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INTRODUCTION TO THE ECONOMICS OF STRUCTURAL STEELWORK
First Edition 2001
The Southern African Institute of Steel Construction PO Box 1338, Johannesburg, 2000, Republic of South Africa Telephone: (011) 838-1665 Fax (011) 834-4301 e-mail:
[email protected]
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Copyright © 2001 SAISC
All rights reserved. This book or any part thereof may not be reproduced in any form without the written permission of the publisher
First Edition First Printing . . . . . . . . 2001
Compiled and published by The Southern African Institute of Steel Construction
Although care has teen taken to ensure, to the best of our knowledge, that all data and information contained herein is accurate to the extent that it relates to either matters of fact or accepted practice or matters of opinion at the time of publication, the Southern African Institute of Steel Construction assumes no responsibility for any errors in or misinterpretations of such data and or information, or any loss or damage arising from or related to its use.
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Contents
CONTENTS ...............................................................................................
3
PREFACE ..................................................................................................
4
REFERENCES...........................................................................................
5
CHAPTER 1 1 1.1 1.2
..................................................................................................... GENERAL .................................................................................... Introduction................................................................................... Main factors influencing cost ........................................................
6-8 6 8 8
CHAPTER 2 2 2.1 2.2 2.3 2.4 2.5 2.6
..................................................................................................... ORGANISATION OF ACTIVITIES ............................................... Introduction................................................................................... Design .......................................................................................... Cost estimating............................................................................. Shop detailing............................................................................... Fabrication.................................................................................... Summary ......................................................................................
9 - 20 9 10 10 12 16 18 19
CHAPTER 3 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8
..................................................................................................... 21 - 33 GENERAL ECONOMIC CONSIDERATIONS .............................. 21 Introduction................................................................................... 21 Materials and material utilisation................................................... 21 Connections ................................................................................. 23 Fabrication.................................................................................... 24 Surface preparation and finishing ................................................. 26 Transportation .............................................................................. 28 Erection ........................................................................................ 29 Summary ...................................................................................... 32
CHAPTER 4 4 4.1 4.2 4.3 4.4 4.5 4.6
..................................................................................................... 34 – 40 STRUCTURAL FRAMING SYSTEMS .......................................... 34 Introduction................................................................................... 34 Main dimensions........................................................................... 34 Design loading.............................................................................. 34 Stabilising systems ....................................................................... 35 Dual function of members............................................................. 40 Summary ...................................................................................... 40
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Preface
The aim of this book is to promote maximum economy in the design, fabrication and erection of steel structures. Competitive pricing, efficiency, quality workmanship and on-time construction are basic requirements for the success of construction projects. Economy is gained not only by savings in steel mass through careful design, but also by the adoption of simple, efficient construction details that can be processed in the workshop at minimum cost. The cost-saving techniques presented in this publication cover a wide range of topics, from the correct choice of an efficient basic structural layout through to economical setting out and production of minor components such as cleats and gussets in the shop. It must be stressed that while the objective is the attainment of a greater degree of economy, the cost-saving proposals put forward do not in any way allow for a reduction in the required level of quality of a structure. The prime objective is to ensure a safe, suitable structure, and then to establish how this may be achieved at the lowest reasonable cost. Although this book is aimed primarily at designers, architects and draughtsmen – since they are the persons most able directly to influence the overall economy of a structure – it should not be regarded as a design manual. The designer is expected to have the necessary expertise in structural analysis to produce a safe structure that meets all requirements and the draughtsman must be well versed in the art of steelwork detailing. It is hoped that the content will be of particular value to persons, including newly-qualified graduates, who may have an understanding of structural principles but who, through lack of instruction or experience, may not have acquired sufficient insight into the finer points of efficient and economical design. It is also hoped that the text will assist all those involved in the production of structural steelwork – designers, estimators, detailers, production personnel and erection staff – to develop a knowledge of and an interest in each others' activities, thereby fostering the establishment of an integrated team, that is able to maintain the unique advantage of steelwork as a construction medium.
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References
(Ref 1)
SABS 1200H – Standardised Specification for Civil Engineering Construction: Structural Steelwork
(Ref 2)
National Building Regulations
(Ref 3)
SABS 0400:Code of Practice for the Application of the National Building Regulations
(Ref 4)
SABS 0162:Code of Practice for the Structural Use of Steel .
(Ref 5)
South African Steel Construction Handbook
(Ref 6)
Southern African Structural Steelwork Detailing Manual
(Ref 7)
Structural Steelwork Connections – Limit States Design
(Ref 8)
South African Structural Hollow Sections Handbook
(Ref 9)
Structural Steel Tables
(Ref 10)
SABS 0160: General Procedures and Loadings to be Adopted in the Design of Buildings
(Ref 11)
Single Storey Buildings – Cost Considerations: Horridge and Morris
(Ref 12)
Design and Construction of Composite Floor Systems
(Ref 13)
AWS D1.1 Structural Welding Code – Steel
(Ref 14)
SAISC Structural Steelwork Specification.
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1
General
1.1 Introduction In any construction activity one of the main objectives is to achieve maximum economy of production whilst maintaining an acceptable level of quality. Establishing the quality level is the prerogative of the purchaser. Thereafter it is in the interest both of the purchaser and of the supplier that the commodity be produced efficiently and at as low a cost as is reasonably possible. In the context of structural engineering this means that the owner or client, usually acting through his engineer, sets out his requirements in a detailed technical specification, and that the contractor then builds the structure in accordance with these requirements by the most efficient means at his disposal. The parameters for different types of structure will vary widely but will always include functional, aesthetic, safety and durability considerations. Basic to all structures is the safety or fitness-for-purpose criterion, the detailed requirements for which are set out in the applicable design code or specification. Functional and aesthetic requirements are more subjective, and should be defined in the client's specification. For example, in the case of a grade-separation bridge over a highway, aesthetics would be an important consideration, calling for clean lines, neat details and an attractive finish. In a crane structure, on the other hand, performance would be the main criterion, along with ease of maintenance. At the lower end of the range, as in the case of a simple storage shed, the only functional requirements might be the provision of adequate volume, dictated by the internal width, length and height of the structure, and the provision of adequate access. Once the detailed parameters have been outlined by the client, the architect and designer are in a position to produce an efficient design for the structure and the steel fabricator can build and erect it in the most economical manner possible. The cost savings resulting from the efficient design and construction of the project will not only benefit the client, but also the Contractor. The sequence of activities and the respective areas of responsibility in a typical steelwork project are set out in Fig. 1.1.
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OWNER Functions:
a)
Briefs architect or engineer as to his requirements
b)
Provides finance
ENGINEER (Employed by owner or appointed by owner's representative, e.g. architect) Functions:
a)
Interacts with client in conceptual phase to indicate functional and other implications of design brief
b)
Prepares conceptual design and possibly also detailed design
c)
Prepares contract specification
d)
Issues enquiry and adjudicates tenders
e)
Supervises contract
CONTRACTOR Functions:
a)
Prepares cost estimate and submits tender
b)
Carries out detailed design (if not done by engineer)
c)
Prepares shop detail drawings
d)
Fabricates steelwork
e)
Erects steelwork
Fig 1.1: Organisation of steelwork project
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1.2 Main factors influencing cost The three main factors that will ensure the economical execution of a building project are the following:
a)
A realistic specification In addition to providing drawings showing the general layout of the building or structure, it is essential that the client spells out his particular needs very clearly. In many cases there are no special requirements and it may be sufficient simply to state that the provisions of an accepted standard specification be complied with, and the South African Structural Steelwork Specification for construction (Ref. 14) e.g. SABS 1200H – Standardised Specification for Civil Engineering Construction: Structural Steelwork (Ref. 1). In other cases where more detailed requirements apply these should be defined clearly: closer tolerances may be called for in fabrication and erection of steelwork to ensure accurate alignment; special finishes may be desired for the sake of corrosion protection; resistance to fatigue may be necessary in structures subject to vibration; attractive appearance and the provision of neat details may be called for where aesthetics are important, etc. Although the client should clearly state his minimum requirements, he should avoid over-specification to prevent unnecessary expense.
b)
An economical design All aspects of design concerned with safety, e.g. the ability of the structure to fulfil its prime purpose of safely carrying the loads imposed upon it, are covered by statutory documents. However, the efficient or economical execution of the design, is entirely in the hands of the designer. Because of his early involvement he is in a position to significantly determine the general, as well as the detailed aspects of the project insofar as they will ensure overall economy. His influence in ensuring cost-savings, from conceptual planning right through to the specifying of suitable details, is all important to the successful completion of the job.
c)
Efficient fabrication and erection These two activities can be considered together since in most steel construction jobs they are carried out by the same contractor. Whereas a client or owner profits from economical design, the steelwork contractor benefits from efficient fabrication and erection. His livelihood depends on being able to produce and erect steelwork more efficiently and therefore at a lower cost than his competitors. He not only needs to make a profit on the contract in hand, but also, to operate so efficiently as to be able to secure further contracts in the face of competitive tendering. There are two main areas where savings can be made in fabrication. These are: i) In efficient production methods in the works, where shop layout, material handling facilities, material processing equipment, fabrication machinery, etc are the governing factors; and ii) In the minimisation of production manhours through the use of simple fabrication details, the elimination of unnecessary drilling and welding, the adoption of standard connections, etc.
