Large Unreinforced Stone Beams

[bigmig]

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.

 

[concretemasonry]

bigmig -

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.

 

[Teguci]

If you are concerned with the stone's structural ability, you can assure yourself a moderate safety factor by setting the stone to span (build short masonry supports in the field) and then filling a bladder with water on top of the stone to check beam action.

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.

 

[bigmig]

Teguci

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.

 

[Teguci]

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.

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.

 

[GregLocock]

I would look at Jacques Heyman book "The stone skeleton" .

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


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[SlideRuleEra]

The 1997 book, Design of Renovations, advises caution in using stone for lintels. Even though there may be no load for this project, continuing the careful approach you are following is wise.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dhengr]

Bigmig:
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.

 

[77JQX]

Can you core the stone and run some steel through it?

 

[bookowski]

About a week ago I came across an existing stone lintel that had failed in shear, nice diagonal up from the bearing point all the way through. It was carrying a good amount of masonry load above so not the same as your situation. I wouldn't do it. You can hide some steel in there.

 

[jike]

One of the best references, that I am familiar with, is from the Indiana Limestone Institute of America

http://www.iliai.com/pages/Handbook.

Their handbook gives you details and design specifications with recommended safety factors.

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!

 

[RareBugTX]

BigMig:

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

 

[racookpe1978]

Divide the two loads into self weight and structural weight (dead weight of the roof and arch. features and roof beams above the stone.

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.

 

[Ussuri]

This thread might also be of some use.

thread507-337254

 

[KootK]

For appropriate stone material choices, I wouldn't be all that worried about changes over time with freeze thaw etc unless we're hoping for durability on a geological time scale. I suspect that your main concer here is simply brittle crack formation initiated by a local flaw and/or stress concentration.

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.

 

[bookowski]

Took a picture of this stone cracked lintel for you. Carrying a lot more than it sounds like you plan on but maybe still informative. Hard to tell in photo but this is supported on a column below and that crack is to the bearing point, can't have more than 3/4" bearing left.

 

[Brad805]

I assume your client is very well to do? If not, their first step should be to talk to a stone mason. Stone masons are not always easy to come by and they know it. Even if they are fine with it, I would still talk to a stone mason. Those still remaining talk great pride in their work and are usually a wealth of knowledge.

 

[Ussuri]

Helpful? Probably not.

http://www.gov.scot/resource/doc/217736/0093791.pdf

Lintels

The structural design of natural-stone lintels is not included within the British or British European Standards and it is therefore impossible to provide structural design calculations to confirm the adequacy or otherwise of a natural-stone lintel for a particular situation. For this reason it is normal practice for a stone lintel to be supported by a galvanised steel, reinforced or prestressed concrete inner lintel or beam. Figures 21 and 23 illustrate some typical examples.

While the adequacy of a natural-stone lintel cannot be confirmed by calculation, such lintels have performed satisfactorily for many centuries and may still have a role in this respect. As a general rule, provided there are no defects in the stone, a stone lintel should be able to support normal wall loads up to a span of 1.5 m without the need for a supporting inner lintel. When the bonded nature of a masonry wall is considered, the actual load on a lintel up to this span is quite moderate and well within the load-bearing capabilities of natural stone.

The minimum bearing length for lintels is defined in the British Standards but will be not less than 150 mm for a lintel spanning in the plane of the supporting wall. Effective tying-in of the ends of lintels (and sills) to the surrounding masonry is essential to the overall stability of the wall by helping to stabilise the opening and preventing its distortion under load. The height of a stone lintel will normally be dictated by the requirements of the wall courses and the facade design. Nevertheless, it is recommended that the minimum height for a lintel, up to a span of 1.5 m, should be at least 200 mm. Note, because of the weight of a stone lintel, the lintel must be handled in
accordance with CDM regulations.