Constructional Details of Formwork

Two objects must be kept  always in mind while evolving a formwork for ally type of construction: 

1. The formwork and falsework  must be sufficiently strong  not to unduly  deflect under  load. 
2. The construction  must be carried out in a way so as to  achieve easy  assembly and dismantling, without  disturbing the remaining  parts  and  avoiding damage to the  concrete. 

• The first object  can be achieved by  adequate design.  
• The second  object can be achieved making the formwork in panels so that they can be easily dismantled. The following  points  need  attention. 

1. Panels should wherever  possible, be held in place by struts, braces or wedges and should not be nailed together or in position to allow for number of re-uses. 
2. Folding  wedges  should he used with braces  and  struts so that sheeting may be eased  away from concrete to avoid sudden stress which may result in damage. These provide correct levels and  makes  stripping easier. 
3. Where panels or supports must be nailed in  position, care must be taken to ensure that the nail heads  are  accessible  for  the  stripping operation. 

Some parts of the formwork may safely be removed earlier than others. Parts removed may be re-used elsewhere thus reducing the total amount of formwork and falsework. On large  works such savings may be considerable and formwork design  should be such as to enable part-stripping to be carried out without disturbing  the remainder. 

Typical  details of simple construction  for the most common type of work  are given in  the following  paragraphs  alongwith  sketches. The sketches  show timber  sheeting only  used for the formwork in contact with concrete, but this can be faced with or replaced by plywood or steel panels is desired. 

i)  Simple Slabs. In case of spans upto 23 metres, joists may be used in single span and carried by ledgers fixed to walls or supported on posts. Holes should be left in  walls at suitable intervals so that  a wall plate may be bolted to the wall to support the joist.  For  greater spans or heavier weight of slab intermediate  lines of posts and ledger should be used. 

ii)  Beam and Slab. Stripping  order should decide the  mode of fabrication and this may take one of three forms: 

a)  Beam sides, slab sheeting and beam bottom separately and at appropriate intervals. This method is  worthwhile when beam sides are required for re-use. 

b)  Bean sides and slabs sheeting together, and beani bottoms later. This is probably the most common method. 

c)  All formwork at the  same time. The most desirable method when no part of the formwork is required for immediate re-use and when leaving it in position does not interfere with the work. 

If part  stripping is to be carried out, careful  thought  must be given to the problem of how this is going to be achieved. Where beam sides are to be stripped before slab sheeting, the  latter should be independently supported. Where beam sides and slab sheeting are to be stripped together, sheeting joists can be carried on beam ledgers as shown in Figure. Beams  are carried on posts fitted with braced capping pieces. When wooden posts are used, they should be braced  to prevent movement or buckling. 

 Typical Beam and Slab Formwork

iii)  Columns. In case of column formwork, panels should be  made by nailing sheeting to yokes or steel panels fixed by bolts as shown in Figure. The two side panels  should be the same width as the columns, whist one end panels should be the same plus the twice the  sheeting thickness. Yokes  on the side panels should project about 15 to 20 on either  side of the sheeting whist those on the  ends should be the  same width as the sheeting. The formwork is held together by bolts passing through the projecting yoke ends and wedges which are driven between the bolts and end pieces. 

iv)  Walls. Wall forms are generally built up in panels. The two  sides are held together by wire ties or bolts and the correct. spacing is ensured by using distance pieces as shown.in Figure. If  pieces of wood  are used as spaces, they should not be fixed  and must. be removed as concrete proceeds. Modern practice is to cat door and window frames into the  concrete to save time. Care must be taken to ensure that openings and conduits  are accurately, located.