[GSBN] Jumbo Bale Question

[martin hammer]

Hello Ben, Derek, and all,


On 5/4/09 4:20 PM, "Derek Roff" <derek at unm.edu> wrote:

> --On Monday, May 4, 2009 4:37 PM -0500 Ben <bobregon at austin.rr.com>
> wrote:
> 
>> There is the possibility that Fort Hood (US Army Base near Killeen TX) will
build a 4 story 120,000 LEED Platinum straw bale facility with a living roof and
a helipad.

What is this "facility"? (Maybe you can't say?)  I wonder if a reason for
the fixation on jumbo bales is ballistics (as John S. also suggested).  A
wall of JBs would make a heck of a bulletproof barrier.

When you say 4 story, do you mean 35-40 ft high walls with a roof only (no
floors)?

> Texas isn't known for earthquakes, but I found reports of a 5.6
> magnitude earthquake as recently as 1995.  A four story wall would
> exceed the recommended height to wall thickness guidelines from
> Martin's draft of the California code, printed in Bruce's book.

I wouldn't let the 6:1 h/w ratio in the draft code decide the height limit.
There are ways to extend that ratio.  The Pakistan houses employ very stable
8:1 ratio walls just by using opposing external bamboo "pins".  Jumbo bales
(laid flat) at 8:1 would yield a 32' high wall (almost 4 story).  You just
need 32' tall bamboo!  Or more likely steel external pins?

More realistic though, is the practice of dividing tall walls into stacked
shorter walls with box beams.  Or to buttress the walls (or long box beams
if necessary) inside or out in some manner.  Whatever the specific solution,
it needs to be engineered (btw, which the draft code allows when exceeding
the 6:1 ratio).

As another point of comparison, the maximum aspect ratio for adobe
construction in the International Building Code (2109.8.4.2) is 10:1. Straw
bale codes to date have used a limit of 5.6 to 1.

>> The same goes for structural data on what a 4'(w) x 3'(h) x 8'(l) bale can
hold up.

Bruce King does an excellent job explaining the factors affecting bearing
capacity on pages 84-85 of Design of SB Buildings.  (You can also review the
various load bearing test results there.)  The mathematical equation shows
bearing capacity as a function of the compressive strength and thickness
(cross-sectional area) of the plaster (times a safety factor).

At least in theory, it is the skins only that determine an SB wall's bearing
capacity, because the plaster skins, not the bales, carry the load (as the
stiffest element of the wall assembly).  So, counter-intuitively, it doesn't
matter if the bale width is 1 ft or 4 ft.  There are probably limits to this
logic (engineers' opinions welcome).  And certainly the relationship between
gravity load and wall height and aspect ratio needs to be considered
relative to potential buckling.  (Similar consideration should be made for
out-of-plane wind and seismic loads.)

800 lbs per lineal foot has come to be the accept-a-bale (I couldn't resist
that pun) bearing capacity of a plastered SB wall (with cement or
lime-cement plaster), regardless of size or orientation of the bales.  But
there are differences in the various US SB Codes.  (Sometimes stated as 360
or 400 lb per sq.ft., and sometimes implying the capacities are valid with
lime or earth plasters as well.  The Austin, TX code says 400 lb/sq.ft. with
any plaster.)

Table L-105A in the draft code states allow-a-bale (pun variation) bearing
capacities, as a function of plaster type.  Cement, cement-lime, and soil
cement all have a stated capacity of 800 lb/ft.  Lime plaster is 450 lb/ft,
and clay plaster is 300 lb/ft.  These values come from detailed analysis by
me and Kevin Donahue (engineer involved in the out-of-plane and other
tests).  They employ the equation in Bruce's book, but use safety factors of
10 or more and take into account many factors.  Eventually those numbers
will be revisited and adjusted, especially if additional testing occurs.
(Note:  All values assume plaster on both sides of wall)

A word should be said about asymmetric skins.  Skins on opposite sides of a
wall and of different stiffness (in particular, clay plaster and any plaster
with substantial cement binder) could present a problem in a load bearing
wall.  It's hard to imagine a serious performance problem, but maybe in some
situations. To be conservative the wall's allowable bearing capacity would
be that of the weaker plaster.

I do think in the case of jumbo bales, it's also worth considering their
unplastered bearing capacity.  This is a great quality of all SB walls,
regardless of bale size.  That they have reserve bearing capacity that would
very likely avoid wall failure if the plaster (or other load bearing system)
is compromised or fails.  But jumbo bales have such a wide footprint, it's
especially worth looking at them unplastered.

Assuming normal bale density, I would say walls of unplastered jumbo bales
laid flat can support 400 lb/ft.  They could support this either
pre-compressed or not.  The only question is how much the walls will
vertically compress under load.  I arrive at this number by using an
allowable load of 100 lb/sq.ft., taken from the low end of a 70-287
lb/sq.ft. range in various unplastered bale load tests (that range includes
a safety factor of 4 relative to ultimate load).

Lots of words, but this question demanded an answer this long.  Maybe I just
should have said - read Chapter 4 and the draft code in Bruce's book.  But I
hope I've provided more than that, including thoughts "between the lines".

Those not in the US, please forgive that my numbers are not metric.
Appendix L in DSBB gives both English and metric, except the tables, which
list conversion factors.  DSBB generally gives both.  Bill C., what's GSBN
policy on this??

And with a reference way back to the beginning of this e-mail . . .

Martin (I won't go ballistic) Hammer