Design and Construction Guidance for Community Safe Rooms
FEMA 361, Second Edition/August 2008
APPENDIX G - DESIGN GUIDANCE ON MISSILE IMPACT PROTECTION LEVELS FOR WOOD
SHEATHING
Reinforced concrete and reinforced masonry have been the most common wall and
roof materials used with success in non-residential safe rooms. The use of wood
panels for exterior wall sheathing in non-residential safe room applications had
been limited. This appendix provides limited information on wood panel testing
that has been performed for both hurricane and tornado safe room applications.
Data from the missile impact tests on walls with plywood and oriented strand
board (OSB) sheathing conducted at Texas Tech University (Carter 1998) and at
Clemson University (Clemson 2000) have been combined to determine the variation
of missile perforation resistance with thickness of the sheathing. In order to
put all the data on a consistent basis, missile weights and lowest impact
velocities for perforation of the sheathing have been extracted from previous
test results. The weight and impact velocity information were used to calculate
the impact momentum {weight (lb) x velocity (feet/second)/acceleration of
gravity (32.2 feet/second squared) = momentum (pound/second)} and the impact
energy {weight (pound) x velocity squared (feet/second) squared/acceleration of
gravity (32.2 feet/second squared) = energy (foot/pound)}. The resulting impact
momentum and impact energy for perforation of the sheathing are plotted as a
function of sheathing thickness (in 1/32 inch) in Figures G-1 and G-2.
The momentum required for a wood 2x4 missile to cause perforation varies
essentially linearly with thickness of the sheathing material for both plywood
and OSB. This suggests, at least for this type of missile and common sheathing
materials, that a desired target penetration resistance (ability to resist a
certain impact momentum) can be achieved by simply adding up the contributions
of the various layers of sheathing. For example, in Figure G-1, sheathing with a
30/32-inch thickness represents two layers of 15/32-inch material.
Figure G-3 provides information on the relative resistance of various common
sheathing materials, in terms of impact momentum absorption, for a compact
impact area such as that associated with a wood 2x4 missile impacting
perpendicular to the sheathing material. Summing the momentum resistance of the
various layers of common sheathing materials is permissible when developing
initial design criteria for walls that provide adequate protection. However,
this process may not work for other types of missiles or for wall materials that
absorb impact energy by undergoing large deformations (i.e., corrugated metal
panels).
For the large design missile of this publication (a 15-lb wood 2x4 missile), the
maximum horizontal impact speed designated in the criteria is 100 mph, and the
corresponding momentum is approximately 68 lb/sec. For vertical impacts, the
maximum impact velocity designated is reduced to 67 mph; the corresponding
momentum to the maximum vertical impact speed is approximately 46 lb/sec.
[Begin figures]
Figure G-1. Variation of impact momentum required for missile penetration vs.
wall sheathing thickness. This is a graph showing the Actual Test Specimen
Thickness (32nds of an inch) compared to the Impact Momentum (lbs-s)
[End figures]
[Begin figures]
Figure G-2. Variation of impact energy required for missile penetration vs. wall
sheathing thickness. This figure is a graph showing the actual test specimen
thickness (32nds of an inch) compared to the impact energy ft/lb).
[End figures]
[Begin figures]
Figure G-3. Impact momentum required for a 2x4 wood missile to penetrate various
common sheathing materials (impact perpendicular to sheathing surface). Note:
All wood products provide less than half the required impact momentum resistance
needed to meet the horizontal surface impact resistance required by the National
Performance Criteria for Tornado Shelters. This figure is a graph showing impact
momentum (ft-sec) for complete penetration of sheathing material by 2x4 type
missile.
[End figures]