In-Depth Discussion of Priority Needs

Some additional details on the four tasks described earlier in Research Priority No. 1:

The TORMIS model developed in the 1970s is still in use today as the Regulatory Guide 1.76, Design Basis Tornado and Tornado Missiles for Nuclear Power Plants, most recently updated and published in March 2007. The prescriptive approach includes three missiles: schedule 40 pipe, automobile, and solid steel sphere. Table A-1 reproduces Table 2 from RG 1.76, which summarizes these missiles.

The tornado wind speed for Regions I through III (in Table A-1) are 230 mph (103 m/s), 200 mph (90 m/s), and 160 mph (72 m/s), respectively, from Figure 1 of RG 1.76. The defined missiles and projectile velocities reflect the higher risk associated with the regional tornado wind speeds. ASCE/SEI 7-22 Risk Category III and IV structures are designed to a much lower hazard level, so corresponding missile definitions are needed, possibly with varying definitions or projectile speeds dependent on the risk category.

ASTM E1886 provides the standard test method and ASTM E1996 the standard specification for windborne debris in hurricanes on exterior components and cladding. Following development of the probabilistic tornado-borne debris model, equivalent industry standards could be developed for tornado debris. These testing standards could then be used to evaluate common materials of construction.

All buildings, whether enclosed or partially enclosed, have an internal pressure that is a function of the distribution of porosity over the exterior of the building and the exterior pressure distribution. In a partially enclosed building, the external pressure at the opening dominates the internal pressure; whereas for an enclosed building, the internal pressure takes on a value that is similar to the average external pressure, depending on the distribution of porosity.

In the case of a tornado event, an additional exterior pressure is associated with the negative pressure within the tornado (also referred to as atmospheric pressure change). As the core of the tornado moves closer to a building, the atmospheric pressure just outside of the building decreases. If the air permeability of the building is minimal and the pressure cannot be transferred into the building, then the building's positive internal pressure will increase. This increase is dependent on the volume of the building and the size of the tornado. For example, in the case of a large building impacted by a small tornado with a low pressure associated with the atmospheric pressure change at its center, the averaging process associated with the internal pressure of the large building will indicate that the effect of the low pressure in the center of the tornado on the internal pressure of the building will be small. The net result being a large upward external pressure on the building at the center of the tornado.

ASCE/SEI 7-22 handles the increased internal pressure due to atmospheric pressure change by increasing the positive pressure coefficient for “enclosed” buildings from +0.18 to the positive pressure coefficient for “partially enclosed” buildings in ASCE/SEI 7-22 wind design, which is +0.55. This value is meant to capture the combined external and internal pressures from the APC and account for the internal pressure based on the building's openings. This research need would investigate the +0.55 coefficient and determine if this is the correct value, or if it is too low for enclosed, partially enclosed, and partially open structures.

Internal pressures do not affect most non-building structures except for tanks. Tanks can be either pressurized (a truly sealed condition) or vented to the atmosphere. While ASCE/SEI 7-22 applies a tornadic internal pressure of +1.0 to account for atmospheric pressure change, this is applicable to the former case and does not capture those vessels that might have venting (e.g., overflow pipes or vents). The vents in a tank are often significantly smaller than the openings in a building; furthermore, the balance of the tank envelope is tightly sealed (e.g., to contain liquids), while the balance of the building envelope is not, as air leakage can pass through the envelope. Thus, a better understanding of internal pressures for tanks is warranted.

It is important to note that while ASCE/SEI 7-22 treats atmospheric pressure change as an internal pressure, Research Need No. 9 assumes the external pressure and atmospheric pressure change are intertwined; this must be considered during research of internal pressures for buildings and non-building structures as it relates to tornadic internal pressures.

For non-building structures, the effects of APC are most notably felt on tanks and some rooftop equipment. Other lattice and open-framed structures do not experience the effects of the differential pressure. In the case of tanks, the area of influence and magnitude of APC are of utmost importance. For large-diameter tanks, the core of the tornado could be smaller than the tank footprint, thus applying the uplift pressure to the entire roof could be overly conservative. A standardized area is recommended for the application of tanks. Researchers should also be directed to Research Need No. 7, as ASCE/SEI 7-22 currently addresses APC via the internal pressure generated during a tornado.

Current design standards for mechanical components in tornado storm shelters have been based on information available for straight-line wind conditions and conventional occupancies where occupants are not sheltering in place close together where exterior debris can trap them in place for extended periods of time. There is a need for research to better understand wind pressures and air flows as air moves into, moves within, and moves out of tornado storm shelters through mechanical components that are necessary to provide ventilation to occupants and to provide air flow to emergency generators during a tornadic event.

Standards issued by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) for determining the required pressure rating on an exterior louver in straight-line winds are very inconsistent with the design wind pressure requirements for ASCE/SEI 7 for straight-line winds. Additionally, these ASHRAE standards do not incorporate recent research in tornadic wind pressures that are the basis of the new tornado load requirements in ASCE/SEI 7, which indicates wind pressure profiles on the exteriors of buildings are different for tornadic wind events as compared to straight-line wind events. Furthermore, current manufacturers of louvers do not have test data indicating what air flows occur and what air pressure drops occur across their products at the significantly higher wind speeds than they have been tested for in straight-line conditions because there are not full-scale wind tunnel testing facilities capable of testing at the higher tornadic wind speeds greater than 250 mph. There is no know research indicating if typical configurations of debris-resistant louvers will continue to have laminar flow, at these higher wind speeds, which is the basis of ASHRAE standard pressure drop coefficients. There is also no standard for determining if tornadic wind pressures on the interior face of an exterior louver system are small enough to be suitable near occupants, ductwork or equipment in a tornado shelter. And there is a need for research to investigate the potential for unbalanced interior pressures that will occur on mechanical components, like ductwork or emergency generators, during the interior pressurization that occurs during a tornadic event.

ASHRAE 62.1 has requirements for sizing and locating openings in interior partitions to provide sufficient ventilation of all occupied spaces within a typical building. However, the area and location requirements of ICC 500 for exterior openings and natural ventilation differ significantly from ASHRAE 62.1 because they are for different conditions and there is no research or standard indicating how openings in interior partitions should be sized and located in a tornado storm shelter.