Technical requirements for safety in metal scaffolding – Tubular Scaffolds
5.2 Tubular Scaffolds
They are constructed in tubes and couplers for the purpose of providing working platforms. Each scaffold should be constructed in accordance with the design and drawings of professional engineer. Recommendations for the design, construction and use of various types of scaffolds can be found in BS EN 12811 or other equivalent national/international standards or provisions. All tubes, couplers and fittings should comply with BS 1139, BS EN 39, BS EN 74, BS EN 1004 and BS EN 12810 or other equivalent national/international standards or provisions. When other tubes or materials are used, structural calculations should be carried out using the properties of the tubes or materials used and the structures assembled so that equal or even higher safety standards than the aforesaid standards can be achieved. In order to calculate the height of a tubular scaffold, the table in Section 5.1.1(i) and the following table should be made reference to:
| Duty |
Max. number of platforms |
Commonly used widths
using 225mm boards |
Max. bay length (m) |
| Inspection and very light duty |
1 working platform |
3 boards |
2.7 |
| Light duty |
2 working platforms |
4 boards |
2.4 |
| General purpose |
2 working platforms
+ 1 at very light duty |
5 boards or
4 boards + 1 inside |
2.1 |
| Heavy duty |
2 working platforms
+ 1 at very light duty |
5 boards or
5 boards + 1 inside or
4 boards + 1 inside |
2.0 |
| Masonry or special duty |
1 working platform
+ 1 at very light duty |
6 to 8 boards |
1.8 |
| Note: (a) The boards are timber scaffold boards of nominal cross sections
38mm x 225mm. Decking units of other types or dimensions but with equal or greater strength can also be used.
(b) The normal lift height for works such as brickwork is 1.35m, and for walk-through scaffolds is 2.0m. For greater lift height or different loading conditions, reference should be made to the design criteria in BS EN 12811 or other equivalent national/ international standards or provisions. |
The following subsections highlight some essential safety requirements for some special types of tubular scaffolds made of steel. A tube made of steel should have a yield stress not less than 235N/mm2, and should have an outside diameter of 48.3mm and a wall thickness of 4mm.
5.2.1 Independent tied metal scaffold
It should consist of a double-row of standards with each row parallel to the building (See Figure 1 for details). The inner row should be set as close to the facade of the building/structure as is practicable. The distance between the lines of standards should be the minimum necessary to accommodate the required boards and toe-boards forming a working platform. The standards should be connected with ledgers parallel to the building/structure and fixed with right angle couplers and with transoms fixed to the ledgers with putlog couplers to give the required platform widths.
(a) Standards
(i) The joints in standards should be staggered. Joints in standards of scaffolds tied to a building/structure should be made with either joint pins or sleeve couplers.
(ii) For scaffold that is free standing or projecting above the level of a building/structure or otherwise subject to forces that would produce tension in the standards, the standards should be joined in manner capable of resisting the applied tension.
(iii) No more than three out of the four standards at the corner of any bay should have joints in the same lift except in the case of the bottom 6.5m of a scaffold where an extended base lift is necessary for pedestrian access or other reason.
(iv) Where any of the standards in a scaffold are at a level lower than the remainder of the standards, the extension downwards should be stiffened by horizontal tubes, in two directions at right angles and fixed at lift heights (i.e. the vertical intervals at which standards are linked to one another).
(v) Where access for the public is required under the first lift, a height of up to 2.7m is permissible, provided that the load in the standards does not exceed the maximum permissible axial stresses and loads for the steel scaffold tubes.
(b) Ledgers
(i) Ledgers should be fixed to standards with right angle couplers and should be horizontal except that a foot lift may follow the slope of the ground at the base of a scaffold. For this situation, the transoms should be attached to the standards and the ledgers to the transoms.
(ii) Joints in ledgers should be made with sleeve couplers or expanding joint pins. Joints in ledgers on the same lift and in adjacent lifts should not normally occur in the same bay.
