Economical structural steel projects
1.
Introduction
There is no way we can compromise on the requirement that a structure
must perform satisfactorily during its lifetime, and that it must be safe. These
requirements define the limit states of design and they are givens. There
may also be additional requirements that have to be met by the structure
of a specific project, such as that it must be aesthetically attractive, or built
under very demanding conditions, or be environmentally sustainable, etc.
But here we talk about economics. Clearly, the owners of buildings want
their completed projects to cost as little as possible, or they want as much
utility as possible from the money they spend, while meeting all the
standards they set. A particular client may or may not be interested in
lifetime costs and the demands of maintenance, but it is generally desirable
for projects to have a reasonable degree of durability and not be
expensive to maintain.
It often happens that the structural engineer feels that he/she is not allowed
to specify the most economical structure, forgetting that the structure is just
one element of the project. Deleting a column may, for example, have a
big impact on the letability of a space and thus the income of the owner,
even while it chases up the cost of the structure. We need to see the
broader picture. Moreover, much of the cost of a project, as well as the
owner's ultimate perception of whether he got value for money, has to do
with the process by which the project is created. If the project runs smoothly
with a minimum of claims and is finished in time and budget the client tends
to be happy. Predictability may be even more important than the quantum
of the cost. If the project can be handled in such a way that the owner gets
the final product considerably quicker, some of the cost can be offset
against an earlier revenue stream.
Delivering highly efficient structures would, however, not be sustainable if all
parties on a project – the owner, the professions and the various contractors
- don't make decent profits. For the structural engineer this means
adequate remuneration for his work, but then a point may arrive where the
professional fees for effecting a saving exceed the value of the saving.
Should the fee of the engineer be based on a percentage of the cost of
the structure he actually does not have a financial incentive to save – it is
just his professional ethics and his relationship with his client that drive him to
design an economical structure.
The question of the economics of a project is clearly much wider and more
complex than designing a cheap structure. And, as we will see below, even
the cost of the structure as such depends on a range of factors. That does
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not mean, however, that the design of the structure is unimportant – a welldesigned and specified structure, properly fabricated and erected, remains
the basis for an economical project that meets the client's requirements.
In what follows we will consider the various things the designer, detailer
and/or steelwork contractor should do or keep in mind to ensure that
economical steelwork projects are delivered.
2.
Handling projects in a standard, predictable manner
 Strive for an earlier, predictable completion date and a predictable
bottom line.
 Follow what is generally regarded as 'best practice' in the steel
construction industry: get on the same wavelength as everybody else
and do what's taught in this course!
 Get the contractor involved as early as possible, find out what his
requirements are and let him make suggestions as to how the project
can be expedited or how savings can be effected. For the contractor
that means having good people with insight who can make a useful
input at the beginning.
 It has been amply demonstrated that if the concept of partnering can
be applied to a series of projects, allowing a whole team to learn
together as they move from project to project, it can result in huge
savings in cost and time.
 Have clear specifications and complete, properly coordinated
drawings.
 Be clear, firm and consistent with quality assurance right from the
beginning, while being fair and not over-specifying.
 Minimise change orders and rework – get things right first time (which
requires some extra work during the early phases).
 Stick to the programme and to promised dates, for example with the
approval of drawings.
3.
Avoiding over-specification
 Design connections, including the welding and number of bolts, for the
actual forces in the connection. Don't just say 'full strength welds'.
 Specify an appropriate level of quality assurance, and ensure that the
inspectors don't insist on a higher level or on unimportant 'nice to haves'.
 Specify only the level of finish that is required for a specific application.
Only steel seen at a close distance by members of the public needs to
be immaculate.
 Use an appropriate corrosion protection system for the corrosiveness of
the environment, the exposure to wear and tear, and the life
expectancy of the project. It is generally not necessary to paint
concealed steel, except in a corrosive environment.
 Use high strength friction grip connections only where required.
 Specify precambering of beams only where really necessary.
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4.
Use washers only where needed.
Standardisation and repetition
 Use standard connections. Section 7.8 in the Red Book shows standard
beam-end connections. Other connections cannot be standardised
completely, but typical connections as shown in Section 7.11 in the Red
Book, or in the SAISC Structural Steel Detailing Manual (the Yellow Book),
or in the SAISC publication Structural Steelwork Connections, can be
used. In general: with 99% of all structures the connections are not the
place where ingenuity should be exhibited.
 Use standard bolts – see Section 6.2.2 (Table 6.11) in the Red Book.
 Use similar details on a project. For example, if some beams have angle
cleat connections, don't let others have welded end plates (except
where a beam has to be attached to a steel face that is not at 90 o to its
longitudinal axis, in which case an end plate or fin plate is called for).
 Repeat the same member size where possible. For example, don't make
one beam smaller than all its mates unless there's a good reason; don't
make every diagonal in a truss a different size; don't change the section
of a truss chord at every node. One of the many advantages of not
switching sizes too frequently is that the amount of waste in the form of
offcuts can be reduced.