The aspects listed under c(i) are all under the control of company management and production supervisory personnel. However, those given in c(ii) are largely governed by the designer and the detailer of the steelwork, and it is these aspects that are addressed specifically in this book.
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Organisation of Activities
2.1 Introduction This chapter deals with various aspects of overall organisation relating to the design, costing and production of structural steelwork, with special emphasis on the efficient utilisation of staff and production facilities. The more technical aspects of economical steelwork design, detailing and fabrication are covered in later chapters. The main activities involved in the execution of a structural steelwork project are: a)
Design
b)
Cost estimating
c)
Shop detailing
d)
Fabrication
e)
Erection
All the persons involved in these functions are in a position to contribute to the overall economy of the project and should ideally work together as a team to ensure maximum efficiency. In the case of large fabricating companies having their own design offices such teamwork between directly employed personnel should exist; the various departments will be familiar with each others' functions and good communication should ensure a smooth working relationship. In most cases, however, such a close relationship does not exist. The designer may be employed by the client or his consulting engineer and may thus not be familiar with the fabrication procedures and erection capabilities of the contractor; he may then not be able to select the form of construction best suited to the contractor's production methods. The detailer likewise may be separately employed, or may be retained by the fabricator on a contract basis; he too may lack an intimate knowledge of the workshop facilities. The erector could also be a separate party working as a sub-contractor. In this case it would be necessary for the main contractor to be aware of the handling and lifting capabilities of the erector so as to gear the manufacture of components to any limitations that may apply. Whatever organisational structure applies, it is desirable that each party gets to know as much as possible about the activities, capabilities, limitations and preferences of all the parties involved. Consulting engineers should encourage their design staff to pay frequent visits to contractors' premises and construction sites during the progress of a job and to discuss techniques, preferred procedures and problems with the workshop and erection supervisors. Likewise contractors should ensure that their design and drawing office 9
personnel become familiar with fabrication procedures through regular visits to the fabrication shops. Regular, informal meetings between office and production staff to discuss current problems and possible improvements in procedures are an effective means of encouraging mutual interest and of ensuring a smoother flow of work through the shops. Efficiency can also be improved by making available technical literature. Every company should have a technical library and subscribe to local and international engineering journals. A senior staff member, preferably in the design office, should have the responsibility of identifying relevant material and this should be made compulsory reading for all design personnel. For those companies which do not have libraries, the SAISC library has excellent and comprehensive information on all aspects of steel design, fabrication and erection.
2.2 Design Of all the players in a typical structural steelwork team, the designer is the one best able to influence the total economy of a project. His involvement comes at a very early stage, and almost every decision he makes will have a bearing on the efficient progress of the job, right through to final completion. He should be aware of this responsibility and should keep the matter of economy in mind in relation to every detail of his day-to-day work. Design responsibility It is a legal requirement that a designer, whether employed directly by the client, the client's consulting engineer or a contractor, be a registered professional engineer or registered technologist, or that his work be done under the supervision of a registered engineer or technologist. It is also mandatory that the design complies with the National Building Regulations (Ref. 2), which in turn refers to SABS 0400: Code of Practice for the Application of the National Building Regulations (Ref. 3) for the 'deemed to satisfy' aspects of the design. One of these requirements is that the structural steelwork must comply with SABS 0162: Code of Practice for the Structural Use of Steel (Ref. 4). The safety or fitness-for-purpose aspects of design are thus covered by statutory documents, but the art of structural engineering lies in the ability of the designer to produce a structure that not only fulfils the safety requirements, but that will meet the client's functional needs, will have adequate durability, can be easily maintained, will be aesthetically pleasing and will be able to be built at an acceptable cost. The designer's overriding responsibility will be to provide a safe design, complying with all statutory requirements, thereafter, his first loyalty will be to his employer, whether client or contractor. Cost of design The cost of design represents a small part of the total outlay on a project, yet the decisions taken by the designer from the conceptual stage through to the final details have a direct effect on the total project cost. The design effort should therefore not be under any undue financial or time constraint. Time and money saved on shorter design times are almost invariably lost in the later stages of construction where delays and losses of economy due to incorrect basic decisions become apparent. The designer should realise the importance of considered, balanced decision-making right through the design process.
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Design philosophy The three components of the design function that have the greatest influence on economy are: a)
The choice of the correct framing system
b)
The efficient design of the structural members comprising the frame
c)
The use of simple connection details
The two basic cost factors in structural steelwork are the mass of steel material involved and the unit cost of fabricating the material. The cost can accordingly be expressed as Mass (in tons) x Cost (in rand) per ton or, put more simply, Price = Mass (tons) x R ton. Both of these components are important, but it is essential to keep them in balance. Some designers are unduly motivated by mass-saving in their designs without realising that the second factor, namely production cost per ton, is the more critical. It must be emphasised that minimum-mass design is seldom the cheapest design. It is not too difficult to produce a design of low mass. This can be achieved in a variety of ways, such as • using a different section size for every member in a lattice girder or for every beam in a • • • • • • • •
floor according to their particular loading; specifying slender plate girders instead of heavier rolled sections; using intermediate stiffeners on plate girders to reduce the web thickness; stiffening column base plates to minimise the bending of the plate; specifying column web stiffeners at beam-to-column moment connections instead of using a thicker column web; making beams fully-continuous by means of site-splicing; curtailing girder flange plates; using groove welds instead of splice plates end plates and fillet welds; adopting non-standard connection details; etc, etc.
However, every one of these mass-saving measures will result in a significant increase in labour input. Whilst it is easy to calculate the steel-mass component of the above equation, it is far more difficult to assess the rate-per-ton component to allow for the extra labour content. Labour costs may account for up to two-thirds or more of the ex-works price of steelwork. It is thus important that consideration be given to minimising labour content in order to reduce overall cost. An argument sometimes put forward in favour of the minimum-mass solution when applied to competitive designs done by or on behalf of contractors is that the estimator will arrive at a lower total price, placing the fabricator in a better position to secure the contract. This is a short-sighted view, since the fabricator will be faced with the problem of having to produce complex steelwork that has been priced at unreasonably low rates. The end result is a reduction in or loss of profit. The better procedure is for the designer to produce a 11
minimum-cost (not minimum-mass) solution and then to point out to the estimator all the labour and cost saving features of the design that will enable him to estimate a lower, more realistic, rate per ton. Without doubt there is hardly a structural design that could not be built better at equal or lower cost through careful attention to efficient, economical details. The savings that can be made by rationalising member sizes, adopting simpler details, using standard connections, etc are discussed in Chapter 3. Computer-aided design The use of computers in design offices has become universal. However, it needs to be understood that a computer cannot 'design' at all – it only processes data that is fed into it. Structural steelwork design is as much an art as it is a science and a good designer will arrive at the best solution not only by instinct, logic and dependence on past experience as by the hard-and-fast application of the rules of engineering. It is true that once the shape, size, framing arrangement, etc of a structure have been determined by the designer, the computer is very useful in analysing the forces in the members and components, in giving the deflected shape of the structure under load and calculating the mass. The cost of design represents a small part of the total outlay on a project and the computer should therefore not be regarded as a means of saving costs, but as an efficient way to perform an accurate analysis and, because of its speed of operation, to carry out a number of alternative analyses for the purpose of comparison.
