SECTION 7 CONSTRUCTION

The steel hollow section column as Architectural proposed in Schematic design is under suggestion to change to open web section to allow the flexible installation of anchor bolts of steel base column. The simple design approach was used as a design criterion. All load combinations were entered into the model, and the combined load effects were compared to the reduced nominal strengths of the members.

31.1.1 Why set tolerances?

Compared to other structural materials, steel (and aluminium) structures can bemade economically to much closer tolerances Compared to mechanical parts,however, it is neither economic nor necessary to achieve extreme accuracy.There are a number of distinct reasons why tolerances may need to be con-sidered It is important to be quite clear which actually apply in any given case, par-ticularly when deciding the values to be specified, or when deciding the actions to

be taken in cases of non-compliance

The various reasons for specifying tolerances are outlined in Table 31.1 In allcases no closer tolerances than are actually needed should normally be specified,because while additional accuracy may be achievable, it generally increases the costsdisproportionately

Table 31.1 Reasons for specifying tolerances

Structural safety Dimensions (particularly of cross-sections, straightness, etc.) associated

with structural resistance and safety of the structure.

Assembly requirements Tolerances necessary to enable fabricated parts to be put together Fit-up Requirements for fixing non-structural components, such as cladding

panels, to the structure.

Interference Tolerances to ensure that the structure does not foul with walls, door or

window openings or service runs, etc.

Clearances Clearances necessary between structures and moving parts, such as

overhead travelling cranes, elevators, etc or for rail tracks, and also between the structure and fixed or moving plant items.

Site boundaries Boundaries of sites to be respected for legal reasons Besides plan

position, this can include limits on the inclination of outer faces of tall buildings.

Serviceability Floors must be sufficiently flat and even, and crane gantry tracks etc.

must be accurately aligned, to enable the structure to fulfil its function Appearance The appearance of a building may impose limits on verticality,

straightness, flatness and alignment, though generally the tolerance limits required for other reasons will already be sufficient.

Table 31.2 Definitions – deviations and tolerances

Deviation The difference between a specified value and the actual measured value,

expressed vectorially (i.e as a positive or negative value).

Permitted deviation The vectorial limit specified for a particular deviation.

Tolerance range The sum of the absolute values of the permitted deviations each side of a

specified value.

Tolerance limits The permitted deviations each side of a specified value, e.g ±3.5 mm or

+5 mm -0 mm.

Table 31.3 Classes of tolerances

Normal tolerances Those which are generally necessary for all buildings They include those

normally required for structural safety, together with normal structural assembly tolerances.

Particular tolerances Tolerances which are closer than normal tolerances, but which apply only to

certain components or only to certain dimensions They may be necessary

in specific cases for reasons of fit-up or interference or in order to respect clearances or boundaries.

Special tolerances Tolerances which are closer than normal tolerances, and which apply to a

complete structure or project They may be necessary in specific cases for

reasons of serviceability or appearance, or possibly for special structural reasons (such as dynamic or cyclic loading or critical design criteria), or for special assembly requirements (such as interchangeability or speed of assembly).

31.1.4 Types of tolerances

For structural steel there are three types of dimensional tolerance:

(1) Manufacturing tolerances, such as plate thickness and dimensions of sections (2) Fabrication tolerances, applicable in the workshops.

(3) Erection tolerances, relevant to work on site.

Manufacturing tolerances are specified in standards such as BS 4, BS 4848, BS EN

10024, BS EN 10029, BS EN 10034 and BS EN 10210 Only fabrication and tion tolerances will be covered here

erec-31.2 Standards

31.2.1 Relevant documents

The standards covering tolerances applicable to building steelwork are:

(1) BS 5950 Structural use of steelwork in building.

Part 2: Specification for materials fabrication and erection: hot rolled sections Part 7: Specification for materials and workmanship: cold formed sections and sheeting.

(2) National structural steelwork specification for building construction NSSS, 4th

edition

(3) ENV 1090-1 Execution of steel structures: Part 1: General rules and rules for buildings.

(4) ISO 10721-2: 1999 Steel structures: Part 2: Fabrication and erection.

(5) BS 5606 Guide to accuracy in building.

31.2.2 BS 5950 Structural use of steelwork in building

The specification of tolerances for building steelwork was first introduced intoBritish Standards in BS 5950: Part 2: 1985 The current edition was issued in 2001.This revision of the 1992 edition updates cross-references to other standards, many

of which are now European Standards (BS EN standards) In addition the tunity was taken to align the code more closely with the industry standard docu-

oppor-ment, the National structural steelwork specification for building construction.

