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Woods and wood products are fairly simple engineering materials. However, the conception, design, and construction process are fraught with problems if proper measures are not considered. In using wood in its many forms and with its unique inherent characteristics, there are problem areas which seem to present easily overlooked.
It is necessary for the designers and constructors to be aware about problematic areas of the wood and its products.
- Common Wood Design and Construction Problems
- 1. Moisture Related Issues
- 2. Building Site Environment Related Issues
- 3. Wood Creep Under Load
- 4. Improper Material Protection at the Job Site
- 5. Investigate and Inspect Species and Grade of Required Lumber
- 6. Inspect the Job Site
- 7. Wood and Within-grade Variability
- 8. Inspect all Timber Connections During Erection
- 9. Lack of Adequate Bracing During Frame Erection
- 10. Orthotropicity of Wood
- 11. Use Metal Joist Hangers to Add Strength and Efficiency in Construction
- 12. Fire Retardant Related Problems
Common Wood Design and Construction Problems
1. Moisture Related Issues
Wood absorbs moisture from air and rots if moisture content increased above 20%. So, such problem shall be tackled using proper perseverance treatment.
Moreover, minor roof leakage often leads to pockets of decay, which may not be noticed until severe decay or actual failure has occurred.
Added to that, stained areas on wood siding or at joints may indicate metal fastener rust associated with a wet spot or decay in adjoining, supporting members.
Ground contact of wood members can lead to decay as well as providing ready access to wood-deteriorating termites. Placement of preservative-treated members between the ground and the rest of the structure is usually a code requirement.
2. Building Site Environment Related Issues
In an area that has high relative humidity, special precautions should be taken. A structure that is surrounded by trees or other vegetation or that prevents wind and sun from drying action, is prone to high humidity nearly every day.
Similarly, if the structure is near a stream or other source of moisture, it may have moisture problems. Home siding in this type of atmosphere may warp or exhibit heavy mildew or fungal stain.
Buildings with small roof overhangs are susceptible to similar siding problems. if the siding is improperly installed, allowing water or condensation to enter and accumulate behind the siding.
Inadequate sealing and painting of a surface exacerbates the problem. Poor architecture, poor site, poor construction practice, and poor judgment combined to create a disaster.
3. Wood Creep Under Load
This has created widespread problems in combination with clogged or inadequate drains on flat roofs. Ponding, with increasing roof joist deflection, can lead to ultimate roof failure.
In situations where floor or ceiling deflection is important, a rule of thumb to follow is that increased deflection due to long-term creep may be assumed to be about equal to initial deflection under the design loading.
In some cases, the occupants of a building report that they can hear wood members creaking, particularly under a snow load or ponding action. This is a good indication that the structure is overstressed and failure, or increasing creep deformation with impending failure, is highly possible.
4. Improper Material Protection at the Job Site
Failure to protect materials at the job site may cause plywood and other panel products to become wet through exposure to rain so that they delaminate, warp severely, or swell in thickness to the point of needing to be discarded.
5. Investigate and Inspect Species and Grade of Required Lumber
Engineers and architects tend to order the lumber grade indicated by mathematical calculations; carpenters use what is provided to them. improper investigation and inspection of ordered lumber may cause future problems.
For instance, certain wood products are suitable for an application like scaffolding whereas others are not suitable for other application. Therefore, suitable investigation and inspection should result in buying suitable lumber for the given project.
6. Inspect the Job Site
Make sure that panel products, such as plywood, OSB, or flakeboard, are kept under roof prior to installation. Stacked on the ground or subjected to several weeks of rainy weather, not only will these panels warp, but they may lose their structural integrity over time.
7. Wood and Within-grade Variability
Commonly, it is wise to inspect lumber to cull out pieces that have unusually wide growth rings or wood that is from an area including the pith (center) of the tree. This material often tends to shrink along its length as much as ten times the normal amount due to an inherently high microfibrillar angle in growth rings close to the pith.
8. Inspect all Timber Connections During Erection
Check on proper plate fasteners on trusses and parallel chord beams after installation. Occasionally, in a very dense piece the metal teeth bend over rather than penetrate into the wood properly. A somewhat similar problem arises if wood frames or trusses are not handled properly during erection.
9. Lack of Adequate Bracing During Frame Erection
This is a particularly familiar scenario on do-it-yourself projects like unskilled erection crews. Thin, 2-by lumber is inherently unstable in long lengths; design manuals and warning labels on lumber or product shipments testify to this, yet the warnings are continually disregarded.
10. Orthotropicity of Wood
A large slope of grain around a knot or poorly placed knot can seriously change bending or compressive strength. Allowable design values for tension parallel to the grain 55% of allowable bending values because test results have indicated that slope of grain or other defects greatly reduce tensile properties.
Different orthotropic shrinkage values, due to grain deviations or improper fastening of dissimilar wood planes, can lead to warpage and subsequent shifts in load-induced stresses.
Care must be taken when using multiple fasteners (bolts, split rings, etc.) to avoid end splits as wood changes moisture content, particularly if the members are large and only partially dried at the time of installation.
11. Use Metal Joist Hangers to Add Strength and Efficiency in Construction
Toe-nailing the end of a joist may restrain it from lateral movement, but it does little to prevent it from overturning if there is no stabilizing decking.
Erection stresses caused by carpenters and erection crews standing or working on partially completed framework are a leading cause of member failure and job site injury.
12. Fire Retardant Related Problems
In recent years, developed fire retardant treatments have reacted with wood when in a high temperature–high relative humidity environment to seriously deteriorate the wood in treated plywood or truss members.
These chemicals, presumably withdrawn from the marketplace, act slowly over time, but have contributed to structural failure in the attics of numerous condominium type buildings.
Preventive measures where such problems may be anticipated include the addition of thermostatically controlled forced-air, the addition of an insulation layer to the underside of the roof to reduce the amount of heat accumulation in the attic due to radiant heat absorption from the sun; and the installation of a vapor barrier on the floor of the attic to reduce the amount of water vapor from the underlying living units.