(iii) When guard-rails are to remain permanently in place, the absence of a joint in the guard-rail in any bay may be accepted as giving sufficient continuity to the scaffold to permit joints in the ledgers above and below it in the same bay.
(iv) Where joints are required, they should be positioned at a distance not greater than 1/3 of the span between adjacent standards.
(v) In the case of curved scaffolds, fittings other than right angle couplers may be used to join the ledgers to the standards provided that they are of adequate strength or otherwise supplemented by a right angle check coupler. Besides, for large radius curves, separate scaffolds with platforms of the same height may be used.
(c) Transoms/putlogs
(i) The length of transoms/putlogs should vary according to the intended use of the scaffold.
(ii) Transoms should be extended inwards and outwards for the purpose of butting the face of the building and fixing the longitudinal bracing.
(iii) Transoms should be fixed to the inside and outside ledgers with right
angle or putlog couplers.
(iv) Boarded lifts: The spacing of transoms/putlogs for boarded lifts should be in accordance with the followings:
| Nominal
Thickness
of Board (mm) |
Max. Span between Transoms/ Putlogs (m) |
Min. Overhang
(mm) |
Max. Overhang
(mm) |
| 38 |
1.5 |
50 |
150 |
| 50 |
2.6 |
50 |
150 |
| 63 |
3.25 |
50 |
150 |
(v) Non-boarded lifts: Transoms/putlogs for non-boarded lifts should be fixed at one per pair of standards, including the pair at each end of the scaffold, and should be fixed within 300mm of the standard. On scaffolds of a height more than 50m, the transoms on unboarded lifts, when fixed at the frequency of one transom per pair of standards, should be fixed to the ledgers or standards with right angle or other suitable couplers provided they are capable of sustaining a safe working slip load of 5kN.
(d) Working platform
Safety requirements for a working platform are the same as described in
Section 5.1.4. Other requirements are as follows:
(i) Any group of boards across the width of the scaffold should be of the
same length, with all boards of the same thickness.
(ii) The spacing of the transoms to support the boards should vary according to the thickness and length of the boards as specified in Section 5.2.1 (c)(iv).
(iii) The ends of a working platform should extend beyond the end of the wall or working face by a distance of 600mm when work is to be carried out up to the end of the wall.
(iv) Gangways and working platforms should preferably be horizontal but may slope at an inclination of up to 1 vertical to 4 horizontal without stepping laths. At slopes steeper than this they should be provided with stepping laths to provide a firm foothold.
(v) Ladders or other suitable means should be provided so as to enable workers to gain access to and egress from one platform to another.
(e) Stair/ladder access to and in scaffolds
Stair and ladder towers should be constructed with one side common with the outside of the scaffold. Bracings should be fixed to the remaining sides except in the bays through which access and egress is required. The superimposed loading adopted in calculations should be not less than
2kN/m2 for all landings and stairs in stair towers and ladder towers.
(i) Every sloping ladder should stand on a firm and level base and be supported only by the stiles. The stiles should be securely fixed to the scaffold by lashings or by other attachments at the top.
(ii) Ladder should be set at an angle of 4 vertical to 1 horizontal.
(iii) Ladders should project at least 1.05m above the top landing with the landing rung level with or slightly above the level of the landing. Ladders should not be extended by lashing two lengths together.
(iv) The vertical distance between two successive landings should not exceed 9m. The landings should be provided with access holes for the user which should not exceed 500mm in width and should be as small as practicable in the other direction.
(v) Where practicable, the ladder access to the scaffold should be with its own ladder tower fixed to the outside of the main scaffold.
(vi) Both ladder towers and stair towers should be fully decked at the landings. Toe-boards should be provided at the landings.
(vii) Every landing and every side of a stair shall be provided with suitable guard-rails of adequate strength. The height of a top guard-rail shall be between 900mm and 1 150mm. The height of an intermediate guard-rail shall be between 450mm and 600mm. (Third Schedule to the CSSR)
(viii) The gaps in the decking to allow access and egress from lift to lift up the ladder or steps should be as small as practicable. Short boards necessary to complete the decking round the access hole should be tied down and supported at the correct centres.
