Large Unreinforced Stone Beams
Large Unreinforced Stone Beams
(OP)
I have a client who wants to place an 18" thick x 6'-5" wide x 10 foot long stone over an entry way....like walking under an arch more or less. The stone is tucked under the roof for 80% of its area so a majority of the load is self weight. Are there any design guides to a situation like this?
I would guess that a stone's ability varies by individual specimen, climate, freeze thaw action, etc.
I would guess that a stone's ability varies by individual specimen, climate, freeze thaw action, etc.






RE: Large Unreinforced Stone Beams
You are rights since this is a matter of the stone's properties.
You never get exposed to the behavior of masonry walls in any engineering school class that I have seen, taken or given. - I don't think you can find a standard in existence to determine the structural properties of the "monster" you have erect and install.
Make sure you have a complete mortar bed for bearing and the compressive strength of the mortar used is not really critical unless a new "standard" has been created.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: Large Unreinforced Stone Beams
Another thought, for deep foundations, when supporting significant axial loads on in-situ bedrock, the local stone axial property is qualified by a tested core sample, a visual inspection for rock seams and aberrations and then the allowable load is divided by 5 or more. ASTM C99 - rupture test will probably get you some sense of what the stone can do.
RE: Large Unreinforced Stone Beams
The in situ field test is a good start, but I'm concerned that the stone will change. In 100 years, after seasons of freeze thaw, it will not have the same fissures etc.
Sure I will be gone, but out of a sense of moral obligation, I need it to be predictable, forever. That, or I need a fail safe system that will catch it, or create a predictable, slow
failure as opposed to a brittle collapse.
Regarding bedrock axial comparison, I see that as very much different than an isolated sample since the bedrock is contained in a stable environment, and is confined by
the surrounding rock mass.
The factor of safety is totally relative and is a shot in the dark. Why not FS of 6? 10? 20? It is a guess that has a pretty good chance of being over conservative.
At the end of the day the strength will be on a per specimen basis. The in-situ field test may be the way to go.
I will look at this ASTM C99 you mention.
RE: Large Unreinforced Stone Beams
I get it, but that is an impossible standard. Let's look specifically at the issues you have presented:
Freeze-thaw - You should probably get a geotech to have a look. If this is a sedimentary rock, then I would be very skeptical of using it as a beam. Igneous would be a good choice (unless it has a high iron content). Metamorphic is probably OK as long as the strata will be mostly horizontal. For surface freeze-thaw, scaling will present itself if susceptible. For surfaces and seams, a coating system may be able to seal the stone from the water.
Brittle collapse - Either accept a brittle failure mode and allow for it with a higher safety factor (like what we do for compressive failure elements) or supplement the stone with Dywidag bars cored through the stone.
Factor of safety - if you want to refine the FoS to a real number, you will need multiple core samples to run a statistical analysis. With the analysis you can determine the standard deviation and then pick an FoS that will land you 3 (or 4) deviations above the average. In my mind, an 18" slab spanning 10 ft is probably going to be around a FoS of 8. I'm all for being overly conservative with one-off designs.
RE: Large Unreinforced Stone Beams
It's probably worth digging around in his bibliography, his lectures on the structural design of cathedrals was a high point at uni.
More practically can you rebate a steel rod into the tension side of the beam, or even drill a hole through it and put in a proper tensioning rod.
Cheers
Greg Locock
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RE: Large Unreinforced Stone Beams
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RE: Large Unreinforced Stone Beams
Stonehenge and other old structures prove it is possible. Yes it is dependant upon the individual stones and their mechanical properties, and a thorough inspection for cracks, etc. 18" deep by 6'-5" wide by 10' long stone of unknown species. That means maybe a 7 or 8' span and two free edges. Is it a rough/natural shaped stone or is it a quarried piece of stone? Self weight maybe 300-350lbs./sq.ft. Why not have the mason turn the stone over, and on the bottom saw 3 or 4 saw kerfs, each 1.25" wide by 2" deep by 10' long, for .75" or 1" dia. tensioning rods, with stl. pl. washers and nuts at each end. Save the stone dust to color an epoxy joint filler. Treat this much like you would design a prestressed conc. beam. The turn of the nut method should be some indication of the rod stretch or tensioning force. You want to carry the dead load, plus a little, without putting tension in to top surface. Maybe cover the 20% roof surface above the stone with a copper roofing sheet, flashed under the main roof, and with small drip edges overhanging the stone, to keep all water/ice, etc. off of the top of the stone. Actually, you want some sort of a gutter edge over the front edge, over the entry. This should help waylay your long term cracking and failure concerns. Any failure will be a slow yielding failure as the stl. rods would start to stretch and yield, rather than a sudden breaking failure.
RE: Large Unreinforced Stone Beams
RE: Large Unreinforced Stone Beams
RE: Large Unreinforced Stone Beams
Not sure what is available for other stone products, but I would guess there must be some guidelines available for granite. There is also available information for Cast Stone Products (made of zero slump concrete) for architectural purposes.
Good luck!
RE: Large Unreinforced Stone Beams
have you looked into the possibility to do a continuous longitudinal cut that allows the embedment of a steel beam. Im sure a good mason can do slit the baby.it might even be a vertical plate with a bottom flange or with no flange but drilled through bars staggered for a composite action. I would then just analize the little guy as a steel member. Design bearing plates and done.
Just an idea
Best,
Rarebug
RE: Large Unreinforced Stone Beams
As hinted above, put a steel beam across the gap, very slightly higher than the top of the stone to carry all dead weight from everything above out and over to the two side columns, then down to the foundation.
Treat the stone lintel as a arch. feature: hollow it out to reduce weight that is not visible. Leave the front exposed face at full height, and the bottom that is exposed. The rest of the "L" can be removed at a tremendous savings of non-structurally strong (low-tensile strength) decoration.
RE: Large Unreinforced Stone Beams
thread507-337254: Structural Design of Stone
RE: Large Unreinforced Stone Beams
Consequentlty, I'm a big fan of Teguci's on site load testing proposal. Load her up to 4+X self weight and let it sit for a week. Additionally, I'd try to detail the supports such that axial restraint is minimized and bearing allows for support and slight rotation without inducing serious local stress concentration. Maybe neoprene bearing pads or, as mentioned above, a soft mortar bed.
I see this as being a bit like overhead glass design. As such, I wonder if you might employ similar redundancy strategies. Perhaps the design could be two, stacked stones (each designed to carry the full load alone) with vertical dowels connecting the two layers such that the lower could hang from the upper in the event of a failure. Ironically, the installation of any reinforcing or dowels is likely to introduce the very localized material flaws that would tend to initiate cracks.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Large Unreinforced Stone Beams
RE: Large Unreinforced Stone Beams
RE: Large Unreinforced Stone Beams
www.gov.scot/resource/doc/217736/0093791.pdf