Technical requirements for safety in metal scaffolding – Falsework

5.4   Falsework

Falsework   is   a   temporary   structure   used   to   support   a   permanent structure while the latter is not self-supporting. In Hong Kong, falsework is commonly used to support formwork for in-situ concrete construction, and from time to time, workers have to work on or in the vicinity of the falsework. Total or partial collapse of falsework may lead to serious accidents. Inadequate design, defective or sub-standard materials, faulty setting out, inadequate supervision and procedural inadequacies such as improper loading and dismantling are the common causes of their collapses.

The design, construction, use and dismantling of falsework should comply with BS 5975 or other equivalent national/international standards or provisions. The falsework should be designed by professional engineer and when the case is warranted, an independent checking professional engineer should be arranged to cross-check the design and the erected falsework. The followings highlight the good practices sometimes overlooked in order to prevent collapse of falsework on construction sites in Hong Kong:

5.4.1  Engineering considerations

(a)     The framing of structural members and details of construction should be justified in accordance with recognized engineering principles to meet the loads to which the falsework may be subjected. The loads include vertical loads and lateral loads, and the common ones are given below:

Vertical Loads from: (i)     Self-weights.

(ii)    Permanent works to be supported.

(iii)    Impact due to placing permanent works (e.g. free fall of wet concrete).

(iv)    Construction operations: A minimum of 1.5kN/m2   should be allowed for the operations.

(v)    Temporary storage of materials.

(vi)   Traffic loads.

(vii)  Plant: The operating loads should include the weight of plant,

dynamic effects and vibration effects. (viii) Induced wind loads.

(ix)   Uplift loads due to wind and floatation.

Lateral Loads from:

(i)     Wind loads.

(ii)     Hydrostatic pressure: It may come from wet concrete or an external source.

(iii)   Lateral earth pressure.

(iv)    Differential    movements    of    supports    such    as    ground movements.

(v)    Vibration  effects  such  as  those  due  to  concrete  vibrations,

concrete pumping operations or piling operations nearby. (vi)   Flowing current.

(vii)  Unsymmetrical distribution of vertical loads, such as effects

due to unbalanced concrete placing.

(viii) Unsynchronized    jacking    of    permanent    works    against

falsework.

(ix)   Sway of falsework.

(x)    Buckling of props.

(xi)   Eccentricity of vertical loads due to construction deviations,

especially for falsework on sloping ground. (xii)   Dynamic effects from plant and equipment. (xiii) Casting up of concrete against existing works. (xiv) Discontinuity in the soffit formwork.

(b)    The minimum lateral loads should be taken as the greater of:

(i)     the most adverse combination of the above lateral loads; or

(ii)     2.5%  of  the  vertical  loads  taken  as  acting  at  the  points of contact between the vertical loads and the supporting falsework.

(c)     Each falsework member should be designed for the most adverse combination of vertical loads and lateral loads. The reduction of permissible axial stress for used materials and the reduction in strength with the increase in the number of stages of scaffold should be considered.

5.4.2  Structural steel works

(a)     Use structural steel in accordance with the Code of Practice for the Structural Use of Steel issued by the Buildings Department or other equivalent national/international standards or provisions.

(b)     Steel hollow sections exposed to the weather should have walls not less than 4mm thick, unless protection against corrosion is effectively provided and maintained.

(c)     Steel members of hollow sections are often used repeatedly. As some damage is expected after each cycle of use, they should be inspected prior to reuse and be discarded if found unsatisfactory. For reused members, an allowance for strength reduction should be considered.

5.4.3  Lateral stability

(a)     The  key  to  keep  falsework  safe  is  the  provision  of  adequate lacing and bracing to prevent the falsework from buckling or sway.

(b)     Lacing  are  horizontal  members  connecting  props  together  to reduce the unsupported length of the props. They may behave as struts or ties, and help to transmit lateral forces to bracing members.

(c)     Bracing   generally   are   inclined   members   connecting   lacing members and   props.   They   transmit   lateral   forces   to   the foundations.

(d)    Lacing and  bracing must   be   recognized   as  critical   members

in   falsework.   They   should   be   adequately   provided   in compliance  with  recognized  engineering  principles.  They  should be clearly shown in the drawings in the three principal directions to  illustrate  professional  engineer’s  intentions.  Undue  movement due to lateral forces, torsion or impact forces should be prevented. A properly planned loading sequence will alleviate torsional effects.

