General Fabrication of of Steel Structures I

STEEL CONSTRUCTION: FABRICATION AND ERECTION

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STEEL CONSTRUCTION:

FABRICATION AND ERECTION

Lecture 3.1.1: General Fabrication of

Steel Structures I

OBJECTIVE/SCOPE

To give a brief outline of the fabrication process for steel structures; to identify the factors

affecting manufacturing costs and to stress the importance of the designer considering the

fabrication process when preparing the design.

PREREQUISITES

None are essential.

The following lectures might be helpful:

Lecture 2.1: Characteristics of Iron-Carbon Alloys

Lecture 2.2: Manufacturing and Forming Processes

Lectures 2.3: Engineering Properties of Metals

Lecture 2.4: Steel Grades and Qualities

Lecture 2.5: Selection of Steel Quality

RELATED LECTURES

Lecture 3.3: Principles of Welding

Lecture 3.4: Welding Processes

Lecture 3.5: Fabrication/Erection of Buildings

Lecture 4A.1: General Corrosion

Lecture 15A.8: Offshore: Fabrication

Lecture 15B.12: Introduction to Bridge Construction

SUMMARY

The lecture gives a brief summary of the forms of contract and organisation used for the

fabrication of steel structures. It reviews fabrication processes with brief descriptions of the

main operations.


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1. INTRODUCTION

The objective of this lecture is to give an insight into the fabrication aspects of steel

structures. Optimum design of steel structures can only be achieved if fabrication and

erection are considered together with the functional, architectural and structural

requirements.

To minimize total costs and optimise the design of the steel structure, it is important that

the various disciplines involved work in a coordinated way as a project team during the

various stages.

Fabrication costs do not depend only on the fabrication itself but are also influenced by the

contract scope, contracting procedures and organisation. Costs are very sensitive to the

labour involved in the fabrication. Good design concentrates on minimising material

handling and preparation; in this regard it should be noted that fabrication procedures and

sequencing may be influenced by the protection required to the steelwork. Careful

attention should also be given to other aspects such as material characteristics, distortions

and tolerances.

2. FORMS OF CONTRACT AND ORGANISATION

2.1 General

Both the form of contract and of the organisation depend heavily on the project, the client

and the contractor involved. It is common practice for the fabrication company to enter

into a contract which involves fabrication, erection and the preparation of the detail

drawings; this can prove most competitive since the fabrication costs of a steel structure

are substantially influenced by the cost-consciousness of the detail-engineering.

Major constructions, such as high-rise buildings, bridges, and offshore structures, are

usually designed by a specialist consulting engineering practice under a separate contract;

however, for commonplace structures (portal frames, etc.) it is not unusual that the total

design is undertaken by the contractor in a package deal with the client.

2.2 Contract Procedures

After receipt of the order and agreement of terms, copies of the principal documents are

passed on to the Drawing Office. These documents usually include:

Engineer's Drawings.

Conditions of Contract.

Technical Specification.

Contract Programme.


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2.3 Planning

The success of any contract generally depends on compliance with the requirements of the

project programme; deviations from the programme can have very serious effects on costs;

delays can be traumatic for the other participating trades and subsequently for the client.

The programme is usually made out in bar-line format and based on network techniques,

including critical path analysis.

Essential elements are:

Erection sequence;

Purchasing the material from the steelmill;

Preparation of fabrication drawings;

Material preparation;

Fabrication;

Assembly;

Protective treatment;

Delivery to site.

Each element is planned to a set timescale, and co-ordinated with parallel actions from

other contracts occurring during the same period. If the erection programme imposes

demands for shop fabrication in excess of the fabricator's capacity, then sub-letting of work

will be necessary, coupled with the requisite QA- and QC-assessment.

2.4 Drawing Office

The drawings can be produced expeditiously and economically only if the consulting

engineer provides all necessary information concerning the geometry of the structure,

member sizes, forces and moments in connections. Without the relevant information there

will be very costly delays not only in the Drawing Office, but in the whole production

chain. Extra costs will be incurred by variations to the design after the drawings have been

completed. Even more significantly, extra costs will arise if modifications have to be made

to work in the fabrication shop; alterations to work on site are generally very costly,

particularly if programme delays result.

