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7/18/2019 Economics of Steelwork Design
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Paper to be read before a meeting of the Institu tion
f
Structural Engineers at Upper Belgrave Street, London
W l X
HB on Thursday
13 October 1977.
The economics
of
steelwork
design
F.
H NEEDHAM
A graduate of Impe rial College. Mr. M eedham spent the early
years of his career gaining experience in design of steelwork with
joini ng the steelindustry in 1962. From 1962 -70 he headed the
fabricators, consulting engineers and a local authority, before
structural engineering section of the British Iron Steel Research
Association, Battersea. and in 1970 was transferred to the
headquarters of Redpath Dorman Long Ltd., Bedford, as headof
Product Research and D evelopment Department. Since October,
1974 he has been Chief Engineer, Design D evelopment at
Constrado, Croydon.
He served on the Council of the Institu tion from 1963-66. and
during th at time was on the Science and Research Com mittee and
the Literature Committee. He wasa founder committee member
of the Bedfordshire
Aao inin g Counties Section of the
Institution. being Chairman in 1973-4. He currently serves on the
Institu tion Code Servicing Panel on Stability. He has been oin t
author of tw o revious papers presented o the Institution. for
one
of whichhe was awarded he Henry AdamsAward
Diploma in 1967-68.
He serves on a number of BSI committees, notably B.716 and
8.20. being chairman of the B.2012 General Princip les
sub-committee.
Synopsis
This paper sets ou t n qualitat ive terms guidance for engineers
n
ach ieving the economic
se
of structuralsteelwork.t
identif ies the major cost i temsn the total construction process
and emphasises the importance of their re lativealues. Then
the cost i tems of the steelwork i tselfre given and discuss edn
detai l and the paper indicateso w m o n e yan be saved or
wasted depend ing on the approach adopted by theesigner.
It ind ica tes ho
w
a knowled ge o f thearious steps involved
n
the steelwork contractor s function can be of great value
n
deriving the most economic structuralsolution.
The Structural Engineer/September 1977/No. 9/Volume
F.H. Needham B S ~
Eng FIStructE
FICE
Chie f Eng ineer, Design Deve lopment C onstrado
ntroduction
In considering the economics of steelwork esign, one is fre-
quently asked to give estimates on a costltonne basis for
various kinds of structural steelwork. Quite apart rom he
impossibility of quoting figures in money terms at a time of
rapid nflation and changing elative values, such nquirers
frequently overlook the fact thathe form of structure and he
manner in which
t
has been designed can have a profound
effect upon the overallost. Theaim of thisaper is to high light
some of the factors which need to be considered n order to
produce designs which satisfy the client andwhich the steel-
work contractor can produce and erect to schedule and at a
profit.
BeforeLproceeding
t is wor th defining 'design'. To he
academic or computer special ist
t
usually means analysis to
determine forces, moments and shears in apredetermined
structure under idealised oading. To the steelwork designer
or detailer i n a abricator's office t means the selection of
member sizes and the determination of beam reactions and
moments which are shown ona line diagram (frequently wi th
no thought eing given o overall behaviour y either of them).
To the workshop a good' design is one which gives a high
tonnage output bonus with the minimum of trouble. To the
erector a 'good' design is one in which allarts are of the ame
weight, and is one with the greatest proportionof nter-
changeable parts, the remainder being so different that they
cannot be wrongly transposed. Particularly good is one in
which only one bolt size is used. The meaning of 'design' as
used in this paper means thedevelopmentof hewhole
project from its inception, takingnto account all the influences
which have an effect upon he orm and cost of he inal
structure.Theeconomics of design must be considered
in
this ast context since he client is mainly concerned wi th
what he ays or, a complete buildingwhich meets his needs.
The Total
Project
In any economic tudy, the first stage is to identify the principal
cost items. In bu ilding work they are as follows:
Design calculations, preparation of contract drawings
and the writing ofdetailed specification
'Site clearance
Foundations andrainage
Construction of load bearingr skeletal tructure
Construction of floors and roofs
Fixing of ladding, and/orbuilding of alls and partitions
Fixing of services and lumbing
Finishes and painting
Site supervision
67
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The relative costs of these items vary substantially depending
on the type of building and its location. Forexample, in a
modern office b lock services and plumbing (inc luding lifts, air
conditioning, etc.) can amount to more than
50
per cent of the
whole-excluding land costs. Conversely, in a farm building
or an open gantry structure, the framework, and cladding i f any,
account for 90 per cent of the cost. Similarly, in some cases
site clearance can dominate-in others foundation costs can
outweigh a l l other considerations. In a prestige public building,
finishes can be the largest single item.Clearly hen,
it
is
inappropriate to consider (iv) load bearing or skeletal structure,
in isolation from the other items. Therefore begin by
putting things in perspective by recognising the kind of
structure, its purpose, its location and the ground onwhich it s
to
be built.