(ix) Every gangway or run in the scaffold shall either be closely boarded, planked or plated, or is a platform consisting of open metal work having interstices none of which exceeds 4 000mm2 in area. (Third Schedule to the CSSR)
(x) Every side of the gangway or run shall be provided with suitable guard-rails of adequate strength. The height of a top guard-rail shall be between 900mm and 1 150mm. The height of an intermediate guard-rail shall be between 450mm and 600mm. (Third Schedule to the CSSR)
(f) Ties
(i) When a working lift (i.e. the assembly of ledgers and transoms forming a horizontal level of a scaffold) is required at 2m height and no firm part of the building/structure has been constructed to attach a tie, scaffold should be temporarily stabilized by raking tubes or other means. Such tubes should also be fixed during dismantling if low level ties are impracticable.
(ii) In the situation where ties may be temporarily removed, they should be maintained at such a frequency that there is always one for every 25m2 of scaffold area and they should be reasonably evenly distributed over the scaffold surface, both horizontally and vertically.
(iii) Ties which will not be removed during the use of a scaffold should be inserted and maintained at a frequency of one for every 40m2 of the scaffold surface and should be reasonably evenly distributed over the scaffold face area, both horizontally and vertically.
(iv) The spacing of lines of ties should not exceed 8.5m, either horizontally or vertically, but at the same time individual ties should still be within the area rule above. Where the building/structure surface permits a staggered arrangement of ties, this should be adopted in preference to a rectangular pattern.
(v) The tie tube should be horizontal or sloping downwards away from
the building.
(vi) At the point where the attachment of the tie tube to the building/ structure is made, the building/structure should be strong enough to resist the forces applied to it.
(vii) Ties should preferably be attached to both the inside and outside ledgers or standards and, if possible, at a point not more than 300mm from a braced standard.
(viii) The attachment of the tie tube to the scaffold should preferably be next to pairs of standards which are ledger braced, as near to a node point as possible.
(ix) The couplers for ties set at an angle to the building/structure should be swivels. The couplers for ties set at right angles to the building/structure and horizontally should be right angle couplers or another such arrangement of couplers which gives similar or adequate strength.
(x) Where wire or banding ties are used, they should be turned round a node point of the scaffold or otherwise prevented from slipping along the ledger or standard by fixing safety couplers beside the point of attachment.
(xi) Each tie should comprise an anchorage to the structure served and tying member connecting this anchorage to the scaffold. If a single anchorage is not strong enough to provide a safe working capacity of
6.25kN, two or more should be used or the design reconsidered.
(g) Bracings
Bracings should be provided to stiffen the scaffold. The plane to be braced should, wherever practicable, be divided into a complete series of triangles by braces. These should be fixed as close as possible to intersections. A check should be made on the reduction in strength of the scaffold where a brace has to be omitted or where it cannot be fixed within 300mm of an intersection.
Ledger bracing:
(i) Ledger bracing should be on alternate pairs of standards. Any pair of standards, which are ledger braced, should be made into a complete series of triangles.
(ii) When the bay length is 1.5m or less, the ledger bracing may be fixed
to every third pair of standards.
(iii) The ledger bracing should be fixed from ledger to ledger with right angle couplers when the lift is not to be boarded but may be fixed to the standards using swivel couplers.
(iv) The ledger bracing on boarded lifts should be from under the outside ledger of a boarded lift down to the inside ledger of the lift below so as to avoid the toe-board.
(v) The ledger bracing from the inside ledger to the guard-rail level of the lift below may be used provided that every pair of standards is so braced instead of every alternate pair.
(vi) In scaffolding over footpaths, the ledger bracing may be omitted from the lowest lift provided the lengths of the standards in the lift are not in excess of 2.7m. When the height of the lowest lift is in excess of 2.7m, a knee brace should be inserted across the top corner of the lowest lift, commencing at approximately 1.8m from the ground. One such knee brace should occur on every pair of standards and be fixed with alternate slopes. On large scaffolds it is sometimes desirable to insert cross knee braces on every pair of standards, and a ledger should be fixed adjacent to where the knee brace meets the standard.
