Economical Structural Steelwork Design of Cost Effective Steel Structures

Transcription

1 Economical Structural Steelwork Design of Cost Effective Steel Structures RAFTER RIDGE KNEE JOINT HAUNCH COLUMN Fifth Edition 2009 Editor John Gardner

2 Economical Structural Steelwork edited by John Gardner Fifth edition

3 AUSTRALIAN STEEL INSTITUTE ABN/ACN (94) Economical Structural Steelwork - Design of Cost Effective Steel Structures Coyright 2009 Australian Steel Insititute Published by: AUSTRALIAN STEEL INSTITUTE All rights reserved. This book or any art thereof must not be reroduced in any form without the written ermissison of the Australian Steel Institute. Note to commerical software develoers: Coyright of the information contained within this ublication is held by Australian Steel Institute (ASI). Written ermission must be obtained from ASI for the use of any information contained herein which is subsequently used in any commercially available software ackages. First Edition 1979 Second Edition 1984 Third Edition 1991 Rerinted 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 (bk.) (df.) Includes index. Steel, Structural. Building, Iron and steel--economic asects. Gardner, J. R. Australian Steel Institute Disclaimer The information resented by the Australian Steel Institute in this ublication has been reared for general information only and does not in any way constitute recommendations or rofessional advice. While every effort has been made and all reasonable care taken to ensure the accuracy of the information contained in this ublication, this informattion should not be used or relied uon for any secific alication without investigation and verification as to its accuracy, suitability and alicability by a cometent rofessional erson in this regard. The Australian Steel Institute, its officers and emloyees, and authors and editors of this ublication do not give any warranties or make any reresentations in relation to the information rovided herein and to the extent ermitted by law (a) will not be held liable or resonsible in any way; and (b) exressly disclaim any liability or resonsibility for any loss or damage costs or exenses incurred in connection with this ublication by any erson, whether that erson is the urchaser of this ublication or not. Without limitation, this includes loss, damage, costs, and exenses incurred as a result of the negiligence of the authors, editors or ublishers. The information in this ublication should not be relied uon as a substitute for indeendent due diligence, rofessional or legal advice and in this regard the services of the cometent rofessional erson or ersons should be sought. ii

4 Preface When considering steel structures it is easy to obtain information on engineering and technological asects, however little information is available on how to choose steelwork economically. Increasingly, the viability of a building roject deends uon critical financial considerations. It is imortant, therefore, for designers to have a good general areciation of the comonents that make u the cost of fabricated steel, and of how decisions made at the design stage can influence these costs. This ublication aims to suly some of this information. It is not a design manual, rather a ublication that discusses from a cost oint of view the matters that a structural steel designer should consider. It takes into account current fabrication ractices and material/labour relationshis, both of which have changed markedly since the last edition of this ublication. Adherence to the rinciles outlined in this ublication greatly assist designers in reaching decisions that will lead to effective and economic structures. This has been udated in its references to Australian Standards and industry ractices, and has other amendments. It continues to rovide useful ractical advice towards the achievement of the otimum result in structural steelwork. This edition follows on from the revious edition by substantially adoting the rationalised aroach to the costing of fabricated steel by using a cost er metre for sections and cost er square metre for lates, deending on the size, in lieu of cost er tonne. The basis for this aroach is rovided in detail in the following references: A Rational Aroach to Costing Steelwork by T. Main, K.B. Watson and S. Dallas (Ref. 1.1), and Costing of Steelwork from Feasibility through to Comletion by K.B. Watson, S. Dallas, N. van der Kreek and T. Main (Ref. 2.13). The costings given in this ublication are indicative examles only and should not be used as absolute costs. We wish to thank all those who have contributed to this ublication through comments and inuts. This includes a secial acknowledgment to all ASI Staff who submitted comments on the technical and editorial content of this ublication. Data for various tables was kindly rovided by Beenleigh Steel Fabrications, BlueScoe Distribution, Industrial Galvanizers Cororation, 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 iii

