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Calculating the Flexural Strength of Granite Facade Panel 3

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AH4

Structural
Mar 28, 2024
3
Hi All,

We are being asked to evaluate a granite facade unit to confirm it is not overstressed spanning 8ft from support clip to support clip. Could someone direct me to the proper equation to use when evaluating flexural forces on granite stone.

Design Information: Stone unit attached to a 8ft tall steel frame (building entrance sign)
Granite Facade Stone
Width = 8ft
Height = 8ft
Max Span = 8ft, the current proposed connection has the stone spanning 8ft with a clip on each side attaching to the steel tube framing.

Stone Section:

Depth = 96 inches
Thickness = 2 inches
Max Span = 8ft, the current proposed connection has the stone spanning 8ft with a clip on each side

Any help would be greatly appreciated.

Thanks,
 
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What is the applied load?
You can calculate the stress using basic plate equations, such as in Roark.
The difficulty may be in finding strength values for granite. I would google in in hopes of finding something.
 
Hi SW,

Thanks for your help. Yeah finding the capacity of the granite is our main issue. I'll see what I can find on google.

Loads:
Laterally: 32 psf (wind load); 256 plf
Vertically: self weight; 227 plf
 
Depending on the criticality you may consider testing the rock. WJE has an article related to stone durability below is an excerpt. Bring in an expert may be a good approach.

Screenshot_iqjmsa.png
 
A Google search can give you basic values. If you need specific value you can get it tested.

Regardless of its strength, what’s the standard for this sort of thing? Aren’t stone claddings like this normally continuously supported?
 
It has been a long time since I designed stone panels, but as I recall, the allowable bending stress controls, and it is the modulus of rupture divided by a safety factor of 8.

DaveAtkins
 
Don't know about granite, but the Indiana Limestone Handbook, free to download, has been the accepted standard for limestone construction for many years. I imagine that much of the advice there is applicable to granite.
 
you should arrange some testing of the particular rock, from the same source, quarry, and batch. properties will vary depending on origin and all kinds of things.

if you're not familiar with the material, suggest apply a huge factor of safety.

The critical thing with stone is the connections - what do they want to connect to? often suppliers want to use super dodgy undercut anchors and all other kinds of risky connection methods not very ideal for brittle materials.

that might be why they have come to you (a guy who has never done stone before) their usual engineer who is experienced in working with natural stones is telling them what they want to do is a bad idea!
 
Some additional thoughts (from an engineer who spends most of his day designing stone cladding) -

First, the details on the lines of responsibility etc. for this project are not clear, but we find most jobs use delegated design through the contractor for this work (which is what we provide). This helps as then the contractor who has to do the work is involved in the design process. Like most things, there are no shortage of different ways to approach these designs, and there are certainly some peculiarities with stone. And in this case, this panel could weigh over 2000 lbs., so it's not a minor piece of cladding.

But if you are going to try and get involved, some "big picture" thoughts -
a) Depending on the specifications for the project / size of the job, specific testing for the project may be required. If not, the quarries should be able to provide test data. But you should require specific stone test data - way too much variance within a stone type to generalize. At a minimum, either C99 (Modulus of Rupture) or C880 (Flexural Strength) for bending, and C170 (compressive Strength).
b) Safety factors for stone can vary quite a bit, with project specifications being the first place to look. If not, ASTM C1242 lists "Generally Accepted Safety Factors for Stone Cladding by Stone Type". Good news for you is that granite is probably the best type, as it is generally stronger, and requires a lower safety factor, as the test results are more consistent than other stone types.
c) Generally, two different safety factors are considered for stone design - the first being for the overall panel stress, and the second being for area around the anchor (to add some additional safety factor to address localized issues with the stone). For granite, we typically see a minimum safety factor of 3:1 for the body of the stone, and 4:1 for the anchorage.
d) Generally, stone panels are anchored along the longer sides of rectangular panels, with (2) anchors at each edge at 1/4 points of the longer panel width, for overall stability and to maximize strength relative to thickness.
e) Almost all stone anchorage is stainless, both for basic corrosion resistance and concerns about the contact of carbon steel to stone. Most typical anchors are either pinned or kerfed (slots) into the outside edges of the panel, with custom bent and/or welded plate anchors.

In my experience, 8'x8' is a very large panel, especially for 2" thickness, and I question if spanning 8' is possible or a good idea. Personally, I would consider advocating adding additional framing, to allow for additional anchorage at midspan of the horizontal 8' span, either by splitting the stone into (2)4'x8' or adding back of stone anchorage for the lateral loads. 4'x8' would be significantly safer, easier to fabricate and install, less prone to break, etc.

 
A few things on this from some notes I previously had:
There is a design manual by the Natural stone institute:
There is also ASTM C615 for the standard specifications for granite dimension stone
The flexural testing is ASTM C880.
Here is a material fact sheet by the old Natural stone council: NSC Fact Sheet
It lists flexural strength as 1200psi. We've received testing that typically allowed for an allowable bending stress closer to 1800 psi.

Also your slab is bending out of plane (lateral loads) and in-plane (gravity loads - self weight). Your stress due to self-weight is going to be very low given its height (really it's a deep beam). Out of plane would be my main concern if it is only anchored at two locations (and the anchorage). You may want to do a quick FEA slab analysis to see what your peak stresses are depending on where your anchors are located (ok maybe that's overkill).
 
This is an interesting discussion. I've often wondered how granite and other types of stone can be used as a structural material. I've always assumed that a slab of granite would behave similarly to a slab of unreinforced concrete, albeit with a very high compressive strength. I've also assumed, similar to unreinforced concrete, that there must be some provision in our applicable codes which disallow the use of this material if it will be stressed in flexure or tension, at least for an application where it's failure might have consequences.

Years ago a client wished to use granite posts to support a first floor assembly area. I didn't allow this, with my reasoning being that an unreinforced concrete column would not be allowed in this application, and neither should granite being that the two brittle materials (I assumed) would behave similarly.

Based on some of the responses above, I get the impression that granite is being used to resist flexure/tension. Is this being limited to only low risk applications, or are there perhaps granite slabs being used as part of a floor construction? The latter seems rather dangerous, considering that a crack could form in the slab and result in a sudden brittle failure.

I'm just curious others thoughts on this. Hopefully this isn't derailing the main discussion.

 
@ Rfreund

"We've received testing that typically allowed for an allowable bending stress closer to 1800 psi."

In my experience, I would say that an ULTIMATE bending stress closer to 1800 psi, not allowable. The strongest granite I have data on has an allowable bending stress of 1327 psi, and that is significantly higher than most other granites I have seen.
 
@ Eng16080

We currently design stone in flexure, primarily in for cladding applications, generally oriented vertically, but sometimes horizontally, for soffit conditions.

ASTM testing criteria are used to develop "ultimate" bending stresses, which are then used with safety factors developed based on stone type and variability in test data.

Almost all stone used as a walking surface is continuously supported on a mortar/grout bed, though in rare cases we have considered horizontal stone to support vertical live loads while in bending.

And we have done stone balustrades and railing systems (often for historic renovation).

But your comments regarding are correct, in that failure mechanisms can be sudden/brittle, though compared to other material, the safety factors can be large (ie. 8:1 for sandstone/limestone)
 
@jjl317 - I think you're right. It looks like I mistakenly pulled a value from porcelain tile.


 
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