Steel Designer''''s Manual Part 14 potx

British Standards Institution 1972 Code of practice for safe use of cranes mobile cranes, tower cranes and derrick cranes.. British Standards Institution 1988 Code of practice for safety

Mobility for non-mobile cranes

Non-mobile cranes can be made mobile by mounting them on rails This has twoadvantages: the positioning of the crane can be more easily dictated and controlled,and the loads transmitted by the rails to the ground act in a precisely known loca-tion Many cranes have collapsed because of insufficient support underneath.However, most rail-mounted crane failures have occurred from overloading Wherethe crane is to work over complex plant foundations the rail can be carried on abeam supported, if necessary, on piles especially driven for the purpose If the rail

is supported only by a beam on sleepers in direct contact with the ground, the loadcan be properly distributed by suitable spreaders In either case conditions must

be properly considered and designed for Problems often occur when too much faith is invested in the capability of the ground to support a mobile crane and itsoutriggers

Non-mobile cranes

Non-mobile cranes are generally larger than their mobile counterparts They canreach a greater height, and are able to lift their rated loads at a greater radius.There are two main types of non-mobile crane: the tower crane and the (nowrare) derrick Due to their great size, the cranes must arrive on site in pieces Thusthe disadvantage of a non-mobile crane is that it has to be assembled on site Havingbeen assembled, the crane must receive structural, winch and stability tests beforebeing put into service

A tower crane with sufficient height and lifting capacity (see Fig 33.11) hasseveral advantages:

(1) It requires only two rails for it to be ‘mobile’ These two rails, although at a widegauge, take up less ground space than a derrick

(2) It carries most of its ballast at the top of the tower on the sluing jib/counterbalance structure, and so very much less ballast is needed at the bottom Indeed,

in some cases, there is no need for any ballast at the tower base or portal.(3) Because the jib of a tower crane is often horizontal, with the luffing of a derrickjib replaced by a travelling crab, the crane can work much closer to the structure and can reach over to positions inaccessible to a luffing jib crane.(4) A tower crane is ‘self-erecting’ in the sense that, after initial assembly at or near ground level, the telescoping tower eliminates the need for secondarycranes

(5) As shown in Fig 33.12 a tower crane can be tied into the structure it is ing, thus permitting its use at heights beyond its free-standing capacity

erect-There exist several types of tower crane, e.g articulated jib, luffing and saddlecranes as illustrated in Fig 33.13 It is essential that manufacturers or plant hirersare consulted in order to make the most appropriate choice of crane

Cranes for the stockyard

Stockyard cranes have to work hard The tonnage per job has to be handled twice

in the same period of time, often with many fewer cranes It is therefore importantthat cranes be selected and cited carefully to ensure maximum efficiency

33.8.3 Other solutions

If there is no suitable crane, or if there is no working place around or inside thebuilding where a crane may be placed, then consideration must be given to a specialmounting device for a standard crane, or even a special lifting device to do the work

of a crane, designed to be supported on the growing structure under construction

In either event, close collaboration between the designer and erector members of

Fig 33.11 Tower crane (courtesy of Delta Tower Cranes)

I_! I' 111111 II —

the management team is of paramount importance Conversely, inadequate munication may prove problematic Once the decision to consider the use of aspecial lifting device has been made, a new range of options becomes available.The most important of these is the possibility of sub-assembling larger and heaviercomponents, thereby reducing the number of labour hours worked at height.This is particularly true in the case of bridgework, where substantial sums would otherwise have to be expended on other temporary supports and stiffening

com-A major disadvantage of special lifting devices is that the apparatus being sidered is often so specialized that is unlikely to be of use on another job Thus

Fig 33.12 Citigroup Tower, London showing tower cranes tied into the building (courtesy

of Victor-Buyck Hollandia)

the whole cost is targeted at the one job for which it has been initially designed (seeFig 33.14).

Where the frame is single-storey, and at the cost of only a slight increase in timeand labour, it is possible to do without an on-site crane With the help of a winch(powered by either compressed air or an internal combustion engine, and someblocks and tackle), a light lattice-guyed pole can be used to give very economic erec-tion (see Fig 33.15) In this instance the pole is carried in a cradle of wires attached

at points on the tower These connection points need to be carefully designed toensure that they will carry the load without crippling the tower structure

It is vital that all poles are used in as near a vertical position as possible, sincecapacity drops off severely as the droop increases This requires careful planningand the employment of a gang of men experienced in the use of the method

In a different context, pairs of heavier poles provided with a cat head to supportthe top block of the tackle can be used inside existing buildings to erect the com-ponents of, for example, an overhead travelling crane, or to lift in a replacementgirder The arrangements for a pole and its appurtenances take up much less floorspace in a working bay than a mobile crane This is because a mobile crane needs

a wide access route and adequate space to manoeuvre itself into position – larly useful where headroom is restricted

Fig 33.13 Tower cranes used in the construction of Citigroup Tower, London (courtesy of

Victor-Buyck Hollandia)

33.8.4 Crane layout

It is important to decide on the type, size and number of cranes that are required

to carry out the work, since each has a designated range of positions relative to thework it is to perform These positions are then co-ordinated into an overall planwhich enables each crane to work without interfering with its neighbours, and at thesame time enables each to work in a position where adequate support can safely beprovided (see Fig 33.16) This plan will then form the basis of the erection methodstatement documentation

A major factor in planning craneage is to ensure that access is both available andadequate to enable the necessary quantity and size of components to be moved Onlarge greenfield developments these movements may often have to take place alongcommon access roads used by all contractors and along routes which may be subject

to weight or size restrictions On a tight urban site the access may be no more than

a narrow one-way street subject to major traffic congestion

33.8.5 The safe use of cranes

Mention has already been made of the UK Statutory Regulations These lay downnot only requirements for safe access and safe working but also a series of testrequirements for cranes and other lifting appliances

