. In locations with a year-round warm climate, buildingswith open sides, windows and door-ways will be more vulnerable to dam-age by high winds and rain. Unless the facility is promptly secured after a storm, broken windows at groundlevel also will provide easy access for looting and vandalism.

Recommendations:Before a storm, remove outdoor furniture, trash containers and other relatively lightweight objects that couldbecome windblown missiles. Shieldwindows with hurricane shutters or plywood. Close and latch exteriordoors and windows, and brace largedoors at shipping and receiving docks.Indoors, relocate equipment away fromwindows, where practical, or cover itwith waterproof tarpaulins. If smalleritems are on the floor, place them onpallets, racks or shelving.

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Consider replacing large windows with smaller ones, filling the remainderof the window space with wall construction equal to the rest of thebuilding. Replace standard glass windows with a laminated glazing system that meets test criteria for missile resistance. In new buildings,increase use of blank wall constructionand narrow profile windows.

When a hurricane is approaching,latch all doors securely. If you have a doorway you no longer need, andeliminating it will not compromiseemergency exiting, remove the doorand fill in the opening with construc-tion equal to that in the rest of thebuilding. In a building with open doorways and sides, use a design with increased resistance to uplift.Long term, when you purchase doors,opt for sturdy products and ruggedframing that will resist wind forceswell. Tightly install windows, doors,air conditioners, vents and similar penetrations through the building’sskin. Maintain well-sealed joints,replacing caulk and joint-covering trim as needed.

Vulnerable Roof FeaturesWhen wind strikes a flat or near-flatcommercial roof and sweeps over it,a negative or reduced pressure results,especially along the windward edge or corners of the roof the wind strikesfirst. (FM Global research and otherstudies of loss experience clearly show wind forces are greater along the periphery of the roof than in thefield, or center, and at a maximum at corners.) The pressure within thebuilding remains positive, which leads

to an upward force against the roofassembly. In addition, wind that entersthe building creates an additional positive force that acts upward againstthe roof system and outward againstleeward walls.

The combination of negative pressureover the roof and positive pressurebeneath it results in uplift pressure that may peel off components of theroof or lift the entire roof assembly off its supporting framing.

Extensive damage to roofs is caused bywinds of lesser velocities if the design,installation or maintenance is deficient.Depending on wind speed, the damagemay include gravel blown off a roof,covering peeled off, insulation rippedoff the deck, metal panels pulled overtheir fasteners, and even sections of the deck lifted off the framing. In manycases, a common denominator is loos-ened flashing, which allows the wind to pry into the exposed edges of theroof covering and insulation. If the roof looks to be in bad condition, itwon’t hold up to hurricane winds.

Many older steel deck roofs have insu-lation that is only adhered to the steeldeck and are very vulnerable to wind-storm damage. In fact, this type of roofsystem is no longer FM Approved forany location. However, newer steeldeck roofs with mechanically fastenedinsulation (you can see the fastenersfrom inside the building) also can bevulnerable if not correctly installed with the proper type and number of fasteners. Additional fasteners arerequired in the perimeter and cornersdue to the higher uplift pressures in

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An inadequately designed parapet wall with masonryblocks was blown off at the roof line.

This glass wall facade was destroyed by high winds,leaving the building interior vulnerable to wind and rain damage.

The damaged roof covering on this seaport building is mostly along the edge, where wind forces are at a maximum.

these areas. FM Global also has foundclay and concrete roof tiles that wereinadequately fastened. In other cases,facilities used only mortar without any fasteners. Loosely or inadequatelyattached tiles were blown off the roof,becoming missiles that broke windowsand damaged wall surfaces.

Roof gravel is another source of missiledamage. Gravel generally provides a benefit in resistance to wear andultraviolet radiation, but loose gravel is easily blown off the roof by highwinds, and will mar building surfacesand break window and door glazing.

Recommendations:The best protection is provided bypoured reinforced concrete, proven to be highly resistant to high winds.Other types of FM Approved roof systems listed in the Approval Guidealso can provide effective protection.

Especially in hurricane-prone areas,select clay or concrete tiles that havetwo or three starter holes. Drive fasteners through every hole providedon each tile. FM Global recommends at least two fasteners and adhesive fortiles in the field of the roof; use threefasteners for tiles at the edge, in cor-ners, and along the ridge of peakedroofs. Screws provide greater upliftresistance than nails. Also, consideradding mortar in the openings at theedge of tiles installed along the firstrow at the eaves.