5 Contents Page 1. Preliminary Considerations Introduction Factors influencing Framing Cost Integrated Design 2 2. General Factors Affecting Economy Steel Grades Economy in use of Material Fabrication Erection Surface Treatment Fire Resistance Secifications Framing Concets and Connection Tyes Introduction Connection Tyes Basic Framing Systems Cost and Framing System Framing Details Conclusion Industrial Buildings Introduction 27 Page 6. Bolting Introduction Bolt Tyes Bolting Categories Factors Affecting Bolting Economy Summary for Economic Bolting Welding Introduction Tyes of Welds Welding Processes Other Cost Factors Economical Design and Detailing Detailing for Economy Detailing on Design Engineer s Drawings Beams Columns Trusses Portal Frames Connection Detailing References & Further Reading Standards Warehouse and Factory Buildings Large San Storage Buildings Heavy Industrial Structures Commercial Buildings Introduction Low-Rise Commercial Buildings High-Rise Commercial Buildings Floor Suort Systems Comosite Construction Summary 42 iv

6 1. Preliminary Considerations 1.1 Introduction It is generally acceted that the objective of engineering design is the achievement of an accetable robability that the structure being designed will retain its fitness for urose during its lanned lifetime. It is also of utmost imortance that the initial costs lus the maintenance costs of the comleted structure be within the limits rovided 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 comly with acceted 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 develo an areciation of the basic sources of exenditure in building construction and their effect on the overall cost of construction. In ractice, the design roblem is an otimisation roblem. The solution to any otimisation roblem involves having some means of judging the overall merit of alternatives. With regard to a building, the measure of overall merit, usually rovided 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 caital and maintenance costs. This ublication deals almost entirely with item (d) above. In the reliminary and final design, the designer often deals rimarily 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 reoccuation with the minimisation of the mass of a steel structure can lead to serious errors of judgement. This ublication 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 Fabricated steel has been traditionally costed on a er tonne basis. Consequently, in discussing the cost of fabricated steel, the question often raised relates to how much is the cost er 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 aroach to the costing of fabricated steel is based on a cost er metre for sections and cost er square metre for lates deending on the size of the member. Fabrication costs for connections and erection costs, etc can then be added on a comonent by comonent basis (Ref 1.1). For multi-level steel construction a cost er 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 rocesses used. (e) Erection techniques used. (f) Secification for fabrication and erection. (g) Other items such as corrosion rotection, fire rotection, etc. The selection of the most efficient framing system is fundamental to achieving an economical framing solution and asects relating to this item are discussed in Sections 3, 4 and 5. Efficient member design remains an imortant cost factor temered by the comments made in Clause 1.1. Detailed consideration of this item does not fall within the scoe of this ublication. One oint that does deserve mention, however, is the avoidance of the individual design of every beam and column in an attemt to achieve least mass. The aim should be to grou similar members (e.g. similar main beams in a floor grid) and adot the one size for all members of the grou. An exerienced designer will otimise the design by being aware that if too much grouing is done, there will be material wastage. However, if little grouing is done, then there is a great waste of time on the art of the draftserson and the erector. Economic fabrication and erection are significantly affected by economical connection details. This ublication 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 oints which need consideration. The secification (item (f) above) is a major influence on the cost of both the fabrication and erection since it secifies the quality of materials and workmanshi required. Similarly, the costs of both corrosion rotection and fire rotection (item (g) above) are imortant influences on the final cost. All these items are discussed in greater detail in Section 2. 1

7 1. Preliminary Considerations 1.3 Integrated Design One of the obstacles to achieving maximum economy is that three of the most imortant activities in steel frame construction, namely structural design, detailing and fabrication, are usually done in isolation from one another. This is artly due to the secialisation in each of the discilines and artly because of a lack of an effective dialogue among the eole involved. As a result of this, there often occurs a total reoccuation with the analytical hase of the design, and a comlete absence of rational thinking about the detailing hase. Consequently, the roblems that arise during the detailing hase are solved by comlicating the detail rather than by modifying the design concet. When the job reaches the fabrication sho, there is little alternative but to carry out whatever haens 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 ossible to modify the structural framing concet to allow the use of simler 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 rocess. In this way, one asect can be altered to enable another to be imroved. This enhances the overall cost efficiency of the final structure. Obviously, such an aroach ideally requires an extensive and u-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 otimum ractical solution. An interchange of ideas among fabricators, erectors and designers is an ideal situation for achieving otimisation. It should be areciated that what constitutes design and good (i.e. economical) design will vary deending on whose viewoint 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 outut 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 roblems. Clearly such different viewoints are best resolved by an integrated and interactive aroach on the art of the steelwork designer. The Steel Detailer, using 3D modelling software, can assist in roviding a service to designers by modelling the steel structure rior to engineering analysis and exchanging data in a Building Information Modelling (BIM) environment. The Steel Detailer can also rovide a range of oututs for the Steel Distributor and/or Fabricator to utilise, seeding u the roduction of structural steelwork. Guidelines on Steel Detailing oututs are rovided in Ref Further, the recent emergence of the Steelwork Contractor who integrates design, detailing and fabrication is roviding 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 rovide a Total Solution. 2