Fig 33.14 A purpose-made lifting beam for cantilever erection

Fig 33.15 An erection pole used to build a transmission tower

It is the responsibility of management to ensure that plant put on to a site has asufficient capacity to do the job for which it is intended, and that it remains in goodcondition during the course of the project Shackles and slings must have test cer-tificates showing when they were last tested Cranes must be tested to an overloadafter they have been assembled The crane test is to ensure that the winch capacity,

as well as the resistance to overturning and the integrity of the structure, is adequate

British Standards lay down the various requirements for safe working Lists ofthose standards, and the necessary forms to enable each of the tests to be recorded,must be provided by management, often in the form of a ‘site pack’ which the siteagent must then display and bring into use as each test is carried out It is the siteagent’s responsibility to ensure that these requirements are fulfilled The site agentmay also be required to produce them from time to time for inspection by thefactory inspector during one of his periodic visits to the site

A crane which has been tested and used safely in many locations might overturn

at its next location Failure is often caused by inadequate foundation provisionunder the tracks or outriggers of the crane In other words adequate support underthe tracks or outriggers is an essential requirement It is equally important that thecrane should work on level ground, since an overload can easily be imposed, eitherdirectly or as a sideways twist to the jib, if the ground is not level

33.8.6 Slinging and lifting

Components, whether they are on transport or are lying in the stockyard, shouldalways be landed on timber packers The packers should be strong enough to

Fig 33.16 Typical crane layout

horizon-it will behave, and also to check that the slings are properly bedded (see Fig 33.17).Most steelwork arrives on site with some or all of its paint treatment Since theinevitable damage which slinging and handling can do to paintwork must be madegood, it is therefore important to try to minimize that damage The same measuresthat achieve this also ensure that the load will not slip as it is being lifted, and thatthe slings (chain or wire) are not themselves damaged as they bend sharply aroundthe corners Softwood packers should be used to ease these sharp corners.

Packers to prevent slipping are even more necessary if the piece being erecteddoes not end up in a horizontal position The aim should always be to sling the piece

to hang at the same attitude that it will assume in its erected position Pieces beinglifted are usually controlled by a light hand line affixed to one end This hand line

is there to control the swing of the piece in the wind, and not to pull it into level.Wherever possible non-metallic slings should be used They will reduce damage topaintwork and are less likely to slip than chain or wire slings

In extreme cases two pieces may have to be erected simultaneously using twocranes Staff, working back at the office, should account for this in the site erectionmethod statement It is too late to discover this omission when the erection isattempted with only one crane, or with no contingency plan to pull back the head

and the need to build assemblies in a jig to represent the various points at whichconnection has to be made in the main framework An additional jig for lifting can

be particularly useful if there are many similar lifts to be made This can be made

to combine the need to stiffen with the need to connect to stiff points in the frame, and the need to have the sub-frame hang in the correct attitude on the cranehook The weight of any such stiffening and of any jig must of course be taken intoaccount in the choice of crane

sub-Some temporary stiffening may be left in position after the initial erection untilthe permanent connections are made This eventuality should also have been fore-seen, and sufficient stiffeners and lifting devices should be provided to avoid anunnecessary bottleneck caused by a shortage of a device for erection of the nextsub-frame

Where a particularly awkward or heavy lift has to be made, slinging and liftingcan be made both quicker and safer if cleats for the slings have been incorporated

in the fabrication Each trial lift made after the first one wastes time until the piecehangs true The drawing office should determine exactly where the centre of gravityis

A chart giving details of standard hand signals is illustrated in Fig 33.18 Their

use is essential when a banksman is employed to control the rear end of the port, thereby bringing the component to the hook as it is reversed The banksman

trans-is needed to relay the signal from the man directing the movements of the crane if

he is out of sight of the crane driver A clear system of signals should be agreed forthe handover of crane control from the man on the ground to the man up on thesteel who controls the actual landing of the component A banksman may also beneeded up on the steel if the crane driver cannot clearly see the top man who is

giving the control instructions: it is vital that there is no confusion over who is giving

instructions to the crane driver.

33.9 Safety

33.9.1 The safety of the workforce

The health and safety regulations require a project safety plan to be drawn up, whichshould include a detailed assessment of anticipated risks

There are a number of standards, regulations and guidance notes for the safety

of the workforce during construction, as referenced in the further reading

Site safety of the workforce is subject to statutory regulation and inspection

by the Health and Safety Executive Regulations lay down minimum acceptablestandards for the width of working platforms; the height of guard-rails; the fixing

of ladders; and so on They refer to the use of safety belts and safety nets They laydown the frequency with which a shackle or chain sling must be tested and therecords that must be kept to show that this was done Reference should be made tothe appropriate regulation for the details of these requirements

TRAVEL FROM ME

DERRICKING JIB

TRAVEL TO ME

EXTEND JIB RETRACT JIB TELESCOPING JIB

TROLLEY OUT TROLLEY IN

33.9.2 Risk assessment

Identification of foreseeable risk should be carried out5, as shown in Fig 33.19,required before the start of work on site These should be categorized into likeli-hood of occurrence (probability) and severity of occurrence (impact)

By carrying out a risk assessment, the risks can be identified and, where priate, avoided and reduced

appro-However, by its very nature, the erection of a structural frame is a process ing a certain amount of risk The work is carried out at height, and until it has pro-gressed to a certain point there is nothing to which a safe working platform can beattached The process of erecting a safe platform can be as hazardous as the erec-tion process itself One solution is to provide mobile access equipment if groundconditions permit

involv-Different access platforms are appropriate in different circumstances One tage of modern composite floor construction is that the decking can quickly provide

advan-a sadvan-afe working pladvan-atform, requiring only the advan-addition of advan-a hadvan-andradvan-ail Figure 33.20shows a safe platform for the erection of bare steelwork – a prefabricated platformslung over a convenient beam In this case weather protection may be added forsite welding

It is the responsibility of the designers and planners to ensure that no platformsare erected in order to carry out work that ought to have been done either in thefabrication shops or on the ground before the component concerned was lifted intoplace

A key planning-stage consideration is to see if the need for a working platformcan be eliminated altogether, i.e can the operation be carried out at ground levelbefore the component is erected? If not, can the platform be designed so that it isassembled on the component while it is still on the ground? It is impractical to have

to consider the provision of a safe working platform in order to be able to safelyerect the main safe working platform

