Erection I

STEEL CONSTRUCTION: FABRICATION AND ERECTION

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STEEL CONSTRUCTION:

FABRICATION AND ERECTION

Lecture 3.2.1: Erection I

OBJECTIVE/SCOPE

To give undergraduates, young engineers and managers an introduction to the trade of steel

erection.

PREREQUISITES

None are essential.

The following lectures might be helpful:

Lecture 3.1: General Fabrication of Steel Structures

Lecture 3.3: Principles of Welding

Lecture 3.4: Welding Processes

RELATED LECTURES

Lecture 3.2.2: Erection II

Lecture 3.2.3: Erection III

Lecture 3.5: Fabrication/Erection of Buildings

Lecture 15A.8: Offshore: Fabrication

Lecture 15B.12: Introduction to Bridge Construction

SUMMARY

The lecture emphasizes the importance of considering erection during all phases of the

project. It outlines the principal requirements for a technical specification and also deals

with the organisational aspects on site.

1. INTRODUCTION

It is important that the erection of the steelwork is considered from the very first stages of a

project. Both design engineer and fabricator must consider the following:

Connections on site: site joints should be bolted rather than welded.


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Preassembly: the fabricator should limit the number of site splices to that

consistent with minimum project cost. The size and weight of structural steel

assemblies will be limited by site and shop capabilities, the permissible weight and

clearance dimensions of available transportation and the site conditions.

Dimensions: all measurements necessary for site assembly should be shown on the

drawings.

Planning: the sequence of erection should be considered as an integral part of the

project process and should be established and documented at an early stage.

Marking: the marking of all parts should be clear and consistent throughout the

project.

Resources: depending on how the site assembly is carried out, it is essential to

ensure that appropriate resources will be available.

The erection team on site must ensure that :

the workforce are aware of and implement the relevant standards and regulations.

changes in procedures, which become necessary during the erection stages are

agreed by the Engineer, and that the technical documents are corrected in order to

be consistent.

hoisting equipment of suitable capacity is available for any preassemblies which

must be lifted.

means of access, such as scaffolding stairs and platforms, are installed to enable

bolting and welding to be carried out satisfactorily.

Erection of structural steelwork is, therefore, an activity that requires detailed

consideration from both an engineering design and organisational point of view. Sections 2

and 3 of this lecture discuss these matters under the headings of: Technical Specification

and Site Organisation.

2. TECHNICAL SPECIFICATION

The Technical Specification for Erection should be developed as early as possible and

should outline the conditions required on site and the technical standards in force. The

documentation should deal with the following matters:

Basic sequence of erection.

Specification for erection, (see below).

Standards in force.

Organization on site.

Basic site accommodation.

Man-hours estimation.

Erection personnel.

Main plant and erection tools.

Erection plan.

Erection drawing.

Safety programme.

Quality Control programme.


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Specification for Erection

The specification for erection should include, in as much detail as possible, the following

information:

1. Requirements for unloading, storage, and handling. 2. Details of any preassembly required on site. 3. Dimensional and levelling tolerances, including those required for foundations and

baseplates.

4. Specification for activities relating to the assembly itself such as bolting, welding and testing.

3. SITE ORGANISATION

3.1 Principal Jobs on Site

The site organisation required depends largely on the size of contract; in the case of a

major project it could, for example, be subdivided into the following sections:

Technical Office.

Scheduling and Planning.

Implementation and Production.

Administration.

Safety.

Quality Control.

Social Management.

3.2 Estimation of needs

Figure 1 gives an example of an Organisation Chart for the extreme case of a project for

the erection of a large steel frame. The chart is applicable to a direct assembly workforce

of approximately 125 men.

3.3 Basic Installations and Site Conditions

The Erector must have adequate facilities available on site for his personnel, including

appropriate office accommodation, sanitary services and storage areas. He must also

ensure that the site is properly supplied with mechanical services necessary for erection

such as electricity, gas, compressed air, etc. These facilities and installations must comply

with the following requirements:

1. They should be situated, where possible, in close proximity to the work site, giving easy access to the site without interfering in any way with the progress of the work;

2. They should be adequately constructed to, at least, the minimum legal requirements;

3. They should have communication facilities; 4. They must be maintained in satisfactory condition throughout the work and

removed on its completion.


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3.4 Direct Manpower

It is essential to estimate the direct manpower resources required in order to properly

programme the erection process and accurately forecast the time of completion and the

costs incurred.

