SECTION SIX:  MITIGATING DAMAGES IN HAWAII'S HURRICANES, A PERSPECTIVE IN RETROFIT OPTIONS

INTRODUCTION

Each time a significant wind event is experienced, a well practiced and oft repeated evaluation procedure begins anew. Each storm brings forth a new sequence of inspection, survey and assessment of damage, a flurry of reports, and, finally, recommendations in preparation for future events. Architects, engineers, representatives of the insurance agency, code enforcement officials, civil defense administrators, politicians, et al, descend on the site of disaster, each group with its unique set of interests and point of view, ready to dissect and examine the remains, the lessons of the post-mortem to be used to mitigate the effects of the next occurrence.

Since this post-disaster survey and evaluation procedure has been part of the response to all significant hurricanes over the past few decades, the types of damage and mechanisms which cause it have been well documented. This sequence of cause and effect, along with recommendations for future corrective action, forms a common thread running through past post-hurricane reports. Thanks to the efforts of these investigative teams, the causes of building damage in hurricanes, as well as the appropriate measures to prevent it, are well understood by design, code and insurance professionals. However well understood, they are but rarely implemented.

It is the task of this paper to establish an overview relating the types of damage caused by hurricane winds, the generic causative mechanisms of that damage, and achievable, economical measures that can be taken to effect mitigation in future storms. Further, qualitative cost conclusions will be drawn which will help to clarify cost vs benefits of mitigation efforts. The emphasis is placed on that which is most readily accomplished and is most cost-effective. In accomplishing these goals heavy reliance will be placed on reports evaluating damage from hurricanes Andrew and Iniki and on the research of independent investigators.

WIND FORCES

First, it is necessary to understand how wind acts on buildings and the relative magnitude of the forces which are generated. The wind pressure increases with the square of the velocity. If the wind blows at 80 mph, the Code mandated hurricane wind speed for Hawaii, the wind pressure on the windward face will be 16 lbs/sq ft. If the wind speed is doubled to 160 mph the wind pressure will be quadrupled to 64 lbs per/sq ft. Considering that floor loads in an office building are designed for 50 lbs/sq ft, these wind forces are enormous and act on walls and roofs that normally carry no other loads.

In general, when the wind blows on a building, the windward wall experiences a pressure pushing inward (positive pressure) while the sidewalls and the leeward wall will experience a suction pressure outward (negative pressure). The roof, depending on its slope, will experience an uplift (negative pressure). Wind forces are significantly increased at corners, ridges, and at abrupt changes in plane as shown in the illustrations below.

Figure 1 shows the airflow over the building and the areas where it separates from the building. The wind strikes the endwall, creates a positive (inward) pressure, and the flow is bent upward and outward around the building, causing the airflow to separate from the building creating outward (negative) pressures.

 

In Figure 2, the wind is unable to accommodate to abrupt changes in direction and so builds large pressures at the corners, ridge, and at roof overhangs. The roof tends to be uplifted, the windward wall to be pushed in, the sidewalls and the leeward walls to be pulled out. Each of these areas requires special concentration during the design and construction period with prescribed anchors provided.

 

 

In the illustrations in Figure 3 above, and Figure 4 below, the basic wind speed is taken at 100 mph. This will produce a wind pressure of 25 lbs/sq ft on the windward side of the building, and higher pressures at the overhangs, ridges, corners and gable ends.

 

These higher pressures can be two to three times greater than that due to the basic wind: approximately 50 lbs/sq ft on the overhangs and 75 lbs/sq ft at the overhanging corners. It is clear that these areas demand special attention to detail during construction.

If the envelope of the building is breached, that is, if there is a failure of doors, windows, siding/sheathing, garage door, or gable end in residential structures, or of the enclosure system in commercial buildings, then the wind will cause internal pressures and effectively double the outward loads on the sidewalls and leeward wall, and double the uplift on the roof. This source of major structural failure due to wind is illustrated below in Figure 5. It allows the intrusion of wind and water which accounts for the vast majority of insurance claims due to hurricane damage.

It has been shown that of damage due to wind in hurricane Hugo, 95% was to the roof. The average damage suffered by a dwelling was 25% of its insured value. In the average insurance claim, however, 75% of the claim was for water damage resulting from intrusion of water following loss of roofing or failures of the enclosure.