The production of the drawings will include three phases:

Preparation and setting out work.

Drawing the details.

Checking the drawings.

The time and cost involved in preparing drawings will largely depend upon the degree of

repetition and the complexity of the design; careful consideration of these matters prior to

starting setting out and detail work should result in drawings that, efficiently and

unambiguously, communicate the structural requirements to the workshop operatives and

the site erection team.


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Work stations equipped with modern computer graphics, when used by trained

draughtsmen, can result in higher rates of drawing production. They can also result in a

higher quality of drawing with modifications being more easily incorporated. Long-

distance transfer by telephone is possible. Automatic listings of materials and tapes for

numerically controlled (NC) fabrication may also be produced advantageously by these

facilities.

3. FABRICATION PROCEDURES

3.1 Workshop Layout

Fabricators range from small general firms to large specialised producers with different

facilities at their disposal. In either case the fabrication must always be organised in such a

way that the material will pass through a one-way system from receipt to final despatch

(Slide 1). A flow chart, as indicated in Slide 2, shows the main areas of activity in a

modern fabrication shop; the specific activities for a simple steel beam can also be

organised as a production line (Slide 3).

Slide 1

Slide 2


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Slide 3

Most fabrication shops are equipped with overhead travelling cranes, sometimes remotely

controlled from the shop floor. Mechanised conveyor systems are common in the larger

shops. They can greatly reduce handling costs.

Special facilities must be provided for the storage of flammable materials; pipelines for gas

and oxygen must be installed. Welding areas require a heavy power supply and screening

to protect eyes from ultraviolet glare. Some operations are very dusty and noisy, such as

mechanical chipping and arc gouging. Where possible, they should be separated, therefore,

from the other production areas.

3.2 Material Handling and Preparation

Material is taken into temporary stock in such a way that it can be easily identified and

moved. Some companies stack the material for easy access and move it by using cranes

equipped with chains and hooks. Other companies use a high degree of automation in their

material handling, using cranes on conveyors with magnetic lifting devices; Slide 4, for

example, shows a travelling Goliath Magnet Crane with the capacity to lift both plates and

sections (Slide 5 also shows a similar operation). Computerised records hold details of

member sizes, lengths, weights and steel quality, all related to an identification mark.


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Slide 4

Slide 5

When required, the steel is shot blasted in a separate location, either by hand or

automatically. Sometimes the automatic installations can sense the size of the members.

Paint-spraying (done either by hand or automatically) may follow directly after blast

cleaning, depending on the production programme; if, for example, welding is required

then painting will take place after fabrication.

3.3 Templates and Marking

Steel may be marked directly by hand with scribe lines and hole centres; nowadays,

however, in most shops pre-programmed automatic plant is in use. Traditionally, full-sized

templates, made of timber or heavy cardboard, were used to mark the steel for cutting and

for centre popping where holes were to be drilled.

Occasionally a drilled and bushed template, made of steel, would be used for mass-

produced items in order to avoid the template wearing out. Templates are still being used,

particularly for small plate fittings and gussets, but the templates themselves are made by


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automatic fabrication methods after plotting in the Drawing Office using computer work

stations. These techniques greatly reducing the work of the traditional skilled template

maker.

3.4 Sawing Line and Rolled Sections

The rolled sections are in most cases sawn to length, the other options being mechanical

cutting or flame burning. Three types of saws are available to the fabricator:

Circular saw;

Band saw;

Motor operated hacksaw.

By far the most popular choice is the circular cold saw, as its productivity is better than

that of the band saw or the hack saw. These saws are, in many cases, integrated in

automatic sawing lines, equipped with mechanised longitudinal and transverse conveyors

and measuring devices, as shown in Slides 6 and 7.

Slide 6

Slide 7


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A saw can perform within an accuracy of a fraction of a millimetre on length and within a

squareness of 0,2% of the depth of the cut. The most accurate type is equipped with a

swivelling arm enabling the blade to descend onto the bar. The blade speed adjusts itself

automatically on its way through the work piece. A fully automated saw system will be

operated through a computer program.

