3.form work design

LARSEN & TOUBRO LIMITED FCCE

FORM WORK DESIGN 10

FORM WORK DESIGN

CONCRETE LOAD AND PRESSURE

LOADS

Formwork for concrete must support all vertical and lateral loads that may be applied

until such time as these loads can be carried by the concrete structure itself.

Loads on forms are:

• Weight of reinforcing steel and fresh concrete

• Self weight of forms

• Various live loads imposed during the construction process

VERTICAL LOADS

Generally the weight of concrete with reinforcement can be assumed as 25 KN / cum.

Self weight of formwork, for ordinary structure, varies between 0.5 KN / sq.m to

0.75 KN / sq.m. A Minimum live load of 2.5 KN / sq.m on plan area is adequate for ordinary

construction. When motorized buggies are used, this is not so commom in India, the

minimum live load should be 3.75 KN/ sq.m. The minimum design load for combined dead

and live load should be 5.0 kN/sq.m or 6.25 kN/sq.m, if motorized buggies are used.

LATERAL PRESSURE ON CONCRETE

The effect of high frequency vibration on freshly placed concrete within formwork is

to keep it in a fluid state so that it behaves almost as a liquid. However, full hydrostatic

pressure may or may not be developed depending on whether stiffening of arching of

concrete occurs before the lift is finished.

For pressure calculation of concrete the following factors are taken into consideration:

• Density of concrete (kg/cum)

• Workability of the mix, Slump (mm)

• Rate of placing R (m/h)

• Concrete temperature (deg. Celsius)


FORM WORK DESIGN 11

• Height of lift H (m)

• Minimum dimension of the section cast, d (mm)

For structures concrete placed at controlled rates ACI committee 347 has developed

the formulae below for the maximum lateral pressure on the form for prescribed conditions

of temperature, rate of placement, vibration, weight-of concrete and slump. They are all

empirical formulae based on experiments and should not be considered theoretically precise.

If Pm = Lateral pressure, kg / sq. cm

Rst = Rate of placement, m/h

Tc = Temperature of concrete in the form, deg. Celsius

h = height of fresh concrete above point considered, m

Then,

1. For columns, Pm = 0.073 + 8.0 Rst Tc + 17.8

Not more than 1.47 kg/sq.cm or.24 h whichever is least

2. For walls, rate of placement not exceeding 2.0 m/h,

Pm = 0.073 + 8.0 Rst

Tc + 17.8

Not more than 0.98 kg / sq.cm or 0.24 h whichever is least

3. For walls, rate of placement ranging from 2.0 m/h to 3.0 m/h

Pm = 0.073 + 11.78 + 2.49 Rst

Tc + 17.8

Not more than 0.98 kg/sq.cm or 0.24 h whichever is least. Concrete pressure on

formwork can also be based on CIRIA Report 108, Excerpts of which are enclosed. This

takes into account the dimension of section cast.

HORIZONTAL LOADS

Bracings and props should be designed for all foreseeable horizontal loads due to

wind, inclined supports, dumping of concrete and equipment. However, in no case this


FORM WORK DESIGN 12

horizontal load acting in any direction at each floor line should be less than 150 kg per linear

meter of floor edge or 2 % of total dead load on the form distributed as a uniform load per

linear meter of slab edge, whichever is greater. Wall forms should be designed to meet wind

load requirements of the local building code. Bracing for wall forms should be designed for a

horizontal load of at least 150 kg per linear meter of wall, applied at top.

SPECIAL EQUIPMENT LOADS

If special methods of placing concrete using equipment like pumps are adopted the

formwork including staging should be designed for additional loads. In the case of pumping,

if the transport pipes are anchored on to the staging, the bends in the pipes will transmit very

high lateral and vertical loads to the staging.

It is also necessary to take into account the effects of starting and stopping of heavy

equipment on the deck. They can be estimated using the expression

F = Wa/g

Where, F = average force

W= weight of loaded equipment

a = acceleration or deceleration of equipment

When large concreting buckets are used, it may be necessary to unload the concrete

in one place for distribution. This can cause impact due to dropping, uplift and unbalanced

loading on the formwork and staging.

