Economical Structural Steel Work

RAFTER

COLUMN

RIDGE

KNEE JOINT

HAUNCH

Economical Structural Steelwork

-Design of Cost Effective Steel Structures

Fifth Edition 2009

Editor John Gardner

Economical Structural Steelwork

edited by

John Gardner

economical structural steelwork _{handbook ii}

AUSTRALIAN STEEL INSTITUTE ABN/ACN (94) 000 973 839

Economical Structural Steelwork - Design of Cost Effective Steel Structures

Copyright © 2009 Australian Steel Insititute

Published by: AUSTRALIAN STEEL INSTITUTE

All rights reserved. This book or any part thereof must not be reproduced in any form without the written permissison of the Australian Steel Institute.

Note to commerical software developers: Copyright of the information contained within this publication is held by Australian Steel Institute (ASI). Written permission must be obtained from ASI for the use of any information contained herein which is subsequently used in any commercially available software packages.

First Edition 1979 Second Edition 1984 Third Edition 1991 Reprinted 1992, 1995, 1996 Fourth Edition 1997 Fifth Edition 2009

National Library of Australia Cataloguing-in-Publication entry: Economical structural steel / editor, John Gardner.

5th ed.

9781921476044 (pbk.) 9781921476051 (pdf.) Includes index. Steel, Structural.

Building, Iron and steel--Economic aspects. Gardner, J. R.

Australian Steel Institute. 624.1821

Disclaimer

The information presented by the Australian Steel Institute in this publication has been prepared for general information only and does not in any way constitute recommendations or professional advice. While every effort has been made and all reasonable care taken to ensure the accuracy of the information contained in this publication, this informattion should not be used or relied upon for any specific application without investigation and verification as to its accuracy, suitability and applicability by a competent professional person in this regard.

The Australian Steel Institute, its officers and employees, and authors and editors of this publication do not give any warranties or make any representations in relation to the information provided herein and to the extent permitted by law (a) will not be held liable or responsible in any way; and (b) expressly disclaim any liability or responsibility for any loss or damage costs or expenses incurred in connection with this publication by any person, whether that person is the purchaser of this publication or not. Without limitation, this includes loss, damage, costs, and expenses incurred as a result of the negiligence of the authors, editors or publishers.

The information in this publication should not be relied upon as a substitute for independent due diligence, professional or legal advice and in this regard the services of the competent professional person or persons should be sought.

Preface

When considering steel structures it is easy to obtain information on engineering and technological aspects,

however little information is available on how to choose steelwork economically. Increasingly, the viability of

a building project depends upon critical financial considerations. It is important, therefore, for designers to

have a good general appreciation of the components that make up the cost of fabricated steel, and of how

decisions made at the design stage can influence these costs.

This publication aims to supply some of this information. It is not a design manual, rather a publication that

discusses from a cost point of view the matters that a structural steel designer should consider. It takes into

account current fabrication practices and material/labour relationships, both of which have changed markedly

since the last edition of this publication.

Adherence to the principles outlined in this publication greatly assist designers in reaching decisions that will

lead to effective and economic structures.

This fifth edition has been updated in its references to Australian Standards and industry practices, and has

other amendments. It continues to provide useful practical advice towards the achievement of the optimum

result in structural steelwork.

This edition follows on from the previous edition by substantially adopting the rationalised approach to the

costing of fabricated steel by using a cost per metre for sections and cost per square metre for plates, depending

on the size, in lieu of cost per tonne. The basis for this approach is provided in detail in the following references:

• “A Rational Approach to Costing Steelwork” by T. Main, K.B. Watson and S. Dallas (Ref. 1.1), and

• “Costing of Steelwork from Feasibility through to Completion” by K.B. Watson, S. Dallas,

N. van der Kreek and T. Main (Ref. 2.13).

The costings given in this publication are indicative examples only and should not be used as absolute costs.

We wish to thank all those who have contributed to this publication through comments and inputs. This

includes a special acknowledgment to all ASI Staff who submitted comments on the technical and editorial

content of this publication.

Data for various tables was kindly provided by Beenleigh Steel Fabrications, BlueScope Distribution, Industrial

Galvanizers Corporation, International Protective Coatings and Promat.

Edited by: John Gardner

BE, MIE Aust., CP Eng., NPER.

ASI State Manager – Qld/NT

ASI National Education

Manager - Technical

economical structural steelwork _{handbook iv}

Contents

1. Preliminary Considerations 1

1.1 Introduction 1 1.2 Factors influencing Framing Cost 1 1.3 Integrated Design 2

2. General Factors Affecting Economy 3

2.1 Steel Grades 3 2.2 Economy in use of Material 4 2.3 Fabrication 5 2.4 Erection 7 2.5 Surface Treatment 9 2.6 Fire Resistance 11 2.7 Specifications 12

3. Framing Concepts and Connection Types 16

3.1 Introduction 16 3.2 Connection Types 16 3.3 Basic Framing Systems 19 3.4 Cost and Framing System 23 3.5 Framing Details 24 3.6 Conclusion 26

4. Industrial Buildings 27

4.1 Introduction 27 4.2 Warehouse and Factory Buildings 27 4.3 Large Span Storage Buildings 34 4.4 Heavy Industrial Structures 34

5. Commercial Buildings 36

5.1 Introduction 36 5.2 Low-Rise Commercial Buildings 36 5.3 High-Rise Commercial Buildings 37 5.4 Floor Support Systems 40 5.5 Composite Construction 41 5.6 Summary 42 6. Bolting 43 6.1 Introduction 43 6.2 Bolt Types 43 6.3 Bolting Categories 43 6.4 Factors Affecting Bolting Economy 44 6.5 Summary for Economic Bolting 45

7. Welding 48

7.1 Introduction 48 7.2 Types of Welds 48 7.3 Welding Processes 50 7.4 Other Cost Factors 51 7.5 Economical Design and Detailing 52

8. Detailing for Economy 56

8.1 Detailing on Design Engineer’s Drawings 56

8.2 Beams 56

8.3 Columns 59 8.4 Trusses 63 8.5 Portal Frames 65 8.6 Connection Detailing 66

9. References & Further Reading 75

10. Standards 77

1. Preliminary Considerations

1.1

Introduction

It is generally accepted that the objective of engineering design is the achievement of an acceptable probability that the structure being designed will retain its fitness for purpose during its planned lifetime. It is also of utmost importance that the initial costs plus the maintenance costs of the completed structure be within the limits provided by the Client.

For the design to be successful in the sense just outlined, the designer should search for design alternatives which consider strength and serviceability on the one hand, and economic feasibility on the other. In other words, out of a number of alternative structural solutions which comply with accepted design criteria for strength and serviceability, the designer should select the alternative likely to be the lowest overall cost. To do this successfully, the designer should develop an appreciation of the basic sources of expenditure in building construction and their effect on the overall cost of construction. In practice, the design problem is an optimisation problem. The solution to any optimisation problem involves having some means of judging the overall merit of alternatives. With regard to a building, the measure of overall merit, usually provided by the Client, will involve one or more of the following criteria:

(a) Functional requirements. (b) Strength and serviceability. (c) Aesthetic satisfaction.