The object of safety procedures is to ensure that everything possible is done toeliminate the risk of an on-site accidents Methods of achieving safety include:(1) An enhanced communication process

Communication of the details of safety procedures to all concerned, the display

of abstracts of the regulations themselves, the issuing of safety procedure documents, and the running of training courses all contribute to safe workingpractice Individuals must be aware of the location of particularly hazardous

Risk Likelihood of occurrence Likely severity

High Certain or near certain to occur Fatality, major injury, long-term disability

Medium Reasonably likely to occur Injury or illness causing short-term disability

Low Rarely or never occur Other injury or illness

Fig 33.19 Risk chart 5

areas and the available protection, the types of protective clothing and ment that are available and how to obtain them, the restrictions in force on thesite regarding the use of scaffolding or certain items of plant, and any accessrestrictions to certain areas They should be encouraged to tell someone inauthority if they see a potential hazard developing before it causes an accident.(2) Adequate equipment provision

equip-It is important to make the necessary equipment available on the site and tain it in good order Equipment includes safety helmets, ladders and workingplatforms, safety belts and properly selected tools

main-(3) Avoidance of working at height

Tasks should be organized to minimize work at height by: (a) the use of assembly techniques; (b) the fixing of ladders and working platforms to thesteelwork on the ground before it is lifted into place; (c) the early provision ofhorizontal access walkways; (d) the provision of temporary staircases or hoistswhere appropriate

sub-The above measures enable some of the hazards of working at height to bereduced by conferring on that work some of the advantages of ground-levelworking

(4) Appropriate fixing of portable equipment

It is important to ensure that portable equipment such as gas bottles andwelding plant is firmly anchored while it is being used The horizontal pull on

a gas pipe or a welding cable being used at height is considerable, and can lodge plant from a working platform, thereby endangering the operator Careshould also be taken to ensure that there are no flammable materials below theworking area, on which sparks could fall

dis-(5) Good design

A well thought-out design can make an important contribution to on-site safety.The positioning of a splice so that it is just above, rather than just below, a floorlevel will reduce the risks associated with the completion of an on-site splice.The arrangement of the splice so that the entry of the next component can besimply and readily completed will reduce the need to complete the splice up inthe air

Lifting cleats and connections for heavy and complex components should bedesigned and incorporated in the shop fabrications, as should fixing cleats, brackets

or holes for working platforms and for safety belts or safety net anchorages Theycan then be incorporated as part of the off-site fabrication, rather than having to beprovided by the erector at a height Access to a level should be provided by attach-ing a ladder and working platform to the member at ground level prior to lifting.Ideally these connections should be designed so that they can be dismantled afterthe erector has left the platform and descended the ladder The erector should nothave to come down an unfixed ladder or stand on an unfixed platform while remov-ing these items after use

Proper consideration of all of the above issues at the drawing-board stage willpreclude resort to risk-laden, hastily improvised on-site solutions by unqualified personnel.

33.9.4 Employees’ first visit to site

A tour of the site should be made during the induction process to aid in the tification and location of key personnel and citing of equipment, fire and first aidpoints, etc Procedures should be laid down and constantly reviewed for anyemployee joining a company

iden-33.9.5 The safety of the structure

The safety of those working on a structure is prescribed by statutory regulations.However, the stability of the structure itself is not prescribed by any regulation.Where a collapse of a partly built structure occurs, the loss of life is generally heavy.Post-collapse investigation and inquest often show gaps in the understanding of thebehaviour of the incomplete structure; lapses in the detailed consideration of eachand every temporary condition; and, most important of all, blocks in the flow ofcommunication of information to all involved In the bridge collapse shown in Fig.33.21 the temporary loading condition was not considered properly At the point offailure of the bottom flange, a splice similar to that shown in Fig 33.22 was unable

to take the compression imposed during erection

A designer must communicate the plan for building the structure to those whowill actually have to do the building In order to realize the design, the designermust be able to successfully translate the planning stage into the construction phase

Columns

The end restraints of a column can change during the construction of a building;each condition must be checked to ensure that column capacity is not compromised.The risks inherent at each stage must be assessed and provided for, taking intoaccount location, height, loads, temporary conditions, etc

Plate girders and box girders

The checks which are applied to the webs and stiffeners of a plate girder during itsdesign normally take account only of conditions at points where stiffeners arelocated and at points where loads are applied to the girder It may not be obvious

to the designer of a bridge girder that it may be subjected to a rolling load when

Safety 1007

Fig 33.21 Bridge failure during cantilever erection

Fig 33.22 Typical splice detail that is adequate in tension but would fail if subject to

sig-nificant compression

the bottom flange is rolled out over the piers, thus subjecting the girder to a pressive load which it is not required to carry in its permanent position.

com-Splices

The effects of stress reversals are most severe on splice details These almostinevitably involve some degree of eccentricity, which can trigger a collapse if thecondition, transient though it is, has not been considered in design

Bracing

Bracing is built into all types of structures to give them the capacity to withstandhorizontal forces produced by wind, temperature and the movements of cranes andother plant in and on the building

Erection cranes carried on the structure produce vibration and load These loadsmay not have been adequately accounted for post building completion Movementsresulting from cranes slewing, luffing and hoisting are carried by the framework sup-porting the crane to ground level These loads and vibrations must be considered,and the structure’s ability to carry them assessed at the outset

Temporary bracings, which may be required at some stages of the work, must haveproperly designed connections and be specifically referred to in the erection methodstatement Early or unauthorized removal of temporary bracings is a common cause

of collapse in a partially completed frame

Having considered the need for installing temporary bracings and the need topostpone fixing permanent bracings, consideration should be given to the overalleconomy of retaining the temporary bracings and perhaps leaving out the perma-nent bracings It is a costly and potentially dangerous business to go back into astructure solely in order to take out temporary members, or to insert componentsthat have had to be left out temporarily