Facade bracing:
(i) Longitudinal bracing should be provided to all scaffolds in which the movement along the facade of the building/structure is not prevented by other means.
(ii) The longitudinal bracing should be achieved by tubes set at between 35o and 55o to the horizontal, reaching from bottom to top of the scaffold. There are three principal forms:
– individual tubes set in zig zag pattern, the top of a tube and the bottom of the next preferably being attached to the same transom;
– a continuous tube, extended as necessary to cover the whole scaffold, only possible for wider scaffolds;
– individual tubes as described in the first form above but all sloping the same way; the top of one is connected at a ledger/ standard intersection, and the bottom of the next is attached to the same pairs of standards.
In most situations, a combination of these should be appropriate. The
bracing tubes should be connected either in the following two ways:
– to every lift of the extended transoms with right angle couplers; or
– to every standard with swivel couplers.
The first way above is to be preferred.
(iii) The brace assembly should be provided at intervals along the scaffold not exceeding 30m.
(iv) The longitudinal bracing should be fixed as near to the standards as
possible.
(v) The longitudinal bracing should include the lower lift being started from the base of one of the outside standards. In the lower lift, when the bracing is started, a guard-rail should be placed through the braced bay to prevent people passing.
(vi) The joints in continuous diagonal bracing should be made by overlapping the two lengths of the tube by a distance of at least
300mm and joining them together with two parallel couplers. Or, the two tubes may be joined by a sleeve coupler or other coupler capable of sustaining the applied load.
Plan bracing:
(i) Plan bracing should be provided to all portions of a scaffold which are
not otherwise stabilized against lateral distortion.
(ii) It may be joined by the same type of couplers used for longitudinal bracing and the same rules with regard to strength apply.
Couplers for fixing braces:
Right angle couplers should be used to fix braces to ledgers or transoms and swivel couplers should be used for the attachment to standards. Other couplers may be used provided that they are capable of sustaining a safe working load of 5kN.
(h) Erection tolerances
(i) Standards should be vertical to within ±20mm in 2m (subject to a max. total deviation of 50mm).
(ii) Bay length and width should be ±200mm on designated lengths,
and level to within ±20mm in 2m (subject to a max. total deviation of
50mm).
(iii) Lift height should be ±150mm on the designated height.
(iv) Nodes should be equal to or less than 150mm between coupler centres.
5.2.2 Metal putlog scaffold
All the requirements are the same as described in Section 5.2.1 above with the following additional points to be observed:
(a) It should consist of a single row of standards parallel to the face of the building/structure and set as far away from it as is necessary to accommodate a working platform same as that required for a double-row metal scaffold, with the inner edge of the platform as close to the facade of the building/structure as is practicable. (See Figure 2 for details)
(b) All the standards should be connected with a ledger fixed with right angle couplers and the putlogs are fixed to the ledgers with right angle or putlog couplers.
(c) The blade end of the putlog tube should be placed horizontally on the brickwork/structure, etc. being built. But for the case of existing building/structure, the old putlog holes (if any) may be reused or others raked out, and the putlog blades may be inserted vertically.
(d) Sole plates and base plates should be used under each standard and their requirements are the same as described in Section 5.1.2 above.
(e) The scaffold should be tied into the building/structure at the manner as described in Section 5.2.1(f) above.
(f) Where a putlog is required for a board support to form a working platform and it is opposite to an opening in the building/ structure such as a window or doorway, etc., the inside end of the putlog should be supported on an underslung bridle tube spacing between adjacent putlogs.
(g) Longitudinal bracing should be required at intervals not exceeding 30m but ledger bracing is not required in the finished scaffold.
(h) The lift height should be no more than 1.35m.
(i) Tie tubes should be attached by right angle couplers to the ledgers or standards.