(d) Economy in relation to capital and maintenance costs. This publication deals almost entirely with item (d) above.

In the preliminary and final design, the designer often deals primarily with member design and consequently tends to consider the minimisation of the mass of the structure as a guiding criterion towards achieving minimum cost. That is, the designer substitutes the more straight forward criterion of mass minimisation for the more involved criterion of minimum cost. In regard to steel structures, a minimum mass solution does not necessarily result in a minimum cost solution. Connection detailing and the resulting cost of fabrication and erection are more often the major influences affecting overall cost. Undue preoccupation with the minimisation of the mass of a steel structure can lead to serious errors of judgement.

This publication is intended to highlight the manner in which a number of factors affect the cost of steel detailing, fabrication and erection. It will also highlight the influence these costs have on the total final cost of a steel structure.

1.2

Factors influencing Framing Cost

much is the cost per tonne of fabricated steel. Such a question usually ignores the fact that a large number of factors have a significant influence on the final cost of fabricated steel.

A more rationalised approach to the costing of fabricated steel is based on a cost per metre for sections and cost per square metre for plates depending on the size

of the member. Fabrication costs for connections and erection costs, etc can then be added on a component by component basis (Ref 1.1).

For multi-level steel construction a cost per square metre can also be used for fabricated steelwork based on each floor area.

In the design, detailing, fabrication and erection of a steel structure, the following factors influence the cost of the framing:

(a) Selection of the framing system. (b) Design of the individual members. (c) Design and detailing of the connections. (d) Fabrication processes used.

(e) Erection techniques used.

(f) Specification for fabrication and erection. (g) Other items such as corrosion protection,

fire protection, etc.

The selection of the most efficient framing system is fundamental to achieving an economical framing solution and aspects relating to this item are discussed in Sections 3, 4 and 5.

Efficient member design remains an important cost factor tempered by the comments made in Clause 1.1. Detailed consideration of this item does not fall within the scope of this publication. One point that does deserve mention, however, is the avoidance of the individual design of every beam and column in an attempt to achieve least mass. The aim should be to group similar members (e.g. similar main beams in a floor grid) and adopt the one size for all members of the group. An experienced designer will optimise the design by being aware that if too much grouping is done, there will be material wastage. However, if little grouping is done, then there is a great waste of time on the part of the draftsperson and the erector.

Economic fabrication and erection are significantly affected by economical connection details. This publication is very concerned with economic detailing of steelwork and the manner in which detailing influences the cost of fabrication and erection. Sections 6, 7 and 8 deal with a variety of points which need consideration.

The specification (item (f) above) is a major influence on the cost of both the fabrication and erection since it specifies the quality of materials and workmanship required.

economical structural steelwork _{handbook 2}

1. Preliminary Considerations

1.3

Integrated Design

One of the obstacles to achieving maximum economy is that three of the most important activities in steel frame construction, namely structural design, detailing and fabrication, are usually done in isolation from one another. This is partly due to the specialisation in each of the disciplines and partly because of a lack of an effective dialogue among the people involved.

As a result of this, there often occurs a total preoccupation with the analytical phase of the design, and a complete absence of rational thinking about the detailing phase. Consequently, the problems that arise during the detailing phase are solved by complicating the detail rather than by modifying the design concept. When the job reaches the fabrication shop, there is little alternative but to carry out whatever happens to be shown on the drawings. A more ideal situation results when the design effort is integrated so that the framework, its members and its connections are considered as a whole. In this way, it becomes possible to modify the structural framing concept to allow the use of simpler and less costly connections in the interest of overall economy.

The cost factors listed in Clause 1.2 should be considered in an integrated manner so that interactions between the framework, its members and its connections are considered during the design process. In this way, one aspect can be altered to enable another to be improved. This enhances the overall cost efficiency of the final structure.

Obviously, such an approach ideally requires an extensive and up-to-date knowledge of the steel fabrication and erection industries. Since such knowledge is not always

easily achieved, communication with fabricators is a useful method of establishing the optimum practical solution. An interchange of ideas among fabricators, erectors and designers is an ideal situation for achieving optimisation.

It should be appreciated that what constitutes “design” and “good (i.e. economical) design” will vary depending on whose viewpoint is being considered. To the designer, an economical design is usually the lightest member to carry the load. To the fabricator, a “good design” means high tonnage output with minimum amount of labour. To the erector a ‘good’ design is one where most members are the same size and can be interchanged without any problems.

Clearly such different viewpoints are best resolved by an integrated and interactive approach on the part of the steelwork designer.

The Steel Detailer, using 3D modelling software, can assist in providing a service to designers by modelling the steel structure prior to engineering analysis and exchanging data in a Building Information Modelling (BIM) environment. The Steel Detailer can also provide a range of outputs for

the Steel Distributor and/or Fabricator to utilise, speeding up the production of structural steelwork. Guidelines on Steel Detailing outputs are provided in Ref. 1.5.

Further, the recent emergence of the Steelwork Contractor who integrates design, detailing and fabrication is providing a building solution which minimises overall costs. The Steelwork Contractor can also integrate following trades in order to minimise risk for the main building contractor and provide a “Total Solution”.

2. General Factors Affecting Economy

2.1

Steel Grades

Throughout the world the least costly and most commonly used grades of steel for structural purposes are those generally referred to as normal strength structural steel. In Australia such steel is covered by AS 3678 or AS 3679 (Parts 1 & 2). It has a typical design yield strength of 250/300 MPa (varying above and below this figure depending on thickness), a tensile strength of at least 410/430 MPa, a minimum elongation of 22% and a carbon equivalent of 0.43/0.44 so as to assure good weldability. AS 3678 and AS 3679 (Parts 1 & 2) are omnibus standards covering a family of structural steel grades including variants of the main grades having superior low temperature toughness.

Plates, rolled sections, welded sections and bars are all produced to these standards, although not every product is available in every grade. This is explained more fully in Table 2.1.

2.1.2 WEATHERING STEEL

AS 3678 and AS 3679 (Parts 1 & 2) also deal with so-called ‘weathering steel’. Weathering steel contains alloying

elements which cause it to weather to a uniform patina after which no further corrosion takes place. By nature of the chemical composition the steel is high strength (Grade 350) steel. However in Australia it is available in only a limited number of products – see Table 2.1.

2.1.3 HOLLOW SECTIONS

In Australia structural hollow sections are produced to the product standard AS 1163. This standard covers a number of cold-formed (C) grades. Rectangular hollow sections are available in Grade C350 and Grade C450. Circular hollow sections (CHS) are available in Grade C250 and Grade C350.

2.1.4 QUENCHED AND TEMPERED STEEL

Steel plates are produced in Australia in very high strength heat-treated grades known as ‘quenched and tempered steel’. These steel plates are useful in special applications where mass reduction is important (e.g. crane booms) or where their high wear resistance is needed (e.g. dump truck bodies).

Australian Standard AS 3597 covers these steel plates for structural steel applications and for use in pressure vessels.