Effects of temperature and wind

On a partly erected and unclad building frame the effects of temperature on theframework can exceed the effects of the wind A tall framework will lean away fromthe sun as the sun moves round from east to west: thus checking the plumb of abuilding should be done only on a cloudy day or after the whole structure has beenallowed to reach a uniform temperature (e.g at night), and then only when the tem-perature is at or near the design mean figure Tightening the bolts in the bracingwhen a building is at non-uniform temperature can lock in an error which may provedifficult to correct later

Wind effects can bring a building down if it is not adequately braced and guyed.The wind can have two effects, via the pressure exerted on anything in its path, orvibrations in a member obstructing its path

The problem is compounded by the variability of the direction and speed of awind, and by the variability of the aerodynamic shape of the structure as each newpiece is added Care must be taken to ensure that these issues have been properlyaddressed at each stage of the erection of potentially problematic structures, e.g.bridges erected by cantilevering Bracings, guy ropes and damping weights may allhave to be considered as methods of changing critical frequencies of vibration and

of limiting movements as the job progresses

33.9.6 Temporary supports and temporary conditions

Much time and effort is invested in the design of the structure However, the design

of the temporary works on which that structure may have to depend while it is beingbuilt may not have been given adequate attention The number of recorded col-lapses that take place after an initial failure in the temporary supports bears testi-mony to this omission For example, a temporary support may be designed only totake a vertical load In practice, the structure it is intended to support may movedue to changes in temperature and wind loading, thereby imposing significant addi-tional horizontal loads

Sufficient consideration should be given to the foundations Settlement in a trestlefoundation can profoundly affect the stress distribution in the girder work that itsupports Settlement under a crane outrigger from a load applied only momentar-ily can lead to the collapse of the crane and its load The Code of Practice BS 597512

for falsework (which includes all temporary works, trestling, guy wires, etc., as well

as temporary works associated with earthworks) deals with a wide range of work types and should be carefully read and observed Particular attention should

false-be paid to the paragraphs dealing with communication, co-ordination and vision since failure in any of these areas can lead to a failure of the falsework itself.Re-used steelwork showing signs of severe corrosion must not be used for tem-porary falsework carrying critical loading In other situations re-used steel should

super-be measured to ensure adequate performance

During construction a structure will move as its parts take up their design load Connections to temporary supports have to be capable of absorbing thesemovements

Unless the design allows for these movements, eccentricities can result which maytrigger a collapse The cross-heads at the tops of bridge trestles have been known

to fail from this cause since they are often called upon to resist wind-induced loads,vibration and temperature-induced movements in the structure, in addition to theirmore obvious direct loading burden For these reasons they must receive a specialdesign study

Very tall buildings and chimneys as well as bridges can be affected by induced vibrations, as can working platforms and those who have to work with them.The force of the wind can make welding impossible without adequate shelter: there-fore the fixings for a working platform must be able to take the load of the windblowing on shelter area

•

Too many examples exist of a collapse following the removal of guy wires beforethe bracing was fitted, or before column bases designed to be ‘fixed’ had been actu-ally grouted and fixed What is needed here is a clear flow of communication fromthe designer to the foreman and the workforce of exactly which sequence of workingmust be followed Supervision alone may not suffice The only way to ensure thatsafe practice is adhered to is to issue a clear directive coupled with an explanation

of why the instruction is being given It also helps to have employed a skilled force who know what they are doing!

work-The need for provision of an organization chart has already been discussed.However, a second chart showing who needs to know what, why, and when shouldalso be produced If the lines of communication and the patterns of responsibilitybetween various management levels and organizations are to be effective, there

Fig 33.23 Humber Bridge during erection

must be a commitment made by all concerned to understand why the links are there,and how best to enhance speedy information exchange.

33.10 Special structures

All structures are, to some extent, special However, there exist particular structureswhich, by their complexity, require special consideration when designing and plan-ning their erection The length, height and relative mobility of a completed struc-ture, or the depth of individual members, may bring forward particular designproblems

Temperature differentials over the depth of bridge box girders will producechanges in the camber of the girder Temperature differentials over the width of astructure will produce changes in verticality Temperature changes will affect thevertical orientation of the columns at each end of a long single-storey factory build-ing or bridge Some of these effects can be, and commonly are, accommodated bythe provision of expansion joints Others must be addressed in the planning and exe-cution phases

The construction of suspension (see Fig 33.23) or cable-stayed bridges providesgood examples where movement and change to the shape of the structure becomeincreasingly apparent as the construction process progresses A radio telescope isthe best example of a special structure which is designed to move and yet must main-tain very close tolerances as the extremities of the structure are reached Otherstructures move as they grow, and their temporary supports can fail as a result Thesefailures are too often the result of a lack of appreciation of construction movements,vibrations from wind, or local loads from erection plant

References to Chapter 33

1 HMSO (1995) The Construction (Design & Management) Regulations 1994

2 Health and Safety Executive (1984) Guidance Note 28 (Parts 1–4), HMSO.

3 The British Constructional Steelwork Association (2002) National Structural

Steelwork Specification for Building Structures, 4th edition, BCSA/SCI.

4 British Standards Institution (1990) Building setting out and measurements Part 1: Methods of measuring, planning and organisation and acceptance criteria Part 2: Measuring stations and targets, Part 3: Check-lists for the procurement of

surveys and measurement surveys BS 5964, BSI, London.

5 CIMSteel (1997) Design for Construction The Steel Construction Institute,

Ascot, Berks

6 Cheal B.D (1980) Design Guidance Notes for Friction Grip Bolted Connections.

CIRIA Technical Note 98 Construction Industry Research and InformationAssociation, London

7 Couchman G.H., Mullett D.L & Rackham L.W (2000) Composite slabs and

beams using steel decking: best practice for design and construction The Metal

Cladding & Roofing Manufacturers Association/The Steel Construction tute, Ascot, Berks

Insti-8 British Standards Institution (1993) Quality systems Part 14: Guide to

depend-ability programme management BS 5750, BSI, London.

9 British Standards Institution (1994) Quality systems Model for quality

assur-ance in design, development, production, installation and servicing BS EN ISO

9001, BSI, London

10 British Standards Institution (1998) Execution of steel structures Part 1: General

rules and rules for buildings DD ENV 1090, BSI, London.

