June 28


Met History [once and found her quite appealing] --

I have never seen bluestone or slate spanning vaults; which isn't to say (1)
they don't exist or (2) that the large bluestone or slate slabs I've seen
from above were not spanning vaults. What I do mean is: "I've never tried to
fix a sidewalk vault with anything other than granite at the combined
structure and wearing surface."

A quick check of the a few historical sources on stone -- excluding Mr.
Merrill's treatise, which I don't have at hand here -- reveals an
interesting comparision between the modulus of rupture for slate, sandstone,
and granite. The modulus of rupture can be used as a measure of the load
capacity of a stone in bending. The modulus of rupture for slate ranges from
7000 to 11000 pounds per square inch (psi) with an average of 8500 psi; for
granite from 1200 to 2200 (average 1600); and sandstone from 500 to 2200
(average 1500).

Whenever I encounter bluestone I assume it is a sandstone with a blue iron
oxide pigment. (Perhaps Ivan could give a rundown on the various
permutations of bluestone for the benefit of all here at BP). Classifying
bluestone as such allows me to sleep at night. If we put any faith in the
values provided by my historical sources, bluestone and granite have
comparable properties; slate rules. I'd be inclined to use granite over
bluestone in sidewalk applications because it is more securely and
predictably bonded (i.e., has more consistent cohesive properties). But, I
suppose it is possible to find a tightly bonded bluestone used as paving
over a sidewalk vault. As for slate, doesn't seem to be any reason not to
use it based on capacity in bending alone. Again, slate that isn't tightly
bonded -- like the slate on the roof of Horace Mann at Teachers College
(which is like blue foam) -- is probably a poor choice for sidewalk slabs.

While I have seen several granite sidewalks with spans of more than 12 feet,
I have seen as many (if not more), where the slabs of stone are supported
along their lengths on ferrous rolled beams (wrought iron or steel).
Supporting the beams along the long axis of the slab reduces the span from
more than 10 feet to less than 4 (typically). Now, whether the beams along
the length of the stone were original specified or part of a prior repair --
who is to know. The only reason I am there is that the beams have rotted
away.

Hope this is a help,

Go, Figure Man!!!

-----Original Message-----
From: Met History [mailto:[log in to unmask]]
Sent: Monday, June 28, 1999 12:27 PM
To: [log in to unmask]
Subject: Why are comm'l sidewalks igneous, resid'l sidewalks bluestone?


I queried Ivan "Resigned-from-BP-to-get-a-life" Myjer about something I have
observed regarding NYC sidewalks.  In the Tribeca/Soho areas (all commercial
buildings, 1860's-1900's) the surviving stone sidewalks are all granite (or
just igneous?), with long spans (no doubt to cover the underground vaults).
In residential/brownstone areas (1870's-1910's), surviving sidewalks are
smaller flags (because they have no need for the vault space, but they are
all SLATE OR BLUESTONE.

I had originally posited that slate or bluestone would fail at such a long
span, but Ivan's message (given below in full), says ix-nay.

Does another subscriber have another theory?

Signed,  Christopher "Once-had-a-life" Gray

BEGIN IVAN
Subj:    Re: granite vs. bluestone flagstones
Date:   6/28/99 8:41:50 AM Eastern Daylight Time
From:   [log in to unmask] (Ivan Myjer)
Reply-to:       [log in to unmask]
To:     [log in to unmask]

Granite and bluestone at 16" to 20" the thickness used for large flagstones
are equally strong in compression. I am not sure about tensile strength but
I would think that bluestone holds up as well.
Over the years I have been asked several times if I knew where to get 12
foot long slabs to replace bluestone pavers in NYC. I do not know if they
were over vaults. I suspect that at that length they were spanning vaults.

I suspect that the time of original construction is a factor. I do not know
when bluestone first appears as a flagstone. Merrill, writing in 1910 says
that in 1880 there were over 150 bluestone quarries in the area west of the
Hudson River below Albany.

Due to innovations is quarrying, large pieces of granite for construction
became available in the 1820's
Currently, bluestone is quarried on the New York side of the Delaware
River. Large slabs are no longer available due to the limitations of the
cutting equipment, not the stone beds.

Ivan
END IVAN