It
is worth at this stage sounding a note of caution. Cost
savings are often attempted by reducing or eliminating (ix)
site supervision, on the supposition that if the contractor and
sub-contractors all know their jobs (and the building work is
subject to periodic inspection by the Local Authority) further
supervision is unnecessary.
It
is false economy. In nearly
everycase of failure either catastrophic collapse or simply
unserviceability leading to unacceptably heavy maintenance
charges,
it
is found that there were defects of supervision.
Failures make poor advertisements, apart. rom the distress,
delay and expense ncurred, and do not make for satisfied
clients.
The Steelwork
Having said al l this
it
falls to most structural engineers to
consider item (iv ) load bearing or skeletal structure, i n detail,
and again it is worth identifying the principal ost items, which
are
:
(a) Basic design-Is the building likely to be fully framed
or with load-bearing walls? If he former does one use
steel, concrete, or timber, or perhaps a combination? A
rational decision can only be taken by doing one or two
sketch designs and using approximate current cost
figures.
On the supposition that the outcome points to steel, or at
(b) Detailed design and workshop details
(c) Cost of steel from mills or stockholder
least shows steel to be feasible, the ensuing cost items are
:
N.B. Steel ex-stock costs some 10 per cent more than
direct from the rollingmills
(d) Fabrication cost
(e) Transport and shipment
(f) Erection
(g) Painting
(h) Cladding and finishing (in the case of industrial
All these facets interact wi th one another, as wil l be seen, bu t
buildings)
for clarity they w il l be considered individually.
(a) Basic design
The relative importance of various factors have to be weighed
up. They are
:
(i ) First cost
(i i) Ease of maintenance
(iii) Aesthetics
(iv ) Likelihood of future extension or change of use
Clearly in most cases, lowest first cost dominates, but some-
times one of the other factors is of greater importance. For
instance, in the case of a public building, such as an airport
concourse or
a
sports stadium, estheticsmay emore
important than first cost ormaintenance.Again, in highly
corrosive environments, such as a chemical plant, first cost
may be subordinated to the need to be able to repaint with the
minimum of trouble. Thus in this case one would opt for
welded portal frames ather than trusses,and rolled beams
rather than light lattice work, or alternatively, hollow sections
68
to minimise surface area of expensive coatings. Aesthetics do
not enter into consideration in that case. Again the need for
future extension, or adaptation to meet possible changes of
use may in other instances be al l important, and indeed may
dictate the choice of structural medium. Forexample, the
motor industry and manufacturers of consumer durables
nearly always use steel structures for this reason. A change of
model usually means a change of structural requirement in
terms of cranage needs and unway layout and capacity.
The answers to a l l these questions can only be obtained by
reference o the client, and t is surprising how rarely an original
inquiry gives any guidance on these points. Early discussions
with him are therefore vital in order to agree a design brief,
which will prevent a design being submitted which, although
it may be perfectly sound structurally, is not what he had in
mind. Such a discussion also serves to establish
a
good
working relationship.
Next structural layout,mustbe considered which wi ll be
influenced by the loads to be sustained, the requirements for
usablespace within the building, and in many cases the
constraints imposed on the structure by architectural matters.
It is
a t
this stage that overall structural behaviour mustbe
considered, particularly overturning due to wind.
LIFT STAIR
AND SERVICES
W E L L
’
‘OPEN PLANF F IC E
F L O O R
POSSIBLE LANESOF
>
BRACING SING
TWO
EXTRAOLUMNS
‘OPEN
PLAN OFFICE
‘,----
FLOOR
Fig 1 Example ofef fect o f brac ing on p lan layout
In high rise multi-storey buildings, sway due to wind s often
the design criterion. One then has to decide whether to use a
rigid-jointed frame,or rely on a stiff core, which can be a
braced steel tower or bu ilt of cast i n - s i t u reinforced concrete.