Manpower resources are normally estimated by calculating the number of direct man-hours

required in steel frame erection. An adequate estimate can be obtained based on the

erection weight broken down item by item (columns, beams, bracings, floors, etc.), and

applying various weight rates (in hours) obtained from past experience.

For a more accurate estimate, each activity can be evaluated, not only in terms of its

duration, but also in terms of personnel required to complete it. The product of the time (in

hours) multiplied by the number of people in the team will give the man hours for each

activity which can be totalled to give the direct man-hours required for the whole job.

3.5 Cranes, Tools and other Equipment

Once the activities involved in the erection have been decided, the tools and equipment

required to carry out the work can be defined; these may include:


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Cranes of various types.

Transport equipment.

Special erection equipment, such as erection masts, erection bridges, etc.

Winches (electric and pneumatic).

Bolting equipment, such as spanners, ratchet spanners, torque wrenches, torque

testing equipment etc.

Welding equipment including cables, guns and drying ovens.

Air compressors.

Electric generators.

Hydraulic jacks.

Measuring equipment such as theodolite, levels, tapes, and laser equipment.

Miscellaneous equipment such as pulleys, spreader beams, etc..

Wire ropes, hoisting slings, shackles, etc..

Handling tools.

The heaviest or highest element to be erected, or the part which requires the greatest lifting

capacity (radius-weight) determines the minimum crane capacity to be used.

Figure 2 shows the various lifting capacities of a tower crane.

4. CONCLUDING SUMMARY

Erection requirements should be considered at the earliest stages of the project, by

both Engineer and Fabricator.

The Technical Specification for Erection should outline the conditions required on

site and the technical standards in force; it should also give a detailed description of

how the structure is to be erected, i.e. unloading, preassembly, tolerances,

connections, etc.

The site organisation must be carefully considered in terms of the requirements for

manpower, tools and site facilities and service installations.

5. ADDITIONAL READING

1. Arch, H., Erection, Chapters 33 - Steel Designers' Manual, Oxford, Blackwell Scientific Publications, 1992.

2. Thornton, W. A., "Design for Cost Effective Fabrication and Construction", Chapter 7.1 Constructional Steel Design, London, Elsevier Applied Science, 1992.

3. Miller, E. W., "Erection of Steel Structures", Chapter 7.2 Construction Steel Design, London, Elsevier Applied Science, 1992.

4. Potter, P. D., "Fast Steel Erection": Steel Fabrication Journal, No. 46, February 1983, Australian Institute of Steel Construction.

5. The Steel Construction Industry Code of Practice for Sage Erection of Building Steelwork: Part 2 - Multi-storey Buildings and Structures, Australia, Australian

Institute of Steel Construction, 1989.

6. Eurocode 3: "Design of Steel Structures" ENV 1993-1-1: Part 1.1: General Rules and Rules for Buildings, CEN, 1992.


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STEEL CONSTRUCTION:

FABRICATION AND ERECTION

Lecture 3.2.2: Erection II

OBJECTIVE/SCOPE

To give more detailed information on the technical matters involved in steelwork erection

introduced in Lecture 3.2.1.

PREREQUISITES

Lecture 3.2.1: Erection I

The following lectures might be helpful:

Lectures 3.1: General Fabrication of Steel Structures

Lecture 3.3: Principles of Welding

Lecture 3.4: Welding Processes

RELATED LECTURES

Lecture 3.2.3: Erection III

Lecture 3.5: Fabrication/Erection of Buildings

Lecture 15A.8: Offshore: Fabrication

Lecture 15B.12: Introduction to Bridge Construction

SUMMARY

This lecture discusses the technical aspects of steelwork erection such as unloading and

handling of materials, foundation checking and adjusting for errors, assembly procedures,

and final bolt-up of connections.

1. INTRODUCTION

Lecture 3.2.1 outlined an ideal approach to erecting steelwork emphasising the need for a

Technical Specification for Erection and good site organisation in order to pre-empt

possible erection problems when the steelwork arrives on site.


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This lecture discusses the on-site activities in more detail. It points out that careful

attention to detail is required to ensure that the specification is adhered to and that good

practice is maintained on site at all times. The operations are viewed sequentially, from the

unloading of the steel elements, through checking and adjusting for foundation errors, to

erecting and bolting together the final structure.

2. ERECTION PROCEDURES

2.1. Reception, Unloading and Handling of Construction Material.

Logistics is a very important part of the erection process and must be constantly kept in

mind.

