Calculating Concrete Formwork Loads and Pressure
Concrete formworks are essential for construction projects, as they hold the fresh concrete mixture in place until it reaches the required strength to support its own weight. There are a number of different loads that act on formworks, including vertical loads due to the self weight of the formwork and the concrete, plus live loads of workers and their equipment. Internal pressures resulting from the behavior of the liquid concrete also act on vertical formworks. Furthermore, lateral bracing is necessary to provide stability against lateral forces, such as wind loads.
Concrete Formwork Loads and Pressure Calculations
Following are the various types of loads and pressures act on concrete formwork:
- Vertical load
- Lateral pressure of concrete
- Horizontal loads
- Special loads
Vertical Loads on Concrete Formwork
Dead Loads: Formwork dead load,embedded steel reinforcement, molded fresh concrete all contribute to the dead load on the formwork. To ensure accurate computation of the dead load, the weight of each material should be calculated separately.
Live Loads: The weight of workers, tools, and equipments need to be taken into consideration when designing horizontal formworks. According to ACI 347-04: Guide to Formwork of Concrete, a minimum live load of 2.4 kPa should be used, and at least 3.6 kPa should be employed when motorized carts and buggies are used. The combined dead and live loads must be at least 4.8 kPa or 6 kPa if motorized carts are used.
Formwork Self-Weight: The weight of the formwork itself is determined by calculating the unit weight and dimensions of the formwork components. Generally, the weight of the formwork is much less than the weight of the fresh concrete and the construction live load. To account for this, an allowance of 0.239-0.718 kPa is typically made per square meter of formwork during the design process. This allowance is based on experience and is tested after the formwork has been sized.
Impact of Vibration on Lateral Pressure of Concrete Formwork
The introduction of vibration during concrete placement has a significant impact on the lateral pressure exerted by the concrete on the formwork. As the concrete is placed close to the top of the formwork, it behaves like a liquid, exerting a pressure in the lateral direction that is equal to the vertical liquid head pressure. However, as the concrete is placed further down the formwork and subjected to vibration, the internal friction between the granular particles of the concrete is eliminated, resulting in a liquid-like state and increasing the lateral pressure.
Factors Affecting Lateral Pressure of Concrete Formwork
Various factors can impact the lateral pressure of concrete formwork, such as the rate of placement, concrete temperature, and internal friction. For instance, if the rate of placement is slow, the fresh concrete may have time to start stiffening before reaching the lower levels of the formwork. Additionally, a higher concrete temperature can reduce the time for concrete to start setting, thus decreasing the lateral pressure. Other factors such as pore water movement, friction creation, cement type, admixtures, cement substitutes, and construction practices can influence the level of lateral pressure as well.
Distribution of Lateral Pressure of Concrete Formwork
The lateral pressure of placed concrete is usually distributed in a way that begins close to the top with a higher pressure and declines as the depth increases, as illustrated in Figure-1. For design reasons, an ultimate pressure is usually assumed to be uniform at a conservative value.
Calculation of Lateral Pressure on Concrete Formwork
The American Concrete Institute (ACI) 347-04 provides guidance on calculating lateral pressure of concrete on formworks according to the slump value and placement depth.
Lateral Pressure on Concrete Formworks for Columns:
For concrete with a slump value greater than 175 mm and placed with normal internal vibration to a depth of 1.2 m or less, the lateral pressure is calculated using Equation-1. However, for concrete with a slump value no larger than 175 mm, the lateral pressure is calculated as follows: Pmax = 30Cw kPa, but in no case greater than the given formula.
Lateral Pressure on Concrete Formworks for Walls:
For walls with a placement rate of less than 2.1 m/h and a placement height of no greater than 4.2 m, the lateral pressure is calculated as Pmax = 30Cw kPa, but in no case greater than the given formula. If the placement rate is greater than 2.1 m/h or the placement height exceeds 4.2 m, then the lateral pressure for all walls with placement rates of 2.1 to 4.5 m/h is also calculated using the same formula.
