5. GENERAL - ERECTION
Whilst steelwork erection may be regarded as the final stage of fabrication, it differs from the latter in two principal ways: firstly, there is the added dimension of height and the time occupied by vertical movement of materials, equipment and labour; secondly, the fact that work has to be carried out in the open means that progress may be hampered by adverse weather.5.1 Site Planning
By its nature, work done on site can become unduly expensive. The primary aim of the programme should be to minimise costs by condensing the time scale realistically. Options and alternatives need to be carefully examined at the preliminary design stage otherwise the scope for reducing the time scale may be unduly restricted.
Clearly the significance of the various issues will vary according to the type of building and any limitations which the site and its environment may impose. Even when structures possess marked similarities, different erection methods and procedures may need to be adopted. For this reason, only the broad principles concerning erection can be stated.
Invariably, erection of structural steelwork has to be closely integrated with other major trades such as flooring, cladding and services. Operations on site where there may be competition for limited resources, are potentially difficult to control. A far-sighted strategy has to be developed and maintained.5.2 Site Organisation
Key objectives and, most importantly, starting and finishing dates must be clearly established and progress reviewed on a regular basis. Failure to meet commitments can result in substantial cost penalties. Further complications may easily arise which are totally disproportionate to the cause.
The maximum size and weight of the various steel members which can be delivered may be restricted on a site with limited and restricted access.5.3 Setting Out
Narrow streets in a busy town centre may cause difficulties with space to manoeuvre. Waiting time to off-load may also be restricted to specific periods. Matters of this kind must be investigated well in advance and decisions made accordingly.
Within site, movement may often be hampered by a variety of obstructions such as scaffolding, shoring, pile caps, excavation, and so on. Service roads and off-loading areas need to be hard cored and adequately drained to support heavy vehicles during the severest winter conditions. The steelwork has to be erected in the general sequence determined by the construction programme. Each consignment of steel has to be strictly regulated to this timetable. Whilst in some instances, a few key components can be lifted directly from the vehicle into position, most of the material will need to be off-loaded and stacked temporarily until needed.
The area of the site allocated for this purpose has to be orderly and well managed, particularly where space is limited. To compensate for minor interruptions in delivery, for example due to traffic delays, a small buffer stock is usually held in reserve.
Space is also required for laying material out and for assembly of frames or girders prior to hoisting into position.
Before commencement of erection, the plan position and level of the column bases should be verified by the erection contractor. This needs to be carried out as soon as possible to ensure that any errors can be corrected in good time or, at least, alternative measures approved and introduced.5.4 Operations
Checks should include not only the centres of the foundation bolts relative to the reference grid lines, but also the projection of the bolts above the base level.
To compensate for minor discrepancies, a limited amount of deviation of the column from its true vertical and horizontal position is provided for by the grout space under the baseplate and by leaving a movement pocket around each bolt during pouring of the concrete. Normally this will allow latitude of about ±25mm in any direction.
Steel erection may appear to be a series of distinct operations when in reality they overlap and merge. Nevertheless, each complete stage of the work has to follow a methodical routine which consists of:
Because minor dimensional inaccuracies can accumulate during fabrication and setting out, it would be impractical to complete the entire structure before compensating for these by adjustment. The work is therefore sub-divided into a number of phases which may be controlled by shape or simply by an appropriate number of bays or storeys. For stability, each phase relies upon some form of restraint to create a local box effect. This effect may be achieved in various ways, such as employment of temporary or permanent diagonal bracing.5.5 Single-Storey Buildings
Initially, end connections and base anchorages are only secured temporarily. After completion of plumbing, lining and levelling, all connections are then made permanent by tightening up all nuts or inserting any bolts initially omitted to assist adjustment. This process allows substantial areas to be released quickly for grouting and following trades are able to proceed much earlier than would otherwise be possible.
Under normal circumstances, single-storey buildings are quickly and easily erected. A high proportion of industrial buildings are rigid jointed. It is common practice to bolt, assemble or weld these joints on the ground and then lift the complete frame upright using a mobile crane.
