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Concrete Formwork Loads and Pressure Calculations

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Concrete formworks are subjected to various loads and pressure. Concrete formwork loads and pressure calculations are described in this article. Formworks or molds are considerably important for building constrictions by holding fresh concrete mixture at place until it get required strength by which the self weight can be sustained. Generally, there are various loads which are possible to act on formworks. Vertical loads are one of the most significant loads that act on formworks and are due to the self weight of the formwork and casted concrete plus live load of worker in addition to their equipment. Moreover, internal pressures which caused by the behavior of liquid fresh concrete, is acted on vertical formworks. Furthermore, it is mandatory to provide lateral bracing to achieve stability against lateral forces for example wind loads. concrete-formwork-loads-pressure-calculation

Concrete Formwork Loads and Pressure Calculations

Following are the various types of loads and pressures act on concrete formwork:
  1. Vertical load
  2. Lateral pressure of concrete
  3. Horizontal loads
  4. Special loads

1. Vertical Loads on Concrete Formwork

Vertical loads are exerted on formworks and may consist of dead loads such as formwork dead load, steel reinforcement embedded in the forms, molded fresh concrete and live loads such as the weight of workers, equipments, and tools. It is recommended to compute weight of materials separately in the case of heavy reinforcement to specify accurate unit weight. ACI 347-04: Guide to Formwork of Concrete specify that, to allow for workers and their placing tools such as screeds, vibrators, and hoses, at least 2.4Kpa live load should be used for the design of horizontal formworks and a minimum live load of 3.6 kPa should be employed in cases where motorized carts and buggies are utilized. Furthermore, ACI 347-04 determines combined live and dead load design of not less than 4.8 kPa or 6 kPa if motorized carts are employed. Finally, formwork selfweight is calculated using unit weight and dimensions of formwork various parts. The weight of formworks is substantially less than fresh concrete dead load and construction live load. That is why an allowance is decided as superimposed load per square meter for form work components during design. An initial assumption is made between 0.239-0.718 kPa based on experience and checked after the member is sized. This estimation is depending on the fact that common weight of formworks is 0.239-0.718 kPa.

2. Lateral Pressure on Concrete Formwork

Internal pressure resulted from accumulated depth of placed concrete is imposed on vertical formworks such as walls and columns. During vibration and for short period after vibration, placed fresh concrete close to the top and to a small depth of formwork behaves like a liquid and impose lateral pressure on the formwork that is equal to the vertical liquid head. Fresh concrete is granular with internal friction but vibrations eliminate bonds in the mixture and generate liquid state. There are different reasons such as placement rate, concrete temperature, and internal frictions that affect lateral pressure of below vibration controlled depth and make lateral pressure smaller than liquid pressure head. When vertical placement is carried out at slow pace, fresh concrete could have time to start stiffening. Moreover, unless concrete temperature is low, the time to start setting is not short. Other factors such as pore water movement, creation of friction and other parameters may lead to decline lateral pressure. Various types of cement, admixtures, cement substitutes, construction practices might influence level of lateral pressure. Mostly, concrete lateral distribution pressure, which based on tests, is depicted as shown in Figure-1. The distribution begins close to the top as a liquid and reaches peak value at lower level. For design reasons, it is suggested that ultimate pressure is uniform at conservative value.

Figure-1: Typical and Assumed Distribution of Concrete Lateral Pressure on Formworks