Fabricators have a strong tendency to deliver the construction elements in the same

sequence as they are fabricated; this is satisfactory provided the fabrication sequence is

similar to the erection sequence. For this purpose detailed planning of the fabrication and

erection processes is required.

It is necessary, therefore, to prepare a delivery schedule, coordinated with the capacity of

the site erection team. Generally the site will have a storage area to allow for times when

the deliveries cannot be exactly coordinated. If possible a "just-in-time" delivery should be

organised for the heavier construction elements in order to avoid the costly manoeuvres of

unloading and intermediate handling.

Any damaged material, which should be repaired or returned, must be separated

immediately after arrival. The project manager must be informed, in order to allow him to

assess what effect this might have on the construction programme.

When handling individual members the following should be noted:

parts with different reference marks must not be mixed or packed together.

on larger or heavy parts the lifting points must be indicated, including where

possible the location of the centre of mass of the component.

all parts that can be easily damaged must be adequately protected during transport

and handling.

components in the storage area must be kept off the ground, protected from damp

and dirt by means of sleepers or battens.

When using steel wire cables as lifting slings the following should be noted:

care must be taken to ensure that the sharp corners of beams are packed with wood

in order to protect the sling, the surface treatment and any prepared edges.

slings must be regularly inspected, and the maximum safe load must be shown on a

steel tag.

when using double-leg slings it must be appreciated that the lifting capacity

diminishes as the angle between the legs increases; the load in the legs can be

calculated from:


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R = P/2 x 1/cos

where R - is the load in a leg

P - is the total leg

- is the half-angle between the legs of the sling.

the maximum safe load should not exceed 1/6 of the breaking load of the steel wire

cable from which the sling is made.

the hoisting slings should be regularly inspected and taken out of service as soon as

the number of broken wires in a metre's length reaches or exceeds 20% of the total.

the slings should not be bent over diameters less than six times the diameter of the

cable. If necessary rounded cushions must be put in place to support the sling.

For the transport and delivery to site of bolts, nuts and washers the following should be

noted:

they must be provided with adequate packing, sufficiently protected to avoid

damage during transport.

in the case of high strength bolts, both bolts and nuts must be adequately greased.

each package shall contain only one type of bolt, nut or washer of the same

diameter, length and quality.

each package shall be provided with a label, indicating the manufacturer's mark,

the type (bolt, nut and washer), the quality and the number.

high strength bolts are indicated by coloured labels, according to the type and

quality (4,6 = green, 8,8 = red, 10,9 = blue).

Regarding the storage and use of welding consumables, the following should be noted:

the electrodes must be packed in closed and sealed packages or tins; storage must

be in a closed and dry place at a temperature high enough to prevent condensation

and special care must be taken to avoid damaging the coating.

when basic-type electrodes are used, they must be dried or baked in accordance

with the welding procedure.

once the sealed packets have been opened, the electrodes must be placed in a

portable drying oven.

mobile drying ovens must be provided close to the welding area; the welder

himself must be provided with an insulated quiver in which he keeps the

electrodes, taken from the nearest drying oven.

in very special cases and in very damp conditions the welders should be provided

with individual portable drying ovens, from which they remove the electrodes one

by one, as they need them.

For general purposes a complete list of erection elements must be available on site

indicating their code-number, weight, size, etc., and specifying the locations where they

are to be used.


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2.2 Foundations and Base Plates (Levelling, Measuring, etc.)

The columns of a steel framework transfer their load to the foundation by means of the

base plates. The foundation bolts constitutes the unifying element between foundation and

framework.

In cases where the column transfers compressive stress only (theoretically no foundations

bolts are needed) the bolts are used to locate the column correctly.

Anchor bolts are either put in place before the concrete is poured, are drill fixed afterwards

in the hardened concrete, or are placed in openings, left in the foundation, which can be

filled later.

An inspection of the foundation, to check the levelling and alignment of the anchor bolts,

must be made before erection commences. A fixed levelling point and three fixed

alignment points are generally established for this purpose. Errors in the concrete

foundation identified at this early stage, can be easily corrected using packer plates.

The foundations must be cleaned prior to erecting the steelwork. It must be ensured that

the cavities for the holding down bolts are completely free from contamination.

2.3 Assembly and Erection

The erection of the steel frame can start after the packer plates (or the base plates) are in

position.

The main aim during erection is to maintain the stability of the structure at all times.