3.5 Drilling and the Beam Line System

The traditional method of drilling involves three operations:

Marking the position of the holes to be drilled;

Moving the member to the drill by crane, by conveyor, or by other means;

The actual drilling of the hole, using for instance, a radial drilling machine (radius

about 1,5 metre).

The better equipped fabrication shops nowadays have automatic beam-line systems (Slide

8) which are generally linked to the conveyors of the sawing line. The beam (Slide 9)

moves by longitudinal conveyors along the Y-axis, denoted V and X for each flange, while

the web drilling heads move along the Z-axis.

Slide 8

Slide 9


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Like the sawing line, this system is controlled by computer programs; some machines are

equipped with multiple drilling heads enabling them to drill several holes simultaneously

in each axis (Slides 10 - 12).

Slide 10

Slide 11

Slide 12


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New twist drills are currently available which are capable of higher speeds and greater

efficiency as follows:

Coolant fed drills, giving a threefold increase in drilling speed.

Titanium nitride coated drills, enabling a six-fold speed increase.

Carbide tipped drills with exceptionally high cutting speeds.

3.6 Cropping, Guillotines and Punching

Cropping shears can be used for cutting small sections of limited thickness.

Guillotines can be used for shearing plates up to 25 mm thick but the plate will usually

distort by the high pressure contact with the bottom blade; these, therefore, may only be

used where the specification allows. New punching machines, however, operating at high

speeds, will induce less distortion of the material.

Punching holes in steelwork is much faster, and therefore less costly, than drilling; its use,

however, is generally limited to predominantly statically loaded structures with limited

thickness, or to secondary members, unless HSFG bolted connections are used or the holes

are reamed out to a larger size. The maximum thickness where punching is applicable

depends on the material grade and quality.

3.7 Flame Burning of Plates

Bevelling and shaping of plates by flame cutting is general practice in most fabrication

shops. Oxygen and propane are usually stored in bulk in areas outside and supplied to the

shopfloor in pipelines. The equipment for flame cutting ranges from the simple hand-held

torch to multi-torch, numerically controlled, profiling machines (Slide 13). For wider

plates several heads can be arranged in order to ensure that equal heat is applied to both

edges, thereby avoiding distortion. The cutting carriage can even be provided with three

cutting heads in order to produce double-bevel edges.

Slide 13


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Single head machines can be operated by an optical controlling head, following a one-in-

ten or full size outline, drawn on paper. Profile cutting is often performed by numerically

controlled machines which also have the capacity to mark hole positions and hardstamp

identification marks.

For accelerated cutting speeds, where edge hardness is not considered detrimental, other

methods, like plasma cutting under water or under an inert powder, are available. Laser

cutting is just starting to come into use, but is, for the time being, restricted to thin plates;

the resulting edge hardness, however, makes it unsuitable for some applications.

The fabricator must be aware that flame cutting will always result in shrinkage, for similar

reasons as for welding.

Symmetrical burning of plates limits distortion. Machining of one side will result in

deformation due to the resulting residual stress.

3.8 Pressing and Forming

For the modern fabricator the most important application of plate forming and pressing is

to add to the available range of rolled sections. The trapezoidal shaped trough (Slide 14),

used to stiffen bridge decks, is a very good example. Other examples are the circular

sections of larger than standard dimensions (Slide 15).

Slide 14

Slide 15


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3.9 Methods of Welding

Three welding processes are most commonly used in modern fabrication shops:

Manual Metal-Arc Welding for fittings and for some profile and positional welding

(Slide 16);

Metal Active Gas Welding (MAG) and Cored Wire Welding with and without gas

(Slide 17);

Submerged Arc Welding (Slide 18) for fully automatic processes; particularly

useful for heavy welding in the flat or horizontal-vertical position and for the long-

run welds in plate and box girders.

Electric Arc Stud Welding, principally used in composite construction of steel and

concrete.

Slide 16

Slide 17


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Slide 18

3.10 Welding Design and Control of Distortion

Because a full penetration weld is more difficult to make than a full strength weld, full

penetration welds should only be used where necessary, such as in connections where high

fatigue stresses can occur. Good welding design reflects the economies and advantages of

different types of weld by choosing types appropriate to the needs of the design.