2.3 Cost estimating Another important activity in the production of economical structures is accurate estimating. Once the designer has completed his basic conceptual design – including perhaps one or more alternative designs for comparison – it is necessary to arrive at a close estimate of the total cost of the structure. For this purpose a list of quantities must be prepared and this should cover not only the main members, but also make allowance for details and connections. The list may be compiled by the designer himself, or by a quantity-writer in the estimating office working from the layout drawings and or calculation sheets supplied by the designer. The procedure whereby the designer lists the quantities is the better one since he, through his involvement with the design, is in a much better position to make allowance for details, connections, splices, fasteners, etc. A quantity-writer, on the other hand, has difficulty in visualising the designer's requirements regarding connections and other details, having to make a percentage allowance to cover these items. This obviously leads to less accurate estimating. As stated earlier, fabricated steel is costed on a rate per ton basis, i.e. the mass of steelwork is multiplied by the estimated fabrication cost per ton to give the total cost, a different cost 12
rate being applied to each category of steelwork in the job, e.g. columns, beams, girders, trusses, purlins, stairs, gutters, etc. Erection is usually estimated separately, either on a rate per ton applied to the total structural mass or by estimating the time required for erection plus crane and equipment hire costs. The above pricing system applies to contracts awarded on a lump-sum basis, i.e. when contractors are required at the tender stage to quote a single, firm price for the fabrication, delivery and erection of the entire steelwork. An alternative system, usually employed by consulting engineers, is for the engineer to prepare a bill of quantities, broken down into the various categories of steelwork, and to issue this, together with the necessary layout drawings, to the contractors for pricing and submission of tenders. In the latter procedure a high level of accuracy in quantity take-off is not called for since the final sum to be paid to the contractor is adjusted by multiplying the agreed rates by the final steelwork masses as measured on completion of the job, these masses being calculated from the steelwork detail drawings. Whichever system of pricing is used, calculating the mass is not difficult since this is based on an already-completed activity, namely the design. However, estimating the cost per ton has to cover activities yet to take place, viz. fabrication and erection, and is consequently much more difficult to do. For any given category of steelwork the cost for supply, delivery and erection is usually assumed to be made up of the following items; a)
Preparation of shop detail drawings
b)
Purchase of steel from mills or merchants
c)
Purchase of bolts and welding electrodes
d)
Shop fabrication, including shop painting
e)
Transportation to site
f)
Erection and site painting
g)
Overheads
h)
Profit
If an accurate estimate is required, the above costing exercise should be applied to each main category of steelwork in the job, as previously explained, with allowance being made for the type of construction involved, e.g. bolting or welding. The differentiated costing according to category is especially applicable to large, complex structures such as power stations and industrial process buildings where a wide variety of components are involved, ranging from simple rolled floor beams through plain columns, purlins and girts, plate girders, latticed columns and welded trusses to more labour-intensive items such as stairways, gutters and special plate work. On the other hand, where the structure is relatively simple and contains an average mix of low to high labour input components, the contractor may prefer to do a simple costing exercise, inserting average rates per ton for items (a), (d) and (f). The following points should be taken into consideration with regard to the costing of steelwork items listed above: 13
a)
Detail drawings The preparation of shop detail drawings can amount to 6 per cent of the total cost and this task should not be underestimated. The rate is usually based on drawing office costs incurred on previous, similar jobs. Alternatively, where a contract draughting service is used, a firm quotation may be obtained. The production of good, accurate shop drawings is essential to the smooth progress of the job and an adequate sum should be included for this service. If the amount allowed is too low, short cuts will have to be taken to save time as the job proceeds and this will have serious repercussions right through the steel ordering, fabrication and erection stages. Included in the drawing estimate should be an allowance for proper checking.
b)
Purchase of steel The detailed steel requirements are contained in the quantity lists referred to previously. They are listed under the various components, viz. columns, beams, trusses, purlins, etc. Steel prices as given in the mills' price lists are made up of the base price, which varies according to the type of mill product, e.g. plate, sections, etc, and extras determined by the size of section, the requirement for standard versus non-standard lengths, and the grade of steel, less discounts for large quantities of a particular size. Merchants' price lists on the other hand are all-inclusive and usually allow for delivery. Discounts are negotiable and depend on the volume of orders placed by the contractor. The prices for standard or stock lengths of section are lower than for sections cut to size, but the latter option should be considered carefully because of the overall savings that could be achieved. In South Africa the steel for most small to medium-sized contracts is ordered by the contractor from steel merchants rather than from the rolling mills. The merchants usually carry large stocks and are able to give quick delivery, and offer a cut-to-length service for sections and a cut-to-size service for plates. For contracts requiring a large tonnage of steel, especially where a large quantity of particular sections is required, it might be cheaper to order the main material from the mills (at a discount for quantity) and to obtain the low-volume material from merchants.
c)
Purchase of bolts Because of the high unit cost of bolts, they should be specifically allowed for in the steelwork price make-up. An estimate should be made for the bolt mass per ton of fabricated steelwork, depending on the category of steelwork involved, and this should be multiplied by the cost per ton of the bolts. As a rough guide, an allowance for bolts of 2,5 per cent to 5 per cent of the steel mass would be realistic, but more accurate percentages may be available where fabricators keep records of the bolt content of previous jobs, graded according to the steelwork category.
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d)
Shop fabrication The cost per ton of shop fabrication varies over a wide range, depending on the complexity of the component. Purlins and girts, which only require cutting and punching, have a very low labour input. This also applies to rolled I-section beams with simple end connections, but once stiffeners and extra cleats are added the amount of labour increases considerably. At the top end of the cost scale are welded plate girders with stiffeners, welded trusses and lattice girders, and latticed twin-leg crane columns. In each case the estimator has to assess the total labour input of the component against its mass in order to arrive at the works cost per ton. Since it is difficult to anticipate what the actual shop production costs will be, a common practice is for estimators to rely on costs of similar work previously carried out. It is important therefore for the shops to keep accurate cost records of all types of work done and to make these available to the estimating department for updating their records. Whatever system is used, it is important that the estimator evaluates the content of the job carefully so as to arrive at a true assessment of the degree of labour intensity. If the designer has gone out of his way to produce a simple, but not necessarily minimum-mass design, he should point this out to the estimator so that a reduced rate per ton can be applied. Shop painting, where called for, usually includes blast cleaning and the application of one coat of primer, the costs of which can vary considerably. The price, which is proportional to the surface area to be treated, must allow for the cost of application, the required dry-film thickness and the applicable spread rates. Surface areas per metre length of all the commonly used sections are given in the Steel Construction Handbook (Ref. 5), so it is not difficult to convert these to areas per ton. The cost of application may be based on previous shop records.
e)
Transportation The transportation of fabricated steelwork from the shops to the site may be undertaken either by the fabricator's own transport or by a transport contractor. In either case an estimate must be made of the number of trips involved, which will depend on the capacity of the vehicles and on how efficiently they can be loaded. Where applicable, allowance must also be made for special or police escorts in the case of abnormal loads, or for having to transport at off-peak times.
f)
Erection and site painting The cost of erecting the steelwork is best obtained by getting the erection department to estimate the time required to complete the job, the site establishment costs and the crane requirements. Here again, past experience can be useful, but due allowance must be made for special factors which may have influenced the erection time on previous jobs, such as bad weather and delays due to outside causes. Painting of the structure after erection is priced in the same way as shop painting, namely by allowing for the cost of the paint and estimating the manhours required for application.
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g)
Overheads Overheads are made up of all costs involved in the operation of the contractor's business that cannot be or are not directly attributed to particular contracts. They cover a wide range of items and include the salaries of managers and non-production personnel, rentals, property maintenance, running costs, etc. By dividing the annual total of such costs by the annual production of steelwork per annum, an average rate per ton can be arrived at and included in the cost make-up. Alternatively, the annual total can be divided by the annual shop manhours worked to arrive at a percentage that can be applied to direct fabrication cost per ton.
h)
Profit When the costs calculated for items (a) to (g) are added up, the total cost per ton for supply, delivery and erection of the steelwork is obtained. The percentage profit to be added depends on the state of the market, the current level of activity of the company's workshops, the suitability of the job to the company's production mix, amongst others.