Standards 919

31.2.3 National structural steelwork specification (NSSS)

The limitations of the tolerances specified in earlier versions of BS 5950: Part 2 have

been extended by an extensive coverage of tolerances in the National structural steelwork specification for building construction This is an industry standard based

on established sound practice The widely accepted document, promoted by theBritish Constructional Steelwork Association (BCSA), is now in its 4th edition

31.2.4 ENV 1090-1 Execution of steel structures

As part of the harmonization of construction standards in Europe, CEN has issued

ENV 1090: Part 1: General rules and rules for buildings, which is available through

BSI as DD ENV 1090-1: 1998

This document includes comprehensive recommendations for both erection andmanufacturing tolerances To a large extent these recommendations are consistentwith BS 5950: Part 2 and the NSSS However, some of them are more detailed

31.2.5 ISO 1071-2 Steel structures: Part 2: Fabrication and erection

This is very similar to ENV 1090-1 and BS 5950: Part 2 It is unlikely to be issued

as a BSI standard

31.2.6 BS 5606 Guide to accuracy in building

BS 5606 is concerned with buildings generally and is not specific to steelwork The

1990 version has been rewritten as a guide, following difficulties due to incorrectapplication of the previous (1978) version, which was in the form of a code

BS 5606 is not intended as a document to be simply called up in a contract specification It is primarily addressed to designers to explain the need for them toinclude means for adjustment, rather than to call for unattainable accuracy of con-struction Provided that this advice is heeded, its tables of ‘normal’ accuracy canthen be included in specifications, except where they conflict with overriding struc-tural requirements This can in fact happen, so it is important to remember that therequirements of BS 5950 must take precedence over BS 5606

BS 5606 introduces the idea of characteristic accuracy, the concept that any

con-struction process will inevitably lead to deviations from the target dimensions, andits objective is to advise designers on how to avoid resulting problems on site byappropriate detailing The emphasis in BS 5606 is on the practical tolerances whichwill normally be achieved by good workmanship and proper site supervision Thiscan only be improved upon by adopting intrinsically more accurate techniques,

which are likely to incur greater costs These affect the fit-up, the boundary sions, the finishes and the interference problems Data are given on the normal tol-erances (to be expected and catered for in detailed design) under two headings:(1) Site construction (table 1 of BS 5606).

dimen-(2) Manufacture (table 2 of BS 5606)

Unfortunately many of the values for site construction of steelwork are only mated No specific consideration is given in BS 5606 to dimensional tolerances nec-essary to comply with the assumptions inherent in structural design procedures,which may in fact be more stringent It does however recognize that special accu-racy may be necessary for particular details, joints and interfaces

esti-Another important point mentioned in BS 5606 is the need to specify methods

of monitoring compliance, including methods of measurement It has to be nized that methods of measurement are also subject to deviations; for the methodsnecessary for monitoring site dimensions, these measurement deviations may in fact

recog-be quite significant compared to the permitted deviations of the structure itself

For example, a 356 ¥ 406 ¥ 235 UC has a nominal size of 381 mm deep by 395 mmwide, but with tolerances to BS 4 may actually measure 401 mm wide by 387 mmdeep one side, and have a depth of 381 mm the other side The same is true of con-tinental sections A 400 ¥ 400 ¥ 237 HD also has a nominal size of 381 mm deep by

395 mm wide, but with tolerances to Euronorm 34 may actually measure 398 mmwide by 389 mm deep one side, and have a depth of 380 mm the other side

31.3.1.2 Fabrication

Variations of cross-sectional dimensions (with permitted deviations) may also need

to be allowed for, either in detailing the workmanship drawings or in the tion process itself, if problems are to be avoided during erection on site

fabrica-Implications of tolerances 921

The most obvious case is a splice between two components of the same nominalsize, where packs may be needed before the flange splice plates fit properly, unlessthe components are carefully matched Similarly variations in the depths of adja-cent crane girders or runway beams may necessitate the provision of packs, unlessthe members are carefully matched.

Less obviously, if the sizes of columns vary, the lengths of beams connectedbetween them will need some form of adjustment, even if the columns are accu-rately located and the beams are exactly to length

31.3.2 Attachment of non-structural components

It is good practice to ensure that all other items attached to the steel frame haveadequate provision for adjustment in their fixings to cater for the effects of all steel-work tolerances, plus an allowance for deviations in their own dimensions Wherenecessary, further allowances may be needed to cater for structural movementsunder load and for differential expansion due to temperature changes

Where possible, the number of fixing points should be limited to three or four,only one of which should be positive with all the others having slotted holes or othermeans of adjustment

31.3.3 Building envelope

It must be appreciated that erection tolerances, including variation in the position

of the site grid lines, will affect the exact location of the external building enveloperelative to other buildings or to site boundaries, and there may be legal constraints

to be respected which will have to be taken into account at the planning and liminary stages of design

pre-These effects also need to be taken into account where a building is intended tohave provision for future extension or where the project is an extension of an exist-ing building, in which case deviations in the actual dimensions have to be cateredfor at the interface