2.1.5 CHOICE OF STEEL GRADE

Table 2.1 lists the availability of various products by steel

In large structures with longer lead times the use of higher grades will often be worth considering at least for parts of the frame. Heavy plate members such as bridge girders are one instance where higher grades may prove economical. Other applications include:

• Multi-storey structures, particularly with composite steel beams; also in maintaining the same column size down a building by varying steel grades; • Trusses and lattice girders.

Grade 350 steel costs around 5% more than Grade 300, and generally about 5% more to fabricate. To offset these cost extras, it provides greater yield strength but no increase in stiffness.

In some frames, significant reduction in steel mass may overcome the increase in material cost and fabrication cost by the use of higher grades. Each individual frame must be assessed on its merits, but there are undoubtedly applications where the use of higher grades is economical.

TABLE 2.1: Availability of products by Grade (check currency of information with steel suppliers)

Steel Grade

Plates (or Floor

plates) SectionsRolled SectionsWelded

Structural Hollow Sections Grade AS 3678 AS 3679.1 AS 3679.2 AS 1163 200 × – – 250 † × – 250L0 × × – 250L15 ‡ × – 300 ‡ † † 300L15 ‡ – + 350 † ‡ – 350L0 × × – 350L15 ‡ – – 400 × – † 400L15 × – ‡ WR350/1 ‡ – – WR350/1 L0 ‡ – – C250 † C350 † C450 †

Quenched & Tempered Structural Steel AS 3597

80 †

Notes:

† Regular grade commonly produced, readily available from stockists.

economical structural steelwork _{handbook 4}

2. General Factors Affecting Economy

While the information presented in Table 2.1 is indicative of the general situation, it must be remembered that the steel suppliers are always willing to discuss special cases where, for example, the economics of a high strength steel has been considered by the designer and the sections required are not normally manufactured in that grade. For a project requiring large tonnage of specific sections, it may be possible to negotiate a special order with the supplier, provided that an arrangement has been agreed at an early enough phase in the design.

Conversely, on average projects the designer should always be careful to keep within the range of readily available products so as to ensure that no problems of steel procurement occur at the fabrication stage.

TABLE 2.2: Indicative cost ratios for different grades of structural steel (per tonne, supply only)

Grade Plates SectionsRolled SectionsWelded AS 3678, AS 3679.1 & AS 3679.2 Grade 250 100 100 – 250L0 – 105 – 250L15 110 105 – 300 100 100 100 300L15 105 – 100 350 105 105 – 350L0 – – – 350L15 110 – – 400 115 – 105 400L15 120 – 105 WR350/1 125 – – WR350/1 L0 135 – – AS 1163 Grade C250 130 C350 130 C450 130

AS 3597 Quenched & Tempered Steel

80 200

2.2 Economy in use of Material

As well as having a knowledge of the factors affecting the choice of steel grade, the designer should also be aware of how design decisions can avoid unnecessary material cost or wastage. This will involve a study of the factors discussed below.

2.2.1 STEEL PRICING

Mill prices are expressed in terms of a base price and various extras. The base price relates to the type of mill

product such as plate or sections, while extras relate to specifics of the particular product or section.

The most common extras for structural quality steel include the size or designation, standard or non-standard lengths, quantity extras or discounts related to the total mass of individual order items, and the grade extras which apply to the quality specification for the material chosen. Quality extras for structural steel relate to the material specifications and reflect the costs of alloying elements, of tighter controls on such elements as carbon, manganese, phosphorus and silicon, and of tighter controls on manufacturing techniques to meet the specified chemical and mechanical properties. The cost of additional tests and greater frequency of testing, necessary for increased stringency of yield strength and notch ductility, are also reflected in increased quality and testing extras.

Designers should recognise that the more exotic the requirements of the steel specification, the greater is the probability that other costs associated with its use, ranging from procurement through all stages of fabrication, will also be increased. Unnecessary demands by specifiers for mill heat certificates for standard sections of known origin to be used on routine projects is another example of unnecessary costs added onto projects.

The foregoing relates to purchases made direct from the steel mill, but in Australia most fabricators obtain their steel through steel distributors. These steel distributors aim to carry comprehensive stocks and are thus able to offer prompter delivery than would be available through the normal steelmaker’s rolling programs. Their stock holding tends to concentrate on popular, high turn-over items.

TABLE 2.3: Preferred steel plate thicknesses (in mm)

3 25 70 4 28 80 5 32 90 6 36 100 8 40 110 10 45 120 12 50 140 16 55 150 20 60 2.2.2 PLATES

In Australia there is a rationalised series of ‘preferred’ plate thicknesses as listed in Table 2.3.

For practically all structures the designer should operate within this standard range. Non-preferred thicknesses incur cost premiums and extended delivery times, and should only be considered on major projects where the overall saving in using a special thickness is greater than the direct and indirect cost penalties.

2. General Factors Affecting Economy

Similarly there are preferred lengths and widths of plates which should be borne in mind. Major plate elements should be dimensioned as far as possible so that they can be cut from standard plates with a minimum of scrap. Smaller plate details such as brackets and gussets should be considered in the same way, especially when there is a large number of them. The most common sizes for plates up to 25 mm thick are 1.8m × 6m, 2.4m × 6m, 2.4m × 9m, 3m × 9m and 3.2m × 12m.

Note: Small plate components may be substituted by flat bars which are considered as sections.

2.2.3 SECTIONS

Australia produces a range of welded products, universal sections, channels, angles, and hollow sections which provide the designer with a reasonable choice without the proliferation which can lead to problems of availability. The lowest weight in each nominal size of universal section is the most structurally efficient and they account for over two-thirds of all UB sales. The designer should therefore make every endeavour to keep to the lowest weights in each size range, although this will not always be possible. Very long lengths of sections become difficult to keep straight and to handle, and the mills impose a price extra for them. It should be especially noted that although universal sections are listed as being available up to 18m long (and up to 22m by enquiry), the usual maximum length found in stock is around 18m. The available lengths of structural hollow sections are usually restricted to 6.5m (circulars) or 12m (rectangulars and squares).

2.2.4 SCRAP AND WASTE

The real cost of material is affected by the quantity of scrap and waste, and designers should be receptive to suggestions for minimising and controlling the generation of waste. This may include greater standardisation of structural sizes, or of plate widths and thicknesses, in order to take advantage of size and quantity discounts. It might also include a more liberal approach to the splicing of beams or other structural sections using standard lengths.

Random splicing, which involves welded splices anywhere within the length of a rolled structural member, can be particularly effective when material is sawn to length and fabricated on a conveyorised production line. When carefully controlled, it can dramatically reduce the accumulation of shorts and thus reduce the total cost. The only real restriction to random splicing applies to

2.3 Fabrication

Fabrication costs are a function of complexity and are influenced by:

• Size of the component

• Size and type of sections involved • Amount of stiffening and reinforcing required • Amount of repetition • Shop and field details • Space requirements in the shop, and • Facilities available for handling, lifting and moving the structural components.

