Erection - Steel Structure: Planning.

1 Design information
Construction planning should start at the design phase since decisions at this stage will dictate the performance of the fabrication and erection teams. Site erection performance depends on many external and internal factors, and it is essential that these areas are properly managed.The site operation process is a complex interrelationship of other disciplines which influence performance, as shown in Fig. 33.1.

The use of value engineering has produced spectacular improvements in on-site performance by identifying the cost of alternative methods and  highlighting the need to minimize change in the site erection programme. The precise methodology it offers provides a unique vehicle for the incorporation of a wide range of construction expertise into the design process, with the ultimate objective of achieving building designs and construction which offer clients better value for money.

 Chart showing the dependence of the erection process on other disciplines
Fig. 33.1 Chart showing the dependence of the erection process on other disciplines

2 Programming
Resources can be best utilized when the delivery and erection operations are well planned to follow a logical sequence.Typically, civil and other trades need to be sufficiently ahead of the steel erection (i.e. one week minimum) to allow for a reasonable flow of uninterrupted work. This will allow the steelwork contractor time to survey the foundation bolts prior to accepting them. It is not unusual for the civil contractor to have to carry out minor remedial work due to poor alignment of the holding-down bolts, and to this end it is advisable to allow the maximum time possible for handovers. Accurate positioning of foundations is essential, and tolerances defined in the specification and the National Structural Steelwork Specification (NSSS) should be strictly adhered to.

Access restrictions or phasing of the works often govern the sequence of  erection, which normally follows a grid pattern, split into zones. The siting of the erection crane will also dictate the construction pattern. Once  the sequence and phasing have been agreed, the steelwork contractor can determine the resources required to meet the programme.

3 Delivery and off-loading of steelwork
A construction sequence programme is agreed prior to delivery of any materials to site, from which a delivery schedule may be produced. A more detailed piece-by-piece schedule for the steel erection method will need to be carried out to ensure smooth working on site. Particular attention should be paid to the logistics of this exercise to prevent members being missed out, double handling of materials on site, lorries being under-utilized, and the positioning of steel in the wrong location.

Delivery of the steelwork is normally in 20-tonne lorry loads, often sub-divided into four bundles of 5 tonnes each to suit tower crane capacity, on a ‘just in time’ delivery sequence in order to alleviate site congestion, and minimize double handling and on-site damage. Material needs to be stacked in the lorry for easy off- loading, with items that are needed first, readily available. Often the loading of thelorry can be dictated by crane off-loading capacity or stability of the load as columns are usually required first, yet they need to be at the bottom of the load because of their weight.

Shakedown areas (areas for separating and selecting steel bundles) are required on site to allow the sorting of the steelwork.The area should be firm and level with a plentiful supply of wooden sleepers or other suitable material.The steel members will have been allocated a unique code related to section, level and piece number: this is usually stamped on the top surface to identify its orientation. In multi-storey high-rise buildings it is usual to create temporary shakeout platforms at various stages of the construction to minimize crane hook time.

Cold-formed roof members and steel decking packs also need to be placed in location as the work proceeds since crane access is often restricted later. Each bundle should be numbered and placed in convenient locations as subsequent  relocation may prove difficult and costly.

4 Sub-assemblies on site
The designer may have been limited by the size and weight of components for transportation: thus it may prove advantageous to erect canopies, roof trusses, lattice girders and the like ‘piece small’ and then lift them as completed components.

If the decision to sub-assemble has been made there will remain the further question of deciding whether an area in the stockyard is to be dedicated for this purpose or whether the work will be done behind the cranes on the erection front in order that the assembly can be lifted straight off the ground and into position as the crane is moved back.

The most common components to be assembled behind the crane are roof trusses or lattice girders, which, because of their size, are almost always delivered to site ‘piece small’. However, there is not always space for this to be done and other locations, such as the stockyard, have to be made available.

Where the potential sub-assembly area, and even the stockyard itself, is remote from the erection site, very careful investigations are needed before a decision to sub-assemble can be agreed. Local transportation size restrictions on the route to the erection cranes may rule this option out.

There are three factors which affect the practicability and economy of subassembling a unit on the ground:

(1) The weight of the eventual assembly, including any lifting beams required
(2) The degree to which the unit is capable of being temporarily stiffened without unduly increasing its weight
(3) The bulk of the unit, i.e. will it be possible to lift it to the height needed without fouling the crane jib?

Sub-assembly is only worthwhile if the unit can be lifted and bolted into place almost as easily as a single beam. In other words, the object of  the exercise  is to avoid carrying out operations at height which can easily be done at ground  level.

The speed of construction is dependent on the number and type of crane lifts and connection.The rate of erection will also vary according to the weight, size and location of the piece being erected. Erection speed is often dictated by the distance between the crane and hook, thereby emphasizing the need to consider lifting bundles closer to the erection face and providing shake-out platforms.

It is impossible to provide a chart showing piece counts per hour or day.However, as a broad rule of thumb, an experienced erection crew working in ideal conditions  at low level with average weight, standard-sized components should be able to erect 40 to 60 pieces per day per hook.

5 Interface management
In order to minimize on-site disruption, the specialist/trade interface must be managed successfully. Failure to understand the requirements of  other trades through unclear specifications and uncertain allocation of responsibility may result in conflict and disagreement. Thus the development of open communication between all parties and the establishment of clear objectives during regular site meetings between overlapping trades are necessary measures to ensure proper component, plant, and work area management. Unfortunately, these measures  constitute some of the most neglected areas in the construction process. Failure  to adequately address these issues can result in cladding fixings being left off or located in the wrong place, holding-down bolts being incorrectly positioned, lift shaft tolerances proving incompatible with the main frame, and so on.

Thus co-operation between all parties is necessary to ensure enhanced efficiency.

This may be achieved via:

(1) An improved communication process
(2) A clear contractual definition
(3) The use of single-source suppliers.

6 Surveying and aligning the structure
Surveying should be carried out in accordance with BS 59644 (1990) Parts 1 to  3  Building setting out and measurement, and adjusted for temperature outside  the range of 5°C to 15°C. Accurate setting-out is crucial for maintaining control  of tolerances and achieving a structure which is acceptable to  all the subsequent  trades.     

The site should be provided with a dumpy or quick-set level and with a simple theodolite. On larger sites, it is common practice to use an electronic digital  measurement (EDM) instrument. The advantage of a theodolite is that the transit of the instrument can later be used for checking the plumb of the columns. The
telescope can be employed for the reading of offsets to check their alignment.Time spent on laying down and marking the cross-centre lines of the  column bases on individual boards at each base and subsequently on to the concrete of the base is time very well spent. The base can then be quickly and accurately set in both  directions and to the correct level on the pre-set pads.The value of this work is that it enables errors in the concrete work to be identified at a time when corrective measures can be undertaken.The accurate setting of grouted packer plates, on which to land the column, serves the same purpose, i.e. it will facilitate the identification of concrete which is high and holding-down bolts which are low.

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