An example of a pricing schedule for the ex-works cost of a particular type of component, viz. welded roof trusses, is given in Fig 2.1. Actual rates cannot be given because of their variability from one contractor to another, so they are given in terms of an imaginary total price of 100 units per ton, excluding profit and VAT. Price per ton for supply ex works of welded roof trusses Shop detail drawings Purchase of steel Purchase of bolts Purchase of electrodes Shop fabrication Shop painting Works overheads Net cost ex works
3,5 42,0 2,0 1,0 22,0 3,5 26,0 100,0
Fig 2.1: Pricing schedule for welded roof trusses
2.4 Shop detailing As is the case with the designer, the draughtsman who prepares the shop drawings is in a position to directly influence the economics of a structural steelwork project. His drawings are the medium by which the designer's intentions and requirements are conveyed to the fabricating shops. If the drawings are clear, unambiguous and above all accurate, fabrication and assembly can proceed smoothly and efficiently. If they are not, queries arise between shop and drawing office, work is slowed down and at worst, mistakes are made that require the re-working of steel and wastage of material. Detail draughting personnel may either be in the direct employ of the steel fabricator, or may be engaged as contract draughtsmen on a short-term basis. Where a group of draughtsmen 16
with a diversity of training and experience are required to work together on a single project, as could be the case with contract draughtsmen, it is essential that management clearly spells out the required level of performance by way of drawing office standards, reference to standard industry procedures, etc. To promote suitable draughting standards the SAISC has published the Southern African Structural Steelwork Detailing Manual (Ref. 6). The adoption of the procedures set out in this publication should go a long way towards meeting the needs referred to above. However, the general procedures should be supplemented by specific company standards covering a) detailing practice, including standardised drawing layout, projections, scales, notation, symbols, etc, and b) preferred workshop practices, e.g. standard end connections, preference for bolted or welded details, workshop machine capabilities, handling facilities, crane capacities, etc. An area of shop detailing that is often not clearly defined is the design of connections. Where the designer has not given specific details of the connections required it is the duty of the draughtsman to select suitable connection types. His selection must be based on the loadings given on the designer's layout drawings for beam end reactions, member forces in trusses (tensile and compressive), column loads, crane loading on columns, etc. To simplify the selection of connections, the Steel Construction Handbook (Ref. 5) presents the design resistances of a large number of standard connections in tabular form. The welds specified on the drawings should be proportioned to transmit the actual forces. The 'short-cut' method of calling for continuous welds in the General Notes often results in items being over-welded and should be avoided. Structural connections are every bit as important as the main members themselves – the failure of an inadequate connection will result in an overall failure or collapse just as serious as that caused by the failure of the member. For this reason it is imperative that the allocation of responsibility for the design or selection of the connections is clearly defined in the contract documents. It is the practice of some engineers who lack a detailed knowledge of connection behaviour and design to pass on the responsibility to the contractor. It is also true, however, that the understanding of connection design in some drawing offices is not as good as it should be. In an attempt to address this failing, the SAISC has published a book, Structural Steelwork Connections – Limit States Design (Ref. 7), which covers the theory and design of the full range of welded and bolted connection types used in the steelwork industry. This book is recommended reading for all steelwork designers and draughtsmen. Note that the design engineer retains responsibility for the structural behaviour of connections; where connections are selected by the contractor they must be approved by the engineer. The use of computer-aided draughting or CAD is becoming the norm in South Africa, especially in the larger design offices. Considerable savings can be achieved if a suitable system is used and this can be important on large contracts where the drawing costs represent a significant part of total costs. Another advantage of CAD is that it imposes a standardised method of detailing on the fabricator, which makes for easier interpretation of drawings in the shop. The introduction of 3D detailing packages by some larger firms allows for project integration between designer, detailer and fabricator. The operator creates a complete, solid 3D product model of the steel structure including all the relevant information required for manufacture and construction. 17
The model of the structure is created by the operator specifying interactively the layout of the elements and associated connections. The elements appear on the screen as real sections, which enables the operator to dictate the necessary detailing requirements to solve any practical problems whilst generating the model, thereby eliminating any unnecessary amendments. All elements are intelligent objects. For example, when the size of a beam is altered, dimensions of adjoining connections change accordingly.
2.5 Fabrication The main operations in the fabricating shop are the drawing of materials from stock; cutting to size; the drilling or punching of holes; assembly by bolting or welding; the attaching of cleats, stiffeners and other fittings; and usually shop painting. Cutting to size will be by sawing, cropping, shearing, flame-cutting or machining, whilst the attachment of fittings will be by bolting or welding. In addition to having workshops equipped for all of the above activities, the larger fabricators will also have facilities for facing column ends at splices, machining base plates, sand or shot blasting, turning shafts, boring large diameter holes, rolling plates and forging special shapes. Smaller fabricators on the other hand are often only capable of carrying out the most basic operations of cutting, drilling and welding. Even so, they are nevertheless able to fabricate a wide variety of the simpler types of structure at competitive prices. The four main requirements for efficient steelwork production are, a) processing machines (preferably computer numeric-controlled — CNC) for the type of work being handled, b) a suitable shop layout that will allow the smooth and orderly progress of work through the fabrication processes, c) adequate handling facilities, such as overhead and jib cranes, roller tables and conveyors, and d) modern automatic and semi-automatic welding equipment. These requirements may of course vary in their particular characteristics according to the type of work being specialised in – whether light or heavy structural work, bolted or welded construction, latticed or plain section members, platework, pipework, conveyor steelwork, etc. Usually, the larger the shop, the wider the variety of work handled. Thus, in a typical medium to large plant there will be several production bays, generally parallel to each other, specialising in light, medium and heavy fabrication; the manufacture of small components, e.g. cleats, base and cap plates; the assembly of sub-components into larger components such as trusses and lattice girders; and for wire brushing or blast cleaning and painting. The larger shops are also more likely to have automated handling equipment and CNC machines, where simpler items such as I-section beams and columns can be handled on a production line incorporating conveyors, automatic saws and punching or drilling machines. In such systems it is possible for the whole sequence of handling and forming to be pre-programmed, eliminating the need to measure members or mark off holes beforehand. The attachment of cleats, fittings and other details is done by hand. An important consideration in planning the flow of work through the shop is to avoid delays in the progression of work. Any hold-up in production, whether because of unusual fabrication processes, awkward shapes or sizes, or the need to correct errors, wastes time and money since other work is being delayed. It is therefore necessary to minimise impediments as far as possible.
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Because of cost implications, fabricators generally prefer welding to bolting for the attachment of fittings in the shop. This requires the positions of fittings on each of the main members to be marked, but even if this is done with care there is the possibility of errors since each marking operation is a once-off activity, except when jigs are used. Larger fabricators with programmed drilling or punching equipment might prefer bolting, particularly for standard connections. Because the repetitive nature of the drilling or punching operation reduces the likelihood of error. The checking of the fabricated steelwork before dispatch to site is an essential activity. Where items are individually fabricated every component should be checked as a matter of course while selected items should be checked where automated and jigged production methods are used. The discovery of errors on site, particularly when a large piece such as a truss, girder or portal rafter is lifted and is found not to fit onto the columns, is extremely disruptive. Correction is usually costly and time-consuming, particularly when it involves returning the piece to the shop for re-working. Even a few mistakes in a single contract can seriously reduce the profitability.
2.6 Summary • All persons involved in a project should work together as a team and should have a
general understanding of each others' functions and responsibilities. • Design:
The design function, even though critical to the efficient progress of a job, represents only a small part of the total project cost and should thus not be placed under undue time or financial constraints. The three components of the design process that affect economy are the choice of a correct framing system, the efficient design of members and the use of simple connection details. Comparative designs should be done to determine the most economical solution. Savings in steel mass should be weighed against simplicity of detail; the lowestmass design is seldom the cheapest solution. • Estimating:
Estimates of material costs should be based on mills' or merchants' price lists, with allowances for extras and quantity discounts, etc, and making provision for the cost of fasteners. Estimates of fabrication costs should be based on man-hours ton recorded on previous contracts per class of steelwork (workshops should keep accurate records for this purpose). Lists of quantities should preferably be prepared by the designer as he is in the best position to make allowance for connections and other details. Adequate allowance should be made for shop cleaning and painting of the steelwork.
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The erection price should be based on a realistic assessment of the on-site costs, such as labour, travelling time, hire of equipment and final painting. • Detailing:
Standard drawing office procedures should be used and should be based on SAISC's Structural Detailing Manual and company standards. Accurate, clear drawings should be produced and all drawings should be checked. Standard connections, as given in SAISC's Steel Construction Handbook or in Structural Steelwork Connections, should be used wherever possible. Steelwork should be detailed in the manner best suited to workshop production facilities. • Fabrication:
Workshops should be laid out in a manner conducive to an efficient flow of work. Suitable machines and equipment (preferably automated or CNC) should be provided to ensure the rapid handling and assembly of components. Adequate craneage should be provided. Where possible separate areas or bays should be allocated for different types of work. A routine inspection procedure should be carried out.