8 2. General Factors Affecting Economy 2.1 Steel Grades STRUCTURAL STEEL Throughout the world the least costly and most commonly used grades of steel for structural uroses 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 tyical design yield strength of 250/300 MPa (varying above and below this figure deending 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 suerior low temerature toughness. Plates, rolled sections, welded sections and bars are all roduced to these standards, although not every roduct is available in every grade. This is exlained more fully in Table 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 atina after which no further corrosion takes lace. By nature of the chemical comosition the steel is high strength (Grade 350) steel. However in Australia it is available in only a limited number of roducts see Table HOLLOW SECTIONS In Australia structural hollow sections are roduced to the roduct standard AS 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 C QUENCHED AND TEMPERED STEEL Steel lates are roduced in Australia in very high strength heat-treated grades known as quenched and temered steel. These steel lates are useful in secial alications where mass reduction is imortant (e.g. crane booms) or where their high wear resistance is needed (e.g. dum truck bodies). Australian Standard AS 3597 covers these steel lates for structural steel alications and for use in ressure vessels CHOICE OF STEEL GRADE Table 2.1 lists the availability of various roducts by steel grade. The indicative relative cost of grades is shown in Table 2.2. For most structures the greatest economy will be achieved by the selection of the least costly and most readily available steel, i.e. Grade 300. In large structures with longer lead times the use of higher grades will often be worth considering at least for arts of the frame. Heavy late members such as bridge girders are one instance where higher grades may rove economical. Other alications include: Multi-storey structures, articularly with comosite 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 rovides 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 alications where the use of higher grades is economical. TABLE 2.1: Availability of roducts by Grade (check currency of information with steel suliers) Steel Grade Plates (or Floor lates) Rolled Sections Welded Sections Structural Hollow Sections Grade AS 3678 AS AS AS L0 250L L L0 350L L15 WR350/1 WR350/1 L0 Notes: C250 C350 C Quenched & Temered Structural Steel AS 3597 Regular grade commonly roduced, readily available from stockists. Regular grade not commonly roduced, availability subject to time limitations and order size. Non-regular grade, availability subject to time limitations and order size. Not manufactured. 3

9 2. General Factors Affecting Economy While the information resented in Table 2.1 is indicative of the general situation, it must be remembered that the steel suliers are always willing to discuss secial cases where, for examle, 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 roject requiring large tonnage of secific sections, it may be ossible to negotiate a secial order with the sulier, rovided that an arrangement has been agreed at an early enough hase in the design. Conversely, on average rojects the designer should always be careful to kee within the range of readily available roducts so as to ensure that no roblems of steel rocurement occur at the fabrication stage. TABLE 2.2: Indicative cost ratios for different grades of structural steel (er tonne, suly only) Grade Plates Rolled Sections AS 3678, AS & AS Welded Sections Grade L L L L0 350L L WR350/1 125 WR350/1 L0 135 AS 1163 Grade C C C AS 3597 Quenched & Temered Steel 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 STEEL PRICING Mill rices are exressed in terms of a base rice and various extras. The base rice relates to the tye of mill roduct such as late or sections, while extras relate to secifics of the articular roduct 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 aly to the quality secification for the material chosen. Quality extras for structural steel relate to the material secifications and reflect the costs of alloying elements, of tighter controls on such elements as carbon, manganese, hoshorus and silicon, and of tighter controls on manufacturing techniques to meet the secified chemical and mechanical roerties. 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 secification, the greater is the robability that other costs associated with its use, ranging from rocurement through all stages of fabrication, will also be increased. Unnecessary demands by secifiers for mill heat certificates for standard sections of known origin to be used on routine rojects is another examle of unnecessary costs added onto rojects. The foregoing relates to urchases made direct from the steel mill, but in Australia most fabricators obtain their steel through steel distributors. These steel distributors aim to carry comrehensive stocks and are thus able to offer romter delivery than would be available through the normal steelmaker s rolling rograms. Their stock holding tends to concentrate on oular, high turn-over items. TABLE 2.3: Preferred steel late thicknesses (in mm) PLATES In Australia there is a rationalised series of referred late thicknesses as listed in Table 2.3. For ractically all structures the designer should oerate within this standard range. Non-referred thicknesses incur cost remiums and extended delivery times, and should only be considered on major rojects where the overall saving in using a secial thickness is greater than the direct and indirect cost enalties. 4