In the case of special structures like shells and domes there are additional factors to

be considered. Due regard should be given to unsymmetrical of eccentric loading which may

occur in the formwork during placement. Stress occurring during erection and decent ring of

the false work should also be considered. Since a three dimensional analysis of these

complex shapes is required, a competent an experienced engineer should do the structural

analysis. It is also essential to consider the necessity on back forms and the load flow from

concrete pressures on inclined form surfaces.


FORM WORK DESIGN 13

DESIGN CRITERIA

Although there is a need for greater accuracy than is frequently used in formwork

design, excessive refinement wastes time. Absolute precision is unwarranted when so many

assumptions have to be made as to loads, lateral pressures, and quality of materials,

workmanship at site and other factors.

Hence following simplifications can be done for computations of bending moment,

shear force and deflection:

• All loads are assumed uniformly distributed

• Beam supported over three or more spans are regarded as continuous and

approximate formulae are used.

The stresses induced in every member of formwork, in bending, in shear and in

bearing should be within the permissible working stress for that material. Forms must be so

designed that the various parts will not deflect beyond the prescribed limits. The permissible

deflection depress on the desired finish as well as the location.

In the absence of job specification to the contrary acceptable and frequently used

values of permissible deflections are:

• for sheathing, 1.6 mm

• for members spanning up to 1.5m is 3.0 mm

• for members spanning more than 1.5m is 6.0 mm or span/360, whichever is less

DESIGN METHOD

Freshly placed concrete comprises a gradation of particles from coarse aggregate

down to fine cement particles, all of which are suspended to a greater or lesser extent in

water. This is not a stable condition. The loss or displacement of an amore fraction of the

total mix water (by settlement, leakage or hydration) can change the structure of the fresh

concrete from a quash liquid to a relatively stiff framework of touching particles with the

water contained within the voids. This change in structure is important. While the aggregates

and cement are suspended in water, the concrete exerts a fluid pressure (Dh) on the

formwork, but once a stable particle structure has been created, further increments of vertical


FORM WORK DESIGN 14

load have an insignificant effect on the lateral pressure. Therefore the maximum lateral

pressure is generally below the fluid head and it is controlled by this change of stricture

(which can take from a few minutes to a few hours).

The following factors affect the change of state (and hence the maximum formwork

pressure):

Concrete

Admixtures

Aggregate shape. Size, grading and density

Cementations materials

Mix proportions

Temperature at placing

Weight density

Workability

Formwork

Permeability/water tightness

Plan area of the cast section

Plan shape of the cast section

Roughness of the sheeting material

Slope of the form

Stiffness of the form

Vertical from height

Placing

Impact of concrete discharge

In air or underwater

Placing method (e.g. in lifts or continuous vertical rate of rise)

Vibration


FORM WORK DESIGN 15

The complex inter-relationships of these factors are not described in this Report. A

rationalized design equation is presented, together with a description of how the variables

should be treated under design conditions.

Using concepts developed at the Cement and Concrete Association and during recent

research on the mechanisms creating formwork pressures. The data for OPC concrete were

analyzed to quantify the relationships between maximum pressure, vertical from height, rate

of rise and concrete temperature at placing. Modifications to these basic relationships were

then developed for concrete containing admixtures. This analysis led to the following

expression for the maximum concrete pressure on formwork:

Pmax = D (C1 √R – C2 R√ (H-C2 √R1)) or Dh kN/m2 whichever is less.

Where

C1 - coefficient dependent on the size and shape of from work (see Table I for values). √ (mh)

C2 - coefficient dependent on the constituent materials of the converts (see Table I for

values). √m

D - Weight density of concrete kN/m3

H - Vertical form height in m.

h - Vertical pour height in m.

