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A Brief Guide to Debris Mitigation and Impact Absorption of Stone Building Envelopes and Facades
The Challenge of Designing a Blast Resistant Stone Facade
It’s an unfortunate fact of life today that buildings are at risk due to terrorism. In a fraction of a second, explosives in a parked car can destroy the face of a building. When designing a building enclosure or facade, the impact of blasts is a force to be considered. This is especially true when designing facades for courthouses, hospitals, government buildings and other high risk facilities. In a blast event, the shock wave of the blast striking the facade cladding can be mitigated by stopping flying debris and absorbing the blast energy. So how do you design a stone facade to be blast resistant? Following are some solutions to the challenge.
Ductile Stone Cladding for Blast Resistant Building Envelopes
In blast resistant cladding and building envelopes, a greater emphasis is placed on ductile behavior of a cladding in response to blast loading. StonePly, with its aluminum honeycomb is an excellent material to consider, since by its very nature it is ductile. Even though it presents a stone facade to the world, the aluminum backing means that StonePly bends and deforms elastically prior to failing. In a blast, StonePly cladding panels perform much better than slab stone. This is because StonePly has capacity both in tension and in compression, where ordinary stone cladding only has capacity in compression. This means StonePly stone panels are up to 60 times more impact resistant than granite or marble slab panels.
Preventing and Stopping Flying Debris with Stone Ply Facade Panels
In a blast, flying fragments can cause as much or more harm than the explosion itself. So how do you avoid fragmentation of stone cladding and the creation of flying debris? Designers of blast resistant buildings are using architectural elements and cladding that do not fragment during an explosion. With StonePly, the entire stone is bonded to a ductile aluminum panel. In lower level blast events, the blast energy is absorbed as the panel flexes under the shock wave and then returns to normal. Under a higher level blast, the stone may fracture, but the aluminum panel, acts as a safety net. Much like in a car wreck when the laminated safety glass holds the pieces of broken glass together, the aluminum holds the stone pieces together.
Slowing Impact Time to Reduce Blast Effects on the Building Structure
A large bomb blast pushes the stone face of the StonePly cladding inward. The aluminum honeycomb panel goes into tension, and the cells of the aluminum honeycomb act as thousands of crumple zones as they absorb the blast energy. By the panels flexing under the load some energy is absorbed and dissipated. In addition, the individual honeycomb cells act as little airbags to further absorb the impact. By slowing the impact time you reduce the force of the impact.
Anyone who has ever hit a bubble wrap padded surface versus a solid concrete surface with their hand understands that while the padding (by slowing the impact time) reduces the damage to your hand, it also reduces the damage to whatever is behind the padding and reduces the impact force applied to it. It is the same principle as an airbag or crumple zones in a car. When it comes to cladding and blast resistance, the ideal cladding is one that can resist smaller blast loads, but when they blast loads become very large, can crumple and deform in an energy absorbing manner, to reduce the blast load on the structure and to prevent flying debris. While StonePly panels flex and bounce back from smaller blast pressures, under extreme blast pressures, the StonePly honeycomb cells crumple like little airbags. Like the crumple zones and airbags of a car protect the driver from the crash impact, the StonePly cladding of the building envelope softens the impact and protects the buildings structure and its occupants from the full force of the blast.
The importance of the StonePly slowing the blast impact down can be explained mathematically. Newton’s second law may be written as mΔv=Ft where m is the mass, Δv is the change in speed, F is the average force, and t is the time during which F acts. So the longer you can make the collision time when the blasts shock wave strikes the buildings cladding, by crumple zones, flexing and air cells (the t in the equation), the smaller the F (the blast waves force transferred to the building) will be.
With their high impact strength, ductile nature and energy absorbing cell structure, StonePly panels can form an important component of a complete system of blast resistant cladding for your building enclosure and the protection of its occupants.
Fire Resistant and Blast Resistant Cladding
In a blast event, fire is an added concern. Conventional bombs and explosives rely of the rapid oxidation of a fuel to create an explosion and fire. Fire and smoke can be as deadly and damaging as the initial blast. Stone Ply offers a non-combustible stone face, with a 0 flame spread surfaces, both on the stone front side and the non-combustible aluminum back. In ASTM standard testing, StonePly resisted over 240 minutes of direct flame against the panel face without any penetration or any burning through. In addition, in the flame exposure test, the back of panel never reached temperatures over 160 degrees, protecting materials behind the exterior building envelope from heat and flame.
And unlike some cladding materials, which have only one side non-combustible, StonePly offers a non-combustible front and back. In the event of a blast where flame penetrates the cladding and into the drainage cavity, StonePly features a non-combustible, 0 flame spread, aluminum back.
A Rich Selection of Stone Colors are Available for Blast Resistant Cladding
Blast resistant stone facades can be beautiful. Compared to ordinary stone veneer cladding, you get the same natural stone exterior but in a stronger, ductile and 60 times higher impact strength panel. StonePly cladding panels are available in natural granite, marble, limestone, onyx and travertine. The blast resistance and strength of the panel are derived from the back of the panel, the aluminum honeycomb core. The stone that is selected for the exterior face has little or no effect on the blast resistance, so the architect designing the facade is free to select from a rich palette of stones from around the world.
Designing for Blast Resistance vs Traditional Building Enclosure Design
Blast designs involve a building enclosure built to withstand a high pressure over a very short period and in a relatively concentrated area. Traditional cladding and building enclosure designs require façades to resist lower pressures and more uniformly distributed loads. Wind loads apply pressure to the facade over extended periods and repeatedly (day after day, year after year). Blast loads are onetime events, with a concentrated load over a small area. The total force acting on the entire building envelope from a 30psf wind load (1.44 kPa) can be much higher than the smaller, but more concentrated load from a blast. The blast creates a concentrated load on a few facade panels, but the smaller but concentrated pressure wave can exert much higher pressures per square foot.
The higher impact resistance of StonePly cladding allows it to resist concentrated impacts 60 times better than a solid granite slab cladding.
The Next Step
If you have questions or need assistance with stone cladding design details or blast resistant stone cladding, please contact us. We are happy to help.