Collapse of structures during erection is often due to lack of understanding of the stability

requirements.

Stresses can be reversed during erection, and every reversal, no matter how transient, must

be considered in the design.

Questions concerning the construction sequence and its effect on stability must be

resolved. The designer should position the braced bays in a way that ensures that they are

the first to be erected. It is essential that the structure is braced and true as the erection

proceeds.

The use of sub-assembled units is a way of reducing the amount of work to be done at

height. There are, however, some factors which affect the practability and economy of

sub-assembling a unit on the ground. The first is the weight of the eventual assembly,

including any lifting beams; another is the degree to which the unit is capable of being

temporarily stiffened without unduly increasing its weight. The bulk of the unit is also a

significant factor as fouling the crane jib must be avoided. It is often necessary to make a

drawing of the crane jib and the sub-assembled unit at the point of highest lift to check

practicability.

Sub-assembly is only worthwhile if the unit can be lifted and bolted in a reasonably easy

way. The object is to save operations at a height which could readily be done at ground


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level; having to loosen and retighten bolts to remove twist, therefore, makes sub-assembly

much less attractive.

Most steelwork arrives on site pre-painted. The paint treatment may be damaged, by the

steel slings, during handling; the damage should be minimised by the use of softwood

packers which will also ensure that the load will not slip as it is being lifted and that the

slings - chain or wire - are not themselves damaged, as they bend around sharp corners

Packers to prevent slipping are even more necessary if the final position of the

construction piece being erected is not horizontal. The aim should always be to arrange the

slinging in such a way that the piece hangs at the same angle as that which it will assume

in its erected position.

Pieces being lifted are usually controlled by a light hand line fixed to one end. This line

controls only the swing of the piece, and is not intended to be used to pull it into level.

When lifting large and heavy parts from a horizontal position to a vertical one, temporary

tiebacks must be used to avoid uncontrolled movements when the part is getting close to

the vertical position.

Some situations may require temporary stiffening to be left in position after the initial

erection and until the permanent connections are made. The need for temporary stiffening

should be foreseen in the erection plan, so that sufficient stiffeners and lifting devices are

available and no delays occur due to shortage of devices for the erection of the next sub-

frame.

Where a particularly awkward or heavy lift has to be made, it may be simpler and safer to

fabricate special cleats for this purpose. A small amount of additional effort in the drawing

office and workshop can save much time and money on site.

2.4 Bolting Connections on Site

Before carrying out any bolted connections checks must be made:

the bolts, nuts and washers to be used must be exactly as indicated on the

drawings.

bolts, nuts and washers must be clean and undamaged.

the parts to be connected must be clean and without defects.

The use of flame cutting to enlarge holes should not be permitted since it will result in an

unacceptable connection and will damage the paintwork.

In connections with tapered flanges, tapered washers must be placed under the nut, the

bolt, or both.

In the case of a hole with a vertical axis, the bolts should be inserted from above, with the

nut at the bottom.

Where specified, the nuts should be secured against loosening by an extra nut or by

applying a special nut or washer.


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2.4.1 Ordinary bolts

Hexagon-headed bolts and nuts are normally available in a range of sizes, and tensile

strengths. Washers were traditionally used under the nut but are now frequently omitted.

The strength grades most commonly used for structural bolting are 4.6 and 8.8, the former

in general applications, the latter where more severe loading applies.

Bolts are normally installed hand-spanner tight, in 2 mm clearance holes for diameters up

to 24 mm, and 3 mm clearance holes for diameters over 24 mm. Where exact location and

prevention of relative movement between the joined parts is required, accurately machined

bolts, fitted in reamed holes, are used. These bolts require precision work and are costly to

install. Where rigidity is required pretensioned HSFG bolts are normally used.

2.4.2 High-strength friction-grip (HSFG) bolts

In HSFG bolted joints the shear load is transferred between the connected parts by

friction. The friction force is provided by the clamping action of the bolts, which are

tightened in a controlled way to provide a specific shank tension. The bolts are installed in

clearance holes and thus there may be no bearing action in transferring the load.

To make practical use of the friction effect, it is necessary to use high-tensile bolts so that

an adequate clamping force can be obtained with reasonably sized bolts. The stress

induced in the bolts by the pretensioning is at, or near, the proof stress.

Two strength grades of parallel shank bolts are available, the General Grade (equivalent to

8.8) and the Higher Grade (equivalent to 10.9). Nuts are designed to develop the full

strength of the bolt. Hardened washers are used under the element which is to be rotated

during tightening.

