PERFORMANCE OF STRUCTURES DURING HURRICANE ALICIA AND THE ALTUS TORNADO by Timothy P. Marshall

(Presented at the Department of Energy Natural Phenomena Hazards Mitigation Conference, Las Vegas, NV, October, 1985.)

Abstract

Investigation of building damage from Hurricane Alicia and the Altus, Oklahoma Tornado revealed similarities in structural performance. Variability in construction practices and inherent uncertainties in structural materials played an important role in determining why one building survived unscathed while an adjacent building was heavily damaged. Attention to detail like the installation of anchor bolts or tie down straps helped explain the large variation in damage.

Observed modes of building failure in the two different windstorms are presented. Distinction between wind and wave damage in Hurricane Alicia is discussed. Suggestions are presented to help minimize structural damage due to wind and wave forces in the future.

The second portion of this paper presents a methodology for computing failure wind speeds using a probabilistic approach which incorporates load and resistance statistics. Upper and lower bound wind speed estimates are obtained with a desired degree of confidence. Calculated wind speeds have been consistent with the type of structural damages observed.

INTRODUCTION

Over the past several decades, there have been numerous advancements in wind engineering research. Before 1950, few studies appeared in the literature deriving wind speed estimates from analysis of structural damage. Windstorms were perceived by many as a hazard for which buildings could not be economically designed. Some people believe that a direct hit from a large tornado or hurricane would level even the sturdiest structure.

The Dallas, Texas tornado, in 1957, gave engineers an opportunity to study windstorm damage to a variety of structures. Segner [1] derived credible wind speed estimates from studying the structural damage in the wake of that tornado.

Since then, there have been a number of studies estimating failure wind speeds on buildings struck by tornadoes and hurricanes. Engineering assessments of tornado damage to buildings have been accomplished by Minor [2] [3], Mehta [4] [5], and McDonald [6] to name just a few. Likewise, hurricane wind speed-damage correlations have been completed by Mehta [7], and more recently by Kareem [8] and Cox [9].

However, several questions arise in determining the wind speed at which a building fails. What effect does the variability in wind load and structural materials have in building response? How credible are the calculated failure wind speeds? What degree of confidence is there in the failure wind speed estimates? These questions and others will be addressed in latter portions of this text.

ADVANCEMENTS IN DAMAGE ANALYSIS

The results of engineering analyses of hurricane and tornado damage have shown that buildings fail similarly. Increased airflow around building corners creates additional outward pressures. As a result, structural failure typically occurs at roof eaves, corners, and ridgelines.

Also, study of tornado damage has revealed that buildings do not explode because of low air pressure or vacuum. Though a small barometric pressure drop does occur inside a tornado, most buildings have adequate ventilation to offset this pressure differential rapidly. Furthermore, flying debris will most likely open buildings to the wind well in advance of stronger tornadic winds. Thus, opening windows in advance of a tornado is not worthwhile.

Maximum wind speeds in hurricanes and tornadoes can approach 225 mph. However, most storms have wind velocities less than 150 mph. Thus, buildings CAN be designed to survive windstorms. Engineered structures which have sustained a direct hit from tornadoes include the Great Plains Life Building and Pioneer Gas Building in Lubbock, Texas. These structures were repaired and are in use today. In general, non-engineered buildings, like residences, usually have less resistance to the wind because minimal attention is paid to connection details. Thus, severe structural damage can occur to these types of buildings in windstorms.

HURRICANE ALICIA

Hurricane Alicia struck the Texas coast near West Galveston Island just after midnight on August 18, 1983. Alicia was a small hurricane by meteorological standards, with maximum sustained wind speeds barely attaining hurricane force (75 mph). Refer to Figure I showing the path of the hurricane eye. As typical with hurricanes in the Northern Hemisphere, Alicias' maximum winds and storm surge occurred on the right side of the hurricane track. Peak wind speeds in Alicia approached 110 mph in a narrow band along West Galveston Island and near Baytown. Likewise, a maximum storm surge of twelve feet (above mean sea level) occurred at both locations.