Where pea gravel has been used on a built-up roof, sweep back any loosestones, flood-coat the roof with hotasphalt or coal tar, and then sweep

the gravel over the hot coating. Allowto set; then remove any loose gravel. In new construction, make sure thenewly applied gravel is embedded inthe asphalt and then remove whateverremains loose.

If you have any doubt about a roof’sresistance to wind forces, contact your local FM Global office to discusshaving the roof deck and roof coversecurement analyzed or having a contractor conduct an uplift test. Infact, to make sure you’re getting theroof you paid for, FM Global now recommends an uplift test be conductedbefore accepting new roofs (for rooftypes that can be tested) where thebasic wind speed is equal to or greaterthan 100 mph (161 kph). See DataSheet 1-52, Field Uplift Tests, for applicability and testing details.

Other Vulnerable Construction FeaturesExterior Insulation and Finish Systems(EIFS) are used extensively in newerconstruction at resorts and hotels.Many times, large pieces of the EIFSassembly are blown off, bringing hurricane winds and rain directly intohotel rooms and offices. The removedEIFS pieces then become flying mis-siles, breaking large windows of nearbybuildings. This was seen extensivelyduring the 2004 hurricane season,even where the gypsum board appeared“well-secured” to the steel studs with a 6-in. (152-mm) screw spacing. (EIFSis multilayered, usually consisting ofgypsum board attached to the exteriorof metal studs, then covered withexpanded or extruded polystyrene,

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Panels of this standing-seam deck were torn off, leavingthe fasteners still attached to the roof framing.

The lightweight insulating concrete on this metal roof wasseverely damaged by a tropical storm.

An insulated steel-deck roof was blown off this building.

which, in turn, is faced with a thin layer of glass-fiber-reinforced polymer-modified plaster or thin sections of brick.)

Similar damage has occurred to plaster-on-metal-lath construction,widely used at resort buildings for balcony dividers (privacy screens),which breaks away under high winds,turning pieces into destructive missiles.Skimpy attachment of steel studs to the building frame and corrosion of studs over time appear to be responsible for such damage.

Recommendations:It is critical to properly maintain the water tightness of EIFS systemsbecause water penetration will greatly weaken the EIFS system. The sealant or caulking of EIFScladding, especially around the win-dows, should be inspected every fiveyears and repaired or replaced every 15 years or when necessary. The wallsalso should be inspected for visualsigns of delamination or deterioration,which may include bowing or a loosefeel when pushed in. Most of all, yourwind emergency response plan shouldaccount for the loss of large sections of these walls.

Anchor exterior wall sections of plas-ter-on-metal-lath to studs on the mainbuilding structure. Interior partitionsare best when built full-height from the floor up to the overhead floor slabor roof framing, and anchored at topand bottom. (Such a design also acts as a fire cutoff for the surrounding area as long as doors are closed.)

Reinforced concrete or reinforced-concrete block walls offer the bestwindstorm protection for new construc-tion. Other wall assemblies should beevaluated for proper pressure and wind-borne debris ratings. EIFS is notrecommended by FM Global for use in high-wind areas. You also shouldconsider if you, your employees, yourguests (especially hotels) and yourbusiness are safe behind the selectedwall assembly, with wall studs and treebranches flying outside during a hurri-cane. Damage and restoration time will vary greatly depending on the system chosen. Will your customers be waiting for you?

Flashing: The Key ComponentThe most important component of aroof system is the flashing. It acts as a termination along the edge of a roofwhere it meets the wall. If high windsloosen perimeter flashing, further dam-age to the rest of the roof—roof cover-ing, insulation and even the deck—isinevitable. Inadequately designed andinstalled flashing will likely lead toadditional damage from water leakage.

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Recommendations:Probably the single most cost-effectivemeasure against wind damage to a roofis to ensure flashing is properly secured.It is vitally important to attach perime-ter flashing well enough to prevent itfrom being pulled loose by wind. When installing new perimeter flashing,be sure to continuously attach its lowerpart to a hook strip that is firmlysecured to the building. For wood nailers and cant strips, give preferenceto high-quality preservative-treated lumber, and anchor it securely to themain structure. (See the Approval Guideand details illustrated in Data Sheet 1-49, Perimeter Flashing.)

When inspecting existing flashing,just pull out on the lower edge. If itfeels loose, resecure it with appropriateweather-resistant fasteners driventhrough washers. For further advice,contact your FM Global engineer.

