Building Construction formwork design

Formwork Design

Formwork Design criteria for the water retaining structures..

1 The erected forms will be watertight in the ingress of external liquids and egress of internal liquids.

2 The look of formwork will consider; height and rate of pour; thickness of member; concrete slump and density; placing temperature; texture of finish; construction joints; wind load.

3 On soffit forms (along with concrete weight) one more live load of two.5 kN/m2, or one more live load of three.75 kN/m2 if your motorised cart can be used.

4 The minimum design load for combined dead and live load will be 6.50 kN/m2, or, if a motorised cart can be used, 7.75 kN/m2.

5 Within the assessment of loads, the worst mixture of self-weight, form work forces, reinforcement weight, wet concrete weight, construction loads, wind loads, incidental dynamic effects brought on by placing, vibrating and compacting concrete, using externally applied vibrators, approach to concrete discharge and access for concrete placement and vibration will be used.

6 Formwork will be made to be demountable without shock, disturbance or harm to concrete, and sufficiently rigid to keep the right position, shape and profile so the final concrete structure is at the dimensional tolerances based on BS 5328.

7 Soffit formwork, properly supported on shores only, will be able to be retained in place during concrete maturing period.

8 Adjustable steel supports and shores shall allow form-boards and framework to become accurately adjusted to line and level.

9 The look will allow free movement and accessibility under formwork.

10 Shores for abnormal ceiling heights will be specially designed.

11 Forms will incorporate 20 mm chamfers on exposed corners of columns, walls and beams, except where plaster or rendered finish is specified.

12 The Contractor will make sure that adequate ground support for shoring and supports is available, and when not, will take measures to ensure they are suitable.

Building Construction formwork design

Slab Form Work

Slab form work

When the   slab form work was considered, it was necessary to consider the all the weight that can be applied to the form work such as total concrete weight, all the live load like labours and equipments, which were used for the concreting work.

Normally for the slab form work 12mm thickness plywood was used (coated plywood).

Method for fixing form for slab

Step 1: First step of the slab form work was the fixing of the collar to the columns, which were in the slab area. When the collar was prepared, it was necessary to minimize the off cut of the plywood board.(Most of the column 600*600, therefore ply wood board was cut the as describe column form work.)

Step 2: Bottom of the beam form works were fixed to each collar of the columns by considering the level of each beam bottom and all bottom of the beam form work

Step 3: Sideboards of the beams form work were fixed to the bottom of the beam form work by considering the level, height of the beams form work.

Levelling of the slab before concreting

Level instrument was used to check the level of the slab. It was necessary to calculate the level of the staff, before levelling the slab by considering the thickness of the board and size of the GI pipe. This has to ensure the slab was in the correct level before concrete. By adjusting the joint of the GI pipe support slab form work was levelled.

Some of the factor considered, when the leveling as follows

I.     Numbers of points were taken for level checking was depending on the size of the panel.

II.    Normally six or seven points were taken to ensure the level and if ever suspected level was noted there also level readings were taken.

III.  Especially in the middle of the slab noticeable sagging was noted.

Normally +5mm allowable was added to the sagging area of the slab.

Stair case form work

Specially, three methods were considered.

  1. Calculating method
  2. Used AutoCAD for finding the required soffit length
  3. practical method
  4. Graphical method

The staircase form work was not simple. Here the frame structure should be erected properly such that it can withstand all the loads that were affected such as dead load of concrete, reinforcement, and loads imposing while concreting. And also it should be properly levelled. To erect the structure we used 2”x 4” timber and to join them steel nails were used.

Inspections of form work

When erection of the form work had been completed and all the reinforcement was laid an inspection was carried out by consultancy.

Form work was inspected to check whether they were in accordance with the drawing and, whether they were in correct positions. In the same time as checking of the form work line and level was checked. The vertical of the wall and column form work was checked, by using a plumb bob. Slab and beam form work levels were checked by using an inverted staff and dumpy level.

Also joints were checked to ensure that grouts could not escape through them when the concrete was begun. Initially before the concreting a final check was made to that there were no wood shavings and dust left inside the forms.

Removing of form work

The striking or removal of form work should only take place on instruction from the engineer or agent. The appropriate time at which it was safe to remove form work can be assessed by rest on cubes taken from a similar batch mixed at the time the concrete was poured and cured under similar condition. The normal stripping times that we allowed in this site was as follows.

Time period of the form work removal,

  1. Sides of beams, walls, columns and piles                           03 day
  2. Soffit of secondary slab (Props left in)                                03 days
  3. Soffit of main slab (Props left in)                                           07 days
  4. Soffit of beams (Props left in)                                                  07 days
  5. Removal of props (Secondary slab)                                     07 days
  6. Removal of props (Beams & main slab)                               16 days
Building Construction formwork design

Form Work.

Inspection item at the site

The following items were checked before the concreting.

(a)                Marking

(b)               Level of bottom form

(c)                Alignment in both directions

(d)               Spacing of support, shoring and joists

(e)                Sectional Dimension

(f)                Required Embedded material

(g)               Gaps in between form work

Form work for column

Normally coated fly wood from grease mixed kerosene were used at the site.

It was necessary to consider available size of plywood board, before decided the way of fixing. Normally available size of the plywood board of this site was 1220*2440 mm. If the column size 600*600 mm, it was necessary to use the proper method for fixing of form work to minimize the wastage of materials. If the concrete height was more than about 2400mm, then fixing of the form work was shown bellow.

One side of the column =600 mm

As plywood size 1220 then fixing of plywood as above figure, plywood could be used directly without cut-off. (1220-100-12*2=1096 mm). Other side of the column equal to 600 mm then plywood could be used two columns by cutting two parts as shown above figure.

Form ties (separators) were used to maintain the spacing.

Method of fixing form work for column

Step 1: Kicker was fixed around the column marking and sponge was also used for avoiding the leakage of grout during concreting.

Step 2: Boards were prepared as above figure, before fixing of board to kicker.

Step 3: Separators (form tie) were fixed as above figure to tie the plywood board. (Separators are fixed in this all four side.

When the separators were fixed, it was necessary to determine the height of the column and width of the column. Normally, vertical distances between the separators were increased from bottom to top. Practical experience decided distance or calculation.

Step 4: galvanized iron Pipes were fixed as shown above figure. (50mm diameter steel pipes)

Method of checking form work and column before and after concerting

Step 1: Before concreting the column, it was necessary to check the dimension of the column and straightness by plumb bob. (By checking top and bottom of the column, it can be ensure throughout the column has same dimension)

Step 2: After concreting of the column, it was necessary to check the column as above mention. Figure of the checking method was shown below.

Besides these checks, hamlet is used to ensure the proper compaction of the concrete during concreting by striking the column form work.