10 2. General Factors Affecting Economy Similarly there are referred lengths and widths of lates which should be borne in mind. Major late elements should be dimensioned as far as ossible so that they can be cut from standard lates with a minimum of scra. Smaller late details such as brackets and gussets should be considered in the same way, esecially when there is a large number of them. The most common sizes for lates u to 25 mm thick are 1.8m 6m, 2.4m 6m, 2.4m 9m, 3m 9m and 3.2m 12m. Note: Small late comonents may be substituted by flat bars which are considered as sections SECTIONS Australia roduces a range of welded roducts, universal sections, channels, angles, and hollow sections which rovide the designer with a reasonable choice without the roliferation which can lead to roblems 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 kee to the lowest weights in each size range, although this will not always be ossible. Very long lengths of sections become difficult to kee straight and to handle, and the mills imose a rice extra for them. It should be esecially noted that although universal sections are listed as being available u to 18m long (and u 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) SCRAP AND WASTE The real cost of material is affected by the quantity of scra and waste, and designers should be recetive to suggestions for minimising and controlling the generation of waste. This may include greater standardisation of structural sizes, or of late widths and thicknesses, in order to take advantage of size and quantity discounts. It might also include a more liberal aroach to the slicing of beams or other structural sections using standard lengths. Random slicing, which involves welded slices anywhere within the length of a rolled structural member, can be articularly effective when material is sawn to length and fabricated on a conveyorised roduction line. When carefully controlled, it can dramatically reduce the accumulation of shorts and thus reduce the total cost. The only real restriction to random slicing alies to its use for beams subject to severe dynamic loads. Of course the savings in scra have to be balanced against the welding costs, and the designer should be recetive to this technique where it is aroriate. 2.3 Fabrication GENERAL Fabrication costs are a function of comlexity and are influenced by: Size of the comonent Size and tye of sections involved Amount of stiffening and reinforcing required Amount of reetition Sho and field details Sace requirements in the sho, and Facilities available for handling, lifting and moving the structural comonents. Fabrication costs are sensitive to simlicity or comlexity of detail, and the degree to which roduction line techniques can be alied. They are controlled by the quality of the sho detail drawings, which must reflect the designer s concet for the structure, but must also ermit the otimum utilisation of the fabricator s facilities and equiment. Sho drawing rearation should be guided by the basic rincile that they must rovide for economy of fabrication and for economy of erection. Sho oerations basically involve cutting material to size, hole-making for mechanical fasteners, and assembling and joining. Other oerations include handling, cleaning and corrosion rotection. All sho oerations require facilities for lifting and for moving or conveying the structural steel. Cutting oerations include shearing, sawing and flame cutting; hole-making oerations include unching and drilling; assembly oerations include welding and bolting. Increased use of comuter numerically controlled (CNC) fabrication rocesses is changing the economics of steel fabrication. Cutting, drilling and welding oerations can now be undertaken by the CNC fabrication rocess. Information from comuter drafted sho drawings can be fed directly into CNC fabrication equiment to further imrove oerational efficiency. Some fabricators are now bar coding steelwork to facilitate control and monitoring of rojects. Generally welding is the referred method for sho assembly, with bolting for field assembly. There are, however, some fabricators with sohisticated holemaking equiment, who refer sho bolting to sho welding for standard connections. Some steel merchants also rovide basic cutting and drilling services to the steel fabricators. Many steel distributors now offer a steel re-rocessing service where steel sections and lates are cut and drilled to size. The fabricators then weld the comonents together in the worksho. 5