In the case of tall multi-storey buildings, the building envelope deviates ingly with height compared to the location at ground level, even though permitteddeviations for column lean generally reduce with height Unless there are step-backs

increas-or other features with a similar effect, it may be necessary to impose particular erance limits on the outward deviations of the columns

tol-31.3.4 Lift shafts for elevators

The deviations from verticality that can be tolerated in the construction of guidesfor ‘lifts’ or elevators are commonly more stringent than those for the construction

of the building in which they operate In low-rise buildings sufficient adjustment can

be provided in association with the clearances, but in tall buildings it becomes essary either to impose ‘special’ tolerances on column verticality or else to impose

nec-‘particular’ tolerances on those columns bounding the lift shaft

In agreeing the limits to be observed with the lift supplier, it should not be looked that the horizontal deflections of the building due to wind load also haveimplications for the verticality of the lift shafts

over-31.4 Fabrication tolerances

31.4.1 Scope of fabrication tolerances

The description ‘fabrication tolerances’ is used here to include tolerances for allnormal workshop operations except welding It thus covers tolerances for:

(1) cross sections, other than rolled sections,

(2) member length, straightness and squareness,

(3) webs, stiffened plates and stiffeners,

(4) holes, edges and notches,

(5) bolted joints and splices,

(6) column baseplates and cap plates

However, tolerances for cross sections of rolled sections and for thicknesses ofplates and flats are treated as manufacturing tolerances Welding tolerances (includ-ing tolerances on weld preparations and fit-up and sizes of permitted weld defects)are treated elsewhere

31.4.2 Relation to erection tolerances

An overriding requirement for accuracy of fabrication must always be to ensurethat it is possible to erect the steelwork within the specified erection tolerances.Due to the wide variety of steel structures and the even wider variety of theircomponents, any recommended tolerances must always be specified in a verygeneral way Even if it were possible to specify fabrication tolerances in such a waythat their cumulative effect would always permit the specified erection tolerances

to be satisfied, the resulting permitted deviations would be so small as to be sonably expensive, if not impossible, to achieve

unrea-Fabrication tolerances 923

Fortunately in most cases it is possible to rely on the inherent improbability ofall unfavourable extreme deviations occurring together Also the usually acceptedvalues for fabrication tolerances do make some limited allowances for the need toavoid cumulative effects developing on site They are tolerances that have beenshown by experience to be workable, provided that simple means of adjustment areincorporated where the effects of a number of deviations could otherwise becomecumulative For example, beams with bolted end cleats usually have sufficientadjustment available due to hole clearances, but where a line of beams all have endplate connections, provision for packing at intervals may be advisable, unless othermeasures are taken to ensure that the beams are not all systematically over-length

or under-length by the normal permitted deviation Other possible means for ment include threaded rods and slotted holes

adjust-Where it can be seen from the drawings that the fabrication tolerances couldeasily accumulate in such a way as to create a serious problem in erection, eithercloser tolerances or means of adjustment should be considered; however, the coin-cident occurrence of all extreme deviations is highly improbable, and judgementshould be exercised both on the need for providing means of adjustment and on therange of adjustment to be incorporated

31.4.3 Full contact bearing

in the member: thus full contact is needed to transmit this stress from the memberinto the plate Only that part of the plate in contact with the member need satisfythe full contact bearing criteria, though it may be easier to prepare the whole plate.Figure 31.1(b) shows two end plates in simple bearing The potential contact area

is substantially larger than the cross-sectional area of the member: thus full contactbearing is not necessary All that is needed is for the end plates to be square to theaxis of the member Another common case of simple bearing is shown in Fig 31.1(c)

By contrast, the case shown in Fig 31.1(d) is one where, if full contact bearing isneeded, it is also necessary to take special measures to ensure that the profiles ofthe two members align accurately, otherwise the area in contact may be significantlyless than the area required to transmit the load Particular tolerances should be specified in such cases, based on the maximum local reduction of area that can be

accepted according to the design calculations Alternatively a division plate could

be introduced; if the stresses are high this may well prove to be the most practicalsolution

Fig 31.1 Types of member-to-member bearing: (a) profile to plate, (b) plate to plate,

(c) flange to flange, (d) profile to profile (accurate alignment necessary)

31.4.3.3 Squareness

If the ends of a length of column are not square to its axis, then after erection eitherthe column will not be vertical or else there may be tapered gaps at the joints,depending on the extent to which surrounding parts of the structure prevent thecolumn from tilting Under load any such gap will try to close, exerting extra forces

on the surrounding members In addition, both a gap or a tilt will induce a localeccentricity in the column

A practical erection criterion is that the column should not lean more than 1 in

x (where x is 600 in NSSS and 500 in ENV 1090-1) This slope is measured relative

to a line joining the centres of each end of the column length, referred to as the

overall centreline The column is also allowed a lack of straightness tolerance of

(length/1000), which corresponds to end slopes of about 1/300 (see Fig 31.2(a)) It

is thus necessary to specify end squareness criteria relative to the overall centreline,rather than to the local centreline adjacent to the end (see Fig 31.2(b))

There is generally a design assumption that the line of action of the force in thecolumn does not change direction at a braced joint by more than 1/250, requiring

an end squareness in a simple bearing connection (relative to the overall axis of themember) of 1/500 (see Fig 31.2(c)) However, full contact bearing generally arises

at column splices which are not at braced points, so an end squareness tolerance of1/1000 is usually specified, producing a maximum change of slope of 1/500 (see Fig.31.2(d))