 

 

STRUCTURAL FAILURE

Major structural failure during a hurricane is the exception rather than the rule, particularly among engineered buildings: buildings which because of their size, occupancy or importance are required to have a structural engineer involved in the design process. Engineered buildings are more likely to suffer a failure of the envelope with consequential interior damage due to the intrusion of wind and water. Major collapse of such structures is rare.

While major structural failure among non-engineered, primarily residential buildings is not the norm, it occurs frequently enough that it demands and receives special consideration in the 1991 Uniform Building Code's prescriptive requirements for wood frame and masonry construction, Appendices Sec 2518 and 2425 respectively.

Sections 2518 and 2425 provide for reinforcing the structure to provide a continuous path of load resistance from the point of application of wind loads to the foundation. This is accomplished by providing metal ties connecting the structural elements together assuring continuity at the ridge, between the roof and walls, between second floor and first floor walls, and first floor walls to the foundation.

Codes, Inspection and Enforcement

In looking to the Building Code for mitigation of damages due to high winds, it is important to remember a few obvious points:

1. The appropriate Building Code edition must have been adopted by the local authority, whether it be the state, county, or municipality. No adoption, no code, no mitigation benefit.

2. The Building Code will have an impact only on future buildings; it does not address the existing stock of buildings except in certain circumstances of addition, renovation, or repair.

3. The Building Code will only be as effective as its enforcement. An inspector's competence in enforcing the Code is no better than his preparation and training.

4. Subcontractors installing anchors and ties crucial to the wind resisting continuous load path and carpenters nailing the roof and wall sheathing will only do it correctly if they have been shown the correct way.

Research has shown that residential buildings built without benefit of the continuous load path approach of the 1991 UBC have experienced a significant proportion of structural failures. Even those buildings built to withstand design wind speeds suffer some severe structural failures. Failures in this latter group are frequently due to inadequate knowledge and training inspectors which results in inadequate inspection and enforcement.

Construction Quality

Another frequent cause of failures in buildings subject to code wind requirements is insufficient knowledge and training of subcontractors and their employees. Subcontractors installing anchors and ties crucial to the wind resisting continuous load path and carpenters nailing the roof and wall sheathing and installing the doors and windows will only do it correctly if they have been shown how.

Cost of Damage

There are two primary sources of loss in any hurricane, the cost of the physical damage to structure and infrastructure, and the economic loss due to decreased economic activity, the loss of jobs, and the multiplier effect as the loss filters down through the economy taking more jobs along the way. While the two sources of loss are approximately equal to one another, the first is well known and understood, but the second, which can be greater, is all too seldom discussed. Nowhere is this more evident than in the case of hurricane Iniki on Kauai.

Iniki cost the state almost $1.6 billion. But that appalling total does not include disruptions in economic activity; those losses amount to almost $900 million, putting the total loss to the state's economy at almost $2.5 billion.

Failures in Commercial Construction

While major structural failures are rare in commercial construction, they are not unknown, particularly in the coastal zone where buildings are subject to flooding, surge and wave action, and the scouring of footings and foundations.

A significant exception to this is in pre-engineered metal buildings which have not fared so well in recent hurricanes. These buildings suffer a loss of the lightweight sheet metal envelope, failure of primary structural members, and failure at the connection to the foundation. Studies of pre-engineered metal buildings are ongoing by the NMBA, engineering groups and code officials and are a special class of buildings beyond the scope and intent of this paper.

Other than as noted above, failures in engineered commercial structures are largely limited to loss of roof ballast (gravel), loss of roofing, loss of components from roof mounted equipment, and breakage of glass in the envelope. It is this last, glass breakage, that causes the preponderance of economic loss in strong winds.

Glass, while exceedingly strong and able to withstand the effects of design wind speeds, cannot withstand the impacts of windborne debris. In a hurricane the air is laden with flying objects: roof gravel, roofing, siding, roof sheathing, tree limbs, etc. These missiles have a velocity approximating that of the wind. They can break exterior glass and lead to a sequence of events causing catastrophic loss to the building interior and contents.

Damages in Commercial Construction

Other than damages due to flood, surge and wave action in the flood zone, the bulk of damage to commercial structures is due to failures in the building envelope, primarily the glass as noted above. With the failure of the glass, there is consequent pressurization of the interior due to the intrusion of wind which doubles the negative pressures on walls and roof; secondary failures occur in the envelope due to the internal pressure as other windows blow out, and the destruction of the interior and its contents is completed by a combination of wind and water blowing through the open building. An opening of only 5% of a wall's area is sufficient to fully pressurize the interior of a building.

