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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 |
U.S.A. |
Germany |
Italy |
Japan |
France |
Spain |
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BS4659 |
AISI/SAE |
W-Nr DIN |
UNI |
JIS |
AFNOR |
UNE |
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
Initially the commonly used structural steel grade in South Africa was 300WA, which has a minimum yield stress of 300 MPa and a minimum ultimate tensile strength of 450 MPa. It was the steel with the most favourable strength-to-cost ratio and is the most readily available from suppliers.
Grade 350WA then became 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 was greater than that of Grade 300.
Now, the new steel grade in South Africa is S355JR. The minimum tensile strength of Grade S355JR is 490 MPa (for Grade 350WA it is 480 MPa), while the minimum yield stress depends on the thickness of the steel.
You should ensure that only commercial grade S355JR SABS approved steel for ALL building components is used by your steel building manufacturer.
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 emphasized 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 properly 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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