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The fact is that traditional methods of building with load-bearing stone blocks would be cost prohibitive for most modern buildings and the "new tradition" of 3 to 4 cm (1 1/8 - 1 ½ ins.) thick non load-bearing veneers have actually only been common for about 40 years. There have been many failures with structures using these systems that testify to the reasons for StoneLite® panels.
Consider the major points:
Strength
Stone has very high compressive strength (irrelevant for a non load-bearing application) but has low flexural and very poor impact strengths due to its brittle nature. These are relevant for cladding as attachment systems rely upon them. In addition, the strength is greatly affected by the flaws and defects prevalent in natural stone.
With StoneLite® panels, the stone is simply a decorative veneer; therefore the natural flaws and defects of the stone are not a factor. The strength of the panel is the fiberglass-reinforced honeycomb backing. The same material used to make aircraft wings. This combination yields vastly improved flexural and impact strength. The panels have been tested to over 400-lbs./sq. ft. (19 kN/sq. m.) wind load and resist large missile impact and dynamic hurricane loading.
Flexibility
Stone is extremely brittle and can withstand minimal structural movement. StoneLite® panels are very flexible and can move with the structure (see Ramtech Seismic Racking Test Report). This makes StoneLite® ideal for high wind-load or seismic situations. This is supported by the favorable results experienced on projects both in south Florida during Hurricane Andrew, and the San Francisco Bay Area following the 1989 earthquake, where StoneLite® clad buildings that encountered no cladding damage.
The Providence Towers in Dallas, clad with 2 inch thick granite, has been stripped and reclad (less than 5 years after being built) due to kerf breakage.
Stability
Amoco's 80-story Chicago headquarters and the Lincoln First Tower in Rochester, NY are the most widely publicized of the many projects that have suffered from bowing of 3cm stone.
The investigators for the Amoco Building failure confirmed the phenomenon that the Earl of Mayo knew in the 60's when he "invented" the StoneLite® Panel. Many stones both naturally warp after they are cut into 30-40mm slabs and also expand under each heating cycle but do not fully contract during the subsequent cooling cycle, therefore the stone gradually warps. Combined with the temperature gradient through the stone the result at Amoco was the stone slabs bowed up to 1 inch (25mm). Obviously the kerfs broke off long before they reached that amount of dish and the building had to have temporary stainless steel straps bolted through the slabs to retain them prior to a complete reclad at a cost of $80 million.
The stone at less than ¼ inch (6mm) thick on the StoneLite® panel performs as a decorative veneer only. It cannot exert any force on the honeycomb-backing panel to cause such a problem. The back up framing to the StoneLite® panel controls the flatness of the system.
We have buildings clad with over 300,000 square feet of our StoneLite® panels that have the exact same stone used on the original installations of the Amoco and Lincoln First Tower Buildings. Our panels are the same today as they were when installed.
Bond Strength
The obvious concern with StoneLite® panels is related to the bonding of the stone to the backing of the panel.
To obtain I.C.C. approval, a sandwich panel must pass many tests. For bond strength the flat wise tension must be a minimum of 50 p.s.i. After an accelerated aging test designed to similate a 30-year life, strength can be no less than 85% of the original value.
Our panels achieved an initial strength of 385 p.s.i. (55,400 lbs/ft2) and a post aging strength of 460 p.s.i. (66,200 lbs/ft2) representing an INCREASE in strength of 20 %, due to post curing of the epoxy resin, which gains strength over time.
Considering that a.) The initial strength exhibits a safety factor over 2,000:1 when subjected to a 100 m.p.h. wind and b.) The post aging strength is HIGHER; obviously extrapolation of such results would only show an infinite life expectancy.
Further investigation also shows that the bond strength always exceeds the cohesive strength of marble and the majority of granites. Therefore, the expected failure in service would only be by fracturing the stone surface.
