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South
African Steel
Unfortunately
the world has a proliferation of standards, which makes
it very difficult for prospective customers to analyse
the steel quality of a particular country. The
International Organization for Standardization (ISO)
seems to argue back and forth on standardisation
world-wide but at present there are multiple
registration schemes all over the world. What is needed
is a combined system for quality, the environment, and
safety to eliminate the current system of multiple
certificates. Maybe within the next three to five years,
we'll see a major change [in certification]. Below are
some examples of present certification.
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Great
Britain
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U.S.A.
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Germany
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Italy
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Japan
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France
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Spain
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BS4659
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AISI/SAE
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W-Nr
DIN
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UNI
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JIS
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AFNOR
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UNE
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In
South Africa our steel is produced by Mittal Steel,
which is the largest steelmaker in the world, with shipments
of 42.1 million tons and revenues of over $22
billion in 2004. The product of a merger announced in
October 2004 between LNM Holdings and Ispat
International, Mittal Steel is the world’s most global
steel producer with steel-making facilities in 14
countries and sales and marketing offices in a further
11.
Mittal
Steel is among the most efficient steel producers in the
world. They encompass all aspects of modern steelmaking,
combining both integrated and mini-mill facilities and
producing much of the iron ore and coking coal used in
their furnaces. They are also among the most advanced
steel makers, operating a range of modern technologies.
They pioneered the use of direct reduced iron (DRI) as a
raw material source and are now the world’s biggest
producer of DRI. With two technical research facilities,
their product development teams are ready to meet the
needs of the most demanding customers.
If
interested you can read more about this huge corporation
by clicking on www.mittalsteel.com.
South
African Steel Grades
The
commonly used structural steel grade in South Africa
is 300WA, which has a minimum yield stress of 300 MPa
and a minimum ultimate tensile strength of 450 MPa. It
is the steel with the most favourable strength-to-cost
ratio and is the most readily available from
suppliers.
Grade
350WA is also available if required. The
modulus of elasticity (E value) for all steels is the
same, so it is obvious that a beam in, say, Grade 350
steel will have a deflection about 17 per cent larger
than that of a beam in Grade 300 of equal depth and
stressed to the same percentage of the yield stress.
But the use of this grade should be considered
carefully. The use of a higher
strength steel could be justified in heavy welded
plate girders, in the columns of large multi-storey
buildings to maintain the same serial size for the
full height of the column and in the construction of
heavy box columns. It must be remembered, however,
that the cost of welding high-strength steel is
greater than that of Grade 300.
Furthermore,
such steels would need to be rolled specially and the
mill would require an order to be placed well in
advance and for a certain minimum tonnage per section
or plate size. For any particular project, the cost
per ton ratios of Grade 350W and Grade 450W to Grade
300W steel must be obtained from the steel mills.
Tolerances
that have a bearing on shop fabrication are:
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mill or
rolling tolerances, which relate to the dimensions of
sections and plates and
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fabrication tolerances,
which have to be observed by the workshop personnel in
the fabrication of the steelwork.
Rolling
tolerances are the allowable deviations from
cross-sectional dimensions, cross-sectional
squareness, straightness, specified mass per metre and
plate thickness.
There
is no South African specification in this regard, but
all sections rolled locally are produced within the
tolerances laid down in BS 4: Part 1, ISO-R657, DIN
1025 and DIN 1026 for the various types of section.
They are reproduced in the SAISC publications South
African Steel Construction Handbook (Ref. 5) and Structural
Steel Tables (Ref. 9).
Out-of-squareness of column
flanges may require shimming of seated beam-end
connections, cross-sectional variations at column and
beam splices can be avoided by matching the two parts
of the member, off-centre webs at beam splices will
require adjustment of the holing in the flange splice
plates, etc. If such variations can be anticipated and
are allowed for there will be a saving in assembly
time.
Fabrication
tolerances are specified in SABS 1200H and should be
carefully observed in the workshop. They include
permissible deviations on the depth and width of
welded cross sections, the flatness of webs, the tilt
and warpage of flanges, the overall length of members
and the straightness of members. The general tolerance
applicable to dimensions of members and components,
and to the location of holes, is ± 2 mm. The
numerical values of tolerances are based on practical
fabrication procedures and are not difficult to
maintain provided the shop personnel are aware of them
and are reasonably careful.
