[Ferro List] Fiberglass mesh for LFC subterranean home: Strength & EMF

[chuck east]

Many thanks, Keith.  Uwe also enlightened me re Faraday's cage putting me at
ease re EMF.  As you've suggested, with so many other issues at play I will
complete a work-up of specs and design during the months to come.  However,
as a primer, my wife and I are looking at property in France but haven't
settled whether to live near Bordeaux or in the Alps but the latter is most
likely where perhaps the advantages of subterranean living will be more
completely realized.  We aim to be as energy efficient as possible (PAHS or
AGS?) with full-up eco-friendly technologies and processes in play.  But as
you've suggested I will focus on greater specificity - which I must do
anyway - and benefit from the fc.net knowledge base and advice you and
others so generously offer.
Best 
Chuck  

-----Original Message-----
From: list-bounces at ferrocement.net [mailto:list-bounces at ferrocement.net] On
Behalf Of Keith B
Sent: Thursday, January 24, 2008 8:43 PM
To: Ferrocement Discussion List
Subject: Re: [Ferro List] Fiberglass mesh for LFC subterranean home:
Strength & EMF

Chuck

Glass fiber is not a preferred reinforcement for FC, for reasons other 
than cost and chemical attack.  Most importantly, its tensile behavior 
is purely elastic, resulting in sudden, brittle fracture when it fails.  
Steel reaches a yield point, then stretches some plastically, giving 
warning and a substantial reserve of energy absorption during failure.  
(Sudden steel structural failure does occur, but almost always when 
bolts, rivets or welds break.  FC doesn't have critical weak spots like 
that.) Unlike steel, you can't bend glass to shape and have it stay that 
way, a major convenience in construction.  Well designed structures have 
working stresses a long way below expected failure levels, but fatigue 
is a problem at any level for pretty much any material other than 
steel.  That doesn't fatigue if the loads are below a critical value.

Perhaps more to the point, if you are building a barrel vault, tensile 
strength is likely to be a minor or non-issue.  You can do perfectly 
well with unmortared bricks or blocks of stone.  Within limits, arches 
are only loaded in compression, so no tensile strength is needed.  
Granted, a drystone arch is a bit leaky, needs to be fairly thick to be 
stable, stability does become a concern with exotic curves, and they 
aren't the easiest things to build even if stable when finished.  
There's certainly good reason to build a concrete arch instead, and more 
to distribute any steel reinforcement used following FC principles 
rather than conventional rebar.  Consider though a double layer of 6" 
WWM (welded wire mesh) offset to give 3" holes, rather than anything 
exotic.  If you worry about EMF, bond it together and across the floor 
and you'll live in a Faraday Cage. 

Look closely at just using PVA fibers with no steel.  Another option is 
a ribbed arch, perhaps a hybrid with a single rebar in each rib and an 
FC web.  Various schemes have been looked at and may be found in the 
archives.  Slip forming is a bit much for a "one off", but a simple full 
arch form repeatedly moved lets all work be done from the top and by 
pouring.  Spray layup onto an inflated form suits such a simple shape, 
and the archives have a lot of neat stuff on making custom inflatables. 

One could go on and on, but perhaps the best advice, since you have time 
to learn and think, is to tell us where you are, what your site 
parameters are (or what you are/will be looking for) and what your 
climate will be like.  Are you looking to be heavily solar, just 
efficient, or planning to go full PAHS?  The better we understand your 
interests, the easier it is to select from almost infinite options.

kb

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