K - Temperature coefficient taken as (36/ (T+16)) 2 R - The rate a: which the concrete rises vertically up the form. m/h T - concrete temperature at placing. ˚C

When C1 √R > H, the fluid pressure (Dh) should be taken as the design pressure.

The term C1 √ R incorporates the effects of vibration and workability, because these

factors are largely dependant on size, shape and rate of rise. All the effects of the height of

discharge, cement type, admixtures, and concrete temperature at placing are incorporated in

the term:

C2 K√ (H-C1 √R)

The design chart. Table 2. quantifies these equations for normal UK conditions

where the concrete placing temperature is between 5 and 15º C. Pressure values shown in

bold on the chart are for placing conditions broadly covered by pressure measurements on


FORM WORK DESIGN 16

site, where the highest recorded pressures were 90 kN/m2 for walls and 166 kN/m2 for

columns. Values not in bold are outside recorded experience. They are in accord with the

general trend, but may be somewhat conservative.

No change is proposed in the design pressure envelope from that given in the CIRJA

Report I design method. The envelope (Figure 1) comprises fluid pressure to the depth where

the maximum pressure obtained from the design equation or chart occurs and then remains at

this value.

Figure 1 Design Pressure Envelope

Figure 2 is an example of measurements of formwork pressure and deflection taken

on a sit. This illustration shows that once a form deflects, it remains in that state until the tie

bolts are released. In theory, rigid forms would experience a reduction in pressure after the

maximum. In practice. Forms are not rigid, and some stress remains between the form and

concrete. For this reason, no reduction in pressure after the maximum is given in the design

pressure envelope.


FORM WORK DESIGN 17

Figure 2 Formwork pressures and deflection measurements

Table 1 Values of coefficients C1 and C2

Concrete Value of C2

OPC. RHPC, or SRPC without admixtures

OPC. RHPC or SRPC with any admixture, except a retarder

OPC. RHPC or SRPC with a retarder

LHPBFR. PBFC.PPFAC or blends containing less than 70% ggbfs

or 40% pfa without admixtures

LHPBFC.PBFC.PPFAC or blends containing less than 70% ggbfs or

40% pfa with any admixtures except a retarder

LHPBFC.PBFC.PPFAC or blends containing less than 70% ggbfs or

40 pfa with a retarder

Blends containing more than 70% ggbfs or 40% pfa

0.3

0.3

0.45

0.45

0.45

0.6

0.6

Walls C1 = 1.0 Columns: C2 = 1.5


FORM WORK DESIGN 18

NOTES FOR GUIDANCE

Cementitious Materials and Admixtures

Coefficient C2 (see Table 1) takes into account the effects of different cementitious

materials and admixtures. The term ‘admixture’ in Table I covers the range of products

commercially available in 1985: Within the grouping ‘retarder’ fall readers, retarding water-

reducers and retarding superplasticisers, also any admixture which is used above the

recommended dosage such that it effectively acts as a retarder.

A major change from existing practice is the recommendation that superplasticised

concrete should be included within the general grouping, and that it does not necessarily

require design pressure equal to the fluid head.

Aggregates

While quantifying the design equation, the effects of the aggregate shape and grading

could not be isolated from the other mix parameters, so these factors are not included in the

design method. With the exception of no-fines concrete the formula apply to all graded

natural aggregates.

The design equations apply to concrete mixes containing maximum aggregate sizes

up to 40 mm. Pressures with larger maximum sized aggregates are likely to be controlled by

the impact on discharge and the heavy vibration required..

MIX PROPORTIONS

The formula and design tables apply to the whole range of normal mix proportions.

No-Fines Concrete

Because no - fines concrete has a particle structure from the moment of placing. It

results in a low formwork pressure. Typical design values are of the order of 2 to 2.5 kN/m2

so that handling stresses are likely to control the design of the form.


FORM WORK DESIGN 19

Workability

Slump is not included as a variable in the design chart for the following reasons:

1. The problems with placing low workability concrete around reinforcement lead to

prolonged vibration and formwork pressures similar to those obtained with more

workable concretes.