Fabrication costs are sensitive to simplicity or complexity of detail, and the degree to which production line techniques can be applied. They are controlled by the quality of the shop detail drawings, which must reflect the designer’s concept for the structure, but must also permit the optimum utilisation of the fabricator’s facilities and equipment. Shop drawing preparation should be guided by the basic principle that they must provide for economy of fabrication and for economy of erection.

Shop operations basically involve cutting material to size, hole-making for mechanical fasteners, and assembling and joining. Other operations include handling, cleaning and corrosion protection. All shop operations require facilities for lifting and for moving or conveying the structural steel.

Cutting operations include shearing, sawing and flame cutting; hole-making operations include punching and drilling; assembly operations include welding and bolting. Increased use of computer numerically controlled (CNC) fabrication processes is changing the economics of steel fabrication. Cutting, drilling and welding operations can now be undertaken by the CNC fabrication process. Information from computer drafted shop drawings can be fed directly into CNC fabrication equipment to further improve operational efficiency. Some fabricators are now bar coding steelwork to facilitate control and monitoring of projects.

Generally welding is the preferred method for shop assembly, with bolting for field assembly. There are, however, some fabricators with sophisticated hole-making equipment, who prefer shop bolting to shop welding for standard connections. Some steel merchants also provide basic cutting and drilling services to the steel fabricators.

economical structural steelwork _{handbook 6}

2. General Factors Affecting Economy

2.3.2 BEAM AND COLUMN FABRICATION

A large part of structural steel fabrication consists of beam and column work. It embraces framing members consisting of standard rolled shapes connected by shear or moment connections, and also includes highly irregular framing members with custom designed built-up sections and complex connections designed for combinations of shear, moment and direct tension.

Simple beam and column fabrication lends itself to production line methods, in which the members are transported on a series of conveyors to saws which cut the material to length, and to hole-making equipment which provide holes in either the web or flange or both. Any additional requirements, such as the attachment of cleats or brackets, are off-line operations. It is important therefore that connections and other details be selected so as to provide the maximum number of members with only cutting and holing. Otherwise the economy of using CNC equipment and the conveyorised beam-line system will be less apparent (see Figures 3.13 and 8.29). Many steel distributors now offer steel pre-processing services where steel sections and plates are cut and drilled to size. The fabricators then weld the components together in the workshop.

2.3.3 GIRDER AND TRUSS FABRICATION

Fabrication of plate girders and trusses differs from beam and column work in that it involves assembly in the shop, and calls for adequate space and handling facilities. Both girders and trusses require special fit-up jigs for assembly and welding, and the availability of heavy lifting equipment. Just as with beam and column work, however, the key to productivity and economical fabrication is the use of simple standard details for stiffeners, splices, gussets, etc. For plate girders all details should be designed for automatic welding, allowing adequate clearances for the welding machines to pass and for termination of welds at the ends of web stiffeners. Maintaining constant width flanges within a shop fabricated length of girder permits splicing of multiple width plate and subsequent stripping to finished width. This will reduce weld set-up time, eliminate weld starts and stops, and require only one set of run-on and run-off tabs. Reductions of flange widths, web depths and plate thicknesses purely to reduce mass should be considered very carefully as they can significantly increase fabrication costs. Control of distortion in plate girder fabrication is a major problem which can be helped by design which minimises the amount of welding and avoids the use of significantly non-symmetrical sections. It is false economy to design for minimum web thickness only to require web stiffeners, thereby increasing the amount of welding and distortion; or to use very light top flanges in composite girders only to compound the problem of camber control. See also Clause 8.2.5.

Trusses can be designed in a large variety of configurations which depend on the truss span, depth and loads to be carried. Therefore, it is impossible to make general statements regarding the most economical design for fabrication, other than to stress again the importance of simplicity of detail. Designers should avoid situations that can cause weld restraint and problems resulting from weld induced distortion. As far as possible trusses in the one project should have the same configuration so that they can all be fabricated from the one jig.

In truss work, the correct selection of chord members can often remove the need to turn the truss over during the fabrication (see Clause 8.4). This will enable the fabricator to complete the entire welding on the truss component without further handling.

2.3.4 SUMMARY FOR ECONOMIC FABRICATION

The key to economic fabrication is the use of standards at all stages. This includes standard procedures, standard schedules, standard drawings, and above all standard connections and details. Non-standard details are usually handled as ‘special job standards’; however, the net effect of any specials is to slow production with some loss of fabrication economy.

In the selection of connections the designer should observe the following principles:

• Select members and connections to provide a maximum of repetition throughout a structure. This provides the fabricator with the opportunity to make up jigs and fixtures to speed up the fabrication process.

• As far as possible, select connections so that the assembly of fitments on a member can be carried out in one position. This will reduce the number of handling or rotating operations during fabrication. • Keep the number of components in a connection

to a minimum.

• Select connections so that assembly of components occurs on the least number of members.

• As far as possible use connections that are standard in the industry (see ASI: Connections Design Guides – First Edition 2007 (Ref. 1)). • Ensure a minimum standard of documentation

in line with ASI’s publication: “A Guide to the Requirements for Engineering Drawings of Structural Steelwork” (Ref. 2.12).

• Most importantly, keep an open mind on the selection of members and connections. Before finally committing a design to the detail design phase, communicate with the industry and try to determine the best solution to optimise the use of material and labour in the fabrication shop. This industry communication can often be facilitated through the services of ASI.

2. General Factors Affecting Economy

2.4 Erection

2.4.1 GENERAL CONSIDERATIONS

The rate of erection of steel in a structure is controlled by five main factors:

1. Connection simplicity 2. Number of members

3. Number of bolts and/or amount of field welding 4. Size and efficiency of erection crew, and the

equipment at their disposal 5. Timely supply of steel.

It is interesting to note that of these factors, the first three are under the control of the designer.

Connections should be simple, and of such a type that the allowable tolerances (in member size and shape, detailing and fabrication) can be accommodated during the placing of the members.

The number of members should be kept to a practical minimum and so should the number of bolts or amount of field welding. There should be sufficient access for welding or for tightening bolts using power wrenches. Bolted connections should be used wherever possible and field welding kept to a minimum. Connection plates should be shop welded to one member rather than field bolted to both, unless other considerations govern. Every endeavour should be made to standardise as far as possible (member sizes, bolt sizes, type of connection, gauge lines, member spacing, etc.), and careful consideration should be given to how a member is to be installed with minimum interference by other members, gusset plates, etc. (see Ref. 1).

With an increasing awareness of the importance of employee safety in the work place, erection methods are changing. Designers and erectors have a duty of care and should consider safe erection methods. The use of equipment such as cherry pickers is becoming more common during erection. Designers need to include anchorage points for safety lines and harnesses for riggers. These issues are resulting in steelwork being erected on the ground and then craned up to final position in many projects to reduce the amount of work done at great heights. This may require alternative design and detail methods and utilisation of additional short term cranage but provides a safer work site. A safer work site will lead to faster and more economical erection.

2.4.2 HANDLING AND TRANSPORT

As a general rule it is more economical to erect fewer large pieces than many small pieces, due to the number of lifts involved and the number of joints to make. Generally this

field splices. For example, with long flexible trusses, the transportation length may have to be curtailed to avoid damage during transfer to site or to avoid obstructions along the way.

