Category CONCRETE FORMWORK

Size Classification

There are three main size categories for lumber:

1. Boards: Lumber that nominally is less than 2 in. (50.8 mm) thick and 2 in. (50.8 mm) or more wide. Boards’ thicknesses refers to the smallest cross-section dimension of lumber and the term width refers to the largest dimen­sion. Boards less than 6 in. (152.4 mm) wide are classified as strips. Boards are used for sheathing, roofing, siding, and paneling.

2. Dimension lumber: Lumber with a nominal thickness of 2-5 in. (50.8-127 mm) and a nominal width of 2 in. (50.8 mm) or more. Dimension lumber ranges in size from 2 X 2 in. (50.8 X 50.8 mm) to 4 – 16 in. (101.6 X 406.4 mm). Dimension lumber is used for general construction where appearance is not a factor, such as studding, blocking, and bracing.

3. Timber: Lumber that is nominally 5 or more inches (127 mm) in the smallest dimension. Timber is used for col­umns, posts, beams, stringers, struts, caps, sills, and girders.

Подпись: Copyright © Marcel Dekker, Inc. All rights reserved.Lumber is also grouped according to size and intended use into several categories.

1. Light framing: 2-4 in. (50.8-101.6 mm) thick, 2-4 in. (50.8-101.6 mm) wide. Typical grades are Construction Standard, Utility, and Economy and are widely used for general framing purposes. Lumber under this category is of fine appearance but is graded primarily for strength and serviceability. Utility lumber is used where a combi­nation of high strength and economical construction is
desired. An example would be for general framing, blocking, bracing, and rafters.

2. Studs: 2-4 in. (50.8-101.6 mm) thick, 2-6 in (50.8-152.4 mm) wide, 10 ft (3048.0 mm) long and shorter. Primary use is for walls, whether they are load-bearing or nonload­bearing walls.

3. Structural light framing: 2-4 in. (50.8-101.6 mm) thick and 2-4 in. (50.8-101.6 mm) wide. Typical grades are Select Structural No. 1, No. 2, and No. 3. This is intended primarily for use where high strength, stiffness, and fine appearance are desired. An example of a use would be in trusses.

4. Appearance framing: 2-4 in (50.8-101.6 mm) thick, 2 in. (50.8 mm) and wider. For use in general housing and light construction where knots are permitted but high strength and fine appearance are desired.

5. Structural joists and planks: 2-4 in. (50.8-101.6 mm) thick and 5 in. (127.0 mm) or more wide. Typical grades are Select Structural No. 1, No. 2, and No. 3. Intended primarily for use where high strength, stiffness, and fine appearance are required.

Grading of Lumber

Подпись: Copyright © Marcel Dekker, Inc. All rights reserved.Lumber is graded visually and by the machine stress-rated system (MSR). The majority of lumber is graded visually by experienced inspectors who are familiar with lumber grading rules. Lumber grading rules establish limits on the size and characteristics of knots, the number of shakes or splits, and the slope of the grain. Visual evaluation also takes into account any imperfections caused by manufacturing such as torn grain or chip marks. Figure 1.4a shows a typical grade for visually graded lumber.

In machine stress rating, lumber is evaluated by mechanical stress-rating equipment. Lumber is fed through a machine that subjects each piece of wood to a nondestructive test that measures the modulus of elasticity and bending stress. The machine auto­matically takes into account size and characteristics of knots, slope of the grain, density, and moisture contents. The machine automat­ically stamps lumber as machine rated and indicates the values for

Grading of Lumber Grading of Lumber
Grading of Lumber
Grading of Lumber

‘Nominal

Bending

Stress

(1650psi)

 

Moisture Content at Time of Surfacing (S-DRY)

Ю

 

Modulus of Elasticity (1.5x10s psi)

 

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Figure 1.4 Gradings of lumber: (a) visually graded; (b) machine rated.

 

fiber stress in bending and the corresponding modulus of elastic­ity. Figure 1.4b shows a typical grade for machine-rated (MSR) lumber.

Another method of grading is to grade lumber according to its use and appearance. Lumber is classified as Select, Finish, or Common. Select and Finish are used when fine appearance is re-

 

quired. Common grades are more suitable for general construc­tion.

Commercial Softwood Lumber

There are two major categories for commercial softwood lumber: construction lumber and lumber for remanufacture. Construction lumber is normally used in construction at the same size as it was graded. Grading for construction lumber is normally decided at the sawmill. No further grading occurs once the piece leaves the sawmill. On the other hand, lumber for remanufacture normally undergoes a number of additional manufacturing processes and reaches the consumer in a significantly different form.