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3
General Economic Considerations
3.1 Introduction The topics discussed in this chapter include many practical matters ranging from efficient material utilisation to detailed workshop practices. They may be applicable to almost every type of structural project and have a direct bearing on low cost steelwork production. Considerations relating to framing systems and the design and detailing of particular components such as columns, beams, girders, bracing systems, etc, are dealt with in later chapters.
3.2 Materials and material utilisation Grouping of members into sizes In selecting the sizes of members in trusses and lattice girders, beams in a floor, columns in multi-storey buildings or members in a bracing system, it is often best not to choose a different size for each member based on its particular loading, but rather to arrange the members into groups of the same size. In this way the ordering of steel is simplified and advantage can be taken of quantity discounts. Short lengths of material can be cut from stock lengths, reducing waste and saving costs. Grouping by size is of special benefit when applied to beams, since many or all of the beams of a given span can be covered by a single detail on the drawing, with savings in draughting time, much larger savings in shop cutting, marking and drilling time, and a reduction in the likelihood of errors. It is obviously much quicker to process, say, ten identical beams than to have them divided into smaller numbers of different sizes. Erection is also simplified since all the beams will have the same erection mark and can be more easily identified prior to being lifted into place. There must of course be a balance between the expected saving in labour and the extra cost of the heavier material. This balance cannot be defined exactly, but in general an increase in mass of up to 5 per cent or even more will be offset by the savings in labour.
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Section and plate sizes - preferred range The range of rolled section sizes produced by South African mills is much smaller than in the United Kingdom and especially the United States, and yet is wide enough to meet the practical needs of most designs. This limitation in product range is obviously of benefit to the industry through economics of scale as it enables prices to be kept down. For the same reasons that apply to grouping by size, the use of a range of preferred beam and angle sizes and plate thicknesses is an advantage. This would be a series more limited than the full range available from the mills and can be decided upon by the individual fabricator on the basis of his purchasing pattern, stockyard space, etc. The designer should thereafter always try to work within these limits. The lightest mass m in each serial size of Iand H-section and the smallest thickness in each angle size represent the most structurally efficient sections, i.e. on a load capacity per kilogram basis, so obviously these sections would make up the bulk of the preferred range. Plate thicknesses in particular should be reduced to a preferred range so as to simplify ordering and to enable plate off-cuts to be used for making smaller items such as gussets and end plates. A suggested range is, in mm: 3, 4, 6, 8, 10, 12, 16, 20 and then upwards in increments of 5 to, say, 60 and then upwards in increments of 10. Plate lengths and widths, too, should be restricted to a relatively limited range selected from merchants' stock sizes. Even where the design is prepared by the owner or his consulting engineer, and includes a wide variety of sections, the fabricator would be well advised to reduce the range by substituting heavier sections in a stepped pattern, thereby achieving an overall saving. Shop splicing Random splicing, i.e. the introduction of a shop-welded splice within the length of a member, is often done in order to use shorter pieces of material to make up a full length. In this way surplus short lengths, possibly off-cuts, can be used up and wastage reduced. This can be an effective way of saving money, but obviously the cost of welding should not exceed the saving in material. The splices would usually need to be full-strength welds (especially in the case of beams) and would involve edge-preparation of the member, which could easily offset the apparent savings. The designer should be referred to before this method is used to ensure that it is structurally acceptable. Steel grades The commonly used structural steel grade in South Africa is 300WA, which has a minimum yield stress of 300 MPa and a minimum ultimate tensile strength of 450 MPa. It is the steel with the most favourable strength-to-cost ratio and is the most readily available from suppliers. (Due to production improvements, Grade 350WA is becoming more readily available. Merchants and Producers should be consulted prior to design) Any departure from this grade should be considered carefully by the designer. The use of a higher strength steel could be justified in heavy welded plate girders, in the columns of large multi-storey buildings to maintain the same serial size for the full height of the column and in the construction of heavy box columns. It must be remembered, however, that the cost of welding high-strength steel is greater than that of Grade 300. The modulus of elasticity (E value) for all steels is the same, so it is obvious that a beam in, say, Grade 350 steel will have a deflection about 17 per cent larger than that of a beam in 22
Grade 300 of equal depth and stressed to the same percentage of the yield stress; this should be checked by the designer. Furthermore, such steels would need to be rolled specially and the mill would require an order to be placed well in advance and for a certain minimum tonnage per section or plate size. For any particular project, the cost per ton ratios of Grade 350W and Grade 450W to Grade 300W steel should be obtained from the mills. Structural hollow sections The use of structural hollow sections (SHS) is becoming more common, especially in structures where appearance is important, but they are more expensive per ton than conventional open rolled sections and are usually more costly to fabricate. On the other hand, these sections are very much more efficient when used for columns or other compression members, especially bracings, and significant mass savings can be made. Their properties and uses are dealt with in detail in the SAISC publication South African Structural Hollow Sections Handbook (Ref. 8). A serious disadvantage of hollow sections it that they are generally only available from merchants in 6,0 m lengths and that butt splicing is required for longer lengths. This is in itself expensive and also generates excessive waste. Some producers and merchants are now stocking lengths greater than 6 m. Relative costs per ton of sections The approximate relative costs per ton for the various types of steel section commonly used are shown in Fig 3.1. The cost factors are based on merchants' retail list prices for 1994, with an allowance for a 20 per cent discount on hot-rolled sections and 30 per cent on coldformed sections (these discounts vary slightly according to the supplier, the terms of payment, etc, but are fairly representative). The base figure for which the factors apply is the cost relating to the range of equal angles and IPE sections. The factors are not exact, but they do give a fairly accurate indication of the relative material costs of the various sections and will serve as a guide to designers when selecting types of section to be used in a structure. For example, comparing the costs of, say, a lattice girder built alternatively in equal-angle sections or in rectangular hollow sections, it will be seen that the use of hollow sections will cost about twice as much on a unit material cost basis. The hollow sections truss will, however, have a much lower total mass.
3.3 Connections Connections should be kept as simple as possible. Both the designer and the detailer should be aware that connections make up a significant part of the cost of a structure and should thus take advantage of standard details wherever practicable. A high level of repetition of simple connections throughout a project makes for economy both in material usage and in labour. It also enables the fabricator to make up a stock of standard items such as angle cleats for beam ends and to employ standard jigs for attaching welded end plates to beams.
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The subject of connections is dealt with in greater detail in the chapters relating to specific topics, viz. beams and plate girders, trusses and lattice girders, columns, bracing systems, etc.
2,05 1,96 305 x 305 x 198, 240, 283
1,70 1,54 1,35
1,00 1,00 1,02
1,09 1,11 1,13
Steel Grade Commercial
Steel Grade: 300WA
IPE
Pl.
Fig. 3.1: Relative costs/ton of steel sections
3.4 Fabrication Specification The most commonly used specification relating to the fabrication and erection of structural steelwork in South Africa is SABS 1200: Standardized Specification for Civil Engineering Construction, Part H: Structural Steelwork (Ref. 1). It covers all aspects of structural steelwork, including materials, fabrication, erection, tolerances and testing. This should be used together with the SAISC, SA Structural Steelwork Specification for construction (Ref. 14). Since it is fully comprehensive there is usually no need for a client to write his own specification on these topics; all that is required is for him to refer to SABS 1200H in his contract documents. In the case of special structures such as bridges, cranes, mining structures and tanks it is, however, necessary for the client to specify additional requirements or to refer to supplementary specifications. All fabricators should be in possession of a copy of SABS 1200H and be fully conversant with its contents, especially the clauses dealing with fabrication and tolerances. Materials handling Every piece of steelwork in a structure, ranging from the smallest components through columns and beams to large items such as shop-assembled trusses and girders, has to 24
move through the shops from the stockyard to the dispatch bay and a variety of fabrication operations along the way. This needs to be borne in mind by both the designer and the detailer, so that the components are made in the most convenient form for easy fabrication and handling. To facilitate the assembly of latticed trusses and girders they should where possible be designed in such a way that all the work can be done from one side when they are lying on the assembly bed. Turning the truss over to allow welding on the other side requires crane handling and is a time-consuming and expensive operation.