The full strength weld is easy to achieve using fillet welds; full penetration welds,

however, without inclusions at the centre, can only be achieved by extensive back gouging

prior to welding the reverse side. The risk of faults in full penetration welds is much

greater and control of distortion more difficult.

Control of distortion is achieved by prestressing the member before welding, or by a

balanced application of heat on each side of the neutral axis of the section. Allowance must

also be made for overall contraction due to welding. The quality of the fitting-up is very

important as any excess gap will affect the distortion and increase the shrinkage.

Maintaining the quality of the weld at the end of the run is difficult. The problem in butt

welds can be tackled by tacking short run-on and run-off plates on each side which are

removed after completion of the weld.

3.11 The Role of the Welding Engineer

Welding procedures are the responsibility of the Welding Engineer, who will produce a

procedure sheet for each weld. He will also make sure that the welder is qualified to the

required standard.

He may also supervise any non-destructive testing (NDT) which may be undertaken by

radiographic, ultrasonic, magnetic particle or dye penetrant means.

3.12 Automatic Production of Plate Girders

The welded plate girder forms a natural addition to the range of rolled sections available.

The typical production sequence is as follows:


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The plates for the girders receive an NDT check for laminations or other defects;

they are then flame cut to the required dimensions and shot blasted.

The girders are firmly clamped into position and tack welded; submerged welding

then follows, the welding heads moving along the weld lines (Slides 19 and 20).

Any stiffeners required are then tacked and welded, usually by MAG welding.

Slide 19

Slide 20

Simultaneous welding of the flanges will reduce distortion.

3.13 Machine Operations

Most fabrication shops are equipped with facilities for edge planing, for end milling and

for surface machining of plate (Slide 21).


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Slide 21

Unacceptable levels of hardness at the edge of the plate, often caused by burning, can be

removed by planing.

End planing of members is used to get a higher standard of squareness than can be

achieved by sawing. Optical laserbeam methods are used to align the axis of the member to

the cutting head.

Surface machining is only necessary for special bearing surfaces and sometimes for the

slab base plates of columns.

3.14 Fabrication Tolerances

Modern fabrication shops have accurate dimensional control over fabricated sections and

have no problems in cutting the rolled material to length. The main problem is coping with

the deviations in the sections and plates received from the steelmills. Euronorm (CEN) and

ISO standards give dimensional tolerances for rolled sections, plates and flats, hollow

sections and angles respectively. The fabricator will use bending rolls to straighten the

material and to "square" flanges of beam sections at critical connection points. As already

mentioned, the control of distortion due to welding during assembly is the important factor

in producing dimensional accuracy in welded sections.

The details and the connections must be designed in such a way that the tolerances will be

met within the limits of good workmanship. An example is given in Slide 22.


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Slide 22

3.15 Trial Erection in the Fabrication Shop

It is sometimes necessary to "prove" the dimensional qualities of the product by a trial

erection of one section of the structure in the fabricator's works.

Parts of bridge structures, particularly those bound for overseas locations, and structures

for the support of intricate industrial plant are likely candidates.

Trial erection is expensive and should be avoided where possible by incorporating methods

of site adjustment into the design and by optimum control of measurements.

3.16 Inspection and Quality Control

Quality Control should commence with the designer and continue through the preparation

of drawings and material procurement; maintaining the quality during the entire production

process will depend heavily on the fabrication details and on the material obtained.


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The larger fabricators have their own Quality Control Department, which will create and

maintain a QC-manual, describing the method of operation throughout the fabrication

process. The Quality Control Department will liaise with the shop management to make

sure that all workers have the skill required for the job on hand and that welders are

qualified to undertake the prescribed welding procedures.

Regular checks are necessary to ensure that:

All materials can be checked against specifications.

Material is checked for laminations.

Welding electrodes are identifiable.

Welding electrodes are stored in the required conditions.

Welding procedures are being followed.

Welding is being inspected during the process.

Correct procedures are in operation for tightening HSFG bolts.

Identification marks are clear and visible.

All equipment is maintained correctly.

Close liaison should always be maintained between the QC staff and the Drawing Office.

4. CONCLUDING SUMMARY

Good design makes efficient use of material and makes proper provisions for

tolerances in fabrication and erection.