Once a column has been erected, it is more practical to measure the remaininggaps in a joint These gaps are affected not only by the squareness of the ends butalso by the second criterion, flatness

31.4.3.4 Flatness

Ends have to be reasonably flat (as distinct from curved or grossly uneven) to enablethe load to be transferred properly Following a history of arguments over appro-priate specifications, the American Institute of Steel Construction (AISC) commis-sioned some tests, which are the basis for their current specifications

It was found that a surprisingly high tolerance was quite acceptable, and thatbeyond its limit (or to compensate for end squareness deviations) the use of localized packs or shims was acceptable Basically similar rules are now beginning

to appear in other specifications including the CEN standard (see section 31.5.6 inrelation to erection tolerances) This is an essentially simple and effective method

of correcting excessive gaps on site (see also section 31.5.6) However, insertingshims into column joints is not a matter to be undertaken lightly It is normally moreeconomic to avoid the need for shimming by working to close fabrication tolerances

in joints where full contact bearing is required

Fabrication tolerances 927

Fig 31.2 Squareness of column ends (a) Bow of 1/1000 giving end slopes of about 1/300.

(b) Squareness of end measured relative to overall centreline (c) Change of tion at a braced joint (d) End squareness at full contact bearing splice

Where sawing is not possible, ending machines (i.e special end-milling machines)can be used for correcting the squareness (or flatness) of ends of built-up (fabri-cated) columns, such as box columns or other welded-up constructions Where base-plates are not flat and are too thick to be pressed flat, either they are milled locally

in the contact zone or else planing machines are used

However, it cannot be overemphasized that the normal preparation for a rolledsection column required to transmit compression by full contact in bearing is by sawcutting square to the axis of the member

It is, of course, unnecessary to flatten the undersides of baseplates supported onconcrete foundations

31.4.4 Other compression joints

Compression joints, transferring compression through end plates in simple bearing,also need to have their ends square to the axis If, after the members have beenfirmly drawn together, a gap remains which would introduce eccentricity into thejoint, it should be skimmed

31.4.5 Lap joints

Steel packs should be used where necessary to limit the maximum step betweenadjacent surfaces in a lap joint (see Fig 31.3) to 2 mm with ordinary bolts or 1 mm(before tightening the bolts) where preloaded HSFG bolts are used

Fig 31.3 Maximum step between adjacent surfaces

31.4.6 Beam end plates

Where the length of a beam with end plates is too short to fit between the porting columns, or other supporting members, packs should be supplied to make

sup-up the difference

Gaps arising from distortion caused by welding, as shown in Fig 31.4, need not

be packed if the members can be firmly drawn together However, they may need

to be filled or sealed to avoid corrosion where the steelwork is external or is exposed

to an aggressive internal environment

31.4.7 Values for fabrication tolerances

The values for fabrication tolerances currently given in the NSSS are reproducedfor convenience in Table 31.4 Each of the specified criteria should be consideredand satisfied separately The cumulative effect of several permitted deviationsshould not be considered as overriding the specific criteria

These values represent current practice and are taken from the fourth edition ofthe NSSS

The clause numbers referred to in Table 31.4 are clause numbers in the NSSS,which should be referred to for further information

31.5 Erection tolerances

31.5.1 Importance of erection tolerances

Erection tolerances potentially have a significant effect on structural behaviour.There are four matters to be considered:

Fig 31.4 End plate with welding (exaggerated)

Erection tolerances 931

Table 31.4 (Extract from National Structural Steelwork Specification 4th edn.)

©BCSA

Table 31.4 (contd )

©BCSA

Erection tolerances 933

Table 31.4 (contd )

©BCSA

Table 31.4 (contd )

©BCSA

Erection tolerances 935

Table 31.4 (contd )

©BCSA

Table 31.4 (contd )

©BCSA

national system, it is usual to set subsidiary site datum points, and often a site datumlevel, and then refer the accuracy of the structure to these.

For any site the use of a grid of established column lines together with an established site level is strongly recommended For a large site it is virtually indis-pensable To help appreciate this, consider what happens on the site of a steel structure

31.5.2.2 Site practice

Normal site practice is for the supporting concrete foundations, and other ing structures, to be prepared in advance of steel erection, generally by an organi-zation separate from the steel erector Depending on the system of holding-downbolts or other fixings to be used, this may involve casting-in of holding-down bolts,preparation of pockets in the concrete, and preparation of surfaces to receive fixings

support-to the steelwork

Even with care, the standard of accuracy achievable is limited, and the concreterequires time to harden to a sufficient strength for steel erection to proceed Onceall the foundations etc are available for steel erection (or at least a sufficient proportion of them on a large site), it is prudent to survey them to review their accuracy

