Hurricane Opal in Florida
A Building Performance Assessment
Federal Emergency Management Agency 
Mitigation Directorate 
FEMA 281/August 1996

2 SITE OBSERVATIONS

2.1 Typical Pre-FIRM and Post-FIRM Construction

Typical pre-FIRM structures in the study area are one-story concrete block or 
wood-frame structures built on slab-on-grade foundations, one- to three-story 
concrete block structures, and one- to three-story wood-frame structures founded 
on timber piles. Many of these structures are behind concrete sheetpile 
seawalls. Many pre-FIRM structures were substantially damaged by the surge 
accompanying the storm event and were often destroyed because of foundation 
collapse, wave attack, or both.

Typical post-FIRM structures in the study area are one-, two-, and three-story 
wood-frame structures elevated on timber or concrete pile foundations. Some of 
these are new structures that either incorporate older, pre-FIRM structures or 
were built over them. Post-FIRM elevated structures sustained some wind and 
flood damage but, overall, performed much better than pre-FIRM structures that 
were at-grade or that were elevated but not to the BFE or CCCL requirements.

2.2 Observations of Wind Damage

Wind damage observed by the BPAT was generally confined to roofing shingles and 
tiles, exterior sheathing, unsecured air conditioning compressors, power poles 
and lines, and signs. However, the wind damage observed did not constitute a 
large portion of the total damage to structures.

2.3 Observations of Flood Damage

Flood damage was observed along the Gulf of Mexico shoreline at all sites 
visited by the BPAT (see Figures 2-1 and 2-2). Structures damaged by flood 
forces generally fell into the following categories:

- pre-FIRM structures founded on slabs or shallow footings and located in mapped 
V-Zones

- post-FIRM structures in mapped A-Zones, B-Zones, C-Zones, and X-Zones founded 
on slabs or shallow footings, but exposed to high-velocity flows, high-velocity 
wave action, flood-induced erosion, floodborne debris, or burial by overwash

- post-FIRM elevated structures not properly constructed or not elevated to or 
above the elevation reached by storm surge and wave effects

- pre- and post-FIRM structures dependent, in part or completely, on failed 
seawalls or bulkheads for protection and foundation support.

[Begin figures]
Figure 2-1 is a photo showing debris washed inland as a result of surge action 
from Hurricane Opal.

Figure 2-2 is a photo showing how beach erosion caused major damage to 
structures, as well as roads and utilities.
[End figures]

2.3.1 Erosion and Scour

Where sand dunes existed before Hurricane Opal, significant loss of dune height 
and width was observed. Many dunes were breached or flattened (see Figure 2-3). 
Those that remained after the storm were scarped and weakened. Duneface retreat 
of 75 feet to 100 feet was observed in several locations. Overwash of eroded 
dune sediments was common, sometimes extending over 500 feet inland and causing 
burial of roads and at-grade construction by 1 to 4 feet of sand.

[Begin figure]
Figure 2-3 is a photo showing how beach erosion eliminated protection for 
structures. It is important that the beach and dunes be rebuilt as soon as 
possible. Arrows point to walkways used to traverse dune that has been washed 
away.
[End figure]

In some cases, an estimated 10 to 20 feet of vertical relief was lost at the 
seaward edge of high dune and bluff areas. Many structures atop high dunes or 
bluffs collapsed because of a loss of support, either from the undermining of 
slab foundations or from inadequate pile embedment (see Figure 2-4).

[Begin figure]
Figure 2-4 is a photo showing how erosion such as this took place along the gulf 
coast, causing structures to be undermined and resulting in damage.
[End figure]

Ground levels at many front-row elevated structures that survived Hurricane Opal 
were typically reduced 3 to 7 feet, or more. In addition, local scour 
depressions were observed at the bases of many piles, indicating that 6 to 12 
inches of additional soil was lost immediately adjacent to the piles. Scour 
during the storm probably rendered greater than 6 to 12 inches of soil around 
the piles unsupporting.