From an economic point of view an ounce of bracing is worth
a
ton of stiff joints in controllingdeflections. For example, in a
recent design for a bank building n Hong Kong, subject to
typhoon wind forces, the cost of the structure could have been
reduced by some 30-40 per cent if bracing had been accept-
able. It is interesting to record that this alternative was offered
to the client, and rejected on the grounds that obstruction
would be caused to an otherwise open plan layout, although
that obstruction was small (Fig 1 .
In factory buildings, cranage needs dominate, and if heavy
lift ing capacity is needed one is designing not a building with
a crane, but a gantry structure with an envelope. n such cases,
each main column and its foundation has to be designed for
the situation when the loaded crane passes over it. Thus the
size and cost of the main columns are almost independent of
The Structural Engineer/September 1977/No. 9/Volume
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Fig 2 Al ternat ive ayouts or heavy worksh op bui ld ing
their spacing. Viewed in isolation, long span gantry girders
appear to incur a cost penalty, but this may be more than
offset by the reduction in the number of columns and their
foundations, particularly if the ground conditions demand
piles.Clearly a balancehas to be struck and several trial
designsmay be needed (Fig
2).
The roof design in such
circumstances is an incidental, its layout having 'occurred',
and again, viewed in isolation
it
may not appear economic.
On the other hand, without substantial cranage, you are
designing only the envelope and here you have to weigh up
whether you use a number of small span bays or one or tw o
of large span. Theormer wi ll give the cheapest steel structure,
particularly if headroom requirements are small, but the latter
may be more convenient and indeed cheaperoverall,The
reduction of obstruction from internal columns, which could
interfere with the layout of machine tools, now and in the
future, coupled w ith the elimination of valley gutters, internal
drainage and foundations may swing the balance. In such
cases if the steel structure is designed in isolation an economic
solution in terms of total buildingcosts is unlikely to result.
In practice
it
is probable that the apparent economy of using
short spans has resulted in waste, and in most cases much
longer spans would havebeenappropriate, possibly using
lattice construction in high tensile steel.
The current trend is towards flatter pitches. This partly
arises from legislation making it necessary to heat and insulate
most factory buildings, and partly from the whims of archi-
tectural fashion. Nonetheless, a truly flat roof wi ll always be
more expensive to make and keep watertight than one w ith
a
moderate pitch. This saving wi ll more than compensate for the
cost of
a
parapet masking he shape of the roof from an observer
a t ground level, which is mostly al l that the architect is con-
cerned about. Further, ideal ly the depth of a lattice truss should
match the shape of the bending moment diagram, to keep the
main chord forces more or less constant throughout the span.
This cannot be achieved exactly, but t is a useful aim as it
avoids changes of section in the main chords. Thus for once
two factors act together and
a
moderate pitch results in
a
good truss design (Fig 3 .
The competition between trusses and portal frames
continues, and
it
is often worthwhile going to tender on
alternatives.Relative fabrication costs depend more than
anything else on shop layout, plant and labour skills. The
third alternative, which can often be cheaper than either, is
often not even considered, i.e. tied portals. The tie need only
be quite light, and can be at eaves height or part way up the
A
B
A . MOR
EFFICIENT
STRUCTURALLY
B MORE
PLEASING VISUALLY
Fig
3
Roof truss designs
rafter and in long spans may need to be slung a t intervals, but
the savings possible in the rafter and column sizes can be very
substantial. This principle is by no means new, having been
used to good effect by the Victorian railway engineers, and its
virtues are still valid. Whether one can se tied portals depends
on whether one can obstruct the roof void and also whether
it is required to sling runway beams. If so, trusses are usually
preferable as they present strong points on the tie, and can
easily be strengthened locally, sometimesyearsafter con-
struction.
However much is put in to the design of the roof principals
it is worth remembering that they only form part of the building
steelwork, sometimes less than half the otal weight. The
remainder consists of purlins, sheeting rails,bracing, door
lintels and posts, and gable framing. Where freedom exists,
it is worth designing the purlins and rails first, and let their
The Structural Engineer/September
1977/No.
S/Volume
55
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design dictate the spacing of main frames. Admitted his
is not always possible, and in any case inept design of the
door arrangements and gable framing can negate any savings
made through careful design of the frames and rails.
It
is also
a t
the basic design stage that one must consider the
choice of steel grade, as this will frequently dictate the layout.