I-section beams that require drilling and the welding on of cleats and fittings require far more handling than beams designed for drilling or welding only. It is usually much cheaper to punch holes than to drill them, but obviously the shop must be properly equipped for a punching operation. The designer should be aware of the hole forming method to be used so that he can make the correct design allowance for holes in tension members. Punching and drilling is usually performed by stationary machines, with the workpiece being moved through. Adjustable stops, engaging in a template, can be used to avoid the individual marking of identical pieces. In smaller shops portable drills may be used, but in this instance extra care needs to be taken in marking off and centre-punching the individual pieces; this procedure is time-consuming and expensive. As crane handling is expensive and slow, the turning over and double handling of large assemblies should be avoided. Designers and detailers should familiarise themselves with the crane capacities in the various bays of the shop, so that they do not call for items or assemblies that are beyond these capacities. Tolerances Tolerances that have a bearing on shop fabrication are a) mill or rolling tolerances, which relate to the dimensions of sections and plates and b) fabrication tolerances, which have to be observed by the workshop personnel in the fabrication of the steelwork. Rolling tolerances are the allowable deviations from cross-sectional dimensions, cross-sectional squareness, straightness, specified mass per metre and plate thickness. There is no South African specification in this regard, but all sections rolled locally are produced within the tolerances laid down in BS 4: Part 1, ISO-R657, DIN 1025 and DIN 1026 for the various types of section. They are reproduced in the SAISC publications South African Steel Construction Handbook (Ref. 5) and Structural Steel Tables (Ref. 9). Fabricators should familiarise themselves with the tolerances since they are large enough to have an effect on fabrication procedures. Out-of-squareness of column flanges may require shimming of seated beam-end connections, cross-sectional variations at column and beam splices can be avoided by matching the two parts of the member, off-centre webs at beam splices will require adjustment of the holing in the flange splice plates, etc. If such variations can be anticipated and are allowed for there will be a saving in assembly time. Fabrication tolerances are specified in SABS 1200H and should be carefully observed in the workshop. They include permissible deviations on the depth and width of welded cross sections, the flatness of webs, the tilt and warpage of flanges, the overall length of members and the straightness of members. The general tolerance applicable to dimensions of 25
members and components, and to the location of holes, is ± 2 mm. The numerical values of tolerances are based on practical fabrication procedures and are not difficult to maintain provided the shop personnel are aware of them and are reasonably careful. The exceeding of tolerances can be cause for rejection of the steelwork by the client and rectification can be very costly.
3.5 Surface preparation and finishing Unpainted steelwork Steelwork that is to be embedded in concrete, e.g. the columns and beams in a multi-storey building that have been designed as composite members, is left unpainted. Other examples are column bases located below the level of a concrete floor and steelwork to be fire-proofed by a sprayed-on coating. In all such cases it is important that the steelwork is properly cleaned and degreased to ensure that the concrete or coating bonds properly to the steel surface. There are many other situations where painting is not necessary. For many years it has been the practice in the United States to dispense with painting on all steelwork that is concealed from view in the final structure, provided that it is not in a corrosive environment. This includes floor beams in a multi-storey building, roof steelwork above a ceiling and even the steelwork in a power station boiler house where the settlement of coal dust on the structural framing will in any case nullify the aesthetic effect of the paint. In South Africa this practice has been slow to catch on and very few structures are left unpainted in spite of the generally favourable conditions that exist in the dry interior regions of the country and benign internal environments created within many buildings. Obviously considerable savings could be achieved by dispensing with unnecessary painting. Standard paint coatings For steelwork exposed to view, but where corrosive conditions are mild, a paint coating is usually required for aesthetic reasons; here a simple specification will suffice since no special corrosion resistance is called for. The usual specification is wire-brushing followed by the application of a primer coat of paint to the steelwork in the shops, and touching-up of the primer and a final coating with enamel or other suitable paint of a specified colour after erection. When properly applied this system lasts well, but the touching-up on site must be done thoroughly, especially on the site bolts and site welds. It should be done as soon as possible after erection to prevent the corrosion of fasteners and steel surfaces. Special paint coatings Where corrosive conditions exist, such as external exposure in coastal areas and inside chemical and certain other process plants, the purpose of the paint or other coating is to provide protection from the corrosive environment. A much more specialized procedure needs to be adopted, including the blast-cleaning of the steelwork in the shops and the application of special paints. It must be appreciated, though, that the cost of such treatment could approach or exceed the cost of the basic unworked steel. 26
In these special circumstances consideration should be given to the use of structural hollow sections, which have the following advantages: a)
Because of their smoother profiles and the absence of re-entrant corners they do not retain as much condensation or dust as open sections and are thus less susceptible to corrosion.
b)
Their surface areas are only about one-third to one-half of the areas of open sections. Much less paint is thus required and the paint is more easily applied.
c)
When they require repainting at a later stage they can be much more easily cleaned down and, for the reasons given in (b), are more quickly and cheaply repainted.
The above considerations apply not only to structures in corrosive environments but also to those where access for maintenance repainting is difficult, such as sign structures over motorways, high roofs in congested plant buildings, mine shaft steelwork, tall latticed towers and conveyor gantries. Surface preparation The way in which the a steel surface is prepared for painting will depend on the kind of coating to be used. For a simple paint coating, hand or power wire-brushing is sufficient, but must be done thoroughly: Grease and loose mill-scale in particular should be removed, as should all rust. For special painting specifications, sand or shot blasting of the steel surface is called for. The required degree of cleanliness of the wire-brushed or blast-cleaned surface is usually stated in the paint manufacture's product data sheets and is normally specified in terms of Swedish Standard SIS 055900-1967. The advantage of this specification is that it gives clear pictorial representations of the various grades of cleaning. A further important factor influencing paint adhesion is the blast profile and this is usually also specified by the paint manufacturer. Hot-dip galvanising Where a high level of corrosion resistance is called for, hot-dip galvanising would be the correct surface treatment. It produces a hard, durable finish that is not as prone to damage during handling and erection as a paint coating. The steelwork, in the form of complete sub-assemblies such as beams, columns, trusses (or half-trusses), etc, is first immersed in an acid pickling bath for the removal of all foreign matter and the production of a clean, matt surface and is then placed in the galvanising bath. Different thicknesses of zinc coatings can be specified, depending on the degree of corrosion resistance called for. Steel is galvanised to SABS 763-1988 and the thickness of the coating depends on the type of article and whether the applications are for general or heavy duty, as defined in SABS 763. The size of component that can be handled is of course limited to the size of the bath, although long members can be galvanised by a two-stage process, a half-length being treated at each dip. The designer or detailer should be aware of available bath sizes and limit the sizes of the components accordingly.
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Structural hollow sections with closed ends (e.g. welded end plates or T-connections) require special treatment. Because of the possibility of a section bursting in the zinc bath due to sudden vaporisation at high temperature of the pickling solution in the enclosed spaces, it is necessary to provide air vents to all such areas. These should be in the form of small holes located at both ends of the member to ensure a free flow of acid solution and zinc coatings in the voids. Designing for corrosion resistance Perhaps the most effective means of preventing corrosion is proper design. If all sections are selected and positioned in such a way as to avoid traps for dust and water, the incidence of corrosion will be much reduced. Examples of traps are the outstanding legs of members in inclined bracing systems, the web-flange hollow in horizontal I-section members and the spaces between the components of double-angle and double-channel members placed back to back with a gap between. Examples of bad and good practice are shown in Fig 3.2. As stated earlier, the use of structural hollow sections is also a good corrosion-resistant measure. Other areas requiring attention are member connections and fittings such as stiffeners and cleats. In all cases these should be designed to be as simple as possible and to allow water to drain off easily.
(a) Bad practice
(b) Good practice
Fig 3.2: Member corrosion resistance
3.6 Transportation The transporting of fabricated steelwork to site is an expensive operation, especially when long distances are involved. Components should be designed and detailed so that they can be stacked efficiently onto road transport vehicles, which should be loaded as close to their tare weight as possible. Heavy items should not be stacked on top of lighter, less robust ones, and all should be arranged for easy off-loading, with allowances being made for the limitations of site handling equipment.
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Where possible, load sizes should be kept within defined limits to avoid having to provide motor car or police escorts, or having to obtain a special permit. The limits are set out in the Road Transport Guidelines of the provincial traffic laws. When extra-large items have to be transported by road, enquiries should be made as to the limits for a one-car, two-car or police escort, and whether any other restrictions, such as a prohibition of abnormal loads an busy roads during school holidays, apply. Steelwork should be delivered to the site in the order in which it is required to be erected. This may seem obvious, yet it is common practice for material to be shipped out in the wrong order, with resulting chaos on site because of inadequate storage space and the need for double handling. An extreme example is when a workshop, wishing to boost its current monthly output, fabricates and delivers the purlins, plain beams and other simple items first. This practice should be avoided at all costs.