The consideration for long-term durability must then be limited to the erosion resistance of the stone surface. The stone most susceptible to this, of all those we've produced to date, is Indiana Limestone. The Indiana Limestone panels with ¼ inch thick stone (average) allows for a serviceable life of some 300 years. A granite surface would far exceed this life expectancy.
I.C.C. approves our StoneLite® panels for Class 1 (unlimited height) structures.
We have never had any failure (delamination or otherwise) of our panels. Neither are we likely to, considering the strengths we achieve.
Attachment Systems
The weakest point of a cladding system is normally its attachment system.
A typical 3-4cm system has kerfs cut into its edges, leaving a short tab of stone approximately 3/8 to ½ inch thick to set onto the anchors. These tabs cover about 1% of the panel area and must resist the total wind load on the full panel. In this situation the tab is subjected to bending stresses, which exposes the weakest property of the stone. When combined with the micro-cracking common in all stones and the action of the freeze-thaw cycling, it is easy to understand the benefits of the StoneLite® Panel System.
Compare this attachment system to the full surface bond coverage using our high strength epoxy adhesive and factory set threaded inserts that may be designed to meet any wind load requirement.
Weatherproof
Natural stone absorbs moisture, some to a greater extent than others. Water will pass through travertine and limestone in a matter of minutes, sometimes seconds. Polished granites, the best performers, will take longer, but eventually weep.
The moisture in the stone has two potentially harmful effects on the solid stone system. The moisture-laden stone under freezing conditions can seriously weaken the structural integrity of the solid stone, particularly significant around the attachment areas. The weepage through into the cavity can build to significant quantities potentially causing water ingress into the building interior. European building codes require all stainless steel attachments, as the moisture is detrimental to lesser quality materials.
StoneLite® has a totally impervious moisture barrier behind the stone, which prevents all the above problems. Independent testing has proven this many times.
Experience
One of the most common misconceptions is that this is a "new" product as compared to solid stone. The fact is we have been producing these panels for over 35 years and they have been used as cladding for the exterior of buildings in the U.S. for over 25 years. The "new tradition" of 3 to 4 cm (1 1/8 - 1 ½ ins.) thick non load-bearing veneers have actually only been common for about 40 years, and have been plagued with problems and failures.
Moreover, consider the technology application in the aircraft industry. Since the 1950's aircraft have used epoxy resins to bond their parts. Every commercial and military aircraft built in the U.S. and Europe utilize honeycomb and epoxy resins for their control surfaces (flaps, ailerons, etc.). The Stealth fighters and bombers are totally built from fiber-reinforced composites (honeycomb and resins). Helicopter rotor blades are now normally similarly structured.
Aircraft are DYNAMIC structures and subjected to enormous forces. A plane leaving a desert airstrip has surfaces around 160/170°F (70°C) which will change to minus 60/70°F (-55°C) within minutes when it reaches 30,000 feet while experiencing 500 to 1500 m.p.h. wind loads, plus tremendous vibration. The designed life of an airliner is normally 20 years but many are in service much longer.
How many buildings are subject to CONSTANT wind loads over 500 m.p.h.? It is certain that "traditional" kerf attached slab would not withstand it.
At an independent test laboratory we were attempting to test StoneLite® panels to destruction. We blew out the test chamber at 413 lbs/ft2 (equivalent to a wind speed over 400 m.p.h.) and the panel did not break.
At the same time a test for a 30mm system failed at 46 lbs/ft2 (135 m.p.h. wind) by the stone breaking away from its kerf, plus its EPOXY-SET dowel anchors added when it first failed at a lower pressure. A second redesign with added fasteners allowed it to just meet the design criteria of 60 lbs/ft2 (155 m.p.h. wind).
We're not suggesting that anyone needs a 2,000:1 safety factor but it is very comforting.
Testing
Consider the project test requirements for StoneLite® panels and the 3-5cm veneer systems.
