Practical Engineering: Resisting Tornado Damage,
continued
Storm Report
The masonry walls of this fire station were inadequately
reinforced to withstand the tornado forces. A lumber nailer
bolted to the top of the walls supported open-web wood trusses
that spanned 50 feet. Despite the enormous loads such large
trusses could be subjected to in high winds, they were attached
to the lumber plate with only toenails.
As a unit, the roof lifted off the building and shifted
laterally. Many of the walls collapsed, covering emergency
equipment and personnel. A worker reported that the building
was 10 to 15 years old.
The severely damaged homes in the Missouri subdivision
visited by the author commonly lost either all or a large
portion of their roof framing. Most of the roofs were
stick-built, with no light-gauge metal connectors tying the
roof framing to the top of the walls. The home shown below was
under construction but completely dried-in at the time of the
tornado. Throughout the subdivision, sections of roof lay
scattered on the ground; most of the rafters had only a few
toenails driven into the supporting wall top plates.
The homes in this subdivision were generally between 2 and 10
years old.
Nearly every home in the subdivision had a two-car garage.
On about half the homes with major damage, the structural
failure started with the garage. In some cases, weak garage
doors were destroyed by wind and flying debris, exposing the
interior of the homes to wind pressure. Inadequate wall bracing
on either side of the garage opening and poor attachment
between roofs, walls, and foundations also made garages
vulnerable to internal pressurization.
The garage walls and the entire roof are completely missing
from this severely damaged home. Note the exposed garage slab
and the drywall on the garage's rear wall, where a storage
shelf still stands. The garage walls were poorly braced and
inadequately tied to the foundation. The bottom plate of the
garage sidewall is missing, leaving anchor bolts jutting up on
8-foot centers. The bolts had been installed with standard
round washers, which have proved inadequate to resist strong
uplift forces.
The bottom plate of this garage was still in place, with the
end nails into the studs sticking up every 16 inches. The brick
veneer was peeled away.
The return wall of this garage was attached with only one
anchor bolt. Though flat on the ground, the sheathed return
wall was relatively intact, with the OSB sheathing still
attached to the corner studs. Note the metal garage door track
on the wall's inside face.