11 2. General Factors Affecting Economy BEAM AND COLUMN FABRICATION A large art of structural steel fabrication consists of beam and column work. It embraces framing members consisting of standard rolled shaes connected by shear or moment connections, and also includes highly irregular framing members with custom designed built-u sections and comlex connections designed for combinations of shear, moment and direct tension. Simle beam and column fabrication lends itself to roduction line methods, in which the members are transorted on a series of conveyors to saws which cut the material to length, and to hole-making equiment which rovide holes in either the web or flange or both. Any additional requirements, such as the attachment of cleats or brackets, are off-line oerations. It is imortant therefore that connections and other details be selected so as to rovide the maximum number of members with only cutting and holing. Otherwise the economy of using CNC equiment and the conveyorised beam-line system will be less aarent (see Figures 3.13 and 8.29). Many steel distributors now offer steel re-rocessing services where steel sections and lates are cut and drilled to size. The fabricators then weld the comonents together in the worksho GIRDER AND TRUSS FABRICATION Fabrication of late girders and trusses differs from beam and column work in that it involves assembly in the sho, and calls for adequate sace and handling facilities. Both girders and trusses require secial fit-u jigs for assembly and welding, and the availability of heavy lifting equiment. Just as with beam and column work, however, the key to roductivity and economical fabrication is the use of simle standard details for stiffeners, slices, gussets, etc. For late girders all details should be designed for automatic welding, allowing adequate clearances for the welding machines to ass and for termination of welds at the ends of web stiffeners. Maintaining constant width flanges within a sho fabricated length of girder ermits slicing of multile width late and subsequent striing to finished width. This will reduce weld setu time, eliminate weld starts and stos, and require only one set of run-on and run-off tabs. Reductions of flange widths, web deths and late thicknesses urely to reduce mass should be considered very carefully as they can significantly increase fabrication costs. Control of distortion in late girder fabrication is a major roblem which can be heled 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 to flanges in comosite girders only to comound the roblem of camber control. See also Clause Trusses can be designed in a large variety of configurations which deend on the truss san, deth and loads to be carried. Therefore, it is imossible to make general statements regarding the most economical design for fabrication, other than to stress again the imortance of simlicity of detail. Designers should avoid situations that can cause weld restraint and roblems resulting from weld induced distortion. As far as ossible trusses in the one roject 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 comlete the entire welding on the truss comonent without further handling SUMMARY FOR ECONOMIC FABRICATION The key to economic fabrication is the use of standards at all stages. This includes standard rocedures, standard schedules, standard drawings, and above all standard connections and details. Non-standard details are usually handled as secial job standards ; however, the net effect of any secials is to slow roduction with some loss of fabrication economy. In the selection of connections the designer should observe the following rinciles: Select members and connections to rovide a maximum of reetition throughout a structure. This rovides the fabricator with the oortunity to make u jigs and fixtures to seed u the fabrication rocess. As far as ossible, select connections so that the assembly of fitments on a member can be carried out in one osition. This will reduce the number of handling or rotating oerations during fabrication. Kee the number of comonents in a connection to a minimum. Select connections so that assembly of comonents occurs on the least number of members. As far as ossible 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 ublication: A Guide to the Requirements for Engineering Drawings of Structural Steelwork (Ref. 2.12). Most imortantly, kee an oen mind on the selection of members and connections. Before finally committing a design to the detail design hase, communicate with the industry and try to determine the best solution to otimise the use of material and labour in the fabrication sho. This industry communication can often be facilitated through the services of ASI. 6

