1 Due to the tolerances allowed for erecting steel structures and pouring concrete, and due to other field conditions, the setting space behind the stone panel may have large variations and other discrepancies such as misplaced or left-out inserts, etc.
Based on this, it is recommended that the design of all anchoring devices be for the worst possible condition and to follow A.I.S.C. specifications for allowable stresses.
2 When testing natural stones, test results in a close range indicate a stone with more consistent physical properties, while test results in a wider range show the weaker and stronger areas in that test specimen.
A wide margin of safety is needed not only to meet the varying strength of the building stones, but also to provide for possible deterioration in strength of the stone after it is placed in the wall due to environmental attacks and normal expansion and contraction, freeze-thaw cycles, or other external forces, and aging.
Since the basic chemical and physical characteristics of natural building stone are determining factors of its strength and durability, it is recommended that when calculating slab thickness for wind load, for handling and for lateral anchoring, different safety factors be used for the sedimentary, metamorphic, and igneous origin rocks, so that the safety factor will reflect not only the range of spread in the test results but will also agree with the general chemical and physical characteristics of the rock.
Using a minimum of five (5) test specimens, preferably from different blocks and slabs, it is recommended that the spread in these test results be converted into safety factors as described in Table 2.
TABLE 2
Safety Factor for Calculating stone Thickness
Spread in Test Results
For Windload
For Lateral Anchoring
IGNEOUS ROCK
Up to 10%
3.0
4.5
l0 to 20%
4.0
6.0
Over 20%
6.0
8.0
METAMORPHIC ROCK
Up to 10%
4.0
6.0
10 to 20%
5.0
7.5
Over 20%
7.0
10.0
SEDIMENTARY ROCK
Up to 10%
5.0
7.5
10 to 20%
6.0
9.0
Over 20%
8.0
12.0
It is recommended that, when possible, the full scale anchoring system be laboratory tested in lieu of relying solely on calculations. Based on a minimum of five (5) pull-out test results for anchoring stone, one can use the same safety factors which are shown in Table No. 2 for calculating the stone thickness for windload.
As a general rule, natural building stones possess higher strength in the direction at a right angle to the bedding plane than to any other direction. Therefore, it is recommended that when testing natural building stone to establish safety factors, the tests should be performed on specimens which are fabricated for testing at parallel direction with the bedding plane.
stone specifications shall specify, and inspection shall control, the fact that all stone blocks are slabbed at the parallel direction with the bedding plane.
Where natural building stone is used as load bearing material, a 100% increase to the safety factors is recommended as shown in Table No. 2 for stone thickness.
The previously mentioned safety factor recommendation is reasonably conservative. It is written as a guide to users in the stone industry for the avoidance of potential failure and litigation. The factor of safety depends upon the Building Code and the judgment of the engineer.
The physical and chemical characteristics of the stone determine its durability, resistance to moisture and atmospheric pollutants (after it is placed in the wall). Consequently, it is not advisable to use the same safety factors for rocks of igneous, metamorphic, and sedimentary origin.
The physical and chemical characteristics of the rock also vary widely within these three groups. Nevertheless, such a simplified grouping will provide some degree of guidance to engineers, architects and designers who are calculating stone thickness for wind and anchoring, but are not thoroughly familiar with all its physical and chemical characteristics.
The basic chemical and physical properties of natural building stone vary according to its geological origin. These characteristics determine:
? elastic properties, compressive and flexural strength
? hardness and resistance to erosion
? resistance to attacks of acidic solutions (weathering)
? resistance to attacks of freeze/thaw cycles
? internal structure, coherence of the minerals
In addition to the wide variation of the weak and the strong zones of the natural building stone, there is an endless list of occasions when damages have caused substantial weakening in some of the installed stone panels, and the stone has become substantially weaker than the previously received test results would indicate.
It is common knowledge that most of the failures occur at, or near, the anchors. Unless properly conducted anchor pull-out tests suggest differently, it is reasonable to use more conservative safety factors when calculating stone thickness for anchoring, than for calculating stone thickness for wind. Of course the flexural and shear strength of the homogeneous metal anchors may be safely calculated regardless of the origin of the loads. However, natural stone is heterogeneous, therefore weak zones at, or close to, the anchors could lead to failure much before the life expectancy of the rock.
source: Building stone Institute