2. The site data show no consistent difference in formwork pressure between low,

medium and high slump concretes.

3. Slump is not a good measure of the factors which affect formwork pressure.

Concrete Temperature at Placing

At low rates of concrete placing, hydration effects become a significant factor in

determining the maximum formwork pressure. Because these effects depend on the concrete

temperature at placing, the design equation including a temperature factor

K = (36/ (T + 16)) 2

Although this only strictly applies to OPC and RHPC concretes, it is sufficiently

accurate for all types of concrete when used in conjunction with coefficient C2. The K factor

represents a ratio of stiffening effects, which are dependent on temperature at placing. Data

for concrete temperatures at placing in excess of 30 ْC or below 5 ْC are rare, and it is prudent

not to extrapolate the design equation beyond these values.

VERTICAL FORM HEIGHT

The vertical form height is important for two reasons:

1. It limits the potential maximum pressure which can develop (in general, the

maximum design pressure is not greater than Dh ).

2. Height of discharges affects the magnitude of the impact forces.

Both these factors affect the maximum formwork pressure, and they have been

incorporated in the design equation as a function of the form height.

Sometimes, the form can be substantially higher than the height of section cast (see

Figure 3). In these cases, the limiting pressure might be the fluid pressure (which is obtained


FORM WORK DESIGN 20

from the weight density times the actual pour height). This should be checked with a separate

calculation.

Figure 3 Height Value to be used in Formulae

Shape and Plan Area of the Cast Section

In a section of small plan area, vibration can be sufficient to mobilize all the concrete

in a layer and to transmit a relatively high amount of energy to the form. This has the effect

of increasing the depth over which vibration is effective, and consequently the pressure on

the form.

In a larger section, all the concrete in a layer is not mobilized at the same time, and

less energy is transmitted into the formwork. The point of concrete discharge and vibration is

normally moved along the section, which allows the concrete a period of rest before the next

layer is placed. The net effect is that in ‘walls’ the maximum pressures are lower than in

‘columns’. In fundamental terms, a wall is where the concrete placed in layers with the point

of discharge and vibration moving along the wall, while, for columns, the point of discharge

and vibration is raised vertically. These conditions can be conservatively defined using the

following simple definitions:

Wall or base – section where either the width or breadth exceeds 2 m

Column - section where both the width and breadth are 2 m or less.

The few size data recorded for small, single- storey columns indicated a fluid

pressure distribution. The formula generally predicts fluid head for small columns. This is

reasonable, because small columns can be placed very quickly and vibrated such that the full


FORM WORK DESIGN 21

fluid head is mobilized. However, an analysis of the forces on column camps indicates that

they would fail if concrete in columns develops full fluid pressure. It is therefore widespread

practice to design small ply and timber column forms assuming less than the fluid head. The

possible explanation of this anomaly has not been experimentally verified.

Formwork Permeability

Formwork pressure decreases as the formwork permeability increases, if all other

conditions are equal. This reflects the extent to which excess pore water pressure can

dissipate through the formwork. The pressures are substantially lower with extremely

permeable form materials such as expanded metal of fabric. In theory, the design equation

should contain a factor for form permeability. Effects such as reduction of permeability

through previous usage and the use of sealers and coating, throw doubt on the ‘practicality’

of such a factor. Because the design equation does not include a factor for form permeability,

the estimated pressures are not applicable to form materials such as expanded metal, where

they effectively act as free surfaces and prevent the build up of pore water pressure.

Formwork Stiffness and Roughness

Study of the data suggests that the use of stiffer forms results in high pressures.

Conversely, independent research work shows that the formwork pressure decreases

substantially if a stiff form is moved slightly outwards. In most practical situations, the

stiltness of a form varies from point to point, and it is difficult to quantify. Formwork

stiffness was not, therefore, included in the design equation.

While the concrete is acting as a fluid, the formwork roughness is immaterial, until a

particle structure forms and the concrete starts to develop internal friction. Compared with

other factors, its influence on the maximum pressure is small, and it has not been isolated in

the design equation.