Large scour depressions were observed where large volumes of water flowed during 
the storm. Depressions measuring 10 to 40 feet in length and width and 2 to 4 
feet in depth were observed around some pile-supported structures and near the 
corners of some at-grade construction. Structures that seemed particularly 
vulnerable included the following:

- structures at the landward termination of roads and driveways that funneled 
floodwaters toward the structures

- structures between drainage basins or lakes and larger bodies of water

- structures near locations where floodwaters crossed or breached the barrier 
islands

2.3.2 Debris

Small debris was widespread, ranging from household items to construction 
materials. These items did not cause structural damage to buildings, 
foundations, or other building components. Evidence of much larger debris 
shifted by floodwaters was also observed, including pier piles and braces, 
concrete slabs, dumpsters, automobiles, boats, and collapsed houses (see Figure 
2-5). Many of these objects washed into buildings, and some caused structural 
damage.

[Begin figure]
Figure 2-5 is a photo showing how debris, consisting of broken concrete and wood 
framing systems, was generated by surge action
[End figure]

2.3.3 Slab Foundations

Many slab failures were noted in all types of structures (see Figure 2-6). The 
major reason for these failures was the loss of support coupled with a lack of 
reinforcing in the slabs. Welded reinforcing wire fabric was observed in many 
slabs but did not prevent failure of the slabs once they were undermined.

[Begin figure]
Figure 2-6 is a photo showing how undermining of concrete decks and floor slabs 
caused the failure of many unreinforced concrete structures
[End figure]

2.3.4 Pile and Pier Foundations

Three to seven feet of vertical erosion at the seaward row of piles was common 
(see Figure 2-7). This erosion, coupled with insufficient penetration of the 
piles on many structures, led to structural damage to or collapse of primarily 
pre-FIRM structures. Undersized piles (6-inch diameter timber in some instances) 
were not sufficient to resist storm forces; they generally failed and resulted 
in structural damage or collapse. Piers constructed of concrete blocks on 
shallow footings frequently collapsed as a result of erosion. Well-designed and 
well-constructed pile and pier foundations withstood the forces exerted by the 
storm. Use of splicing techniques was also observed on some eroded piles (see 
Figure 2-8). Although the splicing of piles placed these structures at increased 
risk of failure, no failures related to spliced piles were observed.

[Begin figures]
Figure 2-7 is a photo showing how these piles were not embedded deep enough to 
survive the erosion of the sand. As a result, there is now a large gap between 
the bottoms of the pilings and the ground surface.

Figure 2-8 is a photo showing how these piles were not long enough and were 
spliced to add depth. The splicing was exposed by storm-induced erosion.
[End figures]

2.3.5 Framing Systems

The BPAT found many examples of poor framing of timber floor beams and joists in 
platform-type construction. In particular, poorly fashioned beam-to-beam and 
joist-to-beam connections were common. Typical problems included the following:

- pile notching greater than 50 percent of pile cross-section

- poor alignment of piles, which resulted in unsupported beams at piles

- use of wooden shims to support beams (i.e., to compensate for notches cut too 
low)

- over reliance on nails and thin metal straps/hangers

Glue-laminated beams and joists were observed in exterior applications in some 
recent post-FIRM residential construction. The use of laminated structural 
members in exterior applications is of interest because this practice has not 
been widely observed by previous BPATs. Although laminated structural members 
rated for exterior use are available, the Hurricane Opal BPAT could not 
determine the rating of those it saw. No failures of these beams and joists were 
observed however.

2.3.6 Connections

Many of the connections observed were deficient. The BPAT observed widespread 
corrosion of galvanized straps, hangers, and joist-to-beam ties beneath elevated 
buildings. Some of the corroded connectors had failed either before or during 
the storm.  

The BPAT observed some galvanized strap connectors between piles, beams and 
joists (in otherwise good condition) that failed as a result of insufficient 
nailing or because storm forces exceeded the design forces (see Figure 2-9). 
This was not a common mode of failure, however. The BPAT also found evidence 
that structural components had pulled away from one another when acted on 
simultaneously by flood and wind forces, despite the presence of the galvanized 
connectors. In some instances, foundation piles and beams were well-connected 
and withstood storm forces, while walls or upper structure components were 
poorly connected and were damaged or destroyed by wind forces, flood forces, or 
both.

[Begin figure]
Figure 2-9 is a photo showing how the floor beam connection and beam support 
illustrates the lack of design and proper construction observed on may damaged 
structures. Note the bolt pullouts. Also, the beam support is attached to the 
pile with lag bolts rather than bolts that extend through the pile
[End figure]

2.3.7 Bracing

The use of 2 x 8 or similar timber cross-bracing between timber piles was common 
beneath elevated wood-frame structures. Some bracing failures were observed that 
were apparently due to horizontal loading from water, debris, or both. The use 
of threaded galvanized rods and turnbuckles as cross-bracing was less common 
(see Figure 2-10). No failures of this type of bracing were noted, but debris 
was trapped by the bracing, and in some instances, the rods were bent by lateral 
loads imposed by the force of flood waters acting on the trapped debris.  

[Begin figure]
Figure 2-10 is a photo showing how this undamaged threaded rod and turnbuckle 
cross-bracing continues to provide additional support as designed.
[End figure]

Use of knee bracing was also prevalent in elevated wood-frame structures. A 
common problem observed with knee bracing was that timber piles had been 
notched, some deeply, to accommodate bearing seats for tie-in purposes. Although 
the BPAT did not observe any structural failures that could be definitively 
linked to this problem, notching of piles can undermine their structural 
integrity and should therefore be avoided.

2.3.8 Breakaway Construction and Enclosures

There were a number of damaged or destroyed enclosures below elevated structures 
(see Figure 2-11), many with electrical wiring attached to breakaway walls. The 
presence of breakaway walls indicates the designer was aware of potential flood 
impacts. However, the placement or attachment of utilities to breakaway walls 
below the elevated portions of the structures demonstrates, at a minimum, lack 
of awareness of local, CCCL, and NFIP regulations by owners or contractors, or 
possibly disregard of those regulations. 

In some post-FIRM structures with breakaway walls below the lowest habitable 
floor, the walls broke away as intended but in doing so, damaged exterior 
sheathing and wall finishes above the lowest floor. The damage above the 
breakaway wall was usually minor but could have been prevented by better design 
and construction of this detail (as shown in Figure 2-11). Rolldown garage doors 
were damaged or destroyed in pre-FIRM and post-FIRM construction alike (see 
Figure 2-11).

[Begin figure]
Figure 2-11 is a photo showing how these breakaway walls functioned as designed, 
lessening the pressures of water, sand, and debris on the structure.
[End figure]

2.3.9 Stairs, Decks, and Porches

Timber stairs and decks were frequent casualties of the storm. Many were 
supported by short, small-diameter shallow posts or piles. Some decks were 
supported by knee braces attached to structural piles supporting the main 
structure. Decks of this design seemed to better resist Opals forces. Loss of 
decks sometimes led to roof damage where roof overhangs were supported by posts 
attached to the decks. In one instance, a deck located seaward of the States 
CCCL and permitted by the State survived the storm, while the landward habitable 
structure (behind the CCCL and not within the States jurisdiction) was 
destroyed.

2.3.10 Utilities

The BPAT noted several problems associated with utilities and utility 
connections at habitable structures:

- Placement of electric meters, panels, boxes, and wiring below a buildings 
lowest habitable floor, rendering that equipment vulnerable to storm surge, 
wave, debris and overwash damage (see Figure 2-12).

- Attachment of wiring, conduit, and electrical panels to breakaway walls (see 
Figure 2-13).

- Failure to adequately support and fasten air conditioning compressor units 
(see Figure 2-14). Many support platforms were destroyed, leading to compressor 
damage.  Some platforms survived, while unfastened compressor units were blown 
or washed away. Units not properly supported and attached were also observed to 
have caused damage to exterior walls of some structures.

- Placement of utility lines, septic systems, and mechanical connections and 
equipment, including air conditioning units, on the sides or seaward of 
buildings, rather than landward of the building. Loss of air conditioning units 
and/or utility/mechanical components sometimes led to damage of the main 
structure.

[Begin figures]
Figure 2-12 is a photo showing utility lines and boxes dislocated by hurricane 
forces. Not that the cross-bracing and pile support system remain in good 
condition after the storm.

Figure 2-13 is a photo showing the interior view of breakaway wall blocked by 
air conditioning unit and support platform. Electric cooling lines were extended 
through the breakaway panel.

Figure 2-14 is a photo showing how breakaway panels functioned as designed. Note 
the loss of the air conditioning unit platform.
[End figures]

2.3.11 Seawalls

The BPAT observed widespread failure of seawalls and bulkheads along the Gulf of 
Mexico shoreline (see Figure 2-15). Damage figures from the State of Florida 
revealed that over 3 miles of seawalls and bulkheads were destroyed by Hurricane 
Opal, including 1.3 miles of concrete walls, 1.0 mile of concrete block walls, 
and 0.8 mile of timber walls (FDEP 1995). Failed walls contributed to damage of 
buildings, pools, and other structures, due to loss of backfill and generation 
of debris.