Good interaction between shop floor and drawing office is indispensable for

economical and efficient fabrication and erection.

Labour should be used as effectively as possible so that labour costs are minimised.

Automatic processes should be used where feasible and appropriate.

Quality control is essential.

5. ADDITIONAL READING

1. Davies, B. J. and Crawley, E. J., Structural Steelwork Fabrication, British Constructional Steelwork Association (BCSA), London, 1980.

2. Arch, W. H., Structural Steelwork - Erection, British Constructional Steelwork Association (BCSA), London, 1989.

3. Firkins, A., Fabrication Cost of Structural Steelwork, Steel Construction, Vol. 24, No. 2, Australian Institute of Steel Construction, 1990.

4. Wardenier, J., Design and Fabrication of Steel Structures, Engineering Design of Welded Construction, IIW 1992, Houdremont lecture, Pergamon Press, 1992.

5. Various authors, Steel Construction Today, Vol. 5, No 3, Steel Construction Institute, May 1991.

6. Eurocode 3: "Design of Steel Structures": ENV 1993-1-1: Part 1.1: General Rules and Rules for Buildings, CEN, 1992.


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STEEL CONSTRUCTION:

FABRICATION AND ERECTION

Lecture 3.1.2: General Fabrication

of Steel Structures II

OBJECTIVE/SCOPE

This lecture can be considered as a supplement to the introductory Lecture 3.1.1. It deals

with the overall management of fabrication in the workshop in relation to the cost of

fabricated steelwork. It also gives a few examples of how to avoid complications during

fabrication.

PREREQUISITES

Essential:

Lecture 3.1.1: General Fabrication of Steel Structures I

The following lectures might be helpful:

Lecture 2.1: Characteristics of Iron-Carbon Alloys

Lecture 2.2: Manufacturing and Forming Processes

Lectures 2.3: Engineering Properties of Metals

Lecture 2.4: Steel Grades and Qualities

Lecture 2.5: Selection of Steel Quality

RELATED LECTURES

Lecture 3.3: Principles of Welding

Lecture 3.4: Welding Processes

Lecture 3.5: Fabrication/Erection of Buildings

Lecture 15A.8: Offshore: Fabrication

Lecture 15B.12: Introduction to Bridge Construction


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SUMMARY

This lecture deals with the cost build-up for fabricated steelwork delivered to site. It

discusses costs under the headings of materials, fabrication, protective treatment, delivery

and commercial factors. It gives examples of how detailing can be improved to minimise

costs.

1. INTRODUCTION

This lecture is a continuation of Lecture 3.1.1 and concentrates more on the costs

associated with material handling and fabrication equipment; it gives several suggestions of

ways to improve designs to facilitate more economic fabrication.

2. COST FACTORS

Modern programs for the calculation of costs determine the costs of steel structures in

relation to their component parts. For every part, the cost of material, handling, preparation,

welding, bolting, etc. is calculated.

The price of the steelwork, however, is often quoted in cost per tonne for material supply,

fabrication, protection treatment and delivery to site. For simple fabrication the material

cost is often equal to the total cost of preparing drawings, fabrication, treatment and

delivery. For complex structures the cost of fabrication alone can be much more than the

material value.

Typical types of steel structures, in ascending order of cost per tonne, are as follows:

Simple beam to column structures, in rolled sections;

Single-storey portal structures in universal sections;

Simply supported bridges in rolled sections;

High-rise structures with some welded sections;

Structures with welded plate and box sections;

Structures in tubular and RHS sections;

Bunker and silo structures;

Plate and box girder bridges in composite construction;

Trapezoidal bridges with orthotropic decks;

Special high weld content structures;

Offshore structures.

The cost build-up of "middle of the range" fabrications could be as follows:

Material 38%

Fabrication 32%

Drawing office 6%

Protection treatment 10%

Delivery 4%


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Commercial factors 10%

Each item, with the exception of the drawing office input which was discussed in the

previous lecture, is examined in the following sections.

2.1 Material

Steel supplies can be purchased directly from the steel mill or from stockholders. Generally

the larger quantities, delivery time permitting, should be obtained from the mills, since

their prices can be 10 - 15% lower than the stockholder's. However, for small quantities,

(say less than 10 tonnes) it might be more attractive to purchase from stockholders.

