Formwork

By

Paul Markham

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Definition of formwork from BS5975:2008

Formwork (also forms, shutters or shuttering): structure, usually temporary, but in some cases wholly or partly permanent, used to contain poured concrete to mould it to the required dimensions and support it until it is able to support itself.

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Formwork

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Formwork

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Formwork

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Formwork

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Formwork

w

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Timber formwork

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Timber formwork

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Formwork

The loads that must be considered in formwork and false work fall into five categories;

1. Self weight of the formwork.

2. Operating loads/live loads.

3. Environmental loads.

4. Horizontal loads.

5. Loads due to concrete.

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Formwork

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Formwork

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Limit state or permissible stress?BS5975 is written in terms of permissible stress but large parts refer to British Standards which were withdrawn on 31 March 2010.It would be possible to design the timber secondaries to BS EN 1995 (Eurocode 5). However, often aluminium secondaries used andPlySoldiers All have to be designed in accordance with permissible stress.Tie rods

Therefore probably easiest to design using permissible stress and using aluminium secondaries. However, sometimes it will be necessary to use timber secondaries in which case mixed permissible and partial factor design is required.

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TimberMost timber is now S4S – sawn four sides, this reduces the section sizes slightly so that a section nominally 100 x 50mm is actually 95 x 47mm (20% reduction in second moment of area). Common timber grades for formwork use are C16 and C24C Conifer (D, deciduous)16 or 24 Characteristic bending strength in MPa.Allowable strength is about a third of the characteristic strength but other factors apply for moisture content and duration of loading etc.

Use charts to EC5 for rapid design.

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Timber design charts

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Single sided formwork

H

Raking prop

Horizontal reaction

Horizontal reaction

Uplift

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Single sided formwork

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Hy-rib

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Hy-rib

Expanded metalLower pressures

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Formwork – supplier’s software

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Formwork

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Formwork

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Formwork

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Software

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Software

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Climbing Formwork

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Climbing Formwork

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Climbing Formwork

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Climbing device ACS 100

upper climbing head

hydraulic cylinder

lowerclimbing head

climbing rail

climbing shoe

PERI ACS – Automatic Climbing System.

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Climbing Formwork

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Formwork

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Formwork

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Formwork

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Formwork

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Formwork – RMD Alsec

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Formwork – RMD Albeam

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Formwork – RMD Alform

Compare these figures with characteristic loads

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RMDK Soldiers – allowable loads

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Plywood

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Timber design to EC5

1200 2no 145 x 47 timbersDesign loads:Bending moment, M = 3.66kNm Shear Force, F = 15.2kNDeflection, d = 0.00541 W L3 / EI (for a two span continuous beam from Formwork a Guide to Good Practice)Second moment of area, I = bd3 / 12 = 2 x 47 x 1453 / 12 = 23.9 x 106mm3

Therefore, d = 0.00541 (15 x 1.2) x 12003 / (7.4 x 103 x 23.9 x 106) = 0.95mm

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Timber design to EC5

C24 timber: Characteristic bending strength, fm,k = 24 MPa (BS EN 338)

Various factors must be applied to this to get the design bending strength:

kh,m Depth factorkmod Modification factor for duration of load and moisture contentksys System strength factorkcrit Factor for lateral bucklinggM Partial factor for material properties

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Timber design to EC5

Clause 3.2 (3)kh,m = lower of (150/h)0.2 and 1.3. Where h is the depth

(150/h)0.2 = (150/145)0.2 = 1.007

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Timber design to EC5

kmod :

Therefore, medium term loading .The falsework will be erected outdoors and subject to rain. Therefore Service Class 3.

Use Table 3.1 to find kmod

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Timber design to EC5

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Timber design to EC5

ksys System strength factor – Clause 6.6

The secondaries can be used to distribute the loads from one member to the adjacent members. They will be overloaded but this is an accidental situation and that overloading would be acceptable. Therefore use ksys = 1.1.

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Timber design to EC5

kcrit is a factor to account for lateral buckling see Clause 6.3.3 (5)

The primaries in this falsework will be located in U-heads which will provide torsional restraint at the supports. There will be sufficient friction with the secondaries to prevent lateral displacement of the top of the timber. Therefore use kcrit = 1.0.

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Timber design to EC5

gM is the familiar partial factor for material properties (Table 2.3)

So use gM= 1.3.

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Timber design to EC5

Design bending strength, fm,d = kh,m x kmod x ksys x kcrit x fm,k / gM

= 1.007 x 0.65 x 1.1 x 1.0 x 24 / 1.3 = 13.3MPa

Therefore the bending resistance, Md = fm,d x wy Where wy is the section modulus. wy = N x b x h2 / 6 = 2 x 47 x 1452 / 6 = 329 000mm3. Therefore Md = 13.3 x 329 000 = 4.4kNm > applied moment, therefore bending is ok.

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Timber design to EC5

Check shear: F = 15.2kN

Applied shear stress td = 3 x F / 2 x A = 3 x 15.2 x 103 / (2 x 2 x 47 x 145) = 1.65 MPa

C24 timber: Characteristic shear strength, fv,k = 2.5MPa (BS EN 338)

Various factors must be applied to this to get the design shear strength:kmod Modification factor for duration of load and moisture contentksys System strength factorgM Partial factor for material propertiesFactor of 1.5 allowed by BS 5975 for falsework

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Timber design to EC5

Therefore, design shear strength,fm,d = kmod x ksys x fv,k x 1.5 / gM

= 0.65 x 1.1 x 2.5 x 1.5 / 1.3 = 2.06MPa > Applied, therefore ok

Deflection:Permissible deflection = smaller of 5mm or span/270

Span/270 = 1200 / 270 = 4.4mm.Therefore, permissible deflection = 4.4mm > calculated deflection (0.9mm) Therefore deflection is ok.

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Formwork

Any questions?