Increases in population, urbanization, and changes in climate have placed increasing numbers of the US population into areas that are subjected to higher and more damaging winds. This increase in risk has prompted a response by the building codes. In fact, the 2015 edition of the International Building Code (IBC) [1]now requires that most schools and emergency facilities located in a significant portion of the Central US contain tornado shelters. Figure 1 shows the areas where these tornado shelters are required. For areas that use the 2015 IBC, this new requirement will impact the majority of new school and emergency facility construction spanning as far north as central Minnesota, as far south as southern Mississippi, and stretching to western Pennsylvania in the east and western Texas to the west. The IBC also requires that these tornado shelters be designed to meet the requirements described in the ICC 500, Standard for the Design and Construction of Storm Shelters[2].
Where sheltering is mandated, the ICC 500requires that a series of design criteria must be met. Tornado shelters have structural, civil, and architectural requirements, along with increased documentation and inspection. As an example, tornado shelters must provide a minimum 5 ft2usable floor area per occupant, minimum ventilation, sanitary facilities, fenestration impact resistance, handicap access, and adequate egress. Structurally, the exterior walls of the shelter and the roof must pass debris impact tests designed to preclude interior surface penetration of wind borne debris. These systems must also be designed to resist wind loads from 250 MPH winds, and the roofs and walls must be designed to resist a 100 psf minimum roof live loads.
In recognition of the fact that many of the schools and emergency facilities have, and continue to be constructed using masonry, a study of how masonry walls can be used to provide safe, practical and cost effective solutions for sheltering from tornados and high wind events was conducted [3].
This study showed that tornado shelters can be provided for schools using a class room wing shelter as shown in Figure 2. This solution has the advantage of providing for student comfort with a familiar classroom appearance. It also provides sheltering in place for many, is cost effective, and uses the space for both classroom space and sheltering use. This space also provides restrooms as part of the typical classroom wing design.
Figure 3 shows a typical design of an exterior load bearing masonry wall for a class room wing. This design was assumed to be in Ohio and to use a 8 inch CMU wall. The walls were assumed to span 12 ft. and have large distributed windows, as is typical in class room configurations. There are two types of masonry wall systems that can be used to provide the debris resistance based on tests [3] [4] [5] [6]. A fully grouted masonry wall will provide more than sufficient debris impact resistance for use in a tornado shelter, with or without a masonry veneer. Figure 4 shows a fully grouted masonry wall after three simulated debris impacts (2 x 4) at the required 100 MPH speed. As can be seen in the photo, there was no damage to the masonry walls.
Figure 5 shows the results of debris impact testing on a partially grouted CMU backed cavity wall system. These tests showed that these wall systems also provide debris impact resistance (no interior penetration, albeit with veneer damage). However, if partially grouted cavity walls are used in this application, the veneer anchor systems must be engineered for 250 MPH wind loads. Analysis of the typical CMU backed veneer and anchors suggest that a heavy duty version of typical anchor systems would be adequate for this application.
A comparison of the wall designs for the classroom wing acting as a shelter versus not acting as a shelter shows an increase in bar size from a #5 to a # 6 bar with a decrease in spacing from 64 inches to 24 inches for typical design conditions in Ohio. These small changes will thus have minimum impact on the cost of the wall systems. Furthermore, if the roof, or the floor of a second story can be readily strengthened to resist the relatively high live load for the shelter roofing.
Another tornado shelter design option in schools is to provide the shelter in the gymnasium area. Gymnasiums tend to have large open floor areas, locker rooms with sanitary facilities, and limited windows. However, if these areas are used, door locations must be carefully considered as double doors or corner door locations can be challenging structurally in high wall systems. In addition, if exterior masonry walls are used, they must either be solidly grouted or have brick veneer cavity walls that have been debris impact tested. The high out-of-plane wind loads and roof uplift forces will require larger wall thicknesses, shorter reinforcing spacing and larger bars sizes. Horizontally spanning walls with highly reinforced wall pilasters may provide a solution. It should be noted that it can also be difficult to design the structural roof system to resist the large 100 psf live load, with the typical long spans used in gymnasium areas.
SUMMARY
New model code requirements will be mandating tornado shelters be incorporated into the design of schools and essential facilities for much of the central United States. As these structures are often designed using exterior masonry wall systems, these walls can readily converted into shelter walls with the required strength and debris resistance, at a minimal cost. Masonry walls can thus be used to provide safe, practical and cost effective solutions for sheltering from tornados and high wind events.
ACKNOWLEDGEMENTS
The funding for studies supporting the discussion presented in this paper was provided by the International Masonry Institute and their support is gratefully acknowledged. Significant input was also obtained from Diane Throop, Bill Coulbourne, Scott Walkowicz and Benchmark Harris, and their input is gratefully acknowledged.
Words:Mark McGinley Ph. D., PE, FASTM, FTMS
REFERENCES1. International Code Council, International Building Code, Washington DC: ICC, 2015. 2. ICC, ICC 500, Standard for the Design and Construction of Storm Shelters, Washington DC: ICC, 2014. 3. McGinley, W. Mark, Throop, Diane, B. and Coulbourne, William, L., “ Tornado and High Wind Sheltering With Masonry”, Proceedings, 13th Canadian Masonry Symposium, Halifax , NS, Canada, June 2017.4. E. W. Kiesling and L. J. Tanner, “Test Report: Investigation of Wind Projectile Resistance of Concrete Masonry Walls and Ceiling Panels with Wide Spaced Reinforcement for Above Ground Shelters,” Texas Tech, Lubbock, TX, 2003.5. Wind Science & Engineering Research Center Debris Impact Test Facility, “Report No. _20120426A,” Texas Tech University, 2012.6. E. J. Kiesling and T. L. J., ” Investigation of Wind Projectile resistance of the International Masonry Instiitute Series 1: Utility Brick Veneer Wall and Series 2: Modular Brick Veneer Wall,” Texas Tech University National Wind Institute, Lubbock TX, 2014.