The storm surge elevation gradually increased as the hurricane approached land. The sea submerged most low-lying areas on barrier islands before the strongest winds arrived. Highest storm surge elevations occurred when the eye of the hurricane made landfall. After landfall, the eye of the storm progressed in a northwest direction just passing west of downtown Houston.

Several small residential communities on West Galveston Island sustained heavy storm damage. However, minimum design wind speeds as specified in ANSI [10] were not exceeded. Inspection of building damage by the author revealed minimal attention had been given to anchorage, bracing, and connections when homes were constructed. This explained why so much damage resulted when wind speeds had been at or below building code requirements.

Structural failure typically originated at the weakest connection and progressed rapidly causing extensive damage. Vast differences were observed in the type and amount of anchoring used in coastal construction. Thus, it was not surprising to see sharp variations in structural damage from one building to the next (Figure 2). It was interesting to note that many people attributed the contrast in damage due to "skipping" tornadoes.

Building failures were observed in coastal structures resulting from both wind and wave action. These failures primarily originated at the ground/pier, wall/floor, or roof/wall connections. Ground/pier movement was observed due to wave action. The other two types of failures were associated with wind effects. In each case, structural breakdown could be traced to an inadequate connection. It was interesting to find that few pier/floor connections failed.

Coastal areas lost up to three feet of sand in vertical elevation. Piers were typically 8 to 15 feet below grade. Thus, soil scouring by wave action caused a substantial reduction in pier stability. Battering by waves caused some piers to shift. A few structures with piers installed less than five feet below grade had collapsed (Figure 3).

Most coastal residences were elevated on piers since shore front property was generally less than 8 feet above sea level. However, some structures were not elevated on piers. Such, was the case in the Brownwood community near Baytown located along north Galveston Bay. Waves superimposed on a twelve-foot storm surge washed away several homes (Figure 4). The high storm surge occurred since the bay was predominantly under southeast winds throughout the hurricane. Ocean water was literally dammed up at the north end of the bay.

Wave forces are greatest at the base of a building. Repeated wave wash undermines structures from the ground upward. In contrast, wind forces are greatest at roof levels since wind velocity increases with height. Lack of roof damage, such as removal of decking and shingles in Figure 5, indicates the primary damage mechanism was from wave action.

A second type of structural failure observed originated at the wall/floor connection. Typically, straight nails connected the bottom plate of the wall to the floor. Lateral forces due to wind effects pivoted walls about the base until collapse occurred.

Coastal residences tended to have large rooms on the ocean side with few shear walls. As a result, failure of the windward wall resulted in a rapid progression of structural damage.

Roofs were uplifted and exterior walls fell outward. The lack of adequate wall/floor anchorage usually meant loss of the entire structure above floor level (Figure 6). This type of failure resulted with wind velocities well under lO0 mph.

The third type of observed failure was between roof/wall connections. Roof systems are typically designed to remain on a structure by gravity force. Thus, minimal attention had been given to roof/wall connections. Light framed roofs were most susceptible to damage by wind effects (Figure 7).

Roof geometry also played an important role in determining the degree of wind resistance. Gable and flat roofs suffered more wind damage than hip or mansard roofs, the latter being more streamlined. Eaves and porch overhangs were particularly vulnerable to being uplifted (Figure 8). Airflow stagnated underneath beneath overhangs generating additional upward pressures.

Steps To Minimize Hurricane Damage

Investigation of structural damage in Hurricane Alicia revealed that gradations in damage were primarily attributable to variations in construction rather than a sudden changes in wind velocity. Structural failure evolved from the weakest connection. Minimizing potential damage from wind and wave action requires strengthening of existing connections. Figure 9 illustrates important connection locations and braces which could be installed to provide a more wind resistant structure. Diagonal braces added to piers, walls, and roof structure will help distribute wind loads to other portions of the structure.