12 2. General Factors Affecting Economy 2.4 Erection GENERAL CONSIDERATIONS The rate of erection of steel in a structure is controlled by five main factors: 1. Connection simlicity 2. Number of members 3. Number of bolts and/or amount of field welding 4. Size and efficiency of erection crew, and the equiment at their disosal 5. Timely suly of steel. It is interesting to note that of these factors, the first three are under the control of the designer. Connections should be simle, and of such a tye that the allowable tolerances (in member size and shae, detailing and fabrication) can be accommodated during the lacing of the members. The number of members should be ket to a ractical 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 ower wrenches. Bolted connections should be used wherever ossible and field welding ket to a minimum. Connection lates should be sho welded to one member rather than field bolted to both, unless other considerations govern. Every endeavour should be made to standardise as far as ossible (member sizes, bolt sizes, tye of connection, gauge lines, member sacing, etc.), and careful consideration should be given to how a member is to be installed with minimum interference by other members, gusset lates, etc. (see Ref. 1). With an increasing awareness of the imortance of emloyee safety in the work lace, erection methods are changing. Designers and erectors have a duty of care and should consider safe erection methods. The use of equiment such as cherry ickers is becoming more common during erection. Designers need to include anchorage oints for safety lines and harnesses for riggers. These issues are resulting in steelwork being erected on the ground and then craned u to final osition in many rojects 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 rovides a safer work site. A safer work site will lead to faster and more economical erection HANDLING AND TRANSPORT As a general rule it is more economical to erect fewer large ieces than many small ieces, due to the number of lifts involved and the number of joints to make. Generally this means fabricating larger ieces in the sho to reduce the number of ieces and field connections. On the other hand, transortation constraints may limit the size of a iece for delivery to the site and require additional field slices. For examle, with long flexible trusses, the transortation 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 transorted using secial vehicles attended by olice escort, and this may add greatly to the final rice of the structure. However, rojects outside caital cities could use this aroach 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 transort costs and site costs is changing. Where rojects require large site crews, minimising time sent on site is essential to economical erection. The erection or trial erection of large comonents in a fabricator s yard before delivery to site is good ractice and a cost savings exercise. Trial erection guards against fabrication errors being discovered on site which may rove exensive to rectify. To minimise transort costs it is imortant that vehicles travel fully laden. The dimensions of a tyical load of structural steelwork which requires no secial escort are in the order of 15m long 3m wide 2m high. It is imortant that like ieces are loaded together to otimise truck caacity, but also that the comonents be delivered to site in the order required by the erection sequence (i.e. columns followed by beams from the ground uwards). This will save double handling on site and also reduce the cost of site storage and ossible damage. The virtue of designing for reetitive comonents has already been stressed. The gains can be artly lost on site if interchangeable arts are given individual mark numbers. This will require the erector to search for a articular number mark on a member when any one of a considerable number of members would fit. After comleting a design it is worth looking at marking lans with this idea in mind. Indicative transortation costs are given in Table 2.4. Costs include the loading of steelwork onto and off the truck. TABLE 2.4: Transortation costs Notes: Transort Section Mass (kg/m) Fabrication Sho to Site (see Note) $/member 0 to to to Allow for twice the cost of transortation if the surface treatment is alied at remises other than the fabrication sho. 2. See also Ref

13 2. General Factors Affecting Economy CONNECTIONS It is in the final fixing of members that the greatest scoe for erection economy lies. Connections selected to ermit flexibility in fit u should be of rime concern to designers. The use of one tye of bolt and one bolting rocedure throughout a structure will allow the use of a minimum variety of tools on site and rovide for seedy erection sequence (see Section 6). Similarly where site welded connections are required, cleats should be incororated to allow mating members to be held together in lace for actual welding. frequently jams. Gusset lates, stiffeners, and other members tend to interfere with its installation. Access for bolting is usually difficult and sometimes imossible. 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 ossible. This is esecially imortant when the column section is comact. 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). Angle seat, angle cleat and web side late connections (see Clause 8.6.2) rovide considerable flexibility in fit-u, and are referred in braced frames from a urely erection viewoint. The flexible end late 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 late deth should be ket to a minimum to reduce the tendency to jam during installation (Figure 2.1). The tolerance between the face of the end late and the face of the column should either be tightly controlled so that the building lumbs itself automatically, or allowance should be made for shimming in order to lumb the building. Shimming, however, can be exensive. In end late connections for ortal frames careful consideration should be given to access for installing and tensioning bolts, (see Table 8.1). If welded connections are referred, the following should be taken into consideration: Welded connections are normally erected using a bolted erection connection. The same criteria should aly to the design of these connections as described above. Substantial erection clearance between the end of the girder and column face should be rovided where ermitted by the design of the connection. Field welding should be ket to a minimum and overhead welding should be avoided. Attention should be aid to access for welding and welding insection. Consideration should be given to lumbing the building. The most significant time delays in the erection of a girder can be exected to occur when it is installed with the end connection against a column web. The girder can normally only be manoeuvred in a vertical lane and FIGURE 2.1: Dee end lates can cause jamming FIGURE 2.2: One examle of how to avoid the roblem of access to column web connections FIELD BOLTING In rojects with a redominance of large connections, threads may be excluded from the shear lane for bearing tye connections as this will hel 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 roblems. In rojects 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 lane. 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). 8