Slope of the Form

The pressure on sloping forms was not specifically examined in the research, and

only a few experimental results were available. However, the CIRIA method described in

this Report can be used conservatively with non-parallel sided walls with and without a

uniform rate of rise. If the volume supply rate is varied so that the rate of vertical rise is


FORM WORK DESIGN 22

constant, the equation of tables can be directly used. The pressure at any level in the pour is

the same on both faces, and the direction of action is perpendicular to the form (see Figure

4).

Figure 4 Pressure envelopes on the Formwork of a wall with sloping face where the fluid head is fully developed

The following method is suggested for calculating the pressure envelope with a

constant volume supply rate:

1. Split the pour into horizontal levels with the vertical distance between each level 1m or

less.

2. Calculate the plan area at each level.

3. Calculate the instantaneous rate of rise at each level from

Uniform volume supply rate (m3 /h)

plan area at the level considered (m2)

4. Calculate the pressure at each level using the full height of the form H either from the

equation or tables.

5. Produce the design pressure envelope acting at right angles to the form.

R level =


FORM WORK DESIGN 23

Placing Method

The design equations do not apply to conditions where the concrete is being pumped

from below of where pre-placed large aggregate is grouted from below. In both these cased,

the formwork pressures are likely to be higher than these given in the Report.

American experience” suggests that the formwork should be designed to withstand

fluid pressure plus 50% for pump surge.

Rate of Rise

The rate at which the concrete rises vertically up the formwork is an important factor,

and it is included in the design equation. In practice, this is never constant, but, the use of an

average rate of rise is normally adequate for vertical formwork. The average rate of rose

might not be applicable when a considerable lift is placed rapidly, followed by a long delay

before the subsequent lift.

As the rate of rise increased, the maximum pressure increased, but the relationship is

not linear. At high rates of rise, changes in the rate of rise have less effect on the maximum

pressure than changes at lower rates of rise (see Figure 5).

Figure 5 Relationship between rate of rise and pressure


FORM WORK DESIGN 24


FORM WORK DESIGN 25


FORM WORK DESIGN 26

Extracts from IS: 456 – 2000 regarding formwork

General

The formwork shall be designed and constructed so as to remain sufficiently rigid during

placing and compaction of concrete and shall be such as to prevent loss of slurry from the

concrete. The tolerances on the shapes, lines and dimensions shown in the drawing shall be

within the limits given below:

1. Deviation from specified dimension of

Cross-section of columns and beams - + 12 mm / - 6 mm

2. Deviation from dimension of footings

1. Dimension in plan - + 50 mm / - 12 mm

2. Eccentricity - 0.02 times the width of the footing

in the direction of deviation but

not more than 50 mm.

3. Thickness - +/- 0.05 times the specified

thickness

These tolerances apply to concrete dimensions only and not to positioning of vertical

reinforcing steel or dowels.

Stripping time

Forms shall not be released until the concrete has achieved strength of at least twice the

stress to which the concrete may be subjected at the time of removal of formwork. The

strength referred to shall be that of concrete using the same cement and aggregates and

admixtures, if any, with the same proportions and cured under conditions of temperature and

moisture similar to those existing on the work.

While the above criteria of strength shall be the guiding factor for removal of formwork,

in normal circumstances where ambient temperature does not fall below 15oC and where

ordinary Portland cement is used and adequate curing is done, following striking period may

deem to satisfy the guideline given below.


FORM WORK DESIGN 27

For other cements and lower temperatures, the stripping time recommended above

may be suitably modified.

The number of props left under, their sizes and disposition shall as to be able to safely

carry the full dead load of the slab, beam or arch as the case may be together with any live

load likely to occur during curing or further construction.

Where the shape of the element is such that the formwork has re-entrant angles, the

formwork shall be removed as soon as possible after the concrete has set, to avoid shrinkage

cracking occurring due to the restraint imposed.