Curtailing ground/pier failure entails the placement of deep piers to resist lateral forces due to wave action. Deeper piers resist horizontal movement. Structures with concrete slabs placed on grade around piers apparently help stiffen the pier structure.

Generally, the force of a wave is tremendous in comparison to the forces of the wind. A one-foot wave traveling at 10 mph has as much hydrostatic force as a wind velocity of 280 mph.

Wall/floor failures can be reduced by the placement of metal bolts in sill plates fastened to the floor. Shear walls should be constructed to react to wind loads.

Avoidance of roof/wall failures involves placement of metal straps between the rafters, roof joists, and the top plate of the wall. These straps are readily available in coastal areas and are sometimes referred to as "hurricane clips".

Some coastal buildings had metal anchors at critical connections. However, Steel Straps, bolts, and nails had rusted from exposure to sea spray. Effectiveness of anchors was clearly reduced. It is important that exposed metal connections be painted and maintained to avoid corrosion.

One residence on West Galveston Island remained undamaged after the onslaught of wind and wave forces from the hurricane. The homeowner was a carpenter who had constructed his home modifying the structure to resist the wind (Figure 10).

This home was several hundred feet inland and away from the wave action zone. Drilled piers extended into the soil about 14 feet below grade. A concrete slab had been poured around the pier structure. Both actions enhanced pier structure stability. Piers were braced diagonally to the floor structure, thus securing the floor. Connections were bolted.

The shortest dimension of the structure faced the ocean, minimizing the amount of wall area exposed to strongest winds. The exterior wall on the ocean side had small-sized windows. Prior to the storm, the owner boarded up the windows with plywood. This prevented wind blown debris from breaching the structure. Walls were fastened to the floor with 16d nails instead of smaller standard 8d or l0d nails. Likewise, roof-to-wall connections were with 16d nails.

There was only an eighteen-inch eave around the roof perimeter, minimizing wind uplift effects. A small porch, with a four-foot roof overhang, was located on the side of the structure facing away from the ocean. A hip type roof provided better streamlining of the wind. Roof decking and shingles were adequately nailed to the roof structure.

These modifications to the structure increased resistance to lateral loads caused by wind and wave action. This house survived the storm amidst homes which were heavily damaged.

TIIE ALTUS, OKLAHOMA TORNADO

A tornado struck Altus Air Force Base in Oklahoma on May 11, 1982. The tornado traveled in a northeastward direction across the base, causing substantial damage to many buildings. Refer to Figure 11, showing the tornado damage path. One structure, heavily damaged by the storm, was the communications facility. Although this building was engineered and hardened for blast, the roof failed with wind velocities below 150 mph. Building contents were damaged by rainwater.

A damage survey by the author initially categorized damage intensity using the F-scale developed by Fujita [11]. On a scale from zero to five, five being the most severe damage, six buildings suffered heavy damage (F3), eleven, moderate damage (F2), and seven, light damage (Fl).

Construction plans were obtained for several heavily damaged buildings. Information from these plans was used to calculate the wind velocity needed to cause structural failure. A method was devised by this author to incorporate load and resistance statistics in wind speed estimates in order that a degree of confidence could be obtained.

Parachute Drying Tower

A parachute-drying tower was located on the right side of the tornado path approximately 650 feet from the tornado center. As the tornado passed, the tower overturned falling toward the northeast, pivoting about a line through the two leeward supporting columns. Anchor bolts, securing the tower to the foundation, failed in tension. Failure wind speed was calculated based on the tensile strength of the anchor bolts.