In order to mobilise the friction effect it is necessary to develop the required bolt

pretension. This may be done either by controlled tightening of the nuts, using torque-

control or part-turn methods, or by the use of load-indicating devices; these may be special

bolts, special fasteners or load-indicating washers.

2.4.2.1 The torque-control method

For this method of tightening a calibrated torque wrench is required which may be hand

operated or, for larger bolt diameters or large numbers of bolts, power operated. It is

essential to check the tightening equipment in combination with the bolts and nuts to be

tightened very regularly, using special prestress-measuring devices.

A certain deviation in the shank tension must be expected: estimates of the result of

tightening, with the objective of achieving a minimum shank tension of 80% of the

specified tensile strength, have shown that approximately 90% of the bolts would be

tightened satisfactorily.


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2.4.2.2 The part-turn method

This method uses the ductility of the bolt material by rotating the bolt sufficiently to take

the bolt well into the plastic state in which the shank tension is comparatively insensitive

to further nut rotation. The maximum shank tension that can be obtained is equal to the

maximum torqued-tension strength of the bolt under the friction conditions occurring at

the time of tightening. Care must be taken with short bolts and with parallel shank bolts

which have only a small amount of thread in the grip.

The part-turn method is not allowed with Higher Grade (parallel shank) bolts. It is not

recommended also for use with M12 bolts.

2.4.2.3 Load-indicating devices

A variety of special load-indicating bolts are available, some simple and some

complicated. A simple device is the load indicating washer, which has a number of

protruding nibs on one surface. As the nut is tightened the protrusions are crushed; when

the gap between the load indicator and the bolt has reached a prescribed value (measured

by a feeler gauge), the required shank tension will have been achieved.

2.4.2.4 Installation

HSFG bolts are installed in drilled holes with 2 mm clearance for bolts under 24 mm

diameter and 3 mm clearance for those over 24 mm. The holes must be sufficiently

aligned so that the bolts can be inserted freely. A hardened steel washer is used under the

nut or head, whichever is to be rotated.

Where there are a number of bolts in a joint they should be tightened incrementally in a

staggered pattern.

Successful achievement of the specified shank tension depends on the threads being in

good condition. Bolts and nuts must therefore be stored and handled in a way which

ensures that the threads are not damaged or contaminated. For a fastener to be in a usable

condition the nut must run freely on the bolt thread.

Bolts which have been tightened using the part-turn method must not be used again; this

restriction also applies to bolts tightened using torque-control methods if plastic

deformation has occurred.

Heating of the bolt itself or heating of the surrounding surfaces, possibly resulting in

heating of the bolted connection, can result in the destruction of the integrity of the

connection and to the failure of the construction, and must be prevented at all times. Any

welding operations, therefore, must take place before the bolt is tightened.

3. CONCLUDING SUMMARY

The good practice guidelines, listed in Section 2.4.1, should be followed when

handling and storing materials on site.


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Anchor bolts are used to attach the steel columns to their foundation. Errors in

foundation level can be corrected by using steel packers under the baseplate.

Stability must be maintained at all times during erection and the erection sequence

should be arranged to ensure this.

Sub-assemblies can be used with advantage to reduce work at height.

Bolted connections use either ordinary or HSFG bolts. The latter can be

pretensioned either using the torque-control, part-turn, or load-indicating washer

methods.

4. ADDITIONAL READING

1. Arch, H., Erection, Chapters 33 - Steel Designers' Manual, Oxford, Blackwell Scientific Publications, 1992.

2. Thornton, W. A., "Design for Cost Effective Fabrication and Construction", Chapter 7.1 Constructional Steel Design, London, Elsevier Applied Science, 1992.

3. Miller, E. W., "Erection of Steel Structures", Chapter 7.2 Construction Steel Design, London, Elsevier Applied Science, 1992.

4. Potter, P. D., "Fast Steel Erection": Steel Fabrication Journal, No. 46, February 1983, Australian Institute of Steel Construction.

5. The Steel Construction Industry Code of Practice for Sage Erection of Building Steelwork: Part 2 - Multi-storey Buildings and Structures, Australia, Australian

Institute of Steel Construction, 1989.

6. Eurocode 3: "Design of Steel Structures": ENV 1993-1-1: Part 1.1: General Rules and Rules for Buildings, CEN, 1992.

Erection I

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Transcript of Erection I