Nominal Size

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Commercial lumber is sold as boards and planks by dimension sizes. However, the dimensions do not match the actual lumber sizes. For example, a 2 X 4 in. (50.8 X 101.6 mm) pine board is cut to the full 2 X 4 in. (50.8 X 101.6 mm) at the sawmill. This is called the nominal dimension. The nominal dimensions are re­duced because of shrinkage and surfacing in both width and thick­ness. For example, the actual dimensions of a nominal 2 X 4 in. (50.8 X 101.6 mm) are 19/16 X 39/16 in. (39.7 X 90.5 mm). Lumber that is not surfaced is referred to as rough-sawn. Most lumber for construction is surfaced (dressed) to a standard net size which is less than the nominal (name) size. Surfaced lumber is lumber that has been smoothed or sanded on one side (S1S), two sides (S2S), one edge (S1E), two edges (S2E), or on combinations of sides and edges (S1S1E, S2S1E, S1S2E, or S4S).

cost. For example, if the cost of pine lumber is $400 per 1,000 board feet, then the cost of a 2 in. x 6 in. x 10 ft (50.8 x 152.4 x 3048.0 mm) piece would be:

— (2 in. x 6 in. x 10 ft) x = $4 12 1000

Wood

Wood is widely used for many construction applications including concrete formwork. Wood is harvested from trees and is classified as hardwood and softwood. Hardwood comes from trees that have broad leaves such as oaks, maples, and basswood. Softwood comes from trees that have needlelike leaves such as pines, cedars, and firs. Softwoods are most commonly used in construction of form­work. It should be noted that the names ‘‘hardwood’’ and ‘‘soft­
wood’’ give no indication of the hardness or the softness of the wood.

FORMWORK MATERIALS

Materials used for the construction of concrete formwork range from traditional materials such as wood, steel, aluminum, and ply­wood to nontraditional materials such as fiberglass. Wood prod­ucts are the most widely used material for formwork. The objective of this section is to introduce wood as an important material for formwork.

Repair and/or Reuse Formwork

Reuse of concrete formwork is a key for economic formwork con­struction. After only five reuses, formwork materials costs drop to 40 percent of the initial cost. Formwork elements must be handled with care and should not be dropped. After repairing, cleaning, and oiling, the used formwork elements should either be stockpiled for future use or reused in other areas.

Before reusing formwork elements, they should be inspected for damage. Defects on the inside face must be repaired or re­moved; otherwise they will reflect on the finished surface of the concrete to show the same defect.

Provide Reshores/Backshores

Reshoring and backshoring are the processes of providing tempo­rary vertical support shores for the stripped structural elements which have not yet developed full design strength. They also pro­vide temporary vertical support for the completed structure after the original shoring support has been removed. Reshoring and backshoring are the two methods used to provide the concrete with support until it reaches its full design strength.

Reshores are shores placed snugly under a stripped concrete slab or structural member after the original forms and shores have been removed from a large area. In reshoring, the concrete slab is allowed to deflect and, thus, formwork can be removed from a large area. This can help reduce stripping costs, which is the main advantage of reshores.

Подпись: Copyright © Marcel Dekker, Inc. All rights reserved.Backshores are shores placed snugly under a stripped con­crete slab or structural member after the original forms and shores have been removed from a small area. In backshoring, formwork is removed from a small area of slab and then backshores are pro­vided. Concrete slabs or other structural elements are not allowed to deflect, and as a result, stripping can be accomplished at an earlier concrete curing age.

1.3.1 Remove Reshores or Backshores

Reshores and backshores can be removed after the supported slab or member has attained sufficient strength to support all loads
transferred to it. Removal of reshores or backshores must be car­ried out with care to avoid subjecting the structure to impact loads.

Strip Forms

As soon as concrete gains enough strength to eliminate immediate distress or deflection under loads resulting from its own weight
and some additional loads, formwork should be stripped to allow other construction activities to start. The operation of removing the forms is called stripping or wrecking the forms. Formwork can either be partially stripped by removing small areas to prevent the slab from deflecting or completely stripped to allow the slab to deflect. As a general rule, formwork supporting members should not be removed before the strength of concrete has reached at least 70 percent of its design value.

Cure Concrete

Подпись: Copyright © Marcel Dekker, Inc. All rights reserved.The hardening of concrete is a chemical process that requires warmth and moisture. This activity involves curing concrete with water, steam, or any other method to prevent shrinkage and allow the concrete to gain sufficient early strength. Steam curing is used where early strength gain of concrete is important. After the con­crete is cured, the rest of the formwork life cycle continues with the strip forms activity. The cure concrete and strip forms activities are interchangeable depending on the type of structural element. For example, columns and walls are cured after stripping of the forms, while slabs and beams are cured before and after the forms are stripped.