by Bryan Readling,
On May 4, 2003, a severe weather system produced strong
tornadoes in eight states: Arkansas, Kansas, Mississippi,
Missouri, Nebraska, Oklahoma, South Dakota, and Tennessee.
Tornadoes in southwest Missouri were particularly severe,
causing loss of life and property destruction along an eastward
path from Pierce City to Battlefield. On May 7, my colleague
Vince Ellebracht of APAThe Engineered Wood Association and
I visited this area, took photos, made observations, and
recorded what we saw.
Fact and Fiction
Too often, conventional wisdom says that tornado winds are
simply too strong for traditional wood-frame construction. In
fact, this is far from reality.
The National Weather Service classified the tornado along its
path through Pierce City as F-3 on the Fujita scale (see table,
below). About 90% of the roughly 1,000 tornadoes reported each
year, however, are classified as F-0 through F-2. The wind
forces associated with these less powerful tornadoes are
similar to those of hurricanes at the coastline. So it's
reasonable to expect that a home built in compliance with
hurricane code requirements could withstand full exposure to
tornadoes classified up to F-2. And since tornado forces
decrease dramatically within a short distance of the funnel, a
well-built home would also be less vulnerable to winds on the
outer regions of an F-3, F-4, or F-5 tornado.
Some damage to chimneys; tree branches are broken;
pushes over shallow-rooted trees.
Peels surface off roofs; unanchored mobile homes
are overturned; attached garages may be destroyed; some
tree trunks are snapped.
Considerable damage; roofs torn off frame houses;
mobile homes destroyed; boxcars pushed over; large
trees snapped or uprooted; debris becomes
Roof and some walls are torn from structures;
non-reinforced masonry buildings are destroyed; most
trees in forest uprooted.
Well-constructed houses are destroyed; some
structures are lifted from foundations and thrown some
distance; cars are blown some distance.
Strong frame houses are lifted
from foundation; reinforced concrete structures are
damaged; automobile-sized missiles become airborne;
trees are completely debarked.
While the Fujita scale
is widely used, it has limitations: It fails to account
for variations in construction quality, is difficult to
consistently, and is not based on a systematic
correlation of damage descriptions and wind speeds. An
effort is now underway by scientists and engineers to
improve the F-scale and make it more consistent and
What we observed in southwest Missouri fit this pattern. The
severe structural damage to homes that we saw could usually be
linked to some sort of structural inadequacy, confirming our
belief that wind-resistant details are well worth the effort in
We concentrated our efforts in Battlefield, where we spent time
in a small subdivision of single-family homes ranging from 2 to
15 years old. A few were still under construction.
Most of the severely damaged homes would likely have fared much
better if the builders had paid attention to the structural
continuity of the framing connections. In general, the failures
appeared to stem from two main deficiencies: inadequate
attachment of the roof framing to the supporting walls and
inadequate lateral strength of garage walls.
Unfortunately, we saw no light-gauge metal connectors —
rafter clips — on the severely damaged homes. We found
many rafters, roof trusses, and sections of roof scattered on
the ground, and we could see that toenails were the only means
of attachment between the roof framing and the top plates.
Toenails offer little resistance to the high-wind uplift forces
from negative pressures on roof surfaces. Plus, once a breach
occurs in the building, additional uplift loads are imposed
from underneath. Toenails are not up to the task.
We saw one extreme example of the improper use of toenails at a
local fire station. Here, open-web wood trusses spanning 50
feet were attached to the top of the walls with just three to
five toenails, driven in such a way that they were only
Most of the homes in the subdivision had attached two-car
garages with no living space above. The garage doors tended to
be constructed of lightweight metal and were no match for the
wind pressures and flying debris accompanying the
Once a garage door goes, the opening allows the wind pressures
to act on interior surfaces in combination with the pressures
on the outside. This sudden combination of pressures resulted
in rapid failures of garage walls and roofs, which then also
caused damage to the walls and roof of the adjacent living
In addition to their vulnerability to breaching, garages are
generally structurally weaker than the rest of the house,
because the typically narrow return walls (parallel to and on
each side of the garage door) are often incapable of resisting
the lateral forces exerted by high winds. We saw many homes in
which the narrow return walls of the garage had failed.
Connections between the garage walls and the concrete
foundations were also weak. In several cases, we found the
bottom plates of garage walls still attached to the concrete
stem wall with the studs missing. Only the nails through the
bottom plate and into the bottom of the studs remained.
Properly lapped and fastened structural panel wall sheathing
could have been an economical way to transfer the forces around
the stud-to-plate joint and then into the foundation.
In other heavily damaged garages, the bottom plate was no
longer attached to the foundation. In these cases, anchor bolts
either split or pulled through the bottom plate. In most
instances, standard round washers had been used between the nut
and the plate instead of stronger plate washers (see
Providing strong connections
between walls and foundation is key to resisting
tornado uplift. While code minimum is a good starting
place, Option 1 improves uplift resistance by adding
more anchor bolts as well as hold-down straps. Option 2
is stronger still even without straps because of the
simple substitution of large square washers on the
anchors. Option 3 is the strongest. (Source: FEMA)
The type of wall sheathing used greatly affected how well
house walls could sustain impact from wind-borne debris. Most
of the homes in the subdivision we visited had non-structural
sheathing — such as rigid foam board or non-structural
fiberboard — and vinyl siding. The few homes we saw that
had been built with plywood or OSB structural wall sheathing
sustained less damage and far fewer breaches.
Several measures can dramatically and economically improve
building performance in tornadoes:
• Fully sheathe houses.
A house fully sheathed and properly fastened with wood
structural panels (plywood or OSB) has better racking
resistance, greater structural redundancy, and can better
resist flying debris.
• Use metal connectors
to tie roof framing to the top of walls. Uplift performance of
straps or clips is better and more reliable than toenails (see
illustration, below). A few more dollars spent on light-gauge
metal connectors could make the difference between structural
failure and good building performance.
Code minimum nailing of rafters or
trusses to top plates is inadequate to resist strong
uplift forces. Hold-down straps are critical. Option 1
shows straps between trusses and top plate and top
plate and studs. In Option 2, where the trusses align
with the studs, a single strap can create a strong
connection between truss and stud. Note that the metal
connectors shown here address uplift forces only: clips
commonly used in seismic and high-wind zones for
lateral strength are not shown. (Source: FEMA)
• Properly fasten structural
panel sheathing in accordance with local building code
and manufacturer recommendations. Close adherence to a maximum
6-inch-on-center nail spacing at supported panel edges helps
the framing behave as a unified boxlike structure. Reducing
perimeter nail spacing from 6 inches to 4 inches increases wall
racking strength by 50%; reducing the nail spacing from 6
inches to 3 inches doubles racking strength.
• Use proper sill
anchorage. Check local building codes for correct bolt
diameter and maximum bolt spacing and placement. Larger washers
between the sill plate and the nut improve performance. In
high-seismic regions, for example, 2x2x3/16-inch washers are
required. These could be adopted in tornado-prone regions as
well. FEMA also recommends greater use of anchor bolts, in
addition to properly fastened structural panel wall
sheathing.•Use APA-designed portal
when narrow return walls are desired at garages.
Such framing details improve the bracing performance of narrow
walls (see Practical
Engineering, Building Strong Garage Door Walls
Bryan Readling, P.E., is
a structural engineer with APA's Field Services Division in
Davidson, N.C., specializing in wind damage and the use of
engineered wood products and building structures to resist
hurricanes and tornadoes.See page two for photos of the tornado damage in