14 2. General Factors Affecting Economy Generally, the smaller the bolt the easier it is to install. Bolt diameters should therefore be ket small if this can be done without comromising the objective of keeing 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 oor; otherwise M24 bolts are accetable. Bolts should be secified as snug-tight unless there are comelling reasons why fully tensioned bolts are necessary. The cost of full tensioning, including associated insection, 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 roject is designed to overseas codes because some of these require high strength structural bolts to be always fully tensioned. It is referable that only one bolting category (see Section 6) be used on any one structure. When a dearture from the general category (e.g. to fully tensioned bolts, to threads excluded from shear lane, etc.) is unavoidable, this should be highlighted on erection and detail drawings to reduce the ossibility of the requirement being overlooked by erection crews. More information on structural bolting is given in Section 6 and Ref 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 u welding equiment on the site. Access for welding is also imortant, and it should be remembered that a welder generally requires a substantial and carefully laced working latform. Otherwise the normal rules for economic welding aly. Fillet welds are referred to butt welds, and down-hand welding to any other osition. In most structural work difficult out-of-osition welds such as overhead are very slow and costly (see also Section 7) BRACING Bracing is usually difficult and time consuming to install. To reduce erection time, the number of braced bays should be ket to a minimum (i.e. fewer braced bays with heavier bracing is referred). Wherever ossible, wall bracing should be connected to columns rather than beams. This allows bracing to be installed before the beam above is in osition, 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 from a gusset late on the to flange of a beam. Connecting wall bracing to the column also usually results in lower fabrication costs. 2.5 Surface Treatment GENERAL CONSIDERATIONS With the develoment in recent years of a large variety of surface treatment methods, the designer may exerience considerable difficulty in selecting the otimum system for a articular alication. Furthermore, it is often not fully realised that the cost of a sohisticated 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 unexected, surface treatment costs. These costs are a function of surface area which can vary with both, the tye of section used and the class of construction. For examle, a structural hollow section has tyically only one-half to two-thirds of the surface area of an oen structural section (UB, UC) of equivalent caacity, for this reason, hollow sections are well worth bearing in mind for alications requiring any significant amount of multi-coat surface treatment. Heavy steel construction such as for ower stations usually averages out with comaratively less surface area (desite the higher tonnage) than a tyical factory or warehouse where light trusswork may have a much greater surface area (desite the lower tonnage). Obviously treatment costs on a er square metre basis will vary widely deending on the actual surface area to be treated STEEL PERFORMANCE Bare steel will corrode only in the resence of both oxygen and moisture. Corrosion will be accelerated if traces of ollutants such as sulhur dioxide or chlorides are resent the so-called aggressive environments. Steel inside a building is rarely a corrosion risk excet in the occasional case where the building houses an aggressive atmoshere as a result of its urose, (e.g. a fertiliser factory). It follows therefore that steel needs no corrosion rotection whatsoever in most interior alications such as multi-storey buildings where the steel framing is eventually concealed. Where the steelwork remains exosed to view as in a factory or warehouse the same negligible risk alies but in these instances the owner may require a surface finish for a more attractive aearance. The designer should distinguish between treatment secified to achieve rotection from corrosion and that secified merely to rovide decoration. In ractice, of course, any surface finish will attemt to do both. Detailed advice on the classification of environments and the selection of aroriate surface treatment systems is contained in AS 2312 Guide to the rotection of iron and steel against exterior atmosheric corrosion (see Section 10). 9

