2. SETTING OUT AND TOLERANCES

 

Before steelwork setting out or erection starts, carefully study the Specification, the general arrangement drawings and as many fabrication drawings as are available. This in-depth study and discussion with the engineering staff on site is to decide a method of dimensional control starting from the ground. Study the critical points, for instance a transition at first floor cantilevering beyond the ground floor columns and then rising vertically from the end of the cantilever. The critical point here is the alignment and level of the end of the cantilever rather than the ground floor columns.

    Elastic compression in the stanchions is very significant in tall steel

structures. Consult the designer on this matter, as floor datum-levels can

vary considerably as the structure is loaded out, and allowances are needed if this is critical to fit up. Similarly on bolted structures, 'take-up' of clearance holes in adjacent bolted connections, combined with deflection of the surrounding steelwork, can affect the levels of floors and especially cantilever beams when loaded out.

    The steelwork sub-contractor must provide a method

statement of setting out and controlling the structure so that all concerned know how the job is being controlled.

    It is normal on most contracts for the main contractor to provide the

main grid lines and main setting out points. The specialist steelwork subcontractor normally carries out his own detailed setting out based on these. Ensure that the steelwork sub-contractor's setting out is properly

checked.

    The essential object in setting out is ACCURACY in line, level and

plumb to meet specified tolerances. The effort involved in achieving a greater degree of accuracy involves greater cost. Before the job starts it is important that the degree of accuracy to be achieved is agreed between the Company and the Employer, and this should be included in the contract documents. Any relaxation of accuracy and tolerance should not exceed that allowed in BS . Reasonable tolerances are given in the following table:

             Permissible deviations (PD)

(a) Position                                                                                                      mm

       PD between designed and actual positions of the first

       erected column.                                                                                        +10

(b)  Linear dimensions. PD from the designed length

       or width of any structure:                                                                       ±10                                                                      

       Up to and including 8m.                                                                         ±15

       Over 8m up to and including 15m.                                                       ±20

       Over 15m up to and including 25m.                                                     ±25

       Over 25m.

(c)  Plumb

       Vertical PD above base of column:

       Maximum in 30m height.                                                                        ±15

       PD to be rounded to the nearest 5mm.

d)   Levels

 

Unless otherwise specified, the level of a beam

should be taken on the top of the upper flange

PD between the designed and actual levels of the

base of the first column to be erected.                                                         ± 5

PD between designed and actual levels above the

transferred bench mark of any beams (in any 30m

length) at the junction with the column.                                                      ± 15

PD between designed and actual levels of any beam

at its junction with a column-to take account of

tolerances  and in relation to

the TBM of the storey in which the beam is located.                                 ± 10

PD between designed and actual levels of the upper

or lower surfaces of two or more beams meeting at

a column.                                                                               ± 5

 

Keep records of any Company/Employer agreements together with site

records of actual levels, setting out dimensions and plumb of elements of

the building. This helps to avoid claims after hand-over for poor function

due to inaccuracies caused by elastic compression in the structure, take-up of bolted connections etc.

    Steelwork is generally erected speedily. A mistake unnoticed until several storeys have been erected may mean drastic action to realign or, as a last resort, dismantling and re-erecting. This can be very costly in manpower, plant and delays. Therefore set out, check and re-check to make sure.

    Level in exactly all baseplates at substructure level from a common fixed TBM within the site. Do not level one baseplate in to another. It is no use repeating or compounding a mistake, especially when baseplates of the same nominal thickness may vary due to rolling margins.

    Check that baseplates are set on shims of varying thickness to give the

exact level at under-side of baseplate. Hold the shims in position with

a haunching of epoxy mortar or similar so that they do not move prior

to or during erection of the baseplate and stanchion. Levels of baseplates

are important: an error at this point will repeat throughout the whole.

structure.

     Check that stanchion baseplates are aligned exactly on the Grid Lines.

It is no use having the stanchion level and plumb if it is twisted in plan: