In New Orleans, floodwaters from Hurricane Katrina lifted homes
off their foundations. The direction the buildings moved depended
on the wind direction as well as on the ebb and flow of the
One year after Katrina, hurricane researchers, engineers, and
disaster specialists are still grappling with this storm. Most urge
that this hurricane must be understood as a flood event. While the
storm reached Category 5 wind speeds of 175 mph in the middle of
the Gulf of Mexico, the storm weakened to Category 3 by the time it
hit land. But wind speed alone does not make a hurricane, reminds
engineer Timothy Marshall, a building failure and damage consultant
with Dallas-based HAAG Engineering: "The setup was perfect to
As Marshall explains it, Katrina was an exceptionally large
tropical storm. After gathering energy as it passed over the Loop
Current — a warm, deep water flow that moves northward
through the Gulf of Mexico — satellite images of the storm
show a very large, symmetrical cloud shield (a perfect doughnut),
which is a clear sign of a powerful storm. The large diameter
displaced a wide area of water over the Mississippi Sound, the
shallow water basin that extends from Louisiana's delta across the
entire coastline of Mississippi. Had the storm blown in over deep
water, much of the wind's energy would have been absorbed by the
ocean. But over a wide shallow area, all of that energy is
concentrated near the surface. The effect, explains Marshall, can
be visualized by putting water on a dinner plate and blowing across
the surface. It's relatively easy to make the water pile up on the
far side and spill over the edge, which is exactly what happened.
The swell of water in front of Katrina piled up to almost 30 feet
on the east side of the eye wall, where the winds were strongest.
Along the entire Mississippi beachfront, most of the surge was more
than 20 feet, and none was lower than 15 feet. Even in the Florida
Panhandle, 200 miles away, water levels reached as high as 10 feet.
And these levels did not include the height of the waves that
danced above the surface of the swell.
An unwelcome visitor, Hurricane Katrina swept into Biloxi,
Miss., with a 20-foot storm surge that wiped homes clean off their
In the face of such an immense water load, ordinary homes had
little hope of surviving. Aerial photos of the region show a
quarter-mile swath of cleared land in from the shore: Nothing but
slabs wiped clean of the structures that stood above them. Most
homes on ground-level foundations disintegrated under the intense
impact of the oncoming water. Those that held their ground were
gutted. Even elevated homes were picked up off their foundations
when floor connections failed. Broken-up buildings were turned into
floating debris that smashed up other homes. When Katrina was gone,
the debris line extended a couple of miles inland. More than
400,000 homes along the tangled shores of the Gulf region had been
destroyed, and an additional 85,000 housing units suffered major
damage. This amounted to almost ten times more physical damage than
any other U.S. natural disaster. Combined with the damages wrought
by hurricanes Rita and Wilma, the record-breaking 2005 hurricane
season caused the largest U.S. migration in the past 150 years,
leaving more than one million people homeless.
FEMA Responds: Not-Your-Average
Faced with such an unprecedented reconstruction need, the
mitigation arm of the Federal Emergency Management Agency (FEMA)
has at last released its own flood of informational assistance to
aid survivors in rebuilding. If this evokes visions of a series of
shallow, flaccid government pamphlets, think again. These materials
have a reassuring depth and forcefulness to them.
FEMA 550, Recommended Residential Construction for the Gulf
Coast: Building on Strong and Safe Foundations, is an
ambitious manual aimed at providing engineers and builders with the
essential information needed to build house foundations that can
stand up to the forces of a major hurricane. It sets a new standard
for practical engineering guidelines, making it a must-have
document for any coastal building professional. In five chapters,
engineers and mitigation specialists have outlined the essential
engineering requirements for foundations inundated by high winds
and deep surge. Here's a look at what's included and what it means
for coastal builders.
While the brunt of Katrina's surge washed harmlessly beneath
this home on Dauphin Island (top), the water scoured away the sand
from the base of the piles, causing the structure to lean
dangerously. Had the piling been longer and buried deeper, as on a
nearby home (bottom), it would have remained upright.
