Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

88 Chapter Five
Downloaded From : www.EasyEngineering.net
In subsequent sections allowable span lengths are calculated
based on the bending, shear, and deflection of beams and of plywood.
These are the equations commonly used for the design of formwork
and temporary structures during construction. Tables 5-3 and 5-4
provide a summary of the equations for the reader to reference for
design purposes, followed by a presentation and discussion of compression
stresses in axially loaded wood columns. Throughout this
book, Tables 5-3 and 5-4 are frequently referenced in example problems
to illustrate the design of wood members for formwork and
temporary structures.
ww.EasyEngineering.n
Lumber versus Timber Members
Sawn lumber is classified into two classifications; dimension lumber
and timbers. Dimension lumber is the smaller sizes of structural lumber,
whereas timbers are the larger sizes of lumber.
Dimension lumber includes the range of sizes from 2 × 2 through
4 × 16. Thus, dimension lumber is lumber that have nominal thicknesses
of 2 to 4 in. Thickness refers to the smaller dimension of the
piece of wood and width refers to the larger dimension. Timbers are
the larger sizes and have a 5-in. minimum nominal dimension. Thus,
the smallest size timber is 6 × 6, and all members larger than 6 × 6
are classified as timber.
The species of trees used for structural wood are classified as
hardwoods and softwoods. These terms represent the classifications
of trees and not the physical properties of the wood. Hardwoods are
broad-leaved trees, whereas softwoods have narrow, needlelike
leaves, like evergreens. The majority of structural lumber comes from
the softwood category.
Lumber is graded by species groups, reference NDS Supplement
2005. Douglas Fir-Larch and Southern Pine are two very common species
groups that are widely used in structural applications. They are
relatively dense and their structural properties exceed many hardwoods.
For Douglas Fir-Larch the species of trees that may be included
in combination are Douglas Fir and Western Larch. For Southern Pine,
the species of trees that may be included in combination are Loblolly
Pine, Longleaf Pine, Shortleaf Pine, and Slash Pine.
The designer must be aware that more than one set of grading
rules can be used to grade some commercial species groups. For
example, Douglas Fir-Larch can be graded under West Coast Lumber
Inspection Bureau (WCLIB) rules or under Western Wood Products
Association (WWPA) rules. There are some differences in reference
design values between the two sets of rules. The tables of reference
design values in the NDS Supplement have the grading rules clearly
identified, such as WCLIB and/or WWPA. These differences in reference
design values occur only in large-size members known as timbers.
Downloaded From : www.EasyEngineering.net