Large sub-assemblies may require to be transported using special vehicles attended by police escort, and this may add greatly to the final price of the structure. However, projects outside capital cities could use this approach as it minimises the size of the site crew required to be mobilised on a remote or semi-remote site. With greater availability of larger mobile cranes and trucks, the balance between transport costs and site costs is changing. Where projects require large site crews, minimising time spent on site is essential to economical erection. The erection or trial erection of large components in a fabricator’s yard before delivery to site is good practice and a cost savings exercise. Trial erection guards against fabrication errors being discovered on site which may prove expensive to rectify.

To minimise transport costs it is important that vehicles travel fully laden. The dimensions of a typical load of structural steelwork which requires no special escort are in the order of 15m long × 3m wide × 2m high. It is important that like pieces are loaded together to optimise truck capacity, but also that the components be delivered to site in the order required by the erection sequence (i.e. columns followed by beams from the ground upwards). This will save double handling on site and also reduce the

cost of site storage and possible damage.

The virtue of designing for repetitive components has already been stressed. The gains can be partly lost on site if interchangeable parts are given individual mark numbers. This will require the erector to search for a particular number mark on a member when any one of a considerable number of members would fit. After completing a design it is worth looking at marking plans with this idea in mind.

Indicative transportation costs are given in Table 2.4. Costs include the loading of steelwork onto and off the truck.

TABLE 2.4: Transportation costs

Transport to Site (see Note)Fabrication Shop Section Mass (kg/m) $/member

0 to 60.5 20

60.6 to 160 70

160.1 to 455 260

Notes:

economical structural steelwork _{handbook 8}

2. General Factors Affecting Economy

2.4.3 CONNECTIONS

It is in the final fixing of members that the greatest scope for erection economy lies. Connections selected to permit flexibility in fit up should be of prime concern to designers. The use of one type of bolt and one bolting procedure throughout a structure will allow the use of a minimum variety of tools on site and provide for speedy erection sequence (see Section 6). Similarly where site welded connections are required, cleats should be incorporated to allow mating members to be held together in place for actual welding.

Angle seat, angle cleat and web side plate connections (see Clause 8.6.2) provide considerable flexibility in fit-up, and are preferred in braced frames from a purely erection viewpoint. The flexible end plate connection is not quite so easy to erect, although its selection may be decided by other considerations.

In rigid frames, the following should be taken into consideration for the design of bolted connections:

• The end plate depth should be kept to a minimum to reduce the tendency to jam during installation (Figure 2.1).

• The tolerance between the face of the end plate and the face of the column should either be tightly controlled so that the building plumbs itself automatically, or allowance should be made for shimming in order to plumb the building. Shimming, however, can be expensive.

• In end plate connections for portal frames careful consideration should be given to access for installing and tensioning bolts, (see Table 8.1). If welded connections are preferred, the following should be taken into consideration:

• Welded connections are normally erected using a bolted erection connection. The same criteria should apply to the design of these connections as described above.

• Substantial erection clearance between the end of the girder and column face should be provided where permitted by the design of the connection. • Field welding should be kept to a minimum and

overhead welding should be avoided.

• Attention should be paid to access for welding and welding inspection.

• Consideration should be given to plumbing the building.

The most significant time delays in the erection of a girder can be expected to occur when it is installed with the end connection against a column web. The girder can normally only be manoeuvred in a vertical plane and

frequently jams. Gusset plates, stiffeners, and other members tend to interfere with its installation. Access for bolting is usually difficult and sometimes impossible. Every effort should be made to get the connection outside the flanges of the column, or at least as far out from the web as possible. This is especially important when the column section is compact. Consideration should always be given to excluding direct girder/web connections even if it involves increasing column weight, and/or fabrication costs (see Figure 2.2).

FIGURE 2.1: Deep end plates can cause jamming

FIGURE 2.2: One example of how to avoid the problem of access to column web connections

2.4.4 FIELD BOLTING

In projects with a predominance of large connections, threads may be excluded from the shear plane for bearing type connections as this will help to reduce the number of bolts. However with Australia’s ISO metric long-thread bolts, care should be taken that the long ‘stick-through’ that occurs does not cause fouling or access problems. In projects with small connections the saving in number of bolts is not so evident and it is more economic to design for threads included in the shear plane. This then means that bolt lengths can be selected so as to avoid excessive stick-through. However the two systems (threads-in, threads-out) should not be mixed on the one job (see Ref. 6.1).

2. General Factors Affecting Economy

Generally, the smaller the bolt the easier it is to install. Bolt diameters should therefore be kept small if this can be done without compromising the objective of keeping the number of bolts to a minimum. M12 bolts are normally adequate for stairs and girts, while M20 bolts are the maximum size which should be considered if access for tensioning is poor; otherwise M24 bolts are acceptable. Bolts should be specified as ‘snug-tight’ unless there are compelling reasons why fully tensioned bolts are necessary. The cost of full tensioning, including associated inspection, is very high and can double the cost of each installed bolt. Access for wrenches is also less critical where only snug tightening is to be carried out. Care should be exercised, however, where a project is designed to overseas codes because some of these require high strength structural bolts to be always fully tensioned.

It is preferable that only one bolting category (see Section 6) be used on any one structure. When a departure from the general category (e.g. to fully tensioned bolts, to threads excluded from shear plane, etc.) is unavoidable, this should be highlighted on erection and detail drawings to reduce the possibility of the requirement being overlooked by erection crews.

More information on structural bolting is given in Section 6 and Ref. 6.1.

2.4.5 FIELD WELDING

Where site welding is used for connections the total amount of welding on the job should be sufficient to justify the cost of bringing and setting up welding equipment on the site.

Access for welding is also important, and it should be remembered that a welder generally requires a substantial and carefully placed working platform.

Otherwise the normal rules for economic welding apply. Fillet welds are preferred to butt welds, and down-hand welding to any other position. In most structural work difficult out-of-position welds such as overhead are very slow and costly (see also Section 7).

2.4.6 BRACING

Bracing is usually difficult and time consuming to install. To reduce erection time, the number of braced bays should be kept to a minimum (i.e. fewer braced bays with heavier bracing is preferred).

Wherever possible, wall bracing should be connected to columns rather than beams. This allows bracing to be installed before the beam above is in position, hence reducing any interference this beam may cause during erection. Connecting the brace to the column at its lower end eliminates interference to the floor system resulting

2.5 Surface Treatment

With the development in recent years of a large variety of surface treatment methods, the designer may experience considerable difficulty in selecting the optimum system for a particular application.

Furthermore, it is often not fully realised that the cost of a sophisticated multi-coat treatment system can easily be more than the cost of the raw steel itself. Thus care is needed to avoid unnecessary, and sometimes unexpected, surface treatment costs.

These costs are a function of surface area which can vary with both, the type of section used and the class of construction.

For example, a structural hollow section has typically only one-half to two-thirds of the surface area of an ‘open’ structural section (UB, UC) of equivalent capacity, for this reason, hollow sections are well worth bearing in mind for applications requiring any significant amount of multi-coat surface treatment.