Fix It RightAfter a severe windstorm, you are likely to face some degree of damageand the need to make repairs. You mayneed to reinforce the fastening of thedeck to its supporting structure, orreplace sections of deck before a newcovering is placed over it. In othercases, only sections of covering andinsulation will need to be replaced.Even when storm damage is not a con-sideration, you will need to performperiodic maintenance—spot repairs or more extensive reroofing—on anaging roof. For proper repair andreroofing, follow the appropriate

data sheet (for installation guidelines)and the Approval Guide (for productselection). Ask your local FM Globaloffice to review plans before youundertake any work.

Periodic inspections and routine maintenance will go far to minimizethe need for major unexpected repairsthat often follows a severe windstorm.Subtropical and coastal regions mostoften subject to severe windstorms such as hurricanes often are exposed to saltwater spray from the ocean.Under such conditions and withoutappropriate attention, steel claddingand framework can be weakened bycorrosion, and wood construction canrot from decay and insect infestation.Concrete and associated steel reinforc-ing bars (not epoxy-coated) in sea walls and oceanfront walkways arelikely to deteriorate.

Your own staff can handle some of these essential repair and mainte-nance tasks if the specified equipmentand materials are used. Other workshould be completed by a contractor. In any case, it is best to discuss allplans for roofing work with your FM Global engineer.

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Probably the single most cost-effective measure against winddamage to a roof is to ensureflashing is properly secured.

Construction in ProgressWind is more likely to damage build-ings under construction for a number of reasons. Steel framing, precast con-crete wall panels and masonry blockwalls are all vulnerable while they arebeing erected and not fully attached to adjacent walls and the roof. A steelbeam is usually attached to a columnwith a minimum number of bolts untilthe frame is completed and aligned.Precast and tilt-up concrete wall panelsare supported only at the bottom untilthey are stabilized at the top by thestructural roof. Concrete block wallsare commonly erected to unsafe heightswithout lateral support, and they areeasily blown over by moderate winds,especially when they have not yet been tied to structural steel framing.

For steel framing, provide cross-bracing between columns with cable.Concrete block walls should be bracedat an incline, from the ground up to the ceiling on both sides of the wall.Tilt-up concrete walls need similarlean-to pipe bracing on both sides untilthey are attached to the roof. On therooftop, fasten deck panels, roof cover-ing and flashing promptly and securelyas construction progresses. If necessary,provide temporary ballast along roofedges and corners. Rooftop equipmentshould be fastened securely to the roof.If high winds are forecast, removebuilding materials and tools from therooftop. Otherwise, they may blow off, delaying construction and possiblycausing damage to nearby structures.

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Steel framing, precast concretewall panels and masonry blockwalls are all vulnerable whilethey are being erected.

The Human FactorThe scope of preparation dependslargely on the potential strength ofwindstorms likely to strike in yourfacility’s location. In areas prone tosevere windstorms, tracking the storm’sdevelopment and movement shouldprovide time to make appropriatepreparations if you have a proper wind emergency response plan inplace. But remember, preparations must be completed before the onset of high winds, which will arrive wellbefore the hurricane eye that the fore-cast concentrates on. It’s recommendedpreparations take no longer than 12hours to complete because, manytimes, a storm’s forward speed willincrease, resulting in the onset of high winds earlier than expected.

Organize StaffIf the path of a severe windstorm suggests it may bear down on yourlocation, be sure your emergencyresponse team (ERT) is equipped to take action before the storm to minimize damage and to deal withpotentially widespread and massivedamage after the storm passes. To befully prepared, the ERT roster shouldinclude alternate team members.

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In areas prone to severe wind-storms, tracking the storm’sdevelopment and movementshould give you time to makeappropriate preparations.

Prepare the BuildingSurvey the condition of your buildingsand ancillary structures such as coolingtowers (see Data Sheet 1-6, CoolingTowers), antenna towers and signs(Data Sheet 1-8, Antenna Towers andSigns), cranes (Data Sheet 1-62/17-16,Cranes) and chimneys (Data Sheet 1-13, Chimneys). Wind loads should be per Data Sheet 1-28, Design WindLoads. Make building repairs as timepermits, giving flashing priority status.Have plywood or hurricane shuttersready to mount over windows byattaching them to hardware previouslyinstalled. (Note: FM Global does notconsider window taping to offer signifi-

cant protection.) Make sure roof drains,outdoor drains and ditches on and nearthe property are free of debris so theycan handle the heavy rain that normallyaccompanies tropical storms.

