ESDEP WG 3
FABRICATION AND ERECTION
To give more detailed information on the technical matters involved in steelwork erection introduced in Lecture 3.2.1
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
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
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.
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.
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.
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:
When using steel wire cables as lifting slings the following should be noted:
R = P/2 x 1/cos a
where R is the load in a leg
P is the total leg
a is the half-angle between the legs of the sling.
For the transport and delivery to site of bolts, nuts and washers the following should be noted:
Regarding the storage and use of welding consumables, the following should be noted:
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.
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.
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 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.
Before carrying out any bolted connections checks must be made:
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.
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.
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.
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.
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.
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.
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.