Heavy steel construction such as for power stations usually averages out with comparatively less surface area (despite the higher tonnage) than a typical factory or warehouse where light trusswork may have a much greater surface area (despite the lower tonnage). Obviously treatment costs on a per square metre basis will vary widely depending on the actual surface area to be treated.

2.5.2 STEEL PERFORMANCE

Bare steel will corrode only in the presence of both oxygen and moisture. Corrosion will be accelerated if traces of pollutants such as sulphur dioxide or chlorides are present – the so-called ‘aggressive environments’. Steel inside a building is rarely a corrosion risk except in the occasional case where the building houses an aggressive atmosphere as a result of its purpose, (e.g. a fertiliser factory). It follows therefore that steel needs no corrosion protection whatsoever in most interior applications such as multi-storey buildings where the steel framing is eventually concealed.

Where the steelwork remains exposed to view as in a factory or warehouse the same negligible risk applies but in these instances the owner may require a surface finish for a more attractive appearance. The designer should distinguish between treatment specified to achieve protection from corrosion and that specified merely to provide decoration. In practice, of course, any surface finish will attempt to do both.

economical structural steelwork _{handbook 10}

2. General Factors Affecting Economy

2.5.3 SURFACE PREPARATION

An important part of any steel treatment system is the preliminary surface preparation. This can range from simple degreasing and brushing to costly chemical or mechanical descaling.

The surface preparation should be matched to the applied finish. Expensive paint systems will not last if applied to only partially prepared (e.g. wire-brushed) surfaces. Conversely it is a waste of money applying a low-cost porous alkyd primer to a descaled ‘white metal’ surface. Various methods of surface preparation are covered by AS 1627 ‘Metal finishing preparation and pretreatment of surfaces’ (see Section 10), and advice on their selection is contained in AS 2312 (see Section 10).

The most commonly used methods in Australia are wire brushing (suitable for low cost paints) and abrasive blasting to Class 2-1/2 of AS 1627 Part 4 (needed for high performance paint systems). Wire brushing is a time consuming and costly preparation method and would normally only be considered if the work was to be performed on site. Acid descaling (‘pickling’) is encountered mainly as part of the hot-dip galvanising process (see Clause 2.5.5).

An idea of the costs of various methods of surface preparation is given in Table 2.5.

TABLE 2.5: Surface treatment costs

Section Mass

Paint Type Hot Dip Galvanise ROZP + Alkyd ROZP

Gloss IOZ Zinc-Rich Epoxy + Epoxy MIO (kg/m) $/m2 _$/m2 _$/m2 _$/m2 _$/m2 0 to 60.5 18 24 29 42 21 60.6 to 160 17 23 28 40 34 160.1 to 455 15 22 27 38 55 Notes: 1. ROZP – single coat of red oxide zinc phosphate primer @ 40µm DFT applied to a Sa2 blast cleaned surface. 2. ROZP + Alkyd Gloss – red oxide zinc phosphate primer

@ 40µm DFT plus alkyd gloss @ 40µm DFT applied to a Sa2 blasted surface.

3. IOZ – single coat of inorganic zinc primer @ 75µm DFT applied to a Sa2½ blast cleaned surface.

4. Zinc-Rich Epoxy + Epoxy MIO – 2 pack zinc rich epoxy primer @ 75µm DFT plus 2 pack high build epoxy MIO @ 150µm DFT applied to a Sa2½ blast cleaned surface. 5. These prices are intended for comparison use only and are not absolute. Please refer to coating contractor for current pricing.

2.5.4 PAINT SYSTEMS

There is a very large selection of paint systems available for structural steel – too many to be discussed within the scope of this publication. However, excellent guidance on the performance and capabilities of various paint formulations is given in AS 2312.

Probably the most commonly used paint is ‘red oxide zinc phosphate primer’, often referred to as ROZP. Paints of this type provide an economic base for possible further decorative coats of conventional oil paint. However being permeable, ROZP cannot be expected to last if left in the open for more than normal construction periods. Another regularly used paint is ‘inorganic zinc silicate primer’ which is applied over a Class 2-1/2 abrasive blast preparation. It forms an excellent base for most high performance paint formulations, or gives good results as a single coat protection for steel in all but the most aggressive environments.

Paint is normally applied to steel by spraying. It is sometimes suggested that better coating is achieved by brush application, but there is little evidence to support this claim. Brush application costs two to three times as much as spraying, and cannot be used at all for some modern paints; inorganic zinc silicate is an example.

If a multi-coat paint system is required then it is recommended that a rapid cure system be specified to allow a quicker turn around of product.

Table 2.5 includes the cost of the finish painting in the surface treatment costs. It should be noted that transportation costs should also be considered if the treatment is done at premises other than the fabrication shop. Table 2.4 gives an indication of transportation costs.

2.5.5 HOT-DIP GALVANISING

Galvanising is carried out by specialist firms and the process requires pre-cleaning and surface preparation, usually by pickling. The cost of galvanising includes these preparatory processes.

Advice on the performance of hot-dip galvanising, either as a single coat protection or as a base for paint systems, is contained in AS 2312.

When considering galvanising the designer should ascertain the scope of local facilities, and in particular the size of the available galvanising baths. The galvanising bath determines how big an individual component can be dipped. (Items larger than the bath can sometimes be galvanised by ‘double dipping’ but at extra handling cost). Information on bath sizes in Australia is given in ‘After Fabrication Hot-dip Galvanising’ (Ref. 2.4).

2. General Factors Affecting Economy

2.5.6 DESIGN AND DETAILS FOR CORROSION RESISTANCE

In a severe environment where steelwork is exposed to aggressive conditions the designer can vastly enhance the corrosion resistance of the structure by careful attention to a few simple principles. Conversely a structure with bad details will not perform satisfactorily no matter how much has been spent on elaborate multi-coat protective systems. Fortunately, the principles of good corrosion detailing are generally much the same as those for economic fabrication. Connections and other details should be kept as simple as possible with the minimum number of components. Depressions, pockets, ledges, narrow crevices and anywhere where water and foreign matter may lodge permanently should be avoided whenever possible. In really severe situations the use of box sections, CHS or RHS might be considered. Several examples of good and bad practice are given in AS 2312.

2.5.7 SUMMARY CHECKLIST FOR SURFACE TREATMENT

1. The required level of surface treatment and/ or corrosion protection should be decided at the very earliest stage of the design, so that all design decisions can be made with this in mind. 2. In benign atmospheres such as the interiors of

most buildings, or exposed steelwork in non-polluted non-marine environments, corrosion rates are generally so low as to not require corrosion protection. Any painting carried out would therefore be only for aesthetics. 3. Where corrosion protection is required,

the extent needs to be carefully evaluated to ensure that it is appropriate to the circumstances. Too much protection is a waste of money, as also is too little. Obviously professional judgement is needed.

4. The degree of surface preparation should match the surface treatment system to be applied (see Clause 2.5.3).

5. As painting is substantially a labour intensive process, the current trend is to replace multi-coat (3 or 4 multi-coat) systems with one or two coat systems. Zinc-rich paint systems are consequently increasingly used, particularly on blast cleaned surfaces. In these systems, however, film thickness build is vital to a satisfactory performance.