The
Basic Fundamentals of Steel Buildings
No matter who supplies your steel building, you're
more likely to have a positive experience if you
follow and understand a few basics:
The
three components of the design function that have the
greatest influence on economy are:
a)
The choice of the correct framing system
b)
The efficient design of the structural members
comprising the frame
c)
The use of simple connection details
The
two basic cost factors in structural steelwork are the
mass of steel material involved and the unit cost of
fabricating the material. The cost can accordingly be
expressed as Mass (in tons) x Cost (in Rand) per ton
or, put more simply, Price = Mass (tons) x R ton. Both
of these components are important, but it is essential
to keep them in balance.
Some
designers are unduly motivated by mass-saving in their
designs without realising that the second factor,
namely production cost per ton, is the more critical.
It must be emphasised that minimum-mass design is
seldom the cheapest design.
It
is not too difficult to produce a design of low mass.
This can be achieved in a variety of ways, such as:
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using a different section size for every member in a
lattice girder or for every beam in a floor according
to their particular loading;
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specifying slender plate girders instead of heavier
rolled sections;
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using intermediate stiffeners on plate girders to
reduce the web thickness;
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stiffening column base plates to minimise the bending
of the plate;
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specifying column web stiffeners at beam-to-column
moment connections instead of using a thicker column
web;
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making beams fully-continuous by means of
site-splicing;
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curtailing girder flange plates;
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using groove welds instead of splice plates end plates
and fillet welds;
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adopting non-standard connection details; etc, etc.
However,
every one of these mass-saving measures will result in
a significant increase in labour input. Whilst it is
easy to calculate the steel-mass component of the
above equation, it is far more difficult to assess the
rate-per-ton component to allow for the extra labour
content.
Labour
costs may account for up to two-thirds or more of the
ex-works price of steelwork. It is thus important that
consideration be given to minimising labour content in
order to reduce overall cost.
An
argument sometimes put forward in favour of the
minimum-mass solution when applied to competitive
designs is that one will arrive at a lower total price,
placing one in a better position to secure
the contract. This is a short-sighted view, since the
manufacturer will be faced with the problem of having to
produce complex steelwork that has been priced at
unreasonably low rates.
Without
doubt there is hardly a structural design that could
not be built better at equal or lower cost through
careful attention to efficient, economical details.
The
savings that can be made by rationalising member
sizes, adopting simpler details, using standard
connections, etc are all invaluable contributors to
creating a cost-effective end product.
Brief
Erection Description
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Level
each column foundation and install suitable flat steel
packing
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Erect
the two columns, eaves tie and vertical bracing on
their side of the braced bay
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Access
to the column tops to be achieved by using either an
extension ladder suitably supported, a scaffold tower
on wheels or a mobile vertical man hoist
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Splice
each of the braced bay rafters on the ground and
install guy ropes at mid-span
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Hoist
the two rafters in sequence using a strongback, bolt
them to their portal columns and tie the guy ropes to
suitable anchorage points each side of the rafter
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Access
to the columns tops to be achieved as noted above
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Install
all braced bay purlins in sequence working from the
eaves to the apex.
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Install
all horizontal rafter bracings
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Access
to install the above components to be by the above
noted method of the column tops and then climbing
along the rafters using suitable safety harnesses and
fall arrestor straps tied around the rafter at all
times
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Check
that the columns are true and plumb and that the
rafters are straight before fully tightening all
connection bolts
(b)
Erection of Remaining Bays:
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Level
each column foundation and install suitable flat steel
packing
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Install
column and eaves tie at each side of structure
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Splice
rafter using a strongback at ground level and install
guy ropes at mid-span
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Hoist
rafter and bolt to columns and tie guy ropes to
suitable anchorage points either side of rafter
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Install
all purlins from eaves to apex
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Erect
balance of bays in sequence following the above
procedure for each bay
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Check
plumb and line of columns as erection proceeds and
adjust where necessary
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Ensure
that no rafters are left supported only on guy ropes
over night in case a strong wind comes up which could
collapse the rafter
(c)
Erection of Gable Ends
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Level
each column foundation and install suitable flat steel
packing
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Erect
gable columns to extent of gable rafter splice
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Install
purlins over extent of erected rafter section
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Proceed
in sequence until the gable and is complete
(d)
Final Bolt Tightening
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After
doing visual inspection on alignment and checking the
verticality of columns, ensure that all bolts are
poorly tightened
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If
any high strength friction grip bolts have been
specified, tighten them as specified, and mark them as
tightened
(f)
Weather
Conditions During Erection
Conditions
under which work should not be done:
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No
steelwork
should be erected when the wind speed at the
level of the eaves of the building exceeds 20 km/h
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No
steelwork
should be erected while it is raining.
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No
person
should be allowed to climb on steel that is wet,
icy or frosty
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A
careful safety-assessment
should
be made before
personnel are allowed to handle any steel that is wet,
icy or frosty.
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