Further, one should also consider whether to use mainly open
rolled sections (UB's,UC's channels and angles) or closed
sections (CHS or RHS) and it may be that the use of tubes in
high tensile steel can produce the best structure for the
particular purpose. (Tubes not being as sensitive to local
buckling as open sections). Again, only by trial designs can
the optimum solution e evolved.
In multi-storey work, particularly when composite action
is used in the beams and slabs, Grade 50
is frequently the most
economic solution. Further, if one can maintain the same basic
column size down the height of thebuilding, by using different
grades of steel as well as different weights of he column
section, money wi ll be saved on the etailing of curtain walling,
glazing, etc., apart from reducing substantially the cost of
stanchion splices. One must beware, though, of the possibili ty
of cross substitution and possibly deliberately introduce de-
tailed differences.
In single storey work, Grade50, or even55, may be economic
for trusses and lattice girders. Deflection and local buckling
may however preclude its use in portal frames. Strictly speaking
then, one should compare lattice work in HT steel with portal
frames or tied portals in mild steel. One wonders how often
this is done.
(b)
Deta i led design and workshop detai ls
The actual cost of design, relative to total building costs, s
really verysmall. It follows therefore that greatsavings in
design costs by the use of the computer, for instance, are
largely illusory, unless it can be made to
opt imise
on some of
the conflicting actors already considered. The cost of detailing,
however, can be significant and can amount to twenty times
the cost of he design process. Great potential cost savings
can therefore be made by computer detailing, which is being
developed in
a
number of quarters.
Whilst
it
may not be intellectually satisfying or philo-
sophically elegant, the aim of detailed design should be to get
the job done fast and efficiently and therefore at lowest cost.
To do this one should make the ensuing activities easy, for
by so doing mistakes are reduced and highproductivity
achieved. I t is fortunate that if a job is easy t o detail,
it
tends
to foll ow that fabrication is easy also. Detailed design there-
fore should aim at simplicity and repetition.
Therefore avoid individual design of everyeam
and every column length, i n an attempt to achieve leastweight.
nly rarely does this achieve lowest cost. Therefore
aim
a t
'grouping' of members to achieve which, it maybe
necessary to adjust the layout, if this is possible.
It
is in this
grouping hat heskill and judgement of the experienced
designer reveals itself. If too much grouping is done, material
is wasted-if too little, draughtsmen,plater's and erector's
time is wasted. Only by having done it or supervised
it
can the
skill be acquired.
By way ofexample, the standardisation of cleats and fittings
can achieve great savings; similarly stanchion basesand
splices should only vary with the column section. Of course
standard details which suit one shop may not suit another,
depending partly on he machine tools available and partly
on shop layout, wh ich may have been inherited. Conversely,
thehip-ended trussed roof which entailed a special half -
depth truss and hip rafters, necessitating setting out ful l size
on
a
template floor for purlin cleats is now
so
expensive that
it is never done. If templating has to be resorted to,
it
should
only be for members which are required in great numbers.
Detailed design then should recognise hat in spite of greatly
increased steel costs, labour costs account for some half to
tw o thirds of ex-works rices, and areikely to continue to do
so
The aim therefore should be to reduce labour costs to the
minimum. Again for example, web cleats for a beam can be
fixed with the beam standing on the work bench, the holes
in the web having possibly been drilled on an automatic
sawing and drilling machine. A welded end plate to perform
the same function requires the beam to be turned over, at least
once. If
it
can be manhandled, well and good, but if the crane
isneededdelay inevitably occurs,sinceone cannot plan a
total workshop activity so well as to ensure that the crane is
always available exactly when i t
is
needed (There ishe further
advantage of web cleats that should the erector discover that
the column flange to which the beam is to be fixed should be
out of square, some adjustment s possible).
Continuous consultation and collaboration between the
designer the estimator, the chief draughtsman,and works
manager and foremen is essential. Theyshould agree on basic
principles of detailed design and review their findings fre-
quently to adjust to varying conditions, including the personal
skills their menpossessor donot possess. In particular,
changes in labour rates for different classes of labour should
be fed back to the designers and estimators, as they are likely
to affect relative costs of different details which may call for
changes in standard procedures.
(c)
Cost
o
steel
It
is usual or the designer to assume that all steel of
a
particular
grade costs he same per ton. At the time of writing the cost of
plate is broadly comparable with that of sections, whereas at
one time plate cost somewhat more. Both plate and section
prices change not infrequently, independently of each other.
