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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:
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.
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.
It is recommended to just go by hydrostatic head pressure as above.
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.
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.
in M. 48xExI
Max. Defl. @ B = (WL3) In M. 3EI
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)
i) | Bending, compression and tension | 84 kg/ sq. cm. |
ii) | Direct compression | 50 kg/ sq. cm. |
iii) | Shear | 9 kg / sq. cm. |
iv) | Modulus of elasticity | 80 T / sq. cm. |
i) | Direct compression | 15 kg / sq. cm. |
ii) | Shear stress | 6 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.
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
Item | Thickness or Depth | M.R. in kg. Cm. /cm.. | S. R. in kg. /cm. |
Planks | 2.5 cm thick | 88 | 15 |
4.0 cm thick | 224 | 24 | |
5.0 cm thick | 350 | 30 | |
Joists | 7.5 cm thick | 788 | 45 |
10.0 cm deep | 1400 | 60 | |
12.5 cm deep | 2188 | 75 | |
15.0 deep | 3150 | 90 | |
20.0 cm deep | 5600 | 120 |
Props: Load capacity reduction factor R.F. (multiplier) For slenderness are as follows:
1 to 10 | 10 to 15 | 15 to 18 | 18 to 24 | 24 to 30 | |
R.F. | 1 | 0.75 | 0.70 | 0.60 | 0.50 |
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