Leading construction consultants recommend some 20 separate tests using over 150 specimens plus full scale (2-stories by one bay) mock-up and wind tunnel tests (proposed building plus surrounding structures) for each project. This test program is necessary due to the highly variable physical qualities of stone. It would, however, take may months prior to construction, plus the expense is not insignificant. However, compared to the cost of a failure it is obvious it should not be avoided (as Amoco's $80 million attests). As the strength of the stone has no effect on the strength of the StoneLite® Panel system almost all of the recommended tests are not relevant. All our tests carried out to date have proven the systems strength and serviceability.
The sealant compatibility test should be run for the particular stone and sealant to be used for the project. The only other test with any relevance, and not already tested, is the Wind Tunnel Test. Applying the appropriate building code standard and considering the safety factors with the StoneLite® system can normally avoid this.
In 1995 we conducted the Acid Freeze-Thaw test devised to establish a method of assessing the relative life expectancy of granites, marbles and limestones in a harsh environment. This test is extremely severe and no marble and few limestones have ever passed. The material is submerged halfway (stone face down) in a bath of dilute sulfuric acid (4ph). It is rapidly cycled 100 times from -13°F to +173°F (-25°C to + 78°C) in a 6-hour cycle, using an aircraft-conditioning chamber. Flexural strengths are taken before and after.
3 to 5cm thick (1 ¼ " -2") polished granite loses 25% flexural strength in this test, flamed granite 45% and marbles average 70% loss. StoneLite® panels lost an average 13.6%. They started with a flexural strength 24 times greater than solid granite and ended 67 times that of solid marble.
WJE engineers are trying to have ASTM adopt this test as a standard. If they are successful, the only marble less than 4 or 6 inches (10-15cm) thick that would pass for exterior use is StoneLite® panels.
Following Hurricane Andrew, the South Florida building code was modified to add another test due to the massive devastation created by flying debris. In the test, the cladding material is subjected to a 90 p.s.f. cyclic wind loading 1,342 times then taken to a maximum of 135 p.s.f.. A 2" x 4" x 8' timber stud is fired several times at the panel at 50 ft/sec. This simulates a house stud being thrown in a 100 m.p.h. wind, following wind cycles of 190 m.p.h. with gusts up to 230 m.p.h..
StoneLite® panels easily pass this test, but the stud passes straight through 1 ¼ " (3cm) thick granite at less than 10% of the force. The stud is barely slowed on its way into the 3cm granite clad building.
Cost
The installed cost of the StoneLite® system will typically save between 20% and 50% of the cost of a well-engineered 3cm to 5cm system. The 80% reduction in weight translates into savings in cost of back up structure and rapid installation without the need for heavy-duty mechanical handling equipment. This provides the majority of the savings.
Supplemental savings are often made in other areas, such as structural steel requirements, reduction in wall thickness can reduce window framing costs and increase interior floor (net lettable) area.
Color
Another distinct advantage of StoneLite® panels is in the area of color control.
All stones, by nature, vary in color. The manufacture of StoneLite® panels requires only about 1/3 the amount of stone used by a 3cm project. This provides greater color consistency for the building. However, it is very easy in most cases to pre-determine the layout of the panels by color and blend them prior to fabrication/installation to achieve the desired aesthetic result.
Product Acceptance
Stone Panels, Inc. is the direct descendant of the inventor and original Irish producer. We have heard of some 30 companies worldwide attempting to produce their versions of our StoneLite® panels. We will not pass comment on these, except to state that only one company, a subsidiary of the Nippon Steel Corporation in Japan, has our technology (through a license agreement) and the ability to produce an equivalent quality product. One of the world's largest companies they have invested some $22 million in their factory, which replicates our production plant. We can show buildings across the world with StoneLite® panels.
StoneLite® panels provide a natural stone façade to a building that ensures it's an extremely strong, safe installation that has withstood the test of time. During the same period, the "traditional" methods have shown shortcomings and major failures.
That we have saved many millions of dollars on the initial cladding, or recladding, cost for property owners is purely a bonus. Amoco's experience (initial cladding cost - $6.5 million; corrective reclad - $80 million) amply demonstrates the long-term benefits of the StoneLite® panels solution.
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