Also the rolling mills charge extra for quantities smaller than
20
tonnes of any one size on a sliding scale and prefer not to
accept orders for less than five tonnes. Having regard to the
likely lapse in time between design and steel supply, in such
inflationary times the designer can be forgiven if he takes the
easy way out and ignores price variations.
It is only in the design offices of the leading fabricators that
steel cost variations are taken into account in design, and not
always then. Clearly though, in small to medium sized obs,
if such variations are ignored, considerable cost penalties can
be incurred quite fortuitously. The pursuit of minimum weight
by the use of a wide variety of beamand column sizes,
necessitating the purchase of most steel from stock, must be
avoided. As a rule of thumb, if the increase in weight through
grouping does not exceed say 10 per cent on small jobs and
say
5
per cent on medium sizedobs, it
is
worth doing,
particularly if by so doing individual section quantities exceed
10
onnes.
It
is obvious that a rolled section should always be used in
preference to
a
plate girder, if possible, because of the con-
siderableextra fabrication cost of the latter. It is not un-
common to find n
a
consultant's design a plate girder used,
incorporating such refinements as curtailed flange plates, and
thin stiffened web.Thedesigner wi ll havespent hours in
achieving the optimum design and is no doubt well satisfied
as
a
result. A much cheaper solution, albeit weighing more,
can often be calculated in minutes. Where for any reason, a
rolled section by itself wil l not serve, consideration should be
given to a compound section with welded reinforcing plates.
This solution often offers the best compromise, although it is
not very popular,perhapsbecause
it
is old fashioned and
reminiscent of riveted construction. It can be valuable none-
theless.
It
is also worth remembering that the constraints
inherent in the roll ing process necessitate, n big beams,
a
web
thickness-much greater than the comparable plate girder. The
effect of this is that rolled beams rarely need web sti ffeners
except at points of support or under heavyoint loads.
In the case of long span lightly loaded beams, where
deflection tends to govern the design of
a
rolled member
castellated beams should beconsideredL:aszthey often offer
370 The Structural Engineer/September 1977/No. 9/Vdum e 55
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the best olution.Whenexposed they are often visually
more acceptable and when masked by false ceilings offer the
facility forservices to be passed hrough theopenings. Stric tly
speaking though the cost of both material and fabrication for
such a beam ystem shouldbe compared wi th minimum
plain beams capable of fulfi lling thedesign requirements.
As indicated earlier, another variable entershe picture when
one considers steel grade. Broadlyne can say that if buckling
or deflection are not design criteria, then Grade
50
should be
considered. Fabrication costs wi ll increase, particularly when
thick sectionshave to bewelded,necessitating different
welding processes, but fabrication estimates on
a
rate per ton
basis are misleading, since different quantities are nvolved.
To draw a t rue comparison between Grade 43 and Grade
50
one should be dealing in money terms for the supply of the
total structure, including transport and erection. If this were
done moreoften,and on
a
rational ather than an intuitive
basis, much more Grade 50 would be specified than is the
case. I n turn this wou ld bring about an mprovement in the
availability of high ensile steel, the supposed supply difficu lty
often being givenas the reason for not adopting
t
(d)
Fabrication cost
On this aspect whole books could be written. The cost areas
are manpower and time. You are familiar wi th the assessment
of man hours for
a
job, but
it
is less often appreciated that
it
costsmoney for a part to have to wait in the shops with
nothing being done to
it
Apart from interest on capital, it is
occupying productive floor space and obstructing other work
from going through. This is particularly true of box girders
which occupy much loor space, to the detriment of other
smaller orders. Part of the rue cost of a box irder therefore s
the oss of profit of such other orders. The aim therefore must
be to minimise abour hours and speed the job through (the
one does ot necessarily ollow from the ther).
It is axiomatic that everyone
a t
heart wants to do is o b well,
including the shop floor plater or welder provided he
is
paid
appropriately. In most abricating shops men enjoy a bonus
based on output onnage, as
a
financial inducement. The great
benefit of repetition is now evident. Each time a plater has to
read a new drawing, each time a welder has to set up some-
thing new, involves a period of 'learning time'. If this'learning
time' can be spread over IO,
20
r maybe
50
ident ical items,
the less significant it becomes. The output bonus is herefore
good, and within limits themen are pleased. If the job consists
of dozens of different sections, wi th a variety of details, all
fussy and difficult to weld, 'learning time' lasts the whole ob
through; productivity s lo w and men become frustrated and
careless; mistakes are made, delay occurs and profits are lost.
