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F alsework/ F ormwork Scaffolding shoring systems. Formwor k?.
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Formwork? • Concrete Formwork is the temporary structure that supports the fresh concrete and steel during the construction process within required Strength and Tolerance. Formwork for concrete structures has a significant impact on the cost, time, and quality of a completed construction project.
Formwork? • The cost of the formwork may exceed the cost of the concrete and reinforcing steel. The structural safety and reliability of formwork is just as important as that of the building itself. Failure of the formwork during construction could pose obvious safety concerns, not to mention the cost of ripping out incorrectly placed concrete.
General type of formwork • The type of formwork being used is related to the forces that it must be able to withstand. Formwork for a wall must obviously have greater strength and rigidity than formwork for say a curb. Design values for wall forms, column forms, beam forms, and footing forms are all different from each other. The form material must also be considered. The type of wood and the percent moisture are both factored into the design process.
The form materials • Wood • Plywood • Steel • Aluminium • Plastic • Plaster • Synthetics materials(others)
flyforms gang forms slip forms suspended slab forms beam forms jump forms single-sided forms cantilever forms arch forms bridge deck forms shaft-lining forms wall and column forms(of non-standard construction or non-standard height in conventional buildings) tunneling forms other types of formwork - the failure of which could, in the opinion of a Safety and Health Officer cause injury to a worker. Type of formwork
Criteria for choosing • Cost Time Quality: Economic Engineering • Durability of Material • Quality of textures • Quantity of works and uses • Cycle of repetitive uses • Pressure and pouring concrete methods • Quantity of bracing system • Skills and easy install and uninstall • Others
design process • The formwork must be inspected prior to use to ensure that it is structurally sound, watertight, and that it has been erected properly. It is also important that the design process is completed with great detail and that the formwork is placed in exact accordance with the design.
Contractor Responsibilities Work Platforms Concrete Placement Overhangs Premanufactured Items Tower Crane Shoring Rotation of Joists & Beams Inadequate Bracing Proper Foundation Single Post Shores Sloping Members Design Considerations
Design and Construction Consideration: Based on EIT. Standard • General • Force • Design calculation • Installation & Shoring • Take off • Reshoring • Tolerance • Special formwork
General • In order to determine the best methods to guard against formwork failure, the causes of failure must first be considered. When concrete is placed, it is in fluid form and lacks the strength to support itself. The forces acting upon the formwork can cause failure due to inadequate bracing or connections, or failure due to inadaquate strength of the forming material.
design process • The formwork must be inspected prior to use to ensure that it is structurally sound, watertight, and that it has been erected properly. It is also important that the design process is completed with great detail and that the formwork is placed in exact accordance with the design.
Any forces to formwork • The method by which concrete is placed can have a negative effect on the formwork as well. The pressure exerted by the concrete on the formwork depends on the rate at which the concrete is placed, the temperature of the concrete, the density of the concrete, and the method of consolidating the concrete. Carelessness on the part of the workers when placing or vibrating the concrete could lead to improper setting or collapse of the formwork.
Any forces(1) • Vertical forces: • DL. of Concrete, Steel, Formwork itself • LL. of Workers, Equipment and others • Impact Load
Any forces(1) • Vertical force
Wind Any forces(2) • Horizontal force Fresh concrete • Unbalance forces (Local load)
Consideration for design: General case • Weight Concrete = 2400 kg/m3 Reinforcement bars= 150 kg/m3of concrete • Any force from vibration, impact, eccentric…. Horizontal forces = 150 kg/m or 2% of DL. • Horizontal force from fresh concrete can be calculated as the following formula.
Column formwork Formula • Pressure from fresh concrete subjected to formwork P = 800+80000 R/(T+20) kg/m2 By P 15000 kg/m2 or 2400H kg/m2 : R = speed of pouring concrete (vert.) m./hr. T = concrete temperature (˚C) H = height of concrete inside formwork (m.)
Wall formwork Formula (slow) • Pressure from fresh concrete subjected to formwork when R<2 m./hr. P = 800+80000 R/(T+20) kg/m2 By P 10000 kg/m2 or 2400H kg/m2 : R = speed of pouring concrete (vert.) m./hr. T = concrete temperature (˚C) H = height of concrete inside formwork (m.)
Wall formwork Formula (fast) • Pressure from fresh concrete subjected to formwork when R ≥ 2 m./hr. P = 800+(120000+25000R)/(T+20) kg/m2 By P 10000 kg/m2 or 2400H kg/m2 : R = speed of pouring concrete (vert.) m./hr. T = concrete temperature (˚C) H = height of concrete inside formwork (m.) • For liquid action P = 2400H kg/m2
Fresh concrete pressure Formwork Y = P/unit con. wt. = P/2400 (normal) P as Eq. above or P = 2400 H H’ =H-Y
Example Pouring rate = 4.8 m3/hr. • Wall as figure, determine pressure • Concrete 1 m3: • Cement 320 kg • Water 160 kg • Sand 800 kg • Rock 920 kg • Temperature 30 ˚c 4m. 0.20m. 10m.
Solution 2m. • Speed (R) = 4.8/(0.2*10) = 2.4 m/hr. • P = 800+(120000+25000R)/(T+20) • P = 800+(120000+25000*2.4)/(30+20) = 4400 kg/m2 2m. • Concrete wt. = 320+160+800+920 = 2200kg/m3 • Y = 4400/2200 = 2 m. • Total pressure = ((4400*2)/2*10)+ (4400*2*10) = 44000+88000 = 132000 kg. = 132 T.
Calculation • It needs to be designed by Engineer. • Strength formwork/shoring system/supporting • Quality: level, vertical, position, texture, covering • Easy to install/uninstall/maintenance
Unload Load Install formwork and shoring system • Double checking • Leak, hole, etc. • Cleaning up…….lubricants, pouring water • Compensate for pressure error : ex. see figure Depended on many factors (1:360)
Take off • Formwork & shoring system can be taken off when concrete strength developing to be able to resist its weight as well as others occurring during construction process by considering method of construction. • In addition, sequence and duration of reinstalling formwork and shoring system depend on quality of concrete, importance of structure elements, any forces, temperature etc.
Table of min. required time for taking off formwork and shoring system (general structure)
Reshoring • Mean: after taking off formwork and shoring then reshoring simultaneously it again to transfer load into underneath structure. • Checking behaviors • Plan and permitted from authority • Avoid live loads accepted rechecking