Dining Hall and Recreation Buildings

Both buildings were single-story masonry structures with flat timber roof structure. The center of the tornado passed about 200 feet to the south of the buildings. The strong winds lifted eaves along windward sides of the building. The dining hall roof structure consisted of 2" x 10" wooden joists spaced 12 inches on center. Roof framing on the recreation building consisted of wooden joists spaced 16 inches on center. Roof joists were toenailed with l0d nails to the top plate of the walls. Failure of the roof systems occurred when nails were extruded and the dead weight of the roofs was overcome by aerodynamic uplift. Failure wind speeds were calculated using the pullout strengths of the toenailed connections and weight of the roof structures.

Communication Building

The communication building was a one-story, steel reinforced, block masonry building with steel roof joists. Roof joists were spaced 30 inches on center, and were bolted to a bond beam with two half-inch diameter bolts. The roof deck had a two-inch thick lightweight concrete deck poured over a fabric-backed wire mesh. The mesh was anchored to the steel joists by twisted galvanized wire. The tornado passed directly over the building. Applied wind loads caused failure of the twisted galvanized wire and the entire roof literally "rolled" off. Failure wind speed was determined using tensile strength of the wire.

LOAD AND RESISTANCE METHODOLOGY

Within the past several years, a concept has emerged in the design of engineered structures which enables engineers to account for the variability in material strength and types of applied loadings. The concept is termed Load and Resistance Factor Design (LRFD). Further explanation of the LRFD method can be found in Ellingwood [12].

LRFD essentially treats the load and resistance properties of structures in terms of continuous probability distributions (Figure 12). In contrast, nominal strengths (or loads), as specified in building codes, are discrete values. Codes include safety factors which need to be excluded when determining failure wind speeds. If safety factors and uncertainties in load-resistance behavior of the structure are NOT considered, failure wind speeds can be overestimated or underestimated.

_ _

In reference to Figure 12, L and R describe the central tendency of randomly applied loads and material resistance, respectively. When L equals R, failure results. A brief summary of the procedure used to calculate failure wind loads employing the load-resistance concept is presented. Further explanation can be found in Marshall [13].

Uncertainties in structural resistance include variations in material strengths, fabrication, and underlying design assumptions. Ellingwood [12] recognized the uncertainties in structural resistance as a function of:

R = Rn M F P (1)

where Rn is the nominal code-specified resistance, and the terms M, F and P represent ratios in the uncertainties of material strength, fabrication and professional design assumptions, respectively.

The simplified expression for wind pressure can be written as:

2

q = c GCp V (2)

where q is the wind pressure in pounds per square foot (psf), c is the air density term, G is the gust response factor, Cp is the pressure coefficient and V is the wind velocity in miles per hour (mph).

The total wind load, L, is represented by the product q A, where A is the area over which the wind pressure q is acting. Then, the failure wind speed can be calculated by setting the structural resisting moment equal to the wind induced moment. At failure:

/ Rn M F P d

V = √ c A GCp e (3)

where d and e are moment arms. In order to establish confidence limits on the failure wind speed, the coefficient of variation (c.o.v.) in each term must be known. Quantifying these uncertainties is difficult, especially for M, F, P and GC terms. Although Ellingwood [12] has presented some data estimating the variability in these parameters, they are just estimates and more research is needed to quantify these uncertainties.

When combining two or more probability distributions, the resultant c.o.v, is determined using the equations of binary operations as shown by Haugen [14]. Using this approach, the c.o.v, in resistance terms can be expressed as:

2 2 2

Vr = √ Vm + Vf + Vp (5)

Similarly, the c.o.v, in the wind speed terms can be expressed as:

2 2 2

Vw = 1/2 √ Vc + Vcp + Vr (5)

A degree of confidence can be selected for the calculated wind speed if the c.o.v, is known. Ghiocel [15] has shown that, for a normal distribution, the upper and lower bounds of the calculated wind speed are expressed as:

ws = V (1 + K Vw) (6)

where K represents the number of standard deviations from the mean which are selected.

