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New Zealand Engineering 1998 March
structural steel </h3>
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Snapshots
from the <FONT COLOR="#FF0000">Edge
<HR>By:
G Charles Clifton
<br />HERA Structural Engineer
<br />
<TABLE CELLSPACING=5 WIDTH="100%" >
<TR VALIGN=TOP>
<TD VALIGN=TOP><strong><FONT COLOR="#FF0000">After a year's break, this is the
first of two articles on structural steel planned for <em>New Zealand Engineering</em>
in 1998. Reflecting the wide range of readers' interests, each of these
will be general interest articles, albeit centred around the use of steel
in structures.</strong> </p>
<p>This article presents snapshots of leading edge developments in three
significant areas relating to structural steel. These cover the ongoing
preparation of estimating aids to enable confident pricing of steelwork,
innovative applications of steel in fire, and two issues relating to steel
and corrosion. </p>
<p>Confident pricing
of steelwork </p>
<p>An increasing number of new multistorey buildings are now using structural
steelwork.To assist project decision makers in evaluating different alternatives
at the preliminary design stage,the Heavy Engineering Research Association,
HERA,through its Steel Structures Analysis Service (SSAS), is developing
an estimating guide. </p>
<p>The release of HERA Report R4-96, <em>Structural Steel Estimating Guide,</em>will
be in June this year. This guide will cover the costs for typical multistorey
steelwork, in terms of material supply, fabrication, surface treatment
and transportation/erection. The principal fabrication costs are in the
connections and the guide will present costs for all common connection
types, for the range of UB and UC sections readily available, and for a
range of design actions. </p>
<p>The draft is currently being reviewed by industry; this is a vital step
to ensure that the data presented therein properly represents current market
rates in New Zealand. Project decision makers can then confidently price
steelwork for preliminary and ongoing project evaluation. </p>
<p>I will leave readers with the following important points with regard
to pricing of steelwork: </p>
<p>• The least weight option is not usually the least cost option </p>
<p>• Using a $/tonne rate can be misleading. </TD>
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<br /> The
burning issue of steel and fire</p>
<p>The 1996 HERA articles in <em>New Zealand Engineering</em> addressed this
topic. The first, in March, looked at individual steel member performance
in fire, the second, in June, at the links between structural fire severity
and structural damage plus the performance of portal frame buildings in
fire. The third and final article, in September, looked at the performance
of multistorey steel frames in fully developed fires.</p>
<p>The performance of steel structures in fire is an area of high interest
and activity within an exciting general topic of rapid growth, that of
fire engineering design (FED). Readers interested in subsequent and more
detailed information regarding steel and fire should study HERA Report
R4-91, <em>Design of Steel Buildings for Fire Emergency Conditions</em>,
while the behaviour and design of multistorey steel framed buildings is
addressed in the Vol. 10, No. 2 issue of the <em>Journal </em>of the New
Zealand Structural Engineering Society.</p>
<p>The new, performance-based Building Control System (BCS) offers New
Zealand designers wide scope to put innovative FED concepts into practice.
Some recent or ongoing examples where this has been done include:<strong>
<p>Sky City Casino,
Auckland</p>
<p>This impressive building occupies a 1.26 hectare site in central Auckland.
It comprises six levels of below ground carparking and between three and
seven levels of above ground gambling halls, restaurants, shops, conference
facilities, theatres and a hotel. The Sky City development used 3,000 tonnes
of structural steel, with another 650 tonnes in the Sky Tower. The bulk
of this (2,200 tonnes) was used in the six levels of below ground carparking,
comprising 70,000 m<SUP>2</SUP> of suspended floor area. Within this area,
concrete filled CHS columns supported a composite floor system, comprising
concrete slab on steel deck supported by secondary and primary steel beams.</p>
<p>Design for fire safety of the structural system under the BCS allowed
the following to be achieved:</p>
<p>• The steelwork in the carparking levels was unprotected (in
conjunction with a sprinkler system)</p>
<p>• Steelwork in the atrium and parts of the theatre were unprotected,
where a combination of low fire load and enclosure characteristics showed
that a collapse temperature would not be reached.</p>
<p>Arthur Barnett
Development, Dunedin</p>
<p>The Arthur Barnett Development is a five level, retail shopping complex
with 30,000 m<SUP>2</SUP> of suspended floor area, containing over 40 shops
and 360 carparks. It is located in central Dunedin.</p>
<p>The gravity frame comprises CHS columns and a composite floor system.