If the loss of a hotel room's interior and furnishings has a value of $75,000, the loss of use of that room could amount to an additional $75,000 loss to the economy over the period of recovery and reconstruction.

Failure and Damage in Light Construction

Since there is no requirement for engineering in most light and residential construction, and there having been no specific requirement for wind resistance in these structures prior to the current code edition's prescriptive requirements, there is considerably more variety to failure modes in light, non-engineered construction than in larger commercial buildings.

Structural failures in light construction, whether bearing masonry or light framing, can be broadly categorized as those events arising from the lack of a continuous path of load resistance from the roof to the foundation, and events arising from the breaching of the envelope, pressurization of the building and consequential blowing out of the leeward walls, sidewalls, windows or roof.

Ultimately all loads on a structure, wherever placed, will be transmitted to and must be resisted by the foundation. If there is a weak link in the path from point of loading to the foundation, that is where the failure will occur. If the roof is not adequately anchored to the walls, an excessive uplifting load on the roof will not be transferred through the walls to the foundation; the roof will be removed and become another missile in the maelstrom. Without the roof to tie the walls together the walls collapse and the loss will be total.

Similarly, if the building is not anchored to the foundation, as in the case of "tofu block" foundations so common in Hawaii, then it will move off the foundation in high winds sustaining major damage. The key to a structure's resistance to wind forces is establishing a continuous path of load resistance wherein the various elements of the building are tied together with metal ties, and the building is solidly anchored to an adequate foundation system.

Having tied the building together into a single load resisting system, the effort expended to mitigate the second category of failure sources -- breaching the building's enclosure system -- will have enhanced effectiveness. Common failures of the envelope are glass breakage or the blowing out of doors and window units due to inadequate fastening to the building itself, failure of garage doors due to inadequate resistance to bending, loss of roofing, and failure at the joint of endwall and endwall gable. In each of the cases the result is the same, substantial loss of the interior and contents due to water damage, pressurization with secondary failures in the roof , sidewalls and leeward walls.

Whichever way the initial failure is initiated, the end result, if not a major structural loss, is going to be loss of the interior finishes and contents, the primary source of loss in hurricanes. The simplest and most cost effective mitigation efforts are those aimed at minimizing the damage due to water penetration into the building.

In considering the costs and benefits of undertaking a program of building retrofitting to mitigate future storm damage, it is important to keep in mind the overall statistical probability of a building being subjected to design wind speeds during its lifetime. The building code bases the design wind speed of an area on a 50 year recurrence period. If the building has a useful life of 50 years, its probability of experiencing winds exceeding the design wind speed of 90 mph (peak gust) for Hawaii, is 64%, in 100 years the probability rises to 87%. The odds, then, favor a building experiencing winds exceeding code values over the its lifetime. The prudent gambler would hedge his bets by improving his odds through mitigation.

RETROFIT OPTIONS AND COSTS FOR LIGHT CONSTRUCTION

Continuous Load Path

If damage is to be mitigated in a building built to older Building Code editions, the first imperative is to tie the building together with, particular emphasis on preventing loss of the roof. Inadequate connections of the roofing and roof sheathing are one of the major causes of damage to residential buildings. Structural materials rarely fail; God resides in the connections.

Tie together the various structural elements of the building using the appropriate metal anchors to connect the first floor to the foundation, the second floor to the first floor walls, the roof to the uppermost walls and the two sides of the roof to one another. This work should be done following, as close as is practicable, the prescriptive requirements of the UBC-91 Appendix Chapters 24 and 25.

The cost of accomplishing this work will vary according to the type of construction and complexity of the building design.

Estimated Cost Unit Cost $3/sf - $5/sf Cost $4800

Roof

The following retrofit options for the roof can most economically be accomplished at the time of re-roofing an existing building.

Remove all of the existing roofing down to the bare roof-deck. Do not cover the existing roofing with the new roofing. The roof deck nailing can now be examined and deficiencies corrected on the body of the roof, the gable-ends, overhangs and ridge.