Load and Resistance Statistics

Statistical data on load and resistance variables were assembled from numerous sources. These data were categorized and arranged into Table 1. Categories entitled good, acceptable, and questionable were designed to help rank the variability of load and resistance data. These categories were initially developed by Mehta [5] as an attempt to establish some degree of credence in a failure wind speed estimate.

Construction materials in the good category are most reliable for wind speed calculations. Under wind loading, these materials will yield failure wind speed estimates with narrow confidence bands. Widest confidence bands will result with questionable structural materials.

Wind Calculation Procedure

A summary of the above methodology used to calculate failure wind loads with a selected confidence level is presented below:

1) Obtain desired sets of construction plans.

2) Determine the average resistance of connections and structure dead load (weight).

3) Calculate the mean resisting moment.

4) Calculate the overturning moment produced by the wind.

5) Equate steps 3 and 4 and solve for the mean wind pressure or velocity.

6) Determine the weighted coefficient of variation of the resisting moment using Equation 6.

7) Determine the coefficient of variation of the wind speed using Equation 5.

8) Establish the desired confidence levels using equation 6.

Wind Load Examples

Using the methodology described above, failure wind speeds were calculated for four buildings which were heavily damaged at the Altus Air Force Base. The structures were a parachute drying tower, dining hall, recreation center, and communications facility. Results are presented in Table 2.

In each case, structural failure initiated at a connection. The parachute drying tower overturned when anchor bolts failed in tension. The remaining buildings had roof failure which initiated from uplift of nailed or tied connections at the roof/wall intersection. Calculations for structural portions which did not fail were completed to provide an upper bound estimate of the failure wind speed. Although each building was considerably different in construction detail, the calculated wind speeds proved consistent and reasonable for the damage observed.

Roof Eaves and Connections

The effect of roof geometry, weight, eave length, and extent of nailed connections will influence the failure wind speed. In order to determine the relative importance of these parameters, failure wind speeds were calculated for a given roof structure measuring 40 ft long by 14 ft wide. Parameters were varied according to the following:

1) Flat roof

2) Gable roof, 10-degree slope.

3) Roof dead load at 10 psf.

4) Roof dead load at 20 psf.

5) No roof connections.

6) Toe-nailed connections.

7) 6 ft eave on windward side.

8) No roof eave on windward side.

Even numbered parameters represent the more wind resistant cases and odd number parameters represent the less wind resistant cases. Assuming the even numbered criteria, failure wind speed for the given structure was about 150 mph. Using the odd number criteria, failure wind speed was only 55 mph. In this case, eave length proved most critical and roof slope least critical in determining structural susceptibility to wind effects. This example illustrates that even minor building modifications can increase wind resistance dramatically.

USE OF THE F-SCALE

The F-scale is used frequently to rate the intensity of damage to structures. The scale is easy to use and is readily accepted by the National Weather Service and others in the scientific community. One of the shortcomings of the F-scale is that it assumes all structures are homogeneously constructed. Variations in structural strength simply aren't considered. In essence, a leveled steel-reinforced concrete building would be assigned the same F-scale rating as a demolished outbuilding.

Incorporation of load-resistance principles in the F-scale rating would result in the modifications listed below:

1) As the mean wind speed increases, confidence bands widen. As a result, wind speed ranges will overlap between F-scale categories. Therefore, damage intensity of a structure may actually lie in more than one F-scale category.

2) Confidence bands widen with increasing variability in the resistance of a structure. Since this is related to the degree of engineering attention in design of a building, a better degree of confidence in estimating the failure wind speed can be obtained.

These two points are illustrated in Table 3.

SUMMARY

The first portion of this paper explored variations in building performance during Hurricane Alicia. Structural failures could be traced back to inadequate connections. Importance of connection details was stressed in hope that in the future, the vulnerability of structures to wind and waves is minimized.

A methodology to calculate failure wind speeds on buildings was presented. The method incorporates load and resistance statistics to obtain a certain degree of confidence in wind speed assessment. Information can be used from construction plans to calculate the possible range of failure wind speeds.