11 British Standards Institution (1972) Code of practice for safe use of cranes

(mobile cranes, tower cranes and derrick cranes) CP 3010, BSI, London.

12 British Standards Institution (1996) Code of practice for falsework BS 5975,

BSI, London

Further reading for Chapter 33

British Standards Institution (1997) Safety nets Part 1: Safety requirements, test

methods BS EN 1263, BSI, London.

British Standards Institution (1998) Safety nets Part 2: Safety requirements for the

erection of safety nets BS EN 1263, BSI, London.

British Standards Institution (1988) Code of practice for safety in erecting structural

frames BS 5531, BSI, London.

British Standards Institution (1993) Code of practice for access and working

scaf-folds and special scaffold structures in steel BS 5973, BSI, London.

British Standards Institution (1990) Code of practice for temporarily installed

sus-pended scaffolds and access equipment BS 5974, BSI, London.

British Standards Institution (1989) Code of practice for safe use of cranes Part 1:

General BS 7121, BSI, London.

British Standards Institution (1991) Code of practice for safe use of cranes Part 2:

Inspection, testing and examination BS 7121, BSI, London.

British Standards Institution (2000) Code of practice for safe use of cranes Part 3:

Mobile cranes BS 7121, BSI, London.

British Standards Institution (1997) Code of practice for safe use of cranes Part 5:

Tower cranes BS 7121, BSI, London.

British Standards Institution (1998) Code of practice for safe use of cranes Part 11:

Offshore cranes, BS 7121, BSI, London.

The Steel Construction Institute (1993) A Case Study of the Steel Frame Erection at

Senator House, London SCI Publication 136, Ascot, Berks.

The Steel Construction Institute (1994) The Construction (Design and Management)

Regulations 1994: Advice for Designers in Steel SCI Publication 162, Ascot, Berks.

Fire safety must be regarded as a major priority at the earliest stage as it can have

a major impact on the design of a building and its structural form Nevertheless, it

should not stifle aesthetic or functional freedom; fire engineering techniques are now

available which permit a more rational treatment of fire development and fire tection in buildings

pro-The strength of all materials reduces as their temperature increases Steel is noexception It is essential that the structure should not weaken in fire to the extentthat collapse occurs prematurely, while the occupants are seeking to make their way

to safety For this reason it is necessary to provide a minimum degree of fire

resist-ance to the building structure Additionally, a measure of property protection is

implied in the current Approved Document B of the Building Regulations, although

in principle the only concern of the Building Regulations is safety of life

There are two basic ways to provide fire resistance: first, to design the structureusing the ordinary temperature properties of the material and then to insulate themembers so that the temperature of the structure remains sufficiently low, or sec-ondly, to take into account the high-temperature properties of the material, in whichcase no insulation may be necessary

34.2 Standards and building regulations

34.2.1 Building regulations

All buildings in the UK are required to comply with the Building Regulations,1

which are concerned with safety of life The provisions of Approved Document B,for England and Wales, are aimed at reducing the danger to people who are in oraround a building when a fire occurs, by containing the fire and ensuring the sta-bility of the structure for sufficient time to allow the occupants to reach safety Gen-erally, in Scotland and Northern Ireland the provisions are similar but not identical.Approved Document B requires that adequate provision for fire safety be providedeither by fulfilling its recommendations given in Appendix A or by suitable alter-native methods

The fire-resistance requirements of Document B apply only to structural elementsused in:

(a) buildings, or parts of buildings, of more than one storey,

(b) single-storey buildings that are built close to a property boundary

The degree of fire resistance required of a structural member is governed by thebuilding function (office, shop, factory, etc.), by the building height, by the com-partment size in which the member is located, and by whether or not sprinklers areinstalled

Fire resistance provisions are expressed in units of time:1/2, 1, 11/2and 2 hours It

is important to realize that these times are not allowable escape times for buildingoccupants or even survival times for the structure They are simply a convenient way

of grading different categories of buildings by fire load, from those in which a fire

is likely to be relatively small, such as low-rise offices, to those in which a fire mightresult in a major conflagration, such as a library Fire-resistance recommendationsfor structural elements are given in Reference 2

34.2.2 BS 5950: Part 8

BS 5950: Part 82permits two methods of assessing the fire resistance of bare steel

members The first, the load ratio method, consists of comparing the design

tem-perature, which is defined as the temperature reached by an unprotected member

in the required fire-resistance time, with the limiting temperature, which is the

tem-perature at which it will fail The load ratio is defined as:

If the limiting temperature exceeds the design temperature no protection is sary The method permits designers to make use of reduced loads and higher-strength steels to achieve improved fire-resistance times in unprotected sections.The second method, which is applicable to beams only, gives benefits whenmembers are partially exposed and when the temperature distribution is known Itconsists of comparing the calculated moment capacity at the required fire-resistancetime with the applied moment When the moment capacity exceeds the appliedmoment no protection is necessary This method of design is used for unusual struc-tural forms such as ‘shelf-angle’ floor beams Some examples of the use of themoment capacity method are given in the handbook to BS 5950: Part 8

neces-Limiting temperatures for various structural members are presented in the

Appendix Limiting temperatures These ‘failure’ temperatures are independent of

the form or amount of fire protection Beams supporting concrete floors fail at amuch higher limiting temperature than columns, for example

load ratio load carried at the fire limit state

The rate of heating of a given section is related to its section factor which is the

ratio of the surface perimeter exposed to radiation and convection and the mass,which is directly related to cross-sectional area:

A member with a low Am/V value will heat up at a slower rate than one with a high

Am/V value and will require less insulation (fire protection) to achieve the same resistance rating Standard tables are available listing Am/V ratios for structural sec- tions (see the Appendix Section factors for UBs, UCs, CHSs and RHSs) These

fire-factors are calculated as indicated in Fig 34.1

Sections at the heavy end of the structural range have such low Am/V ratios, and

therefore such slow heating rates, that failure does not occur within 1/2hour understandard BS 476 heating conditions even when they are unprotected

Limiting section factors for various structural elements are given in Fig 34.2 responding to a load ratio of 0.6)

(cor-section factor = exposed surface area of section per unit length m

cross-sectional area of the section per unit length m

Fig 34.1 Some different forms of fire protection to I-section members

Fig 34.2 Maximum ratios of Am/V (m-1 ) of exposed steel to give 30 minutes fire resistance

H \

BS 5950: Part 8

i1.5 strain

—0.5 strainEurocode 3 proposal

_ strain)

200 400 600 800

teniperature (°C)

0.9 0.8

0.6 U 0.5

a)

L4-, V)

0.2

Manufacturers of fire-protection products now give guidance on the requiredthickness of fire protection depending on the section factor of the member Theexample of a table for a typical spray applied (profile protection) shown in the

Appendix Minimum thickness of spray protection is taken from Reference 3.