Most big fabricating hops today possess automatic sawing
and drilling plants. These can saw members exactly to length
and dril l flange andweb holes wi thout the necessity of marking
out on the bar. I f members do not require cleats and fittings
to be fitted in he shops, the bars cango straight through to the
painting or despatch bays. Economy flows therefore if
al l
the
fittings can be fixed either to the beams or to the columns.
It
is for this reason that the old method ofseating bracket on
the column and a top cleat on the beamhas gone out of
favour.Holes only in the columns,and web cleats on he
beams are now preferred. There is the added advantage too,
of the brackets being less liable to damage in handling and
transport.
Regardingwelding,modernautomaticorsemi-automatic
processes give
a
deposition rate up to twenty times that of
manualmetal arc (M MA or stick welding). A continuous
automatic run is preferred to intermittentwelding which
should only be specified if some positive advantage accrues
thereby. Web/flange welds in plate girders should therefore
always be welded automatically. Stiffeners and other fittings
are fixed later andwelded manually.
A
significant part of the cost of a welded oin t is the cost
of
n
Fig
.
Weld detai ls
the weld preparation and the attendant setting up (Fig 4 . If
fil let welds can be used and preparation avoided altogether,
money wi ll be saved. f buttwelds are vital, it is best if only ne
edge has to be prepared, and that on the smallest piece, to
facilitate handling Fig 5).
MA1 FLA NGE TAPERED IN WI D TH
AND THICKNESS IN SECTION GROUND
/ / / / / / / / / / / / / / / / / / / / / / /
CHANGE
FLUSH
f i g
6.
Weldedsplice in rai l way brid ge girder
In plate girders, because of the cost of weld preparations,
it
is usually cheaper o run the ame flange plate hrough rather
than to curtail
it
at the nds, if by curtailing butt weld as to be
introduced. Only i f the plate has to be joined anyway, due to
availableplate engths, is
it
worthwhile curtailing.Then it
should be done y reducing the wid th rather than thehickness,
by flame cutting and grinding. The weld preparation can then
be confined to the short end piece. Only in quite exceptional
circumstances as for instance in a railway bridge girder where
fatigue governs the design, should one taper the thickness o f
the larger plate by machining (Fig 6 . n general it can be said
that he ight amount of preparationand weldings he
minimum possible consistantwi th structural requirements.
It
is often assumed that allshop joints should be welded, as
member sizes do not ave to allow fordeductions for holes. In
general this is true, but there are exceptions, notably in l ight
latticework where the savings throughwelding are more
apparent than real. Angle memberscanbecropped to any
length (square ends) and have holes punched on a Vernet or
SEG machine. Truss and lattice girder members thus prepared
accurately are self-jigging and cannot be wrongly assembled,
when bolted or riveted together. Welded trusses of the same
form require either accurate manual setting up and clamping,
or a separateassembly ji g to ensure geometrical accuracy.
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The cost of such a jig can only be justified if identical frames
are needed n great numbers.
(e) Transpor t and s h ipment
Transport costs are high and rising and
it
is therefore important
to the contractor that vehicles travel fully-laden. The shipment
of tender light weight assemblies should be avoided for this
reason. If great distances are involved
it
can be economic to
despatch small lattice work pieces for assembly on site (as in
the case of electricity transmission towers)-another argument
in favour of bolted attice work. On the other hand oversize
loads involving police escorts should be very carefully
considered.
It
can be economic to do welded splices on the
site prior to final erection, rather than transport 1OOft long
girders in one piece.
Even greater attention needs to be given if the work has to
be shipped overseas.Early consultation with the shipping
agents to establish the basis of freight rates may well have an
influence on the form of the structure. Frequently, the ikeli-
hood of accidental damage is underestimated, and long slender
members are particularly susceptible. If such are essential they
should be firmly bundled at the works. Remembering that the
steelwork may be piled
2 ft
deep in the hold, it is sensible to
desptach the robust items from he works first. Small loose
items should be avoided if possible, but if ssential they should
be wired together in bundles or crated, to avoid loss Pressure
for steelwork to go deck cargo should be resisted. Apart from
likely damage to any corrosion protection, there is a real
possibility of items being lost overboard in heavy seas, as
has happened o an expensivebox girder crane bridge.
