3.Form Work Design

LARSEN & TOUBRO LIMITED

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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)

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LARSEN & TOUBRO LIMITED •

Height of lift H (m)

•

Minimum dimension of the section cast, d (mm)

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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

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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.

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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

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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

Impact of concrete discharge

In air or underwater

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

Vibration

Placing

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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

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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.

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Figure 2 Formwork pressures and deflection measurements

Table 1 Values of coefficients C1 and C2 Walls

C1 = 1.0

Columns: C2 = 1.5 Concrete

Value of C2

OPC. RHPC, or SRPC without admixtures

0.3

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

0.3

OPC. RHPC or SRPC with a retarder

0.45

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

0.45

or 40% pfa without admixtures LHPBFC.PBFC.PPFAC or blends containing less than 70% ggbfs or

0.45

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

FORM WORK DESIGN

0.6 0.6

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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 waterreducers 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.

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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 30ْ C or below 5ْ 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

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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

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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

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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 R level =

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.

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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

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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

1. Dimension in plan

-

+ 50 mm / - 12 mm

2. Eccentricity

-

2. Deviation from dimension of footings 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.

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Sl. No. 1 2 3

Type of Formwork Vertical formwork to columns, walls, beams Soffit formwork to slabs (Props to be refixed immediately after removal of formwork) Soffit formwork to beams (props to be refixed immediately after removal of formwork)

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Minimum period before striking of formwork 16 – 24 hours 3 days 7 days

Props to slabs: 4

1) Spanning up to 4.5 m

7 days

2) Spanning over 4.5 m

14 days

Props to beams and arches: 5

1) Spanning up to 4.5 m

14 days

2) Spanning over 4.5 m

21 days

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.

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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 M = w x L2/8

= 0.2 kN m = 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 Q = (5/8) x w x L

= 6.16 kN = (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 FORM WORK DESIGN

= l/360 or 1.5 mm whichever is less 28


FCCE = 1.05 kN m2

Modulus of rigidity 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

Bending moment consideration 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

FORM WORK DESIGN

= 4210 mm

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Deflection consideration Allowable deflection

= L/360 or 6 mm whichever is less,

Maximum deflection is given by 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 Simply supported end condition is assumed Spacing of primary member

= 2000 mm

Width of lading

= 2000 mm

W = loading intensity = 2.0 x 5.7

= 11.4 kN / m

Bending moment consideration 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) FORM WORK DESIGN

= (5 /384) x 11.4 x L ^ 4 / 145 30


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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 m2

=

5.7 kN/ m2

=

11.4 kN

Column side

=

600 mm

Steel waler spacing

=

1300 mm

Design Concrete pressure for columns

=

90 kN/ m2

Width of loading on waler

=

1300 mm

=

(2.0 x 1.0) m2

Design load intensity Actual load on prop

=

5.7 x 2.0 Hence Safe

COLUMN Force in diagonal tie rod of column For a 4.8 m height column box upto 600 mm size

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 Fy the following forces Fx

Fx Fy Where Fx = 70.2 / 2

= 35.1 kN

Fy = 70.2 / 2

= 35.1 kN

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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

=BxD

= 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 = 64.17 kN m2

EI 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

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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

2

= 5.7 x L2/8

M = w x L /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

Maximum deflection is given by 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

FORM WORK DESIGN

540 mm

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Secondary Beam Simply supported end condition is assumed Spacing

= 54 cm

Width of loading

= 54 cm


= 3.08 kN / m

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 Allowable shear force

= 6 kN

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


= 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


= 1640 mm

Therefore max. span of secondary member is

= 1640 mm

Say

1600 mm

Primary Beam Simply supported end condition is assumed Spacing

= 1600 mm

Width of lading

= 1600 mm


= 9.12 kN / m

FORM WORK DESIGN

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FCCE

Bending moment consideration Allowable Bending moment

= 1.167 kN m

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 Allowable shear force

= 6 kN

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


= L / 360 or 6 mm whichever is less

Maximum deflection is given by 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


= 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, ¾ Self weight of floor ¾ Floor finishes ¾ Partitions ¾ 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

35


FCCE

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.

FORM WORK DESIGN

36

3.Form Work Design

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