Lattice girders and trusses are also erected in a similar manner but temporary stiffening may be required to prevent lateral buckling. Care should also be taken, by provision of lifting eyes or similar at specific positions, to ensure that slender members are not subjected to undue compressive stresses.
Ideally, erection should commence at an end which is permanently braced. When this is not possible, temporary bracings should be provided at regular intervals as a safeguard against collapse or deformation (Figure 7).
Space frames are designed to span in two directions. Because of the number of connections required, it is much more economical to assemble the modules at ground level where the joints are readily accessible and then hoist the complete framework. Two or possibly four cranes may be needed depending on the size of the building. Meticulous co-ordination is essential.5.6 Multi-storey Buildings
In most cases, multi-storey buildings are erected storey by storey enabling the lower floors to be completed earlier, offering access, overhead safety and weather protection. Depending upon the site, a single tower crane may be the sole lifting facility. In this case use of the crane has to be shared between a number of sub-contractors, thereby limiting available "hook" time for any given trade.
Since the position of a tower crane is fixed (Figure 8), it is completely independent of any obstructions, such as basements or ground slabs, which could deny access to a mobile crane. This independence allows useful freedom in overall planning. However, the fixed location also means a fixed arc of lifting capacity where the load will be minimum at the greatest reach. As a result the steelwork may have to be provided with site splices simply to keep the weight of the components within such limits.
One of the major virtues of a mobile crane (Figure 9) is its flexibility and independence which enables it to keep moving with the flow of the work. These cranes are generally fitted with telescopic jibs which allow then to become operational very quickly. The vehicles are stabilised during lifting by extended outriggers equipped with levelling jacks.
Whilst permanent stability in the completed building may be introduced, in a number of ways, including braced bays, rigid joints and stiff service cores (Figure 10) and via diaphragm action of the floors, stability must also be ensured throughout the entire construction programme. It may therefore be necessary to install temporary bracings solely for this purpose, which must not be removed until the permanent system has been provided and has become effective.
The rate of steelwork erection is governed by a wide range of factors some of which are beyond the influence of the design engineer. The factors which he can control include:
Simple connections for shear force are straightforward and employ Grade 4.6 or 8.8 bolts. The bolt diameter should be selected with a degree of care. For example, whilst a single M30 bolt has more than twice the shear capacity of two M20's, the effort required to tighten an M30 bolt is some 3½ times greater. An M20 bolt can be tightened without difficulty using ordinary hand tools, a considerable advantage when working at height.5.8 Safety
Joints which are required to transmit bending moments are inherently more robust and may require stiffening ribs and haunches; if this is the case careful attention is required to ensure access for the bolts. For such applications pre-tensioned bolts are often used. They are normally tightened to a minimum torque using a power operated wrench.
Compared to bolting, the site welding of joints is time-consuming and expensive for conventional structures. There may be occasions, however, when site welding is the only realistic way to form a joint, as, for example, in alterations or remedial work. In this case, joint preparation, fitting, inspection and the provision of purpose made enclosures (for access and weather protection) are additional cost factors that must be taken into account.
As a rough guide, about 50% of erection man hours are occupied with lining, levelling, plumbing and final bolting and the remainder of the time is spent hoisting members into position. However, in suitable cases, beam and column elements may be pre-assembled at ground level and lifted directly on to their foundations.
The erection of a building framework is potentially hazardous. Many serious and fatal accidents occur each year on construction sites and most of these are caused by falling from, or whilst gaining access to, heights; handling, lifting and moving materials, however, are also hazardous.
Risks can be minimised considerably by measures such as adequate provision for stability throughout construction, accessibility of splices and connections, guard rails and attachments for safety harnesses and so on.
In addition, safety, need not be compromised on grounds of cost. For example, it will prove cheaper to assemble frames at ground level (Figure 11) rather than bolt them together in mid-air. Metal decked floor systems are not only economical but offer rapid access for all trades whilst providing overhead protection. Safer access is also promoted by the immediate provision of steel stair flights at each floor level as steelwork erection proceeds.
Current and future legislation may place greater responsibilities upon the design engineer because of the influence of design and details on the method and sequence of erection.