DESIGN OF FORMWORK

1.   INTRODUCTION

Formwork is an essential part of concrete construction. It is to give FORM to green concrete as per the structural and Architectural requirements. For concrete construction at higher elevations, FORMWORK supporting structure called centering (scaffolding) is necessary. Both can be called as enabling facilities to create permanent Members of a Structure. Design of formwork is the theme of this presentation and the same only will be dealt with hereafter.

For small and medium size works, provision of formwork is left to the carpenter’s / contractor’s hitherto experience at site. Naturally this method is more by experimentation rather than proper structural design. For safe, economical and sound provision of formwork, it is essential to design the same as structural member even though it is of temporary nature. Assessment of correct loads from the stage of pouring green concrete to the end stage of concrete member gaining self-supporting strength is most important. Secondary effects of above loads need to be duly considered and provided for in the design

Formwork can be designed and provided for permanent construction in different materials as follows:

  1. Timber in natural form or in plywood form. Plywood is used to achieve the shapes and forms
  2. Structural steel is quite versatile and useful for It is economical, durable, accurate and reusable. Use of shuttering plates made in mild steel is quite common due to their repetitive use.

It is essential to study the local environment to arrive at the best solution for best usage of right materials for formwork. Essence of safety, time and cost should govern the right choice for the materials to be used.

2.   PRIME REQUIRMENTS OF FORMWORK DESIGN

  1. Quality: The forms shall be designed and constructed to the desired size, shape and finish of the concrete required. The accuracy in the form, makes the structure stable and economical.
  2. Safety: Formwork should be capable of supporting all dead and live loads without collapsing. Safety to workmen and wet concrete shall be focus of control.
  3. Economy: Economy in material as well as time shall also be considered.

3.   IMPORTANT PARAMETERS FOR FORMWORK DESIGN

  1. Correct assessment of vertical loads over forms due to
    1. Weight of
    2. Weight of fresh concrete, with impact due to drop height
  • Weight of workmen and equipment
  1. Self-weight of
  2. Correct assessment of lateral forces exerting pressure on side forms and bracings.
  3. Wind forces on side
  4. Concrete, concreting methodology and member data
    • Density of concrete
    • Slump of concrete
  • Rate of pour
  1. Method of discharge
  2. Height of discharge
  3. Temperature of concrete
    • Dimensions of sections to be
    • Reinforcement detail
  4. Type of vibrations of concrete used for compaction of
  5. Formwork data
    • Formwork material
    • Stiffness of forms

4.   LOGIC OF FORMWORK DESIGN

  1. a) Green concrete exerts hydrostatic pressure on forms which is a function of its density D (gr) and height of pour H (gr).
  1. For horizontal forms for slabs. design vertical load will be

D (gr) x H (gr) + D (dr) x H (dr) + allowance for heaping of concrete & impact + self weight Where; D (gr) = Density of green concrete

H(gr) = Height of the pour of green concrete D(dry)= Density of dry concrete.

H(dry)= Height of temporary heap of concrete.

  1. For vertical forms for walls, columns and similar contained sections design hydrostatic pressure will vary from zero at top to D (gr) x H (gr) at lowest level of green concrete + impact pressure (uniform) of 1 T/sq. m. on account of falling concrete from height of about 2m. For pour rate of 0.6 m/hr hydrostatic pressure at lowest level will be 1.8 to 2.5 /sqm & double these valves for pour rate of 1.5 m/hr. (lower value will be for tropical climates). Above figures are given for the use of ordinary Portland Cement. Hydrostatic pressures on shuttering will lower for quick setting cements as well as for higher temperatures and vice versa.

It is recommended to just go by hydrostatic head pressure as above.

  • Allowable deflection for shuttering as per S. Code is Span/270 where span is spacing between bearers.

5.   IMPORTANT REFERENCE

  1. IS 4990 : 1993 for use of plywood for Concrete Shuttering
  2. IS 800 : Latest for use of structural Steel Shuttering
  3. The code of practice for Design and Construction of Formwork for Concrete by W.D. Govt. of Maharashtra..