Some fire protection materials are assessed at the single limiting temperature of550°C However, there is an increasing trend for manufacturers to provide thick-ness recommendations for a range of temperatures This allows designers to mini-mize cost by tailoring protection thickness in an individual project to the limitingtemperature derived from BS 5950 Part 8

34.3 Structural performance in fire

34.3.1 Strength of steel at elevated temperatures

Steel begins to lose strength at about 200°C and continues to lose strength at anincreasing rate up to a temperature of about 750°C, when the rate of strength lossflattens off This relationship is shown in Fig 34.3 An important parameter is thestrain at which the strength is assessed It is reasonable to take a higher strain limitthan in normal design, because fire is an ultimate limit state and much higher deflec-tions are allowed in fire tests than in normal structural tests

Fig 34.3 Strength retention factor for grade 43 steel at elevated temperatures

I(b)

BS 5950 Part 8 specifies the use of 2.0% strain values for design of compositebeams and 1.5% strain for non-composite beams that are unprotected or protectedwith ‘robust’ (i.e deformable) fire protection materials For columns and tensilemembers or beams protected with ‘brittle’ material the use of 0.5% strain is speci-fied for design calculations

Design to Eurocode 3 (as proposed in the 2001 draft prEN 1993-1-2) is slightlyless conservative, with 2.0% strain being specified for beams and 1.0% when con-sideration of deformation is required

34.3.2 Performance of beams

Beams supporting concrete slabs behave better than uniformly heated sections, forwhich the material performance is the dominant factor The concrete slab causesthe top flange to be significantly cooler than the bottom flange and thus, as thesection is heated, the plastic neutral axis of the section rises towards the top flange(see Fig 34.4) The section resistance is determined by the strength of steel at 1.5%strain, but in this case more of the web is effective in resisting tension

The limiting temperatures of beams supporting concrete slabs are shown in Fig.34.5 with the test results for a range of beam sizes and load ratios These limitingtemperatures are increased by about 60°C relative to a uniformly heated section.This temperature differential effect, and its beneficial influence on moving theneutral axis, can be enhanced by partially or fully embedding steel beams into thefloors that they support The shelf-angle beam and the Slimdek beam (shown in Fig.34.6) are methods by which 30 minutes or 1 hour fire resistance can be achieved bydesign, without the need for subsequent protection

Fig 34.4 Temperature and stress variation in I-beam supporting concrete slab when

limit-ing temperature is reached (a) Section through beam and slab (b) Temperature variation (c) Stress variation

of 70 normally encountered in buildings.

The fire resistance of columns can also be increased by partial protection in theform of concrete blocks or bricks, either by building into a wall or by fitting blocksbetween the flanges A minimum fire resistance of 30 minutes can usually beachieved in this way

The fire resistance of columns can be increased to 1 hour or more by partial protection in the form of concrete poured between the flanges before delivery tosite, as shown in Fig 34.7 Alternatively, tubular CHS or RHS sections can be filledwith concrete on or off site For higher ratings orthodox protection methods can beused

34.3.4 Fire resistance without protection

Table 34.1 summarizes the methods of attaining fire resistance given in the SCI

pub-lication Design of steel framed buildings without applied fire protection.4 Partial

Fig 34.5 Limiting temperatures for I-section beams supporting concrete floors on the

upper flange

Concretefloor slab

exposure of members reduces heat input and creates a temperature gradient thatallows stress redistribution from hot to cold regions of the section Ratings of up to

1 hour can be achieved without the application of fire-protecting materials

34.3.5 Performance of composite slabs

Modern steel frames often involve the use of composite slabs comprising steeldecking acting compositely with the concrete floor (see Chapter 21) A large number

Fig 34.6 (a) Universal beam with shelf angle to support precast concrete slab

(b) Asymmetric beam with deep metal deck

(a)

(b)

of fire tests have now been carried out to justify the use of 90 minutes’ fire ance for composite slabs with standard mesh reinforcement and no additional fire

resist-protection Guidance is given in Handbook to RS 5950: Part 8.5

In addition, it has been determined from tests that it is unnecessary to fill thevoids between the beam top flange and the floor decking for ratings up to 90minutes, although some increase in protection thickness on the beam may be

required when a trapezoidal deck is used (See Appendix – Steelwork in fire

infor-mation sheet no 6.)

34.3.6 Eurocodes

All of the methods given in this chapter are compatible with current (2001) drafts

of Eurocodes:

Fig 34.7 H-section columns in-filled with concrete have 1 hour fire resistance

TP

Table 34.1 Table 2.1 from SCI publication 186 Design of steel framed buildings without applied

fire protection

Fire resistance (mins) Column type:

prEN 1991-1-2: Actions on structures exposed to fires

prEN 1993-1-2: Design of steel structures: Structural fire design

prEN 1994-1-2: Design of composite structures: Structural fire design.

