Erection - Steel Structure: Site practices.

1 Erection sequence
Columns are placed in grids to suit the grid/zone areas as previously agreed. The columns are lifted with a Dawson Ratchet (see Fig. 33.2), nylon slings or chains.
Care must be taken to minimize damage to painted surfaces as repairs can often be difficult. Once placed into position the column is roughly positioned and plumbed. In extreme cases the columns may need guying off to ensure stability during the erection sequence. This procedure is repeated until a grid is formed.

Beams are then placed in location and secured by two bolts at each end connection throughout the designated area. Once all of the beams are located, the remaining bolts are placed. Since many structures employ composite slab construction, an intricate part of the site erection is the placing and fixing of the decking bundles.A grid system should have been agreed for the placing of bundles which the erection crew situates in convenient locations to ensure that connections are not obstructed, nor the structure eccentrically loaded. Heavy wires are then placed and left in position.

When this is achieved the erection crew can move on to the next area.The engineer with the line and levelling crew then follows on behind, pulling the structure to its correct alignment. Once this task is completed, the bolting-up crew can tighten all of the bolts and complete.The decking is then placed, followed by the stud welding.

Dawson Ratchet
Fig. 33.2 Dawson Ratchet

A typical flow chart of the process is shown in Fig. 33.3.

Flow chart of the erection process
Fig. 33.3 Flow chart of the erection process
2 Lining, levelling and plumbing
If, in spite of having taken care over the initial levelling of packers and carefully positioning the column bases to the pre-set lines on the foundations, the structure still needs adjustment during lining, levelling and plumbing, then something may be wrong. The first thing to check is that no error has been made in the erection. If nothing is out of place then the drawings and the fabrication need to be checked.

If there is an error it will be necessary to take careful note of all the circumstances and advise all concerned immediately.

If the steelwork has just been thrown together with no great regard for accurate positioning it will be necessary to provide equipment and manpower to carry out the lining, levelling and plumbing.The equipment will include jacks, tirfor-type wire pullers with wires, straining screws, wedges, piano wires, and heavy plumb bobs with damping arrangements, all in addition to the level and simple theodolite which are needed for the basic check.

It is necessary to do the lining and levelling check before final bolting up is done.
In practice this means that it must be done immediately following erection since it is inefficient to slacken off bolts already tightened in order move the steelwork about.There is no way that erection can go ahead until the braced bay is bolted up, and that cannot be done until it is level and plumb.

A supply of steel landing wedges and slip plates is needed to adjust the levels.
They must be positioned in pairs opposite to each other on two sides of the base plate of the columns to be levelled. If the wedge is placed on one side only the column will be supported eccentrically and can in fact be brought down, especially if more than one column has been lifted on a series of wedges all driven in from the same direction. Alternatively toe jacks (jacks that can be used in tight areas) can be used in pairs to lift the columns. A temporary bench-mark should be established, and agreed with the client’s representative, in the vicinity of the columns in a position where it cannot be disturbed. The level will then be used to check the final setting with the seating packers inserted, and the column landed back on them and bolted down with the holding-down bolts. The column  can then be moved about in plan on these packers to bring it into line and bay length from its neighbours.

The position of a line, offset from the column centreline in order to clear the columns, should be marked and agreed. This line is then used either to string and strain a piano wire or to set and sight a theodolite telescope. The readings from it will then give the amounts by which movements must be made to keep the structure in tolerance.

Running dimensions from the previous column will give required  longitudinal movement. However, care should be taken to watch the plumb of the columns in this direction in view of the tendency of steelwork to ‘grow’ due to cumulative tolerances. Regard should be paid to column-to-column dimensions rather than to running dimensions from the end of the building.

Having levelled the bases of the columns in their correct positions, it is possible to check the plumb. Attention to temperature effects is necessary as it is futile to check the plumb of a building which is not all at the same temperature and, in the case of a long building, if the temperature is not standard.

The fact that the column bases are level does not mean that a level check is not
needed on the various floors in a tall building. It is important that the levels of any one floor are checked for variations from a plane rather than on the basis of running vertical measurements from the base, since these are affected by temperature and the variable shortening effect on the lower columns as weight is added to them.

Plumb can be most readily checked with a theodolite using its vertical axis and reading against a rule held to zero on the column centreline. This eliminates the effect of rolling errors and is a check against the same centreline used in the fabrication shops. If a theodolite cannot be used, a heavy plumb bob hung on a piano wire and provided with a simple damping arrangement, such as a bucket of water into which the bob is submerged, is a second-best alternative. Measurements are made from the wire to the centrelines in the same  way as before. The disadvantage of using a plumb wire is that all the operatives have to climb on the steel to take and then to check the readings. Optical or laser plumbing units are available; these are particularly useful for checking multi-storey frames.