Engineering calcs. Crunch time in
engineering a coastal foundation begins with calculating the loads
imposed by each of the identified hazards: high winds, storm surge,
wave action, flood-borne debris, and tidal scour. All of these
loads must be accounted for, but if tallied separately, they'd lead
to unnecessary overdesign. So the next step is to determine
appropriate load combinations for the building site. Load
combinations are an engineer's shorthand for computing the relevant
forces from a wide range of design loads — dead, wind, wave,
uplift, flotation, overturning, and so on. The load combinations
used in this manual are those specified by ASCE 7-02, the standard
referenced in the 2003 International Building Code (IBC).
The rationale for outlining this engineering process, explains
project manager Matthew Haupt, stems from the idea that many of the
engineers currently assisting in the massive rebuild effort have
come from noncoastal regions. They have the design expertise but
may not be familiar with the specific forces imposed by high winds
and velocity waves.
Plug-and-play designs. What makes FEMA
550 unusually rich is that it goes a step beyond summarizing how
engineers determine foundation design loads: It provides the
designs for seven pre-engineered foundations. These are presented
as foundation templates — what Haupt refers to as "model
layouts." If you stick within the design parameters —
limiting the size, weight, and roof pitch of the home to a
prescribed range — you don't need to run through the
engineering calcs. It's plug and play: All the design details are
included in a set of construction drawings in the document.
Homes whose dimensions, weights, or roof pitches fall outside the
ranges provided in FEMA 550 should have a licensed professional
engineer's consult. (Don't forget to bring a copy of FEMA 550 when
you meet with the engineer.)
FEMA 550 includes construction details for seven different
foundation types. As long as the home is built within certain
limitations for size, weight, roof pitch, and footprint complexity,
and the site conditions match the assumptions engineers have
assigned, the foundations can be built without engineering
Open vs. closed. The seven foundation
designs offered in FEMA 550 provide a range of open and closed
foundations suitable for rebuilding in different flood zones. In
general, open foundations with deeply driven piles are needed for
homes in V zones — beachfront sites that see direct action
from breaking waves. Piles present very little face to the impact
of the oncoming waves; instead, the brunt of the wave washes
through. Closed foundations, which are much less expensive and much
more familiar to Gulf Coast builders, are subject to the full force
of a breaking wave. They should be used only inland, where the
structure may see high water without wave action.
The foundation designs in FEMA 550 are based on a 14-foot-wide
(maximum) by 28-foot-deep (minimum) "module." While the dimensions
of this module are limited, there's still considerable design
flexibility. For example, if a builder sets out to build a
30-foot-deep, 42-foot-wide home, the foundation can be designed
around three 14-foot-wide by 30-foot-deep sections. A 28-foot-deep
by 50-foot-wide home can be built on four 121/2-foot-wide by
28-foot-deep foundation sections.
While FEMA's guidelines clarify the foundation design process, the
water remains murky around the issue of elevation. How high a home
should be raised is a question that elicits strong opinions on both
sides of a growing debate.
On one side of the argument are those yelling that it's insane to
rebuild too low in a region obviously prone to severe storm surge.
This party includes those calling for a limit on subsidizing
substandard housing that, in all likelihood, taxpayers will have to
pay to rebuild again when the next Katrina-sized storm roars
ashore. On the other side of the debate are those anxious to get as
many homes rebuilt as quickly and inexpensively as possible, so
people displaced from their homes can put their lives back
together. This party includes most of the displaced themselves and
all those fearful that the local residents will be forced to give
up their properties to a more gentrified populace.
Piers on discrete footings (foreground) failed by rotating and
overturning, while piers on more substantial footings — in
this case, a concrete mat — survived (background). However,
the surge near Pass Christian, Miss., reached almost 30 feet,
washing the home off of its foundation.