3.7 Erection Planning Proper attention to the erection aspects of a job at the design and detailing stages will go a long way towards ensuring fast and trouble-free work on the site. It must be remembered that sites are sometimes remote and always lack the convenient back-up facilities that are available in the more orderly environment of the workshop. Everything possible should be done to avoid problems, delays and other crises on site, and to make the erector's task as simple as possible. It is generally quicker to erect a few large pieces than a lot of small ones, but ease of handling must also be considered. Items that are large and very flexible will be difficult to handle and should perhaps be dispatched as subassemblies. Long-span trusses and lattice girders are especially difficult to lift because of their slender chords and will usually require multiple slinging or the use of lifting beams. The designer can avoid undue flexibility by using chords that have adequate lateral stiffness and by providing pitched trusses with apex splices that are continuous over the top. Consideration might be given to erecting trusses in pairs, complete with purlins, longitudinal ties, roof walkways (where applicable), etc. This would require the assembly of these components on the ground and then lifting them with a crane of sufficient capacity and reach. Such a procedure would only be practicable where the size of the job warrants it, and where sufficient assembly space is available, but could result in significant time saving. Where crane gantry girders are provided with lateral plates to the top flange, the plates should be attached and fully bolted on the ground to enable the girder-and-plate assembly to be lifted as one unit. The design and detailing would have to make allowance for suitable connection of the outer flange of the lateral plate to the column roof leg. Erection marking It has already been stated that as many parts or components as possible should be made identical, because of the cost savings in manufacture. The benefits, however, also extend to 29
the site, provided identical items are given identical erection marks. In a group of beams in a floor it will be far easier for the erector to find the one he is looking for if it is one of several identically marked beams than if it has its own distinctive mark and happens to be at the bottom of the pile. This is more important than is generally recognised as material handling facilities on site tend to be rather basic and much time can be taken up in extracting the required member. For main columns the same procedure can be followed, although here there may be some merit in giving each column a different mark corresponding to the grid-marking system used for the building layout. Connections and bolting The designer should ensure that site assembly is as simple as possible. Making connections between members in the air is difficult, time-consuming and potentially dangerous, so every effort should be made to ease the erector's task. It is easier to erect a member by allowing its ends to rest on a solid surface than by having to hold it exactly in position from a crane hook while inserting the bolts. Thus beams that have seating cleats are easier to erect than when web connections are used; likewise trusses and beams that are supported on the top of a column rather than by cleats to the column face. Where seating cleats are used, fabrication and rolling tolerances should be allowed for in the dimensioning of bolt hole positions (see Fig 3.3). Where secondary beams span between main beams and form a continuous line, it is usual practice to detail them slightly under-length and to provide end packings every third or fourth span to make up for the loss in overall length. Also, the provision of a seating cleat under one of the secondary beams to support it while the next one is being placed will speed up erection, as shown in Fig 3.4. Regarding site bolting, the following guidelines should be followed: a)
Consistent with sound connection design they should be designed to use as few bolts as possible.
b)
If practical, all bolts should be of the same type and size.
c)
Friction-grip bolting should only be specified where really necessary because of the high cost of the bolts and installation. Friction-grip connections are normally only specified where load reversal may occur or where slip in the connection may lead to undesirable deformation.
d)
The dimensioning of connections should allow for the easy insertion and tightening of the bolts by hand spanner or power wrench. Avoid bolts fouling one another or other components.
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(a)
(b)
(c)
(d)
Fig 3.3: Beam and truss connections
Erection tolerances The accepted tolerances within which typical structures must be erected are given in SABS 1200H (Ref. 1) and SAISC Structural Steelwork Specification (Ref. 14). As with fabrication tolerances they are based on accepted norms within the industry and are not difficult to meet. For example, member out-of-straightness, i.e. bow must be within 1 1 000 of the member length, while out-of-plumb of columns must be within 1 500 of the height, or 12 mm maximum. Contractors should ensure that the tolerances are in fact worked to as erection proceeds, as it can be extremely time-consuming to re-align, level and plumb a structure that has been found, on final engineer's inspection, to be out of tolerance. On the other hand, the owner should be aware of the tolerance values and thus accept the structure if it does comply with these.
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Direction of erection
Pack
No holes in this leg
Fig 3.4: Secondary beam connections If tighter limits are required for any reason, e.g. in crane gantries, in certain process buildings, to provide accuracy for service installations, to ensure straight alignment for face brickwork, etc, the owner should incorporate such special requirements in his specification. Other erection considerations There are many other matters that have a bearing on erection, but these are discussed within their area of application, in the sections dealing with connections and the detailing of specific items such as columns, beams, trusses and other items.
3.8 Summary • Material utilisation:
When selecting the sizes of members they should be grouped by size rather than different sizes being chosen for individual load cases. Depending on the fabricator's volume of work and stockholding capacity, a limited range of preferred sizes of members, and of sizes and thicknesses of plates, should be stocked. Random shop splicing within the length of a member should be done with discretion as the cost of the splice may outweigh the saving in material. The standard grade of steel, viz. 300 WA/350 WA, should be used wherever possible. Structural hollow sections are usually only available in lengths of 6,0 m when bought from merchants. •
Fabrication: Steelwork should be fabricated to SABS 1200H. There is no need for clients to write their own specifications. Components of structures should be designed and detailed to ensure easy assembly and welding in the shops and a smooth flow from one operation to the next. Punched holes are usually cheaper to produce than drilled holes. 32
The turning over and double-handling of large components or assemblies, involving crane handling, should be avoided where possible. •
Tolerances: Fabricators should observe the fabrication tolerances laid down in SABS 1200H and the SAISC Structural Steelwork Specification (Ref. 14). Fabricators should make allowance for rolling tolerances on section sizes and an out-ofsquareness of column flanges when detailing connection components.
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Surface preparation and painting: Steelwork may often be left unpainted, or be prime-painted only, when not subject to corrosive conditions. Simple painting specifications are sufficient when painting is required for aesthetic reasons only. Where steelwork is used in severely corrosive environments the use of structural hollow sections should be considered providing they are properly sealed. The type and degree of surface preparation called for should be consistent with the type of paint coating to be used.
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Galvanising: Galvanising should be done when a high degree of corrosion resistance is called for. The steelwork components must be of a size that will fit into a galvanising bath. The galvanising of hollow sections requires special precautions to be taken when detailing them to prevent bursting in the zinc bath.
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Transportation: Steelwork items should be loaded efficiently onto vehicles, with abnormal loads being avoided wherever possible. Steelwork should be delivered to site in the order in which it is to be erected.
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Erection: It is preferable to erect a few large pieces than a number of smaller ones, but unwieldy long-span members require special lifting arrangements. Erection marking should be simplified and as many components as practicable should be made identical. Site assembly should be kept simple. Splices should be detailed for ground rather than aerial assembly. Bolting is the simplest form of site connection. Welding should only be used when really necessary. Preferably only one, or at most two, grades and diameters of bolt should be used. Friction-grip bolting should only be specified in special circumstances. Erection tolerances should be observed at all times. In tall buildings this is especially important.
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4
Structural Framing Systems
4.1 Introduction The most important factor in the overall economy of a structure is the choice of framing system. Theoretically, for any given set of functional requirements for a structure there is a wide range of framing solutions. In practice, however, for the great majority of typical structures, the number of solutions is very much reduced and the designer usually has to investigate only two or three types, which he knows by intuition or previous experience will yield the most cost-effective solution.
4.2 Main dimensions The general shape and the leading dimensions of a building or similar structure, i.e. the column centres in plan and the vertical clearances or storey heights, are usually dictated by functional or architectural requirements. Frequently these decisions have been taken by the owner (or his architect) and the structural steelwork designer has little opportunity to modify them. Economy of material usage, however, is influenced by the balance achieved between the column and the beam or truss content of the building, and this in turn is directly affected by the selection of the optimum column centres. Early consultation between the owner and the structural designer will help to ensure the best proportions. Larger column centres will often result in an increase in total steelwork mass, but the rate per ton for the heavier steelwork will be lower because of the heavier sections used and especially because of the reduced number of connections. Furthermore, significant savings can be achieved by virtue of the fact that the number of foundations is reduced. In multi-bay buildings repetition of a common bay width will obviously yield greater economy than the use of bays of different widths, since the trusses or rafters and the floor beams will be the same for each bay, resulting in less time required for design, detailing and fabrication. Symmetry, whether it be in a bracing system within a bay or in the building cross section as a whole, also promotes economy as the components in each half are either identical or opposite-hand to each other.
