1. Finite Element Analysis of Saferooms Subjected to Tornado Impact Loads
YEVGENIY PARFILKO
Fernando Amaral de Arruda
Benjamin Varela
Dept. of Mechanical Engineering, Rochester Institute of Technology, Rochester, NY, USA
3rd International Conference "Innovative Materials, Structures and Technologies"

2. Introduction
Tornadoes are high speed windstorms that occur multiple times per year in regions of the US. Tornado saferooms are designed to protect occupants from high winds and windborne projectiles that occur during severe storms (EF4 and EF5 tornadoes).
A projectile may be construction material, debris, trees, vehicles, ice, power lines, and so on. Vehicle impacts are of special concern since they are usually present near a saferoom, are heavy, and can roll easily.
Our team conducted a study of vehicle impacts into a monolithic concrete saferoom, since this work had not been done before.
3rd International Conference "Innovative Materials, Structures and Technologies"

3. Introduction
Shelters designed to comply with NSSA Code must be able to withstand projectile impact as well as wind loads.
3rd International Conference "Innovative Materials, Structures and Technologies"

4. Methodology
We define the problem: complex interaction of vehicle, structure, and soil
Saferoom-soil component
Identify material model, appropriate mesh, rebar interaction
Define soil contact and boundary conditions on soil only
Vehicle component
Identify vehicle, mass, velocity range, contact surfaces
Determine the vehicle orientation and impact location
Concrete material models, vehicle models, projectile speeds, boundary conditions
Simulation parameters
Control over initial velocity parameter
Select termination time, time step, stability controls
Material-specific damage variable tracking enabled
Contact forces from vehicle
Internal forces from rebar
Reaction forces from soil
3rd International Conference "Innovative Materials, Structures and Technologies"

5. Model Parameters Concrete material properties:
35.4 MPa compressive strength, 16.0 MPa shear
Reidel-Hermaier-Thoma (RHT) material model
Elastic steel material model for rebar
Coupled via Constrained-in-Lagrange formulation
Finite element simulation properties:
30-50 mm element size; 210k-470k elements
Simplified explicit dynamics solid elements (Puso) for concrete
Shell and beam elements for vehicle and rebar steels
RHT Damage variable defined for all concrete elements
0.200 s of impact simulated at internal time step of 9.00E-07 s
Car model properties:
NCAC Chevrolet C2500 Pickup; 2000 kg at 15.6 m/s (56.3 km/h)
3rd International Conference "Innovative Materials, Structures and Technologies"

6. Animation: Frontal wall impact
3rd International Conference "Innovative Materials, Structures and Technologies"

7. Results: Frontal impact into wall
Results show measureable and reasonable deflection
1.65 mm total dynamic displacement
1.13 max. structure displacement
0.68 max. wall deflection relative to structure
Clear indication of elastic response of structure
Plate-like vibrational mode can be extracted
Damping properties can be approximately extracted
Identifiable damage pattern
Zones of compaction for chassis rails, engine block
Similar results expected with other vehicle models
3rd International Conference "Innovative Materials, Structures and Technologies"

8. Animation : Frontal impact into roof
3rd International Conference "Innovative Materials, Structures and Technologies"

9. Results: Frontal impact into roof
Displacements are less due to dampening mass
1.03 mm max. dynamic displacement
0.26 mm max. structure displacement
0.77 mm max. relative displacement
History variable tracks significant surface damage in impact zone.
40% damage averaged between affected elements
Interpretation: elastic yield exceeded, with 40% of element considered to reach fracture.
3rd International Conference "Innovative Materials, Structures and Technologies"

10. Animation: Side impact into door
3rd International Conference "Innovative Materials, Structures and Technologies"

11. Results: Side impact into door
Results show evidence of damage and plastic deformation
Nonzero relative displacement after transient
“Messy” power spectrum – change in domain properties
High relative deflection (1.98 mm), similar global displacement
3rd International Conference "Innovative Materials, Structures and Technologies"

12. Conclusions
Three distinct crash scenarios identified: frontal, side, and top impacts. The damage pattern is unique for each case.
Distributed damage pattern along a side wall, plate-like deflection.
Less severe damage in roof due to dissipation, lowest displacement of all three.
Possible cracking along free surface of doorway, with higher displacements.
Protective specification of saferooms can be validated by simulation. Stability of saferoom in soil can also be validated.
Saferoom design proved to withstand impact from all sides, with varying levels of damage.
Highest damage and deflections observed in front doorway.
Soil and concrete displacements are comparably scaled.
Along free surfaces, cracks develop readily. At the centers of faces, internal damage often develops in reinforced concretes before external cracks appear.
3rd International Conference "Innovative Materials, Structures and Technologies"

13. Further work: Simulation
Comparative analysis on other saferoom designs: ensure comparable performance
Integration of impact analysis into codes and certifications: prolong life cycle of saferooms
Extend analysis method to schools, roadside buildings, hospitals, and other buildings
3rd International Conference "Innovative Materials, Structures and Technologies"

14. Further work: Testing
Real-world validation of safety and impact resistance
Integration of accelerometers and force gauges into structural surfaces
Assessment of damage by nondestructive means such as a rebound hammer or ultrasonic transducer
3rd International Conference "Innovative Materials, Structures and Technologies"

15. References
United States. Federal Emergency Management Agency., National Association of Home Builders of the United States., and Texas Tech University., Taking shelter from the storm: building a safe room for your home or small business, 4th ed. Washington, DC: FEMA, 2014.
S. P. Center, "U.S. TORNADOES* ( ) ", ed. Online.:
United States Federal Emergency Management Agency, "Design and Construction Guidance for Community Saferooms", FEMA P-361, Second Edition, August 2008.
Y. Parfilko and B. Varela, "Simulating Debris Impacts Into Tornado Saferooms with LS-DYNA," Canadian Congress on Applied Mechanics, London, On. Canada, 2015, pp
T. Borrvall and W. Riedel, "The RHT Concrete Model in LS-DYNA," LS-DYNA Users Conference, 2011.
US Department of Transportation. "Users Manual for LS-DYNA Concrete Material Model 159". Federal Highway Administration FHWA-HRT May 2007.
"LS-DYNA Keyword User's Manual," R7.1 ed: Livermore Software Technology Corp., 2014.
NCAC. Finite Element Model of C2500 Pickup Truck. Model Year 1994, Version 7; October
3rd International Conference "Innovative Materials, Structures and Technologies"

16. Thank you for your attention!
Correspond with us:
Geopolymer Research Lab
Benjamin Varela, Ph.D.
Rochester Institute of Technology
Rochester, NY, USA
Special thanks to OZ Saferooms for financial support of this project.
3rd International Conference "Innovative Materials, Structures and Technologies"