Since there are no asphalt shingles that can reliably withstand winds in excess of 90 mph, a second line of defense can realized by installing 30 lb. saturated felt underlayment instead of the usual 15 lb. felt and using simplex nails, roofing nails with an integral washer to distribute the load over a greater area. This will provide a secondary back-up waterproof surface beneath the primary roofing.

The new roofing can now be applied using connections of suitable strength, i.e., carefully bedded tiles with appropriate nailing, properly installed cedar shakes, or heavyweight asphalt shingles using simplex nails, six to the shingle.

Estimated Cost Unit Cost $1.50/sf Cost $1800

Protect Openings:

Windows

Glass rarely fails due to simple wind pressure. If glass is going to fail in a hurricane, it is far more likely to fail due to missile impacts, a combination of missile impacts with oscillations due to gusts and cycling inward and outward pressures, or a pulling of the entire window unit out of its opening due to poor attachment.

Shutters

To protect glass against the impacts of windborne debris, the simplest and most cost effective mitigation option is the installation of shutters with proper connections to withstand both positive and negative pressures. (Shutter failures in hurricane Andrew were not uncommon, but were almost always due to being pulled off of the building (poor connections) rather than pushed in. Nevertheless, shutters are considered so effective that without them one can no longer obtain insurance in Dade County.)

An alternative to shutters, particularly in commercial applications, would be a system of movable louvers doubling as sunshades and capable of withstanding impacts of 8 to 10 lb objects with a velocity of 80 mph.

Estimated Cost Unit Cost $150/window Cost $1500

Re-glazing

Where shutters are not a desirable alternative the glass can be replaced using a tested system of tempered, laminated glass replacing the original glass. While this is expensive, it may be the only alternative for many commercial and office buildings with curtain walls of fixed glazing.

Estimated Cost Unit Cost - 5% to 10% of Building Cost

Taping windows has little or no positive benefit and is a waste of time. However, a permanent installation of solar film on the inside of the glass and adequately anchored at its perimeter is a less expensive option which has been shown to be effective in protecting the interior in event of glass breakage.

Estimated Cost Unit Cost $150-$250/Window

Attachment

Normal attachment of doors and windows is to resist inward rather than outward pressures and installations are often inadequate to loads generated by hurricane winds. Units being pulled out of the building is the second most common mode of failure of doors and windows after missile impacts. Exterior doors and windows can have improved anchorage to the opening in the building wall by adding anchors with increased penetration into the framing.

Estimated Cost Unit Cost $$25/Opening Cost $250

Doors

Doors should be anchored to the building opening as stated above. An interesting note is that one door in Andrew resulted in $3,000,000 damage, an incentive to undertake simple and inexpensive mitigation measures.

Estimated Cost Unit Cost $25/Opening Cost $50

Garage Doors

Garage doors, because of their light construction and the typically large openings they must cover, are prone to failure due solely to wind pressure; they fail in bending due to inadequate thickness and a resulting low strength in resisting bending. Virtually no large overhead doors are made that will resist design loads in a hurricane and they are frequently the primary cause of major structural failures. Once they fail, the building is pressurized and a further chain of failure is likely.

The simplest retrofit measure is to strengthen the existing door by attaching vertical struts behind and attached to the door which run from the ceiling structure above into the slab below, steel bars or angles of appropriate size could prevent the failure of the door.

An alternative buttressing against inward forces can be accomplished by simply backing the family car against the door to hold it. However, since a characteristic of hurricane winds is changing direction as the storm passes, this latter strategy might fail when the wind reverses its direction and the pressures change from positive to negative.

Estimated Cost Unit Cost $200 Cost $200

Brace Gable Ends

A frequent cause of failure in light frame construction is the inadequately attached and braced gable end walls. The gable end is commonly built on top of the wall below with no members running the full height to reinforce against lateral loads. The joint behaves like a hinge and collapses. This critical joint is simply corrected by lateral bracing running back into the roof construction.

Estimated Cost Unit Cost $150/Gable Cost $300

Summary

For an approximate cost of $8,000 to $10,000, or approximately 5% of the replacement cost, the average single family residence in Hawaii can be brought to a condition which will give reasonable assurance that it will survive an Iniki type hurricane with only superficial damage and no significant insurance claim.