31.5.2.3 Established column lines and established site level

From this survey it is convenient to introduce a grid of established column lines(ECL) and an established site level (ESL) of the foundations and other supportingstructures in such a way that the positions and levels of steel columns etc can readily

be related to the site grid and site level

The established column lines are defined as that grid of site grid lines that bestrepresents the actual mean positions of the installed foundations and fixings Simi-larly the established site level is defined as that level which best represents the actualmean level of the installed foundations Of course it should also be verified that thedeviation of the ECL grid and the ESL from those specified are within the relevantpermitted deviations

31.5.3 Erection – fixing bolts

31.5.3.1 Types of fixing bolts

Fixing bolts include both holding-down bolts for columns and various types of fixingbolts used to locate or to support other members, such as beams or brackets carried

by walls or concrete members

Erection tolerances 937

Holding-down bolts and other fixing bolts are either:

(1) fixed in position, or

(2) adjustable, in sleeves or pockets

31.5.3.2 Fixed bolts

Fixed bolts used to be solidly cast in, an operation requiring care and the use of jigs

or templates to achieve accurately However, they are now also commonly produced

by placing resin-grouted bolts in holes drilled in the concrete after casting It mayalso be possible to use expanding bolts

In whatever way fixed bolts are achieved, they need to be positioned accurately,

as the only adjustment possible is in the steelwork, so relatively close tolerances arenormally specified

31.5.3.3 Adjustable bolts

Adjustable bolts are placed in tubes or in tapered trapezoidal or conical holes cast

in the concrete, so that a degree of movement of the threaded end of the bolt is sible, while the other end is held in place by a steel washer or other anchoring deviceembedded in the concrete

pos-This alternative permits the use of more easily achieved tolerances for the bolts,while using relatively simple details for the steelwork Adjustment of the bolt neces-sitates its axis deviating from the vertical to some extent, and the holes in the steel-work need to be large enough to allow for this, particularly if the baseplate is thick.The use of loose plate washers is recommended to span oversize holes if necessary

If required they can be welded in place after the bolts are tightened, but this shouldnot normally be necessary ‘Particular’ tolerances need to be worked out for eachcase, depending on the details, including the length of the bolts, because this affectstheir slope

31.5.3.4 Length of bolts

The level of the top of an HD bolt is also important to ensure that the nuts can befitted properly after erection To provide the necessary tolerances for the fixing ofthe bolts they should be longer than theoretically required, long threaded lengthsshould be provided, and the nominal level for the top should be above the theoreti-cal position

Similar considerations apply to the lengths of fixing bolts located horizontally

31.5.4 Erection – internal accuracy

In terms of structural performance, the main erection tolerance is verticality ofcolumns; positions of beams etc on brackets may also be important Levels ofbeams, particularly of one end relative to the other end and of one beam relative

to the next one, are important in terms of serviceability

Otherwise the internal accuracy of one part of the structure relative to another

is largely a matter of assembly tolerances, provided that these do not cause anyproblem of fit-up, interference or clearances Where the structural accuracy result-ing from the assembly tolerances is liable to infringe any of these limits, ‘particular’tolerances should be specified

The necessary tolerances are specified in relation to readily identifiable pointsand levels For columns and other vertical members, the reference points are con-veniently defined as the actual centre of the member at each end of the fabricatedpiece For beams and other horizontal members the reference points are more con-veniently defined by the actual centre of the top surface at each end Either thecolumn system or the beam system should be used for any other cases, and the rel-evant system should be indicated on the erection drawings The tolerances are thendefined by the permitted deviations of these reference points from the establishedcolumn lines ECL and established floor level EFL

The concept of an ECL grid and an established site level ESL have already beenexplained in section 31.5.2.3 The established floor level EFL is defined as that levelwhich best represents the actual mean level of the as-built floor levels The EFLmust not deviate from the specified floor level (relative to the ESL) by more thanthe permitted deviation for height of columns

The reference points for each beam must then be within the permitted deviationfrom the EFL In addition the difference in level of each end of a beam and the difference in level between adjacent beams must also be within their respectivelimits

In the case of columns, the permitted deviations at each level form an ‘envelope’within which the column must lie at all levels In addition, the permitted inclination

of each column within a storey height is limited, but except where columns are ricated as individual storey-height pieces, the overall envelope normally governs

fab-31.5.5 Erection – external envelope

Generally the same erection tolerances for verticality apply to external columns as

to internal columns When the envelope of extreme permitted deviations is plottedfrom the extreme position of the base (allowing for the permitted deviation of theECL from the theoretical position as well as the permitted deviation of the columnbase from the ECL), it may be found that this is unacceptable in terms of site bound-aries or building lines, especially for a tall multi-storey building If so, ‘particular’tolerances should be specified

Erection tolerances 939

Fit-up problems with cladding could also occur if alternate adjacent columns atthe periphery were allowed too large a deviation alternately in and out from thetheoretical line of the building face Even if the fit-up problems could be overcome,the visual appearance might be affected Again, ‘particular’ tolerances should bespecified if necessary.

