tranverse loading values for unreinforced CMU infill 1

[bigruledsurface]

While doing the demolition for a sidewalk replacement, the contractor uncovered an abandoned underground vault 6’ deep, projecting four feet from the building. The 6’ tall by 4’ wide opening from the vault to the basement was blocked in with 6” CMU set in a running bond. City requires that the vault be filled with compacted fill. Assuming the horizontal diaphragm above is rigid the required controlled fill will have the at rest pressure of 60 PSF/ft-of-depth. The basement walls for the 75 year old building are CIP concrete with the opening supported by steel above so boundary stiffness is not an issue. Likewise all perimeter gaps of the CMU panel are mortared [and will be pinned to the CIP if needed.] Failure mode likely would be through an overstressed horizontal joint acting as a plastic hinge. Even after the cohesive failure of that mortar course, intuitively the panel would still have plenty of strength. [design lat loading just north of 1k/lf] However I am lost how to calculate ultimate failure values.

 

[ishvaaag]

These values that follow are design strengths for masonries extracted from 5 references, to be compared with factored level stresses that account for any stability issues present.

Usual Values of fd
Ladrillo macizo (Brick till 10% voids): 20 kgf/cm2
Ladrillo perforado (Brick over 10% voids): 16 kgf/cm2
Ladrillo Hueco (Brick with transverse voids): 10 kgf/cm2
Concrete Block: 14 to 20 kgf/cm2
Ceramic Block: 16 kgf/cm2
Adobe and Tapial: 1 to 3 kgf/cm2
Cut Hard Stones such Granite and Basalt: 40 to 80 kgf/cm2
Rough Hard Stones such Granite and Basalt: 7 to 25 kgf/cm2
Cut Limestone, Sandstone: 20 to 40 kgf/cm2
Rough Limestone, Sandstone: 6 to 12 kgf/cm2
Cut Soft Limestone or Sandstone: 10 to 20 kgf/cm2
Rough Soft Limestone or Sandstone: 5 to 8 kgf/cm2

1 kgf/cm2 = 14.223 psi

Most likely can be used directly for your check against a plate representation of the masonry under the earth pressure.

 

[ishvaaag]

I include a nice australian (I think superseded but technically beautiful) reference

 

[ishvaaag]

And another beautiful (and it seems standing) document dealing with structural issues that also covers and has example on out of plane shear.

http://eurocodes.jrc.ec.europa.eu/doc/WS2009/3_am_Jaeger_a.pdf

 

[ishvaaag]

Still, of course to comment (and important thing) the former given values are for compression stresses. So essentially you are limited to pressures inside the central nucleus of inertia by that hypothesis. A former senior colleague in charge of the maintenance of the biggest temple in town (Basílica del Pilar, Zaragoza) repeated humorously that the tensile strength of masonry is nil. Well we can do something about that with the standing info but it is not as bad a statement as it seems.

So I was incorrect at proposing my first values as check against plate stresses if the plates made of the true thickness of the wall; it could be true for notional plates of included thickness and less than the wall thickness that follow a funicular or cuasifunicular surface of the standing pressures. For the flexural aspects etc use the other two references ... have more to look at so keep asking if not enough to cover what you want.

 

[ishvaaag]

When referring to "the former" I meant those in my first answer. The other two would be more to the point for flexure and shear, in and our of plane.

 

[ishvaaag]

out of plane ... not "our".

 

[BAretired]

Why not fill slowly with low strength concrete fill and avoid compaction?

http://www.google.co.uk/search?q=fi...crete+fill&gs_rfai=&pbx=1&fp=f8ad8039a5fcb36d

BA

 

[ishvaaag]

I really agree with BAretired in his proposal, and in fact proposed such thing when a worse case of the same was presented to me the past summer in Maluenda, Spain. A moorish village in middle ages, the village seems to have quite more than average subterranean vaults, one of which when affected by works at some house or some recent works at the roadway (two lanes, national) were showing pathologies related to settlement, and this one was discovered to be partly under the roadway itself. Since not a commission but just advice on behalf of my engineer brother I don't know what they have made about, except that investigative topographical work related to the exact position of the vault was in line. Of the inspected vaults, the faulty one had half of its ojival lime or concrete casing tumbled, and I quickly advised our onlooking advising team of no less than 10 people more or less qualified leave as quickly as they were wanting to comply, for there's never a reason to expose such quantity of people on such limited work if unnecessary.

 

[BAretired]

Another advantage of using concrete fill is that it prevents water infiltration which, over time could produce a hydraulic head against the masonry wall. If freezing conditions are present, frost pressure would likely destroy the wall in due course.

BA

 

[bigruledsurface]

Thanks for the helpful responses.

I did find a NIOSH study which conceptualizes transversely loaded dry-stack blocks as a three hinge arch and then empirically derives a formula through testing different blocks, thicknesses, and heights when stacked blocks are placed between two platens. the upper platen stays fixed
and the lower platen moves at a constant velocity [perpendicular to the wall] until crush zones are created on opposite edges of the block wall

http://198.246.98.21/niosh/mining/pubs/pubreference/outputid2871.htm

With a boundry condition of infinite rigidity the authors Barczak-TM, Batchler-TJ (2008)strongly correlated the following:

transverse wall strength = f_cx * (t/L)^2 where

f_cx = unit block compressive strength
t = block thickness
L = wall height

just for general interest: the site is in NYC and special permits, controlled inspectors, and specially licensed contractors are pretty much required any time over 50 yds of concrete gets poured. that alone might make the owner opt against concrete fill.