Design Criteria for Wooden Formwork: Wooden formworks are commonly used for construction due to their ease of use. As such, it is important to properly design them to ensure their stability and safety. There are several criteria that should be considered when designing wooden formworks, as discussed below.
Strength and Stability: When designing a wooden formwork, it is important to ensure that it is sufficiently strong and stable. This involves taking into account the type of wood being used, its load-bearing capacity, the weight of the concrete being poured, and the size and shape of the formwork. It is also important to consider the loading conditions, such as uniform, concentrated, or point loads.
Durability: Durability is another important factor to consider when designing a wooden formwork. The wood should be treated with a preservative to ensure that it is resistant to rotting, mold, and other damage. The formwork should also be regularly inspected for signs of wear and tear, and any damaged or weak members should be replaced as soon as possible.
Ease of Use: It is also important to consider the ease of use of the wooden formwork when designing it. The formwork should be designed in such a way that it can be easily assembled and disassembled, as well as moved around the construction site. The formwork should also be designed so that it is easy to level and align, and so that it can be securely fastened in place.
Wooden Formwork Design Criteria for Concrete Construction
Following are the different criteria for design of wooden concrete formwork
- ASD adjustment factor for lumber stresses
- Factor of overall member size
- Stability factor for beam
- Column stability factor
- Factor of load duration
- Bearing area factor
- Moisture factor
- Flat application factor
- Safety factor for formwork accessories
- Repetitive factor of member utilization
- Manufactured wood products
- Adjustment factor for plywood stresses
Adjustment Factors for Lumber Stresses
The National Design Standard for Wood Construction 2015 provides adjustment factors for reference design values (F) to calculate permissible design values (F’) for bending stress, shear stress, bearing stress, compression stress, and elastic modulus. The adjustment factors in parentheses are used for truss members.
Adjustment Factors for Reference Design Values
The National Design Standard for Wood Construction 2015 provides adjustment factors for reference design values (F) to calculate permissible design values (F’). These adjustment factors are used for bending stress, shear stress, bearing stress, compression stress, and elastic modulus (with additional factors in parentheses for truss members).
Stability Factor for Beam for Wooden Formwork Design
The National Design Specification (NDS) provides recommendations for the value of beam stability factor (CL) for rectangular bending members. In the case of unstable compression edges, the stability factor for beam can be calculated according to the equation provided by NDS 3.3.3.8. Additionally, the NDS recommends values for beam stability factor of 1 for sawn lumber depending on lateral support condition and member depth to width ratio, as outlined in Table-1.
| Depth to Width Ratio (b/d) | Lateral Support Condition | Beam Stability Factor (CL) |
|---|---|---|
| b >= d < 2 | Lateral support is not needed | 1 |
| 4>=b/d >2 | Formwork ends should be supported by nailing, or bridging, full depth solid blocking, or other means | 1 |
| 5 >= b/d >4 | Member compression edge must be kept in position by suitable for its full length to prevent lateral displacement, and rations should be prevented at the end bearing point of the element | 1 |
| 6 >= b/d>5 | Compression edge of the member should be kept at its position by subflooring or other means, lateral displacement at end bearing point must be avoided, and at spacing of 20 cm bridging or diagonal cross bracing or full depth solid block should be provided | 1 |
| 7>=b/d >6 | Not only does the point of end bearing need to be supported to prevent rotation but also both compression edge of the member should be kept at their original position | 1 |
What is Column Stability Factor for Wooden Formwork Design?
Column stability factor, also known as Cp, is used in wooden formwork design as per the provisions of national design standard. It is calculated by taking into account the lateral displacement of compression members and the buckling of shores or braces. The value of Cp is taken as 1 if the compression member is supported throughout its length.
How is Column Stability Factor Calculated?
Column stability factor is calculated using the following equation: Where: Fc*: reference compression design value parallel to natural line on wood (grain) multiply by applicable adjustment factor apart from Cp. c: taken as 0.8 for sawn lumber, 0.85 for round timber poles and plies, and 0.90 for structural glued laminated timber, structural composite lumber, and cross laminated timber FcE: is computed by applying the following expressions. Where: le: is the effective length le?d: is the larger of slenderness ratio about the possible buckling axis and usually do not surpass 50 apart from short loading during construction and in this case, it can be up to 75.
What is the Maximum Slenderness Ratio for Column Stability Factor?
The maximum slenderness ratio for column stability factor is usually not greater than 50, except in cases where short-term loading during construction is required, in which case it can be up to 75.
Load Duration Factor for Wooden Formwork Design
Wood has been demonstrated to have a significantly higher maximum load capacity when subjected to short duration loads compared to normal duration loads. To account for this, a load duration factor is applied to the design of wooden formworks. Table 2 provides the load duration factor for different cumulative maximum load durations. In general, the load duration factor for most formworks is 1.15, however if the formworks are reused for longer cumulative durations, the load duration factor must be adjusted accordingly.
| Duration of Loading | Load Duration Factor (CD) |
|---|---|
| Load Duration > 10 Years | 0.9 |
| 2 Months < Load Duration ≤ 10 Years | 1.0 |
| 7 Days < Load Duration ≤ 2 Months | 1.15 |
| Load Duration ≤ 7 Days | 1.25 |
| Wind/ Earthquake | 1.6 |
| Impact | 2.2 |
The load duration factor, CD, is used to adjust the maximum load capacity of the wooden formwork for different cumulative maximum load durations. By taking this into account, the formwork design can be optimized according to the expected load duration.