6.   IMPORTANT FORMULAE

It is very important for the engineers to revise basics and fundamentals of design to understand the logic of structural behavior. Although typical examples in design are given hereafter, engineers shall be able to apply their mind and logic understood, to variety of problems in the field. Hence, it is absolutely necessary that engineers are clear about fundamentals.

Legend:

W          = Point Load in T, w = UDL T/M, (uniformly distributed load) L  = Span in Meter,  I = Moment of Inertia of section in M4

Z           = Section Modulus in Cub.M.,  E = Mod. Of Elasticity T/Sq.m A  = Area of Cross Section in Sq. m.

Fb         = Permissible stress in bending in T/Sq.m. Fs = Permissible stress in shear in T/Sq.mm.

M.R. = Moment of Resistance in T.M. = Fb x z

S.R. = Shear Resistance in T = Fs x A

All the above units used are in Tonne and Meter. Proper multipliers should be used while changing T to kg. and M to cm.

To calculate the maximum bending moment, following formulae are useful.

  1. B.M. @ Centre =            (w L2/8 )T. m
Max. Shear@ A & B=(wL/2) T
Max. Defl.@ center=5WL4
in M.  384xExI

If partial fixity or continuity over support is assumed, design B.M. can be derated to (WL2 /10) T.M.

  1. B.M. @ Centre = (WL/4) T.M. Max. Shear  @   A & B              = (W/2) T Max.  Defl.  @ Centre              = W L3

in M.                                                    48xExI

 

  1. B.M. @ A = (Wx L) T.M. Max. Shear @ A = (W) T

Max. Defl. @  B   = (WL3) In M.     3EI

 

  1. Typical vectorial Resolution of Forces following principle of static Equilibrium at any Junction of Forces:

Resolving along ‘X’ F1 =  F2 Cos Ø  = 0 Resolving along ‘Y’ F2 Sin Ø – W  = 0 Solving above simultaneous equations, F2 W/ Sin Ø ,F1 = W Cos Ø / Sin Ø (COMPRESSION)                    (TESNION)

7.         DESIGN EXAMPLES

  1. When Timber is used in Natural form for shuttering Form (modulus of elasticity 80 T/sq. cm.)
  2. Permissible stress parallel to grains in-
i)Bending, compression and tension84 kg/ sq. cm.
ii)Direct compression50 kg/ sq. cm.
iii)Shear9 kg / sq. cm.
iv)Modulus of elasticity80 T / sq. cm.
  1. Permissible stress perpendicular to grains in:
i)Direct compression15 kg / sq. cm.
ii)Shear stress6 kg / sq. cm.

Note : Above values should be reduced by 20% for wet timber .

: It is important to use GOOD QUALITY of timber to match with above minimum permissible stresses.

: It is preferable to be conservative about unknown quality of timber.

c) Moment of Resistance (in bending) for different depths for one cm width

M.R. = 84 x (bd2) / 6 = 14d2 kg. cm. shear Resistance for different depths for one cm. Width S.R. 6 (conservative) x d kg

ItemThickness or DepthM.R. in kg. Cm. /cm..S. R. in kg. /cm.
Planks2.5 cm thick8815
 4.0 cm thick22424
 5.0 cm thick35030
Joists7.5 cm thick78845
 10.0 cm deep140060
 12.5 cm deep218875
 15.0 deep315090
 20.0 cm deep5600120

Props: Load capacity reduction factor R.F. (multiplier) For slenderness are as follows:

Effective Length ‘L’ Least Lateral Dimension ‘d’ 

 1 to 1010 to 1515 to 1818 to 2424 to 30
R.F.10.750.700.600.50
  • Plywood formwork will have different permissible stresses depending upon Type of timber Hence, recommendations of Approved Manufactures /suppliers of plywood for permissible stresses etc. should be used in design.
  • Formwork in structural steel work will have reference to. I.S. 800 for Permissible stresses. Shuttering plates of 3mm thickness with angle Framing are commonly used directly below slab. For joists columns and channel or beam sections are used. Alternatively, ACROW make or equivalent Spans (steel trusses) and steel props are

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