Lateral load resistance is provided by eccentrically braced framing. The
CHS columns are concrete filled for load carrying capacity under fully
developed fire conditions. This building occupies a full city block, with
no immediate neighbour. In addition, the fire detection and suppression
systems and evacuation procedures were given special emphasis, because
of the building's retail function. The probability of full fire development
was therefore assessed as so low that the consequences of its occurrence
were negligible in any risk/benefit study. An outcome of the FED was, therefore,
elimination of the need to passive fire protect any of the steelwork, in
exchange for upgrading the single-supply sprinkler system to a dual-supply
system with on-site water storage. The net result was a saving in cost
and simplifying of the construction, with an increased level of safety
and property protection provided against the effects of fire.</p>
<p>St James Theatre
Refurbishment, Wellington</p>
<p>The St James Theatre, Courtenay Place, Wellington has recently undergone
a major refurbishment and redevelopment. The building is fully sprinkler
protected. Innovations in the design for fire safety of the structural
system include:</p>
<p>• Steelwork in the stage house is unprotected where calculations
determined that a collapse temperature would not be reached</p>
<p>• Two new composite floors of approximately 2000 m<SUP>2</SUP>
area are supported on unprotected steel beams and protected columns, with
the floor system designed and detailed for dependable inelastic response
in the event of fully developed fire occurring following sprinkler failure.</p>
<p>Apartment and
hotel buildings</p>
<p>These have the following characteristics with regard to fire:</p>
<p>• Each room is typically a separate fire enclosure and each apartment
a separate fire cell</p>
<p>• The fire loads are relatively low</p>
<p>• The structural steelwork is hidden behind dry-wall linings.</p>
<p>These factors make it feasible to often use structural steelwork without
passive fire protection and this has been achieved on several recent apartment
and hotel buildings.</p>
<p>Rust never sleeps</p>
<p>Just how corrosive is the New Zealand environment? Is the severity of
the corrosion environments in New Zealand unusually severe, as proponents
of, for example, concrete roof tiles would have us all believe?</p>
<p>The answer is no. Corrosion environments are classified under AS/NZS
2312, using the environmental classifications given by ISO 9223. These
classifications are based on the measured first year corrosion rate of
steel obtained from specified weathering tests on steel coupons. They are
then linked into atmospheric conditions such as the level of atmospheric
pollutants, rainfall, temperature and humidity.</p>
<p>BRANZ has undertaken a comprehensive testing programme on mild steel
coupons, such that the first year corrosion rates for mild steel throughout
New Zealand are known. NIWA has recorded the atmospheric conditions at
many of these test sites. Using this data, it has been possible to compare
mild steel corrosion rates at similar locations (eg. distance from sea,
type of local environment) between New Zealand and the United Kingdom.
The results show the New Zealand environment to be significantly less corrosive
than the UK environment.</p>
<p>Why might this be so? The probable answer lies in the fact that the
corrosivity is strongly influenced by the concentrations of both chloride
and sulphur dioxide in the atmosphere. New Zealand, like the UK, has high
levels of chlorides (salinity) near the coast, however these levels decrease
rapidly with distance inland.</p>
<p>Atmospheric sulphur dioxide concentrations are reasonably constant across
all regions in both countries, however, ambient levels in New Zealand are
only about four percent of ambient UK levels. Thus the corrosion-inducing
effect of atmospheric sulphur dioxide is much lower here than in the UK.</p>
<p>ISO 9223 requires the environmental classification for corrosivity of
a site to be determined using measured steel corrosion rates or atmospheric
conditions. AS/NZS 2312 also gives this option, for which the BRANZ test
data mentioned earlier can be used. HERA recommends this as the preferred
method of assessing site corrosivity and can provide further guidance.
AS/NZS 2312 provides a second option, through generic descriptions for
each environmental classification. These tend to be conservative, in that
they overestimate the atmospheric classification to be used for a given
site compared with that which would be obtained using measured corrosion
rates.<strong>
<p>Inorganic zinc
silicate paints</p>
<p>Inorganic zinc silicate (IZS) paints consist of zinc in finely divided
metallic form, dispersed in a self curing, inorganic silicate medium.</p>
<p>They have been commercially available since the 1940s and used in the
protection of steel in all climate conditions, either as a single coat
system or as the primer base in multi-coat systems.</p>
<p>Their principal role has traditionally been the latter, however, observed
performance in practice and comparative weathering tests have shown them
to be most cost effective when applied as a single coat system. In this
role, they provide sacrificial protection to the steel surface, functioning
as a "slow-release" zinc layer.</p>
<p>In this single coat role, they are easy to apply and very cost effective
to maintain. Further details on single coat application, the preferred
mode of use for IZS paints, are available from HERA.</p>
<p><strong>Charles Clifton</strong><em> is the HERA structural engineer</em></p>
<p><em>Fax: 0-9-262 2856, Email: <a href="mailto: structural.hera@wave.co.nz">structural.hera@wave.co.nz</A></em></p>
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