Scaffold Towers & Birdcage Towers
ACCESS BIRDCAGE SCAFFOLDS
A birdcage scaffold consists of a mass of standards arranged at regular intervals in parallel lines, usually evenly spaced apart. These standards are laced together with a grid of ledgers and transoms at every lift height. The top lift is boarded to form the access platform for work on ceilings and soffits, e.g. fix lighting, ventilation or sprinklers over an inside area.
The side bays of the birdcage may also be required to form a normal access scaffold to the walls supporting the soffit.
There are two types of birdcage:
1. Birdcages with more than one lift height.
2. Single lift birdcages.
The standard should be a maximum of 2.5 M in each direction.
The first lift may be up to 2.5 M high if necessary for access below the scaffold otherwise the lift heights should, in general, be at 2 M intervals.
Except for the working lift in the outside bays, a birdcage should have only one working lift, i.e. the top lift, and the loading on this should not exceed 0.75 kn/M2.
In the case of a birdcage scaffold covering a large plan area, the width of the edge bays may be reduced to three, four or five boards to meet the requirements for the particular type of access required round the edges.
Ledgers and transoms should be fixed to the standards with right angle couplers. In the top lift, transoms should be fixed to the ledgers with putlog couplers. Transoms to the working lifts in the side bays may be fixed to the ledgers to provide level support for the decking.
All birdcage scaffolds require to be braced and/or tied to ensure they are stable. The standards have to be adequately braced to be strong enough to carry the vertical loading. Birdcages should be stiff enough to resist lateral sway and horizontal loads and be safe from overturning. External birdcages should be braced to resist wind forces and anchored when necessary.
Bracing should be provided at the rate of one brace for every six standards in each line.
Bracing in each direction should preferably be attached to the ledgers or transoms by right angle couplers or, if this is not possible, to the standards by means of swivel couplers.
Where an adjacent structure is available, a birdcage should be stabilised from it. If possible, all four sides should be secured. Where two opposite walls are available, it is sufficient to arrange tubes to butt against them from each side, but if only one is available, it is necessary to provide a two-way tie.
The most satisfactory details is to extend the transoms or ledgers. Where they are to butt both ends, timber packing pieces and reveal pins may be used.
There are no permissible gaps in decking for the platform. Platforms should be no further than 150 mm from the working edge, unless provided with guard rails and toe boards. Where scaffolds have more than 11 standards in line, it is not possible to rely on wall support only, because some standards will be too far from a support in at least one direction. Thus some bracing will be essential, but can be used in conjunction with tying and butting using intermediate parts of the structure, e.g. columns.
Any birdcage higher than 50 M should be specially designed. As a general rule alternate rows of standards would be braced.
Single lift birdcages should have special attention paid to the bracing of any height less than 2.5 M. The absence of the braced upper lifts requires that the stability of the deck is derived from one lift of bracing only, which should therefore be correctly positioned and fixed.
The standards should be supplied with foot ties in at lease alternate bays in both directions to form boxes. Diagonal bracing of alternate boxes in two directions should be fixed. Ledgers or transoms should be joined with sleeve or parallel couplers. No materials should be stored on a birdcage unless the storage areas have been specially designed.
Free Standing Towers
Free standing structures which are self-supporting and do not depend totally on other structures for their rigidity and stability. Three classes of these structures are generally recognised.
1. Light Duty Access Towers
Stationary and mobile for use inside buildings (imposed load not greater than 1.5 kN/M2).
2. Light Duty Access Towers
Stationary and mobile for use in the open (imposed load not greater than 1.5 kN/M2).
3. Heavy Duty Towers
These are always built to design details.
Mobile Towers Only
Joints in standards should only be made with sleeve or parallel couplers.
Wheels or castors not less then 125 mm in diameter and fitted with brakes which cannot be released accidentally should be locked into the base of standards. The safe working load should be marked on the castor wheel.
Ladder should be fixed to narrowest side of the tower, preferable inside the base area. It may be upright or angled. All usual ladder regulations apply.
Mobile and Static Towers
Dimensions will vary according to need, but standards should never be less than 1.2 M or more than 2.7 M apart.
Foot ties should be fixed approximately 150 mm from the bottom of the standards.
Except for the working platform, ledgers and transoms should be fixed to the standards with right angle couplers.
Lifts should not exceed 2.7 M or be greater than the smallest base dimension.
Bracing is in two forms:
1. Plan bracing at the base, the working platform and at alternate lifts.
2. Diagonal bracing to the full height of the scaffold on all four sides.
Plan bracing should be fixed to the standards with right angle couplers. Diagonal bracing should be fixed to the ledgers and transoms by right angle or swivel couplers. The height to base ratio should not be greater than shown below for each type of structure.
Stationary internal tower 4 to 1
Stationary external tower 3.5 to 1
Mobile internal tower 3.5 to 1
Mobile external tower 3 to 1
The recommended maximum freestanding height for static towers is 10.8 M, except that this may be increased to 50 M if it is tied to a structure at 6 M intervals.
The working platform should be fully and closely boarded to an area of at least 1.2 M x 1.2 M.
Transoms should be spaced at not more that 1.5 M intervals.
A distributed load on the platform of a light duty tower should not exceed 1.5 kN/M2.
Guard rails and toe boards will be necessary on all four sides, and the toe boards must rise at lease 150 mm above platform level. Guard rails must be at least 910 mm above the platform width with the distance between the top of the toe board and the guard rail not exceeding 470 mm.