31.5.6 Shimming full contact bearing splices

As mentioned in section 31.4.3.4 in relation to fabrication tolerances, tests missioned by AISC, and used as the basis for several modern standards, showed thatshims can be used to reduce gaps in full contact bearings to within the specified tol-erances Shimmed gaps up to 6.35 mm were tested, so it is not prudent to permitshimming for gaps exceeding 6 mm; gaps larger than this should be corrected byother means

com-Gaps which would otherwise remain over the specified tolerance when themembers are in their final alignment should be shimmed As the tests were on flatshims, it is acceptable to use flat shims in practice In the tests the shims were ofmild steel, and this is permitted in the AISC specification and ENV 1090-1.The shims should be inserted such that no remaining gap exceeds the specifiedpermitted deviation Short lengths of shim are appropriate in a variety of thicknesses

Fig 31.5 Shims for full contact bearing (a) Shims with partial penetration butt weld.

(b) Finger shim

Erection tolerances 941

Table 31.5 (Extract from National Structural Steelwork Specification 4th edn.)

©BCSA

Table 31.5 (contd )

©BCSA

Erection tolerances 943

Table 31.5 (contd )

©BCSA

Table 31.5 (contd )

©BCSA

Erection tolerances 945

Table 31.5 (contd )

©BCSA

in steps not exceeding the permitted deviation No more than three layers of shimsshould be used at any point, and preferably only one or two The shims (and thelengths of columns) may be held in place by means of a partial penetration buttweld extending over the shims (see Fig 31.5(a)).

In bolted compression splices, bolted ‘finger’ shims (shaped as indicated in Fig 31.5(b)) can be used

In some cases shims can be driven in, but if so they need to be fairly robust(usually over 2 mm thick), so shims of various thicknesses are needed throughoutthe joint Driven shims are best limited to vertical joints e.g between a beam endplate and a column More commonly the joint must be jacked or wedged open (orelse the upper portion lifted by a crane) so that the shims can be inserted Taperedshims are particularly difficult to insert; as they are not necessary they are bestavoided

31.5.7 Values for erection tolerances

The values for erection tolerances are given in Table 31.5 Each of the specified teria should be considered and satisfied separately The permitted deviations shouldnot be considered as cumulative, except to the extent that they are specified rela-tive to points or lines that also have permitted deviation These values representcurrent practice and are taken from the fourth edition of the NSSS

cri-Table 31.5 (contd )

©BCSA

Further reading 947

The clause numbers referred to in Table 31.5 are clause numbers in the NSSS,which should be referred to for further information

Further reading for Chapter 31

British Standards Institution (1993) Specification for hot rolled sections BS 4: Part

Extracts from the National structural steelwork specification 4th Edition are

re-produced with the kind permission of the British Constructional Steelwork Association

by DAVID DIBB-FULLER

948

32.1 Introduction

The steel-framed building derives most of its competitive advantage from the virtues

of prefabricated components which can be assembled speedily on site Additionaleconomies can be significant provided the designer seeks through the design to minimize the value added costs of fabrication This is proper ‘value engineering’

of the product and is applied to perhaps the most influential sector of the to-site cost of structural steelwork

delivered-This chapter explains the processes of fabrication and links them with design decisions It is increasingly important for the designer to understand the skills andtechniques available from different fabricators so that the design can be tailored tokeep overall costs down The choice of fabricator can then be made on the basis ofability to conform in production engineering terms to design assumptions mademuch earlier It is unacceptable to allow the fabricator or designer to undertake theproduction engineering element of design in perfect isolation; the dialogue betweendesigners and fabricators must be a continuous one The effects that fabrication and assembly have on design assumptions and vice versa and, in particular, theachievable fit-up of components and permissible limits of tolerance must beaddressed

Design must be viewed as a complete process, covering strength and stiffness aswell as production engineering, to achieve the most economical structures

32.2 Economy of fabrication

Structural form has a significant effect on the delivered-to-site cost of steelwork.This is due to a number of factors additional to the cost of raw steel from steel sup-pliers Some forms will prove to be more costly from some fabricators than others;they tend to attract work by aligning their production facilities to specific marketsectors For example, the industrial building market was largely taken over by theintroduction of the portal-framed structure The fabricators in this sector adoptedhigh-volume, low-cost production and concentrated primarily on this area of themarket By the use of pre-engineered standards they were able to maximize repe-tition and minimize input from both design and drawing activities: a classic example

Economy of fabrication 949

of a combination of design and production engineering Other steelwork tors specialize in tubular structures, lightweight sections and heavy sections

contrac-32.2.1 Fabrication as a cost consideration

Figures 32.1 to 32.4 give an indication of the proportional costs associated with thefabrication and erection of structural steelwork Actual costs in monetary termshave not been included as they will change with the demand level of the market.The proportions of cost will vary a little from fabricator to fabricator; those shownrepresent a reasonable average