34.4 Developments in fire-safe design

In the mid 1990s, a series of realistic fire tests were carried out on a full-sized storey steel-framed building at the Building Research Establishment’s large-scaletest facility at Cardington, Bedfordshire (see Fig 34.8) Full analysis of the testresults has shown that the behaviour of steel members in a whole building framewith all of the restraint, continuity and interaction that can occur is very differentfrom the behaviour of single members in unrestrained standard fire tests Columnswill need protection because deformation of columns can cause damage beyond thecompartment of fire origin, but unprotected composite beams were able to survive

eight-a tempereight-ature of 1100°C without colleight-apse (see Fig 34.9) It beceight-ame cleeight-ar theight-at brane action in the composite slab was responsible for the observed high perfor-mance and that it would be possible to design steel-framed buildings in such a way

mem-as to allow the beams to remain unprotected

A further floor test was conducted at BRE’s Garston laboratory to quantify theeffect of tensile membrane action, and Bailey and Moore6 showed that the strength

Fig 34.8 Realistic fire tests in a modern steel-framed office building showed that stability

requirements can be maintained without beam protection

and location of the reinforcement in a composite slab and the aspect ratio of theslab panel itself largely govern its load capacity.

This analysis has been incorporated into simple design guidance by the UK Steel

Construction Institute (Newman et al.7), which allows secondary beams to remainunprotected in composite slab panels up to 9 metre span for fire ratings up to 60minutes These limitations may be extended by use of the Bailey and Moore6 cal-culation method from first principles or by application of a specialized finite elementprogram such as VULCAN,8 which is adapted to deal with fire conditions

34.5 Methods of protection

Basic information on methods of protection is summarized in the Appendix

34.5.1 Spray-applied protection

Sprays are the cheapest method, with costs commonly in the range of £4 to

£12/square metre applied (2001 prices), depending on the fire rating required andthe size of the job As implied, spray protection is applied around the exposedperimeter of the member, and therefore the relevant section factors are for profile

protection (see the Appendix Limiting temperatures) Application is fast, and it is

easy to protect complex shapes or connections However, sprays are applied wet,

Fig 34.9 Unprotected beams were heated to 1100°C without collapse

which can create problems in winter conditions, they can be messy, and the ance is often poor For this reason they are generally used in hidden areas such as

appear-on beams above suspended ceilings, or in plant rooms

34.5.2 Board protection

Boards tend to be more expensive, commonly in the range £6 to £20/square metrewithout a decorative finish (2001 prices), because of the higher labour content infixing The price depends on the rating required and the surface finish chosen buttends to be less sensitive to job size Board systems form a box around the sectionand therefore have a reduced heated perimeter in comparison to spray systems (see

the Appendix Limiting temperatures and Fig 34.1) They are dry fixed by gluing,

sta-pling or screwing, so there is less interference with other trades on site, and the boxappearance is often more suitable for frame elements, such as free-standingcolumns, which will be in view

34.5.3 Intumescent coatings

Intumescent coatings have become more widely used in recent years Unlike tional protection materials their insulating layer is formed only by the action of heatwhen the fire breaks out

tradi-The coating is applied as a thin layer, perhaps as thin as 1 mm, but it contains acompound in its formulation which releases a gas when heat is applied This gasinflates the coating into a thick carbonaceous foam, which provides heat insulation

to the steel underneath The coatings are available in a range of colours and may

be used for aesthetic reasons on visible steelwork

Two types of intumescent coating are currently available The first is commonlyused for ratings up to 11/2hours used in dry interiors and is not recommended forwet applications such as swimming pools or in exterior conditions Costs range fromabout £5 to £25/square metre (2001 prices) according to fire rating The second type,which is water resistant, has a maximum rating of 2 hours but is expensive, with a

1 hour coating costing around £40/square metre (2001 prices)

34.5.4 Pre-delivery protection

Recent developments in intumescent coating formulation have made it practicable

to merge two operations, fabrication and fire protection, into a single off-site tract It is a method of improving efficiency that is rapidly gaining ground

Application of intumescent coatings in the fabricator’s works before deliverymeans that steelwork can arrive on site finished (see Fig 34.10), eliminating a wholetrade on site This reduces construction time and can cut overall construction costs.

Further information can be found in the Corus publication Off-site fire protection

using intumescent coatings.9

34.6 Fire testing

Fire resistance requirements are based largely on the Fire Grading of Buildings

report of 1946.10 The fire-resistance periods refer to the time in a standard fire,defined by BS 476: Parts 20 and 21,11 that an element of structure (a column, beam,compartment wall, etc.) should maintain:

(1) stability – it should not collapse under load at the fire limit state,

(2) integrity – it should not crack or otherwise allow the passage of flame to anadjoining compartment,

(3) insulation – it should not allow passage of heat by conduction which mightinduce ignition in an adjoining compartment

The ‘standard’ fire time–temperature relationship defined in BS 476 is presented

in Fig 34.11 Most attention (and cost) is directed at satisfying the stability (orstrength) criterion

Fig 34.10 Pre fire-protected sections awaiting erection

510 20 30 60 90

time (mm) (a)

1000 L)

E

Fig 34.11 Temperature–time curves: (a) standard ISO fire test, (b) standard and natural

fires in small compartments

In a fire test columns are exposed to fire on all four sides and axially loaded tically, whereas beams are loaded horizontally in bending and are exposed on threesides, the upper flange being in contact with the floor slab, which also acts as thefurnace roof.

ver-The stability limit is deemed to have been reached for columns when ‘run-away’deflection occurs For beams this is more accurately specified when the deflection

rate reaches L2/9000d (where L = beam span, d = beam depth), within a deflection range of L/30 min to L/20 max.