Sl. No. Type of Formwork Minimum period before

striking of formwork

1 Vertical formwork to columns, walls, beams 16 – 24 hours

2 Soffit formwork to slabs (Props to be refixed

immediately after removal of formwork) 3 days

3 Soffit formwork to beams (props to be refixed

immediately after removal of formwork) 7 days

4

Props to slabs:

1) Spanning up to 4.5 m

2) Spanning over 4.5 m

7 days

14 days

5

Props to beams and arches:

1) Spanning up to 4.5 m

2) Spanning over 4.5 m

14 days

21 days


FORM WORK DESIGN 28

System Formwork Design

The design procedure will be explained through the following examples and design charts:

Data

Slab Thickness = 150 mm

Use 12 mm plywood as sheathing

H-16 Timber beams as secondary and primary beams

Floor Props CT as the staging (floor to floor ht. 3.3 m)

Loads

Slab self weight (0.15) x (25+1) = 3.9 kN/ m2

Live load = 1.5 kN/ m2

Formwork load = 0.3 kN/ m2

Total Load = 5.7 kN/ m2

Analysis & Design

Plywood

End condition is assumed that of a propped cantilever

Let “L” be the effective span

Note: effective span = clear span + t

Where t is the thickness of plywood

For analysis 1 meter width of plywood is assumed

Bending moment consideration

Allowable Bending moment = 0.2 kN m

M = w x L2/8 = 5.7 x L2/8

By equating Max moment M to the allowable bending moment, we get

L = (0.2 x 8/5.7)^0.5 = 530 mm

Shear force consideration

Allowable shear force = 6.16 kN

Q = (5/8) x w x L = (5/8) x 5.7 x L

By equating Max shear force to the allowable shear force

L = (8/5) x 6.16 / 5.7 = 1730 mm

Deflection consideration

Allowable deflection = l/360 or 1.5 mm whichever is less


FORM WORK DESIGN 29

Modulus of rigidity = 1.05 kN m2

Maximum deflection is given by

d = (1/185) x wxL^4 / (EI) = (1 / 185) x 5.7 x L^4 / (1.07)

By equating the maximum deflection the allowable or limiting deflection,

L = ((185 x 1.07) / (5.7 x 360)) ^ 0.333 = 460 mm

Effective span is taken as the minimum of the above values

Therefore effective span = 460 mm

Using H-16 beams the c/c span will be Effective span – thickness of plywood + width of