6. Good design practice is essential – e.g. avoid pockets where water and debris can lodge and accelerate coating failure (see Clause 2.5.6).

be handled, transported and erected without damage to the coating from crane slings, etc. Touching up of the base coats and the final top

coat must therefore be done on site. 9. Hot-dip galvanising is a high performance

protective system which is not prone to damage during transport and handling. In some circumstances it may cost the same as an alternative paint system (see Table 2.5). 10. Recent developments in the field of corrosion

protection have evolved protective systems greatly superior to those available some years ago. These systems are expensive but are invaluable when appropriate, as in exposed structures in severe industrial or marine environments. However, this has led to waste of money by the specification of such sophisticated treatments in circumstances where they are not necessary.

11. Some paint systems require special application techniques, controlled temperature and humidity when being applied, long drying times or may have a tightly constrained time interval between successive coats. Designers should be careful of such sensitive systems as experience has shown that they are almost impossible to apply correctly in normal construction industry conditions.

2.6 Fire Resistance

2.6.1 GENERAL CONSIDERATIONS

All structural material can be damaged in severe fire conditions and steel, although non-combustible and making no contribution to a fire, can have its function impaired. For this reason, building regulations require it to be protected, usually by a non-combustible insulation, when used for certain elements of construction in some types of building. Building regulations prescribe statutory levels of fire resistance for structural steel members in many types of applications.

The fire resistance level of a building element or structure is determined by constructing a truly representative prototype of that element or structure incorporating fire protection materials, systems or coatings where necessary and submitting that prototype element or structure to the Standard Fire Test. The Australian Standard Fire Test is given in AS 1530 Part 4 which enables a fire tested element or structure to be assigned a fire resistance level in accordance with the criteria laid down in the fire test standard. Fire resistance ratings are expressed in minutes such as 30 min, 60 min, 90 min, 120 min, 180 min or 240 min.

economical structural steelwork _{handbook 12}

to predict from a particular test the fire performance of a similar but slightly different configuration – calling perhaps for further expensive tests. Secondly, it has been shown that the conditions of the standard fire test do not replicate the observed behaviour of actual building fires. The present day trend is toward the development of fire engineering design rules whereby the engineer can design for fire performance in the same way as he or she does for structural performance. The Australian design code AS 4100 contains a comprehensive section on design for fire and it seems likely this approach will become a more common procedure.

2.6.2 REGULATORY REQUIREMENTS

Australian Building Regulations require that elements of a structure achieve specified fire resistance levels (FRL). The level of fire resistance required for a particular application is related to the expected fire load within the building (which is in turn related to type of occupancy), to the building height and area and to the fire zoning of the building locality and the on-site positioning. It is not within the scope of this publication to repeat the requirements of the various Building Regulations.

The fire ratings of common building elements have become well established by virtue of accumulated testing and accepted values are specified in the various Codes and Regulations. Unprotected steelwork does not normally attract any FRL, except where specialised approaches are adopted. One example is in open car parks where full scale tests have demonstrated that bare steel will not reach a critical temperature should a car catch fire (Ref. 2.5). Another example is composite steel deck floor systems utilising fire emergency reinforcement (Refs 2.6, 5.4, 5.5).

2.6.3 MATERIALS FOR FIRE PROTECTION

Where steel has to be protected, the most practicable way is to cover or encase it in a protective material. Such material should be:

• Fully tested and approved • Non-combustible • Unable to produce smoke or toxic gases at elevated temperature • Able to be efficiently and uniformly applied • Durable to prevent dislodgment • Thermally protective • Fully supported by the manufacturer with regards to full applicator training, work auditing and quality assurance inspections.

Another important factor to consider is that dry systems are applied onsite, whilst intumescent coatings may be applied off site. Intumescent coatings also impart anti-corrosion protection in addition to passive fire protection. Overseas experience has shown that Intumescent coatings applied off-site lead to substantial cost savings

and improved quality control of the installed fire protection and have the added benefit of less trades required onsite and shorter overall construction time.

Table 2.6 compares passive fire protection products and gives an approximate indication of their costs. These costs may not tell the whole story where a protected member is exposed to view and will be given a decorative finish – some systems are less costly than others to decorate.

Another important factor to be borne in mind is that dry systems cause less disruption to other trades and the building schedule, and therefore can bring significant indirect cost savings in terms of shorter overall construction time.

Commercially available materials must be able to demonstrate their capability of achieving a fire resistance level as part of building systems. The various manufacturers can supply the necessary accreditation and technical data by reference to tests conducted at recognised fire testing stations (see also Ref. 2.6 and Ref. 2.11).

TABLE 2.6: Passive fire protection costs

Section Mass

Intumescent

Coating Intumescent Coating Vermiculite Spray Vermiculite Spray Vermiculite Spray FRL 60

Minutes FRL 120 Minutes MinutesFRL 60 FRL 120 Minutes FRL 180 Minutes (kg/m) $/m2 _$/m2 _$/m2 _$/m2 _$/m2 0 to 60.5 60 200 40 50 80 60.6 to 160 55 180 40 46 60 160.1 to 455 50 150 40 40 50 Notes:

1. Rates are for supply and installation by specialist applicators. 2. Intumescent coating costs include epoxy anti-corrosive

primer and abrasive blast cleaning to Sa2½ (AS1627.9) in accordance with AS1627.4.

3. These prices are intended for comparison use only and are not absolute. Please refer to fire protective coating contractor for current pricing.

4. Data in table was supplied by Promat.

2.7 Specifications

2.7.1 GENERAL CONSIDERATIONS

The specification is important because it forms part of the tender documents and ultimately becomes part of the contract documents. Its purpose is to cover aspects of the work that fall between the legal contract clauses and the technical data shown on drawings.

Such aspects may include: • Workmanship standards • Tolerances

• Inspection levels, etc.

In past years the specification was essential for the designer to convey to the contractor exactly what was wanted. Nowadays so many of these matters have been codified that a detailed specification has become less necessary. The specification should not repeat material that is already

in the relevant codes or standards. Nor should it become a repository for information which should more properly be shown on the drawings – nowadays most design offices use standard notes on their drawings in order to handle this aspect more efficiently. A set of guideline notes are provided in AISC’s Steel Construction Journal, Volume 29, Number 3, September 1995 (Ref. 2.1). However, such standard notes should always be checked as each drawing is prepared to ensure that they are relevant. A specification should be precise so that both parties to a contract know what is required and should clearly state what the contractor is required to do and what he/she is to refrain from doing. Great care must be taken in the wording, with definitive requirements being stated and all allowable alternatives clearly specified. Vague general statements which could mean different things to different people should be avoided.

The requirements specified should be designed only to produce work of appropriate quality to the building requirements, while avoiding unnecessarily tight requirements which only add to the cost.

Experience has shown that short and precise specifications help considerably in the smooth flow of the work and thus have a beneficial influence on costs. Conversely, long and repetitious documents can easily lead to misunderstanding, contractual arguments and expensive delays.