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 |
Retarding Admixtures for Concrete
Retarding admixtures are added to concrete to slow down the setting process and allow for greater workability of the mix. These admixtures can include retarders, retarding water reducers, retarding mid-range water reducing admixtures, or high-range water-reducing admixtures (superplasticizers). This allows for the concrete to be used in a variety of applications and to be formed into more intricate shapes and structures.
Pressure Equation Utilization in Column and Wall Forms
Column forms are defined as vertical elements with no plan dimensions exceeding 2 m, and wall forms are vertical elements with at least one plan dimension larger than 2 m. In column forms, internal pressure is transferred to the external tie elements on adjacent sides of the form, which are used as links between opposite sides of square or circular columns. For wall forms, internal pressure is transferred from plywood, studs, or wales to the tension ties that link two opposite sides of the form. Additionally, providing resisting elements such as braces is necessary in order to resist external horizontal loads, which tend to overturn wall, column, and slab forms.
3. Horizontal Loads on Concrete Formworks
Wind Loads:
When constructing a building, horizontal loads from wind forces should be taken into account and counteracted with properly designed braces and shore. According to ACI 347-04, the assumed value of these loads should not be lower than the greater value of either 1.5 KN/m of floor edge or 2% of the total dead load spread uniformly along the slab edge. For wall forms, the minimum wind design load should be 0.72 kPa or greater as specified by ASCE 7-10 with adjustments for shorter recurrence intervals as stated in ASCE 37-02. The bracing must also be designed to support a minimum of 1.5 KN/m of wall length applied at the top.
Concrete Dumping and Equipment:
When constructing a building, horizontal loads from concrete dumping and equipment starting and stopping should be taken into account and counteracted with properly designed braces and shore. According to ACI 347-04, the assumed value of these loads should not be lower than the greater value of either 1.5 KN/m of floor edge or 2% of the total dead load spread uniformly along the slab edge. For wall forms, the minimum wind design load should be 0.72 kPa or greater as specified by ASCE 7-10 with adjustments for shorter recurrence intervals as stated in ASCE 37-02. The bracing must also be designed to support a minimum of 1.5 KN/m of wall length applied at the top.
Inclined Supports:
When constructing a building, horizontal loads from inclined supports should be taken into account and counteracted with properly designed braces and shore. According to ACI 347-04, the assumed value of these loads should not be lower than the greater value of either 1.5 KN/m of floor edge or 2% of the total dead load spread uniformly along the slab edge. For wall forms, the minimum wind design load should be 0.72 kPa or greater as specified by ASCE 7-10 with adjustments for shorter recurrence intervals as stated in ASCE 37-02. The bracing must also be designed to support a minimum of 1.5 KN/m of wall length applied at the top.
4. Special Loads on Concrete Formworks
Design Considerations for Unconventional Construction Conditions
When designing formworks for construction conditions that may not be normal, such as concentrated reinforcement loads, unsymmetrical placement of concrete, machine-delivered concrete impact, uplift, and form handling loads, it is important to consider the various loading patterns that may occur before the concrete hardens. An example of this is constructing walls over spans of slabs or beams, which can impose different loading patterns than those that the supporting structure is designed for.
Impact of Unconventional Loads on Formworks
When constructing formworks in unconventional conditions, it is important to take into account the various loading patterns that can occur. Different loading patterns can have an impact on the formworks, such as the reinforcement concentrated loads, unsymmetrical placement of concrete, machine-delivered concrete impact, uplift, and form handling loads. These different loading patterns can cause the formworks to be subject to different levels of stress and strain.
Designing for Unconventional Loads
When designing formworks for unconventional loading patterns, it is important to consider the various loading patterns that can occur. The formworks should be designed to withstand the various loading patterns, such as the concentrated reinforcement loads, unsymmetrical placement of concrete, machine-delivered concrete impact, uplift, and form handling loads. The formworks should also be designed to be able to handle any changes in loading patterns that may occur over time.