This debate erupted in November 2005 when FEMA issued advisory
flood maps for Mississippi, and it was reignited when the advisory
maps were released for Louisiana earlier this year. In keeping with
long-standing policies used to administer the National Flood
Insurance Program, the new maps are based on an average of recorded
flood elevations for the last 20 years. The Advisory Base Flood
Elevations (ABFEs) account for the surge levels from Katrina, but
there's an averaging down, so the elevations do not match Katrina
inundation levels. Those on one side of the debate fret that the
advisory elevations may be too low if a Katrina-sized storm blows
through again, and those on the other fret the increase will drive
the cost of construction too high. The advisory status of the
elevations is a halfway measure that puts the onus on local
jurisdictions to decide how to enforce elevation requirements.
However, once the elevation maps become final (and in all
likelihood the elevations will only increase on the official maps),
FEMA has the option of restricting municipalities from
participating in the flood insurance program if they do not enforce
codes consistent with the maps.
The seven pre-engineered foundation designs (referred to by
case letter in FEMA 550) cover a range of coastal flood conditions.
The deep open foundations are suitable for elevating homes 10 to 15
feet above ground level, while the shallower foundation types have
a practical upper limit of 8 feet above grade.
In Search of a Reasonable
Caught in the middle are the mitigation engineers who recognize
immediately the design ramifications of building too high with
generic foundation plans rather than with custom engineered
designs. FEMA 550 offers a reasonable compromise, opting for closed
foundation designs that are sufficient up to 8 feet above ground
level and open foundation designs up to 15 feet.
These upper limits represent practical limitations of the materials
and techniques: When faced with resisting A-zone flood forces,
8-foot-tall foundations are a practical upper limit for
8-inch-thick reinforced block walls. The open design using timber
piles is limited to 10 feet above ground level primarily by the
availability of longer piles. Steel-reinforced concrete columns are
limited to a height of 15 feet above ground level. Above this
elevation, the amount of reinforcing steel required in the piles
would squeeze out the amount of concrete needed. To go much higher
would require steel-pipe piling and individualized engineering.
There is some consolation for these practical limits, however:
While storm surges from Hurricane Katrina far exceeded these limits
in many areas, the added height would have mitigated most of the
disaster. FEMA's Mitigation Assessment Teams concluded that if
homes facing the brunt of Katrina's surge had been elevated to 15
feet, 80% of the damage could have been avoided.
The issue boils down to money. If you want to save the cost of
engineering and use a plug-and-play foundation design, you have to
accept the elevation limits, which come with certain risks. This
message offers a not-so-subtle hint for those who build high-end
custom homes: The safest foundations are custom-engineered
foundations that elevate homes higher than FEMA's advisory
elevations. Look not to the ABFEs but instead to the surge
inundation reports (see http://www.fema.gov/pdf/hazard/flood/recoverydata/ms_overview.pdf
), and build above those levels. Unfortunately, that's probably not
a viable option for a majority of landowners who lost their Gulf
Coast homes, and their fears of the land passing to a more
gentrified population probably have merit. ~
Clayton DeKorne is editor of Coastal
Gulf Coast Builder's Review
When compiling the foundation designs for FEMA 550, engineers
surveyed Gulf Coast builders for their impressions of the various
foundation types presented in the manual. Highlights of this
builder analysis follow.
Deep Open Foundations (Cases A, B &
Deep timber piles are considered a "new approach" for residential
construction in many Gulf Coast areas.
- Pile-driving contractors are not abundant in all areas of the
- Timber piles are not anticipated to be widely used, but the
option is available.
Shallow Open Foundations (Cases D &
- Foundation type similar to existing Gulf Coast building
- Concrete columns are recommended where masonry has historically
Enclosed Foundations (Cases E & F)
- Familiar construction technique for Gulf Coast builders.
- Common practice for elevation in A zone.
Gulf Coast contractors also evaluated the relative costs for the
various foundation types. Findings include the following:
- Costs for pile driving vary widely throughout the Gulf
- Labor prices are rising as a result of increased demand.
- Material prices are also rising with increased demand.
- As in most markets, an economy of scale from building in
quantity can help to lower construction costs.
Note: The manual includes a breakdown of costs for each design
based on May 2006 prices. These prices reportedly include taxes,
overhead, and profit, but the summaries do not break these out,
rendering them ballpark figures at best.