When preparing fabrication schedules, the cyclic production plan of the steel mill must be

considered since certain sections are produced more rarely than those in common use.

The fabricator will aim at minimum waste when ordering steel. He will usually allow, say,

an extra 2,5% to account for waste which he will eventually sell as scrap.

The steel mills base their price per tonne on quantities of 20 tonnes or more of mild steel,

and lengths up to 15 metres, cut to a tolerance of 50 millimetres.

Extra costs per tonne are incurred for small quantities of sections, and for higher grade

steels with tighter specifications.

Plates are ordered to the required thickness and in a range of widths and lengths, with

sufficient cutting and machining margins, minimizing waste as much as possible.

Circular hollow sections (CHS) and rectangular hollow sections (RHS) can be purchased at

a standard price per tonne. Extras are charged for small quantities, higher specifications,

and for a smaller length tolerance.

The cost of steel also depends on the certification required. Additional requirements with

regard to chemical composition, mechanical and toughness properties, e.g. Through-

Thickness-Properties (Z grade), result in extra cost.

Bearing these points in mind, the fabricator will make an analysis of all the material

required for the project to ensure minimal extras for small quantities. For the same reason

the experienced Engineer should avoid the use of too many different sizes in his design.

Material costs must also include such stock items as bolts, welding consumables, etc.

2.2 Fabrication

Fabrication will depend on the facilities and equipment available in the works. Working

under a roof will boost productivity because of independence from the weather. The larger

fabricator will install efficient, numerically controlled, production lines (which require no

marking of the steel) for burning, sawing, cropping, drilling, punching and welding. These

machines can be very efficient when run at high utilization rates. Smaller fabricators may

still make templates or mark the steel directly before cutting or drilling.


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Powered roller conveyors are now extensively in use in modern plants; the final weight of

the pre-assemblies is usually limited by the capacity of the overhead cranes or by the load

capacity of the available multiwheel-rollers. In the latter case the yard floor must have

sufficient bearing capacity.

Some fabricators have facilities for automatic blast cleaning and paint spraying.

Preheating (Slide 23) or post-weld heat treatment (Slide 24) is sometimes necessary, for

example when using thick material, but can often be avoided by appropriate specification

of the steel quality and welding procedures.

Slide 23

Slide 24

2.3 Protection of the Steelwork


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Protection costs will vary depending on the chosen protection system; for example, the

following systems are listed in order of increasing cost:

No treatment for covered internal steelwork;

Wire brush and prime, as a preparation for later painting;

Dip galvanizing;

Blast clean and metal spray;

Blast clean and a four-coat paint treatment.

Though it may add considerably to the transport costs, it is very important to handle the

material with great care after painting.

Extra costs are also incurred by having to provide areas which are to be left unpainted, such

as the bare steel surfaces required for high strength friction grip bolted (HSFG) connections

or for site welding.

2.4 Delivery

It is generally more expensive to transport pre-assemblies than to transport individual parts

of the structure. Extra costs may be incurred if weights or dimensions exceed those

Slide 25


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accommodated by standard vehicles. The rules differ in each country. Slides 25 and 26

show the present UK Ministry of Transport Rules.

Slide 26

The shipping costs of steelwork sent to overseas destinations can sometimes be more than

10% of the total production costs. Shipping costs are based either on weight or volume of

Slide 27


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the structure. It is advisable to consider these rates at an early stage. It is not uncommon for

a structure to have a "shipping weight" (Slide 27) which is five times the actual weight. To

reduce such costs it is desirable to do as much assembly as possible on site.

2.5 Commercial Factors

Fabrication costs must allow for commercial factors including costs for insurance, risk and

profit; they may also include cashflow, interest charges, bank guarantee charges and

retention costs.

In many contracts the client will make the payment in stages; depending on the contract

conditions payment may be made after receipt of the steel, after completion of drawings,

during fabrication, after delivery to site, or after erection, thereby helping to finance the

work. However, the fabricator must make provision for the cost of interest on his payments

and on the cost of retention, which can be held by the client for one or two years.

3. IMPROVEMENT OF DESIGN: EXAMPLES

The cost-conscious fabricator will always assess the quality of the design under the

following headings:

a. Fabrication,

b. Transport,

c. Erection,

d. Inspection,

e. Maintenance,

f. General costs.

a. The plate in the HE-section shown in Slide 28 cannot be welded with double fillet welds.

A single fillet weld, or a partial or full penetration weld, welded from one side, should be

used.

Slide 28


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The dimensions shown in Slide 28b are too small for proper inside welding. A single

fillet, partial or full penetration weld should be used and another solution for the

transverse stiffening panel should be considered.

Accessibility for welding is very important, (Slide 29). Overlapped joints in trusses of

tubular members (Slide 30) are less flexible when adjustment for dimensional tolerances

is required. If welding of the hidden locations between braces and chords is required, the

verticals cannot be positioned unless they are made in two pieces, each requiring

additional cutting, welding and inspection.

Slide 29

Slide 30

These complications will not occur if gap joints are used. An eccentric diagonal can be

used in this case, if eccentricity would otherwise exceed that allowable.


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b. The dimensions of pre-assemblies are limited by:

the width and height of bridges and viaducts, and the weight restrictions and traffic requirements in the case of road transport.

the capacity of barges (load and stability), water depths, height and width of bridges, capacity of locks, etc in the case of transport over water.

the load capacity of the yard and of the harbour, the tide during the loadout period, etc. in the case of loadout from fabrication shop or site. by transport barge.

In general the loadings on the structure, resulting from the particular transport system,

must be considered, e.g. inertia forces, supports, stability of the integrated system, etc.,

c. The erection and installation methods must be checked with regard to the availability of

the site and the heavy lifting equipment required, such as mobile cranes, shearlegs,

floating cranes, etc.

It is important to consider the nett capacities, taking into account the lifting radius, the

lifting height and the total weight of the lifting equipment including shackles and slings.

d. The inspection of the hidden welds of joints, such as those shown in Slide 8, presents a

serious difficulty. This is another reason for considering an alternative solution, as

mentioned in (a) above.

Slide 8

e. Crevices, sharp corners and details permitting ingress of water and dust should be

avoided from a maintenance point of view. The structural design should allow for

blasting, painting or other protective treatments.


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f. Avoiding stiffeners in deck beams, as shown in Slide 31, will cut costs considerably.

Slide 31

In many specifications, the inspection method is related to the type of weld without

considering its structural importance. A full penetration weld, requiring a rigorous

inspection, can in many cases be replaced by a more economic fillet weld.

The choice of plate thickness can have considerable impact on the requirements for steel

quality, preheating temperatures, welding electrodes and the inspections required.

4. CONCLUDING SUMMARY

The design engineer should be mindful of the processes used in fabrication and

erection and should ensure that unnecessary cost is avoided.

Material is cheaper when ordered in bulk. Small quantities of different sizes should

be avoided as far as possible.

The number of pieces to be handled should be reduced to a minimum and excessive

stiffening of members avoided.

Allowance should be made for weld distortion and fabrication tolerances.

Automatic fabrication techniques reduce costs.

The cost of delivery, particularly overseas, can be reduced by careful design.

Good quality control is essential but specifications should not be unnecessarily

stringent, since this will increase costs.

5. ADDITIONAL READING

1. Davies, B. J. and Crawley, E. J., Structural Steelwork Fabrication, British Constructional Steelwork Association (BCSA), London, 1980.

2. Arch, W. H., Structural Steelwork - Erection, British Constructional Steelwork Association (BCSA), London, 1989.


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3. Firkins, A., Fabrication Cost of Structural Steelwork, Steel Construction, Vol. 24, No. 2, Australian Institute of Steel Construction, 1990.

4. Wardenier, J., Design and Fabrication of Steel Structures, Engineering Design of Welded Construction, IIW 1992, Houdremont lecture, Pergamon Press, 1992.

5. Various authors, Steel Construction Today, Vol. 5, No 3, Steel Construction Institute, May 1991.

6. Eurocode 3: "Design of Steel Stuctures": ENV 1993-1-1: Part 1.1: General Rules and Rules for Buildings, CEN, 1992.

Documents

General Fabrication of of Steel Structures I