What is the Bearing Area Factor for Wooden Formwork Design?
The bearing area factor is used to increase concentrated load design values on wood that is perpendicular to grain or natural lines of wood. As per the National Design Specification for Wood Construction, this factor is applied to all bearings of any length at the end of the member, and to all bearings that are six inches or more in length at any other location.
How is the Bearing Area Factor Calculated?
The bearing area factor is calculated using the following formula: lb is the bearing length measured parallel to the wood grain. If the above conditions are not met, then the factor is equal to 1.
Moisture Content and its Impact on Wood Strength
Wood strength is determined by its moisture content. If wood loses about thirty percent of its moisture content, the strength of the wood increases. Generally, for a moisture content of 19% or below, the essential design values have been established. However, wood may be exposed to external conditions, thus increasing the moisture content. In such cases, the moisture factor must be taken into account to adjust the design values.
Flat Use Factor for Wood Subjected to Loading on its Wide Face
When wood with a thickness between 50.8 cm and 101.6 cm is subjected to loading on its wide face, it will deflect around its weakest axis. As a result, the stress which causes failure will be larger than it would be otherwise. To adjust for this, a flat use factor (Cfu) is used to adjust the basic design values of bending stress. Table 3 provides the flat use factor.
Safety Factor for Formwork Accessories
Hangers, anchors, and ties are all examples of accessories used in formwork. These accessories are commonly made of steel with either ultimate or allowable strength values being provided by manufacturers. Therefore, if the ultimate strength is given, it must be multiplied by a safety factor to obtain the permissible strength.
Safety Factors: The minimum safety factors for formwork accessories are outlined in the American Concrete Institute’s (ACI) 347-04 code and are shown in Table 4.
Table 4: Minimum Safety Factor Used for Accessory Formwork: The ACI 347-04 provides the minimum safety factor for formwork accessories, as outlined in Table 4. The table outlines the safety factor that must be applied when using accessories such as hangers, anchors, and ties in order to obtain their permissible strength.
| Accessory | Type of Construction | Safety Factor |
|---|---|---|
| Form Tie | All Applications | 2 |
| Form Hangers | All Applications | 2 |
| Form Anchor | Formwork supporting form weight and concrete pressure only | 2 |
| Form Anchor | Formwork supports form weight, concrete, construction live load, and impact | 3 |
| Anchoring Inserts used as Form Ties | Precast concrete panel when employed as formwork | 2 |
Repetitive Factor of Member Utilization
WHAT IS REPETITIVE USE FACTOR?
Repetitive use factor is a multiplier used to calculate the design values of members when minimum three lumbers with a thickness of 5.08cm to 10.18cm are employed and connected together. This factor is applicable to plans, joists, studs, rafters, decking, or other similar members with a maximum spacing of 60.96cm. The repetitive use factor generally used is 1.15.
HOW TO APPLY REPETITIVE USE FACTOR?
Repetitive use factor is applied when flooring, sheathing, or subflooring is used to connect lumbers in order to distribute and withstand design loads. In such cases, the design values of the members must be multiplied by the repetitive use factor. This factor is generally 1.15 and is used to take into account the increased load on the members due to the repetitive use.
Manufactured Wood Products
Laminated Veneer Lumber: Laminated veneer lumber (LVL) is a type of manufactured wood product used for formworks. It is composed of layers of thin wood veneers that are glued together to form a stronger and more durable product. LVL is more resistant to warping and shrinking than solid wood, making it an ideal choice for formworks.
Parallel Strand Lumber: Parallel strand lumber (PSL) is another type of manufactured wood product used in formworks. It is made up of strands of wood that are arranged in parallel and bonded with adhesive. PSL is stronger and more dimensionally stable than solid wood, making it ideal for formworks.
Laminated Strand Lumber: Laminated strand lumber (LSL) is a type of manufactured wood product used for formworks. It is composed of strands of wood that are arranged in a cross-laminated pattern and bonded together with adhesive. LSL is more resistant to warping and shrinking than solid wood and is a great choice for formworks.
Adjustment Factors for Plywood Stresses
Plywood is an important material used in modern building and engineering applications, due to its strength and durability. However, like all other materials, it is subject to wear and tear, and its strength must be adjusted accordingly. The Engineered Wood Association provides permissible stress values for plywood, which can be adjusted using three key factors: load duration, wet use, and experience factor.
Load Duration Factor: The load duration factor is the same for both wood and plywood, and takes into account the amount of time the material will be in use. The longer the material is subjected to a load, the greater the stress it will experience. This factor should be adjusted accordingly to ensure the plywood is able to withstand the expected load for the duration of its use.
Experience Factor: The experience factor is an adjustment made to the permissible stress values based on the user’s experience with the plywood material. It takes into account the user’s familiarity with the material, as well as their ability to accurately assess the stress it will experience. If the user has little experience with the material, the permissible stress values should be adjusted accordingly.
Wet Use Factor: The wet use factor is an adjustment made to the permissible stress values to account for the plywood’s use in a wet environment. If the material is going to be exposed to water or moisture, the permissible stress values should be adjusted accordingly to ensure the plywood is able to withstand the expected load in a wet environment.