34.7 Fire engineering

There are many special building forms which can take advantage of a rational

approach, called fire engineering,12,13which takes account of the temperatures oped in a real fire, as opposed to a standard fire (see Fig 34.11)

devel-Essentially the fire engineering design method can be divided into two main steps:(1) Determination of the fire load

The fire load of a compartment is the maximum heat that can theoretically

be generated by the combustible items of contents and structure i.e weight ¥calorific value per unit weight

Fire load is usually expressed in relation to floor area, sometimes as MJ/m2

or Mcal/m2 but often converted to an equivalent weight of wood and expressed

as ‘kg wood/m2’ (1 kg wood ∫ 18 MJ) Standard data tables giving fire loads ofdifferent materials are available.12,13

Examples of typical fire loads (wood equivalent) are:

(2) Prediction of maximum compartment temperature

The heat that is retained in the burning compartment depends upon thethermal characteristics of the wall, floor and ceiling materials and the degree

of ventilation Sheet steel walls will dissipate heat by conduction and tion while blockwork will retain heat in the compartment and lead to highertemperatures

radia-It is assumed that window glass breaks in fire conditions, and calculations takeinto account the size and position of such ventilation Openings close to theceiling level of a compartment (or disintegrating roofing materials) will tend todissipate heat whereas openings close to the floor will provide oxygen to feedthe fire

Since a great deal of data have been gathered over the years on the formance of materials in the standard fire test, methods have been sought torelate real fire conditions to standard fire performance in order that the exist-

per-ing data can be used in fire engineerper-ing The time equivalent or equivalent

required fire resistance is given by:

Teq = CWQf

where Qf= fire load density in MJ/m, i.e the amount of combustible material

per unit area of compartment floor,

W= ventilation factor relating to the area and height of door andwindow openings,

C= a constant relating to the thermal properties of the walls, floorand ceiling

Detailed methods for calculation of temperatures in natural fires are given inEurocode 1 (prEN1991-1-213)

Fire engineering is not a concept that can be recommended for buildings that aresubject to frequent change of use, such as advance factory units, but many buildingsare ‘fixed’ in terms of their occupancy (car parks, hospitals, swimming pools, etc.)and in such cases fire engineering is a valid approach It is most appropriate forbuildings of large volume with low fire load

In the UK a number of buildings have been built using unprotected steel on fireengineering principles, one example being the north stand at Ibrox football ground

in Glasgow

Other examples of the use of calculation methods in determining structuralresponse in fire are external steel in framed buildings,14and portal frames with fire-resistant boundary wall.15

3 The Steel Construction Institute/Association of Structural Fire Protection

Contractors and Manufacturers (1989) Fire Protection of Structural Steel in

Buildings, 2nd edn SCI/ASFPCM.

4 Bailey C.G., Newman G.M & Simms W.I (1999) Design of Steel Framed

ings without Applied Fire Protection The Steel Construction Institute, Ascot,

Berks

5 Lawson R.M & Newman G.M (1990) Fire Resistant Design of Steel Structures

– A Handbook to BS 5950: Part 8 The Steel Construction Institute, Ascot, Berks.

6 Bailey C.G & Moore D.B The structural behaviour of steel frames with

com-posite floor slabs subject to fire The Structural Engineer, 78 No 11, 19–26 &

28–33, 6 June 2000

7 Newman G.M., Robinson J.T & Bailey C.G (2000) Fire Safe Design – A New

Approach to Multi-Storey Buildings The Steel Construction Institute, Ascot,

Berks

8 Papers on VULCAN are published on the website http://www.shef.ac.uk/fire-research

9 Dowling J.J Off-Site Protection Using Intumescent Coatings Corus

Construc-tion Centre, PO Box 1, Brigg Road Scunthorpe http://www.corusconstrucConstruc-tion.com

10 Ministry of Public Buildings and Works (1946) Fire Grading of Buildings, Part

1, General Principles and Structural Precautions Post War Building Studies No.

20 HMSO

11 British Standards Institution (1987) Fire tests on building materials and

struc-tures Part 20: Method of determination of the fire resistance of element of struction (general principles) Part 21: Method for determination of the fire resistance of load-bearing elements of construction BS 476, BSI, London.

con-12 Report of CIB Workshop 14 (1983) Design guide: structural fire safety Fire

Safety Journal, 6 No 1, 1–79.

13 Eurocode 1 – Actions on structures Part 1.2 Actions on structures exposed to

fire CEN, Central secretariat, rue de Stassart 36, Brussels.

14 Law M & O’Brien T.P (1981) Fire and Steel Construction – Fire Safety of Bare

External Structural Steel Constrado.

15 Newman G.M (1990) Fire and Steel Construction – The Behaviour of Steel

Portal Frames in Boundary Conditions, 2nd edn The Steel Construction

Institute, Ascot, Berks

Further reading for Chapter 34

References 4, 7 and 9 are recommended further reading

EUROFER (1990) Steel and Fire Safety – A Global Approach Eurofer, Brussels

(available from The Steel Construction Institute, Ascot, Berks)

to achieve the desired protection requirements for specific structures.

Much guidance for the protection of steel structures has, over the years, been

sought from BS 5493: 1977 Code of Practice for Protective Coating of Iron and Steel

Structures Against Corrosion This document has now been superseded by a series

of new ISO standards One of the most important is ISO 12944 Paints and Varnishes

– Corrosion Protection of Steel Structures by Protective Paint Systems This standard,

which is published in eight parts, should be referred to when drafting protection

specifications for steel structures Part 5 of the series Protective paint systems

con-tains a range of paint coatings and systems for different environmental categories

that are defined in Part 2 Classification of environments However, specifiers

con-cerned with UK projects should be aware that not all of the paints listed are pliant’ with current national environmental legislation, and further advice should

‘com-be sought from the paint manufacturer

35.1.2 General corrosion

Most corrosion of steel can be considered as an electrochemical process whichoccurs in stages Initial attack occurs at anodic areas on the surface, where ferrousions go into solution Electrons are released from the anode and move through the

(hydrated ferric oxide or rust)

metallic structure to the adjacent catholic sites on the surface, where they combinewith oxygen and water to form hydroxyl ions These react with the ferrous ions from the anode to produce ferrous hydroxide, which itself is further oxidized in air

to produce hydrated ferric oxide: red rust (Fig 35.1)

The sum of these reactions is described by the following equation:

4Fe + 3O2 + 2H2O = 2Fe2O3H2O

(iron/steel) + (oxygen) + (water) = rust

Two important points emerge:

(1) For iron or steel to corrode it is necessary to have the simultaneous presence of

water and oxygen; in the absence of either, corrosion does not occur

(2) All corrosion occurs at the anode; no corrosion occurs at the cathode

Fig 35.1 Diagrammatic representation of the corrosion of steel