Calculation of Lateral Pressure on Concrete Formwork

ACI 347-04 specify that, concrete lateral pressure is computed as per Equation-1 if fresh concrete slump value is greater than 175 mm and does not placed with normal internal vibration to a depth of 1.2 m or less. Where: P: Lateral pressure of concrete, kPa : Density of concrete, Kg/m3 g: Gravitational constant, 9.81 N/kg h: Depth of fluid or plastic concrete from top of the placement to the point of consideration in the form, m However, ACI 347-04 stated that, if concrete slump value is no larger than 175 mm and placed with normal vibration to a depth of 1.2 m or less, then lateral pressure of concrete is calculated as follows: Lateral Pressure on Concrete Formworks for Columns With a minimum of 30Cw kPa, but in no case greater than . Where: Pmax: Maximum lateral pressure of concrete, kPa Cw: Coefficient of unit weight which is provided in Cc: Coefficient of chemistry that is provided in R: Concrete placement rate, m/h T: Concrete temperature during placing, oC Lateral Pressure on Concrete Formworks for Walls Concrete lateral pressure for walls with placement rate, smaller than 2.1 m/h and placement height is no greater than 4.2 m. With a minimum of 30Cw kPa, but in no case greater than . Concrete lateral pressure for walls with placement rate of greater than 2.1 m/h and placement height exceeds 4.2 m, and for all walls with placement rate of 2.1 to 4.5 m/h. With a minimum of 30Cw kPa, but in no case greater than . Table-1: Unit Weight Coefficient, Cw
Density of concrete, Kg/m3 Cw
Less than 2240 Cw=0.5[1+(w / 2320 Kg/m3)] but not less than 0.80
2240 to 2400 1.0
More than 2400 Cw=w / 2320 Kg/m3
Table-2: Chemistry coefficient, Cc
Type of cement or blend Cc
Type I, II, and III without retarders1 1.0
Type I, II, and III with a retarder1 1.2
Other types or blend containing less than 70 percent slag or 40 percent fly ash without retarders1 1.2
Other types or blend containing less than 70 percent slag or 40 percent fly ash with a retarder1 1.4
blend containing more than 70 percent slag or 40 percent fly ash 1.4
1Retarders include any admixture, such as a retarder, retarding water reducer, retarding mid-range water reducing admixture, or high-range water-reducing admixture (superplasticizer), that delays setting of concrete. Moreover, for pressure equation utilization, columns are defined as vertical elements with no plan dimensions surpass 2 m, and walls are vertical elements with at least one plan dimension larger than 2 m. Finally, in column forms, internal pressure in transferred to the external tie elements on adjacent side of the form which used as links between opposite sides of square or circular column. Furthermore, internal pressure in wall forms is transferred from plywood, studs, or wales to the tension ties that link two opposite sides of the form. In addition to provide aforementioned techniques to withstand internal pressures, providing resisting elements for example braces are essential for resisting external horizontal loads which tend to overturn wall, column, slab forms as shown in Figure-2 and Figure-3.

Figure-2: Schematic Bracing in Slab Formworks

Figure-3: Schematic Bracing in Walls Formworks

3. Horizontal Loads on Concrete Formworks

Horizontal loads might result from forces like wind, concrete dumping, equipment starting and stopping, and inclined supports should be opposed by properly designed braces and shore. For building construction, assumed value for these loads should not be less than the larger of either 1.5 KN/m of floor edge or 2% of total dead load spread as uniform load per slab edge linear meter, these assumptions is specified by ACI 347-04. Bracing for wall forms should be designed to meet requirements of minimum wind loads of ASCE 7-10 with adjustments for shorter recurrence intervals which could be found in ASCE 37-02 For wall forms exposed to elements 0.72 kPa or greater is used as minimum wind design load. Wall from bracing need to be designed for loads no less than 1.5 KN/m of wall length which is applied at the top.

4. Special Loads on Concrete Formworks

It is required to design formworks for uncommon construction conditions that could occur such as reinforcement concentrated loads, unsymmetrical placement of concrete, machine-delivered concrete impact, uplift, form handling loads. Constructing walls over spans of slab or beams that could impose different loading pattern before concrete hardening than that for which the supporting structure is designed for, is an example of special conditions that should be taken into consideration b form designer. Read More: Types of Formwork (Shuttering) for Concrete Construction Plastic Formworks for Concrete – Applications and Advantages in Construction Concrete Formwork Design Considerations – Basis for Concrete Formwork Design Wooden Concrete Formwork Design Criteria with Calculation Formulas Formwork Removal Time & Specifications Measurement of Formworks Formwork (Shuttering) for Different Structural Members -Beams, Slabs etc Formwork Safe Practices Checklist
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