The National Design Specification for Wood Construction
(NDS) (AF&PA, 1997) makes comprehensive recommendations for engineered uses
of stress-graded lumber. Stress values for all commercially available species
groups and grades of lumber produced in the U.S. are tabulated in the NDS.
The moduli of elasticity for all species groups and grades
are also included in these tables. These tabulated values of stresses and
moduli of elasticity are called base design values. They are modified by
applying adjustment factors to give allowable stresses for the graded lumber.
The adjustment factors reduce (or in some cases increase)
the base design stress values to account for specific conditions of use that
affect the behavior of the lumber. A list of these adjustment factors and a
discussion of their use follows.
Load Duration — CD
The stress level that wood will safely sustain is inversely
proportional to the duration that the stress is applied. That is, stress
applied for a very short time (e.g., an impact load) can have a higher value
than stress applied for a longer duration and still be safely carried by a wood
member. This characteristic of wood is accounted for in determining allowable
stresses by using a load duration factor, CD.
The load duration factor varies from 20 for an impact load
(duration equal to one second) to 0.9 for a permanent load (duration longer
than 10 years). ACI Committee 347 recommends that for concrete formwork, a load
duration factor appropriate for a load of 7 days should be used. This
corresponds to a value for CD of 1.25.
ACI Committee 347 says this load duration factor should only
be applied to concrete forms intended for limited reuse. No precise definition
of limited reuse is given by the ACI committee, but the no increase for
duration of load should be used for concrete forms designed to be reused a high
number of cycles.
Moisture — CM
Wood is affected by moisture content higher than about 19%.
Higher moisture content significantly softens the wood fibers and makes it less
stiff and less able to carry stresses. The reduction in allowable strength
depends on the type of stress (e.g., shear stress is affected less than
perpendicular to grain compressive stress) and the grade of the lumber.
Size — CF
Research on lumber allowable stresses has shown that as
cross-sectional size increases, allowable stresses are reduced. A size factor,
CF, is used to increase base design values for different sizes of lumber.
Repetitive Members — Cr
The NDS allows bending stresses to be increased for beams
that share their loads with other beams. The increased allowable stress is
referred to as a repetitive member stress. For a beam to qualify as a
repetitive member, it must be one of at least three members spaced no further
apart than two feet and joined by a load-distributing element such as plywood
sheathing.
When these three requirements are met, the allowable bending
stress can be increased by 15%. This corresponds to a value for Cr of 1.15.
Repetitive member stresses may be appropriate for some formwork components.
Because the intent of allowing increased stress for repetitive flexural members
is to take advantage of the load sharing provided by continuity, gang panels
assembled securely by bolting or nailing and intended for multiple reuse would
seem to qualify for this increase.
ACI Committee 347 specifies that they should not be used
where the bending stresses have already been increased by 25% for short
duration loads.
Perpendicular to Grain Compression — Cb
Allowable perpendicular to grain bearing stress at the ends
of a beam may be adjusted for length of bearing according to: lb is the length
of bearing parallel to grain.
Horizontal Shear Constant — CH
Shear stress in lumber beams used as components of concrete
forms is usually highest at the ends of the members. For beams having limited
end defects (e.g., splits, checks, cracks), the values of allowable shear
stress can be increased. This is done by using a shear constant CH that depends
on the size of end defects and varies from 1 to 2.
Temperature — CT
Sustained high temperatures adversely affect some properties
of wood. It is unusual for concrete forms to be exposed to temperatures high
enough to require the use of a temperature adjustment factor. For temperatures
in excess of 100°F, the stresses and moduli should be adjusted using CT.
Stablity — CP
Like all columns, wood shores will safely carry axial loads
in inverse proportion to their effective slenderness. The more slender a wood
shore is, the less load it will support because of the increased influence of
buckling. Prior to the 1997 edition of the NDS, wood columns were divided into
three categories (short, intermediate, and long) according to their
slenderness.