To facilitate repairs after a storm,renovations when needed and assess-ment of the roof’s ability to withstandstrong winds, keep up-to-date buildingplans and specifications securely filed.

Maintain CommunicationsYou will need reliable communicationsbetween ERT personnel and any operating personnel remaining at your facility during a storm, especially at a sprawling complex and between your facility and sister plants or publicservices. Look to redundant means of communication, such as internal and Internet e-mail, cellular phones,two-way radio, CB radio, and even

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You will need reliable communications between ERT personnel and any operating personnel remaining at your facility during a storm.

ham radio. Under severe weather conditions, utility poles, satellite dishesand transmission towers may be dam-aged and unreliable, and phone linesmay be constantly busy. Portable radios with fresh batteries will help you keep in touch with announcementsfrom the weather service and local public authorities.

Check UtilitiesConsider how dependent you are on electricity. Plan to purchase or rent portable generators ahead of time. Make sure the equipment willstart and you have an adequate fuelsupply. Have portable lanterns andflashlights with fresh batteries ready.You may need to shut down computersystems and some industrial processesin an orderly fashion as the storm gets closer, to avoid damage caused by erratic supply, surges and abruptloss. For those computer or production operations that cannot tolerate a shutdown, however, prepare an uninterruptible power supply.

At the same time, you may need to shut down some nonessential circuits to avoid short-circuiting and ignition of any exposed combustible material.Piping that carries gas, liquid orprocess water may need to be shut off. Know where the shutoff valves are located.

Protect the ProtectionAssign ERT personnel to check the fireprotection system before, during andafter a storm. Make sure sprinklersystem control valves are open, andknow which valve shuts off only thatpart of the system affected by brokenpiping. Shut off as little of the sprinklersystem as necessary—shutting off toomuch of the system will leave yourfacility unprotected if a fire occurs—and contact your local FM Globaloffice, taking necessary precautions by following the Red Tag PermitSystem, FM Global’s sprinkler impairment program. Emergencypower for electric-motor-driven firepumps and ample fuel for internal-combustion-driven pumps should

be available. Have special protectionsystems, such as carbon dioxide andfoam, designed to switch over to battery backup in case of power loss.Similarly, have a backup for deep-well pumps to ensure a water supplycontinues for fire protection and critical processes.

Protect EquipmentWith the possibility that water will leak into the building from rain andnearby overflowing streams or in-adequate drainage of surrounding soil, ERT personnel must be ready to relocate equipment and storage—especially away from unprotected windows and ground-level doors—or cover it with waterproof tarpaulins.Have water vacuums, pumps, mops,buckets, water absorbents and dehumidifiers ready for cleanup and salvage after the storm.

Also, keeping good records and stockpiling critical spare parts for key machinery and equipment canmake a difference in expediting repairs and returning to service.

You may need to take other emergencyflood damage mitigation measures,depending on the facility’s location.For long-term planning, talk to yourFM Global engineer about the threat of flood waters or surface runoff toyour facility. Refer to Data Sheet 9-2,Surface Water, and 9-13, Evaluation of Flood Exposure, for appropriatemeasures inside and outside buildings.

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For a more complete checklist of precautions to take before, during and after a windstorm, refer to the FM Global publication, SevereWindstorm Checklist (P9308).

Cost-effective loss prevention guide-lines, appropriately applied to yourfacility, are available from your FM Global engineer. When you follow them, you will greatly minimizeor even prevent windstorm damage to your facilities. Although severewindstorms cannot be prevented—nor their courses changed, nor theirstrength diminished—our knowledge of how to minimize damage by theseand lesser storms continues to expand.

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Although severe windstormscannot be prevented,the severity of windstorm damage can.

For additional copies of this publication or other FM Global resources,order online 24 hours a day, seven days a week at www.fmglobal.com/catalog.

Or, for personal assistance worldwide, contact our U.S.-based customer services team:• Toll-free: (1)877 364 6726 (Canada and United States)• By phone: +1 (1)781 255 6681• By fax: +1 (1)781 255 0181

P9811 (Rev. 5/05) Printed in USA (5/05) © 2002, 2005 Factory Mutual Insurance Company All rights reserved. www.fmglobal.com

In the United Kingdom:FM Insurance Company Limited, 1 Windsor Dials,Windsor, Berkshire, SL4 1RSRegulated by the Financial Services Authority.