15 2. General Factors Affecting Economy SURFACE PREPARATION An imortant art of any steel treatment system is the reliminary surface rearation. This can range from simle degreasing and brushing to costly chemical or mechanical descaling. The surface rearation should be matched to the alied finish. Exensive aint systems will not last if alied to only artially reared (e.g. wire-brushed) surfaces. Conversely it is a waste of money alying a low-cost orous alkyd rimer to a descaled white metal surface. Various methods of surface rearation are covered by AS 1627 Metal finishing rearation and retreatment 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 aints) and abrasive blasting to Class 2-1/2 of AS 1627 Part 4 (needed for high erformance aint systems). Wire brushing is a time consuming and costly rearation method and would normally only be considered if the work was to be erformed on site. Acid descaling ( ickling ) is encountered mainly as art of the hot-di galvanising rocess (see Clause 2.5.5). An idea of the costs of various methods of surface rearation is given in Table 2.5. TABLE 2.5: Surface treatment costs Section Mass ROZP ROZP + Alkyd Gloss Paint Tye IOZ Zinc-Rich Eoxy + Eoxy MIO Hot Di Galvanise (kg/m) $/m 2 $/m 2 $/m 2 $/m 2 $/m 2 0 to to to 455 Notes: ROZP single coat of red oxide zinc hoshate 40µm DFT alied to a Sa2 blast cleaned surface. 2. ROZP + Alkyd Gloss red oxide zinc hoshate 40µm DFT lus alkyd 40µm DFT alied to a Sa2 blasted surface. 3. IOZ single coat of inorganic zinc 75µm DFT alied to a Sa2½ blast cleaned surface. 4. Zinc-Rich Eoxy + Eoxy MIO 2 ack zinc rich eoxy 75µm DFT lus 2 ack high build eoxy 150µm DFT alied to a Sa2½ blast cleaned surface. 5. These rices are intended for comarison use only and are not absolute. Please refer to coating contractor for current ricing PAINT SYSTEMS There is a very large selection of aint systems available for structural steel too many to be discussed within the scoe of this ublication. However, excellent guidance on the erformance and caabilities of various aint formulations is given in AS Probably the most commonly used aint is red oxide zinc hoshate rimer, often referred to as ROZP. Paints of this tye rovide an economic base for ossible further decorative coats of conventional oil aint. However being ermeable, ROZP cannot be exected to last if left in the oen for more than normal construction eriods. Another regularly used aint is inorganic zinc silicate rimer which is alied over a Class 2-1/2 abrasive blast rearation. It forms an excellent base for most high erformance aint formulations, or gives good results as a single coat rotection for steel in all but the most aggressive environments. Paint is normally alied to steel by sraying. It is sometimes suggested that better coating is achieved by brush alication, but there is little evidence to suort this claim. Brush alication costs two to three times as much as sraying, and cannot be used at all for some modern aints; inorganic zinc silicate is an examle. If a multi-coat aint system is required then it is recommended that a raid cure system be secified to allow a quicker turn around of roduct. Table 2.5 includes the cost of the finish ainting in the surface treatment costs. It should be noted that transortation costs should also be considered if the treatment is done at remises other than the fabrication sho. Table 2.4 gives an indication of transortation costs HOT-DIP GALVANISING Galvanising is carried out by secialist firms and the rocess requires re-cleaning and surface rearation, usually by ickling. The cost of galvanising includes these rearatory rocesses. Advice on the erformance of hot-di galvanising, either as a single coat rotection or as a base for aint systems, is contained in AS When considering galvanising the designer should ascertain the scoe of local facilities, and in articular the size of the available galvanising baths. The galvanising bath determines how big an individual comonent can be died. (Items larger than the bath can sometimes be galvanised by double diing but at extra handling cost). Information on bath sizes in Australia is given in After Fabrication Hot-di Galvanising (Ref. 2.4). 10