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A
Little Gem About Steel Building Foundations |
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Your steel building will need some type
of foundation on which to anchor the
columns and thereby tie the whole
structure together. Some agricultural
buildings are designed to have dirt or
gravel floors and consequently use
concrete piers (footings) instead of a full slab;
however, a concrete slab is best for
most situations.
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See
also About
Concrete.
Whatever foundation you
use, we strongly recommend that you
employ an experienced local foundation
engineer to design it. Your erection
drawings will include an
"anchor-bolt setting plan with
reactions," which will give a
qualified engineer the necessary data to
design a suitable foundation for your
building. You
should note that the engineer will also take a
good, hard look at the soil. The soil conditions
are of utmost importance and should under no
circumstances be ignored. To
give you a slightly better insight into this aspect, read
a little about ground conditions below. And be
sure to also read our section on "How Does This
Relate to Steel Structures" further
down on this page.
Ground
Conditions
If
a building is to be constructed in an area with
clay soils one should be aware that clay soils
have been found to expand 23 cm or more if
subjected to long cycles of drying or wetting,
thus producing powerful forces that can shear
foundations and lift lightweight buildings. Some
soils with high organic content may, over time,
compress under the building load to a fraction
of their original volume, causing the structure
to settle. Other soils tend to slide under
loads.
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Soils
that have been modified in some way often perform
differently, especially when other soil has been added
to or mixed with existing soil, or when the soil has
been made wetter or drier than normal, or when cement or
chemicals such as lime have been added. Sometimes the
soil under a proposed building varies so greatly over
the entire site that a building simply cannot be
constructed safely or economically.
Soil
and geological analyses are necessary, therefore, to
determine whether a proposed building can be supported
adequately and what would be the most effective and
economical method of support.
If
there is sound bedrock a short distance below the
surface of the construction site, the area over which
the building loads are distributed can be quite small
because of the strength of the rock. As progressively
weaker rock and soils are encountered, however, the area
over which the loads are distributed must be increased.
So,
if you are perhaps starting from scratch and looking for
some land to build on, do
not purchase an Erf if you have even the slightest
suspicion of soil condition problems - it can cost you a
small fortune. Remember; the cheapest stand may not
always be the least expensive. |
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Take
a look at this little story:
An
important institution and had the desire to build a
fitting monument to its achievements. They decided that
a tower of great architectural beauty would be built in
close proximity. Since the site was close by, ground
conditions were well known and were taken for granted.
Besides, soil investigations always take time and add
considerably to the cost. Was this a recipe for failure?
Of course it was.
Construction
commenced but when the tower reached four storeys
tilting became evident and construction was halted.
Years later, construction was resumed only to be halted
after adding four more storeys. More years later, a
penthouse is added off-center in an attempt to
redistribute the load. Even so, the penthouse floor was
so out-of-level that one side had to be raised by over
30cms (12 inches). Under the added load the tower
continued to incline, and then became several degrees
more out of plumb, and was consequently never used for
its originally intended purpose.
Today,
the tilting continues to progress, albeit very slowly;
but by now the tower has engendered a great fondness so
that every effort is made to save it. Many and various
are the efforts to stabilize the tower and even to right
it, but more often the condition is aggravated rather
than improved. Needless to say, dignitaries and
politicians wade in with threats, counter threats and
law suits. |
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If
you haven't recognized it, this is the story of
the Leaning Tower of Pisa in central Italy.
Begun in 1173, the tower was finally finished in
1370 and still exists to this day. The tower is
renowned world-wide and thousands of tourists
come to see it every year. Recovery has engaged
the best architectural, engineering and
geo-technical minds in the business, and
hopefully the delicate engineering works
currently underway will save it from final
collapse. That the building has survived for
over 800 years is remarkable in itself,
considering the short life span of most of our
modern buildings. By these measures, this
initially disastrous project has become a
valuable asset and an outstanding success.
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Our
point is however that all this drama occurred ONLY
because the soil conditions were never properly checked.
For
a detailed history of the Pisa tower, visit: http://www.endex.com/gf/buildings/ltpisa/ltpisa.html
Purely
as a matter of interest, its lean is 4 metres (13 ft)
from the vertical. Legend has it that Galileo took
advantage of this inclination to carry out experiments
with weight in free fall, dropping weights from the top
of the tower.
How
Does This Relate to Steel Structures?
The
portal frames are the main structural elements of the
building, being so to speak its "skeleton".
They consist of S355JR columns and rafters, and the
column (footings) foundations. The frame joints at B, C
and D are shop welded and site bolted, and are designed
and fabricated to achieve a rigid structural unit.
The
frames are designed for the following loads:
A
= Roof loads such as workmen, snow or hail.
B
= Wind - from the sides or as an up-draught.
Vertical
loading on the frame as depicted in figure "A"
above results in the columns (poles) A and E tending to
be pushed outwards. If the foundation cannot resist this
horizontal push, outward movement will occur, and the
frame will lose structural strength.
Wind
(as depicted in figure "B" above) subjects the
portal frame to uplift forces (the roof tries to
"fly-off - just like an aircraft wing), to
overturning forces on the sides and ends of the
building, and also to "drag" forces on the
roof and sides of the building.
The
effect of wind on a steel building cannot be
overemphasized. The destabilization it causes is a major
design consideration, and in this context, foundations
can be regarded as the building’s "anchors". |
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Trust
our years of positive construction experience
and craftsman expertise to confidently guide your
specific building project smoothly to sure success.
You
may just experience enormous benefits.
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And
please always remember John Ruskin's famous words:
The
common law of all business prohibits paying a little and
getting a lot - it cannot be done. Although it is unwise
to pay too much, it is far worse to pay to little - for
when you pay to little, you sometimes lose everything.
Therefore, if you deal with the lowest bidder, it would
be wise to add something onto the price for the risk you
are running; and - if you do that, you had enough to pay
for something better in the first place.
Do
You Perhaps Require A Quotation?
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you would like us to provide you with a quotation on
a product or service please click
here to complete the quotation form.
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you should have any difficulty or objection to complete
the request form, just send us a normal e-mail. The
e-mail link is just a little further down.
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Bresbou Construction
CC, Pendulum Construction CC,
Building
- The Trust Consortium CC and Universal Coverings (Pty) Ltd
Tel:
+2711 453 4401
Cell:
+2784 303 8179
Fax:
086 660 0845 (Local SA Faxes ONLY)
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