Steel erection

Erection techniques

Cranes and MEWPs (Mobile Elevating Work Platforms) are predominantly used for the erection of structural steelwork for buildings and bridges in the UK, although other techniques are sometimes used for steel bridge construction. Generally, cranes may be divided into two broad categories, mobile and non-mobile. The first category includes truck mounted cranes, crawler cranes and all-terrain cranes, whilst the second category primarily covers tower cranes.

MEWPs are used to access the steelwork during erection, i.e. to bolt-up the pieces being lifted in by the crane. However, the MEWPs themselves can be used both on the ground or on the partly erected steelwork to erect lighter steel elements directly provided special measures are taken to support the MEWP (e.g. steel sections to act as rails supported on the partly erected steel). Also the steelwork will need to be checked that it can support the weight of the MEWP.

Mobile cranes

Normally, truck mounted cranes do not require a back-up crane for site assembly, and require very little set-up time. These two attributes mean that they are suitable for one-off, single day commissions. Their main drawback is that to achieve a high lifting capacity from a light vehicle, a larger footprint is required than for an equivalent crawler crane. The size of the footprint can be increased using outriggers, but good ground conditions are necessary to provide a solid base and ensure adequate stability.

Crawler cranes are more rugged than truck mounted cranes. Ground conditions are therefore less critical. Crawler cranes may travel with suspended loads on site, because they are stable without the use of outriggers. They also have a relatively high lifting capacity. Daily hire is not possible for crawler cranes, because transportation to and from site is expensive, and they require site assembly. They are however more competitive than truck mounted cranes for long periods on site in a relatively fixed location.

Typical mobile cranes, be they crawlers, truck mounted cranes, or all-terrain, have a rated capacity of around 30 t to 50 t. The largest examples are rated at over l000 t. However, actual lifting capacity is a function of radius, and may be much less than the rated capacity for a given situation. ‘Heavy-lift’ rigs can be used to increase the capacity of large cranes for one-off applications.

Tower cranes

Tower cranes must be assembled on site, because of their size, and this operation often requires a second (usually truck mounted) crane. Set-up, and similarly dismantling, is therefore expensive. They also have a relatively slow lifting rate, which means they are only used when site conditions preclude an alternative. A further consideration when specifying a crane is that tower cranes are ‘vulnerable’ to wind loading, which may prevent crane use at times. Their advantages are an ability to lift to greater heights than a mobile, and to lift their rated capacity over a significant proportion of their radius range. Crane geometry means that a tower crane can be erected close to, or within, the building frame. A tower crane may even be tied to the building frame to provide stability as height increases. Alternatively, climbing cranes may be used. These are supported off the steel frame itself.

Typical erection rates

Typical erection rates, and hence the site programme are highly dependent on the number of crane lifts which are needed. To reduce this number, maximum use should be made of pre-assembled units. Alternatively, if crane availability is a problem, the use of steel decking, which can be placed by hand, is preferable to precast concrete units requiring a crane for individual placement. A ‘piece count’ is a useful way for the designer to assess the number of lifts needed and hence the erection duration. An example is given in SCI-P178.

Lining, levelling and plumbing

Lining, levelling and plumbing consists of an interaction between the site engineer using the survey instrument and the erection gang doing the final bolt tightening and shimming. By the progressive use of wedges, jacks, pull-lifts and proprietary pulling devices such as Tirfors, the erection gang persuades the frame to move to a position acceptable to the checking engineer and then bolts it up firmly. Some lack-of-fit is overcome in this process, and some is created. If the latter is adverse, local corrections are made. The team rarely returns to a frame once that it has been checked, plumbed and bolted up.

Tolerances

The aim of the Essential tolerances specified in BS EN 1090-2 is to ensure that ‘as built’ imperfections are no greater than those assumed in the structural design calculations. Compliance guarantees that frame deviations will not cause secondary forces greater than those allowed for in the design. It also guarantees that lack of fit between the frame members will not be excessive. Limited lack of fit can be accommodated using appropriate packing, without adversely affecting the performance of the connections. Compliance with BS EN 1090-2 does not ensure that the frame components will fit together within an envelope which is suitable for the other building components. Secondary systems are required to accommodate cladding systems which may require tighter tolerances that the steelwork for the main structural frame.

Site bolting

Structural bolting practice (for buildings) in the UK is based predominantly on property class 4.6 and 8.8 non-preloaded bolts to BS EN 15048[2], generally used in 2 mm clearance holes. The recommended option of M20 8.8 fully threaded bolts is readily available. Property class 4.6 bolts are generally used only for fixing lighter components such as purlins or sheeting rails, when 12 mm or 16 mm bolts may be adopted. Generally, only system HR bolts are used in the UK, as recommended in the NSSS

Site welding

Site welding is not normally preferred if a suitable bolted connection is possible. When site welding is adopted, provision must be made for protection against inclement weather, and good access is needed for both welding and inspection. Providing such protection and access may have programme implications, as well as the direct costs involved.

Erection handover

Demonstration of compliance using a full three-dimensional survey of the complete structure as a final acceptance test is not practical, because of difficulty, time and expense. Neither is it necessary if the purpose is to ensure the stability of the frame. When tolerances are satisfied over a representative part of the frame, deviations in the rest of the frame can be assumed to be acceptable based on a visual inspection alone.