4.3 Design loading The design loading for buildings and similar structures is given in SABS 0160: General Procedures and Loadings to be Adopted in the Design of Buildings (Ref. 10). In the 1989 34
edition of this code (as amended in 1990) the partial load factors and load combination factors used for the ultimate limit states have been modified as compared with earlier design practice. For dead loading acting in combination with other loads the partial load factor γi is 1,2. For wind loading γi is 1,3 and for wind acting as a non-dominant combination with other loading the combination factor ψi is zero.
imposed
load in
For floor loading on non-storage floor areas, when it is a non-dominant imposed load acting in combination with other loads, ψi has been reduced to 0,3. The judicious selection of combined loading factors will result in more economical design in future.
4.4 Stabilising systems Designing a building structure to resist gravity loading is relatively straightforward, but providing resistance to horizontal loads introduces a number of more complex considerations. Horizontal stiffness is provided both to stabilise a gravity-loaded structure (i.e. to prevent it from collapsing sideways) and to resist wind, earthquake or other loads, which act in the horizontal plane. Figs 4.1 to 4.3 illustrate the principles involved. They are diagrammatic only and are intended to show how any structure, whether it be a single or multi-storey industrial or commercial building, a mine headgear, a tower or any other above-ground structure, has to be provided with a stabilising system to maintain it in an upright position. Two basic methods of stabilisation are employed, namely bracing and stiff or rigid framing, and the three figures show how the systems are used, viz. two-way braced, two-way rigid and a combination of these, i.e. one-way braced and one-way rigid. In all cases it has been assumed that the interior of the structure has to be kept clear of vertical bracing for maximum clearance. a)
Two-way braced structures Fig 4.1 shows the simplest and most effective stabilising system, that is vertical bracing on all four of the exterior faces of the structure. Each column is continuous from floor to roof (although joints at intermediate floors would be permissible), but all other members are assumed to be pin-ended. The columns are axially loaded and so can be compact in section (e.g. H-sections), the beams are simply-supported and the bracings are axially-loaded struts or ties. At the intermediate floor and roof levels horizontal bracing is necessary as shown in the plan because the interior columns are not held laterally in the N-S or E-W directions. Diagonal bracing can be provided as shown by the dotted lines, on the floor and roof plan and if of suitable construction, can be designed to furnish the necessary in-plane resistance.
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Side Elevation
End Elevation
N Vertical bracing in
Roof or floor plan
Fig 4.1: Two-way braced structure
Because of the inherent stiffness of the vertical bracing systems the lateral deflection of the structure under wind loading will be small. The total structural system is statically determinate and is easily and quickly analysed since there is no moment interaction between the beams and the columns. b)
One-way braced, one-way rigid structures The structure shown in Fig 4.2 is provided with vertical bracing in the E-W direction, but depends for stiffness in the N-S direction on the rigid or moment-resisting transverse frames. The E-W beams are thus pin-ended, but those in the N-S direction have moment connections to the columns. Consequently, the columns are I-sections rather than H-sections, with their strong axes E-W, and must be continuous from floor to roof. They will also have their bases fixed in the N-S direction. At the floor and roof levels bracing spanning N-S is required, but the floor and roof can be used for this purpose if of suitable design.
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Side Elevation
End Elevation
N Vertical bracing in N. and S. faces only
Roof or floor plan
Fig 4.2: One-way braced, one-way rigid structure
In resisting wind loading in the E-W direction the bracing system is inherently stiff, but in the N-S direction the rigid beam-column system is much more flexible and must be checked to ensure that lateral deflections are within acceptable limits. The frames in the N-S direction are statically indeterminate and so must be analysed as moment-resisting frames subject to both vertical and horizontal loading, with allowance where necessary for second-order or P − ∆ effects. Plastic design may be used, with consequent mass savings, but elastic deflections must be carefully checked. In the E-W direction the analysis for wind loading is simple. Since the beams in the N-S direction generate end moments due to their fixity, they can be of smaller section than if they were simply-supported. The columns, on the other hand, attract moment in this direction and have to be of larger size. Because of the moment connections, the system is relatively more expensive than the two-way braced structure of Fig 4.1, but the general principle of moment stiffness in one direction and braced stiffness in the other is common to a wide range of structures. c)
Two-way rigid structures A structure that depends on moment stiffness for its rigidity in both rectangular directions is shown in Fig 4.3. Here the beams are made continuous via moment-connections to the columns, in both N-S and E-W directions. Because of the two-way stiffness of the framework no diagonal bracing is required, but deflections under wind loading may be significant. The columns may be fixed-based and should be 37
continuous from floor to roof. Since they must be moment-resistant in both directions, I-sections are not suitable and a two-way symmetrical section such as a square box or a double I-section may be necessary.
End Elevation
Side Elevation
N No vertical bracing
Roof or floor plan Fig 4.3: Two-way rigid structure As each frame is stiff in both directions, floor bracing is not necessary, although the stiffness of the floor and roof would usually be mobilised to ensure equal lateral deflection at all the cross frames. The analysis of two-way stiff frames is more complex than for the two types described above, but can either be done separately for each of the E-W and N-S directions or as a three-dimensional frame. Again, beam continuity makes for reduced beam mass, but the column mass will be increased because of the need for strength and stiffness in two directions. A two-way stiff structure is the most expensive of the three systems discussed on account of the two-way beam moment connections and the more complex column construction. It would only be used where special circumstances require a complete absence of bracing. d)
Leaned frames Multi-bay structures, whether single or multi-storey, may be designed as 'leaned frames', where one or more of the bays are designed as rigid frames and the others as pin-jointed frames 'leaning' against the rigid frame. As can be seen from Fig. 4.4 considerable economy can be gained through using a minimum number of rigid or moment-resisting joints, and making the remaining joints nominally pinned. 38
Stiff frame
Pinned or fixed bases (a) Stiff frame
Pinned or fixed bases
(b) Fig 4.4: Leaned frames The following comments apply to all four stabilising systems discussed above: a)
The simple or flexible beam end connections can be standard bolted cleats using Grade 4.8 or 8.8 bearing bolts. Such connections are inexpensive and easily made on site.
b)
With fixed-ended connections the beams can be lighter, but the columns will need to be heavier. The moment connections may require thick end plates and Grade 8.8S bolts. They would thus be more expensive and take longer to erect.
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c)
With simply-supported beams deflections may be a problem, but these may be reduced by designing the beams compositely with the concrete floor slab, where applicable.
d)
Vertical bracing may cause an obstruction in an otherwise open space, but where it is acceptable it will yield the lowest-cost solution. It also has the advantage of ensuring that the building is erected plumb.
e)
In multi-storey structures the floors, if of solid construction such as reinforced or slabdeck construction, can be employed as diaphragms for transmitting horizontal loading to the vertical bracing. Consideration should be given, however, to providing light triangulated bracing to ensure the squareness of the floor in plan during erection and prior to the casting of the floors.
4.5 Dual function of members Almost every member in a structure performs at least one function in addition to its main function. For example, floor beams carry vertical or gravity loading, but also provide lateral support to the columns; purlins carry cladding load, but also support the truss top chords against lateral buckling and may act as part of a rafter bracing system; a crane gantry surge girder resists horizontal loading from the cranes and also provides lateral stability to the crane girder; and a purlin sag bar prevents both down-slope deflection and lateral-torsional failure of the purlin. The designer should be aware of the multi-functional role of structural members and should make optimum use of their ability to contribute to the strength of the structure.
4.6 Summary •
The most economical structural framing system should be selected. This may require several trial designs.
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Suitable plan dimensions as far as main member spans and column centres are concerned, should be chosen to ensure economy. Within limits, larger dimensions are preferable to smaller ones. Repetition of equal spans, bay widths and storey heights leads to economy.
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Stabilising systems, i.e. vertical bracing, should be selected for maximum efficiency, while allowing for the required clearances between columns. These may be two-way braced, one-way braced one-way rigid, or two-way rigid, in ascending order of cost.
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Leaned frames will result in savings as bracing or stiff connections can be omitted in certain bays.
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Whenever possible, members should be designed to perform a dual function in order to increase efficiency.
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