flange of secondary member

Effective span – 12 + 65 = 512 mm

Say 500 mm

Secondary member

H 16 Beam is used as a secondary member

Simply supported end condition is assumed

Spacing of H - Beams = 500 mm

Width of loading = 500 mm

Loading Intensity is the product of width of loading and total load,

W = loading intensity = 0.5 x 5 = 2.85 kN / m


Allowable bending moment = 3 kN m

Max moment is given by,

M = w x L^2/8 = 2.85 x L ^ 2/8

By equating the allowable bending moment to the maximum moment, we get

L = (3.0 x 8 / 2.35) ^ 0.5 = 2900 mm

Shear force consideration,

Allowable shear force = 6 kN

Max shear force is given by,

Q = W x L/2 = 2.85 x L/2

By equating the allowable shear force to the maximum shear force

L = 2.6 / 2.85 = 4210 mm


FORM WORK DESIGN 30

Deflection consideration

Allowable deflection = L/360 or 6 mm whichever is less,


d = (5/384) x W x L^4 / (E I) = (5/384) x 2.85 x L^4 / 145

By equating the maximum deflection to the allowable deflection,

L = ((384 / 5) x 1.45 / (2.85 x 360)) ^ 0.333 = 2210 mm

Minimum of the above values is 2210 mm

Therefore max. Span of secondary H 16 is 2210 mm

Say 2000 mm

Primary member

H 16 Beam is used as a primary member


Spacing of primary member = 2000 mm

Width of lading = 2000 mm

W = loading intensity = 2.0 x 5.7 = 11.4 kN / m


Allowable moment = 3 kN m

Maximum moment is given by,

M = w x L^2/8 = 11.4 x L ^2/8

By equating the allowable moment to the maximum moment, Effective span is

L = (3.0 x 8 / 11.4) ^0.5 = 1450 mm

Shear Force consideration

Allowable shear = 6 kN

Maximum shear is given by the equation,

Q = W^L/2 = 11.4 x L/2

By equating the allowable shear to the maximum shear,

Effective span, L = 2.6 / 11.4 =1050 mm

Deflection Consideration

Allowable deflection = L / 360 or 6 mm whichever is less,

Maximum deflection is given by the equation

d = (5/384) x w x L^4 / (E I) = (5 /384) x 11.4 x L ^ 4 / 145


FORM WORK DESIGN 31

By equating the allowable deflection to the maximum deflection

Effective span, L = ((384/5) x 145 / (11.4 x 360)) ^0.333 = 1390 mm

Minimum of the above values is 1050 mm

Therefore max. span of primary H-16 is 1000 mm

Check for prop

Prop = CT 410

Floor to floor ht. = 3.3 m

Ht. of prop = (3300 – 12 – 160 – 160) mm = 2968 mm

Load carrying capacity of CT 410 at 2.968 m = 26 kN

Loading area = (2.0 x 1.0) m2 = 2.0 m2

Design load intensity = 5.7 kN/ m2

Actual load on prop = 5.7 x 2.0 = 11.4 kN

Hence Safe

COLUMN

Force in diagonal tie rod of column

For a 4.8 m height column box upto 600 mm size

Column side = 600 mm

Steel waler spacing = 1300 mm

Design Concrete pressure for columns = 90 kN/ m2

Width of loading on waler = 1300 mm

Horizontal force due to pressure on each of the four walers = 0.6 x 1.3 x 90

= 70.2 kN

At each side of the two open corners of the column box, the waler end will be subjected to

the following forces

Where Fx = 70.2 / 2 = 35.1 kN

Fy = 70.2 / 2 = 35.1 kN

Fx

Fy

Fy

Fx


FORM WORK DESIGN 32

Resultant diagonal force R = 1.414 x Fx or Fy = 49.6 kN

This Force is resisted by the tie rod which has the Safe bearing capacity of 50 kN.

Design of Slab Formwork - Conventional

Data

Slab Thickness 150 mm

Use 12 mm plywood as sheathing

4” x 4” Timber beams as secondary and primary beams

Floor Props CT as the staging

Section property of 4” x 4” (i.e., 10 cm x 10 cm) Timber

Breadth, B = 100 mm

Depth, D = 100 mm

Area, A = B x D = 10000 mm2

Moment of Inertia, I = (1/12) x100x1003 = 833.33 x 104 mm4

Section modulus, Zxx = I/ (D/2) =166.67 x 103 mm3

Modulus of elasticity = 7700 N/mm2

Allowable Bending Stress fb = 700 N/mm2

Allowable Shear Stress fq = 60 N/mm2

Bending Moment capacity = fb x Zxx

= 700 x 166.67 x 103

= 1.167 kN.m

Shear Force Capacity = fq x A

= 60 x 10000

= 6 kN

E I = 64.17 kN m2

Loads

Slab self weight (0.15) x (25+1) = 3.9 kN/ m2

Live load = 1.5 kN/ m2

Formwork load = 0.3 kN/ m2

Total Load = 5.7 kN/ m2


FORM WORK DESIGN 33

Analysis & Design

Plywood

End condition is assumed that of a propped cantilever

Let “L” be the effective span

Note: effective span = clear span + t

Where t is the thickness of plywood

For analysis 1 meter width of plywood is assumed



M = w x L2/8 = 5.7 x L2/8

By equating Max moment M to the allowable bending moment, we get

L = (0.2 x 8/5.7)^0.5 = 530 mm

Shear force condition

Allowable shear force = 6.16 kN

Q = (5/8) x w x L = (5/8) x 5.7 x L

By equating Max shear force to the allowable shear force

L = (8/5) x 6.16 / 5.7 = 1730 mm

Deflection condition

Allowable deflection = l/360 or 1.5 mm whichever is less

Modulus of rigidity = 1.05 kN m2


d = (1/145) x wxL^4 / (EI) = (1 / 185) x 5.7 x L^4 / (1.07)