The cost headings are:

steel-of stockholders’ steel or the extremes steel-of section variation

Fabrication cost covers a number of different sized jobs and incorporates ing of the raw steel by shot blasting, preparation of small parts (cleats and platesand connection components), and assembly of the components into complete struc-tural members ready for shop-applied paint treatments It also includes anallowance for consumables

clean-Paint cost covers the shop application of 75 mm of primer by spray immediatelyafter fabrication No allowance has been made for blast cleaning of areas affected

by welding A coverage allowance of 28 m2/tonne at the rate of 3 m2/litre has beenmade, which represents the likely consumption for rolled section beams andcolumns This allowance has been adjusted for various specific work types asdescribed in the accompanying text

Transport cost is based on 20 tonne loads per trailer which delivers finished ucts to sites within a 50 mile radius of the fabrication shop Transport costs will risefor loads of less than 20 tonnes or when oversize components need special policeescort or permission

prod-The erection cost is average for the work type and includes preliminaries for high-rise multi-storey work

The cost of site painting has been included but it only covers the average cost fortouching-up damage to the primer coat Other site-applied protection systems varyenormously in cost and so have not been considered

Portal-framed industrial buildings

The breakdown shown in Fig 32.1 follows the assumptions given above There is

no adjustment in either the shop painting or the transport cost as this type of work fits the basic parameters well It can be seen that design economies come principally from the weight of the structure combined with efficient fabricationprocesses

Fig 32.1 Cost breakdown: portal-framed industrial buildings

Fig 32.2 Cost breakdown: simple beam and column structures

Simple beam and column structures

The breakdown shown in Fig 32.2 incorporates the following limitations:

(1) the maximum height of the building is three storeys,

(2) erection is carried out using mobile cranes on the ground floor slab

It can be seen that again tonnage and fabrication efficiencies are the dominant teria; 83% of the costs arise from these elements, with a slightly greater emphasis

cri-on tcri-onnage than was the case with portal frames

High-rise multi-storey

The breakdown shown in Fig 32.3 incorporates the following adjustments:

(1) the steel grade has been taken as S355 with an allowance for cambering,(2) the paint system generally is shot blast and 75 mm primer, 10% of steel coatedwith 100 mm primer,

(3) no allowance has been made for any concrete-encased beams or stanchions,(4) transport includes for off-site stockpiling, bundling and out-of-hours delivery tosite (city centre sites often incur these costs)

This sector of the market has a very different cost profile to those already shown.Raw steel still dominates but erection charges have now overtaken the fabricationelement This type of steelwork lends itself particularly well to automated fabrica-tion techniques featuring drilling lines

(1) angle booms and lacings,

(2) welded joints without gusset plates,

(3) transportable lengths and widths with full-depth splices only

Here fabrication is virtually the same cost as the raw steel The designer should work very closely with the steelwork contractor to ensure simplicity of assembly for this form of structure, in particular checking joint capacities at the design stage.

32.2.2 Design for production

The detailed design of any steelwork construction will have a substantial impact

on its cost of fabrication It is therefore very important that the designer has a basic understanding of the implications of his design on construction in practice.Key points need to be addressed if the design is to be fabricated economically and efficiently This important topic is the subject of Reference 1, from which thefollowing key points are extracted

Fabrication processes

• Modern computer numerically controlled (CNC) fabrication equipment is moreeffective with:

(a) Single end cuts, arranged square to the member length

(b) One hole diameter on any one piece, avoids drill bit changes

(c) Alignment of holes on an axis square to the member length, holes in websand flanges aligned not staggered to reduce piece moves between drill times(d) Web holes having adequate side clearance to the flanges

• To allow efficient production of fittings:

(a) Rationalize on the range of fittings sizes – use a limited range of flats and angles(b) Allow punching and cropping wherever possible

Fig 32.4 Cost breakdown: lattice structures

Economy of fabrication 953

• If possible select connections which avoid mixing welding and drilling in any onepiece This avoids double handling of the member during fabrication

Materials grade and section selection

• The designer should rationalize the range of sections and grades used in any onestructure This will lead to benefits in purchasing and handling during all fabrication, transportation and erection phases of manufacture

• Make maximum use of S355 material for main sections This is typically 8% more expensive but up to 30% stronger than S275 steel The exception is wheredeflection governs section selection

• The specification of small quantities of S355 or other ‘special’ grade materialshould be avoided, particularly if the proposed material has poorer welding qualities

• Choice of fittings material grade should be left with the fabricator wherever sible

pos-• Structural hollow sections are approximately 60–80% more expensive thanequivalent weight open sections and have additional problems associated with the connection requirements Limitations on mill lengths should also beremembered

Connection design considerations

• Connections directly influence 40–60% of the total frame cost They must fore be taken into account during the frame design

there-• Least-weight design solutions are rarely the cheapest Increasing member ness to eliminate stiffening at connections will often be an economic solution

thick-• The cost benefits from an integrated approach to frame and connection designwill only be realized if the fabricator is given a full package of information attender stage Connection styles and design philosophy must be clearly marked

on drawings

Bolts and bolting

• Non-preloaded bolting is the preferred method for site connections Preloaded(friction grip) bolts should only be used where joint slip is unacceptable or wherethere is a danger of fatigue

• The use of different grade bolts of the same diameter on any one contract should

be avoided

• Threads should be permitted in the shear plane and in bearing

• Direct and indirect cost savings can accrue by using only a small range of dard’ bolts

‘stan-Recommended standards are:

M20 grade 8.8 for shear connections

M24 grade 8.8 for moment connections

Mechanical properties to BS 3692, dimensions to BS 4190

Fully threaded for shanks up to 70 mm long

• The use of fully threaded bolts generally means additional thread protrusion isvisible; specifiers should be aware of this and state at tender stage where this is

not acceptable.