Mitigation priorities for light construction should be to:

1. Keep the roof on by improving anchorage and establishing a continuous load path.

2. Protect the openings: doors, garage doors and windows.

3. Tie in the gable ends.

RETROFIT OF HOTELS AND COMMERCIAL STRUCTURES

Protection of the Envelope

The most important single mitigation measure that can be taken by the hotel industry as well as other high rise buildings is to protect the glass either by means of shutters, protective movable-louver solar-screens, or other covering device, or by replacement of the glass with tempered, laminated glass units which have been successfully tested for the application. Such a system was developed, tested and reported by Minor and Behr and consisted of inner and outer layers of 7/16 tempered glass with an interlayer of .060 PVB and PET plastic films. In this design the outer layer is sacrificed to the airborne missiles, the interlayer holds it all in place, and the inner layer remains sufficiently strong to resist the wind pressures.

In hurricane Andrew there were five distinct modes of failure in the glass. The causes were: windblown debris, wind-induced pressure, large deflections leading to pull-out of the glass from the stops, failure of vertical mullions and failure of the connections anchoring the frame to the building. Of these, the most common was breakage due to windborne debris and due to outward acting pressure. The missile impact broke the glass, the building became pressurized and the other glazing on that floor was blown out.

It was noted by investigators following Andrew that "with few exceptions, glazing systems performed poorly and damage to building contents was extensive. Most building occupants were forced to move out and conduct business elsewhere for time periods likely to exceed a year." These losses are both foreseeable and avoidable.

The airborne debris most prevalent at the lower levels of a building, up to 30 feet, included 2x4's, panels of sheet metal pieces of masonry. The mid level, from 30 feet to a level near the top of adjacent buildings was primarily roof gravel from those buildings, and at the upper level there is little or no flying debris. By eliminating gravel as a ballast and ultra violet protection, or by containing it on the roof, one of the largest sources of storm damage can be largely eliminated.

Protection of the Roof

Failures in roofing due to high winds are common and the consequential damage to interiors and contents caused by the intrusion of water is extensive. Two alternatives to reducing this source of damage are much greater attention to details of installation of roofing materials and "requiring a secondary roof membrane which will not be susceptible to removal by wind action. Otherwise, major water damage can be expected to continue to occur even when the structural systems and roof sheathing remain intact." One method would be to provide a waterproof membrane over the roof-deck but under to insulation. If the roofing were breached, the insulation would remain protecting the secondary membrane below.

Elevation of First Floor Above the Flood Plain

In those cases where it is feasible, particularly on ocean-front hotel properties, the first floor spaces, where they are below expected levels of flood and surge, should be opened for free wash through and dedicated for outdoor uses. The wisdom of such a strategy is apparent to even the most casual observer of the damage to the hotels at Poipu.

Mitigation priorities for commercial construction should be to:

1. Protect the glazing with protective devices or by reglazing using tempered, laminated glass systems.

2. Eliminate gravel as a ballast and ultra-violet protection for built-up or membrane roofs.

3. Provide a second roofing membrane to prevent roofing failures.

4. Eliminate ground floor occupancies in the flood plain.

OTHER RECOMMENDATIONS

State Code

The state should adopt the latest edition of the UBC in its entirety as a single uniform statewide building code with administration and compliance at the county level and central administration, interpretation and enforcement by the state.

Training of Code Enforcement Personnel

The state should establish mandatory training courses for county code enforcement personnel ensuring familiarity with and enforcement of code wind provisions.

Training of Contractors

The state should establish mandatory short training courses for state licensed contractors and their supervisory personnel ensuring familiarity with and correct implementation of code wind provisions.

Quality Assurance

Private inspection and certification of the wind resistive construction by registered design professionals should be a required part of all residential construction and should be paid for by the owner, or, in the case of speculative work, by the contractor or developer.

CONCLUSIONS

Research and post-hurricane evaluations have demonstrated that most actual and consequential damages in hurricane winds are due, ultimately, to water damage where the water penetration was due to failures in roofing and the envelope. Economic analysis demonstrates that mitigation efforts in reducing these failures in the existing building stock can produce savings of from three to five times the cost of retrofitting.

It is difficult to persuade people to expend precious and scarce resources to prepare for events which may never happen. While it is true that an individual building in a particular location may never experience hurricane damage, it is also true that, in the aggregate, all buildings in Hawaii share the same risks and statistical probabilities of damage, and that, in the long-haul, another Iniki-type hurricane is a virtual certainty. Mitigation will pay off!!