Travertine Safety Factor

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Author Message Jerome J. Lazar, RA, CCS, CSI, SCIP Senior Member Username: lazarcitec Post Number: 689 Registered: 05-2003 Posted on Tuesday, June 16, 2009 - 09:11 am:    On a recent project where we specified Travertine Stone Exterior Cladding we followed MIA Design Manual and called for a Safety Factor of 8. The stone subcontractor has advised the Architect that it is difficult to meet the safety factor with 3cm stone selected and suggests increasing the thickness to 4cm, of course this will also increase the cost of the stone. A question for all you Travertine Mavens (Experts)is the safety factor of 8 set in stone? Can someone shed some light on this, perhaps someone knowledgeable on Safety Factors and how they are determined? Thanks in advance for the input. Mark Gilligan SE, CSI Senior Member Username: mark_gilligan Post Number: 166 Registered: 10-2007 Posted on Tuesday, June 16, 2009 - 12:43 pm:    In order to answer your question more information will be needed. Your ultimate goal is to reduce the liklihood of failure. For more consistent materials such as steel a FS of 1.5 to 1.7 is common. The only reason for larger factors of safety is if there is a greater liklihood of a piece of stone with a low strength. Thus to decide whether FS=8 is appropriate you need to understand the likely variation in stone strength which may vary with the type of stone and possibly the quarry. This requires an expertise that you will not likely find on this site. "Marble and Stone slab Veneer" by the MIA states that "Normal factors of safety range from 3 for stones with uniform test values to as high as 10 for widely scattered test results" I am assuming that the design of the cladding system is design built. I would recommend that you find an engineering firm/expert that commonly designs stone cladding, but will not design this project, to advise you. Firms such as Wiss Janey will likely also have this expertise. Dale Roberts CSI, CCPR, CTC Senior Member Username: dale_roberts_csi Post Number: 69 Registered: 10-2005 Posted on Wednesday, June 17, 2009 - 10:12 am:    If we new exactly what the performance and strength of a product would be, and knew exactly the loads that it would be subjected to, we could design without a factor of safety at all. The factor of safety of 8 is pretty common for any limestone in a non-constant stress state. Limestones (and travertine is a type of limestone) generally produce very noisy data sets in testing – high standard deviations and high coefficients of variation. That makes their predictability very uncertain, hence the high factor of safety. There is also the concern of strength degradation during service life, where 20 years down the road we probably won’t have the same strengths as we did at installation. I personally wouldn’t stray far from the 8 – no it’s not a code, and if you had 7.5 I’d probably say that’s close enough, but I would want to be in that neighborhood. Steven Bruneel, AIA, CSI-CDT, LEED-AP Senior Member Username: redseca2 Post Number: 179 Registered: 12-2006 Posted on Wednesday, June 17, 2009 - 12:33 pm:    An FS of 8 is the the default in the MasterSpec template doc: A. Safety Factors for Stone: Design dimension stone cladding system to withstand loads indicated without exceeding allowable working stress of stone determined by dividing stone's average ultimate strength, as established by testing, by the following safety factors: 1. Safety Factor for Granite: [3] <Insert number>. 2. Safety Factor for Oolitic Limestone: [8] <Insert number>. 3. Safety Factor for Dolomitic Limestone: [6] <Insert number>. 4. Safety Factor for Marble: [5] <Insert number>. 5. Safety Factor for Quartz-Based Stone: [6] <Insert number>. 6. Safety Factor for Serpentine: [6] <Insert number>. 7. Safety Factor for Slate: [5] <Insert number>. 8. Safety Factor for Travertine: [8] <Insert number>. 9. Safety Factor for Concentrated Stresses: [4 for granite] [and] [10] [for stone varieties other than granite]. As previously noted, Travertine is one of the least "homogeneous" of stone types.