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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.