The cost of replacement parts, or repair of damaged parts
can be considerable, apart from the delay incurred. The intro-
duction of a few extra splices in, for example, rafters can do
much to ensure that the works arrive on the site in good order,
and can thus be erected immediately
a t
minimum cost and to
time.
(f) Erect ion
Again,
it
is only possible to touch on
a
few aspects. The range
of cranage available today is much wider than used to be the
case. The Scotch derrick and the Guy derrick are still with us,
but are now supplemented by
a
whole range of mobile and
tower cranes al l of which are available for hire, if need be for
short periods. Nonetheless certain basic principles are still
valid.
The capacity of crane required is dictated by the weight of
the heaviestpiece.Therefore a design which incorporates
one heavy girder of say 1 0 tons, with the remainder of the
work not exceeding
2
tons presents difficulties. If the heavy
plate girder can be replaced by one of lattice construction,
the cost of keeping a heavy craneon thesite, or hiring one ust
for one l ift can be avoided.
The virtues of repetitive parts have already been stressed.
The gains to be made can be partly last if interchangeable
parts are given individual marknumbers.
It
maymake the
marking diagram look consistant to do so, but
it
may entail
a
careful search by the erectors for a particular member when
one readily to hand would fit. After the completion of the
detailed design then i t is worth looking at the marking diagram
with this point inmind.
The erector's task is twofold, namely hoisting into position
and effecting connections.
It is in the design of the onnections
that the erector's work needs thought. Much has been written
on the undoubted virtues of HSFG bolts, unfortunately to the
extent where they are currently being specified when they are
quite unnecessary. Not only areHSFG bolts expensive in
themselves, but the on-site orquing process costs as much
again, regardless of diameter. This is because staging is nearly
always necessary, whereas
4.6
and
8.8
bolts can be fixed wi th
a hand spanner, mostly without even the use of a ladder.
HSFG bolts should only be specified if a large number are
needed for one connection, as for example a splice in a big
girder, or i f some other positive benef it accrues. They are uite
inappropriate for beam/column connections in ightmulti-
storey work. In such cases economy derives if the service bolts
used to tack the structure in position initially can become the
final fixing with nother pull on spanner.
The design of connections should recognise that no joint
wil l ever give an exact fi t. Reference should be made to the
tolerances on dimensions of sections given in B S 4 and allow-
able lack of straightness of members. The situation should not
arise that members within those tolerances give rise to trouble
in effecting connections.
It is only common sense that to avoid errors site bolts should
be either 4.6 ar
8 8
throughout any one job, and that the tota l
number of different diameters should be kept to
a
minimum.
Grip lengths however can scarcelye controlled, they
merely occur.
Different views are expressed regarding site bolt ing or site
welding. As in the case of shop lattice work, a correctly
fabricated bolted ob is self aligning. On the other hand no one
can weld
a
member which is hanging from a crane hook, and
it
is not uncommon in a job n which site welding has been
specified for the fabricator to weld on lugs with holes in the
shops for the purpose of tack-bolting prior to site welding.
That said, in certain circumstances site welding can be
economic; box girder bridges for example and also tubular
structures where bolted oints can be very diff icul t and look
unsightly. What is not economic is to have
a
mainly bolted
job in which, because the designer finds himself in a corner,
site welding has been specified only in one or two connections.
If the ntroduction of on-sitewelding equipment is really
necessary,
it
might as well be used in any connections in
which benefit wi ll be shown. The difficult corner though often
arises through last minute changes of requirement or loading.
If this occurs and time permits
it
is worth reviewing the whole
job for the reason given.
Thedesigner should give early thought to the need for
bracing. No great harm is done if too much bracing is incor-
porated. Too litt le can result in disaster Frequently a good
fabricator will add bracing in the detailing stage, butwill
content himself with temporary rope guys if he has already
been screwed into the ground on price.
To the inexperienced designer, much bracing appears to be
superfluous, particularly once the structure has been clad and
the floors cast. This is probably true, but bracing is cheap as
an insurance premium for stabiltiy during erection. Also it
costs money to take it out later and throw it away. Since the
client is paying for it, if it is not causing obstruction
it
can be
left in. (Incidentally, i t can prove invaluable during subsequent
alterations) (Fig 7).
(g)
Paint ing
Awhole range of different treatments is now available.