 Use a standard paint system.
 Modularise; get a theme for the project and stick to it. Figure 1 shows a
heavy industrial building where even the spacing of the stiffeners on the
crane girders match the spacing of the verticals on the lattice girder in
the roof. The result is a high degree of repetition, a smaller chance of
making errors, and most probably a cleaner structure with easier
fabrication.
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Figure 1 – Heavy industrial building with a high degree of modularity
5.
Being in line with what's easiest and most cost-effective for the contractor
 The best option is to let the contractor speak for himself by consulting
him early in a project. Here we can only make general remarks.
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6.
The engineer should listen to the contractor, rather than feel that his ego
is threatened by any suggestion of the contractor. At least some of the
contractor's ideas may be beneficial to a project.
With the existence of CNC equipment that can cut to length, punch,
drill, make notches, and make splice or gusset plates, most contractors
prefer bolting to welding. Thus, as far as possible, specify bolted
construction. Alternatively, specify shop welding and site bolting. Only
rarely should site welding be required.
Be aware of the effect of tolerances. Steelwork can be fabricated very
accurately, but not exactly, and there must be room for adjustment.
With things like crane girders there may also be a need for adjustment
during the lifetime of the structure. It is even more important to be aware
of the construction tolerances of other materials such as concrete that
the steel structure has to interface with, and to allow adequate scope
for adjustment.
Design structures that can be fabricated, transported to site and
erected without special measures, if possible. This implies that the
engineer should give some consideration to how the steelwork will be
fabricated, transported (ie. broken into pieces for transportation) and
erected, unless it is just a typical structure. If special measures are
required during fabrication, transport or erection, these should be
brought to the attention of the steelwork contractor and discussed with
him, or the contractor has to bring his ideas to the attention of the
engineer.
Optimisation of the design and detailing
 The general rule is that it is not the mass of the structure that's to be
minimised, but the cost that's to be optimised. 'Optimise' means
minimise under a set of constraints, which includes a host of things such
as meeting the requirements of the owner and the other professions,
aiming for a shorter construction time, ensuring ease of manufacture
and construction, etc. The mass of the material is a significant part of the
cost, but supervision, labour, stockholding, handling, quality assurance,
consumables, transport and erection normally add up to a bigger figure.
As one tries to optimise the bottom line, you should also keep these
other issues, which can be affected by the design, in mind. (There is one
proviso here, namely that the steelwork contractor should be
sophisticated enough to distinguish between jobs with a high labour
content and those with a lower content; if the contractor gives the
same rate for a portal frame than for a truss, the designer can be
expected to go for least weight.)
 There are a number of cases where it may not be obvious what the most
economical solution is, and where the designer needs to do some
thinking and sometimes even get comparative prices to decide what
the best solution is. We present the choices to be made as two extremes
in the table below, acknowledging that using the term 'extreme' may
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not always be appropriate; the term is used to indicate that in many
cases there are possibilities between the two options.. In most of these
cases the higher mass solution may be the better one if labour is taken
into account.
Higher mass extreme
Smaller mass extreme
Making all beams in the structure Making each member in the
the same size, same with columns
structure just strong enough for the
force on it.
Use an individual hot rolled section Use sections welded up from thin
for each element. For bigger plates, latticed members, or
elements, use a box or heavy plate elements consisting of more than
girder or column.
one section interconnected by
battens, cables, etc.
Use thick enough plate for plate Use thin plate with stiffeners.
girder webs, hoppers, chutes, etc.
Use thick column base plates.
Use thinner base plates with
stiffeners.
Use columns with thick webs that Use web stiffeners and other
can resist concentrated lateral methods to strengthen webs.
forces.
Let the flanges of a plate girder Attach web plates to the flanges
remain constant over its entire over part of their length (see Figure
length.
2).
Use simple connections at the ends Make elements continuous by
of
all
beams
(ie,
simple using moment connections, to
construction).
yield continuous beams, sway
frames, etc.
Use portal frames.
Use column and truss construction.
Use solid crane columns made from Use lattice crane columns.
plate.
Use a hot rolled section as a strut.
Use a hollow section.
Use more stable purlins with no sag Use lighter purlins with a proper sag
system.
system.
Specify intermittant welding.
Specify continuous welding.
Use X-bracing that acts in tension Use tubes for bracing, to resist both
only.
tensile and compressive forces.
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It is good to design members in such a way that they don't need to be
turned during fabrication, thus reducing the amount of handling, and to
enable welding to be done in the down-hand position. Both these
principles are illustrated in Figure 2.
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Figure 2 – Plate girder with flange reinforcing plates, avoiding
turnover and allowing down hand welding
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If fire protection is required, it is advisable to make sure good fire
engineering principles are applied. This can lead to significant savings
without impairing the ability of the structure to meet the requirements of
the building regulations.
Starting with a good structural concept and lay-out is the key to an
optimal design.
Sometimes, there is the opportunity to come up with something really
bright that knocks the socks off the competition or makes your client
very happy. Then you can get real economy.
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