By equating the maximum deflection the allowable or limiting deflection,

L = ((145 x 1.07) / (5.7 x 360)) ^ 0.333 = 460 mm

Effective span is taken as the minimum of the above values

Therefore effective span = 460 mm

Using 100 mm x 100 mm Timber Beams the c/c span will be

Eff. Span – 12 + 100 = 547 mm

Say 540 mm


FORM WORK DESIGN 34

Secondary Beam


Spacing = 54 cm

Width of loading = 54 cm


Bending moment Consideration

Allowable bending moment = 1.167 kN m

Maximum moment of the member is given by,

M = w x L2/8 = 3.08 x L 2/8

By equating the maximum moment to the allowable moment, effective span is given by

L = (1.167 x 8 / 3.08) ^ 0.5 = 1740 mm

Shear Force consideration


The maximum shear force is given by,

Q = W x L/2 = 3.08 x L/2

By equating the maximum shear force to the allowable shear force, effective span is given by

L = 2.6 / 3.08 = 3900 mm


Allowable deflection = L / 360 or 6 mm whichever is less

The maximum deflection is given by,

d = (5/384) x W x L^4 / (E I) = (5/384) x 3.08 x L^4 / 64.17

By equating the maximum deflection to the limiting deflection,

L = (((384 / 5) x 64.17 / (3.08 x 360)) ^ 0.333 = 1640 mm

Minimum of the above values is = 1640 mm

Therefore max. span of secondary member is = 1640 mm

Say 1600 mm

Primary Beam


Spacing = 1600 mm

Width of lading = 1600 mm



FORM WORK DESIGN 35



The maximum moment is given by the equation

M = W x L2/8 = 9.12 x L 2/8

By equating the maximum moment to the allowable moment, effective span is

L = (1.167 x 8 / 9.12) ^0.5 = 1010 mm

Shear force consideration


Maximum shear force is given by,

Q = WxL/2 = 9.12 x L/2

By equating the maximum shear force to the limiting shear force, effective span

L = 2.6 / 9.12 = 1320 mm


Allowable deflection = L / 360 or 6 mm whichever is less


d = (5/384) x w x L^4 / (E I) = (5 /384) x 9.12 x L ^ 4 / 64.17

By equating the maximum deflection to the limiting deflection, effective span

L = (((384/5) x 64.17 / (9.12 x 360)) ^0.333 = 1140 mm

Minimum of the above values is = 1010 mm

Therefore max. Span of primary member is 1010 mm.

EARLY STRIPPING

Any floor is designed for Dead Loads + Live Loads

Dead load includes,

Self weight of floor

Floor finishes

Partitions

False ceiling etc

The percentage of self weight alone when compared to the total design load could vary

from 30 % to 60 %. After concreting of the floor the loads are only the sum of self weight


FORM WORK DESIGN 36

and nominal live load. This could be between 40 % and 70 % of the design load. This

reduction in load is more advantageous for early stripping. This is more helpful when the

percentage is less. Hence when the concrete attains the above percentage of its 28 day

strength it is possible to remove the form work fully. Normally concrete attains 40 % strength

in 3 days and 70 % strength in 8 days.

It is also possible to remove the formwork after leaving props known as “Reshores”

to reduce the span of the beam and slab even when the concrete has not attained sufficient

strength for getting fully deshuttered. This will help it getting more number of uses from the

available form work. In case the floor above is to be supported when concreted, then

additional supporting props or scaffolds will have to be provided in the slab below to

withstand the additional load due to weight and construction live loads from floor above.

3.form work design

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