Adjustments, and in extreme cases provision for holding the framework in its correct position, are normally necessary only if the frame is not self-stable. For instance, if structural integrity depends on concrete diaphragm panels for stability, consideration should have been given at the design stage to one of two alternatives: either the concrete panels should be erected with the steel frame, or temporary bracing should have been provided as part of the original planning.To ensure proper function, any bracing must be positioned so that it can be left  in place until after the concrete panel has been placed and fixed.

If diagonal wires have to be used they should be tied off to the frame at node points rather than in mid-beam, in order to avoid bending members.Also, their ends should be fixed using timber packers in the same way as is done when the pieces are slung for lifting. Turnbuckles or tirfor-type pullers provide the effort necessary to tension the wires. The sequence of placing, tensioning and ultimately removing these temporary arrangements should form part of the method statement prepared for each particular situation.Once the components of the building have been pulled into position the final bolting-up can be completed.

Portal frame construction in its temporary state often has the legs pre-set out of vertical, and becomes vertical only after the final loading has taken place. It is the responsibility of the designer to calculate the extent of pre-set. This often proves a difficult process to assess. Further, allowance must be made if the final position is needed for aesthetic purposes.

It is essential to ensure that the lining and levelling keep pace with the erection, since when additional floors are added it may become increasingly difficult to adjust, pull, or move the structure. As a rule of thumb, the lining and levelling of heavy sections should be within two to three bays of the erection face, whereas the lining and levelling of lighter sections should be within four to six bays of the erection face.

33.4.3 Tolerances
Prior to determining tolerances it is important to understand why they are needed. Rolling tolerances of the steel and manufacturing tolerances need to be accommodated. Tolerances must accommodate preceding work and subsequent building components.Tolerances must meet design requirements and architectural/ aesthetic requirements, and maximize buildability.

Tolerances can be categorized as structural (necessary for the  integrity of the intended structure), architectural (necessary for the aesthetics of the structure), and buildability (necessary to construct all of the components in a building). For a detailed coverage of tolerances in steel construction.

It is important to inform the designer of any deviation from specified tolerances, as failure to comply can often adversely affect both the structure and subsequent operations.

33.4.4 Holding-down bolts
To facilitate the erection of the structure, holding-down bolts  are normally positioned by the civils contractor. Movement tolerances must be accommodated in the foundation to ensure that correct alignment can take place (see Fig. 33.4).

 Holding-down bolts
Fig. 33.4 Holding-down bolts
Conical sleeves maximize movement without reducing the area of anchorage. It is important to have the bolts set both vertical and loose in the sleeve. The key benefit of using sleeved cast-in bolts is to allow movement for the alignment of the structure to meet tolerances.After checking and confirming that the holding-down bolts are acceptable, typical 100mm x 100mm steel packs (1–20mm thick) or similar are placed on the base to the required level.

33.4.5 Site bolting
Wherever possible, site splices should be designed to be bolted. This process is less affected by adverse weather than welding, uses simple equipment, and presents much less complex work access and subsequent inspection problems.

Bolts should be chosen to minimize size variation, and ease of repetition and selection. Design for Construction makes the following recommendations when selecting bolts:

(1) Preloaded bolts should only be used where relative movement  of connected
parts (slip) is unacceptable, or where there is a possibility of dynamic loading.
(2) The use of different-grade bolts of the same diameter on the same project should be avoided.
(3) Where appropriate, bolts, nuts and washers should be supplied with a corrosion
protection coating that does not require further on-site protection.
(4) Bolt lengths should be rationalized.
(5) Bolts should be threaded full length where possible.
(6) Connections should be standardized where possible.
(7) ‘Just-in-time’ delivery should be used, based on simple, easy-to-follow bolt lists.
(8) Approximately 90% of simple connections could be made using M20 60mm
long bolts.With a choice of three lengths, 95% of connections are possible.

Impact wrenches and powered nut runners speed the tightening of bolts.When high strength friction-grip bolts (HSFG) are necessary the Construction Industry Research and Information Association technical note for friction grip bolted connection should be used. If small quantities of HSFG bolts are to be used, manual torque wrenches are available.Where large quantities of friction-grip bolts have to be tightened, an impact wrench is an essential tool. Where access is  difficult, wrenches are made which run the nut up and are then used as manual ratchet wrenches to finally tighten the bolt.

Friction-grip bolts must be tightened first to bring the plates together and there-after be brought up to the required preload either by applying a further part turn of the nut or by using a wrench calibrated to indicate that the required torque has been reached. The manual wrench has a break action which gives the indication by means of a spring without endangering the erector by a sudden complete collapse.

Alternatively, load-indicating washers may be used to demonstrate that the bolt has been tightened to the required tension.

0 comentarios:

Post a Comment