2.7.2 WORKMANSHIP STANDARDS

Standards of workmanship and quality are extremely difficult to define in words. In the past many specifications attempted to do so by incorporating such phrases as ‘workmanship shall be of first class quality’ or ‘members shall be true to line and neatly finished’. However, when tested such clauses are meaningless and fortunately are becoming rare in modern specifications.

In practice the owner’s and designer’s interests are best protected by observing these three principles:

• Use the tolerance and workmanship standards specified in the appropriate Code, (e.g. AS 4100). • Select inspection procedures and frequencies

appropriate to the class of work, using Code guidance (e.g. AS 1554) where available.

• Select the fabrication and/or erection contractors on the basis of proven capability, using their previous work as the most reliable indicator of their quality. Check that they have quality assurance programs.

2). The necessity for these tolerances arises because of factors in the steel-rolling process, including rolling speed, roll wear, roll adjustment and differential cooling. A study of the Standards shows that these dimensional tolerances can be significant enough to warrant consideration in detailing and fabrication; Figure 2.3 gives some examples.

(a) Allow for variation in beam depth in flange splice and for off-centre of webs in web splice.

(b) Any connection to column web or column flange must make allowance for out of square, especially end plate connections – allow

for shimming where necessary (may involve tapered shims).

(c) Web side plate connection – allow for out of square of column flange and off centre of beam web.

FIGURE 2.3: Typical connections where allowance for mill tolerance is needed

Experienced fabricators are aware of the possibility of dimensional variations and it is normal practice to match members at splices in such a way as to minimise the effect of these variations.

Tolerances on the dimensions of fabricated members and erected frames are given in AS 4100.

The tolerances specified can be considered as related to the design provisions of the Code. Thus for structures designed in accordance with AS 4100, there is no case for specifying tighter tolerances since the tighter tolerances are not then consistent with the design assumptions, nor with the manufacturing tolerances of the raw steel.

economical structural steelwork _{handbook 14}

It must be particularly noted that the specifying of tighter tolerances can be a costly decision which, in most applications, will serve no purpose and destroy consistency. It is also recommended that tolerances be specified by simple reference to the provisions of AS 4100. Where dimensional tolerances are not defined, there is plenty of room for argument and contractual dispute, as most experienced designers and fabricators know. Conversely, where allowable tolerances are clearly stated, it is a simple matter to decide whether a component or structure complies or not.

2.7.4 CAMBERING

The practice of cambering beams is intended to provide an upward ‘set’ that will counteract the downward deflection due to normal working loads. Several obvious problems present themselves with this procedure:

• It is difficult to calculate accurately the true deflection of a member under working loads. • It is difficult to control accurately the degree

of camber induced in a member.

• Cambering requires the fabricator to perform a difficult, and hence expensive, fabrication operation. There are two main methods by which rolled sections

are cambered. The first involves the use of some form of heavy press, such as a hydraulic side-press. These machines are massive and costly and are found in the shops of only the largest companies.

Most fabricators employ the alternative method of controlled heating and shrinking using a standard flame-cutting torch. Both of these methods involve a degree of trial-and-error in the setting of the member so that cambering is a slow, labour-intensive and therefore rather costly procedure in the fabrication process. On simple, well-detailed beams it can more than double the actual fabrication cost. It is therefore an operation to be called for only when absolutely necessary.

Generally, where members are ultimately concealed from view, or if exposed are unlikely to cause visual offence, cambering is pointless. An exception is sometimes found in steel beam/metal deck composite floor systems where it is desirable to camber against the deflection due to the wet concrete because of the ‘springiness’ of the whole system during pouring.

If the requirement to camber is based on a need to offset increased deflections in light members, consideration should be given to using a stiffer member without camber. There is certainly scope to do this, as the saving on cambering costs would, to a large extent, offset the increase in the cost of the heavier member.

Camber is measured with the member flat on the floor with the web horizontal. Where a member is specified to

be cambered, it is reasonable to accept a tolerance on the specified camber similar to the out-of-straightness tolerance of AS 4100. To maintain tolerances closer than this can be very costly indeed (Ref. 2.10).

2.7.5 TEMPORARY BRACING

Problems often arise when the specification requires the erector to supply temporary bracing for a structure. Sometimes the erector is required to design this bracing and be responsible for its performance. In line with new occupational health and safety regulations, erectors should develop erection plans including temporary bracing requirements with the principal contractor. These plans may need to be checked by the design engineer. So-called ‘temporary bracing’ actually falls into two categories:

(a) Erection Bracing – the bracing or guys required to support individual members during their erection.

(b) Temporary Bracing – required in order that the steel skeleton remains plumb and in a safe condition after erection is completed, until permanent bracing elements such as shear walls are built.

Erection bracing is the principal contractor’s and erector’s responsibility in relation to the supply and its removal on completion.

However, temporary bracing which is to be left in place until other stabilising elements are built is a different matter. Its design requires knowledge of the building sequence and of other factors. Normal prudence would suggest that it must be designed by the Engineer. Any special or unusual features of the structural design that may limit or affect stability during erection should be emphasised on the construction drawings.

2.7.6 INSPECTION

Whilst some level of routine inspection is obviously necessary in the owner’s interest, it should always be remembered that inspection in itself is a non-productive expense. It should therefore be specified with discretion. In most contracts most of the inspection is directed at high-strength bolting, welding and surface treatment. Guidance on inspection levels and methods is given in the relevant codes and standards:

AS 1554 Structural Steel Welding

AS 2312 Guide to the Protection of Iron and Steel against Exterior Atmospheric Corrosion AS 4100 Steel Structures

The specification should define the nature of inspection to be carried out and the methods to be used. This latter is especially important in the case of non-destructive weld testing where there is a range of methods available with widely varying costs. Specifications requiring 100%

x-ray testing on all butt welds in standard industrial buildings impose significant and wasteful costs on projects. The welding test requirements for the oil and gas industry should not be applied on everyday industrial or commercial structures. Appropriate testing levels are essential for economical structures.

Where an independent inspection authority is to be engaged it should be made clear in the tender documents whether or not the fabricator is to cover the cost in his price quotation.

The following guidelines will assist in setting up effective and economic inspection procedures:

• Inspection methods and levels should be compatible with the quality and tolerance requirements of the codes applying to the particular class of work. Inspectors should not seek to impose higher standards.

• Early inspection efforts should be directed towards checking that the fabricator’s procedures will produce the required results. Thus inspection will be more intensive at the start of the job and can be relaxed to a nominal level when production methods are proven.

• The inspectors themselves should not only be experienced in their particular fields but should also have a steel fabrication background. This allows the inspector and fabricator to come to agreement quickly on many day-to-day matters on the basis of common experience, rather than hold up the work unnecessarily on minor details.

2.7.7 SUMMARY FOR

SPECIFICATION WRITERS

• Specifications are not as important as in previous years because so much has now been codified. • Omit meaningless clauses, no matter how

well-sounding. They can achieve nothing but may exacerbate disputes.

• Do not include information in specifications that should be more properly shown on drawings. • Call up AS 4100 and associated documents. • Keep it brief.