Capacity of limestone Stair Threads 1

[P205]

Does anyone have references for calculator the capacity of limestone steps? I'm looking at a project where the client wants 7"x10" limestone pads to form the steps which will span roughly 5 ft. I know it would depend on the quality of the rock itself.

Thanks

 

[P205]

I found the attached in an old (1889) textbook. Any thoughts on the method being presented?

 

[KootK]

You, my friend, need a copy of the Indiana Limestone Handbook

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.

 

[Ron]

A few things to consider:

Limestone is a natural material and has inherent imperfections that are sometimes not apparent, particularly in bulk sections

Even though Indiana limestone is much more consistent than limestone in the lower US, it remains a sedimentary material that is relatively soft...not yet morphed to marble

When you analyze the section, don't forget the concentrated load check as well, keeping in mind that a flexural failure will be brittle, not ductile

Steps are often on a critical life safety path...consider this

Think about bonding a 1/4" thick aluminum plate to the tension side of the section to add ductility to any failure that might occur. It can be completely hidden so that it doesn't detract from the appearance

 

[NITTANYRAY]

I agree with Ron that the treads should be reinforced. Rather than aluminum I would recommend carbon fiber strips.
Sika Carbodur strips is a product that I have had success using.
On stairs, impact must be considered as someone might run up or down the stairs hence the need for tensile strength on the bottom side of the treads since brittle materials don't always perform well for impact. That is one reason why I discourage architects from using glass hand rails on balconies and stairs.
I saw one shattered once one someone fell against the railing.

 

[Ron]

Nittanyray....agree that carbon fiber would be better than aluminum for this application. Good call.

 

[P205]

Thanks everyone for the responses.

Considering the application is a residential house, and the small span, I'm tempted to not reinforce the bottom. Still considering it though.

I think I'll keep the reinforcement option in my back pocket, and aim to get mechanical properties data from the quarry they are sourcing the limestone from. I've also requested a visual inspection of each slab prior to placement.

I know I can't hang my hat on this, but this project is in "Limestone City" (Kingston Ontario) where many many buildings are built out if this limestone which is considered to be of high quality.

 

[oldestguy]

Is this interior or exterior? If exterior, I'd go for reinforced concrete and the stone for a surfacing only. With interior and reinforcing or supplemental support below, I question a suitable means of securing to the stone for a lifetime without the need for maintenance or suitable replacements as needed. .

 

[P205]

It is exterior application. Originally I had designed it as a concrete stair and landing with a stone veneer but they later changed their minds and want solid 7" thick stones.

We may opt to backfill the underside of the stairs to provide continuous bearing for the stone slabs.

 

[Scratch3r]

I have worked with thin stone veneer. I came across this handbook. [URL unfurl="true"]https://www.indianalimestonecompany.com/technical-data/ili-handbook-and-technical-notes/[/url]

I have another book "Stone Cladding Engineering" by Rui de Sousa Camposinhos that has a modern approach in line with Eurocode.

Try running FEA on the panels and check the stress concentration. You should get stone specifications from the supplier. They meet ASTM standards. Stone is not man-made or highly tested and graded, it's like the wild west. You get a few properties and soldier on.

If you're limited to 1 1/4" thick I would say you are going to have to add a stainless angle support. 25 PSF was the practical limit for wind loads +/- at about that span. Its cheap insurance.

 

[oldestguy]

Don't forget that area will have frost heave. Also, What keeps the "backfill" from gradually eroding or sloughing down onto the next step or, as risers from tipping outward? Maybe use the stone also as risers also and set the whole thing on reinforced concrete that won't be seen and will be uniform in dimensions. For all this fancy a stairway, some extra spent on the support is cheap insurance of a good long term job.