• Washers are not required for strength when using non-preloaded bolts in normal

clearance holes; they may still be specified to provide a degree of protection tosurface finishes

• When used with corrosion-protected steelwork, bolts, nuts and washers should

be supplied with a coating which does not require further protection applications

Welding and inspection

• The welding content of a fabrication has a significant influence on the total cost

of fabrication

• In designing welded connections consideration should be given to the weldability

of materials, access for welding and inspection, and the effects of distortion Access

is of primary importance – good welds cannot be formed without adequate access

• Fillet welds up to 12 mm leg length are preferred to the equivalent-strength buttweld Generally two fillet welds whose combined throat thicknesses equal thethickness of the plate to be connected are considered as equivalent in strength

to a full penetration butt weld

• Weld defect inspection and defect acceptance criteria should be defined; the use of the National Structural Steelwork Specification criteria is strongly recommended

Corrosion protection

• In selecting a corrosion protection system the designer must consider the environment in which the steelwork will be placed and the design life of the corrosion protection system

If the environment does not require a corrosion protection system don’t specify one.

• If a protection system is required, significant advantages are gained by use of asingle-coat protection system applied during fabrication These should be speci-fied where possible

• Wherever possible avoid using ‘named’ product specifications; allow the tor to use his preferred supplier or even alternative preferred coating system ofequal capability

fabrica-Specification of surface conditions should relate to the condition immediatelyprior to painting, not bound by any time-limit from shot blasting operations

Example coating specifications for a range of environments are given, togetherwith an indication of relative costs.

Trusses and lattice girders

• Lattice girders and trusses are effective for medium to long spans where tion is a major criterion and are able to accommodate services within their depth,but always consider the use of a plain rolled section beam first

deflec-• Most lattice frames are joint critical Never select a section for the chords or nals without first checking whether it can be effectively joined – preferablywithout recourse to stiffening

inter-• Always check the limits on transport before starting the design

• Be aware that SHS are only available in limited standard lengths, normally fromstockists Long lengths may therefore need additional butt welding

• For internal members try to detail single bevel end cuts; for angles square-cutends are better to allow use of an automatic cropping process

• In tubular construction use of RHS chords leads to simpler end preparation forinternals than that required if CHS chords are used

• Think about access provisions for welding of internals to chords

• Access for painting is difficult for double angle or double channel members; use

of SHS reduces paint area and provides fewer locations for corrosion traps to beformed

At one time it was common practice to assemble components in the workshopusing bolts or rivets With the increased implementation of welding, this practice hasdeclined due to the costs associated with bolted fabrications Instead of a simple run

of fillet weld, holes need to be drilled and bolts introduced, increasing total labourhours and cost In many respects the ease with which welding can be undertakenhas diverted designers’ attention from the use of shop bolting Today, steelwork con-tractors are looking at increased automation to keep costs down, and machines havebeen developed which considerably speed up hole drilling

32.4.1 Shop bolting

There is still a demand for structural members to be bolted arising from a ment to avoid welding because of the service conditions of the member under consideration These may be low temperature criteria, the need to avoid weldingstresses or the requirement for the component to be taken apart during service (e.g.bolted-on crane rails) For lattice structures, the designer should specify the bolting,bearing in mind the effect of hole clearances around bolt shanks HSFG bolts willnot give problems but other bolts in clearance holes will allow a ‘shake-out’ whichcan cause significant additional displacement at joints Typically, a truss with boltedconnections may deflect due to the take-up of lack of fit in clearance holes to such

require-an extent that it loses its theoretical camber The use of HSFG assemblies avoidsthis risk

Large and complex assemblies which are to be bolted together on site may betrial assembled in the fabrication shop This increases fabrication costs but may payfor itself many times over by ensuring that the steel delivered to site will fit Restrict-ing trial assembly to highly repetitive items or items critical to the site programme

is to be recommended

32.4.2 Types of bolt

The choice of which type of bolt to use may not necessarily be made on the basis

of strength alone but may be influenced by the actual situation in which the bolt isused, e.g in non-slip connections

There are four basic types of bolts They are structural bolts, friction-grip boltsand close-tolerance bolts

Structural bolts

Bolts with low material strength and wide manufacturing tolerance were untilrecently known as ‘black bolts’ because of their appearance Now they are called