5.2.3 General free-standing metal towers
(a) These metal towers are free-standing structures which are self-supporting and do not depend totally on other structures for their rigidity or stability. There are mainly three types of such towers:
(i) Light duty access towers, stationary and mobile for use inside
buildings (imposed load not greater than 1.5kN/m2).
(ii) Light duty access towers, stationary and mobile for use in the
open area (imposed load not greater than 1.5kN/m2).
(iii) Heavy duty towers, such as camera towers and welding
platforms (imposed load in excess of 1.5kN/m2).
(b) Free-standing towers situated externally and likely to be subject to wind forces should be the subject of calculations for wind forces and overturning.
(c) All free-standing towers should be vertical and built on firm foundations. If on sloping ground, they should be prevented from slipping. Towers inside buildings should be on level floors or adequately compacted sub-bases.
(d) The towers should be adequately stiffened on all sides and in plan at
every alternate lift, starting at the base lift of mobile towers.
(e) Access to and egress from the top of towers should be by stair/
ladder.
(f) The working deck should be of adequate thickness. If boarded with scaffold boards, the supports of the boards should comply with the recommendations of the table in Section 5.2.1 (c)(iv) and the boards, if short, should be prevented from sliding by battens nailed beneath the deck. The deck should be provided with toe-boards and guard-rails complying with the recommendations of Section 5.1.4. Generally, the deck should have at least one edge in the same vertical plane as one side of the tower base so that this edge can be placed up against the work to be done. The worker is thus not required to lean out over the guard-rail.
(g) All types of free-standing structures depend for their stability either on their self-weight or on additional guys, anchors, outriggers or kentledge. The factor of safety for scaffold structures, i.e. the ratio of the overturning moment to the stabilizing moment, should be not less than 1.5. The overturning moment is due to eccentric weight, imposed loads and environmental loads. The stabilizing moment is due to the self-weight, if suitably centered, added kentledge and the anchor, gut or strut forces, if any.
(i) Where kentledge is used, it should be fixed round the perimeter of the foot lift and a tube and fittings grid should be installed to receive and locate it. If castors are used, their capacity to take the extra load should be checked.
(ii) Where anchor is used, anchor capacities are dependent on ground conditions and reference should be made to the manufacturer for the type, number and location of anchors. There are four types of anchorage commonly used :
Cross tubes attached to the foot lift :
Temporary stability can be achieved by using cross tube anchors attached directly to the bottom of the structure. The forces involved should be calculated and the necessary number of anchors inserted. The necessary number of safety couplers should be added to the base frame of the structure and the tensions in the standards catered for by sleeve couplers and lapping where necessary. (See Figure 3a for details)
Driven tube anchors attached to guys :
Driven tube anchors should not be used on a down slope towards the structure. They can be used in clay, sandy or gravelly ground. Tubes should be 1.75m long and penetrate
1.25m into the ground. They should be fixed together with tubes and fittings connected with right angle fittings in preference to lashings. The tubes should be set at right angles to the guy. The guy should be attached at the bottom of the front tube and prevented from slipping up by a scaffold fitting. (See Figure 3b for details)
Screwed in flight anchors :
They should be set in line with the guy and should be screwed in using a short length of scaffold tube through the ring. They will not penetrate so deeply on an uphill slope towards the guy and allowance for this should be made. (See Figure 3c for details)
Plate and pin anchors :
Plate and pin anchors should be used where the ground is too stoney or has shattered rock, limestone or chalk near the surface. The pins should be driven in at right angles to the guy and the anchors should be set so that the guys are flatter than
40o to the horizontal. For square towers, a separate anchor
should be provided for each corner. (See Figure 3d for details)
(iii) Guys for the metal towers should be of 10mm or 12mm diameter wire rope which should be attached to the scaffolding structure and to the ground tube or anchor by a single round turn and three bulldog grips. The recommended safety factor for guy ropes is 3:1. No tensioning device should be pulled up too tightly since the force required to pull a wire guy tight results in very considerable tension being placed on the ground anchorage and the structure before it has been loaded with the wind forces. All the guys should be attached to node points of the scaffold structure.