[Begin figure]
Figure 2-15 is a photo showing a fractured seawall, damaged by storm forces, 
Note the erosion of the bank behind the wall.
[End figure]

Many walls appeared to have failed because wing walls or return walls were 
flanked by erosion and scour. Many seawall returns flanked by erosion and scour 
were no more than 20 to 30 feet long, although some longer returns (50 feet to 
75 feet) were also flanked.

Seawalls were usually destroyed when backfill was washed from behind the walls 
because of overtopping, insufficient wall embedment, or return wall flanking.  
Habitable structures founded on slabs or shallow foundations, swimming pools, 
and other structures that relied on seawalls to retain supporting soil, were 
frequently undermined and destroyed when seawalls failed.
The BPAT noted that retaining walls constructed of concrete blocks were 
particularly vulnerable to damage by Hurricane Opal. Walls most likely to have 
survived were observed to have:

- reinforced concrete slab or sheetpile construction

- sufficient wall height or backfill protection to prevent significant 
overtopping and loss of backfill

- sufficient anchorage and embedment to prevent collapse from seaward rotation 
of the cap or toe

- return walls extending landward of the seaward face of the building or 
structure being protected and landward of the effects of erosion and scour

2.3.12 Drainage and Drainage Structures

The BPAT observed the remains of several new stormwater discharge structures 
adjacent to or between multifamily buildings. These structures consisted of 
large-diameter corrugated plastic pipes, probably intended to carry stormwater 
runoff from parking areas and other impervious areas to the beach.  
Unfortunately, the seaward portions of these pipes were destroyed during the 
storm and their pre-storm configurations are not known with certainty.

It did appear, however, that erosion beneath habitable structures near these 
damaged discharge pipes was more severe than at areas away from the pipes, 
possibly a result of direct discharge of upland stormwater runoff adjacent to or 
beneath the habitable structures. It is likely that the pipes failed because of 
erosion and scour caused by the storm or because of the loss of protective 
seawalls and bulkheads. It is possible, but not known for certain, that the pipe 
failures and discharge adjacent to the multifamily buildings contributed to 
foundation damage at those buildings.

2.4 Incorporation of Pre-FIRM Construction into New Construction

Many single-family structures appeared to have been constructed above or 
adjacent to portions of older pre-FIRM structures and probably resulted from 
efforts to expand and/or reconstruct older, smaller structures. This type of 
construction is vulnerable to storm damage because the foundations of the pre-
FIRM and post-FIRM sections can respond differently to storm forces and erosion. 
For example, the BPAT found a damaged house in Mexico Beach that was supported 
by two types of foundations. One part of the house was supported on concrete 
block piers placed on the old pre-FIRM slab-on-grade. The remainder of the 
house, which extended beyond the original pre-FIRM footprint, was supported on 
timber piles set in concrete encasements. Although the piles and slab survived 
the storm, the concrete block piers did not. With the loss of the piers, the 
house listed to the unsupported side and the floor beams separated from the 
newer, pile foundation. Had the entire house been supported on timber piles, it 
may have survived with little or no damage.

2.5 Design, Construction, and Workmanship

After observing hundreds of damaged or destroyed structures, the BPAT has 
concluded that many structures seem either to have been built without the aid of 
detailed design plans (prepared by a design professional) or not to have been 
built in accordance with plans that were available. Failure of non-engineered or 
poorly designed foundations, structural systems, and critical connections often 
led to major damage or complete loss of structures. Such losses are preventable.

Numerous instances of poor workmanship were also noted by the BPAT during its 
inspections. In particular, the BPAT found several examples of misalignment of 
timber foundation piles and poor framing practices in platform-type 
construction. The BPAT also noted recurring problems with concrete construction. 
For example, reinforcing steel was missing from or misplaced in slabs, footers, 
and wall grade beams, and welded wire fabric reinforcement was frequently at the 
bottom of, not centered in, the slabs. Although no damage was observed that 
could be definitively linked to these examples of poor workmanship, such 
practices should be avoided in any construction, especially in areas subject to 
coastal storm forces.