(e)    If  possible,  the  falsework  should  be  tied  back  to  stiff  parts  of

completed permanent structures to enhance lateral stability.

(f)     The  framing  of  falsework  should  give  a  robust  and  stable structure, especially for falsework near vehicular traffic. The structure                     should   be   designed   and   constructed   so   that it   is   not   unreasonably  susceptible  to  effects  of  impacts  or vibrations. Damage to small areas of a structure should not lead to collapse of major parts of the structure. To avoid accidents, adequate headroom, lighting, warning signs and signals, and impact protection measures should be provided.

5.4.4  Cantilever members

(a)     The end portion of a prop protruding beyond a lacing member should be considered as a cantilever member unless adequate means is used to brace the end portion. Such end portion often occurs at the top or at the base of a prop.

(b)     If a prop has an extensible portion at the end, the joint between the  extensible  portion  and  the  prop  itself  allows  a  little angular  movement.  Such  movement  constitutes  a  weak  point in the falsework. Unless otherwise justified by recognized engineering    principles,    the    extensible    portion    should    be adequately laced and braced at the end where the extension exceeds 300mm.

5.4.5  Fastenings to concrete or masonry

All fastenings to concrete or masonry for structural uses should be designed  in  accordance  with  recognized  engineering  principles  and the manufacturer’s recommendations.   The construction details and instructions for use should be clearly specified in the drawings and specifications.

5.4.6  Lacing, bracing & wedging

Falsework  will  not  be  safe  without  adequate  lacing,  bracing  and wedging. Workmen should not be permitted to install lacing, bracing or wedging in favour of their own decisions. All the details shown in the drawings and specifications should be followed.

5.4.7  Test on falsework equipment

(a)     In  Hong  Kong,  much  of  the  falsework  equipment  in  use  is  of proprietary design that has been purchased or hired. Detailed information, such as that provided by the manufacturer, is of great importance  in  inspecting  such  equipment  if  the  inspection  is  to be carried out by those not familiar with its usage. Very often, technical                    information   relating   to   the   performance   of   such material  has  been  compiled  from  test  carried  out  during  the

development   of   the   equipment.   It   is   desirable   that   test procedures for similar systems or components should be standardized in such a way as to make the critical properties comparable. Worldwide accepted methods of test for falsework equipment such as those laid down in BS 5507 and BS EN 1065 can be used by manufacturers in compiling the necessary design data.

(b)     Where  the  strength  of  a  manufactured  component  cannot  be ascertained  by  applying  design  criteria  recommended  in  this Code, testing should be carried out at the prototype stage of development    in   order   to   obtain   results,   including   ultimate behaviour, on which design data for the component or system can be based.

5.4.8  Loading sequence/pattern

(a)     Sequence of placing loads on the falsework including loads due to temporary storage and prestressing should be planned and taken into account in the design.

(b)     The   sequence   of   placing   permanent   works   such   as   wet concrete should comply with professional engineer’s intentions expressed in the drawings and specifications. If such a sequence has not been specified, advice on the loading sequence should be sought from the professional engineer. If the professional engineer considers that   no   specific   sequence   is   needed,   then   the sequence of working should be planned by spreading the loads evenly  on  the  falsework.  Uneven  distribution  of  loads,  such as       out-of-balance   effects   due   to   unsynchronized   jacking   of permanent works against the falsework by more than one jack may lead to uplifting or instability.

(c)     Concrete  pouring  by  crane,  skip,  barrow,  dumper  or  pumping produces  impact  forces.  The  free  fall  should  not  exceed  0.5m unless otherwise permitted by professional engineer. Heaping of wet concrete within a small area should be avoided. Unless otherwise permitted by the professional engineer, equipment for concrete pumping should not be fastened to the falsework.

5.4.9  Undue movement of falsework

The   works   under   construction   should   be   suspended   immediately when any undue movement of the falsework occurs. In addition, the falsework should be labelled to show that it is unsafe and should not be used. Investigation on the causes of the undue movement should be carried out immediately by competent person with the help of the drawings and specifications. If any doubt still exists, the competent person should immediately seek professional engineer’s advice.