It
is not uncommon for a fabricator to stock forty different paints.
Early consideration should be given t o the corrosion environ-
ment to arrive at the appropriate total treatment, and the pro-
portion of shop and site painting. It is no longer sufficient
to
decide on the treatment after detailed design has been com-
pleted, since as indicated earlier, the corrosion environment
may affect the structural form. The ife and effectiveness of any
treatment materially depends upon the surface preparation.
Many fabricating shops have shot blasting plants in their
input bay and can carry out blasting and prefabrication
priming quite cheaply as routine. Such treatment is
a
good
investment in the long term whatever subsequent coating is
adopted.
Shop painting is always cheaper and more effective than site
painting, but no steel can be handled, transported and erected
without damage to the coating from crane slings, etc. Touch-
ing up of the base coats and the final top coat must therefore
be done on site.
Regarding the choice of system to adopt in any particular
372
The Structural Engineer/September 1977/No. S/Volume 55
7/18/2019 Economics of Steelwork Design
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Fig
7 .
Layout for por tal shed
case useful guidance can be found i n Constrado publication
No 3/74, Protection of s tructural steelwork from atmosp heric
corrosion
prepared by the Corrosion Advice Bureau of the BSc.
In a difficult case this unitcan be approached or more detailed
advice.
If
a
general comment can bepassed
it
is that he recent
developments n the field of corrosion protection have evolved
protective systems greatly superior to those available some
years’ago.Thesesystemsareexpensive but are invaluable
when appropriate, as in exposed structures in severe industrial
or
marine environments. However, this has led to some waste
of money by the specification of such sohpisticated treatments
in circumstances where they are not necessary. The ate of
corrosion of steel inside a normal heated building is really
very low, and the treatment used should be appropriate to the
situation.
(h)
Cladding and f in ishin g indust ria l bui ld ing s)
The weight of claddings for industrial buildings usually does
not greatly affect the structural design, being much less than
the design superimposed load. Overseas, where no snow load
is experienced his may no t be the case. The spacing of purlins
and sheeting rails is determined by the load capacity of the
sheeting, which can vary. As the purlins and rails also provide
lateral restraint to their supporting members, last minute
changes of mind as to the form of sheeting can result in major
redesign of the rincipals; clearly o be avoided.
Before finalising the design of a shed or mill building, the
available lengths and widths of the esired cladding should be
obtained.
It
should then be determined whether any non-
standard lengths or widths w il l be needed, and if so whether
they can be cut on site or need to be specially ordered.
It
is
often he case that slight adjustments in salient dimensions
or eaves and ridge details can eliminate the need for ‘specials’
entirely. Alternatively, or perhaps in addition, the specified
laps can be increased, either of which expedient wi ll save cost,
and particularly time.
The Structural Engineer/September 1977/No S/Volume
55
It
is usual for roofing and sheeting to be effected by a
separate sub-contractor, and savings in time can be made by
proper phasing of the different activities, which is a manage-
ment problem. The designer can assist, though, by giving an
indication on his drawings where the roofing contractor can
placeheavy loads (such as ‘blocks’ of sheets) ather than
eaving him to findut the ard way. Failures haveccurred asa
result of overloading from this cause, and cold-formed purlins
need consideration in this regard..
The use of stressed skin design should be considered as an
aid to economy. When metal cladding is used, its considerable
shear strength when adequately fixed, can be used to absorb
wind forces, obviating the need to provide separate wind
members.
Conclusions
This paperhas tried to give an insight into the factors
governing the economy of steeltructures.ecognise
the relative proportions of the major cost areas for any project,
and not ry to consider the steelwork in isolation. Ideally
a
number of trial designs should be carried out to explore
alternative arrangements.
Cost and time should be minimised by making the various
activities carried out by the contractor, viz, design, detailing,
fabrication and erection, slick and easy to perform efficiently.
This is most likely to achieve the aim of satisfying the client
and
a t
the same time make the job profitable to the contractor.
Conversely, the economic value of highly sophisticated
design techniques, whether computerised or not, will be
largely illusory if not complemented by a sound working
knowledge of the various processes involved in structural
engineering. Great nicety of calculation is therefore misplaced
if needless expense is incurred by lack of attention t o basic
principles of design. Most of the information given here is
really only common sense but it is not claimed as exhaustive.
Further enquiry from appropriate sources early in the develop-
ment of
a
project can yield substantial economic benefit.
373