(h) When metal towers are required to be a height exceeding the height to the least base dimension ratio recommended in Section
5.2.4 (a) to 5.2.4 (b) and Section 5.2.5 (a) to 5.2.5 (b) and a larger base cannot be built or extension buttresses cannot be fixed at the base, the tower should be constructed up to the maximum height allowed by the height to the least base dimension ratio and then tied, roped or guyed in four directions to the main structure which is being serviced. The tower may then be increased in height and should be additionally tied at levels of approximately every 6m.
(i) Operation of free-standing metal towers :
(i) The user should apply no horizontal force at any working deck, e.g. by hauling heavy ropes or cables, and should not lift significant loads up the outside of the tower or attach a gin wheel on a cantilever tube unless the tower is specifically designed for this purpose.
(ii) If large weights are to be hoisted to the top deck by block and tackle, adequate davits or brackets should be provided and the stability of the tower calculated for the suspension reaction at the top block which might be twice the lifted weight. If the towers are rectangular, the lifting tackle and ladders should be on the shorter side.
(iii) Mobile towers should only be used on even ground, never on a slope which is sufficient to allow them to run away. Castors should normally be kept locked except when the tower is being relocated. When used on surfaces which have a cross fall and/ or a longitudinal fall, the user should be particularly careful to see that the brakes are on at all times other than whilst moving the tower. If there is any doubt as to the adequacy of the brakes, the wheels should be chocked.
(iv) No worker or heavy material should be permitted on any mobile scaffold during its movements. The force to achieve resiting should be applied at the base.
5.2.4 Stationary metal towers
This is one of the commonly used free-standing metal towers in Hong Kong. There are different requirements when being used within and outside buildings:
(a) Within buildings:
The height limit of these towers is achieved by restricting the ratio of the height to the least base dimension. Within buildings there are no environmental loads, but nevertheless some tendency to overturn a tower may occur from raising weights outside its base area, wrongful application of force at the top and normal operations on the top deck. To cater for this overturning moment, the height to least base dimension ratio should not be greater than 4. The height is measured from the floor to the level of the working deck or top lift whereas the least base width is the dimension, centre to centre, of the shortest side of the tower if it is rectangular.
(b) Outside buildings:
(i) The height to the least base dimension ratio for stationary towers outside buildings without special means of anchoring should not be greater than 3.5.
(ii) Stationary towers outside are usually exposed and are therefore subject to wind forces. Towers, even with a height to the least base dimension ratio less than 3.5, are unstable in locations exposed to high winds. For these circumstances, the wind forces should be calculated and the tower restrained by kentledge or guys to give a factor of safety against overturning of 1.5 in any direction.
(iii) Besides, when the ground is soft, sole plates should be used and the tower should be maintained in the centre of the sole plate by the use of substantial nails or other means. When the ground is sloping, the sole plates should be dug in flat.
5.2.5 Mobile metal towers
This is also one of the commonly used types of free-standing metal towers in Hong Kong. They are fitted with castors at the bottom of the standards. The castors should be of the swivel type and fixed to the standards of the scaffold so that they cannot fall off if the leg is out of contact with the ground (See Figure 4 for details). There are different requirements when being used within and outside buildings:
(a) Within buildings:
The height to the least base dimension ratio should be limited to 3.5. (b) Outside buildings:
(i) The height to the least base dimension ratio should not be greater than 3. When in use in exposed situations, the scaffold should be tied to the building it is serving.
(ii) When a scaffold is used in location exposed to high winds, the wind forces should be calculated and the scaffold restrained by kentledge or guys etc., to give a factor of safety of not less than 1.5. Also the capacity of the castors to take the extra load should be checked.
No more than one working platform should be permitted on all mobile metal scaffold at any one time.
Metal Scaffold Systems | Access Tower | Working Platform | Repropping| Formwork 