Screw and nail pull strength in wood materials exposed to salt water and fresh water.
Screw and nail pull tests are conducted to evaluate the behavior of screw and nail connections when subject to pull out and shear forces experienced in a severe storm. Test results help engineers design stronger methods to attach metal and wood components in many kinds of structures. Two studies were recently conducted to aid investigators in their determinations.
The first study measured the screw and nail pull strength of wood materials—e.g., 7/16-inch oriented strand board, one-inch pressure-treated pine, 5/8-inch pine planks, and 11/16-inch exterior grade plywood—after complete immersion in either salt water or fresh water for periods ranging from one to six weeks. The screws and nails were inserted after the wood samples were immersed and dried. Thus, exposure effects on the fastener materials were removed as a variable.
A second study examined the same wood samples plus 3/4-inch cypress and 1 7/10-inch cedar to determine if pullout strength (screw pull only) changed when screws were inserted into the wood samples before immersion (fresh water only) for a period of one week. Screw pull tests were conducted when wood samples were either wet or dry for comparison.
The results provided insight into the selection of building materials for structures built in flood-prone areas based on wood samples that exhibited measurable changes in physical dimension and fastener pullout strength when exposed to normal (eight to 14 percent wood moisture equivalent) and saturated (23 to 35 percent WME) moisture conditions. Test results obtained from wet samples demonstrated that fastener strength varied inversely proportional to the moisture content of the wood.
Materials and Methods
Wood was cut into six-by-12-inch samples and labeled according to type, intended duration of exposure, and exposure condition. The moisture content and material thickness were measured for each sample using a moisture meter and micrometer before testing. After labeling and measuring, all test samples (two samples per wood type) were immersed in five-gallon plastic containers filled with either tap water or filtered Tampa Bay seawater (36 parts per thousand) and covered with a lid.
Control samples, one sample per wood type, were exposed to water vapors under ambient temperature and suspended inside a sealed plastic container above one inch of fresh water.
After a week, the first study’s one-week test and control samples were removed from both water samples, photographed, measured for moisture content and dimensional change, and then placed on a rack and dried at approximately 140 F for one week. After drying, the samples were removed, photographed, measured for moisture content and dimension, and tested for screw and nail pull strength using American Society of Testing and Materials (ASTM) methods (e.g., nail and direct screw withdrawal). Twenty screws (one-inch sheet metal, No. 10, Type AB) and two nails (six penny common) were inserted into each wood type after drying and tested using a calibrated pulling device. This process was repeated after weeks two, three, four, five, and six.
The second study compared the change in screw pull strength among its six materials before and after one week of total immersion in tap water. Before immersion, screws were placed into the wood and tested at ambient moisture content. The screw pull strength was tested again after the samples were submerged for one week without drying.
Results: Control Samples
Temperature and relative humidity monitoring devices placed inside each control test chamber reported ambient temperatures from 45 F to 89 F and relative humidity from 80 to 100 percent during the six-week study period.
Moisture content increased from 12 to 35 percent WME over the study period. A corresponding dimensional increase in height was observed in OSB, which exhibited the largest change (33 percent), while pressure-treated pine exhibited the least (zero percent) when measured wet before drying (Figure 1). Results obtained from the salt-water control samples were comparable.
Results: Test Samples
When test samples were immersed in either fresh water or salt water, several samples (OSB, plywood, and pine plank) exhibited an initial dimensional increase that was sustained for the remaining six-week test period (Figures 2 and 3). When measured wet, OSB exhibited the largest dimensional change (33 to 50 percent) while pressure-treated pine exhibited the least (zero to one percent). After drying, the dimensional change decreased.
Nail Pull Test – Wet Then Dry
For the nail pull test, a nail was inserted within 3/4 inch from the edge of each wood sample and pulled sideways (shear force) in an effort to pull it out of the material. Following six weeks of immersion, the vast majority of nails (98 percent) could not be pulled out of the samples and were bent following testing. The results indicated that the nail fasteners and wood maintained their competency following one long-term immersion period.
Screw Pull Test – Wet Then Dry
Screw pull test (SPT) results revealed that materials saturated in either salt water or fresh water and then dried showed no statistical difference in SPT strength when screws were pulled vertically from the surface. For example, plywood immersed in salt water expressed an average SPT strength of 345 pounds, while those samples immersed in fresh water reported an average SPT strength of 331 pounds. There was no statistical difference between the same materials immersed in either salt water or fresh water over a duration of one to six weeks. The standard deviations showed that the results described statistically similar strength ranges among the same materials.
Among the four materials tested, pine planks exhibited the highest average SPT results at 445 pounds followed by pressure-treated lumber at 426 pounds, plywood at 330 pounds, and OSB at 130 pounds (Photo 1). SPT results were most variable among solid wood products (pine planks and pressure-treated pine).
At least two conditions contributed to variable SPT measurements. First, variations in wood grain width, composite composition, proximity to knots, and the age of the capillary system were evident. Second, the moisture content in the dried samples was not uniform. Based on comparisons of moisture content and screw pull strength, samples with higher moisture content corresponded to lower SPT strength. These results prompted a second study to compare screw pull strength between saturated and dry wood materials in fresh water.
Wet vs. Dry Screw Results
For the second study, all six wood materials reported lower SPT strength when saturated as compared to the dry samples (Figure 4). OSB wood samples showed the largest decline (38 percent), while pressure-treated lumber showed the least (22 percent).
Wet Wood vs. Dry Wood
Can we conclude that wet wood is weaker than dry wood? Yes, according to these results and other studies. A study conducted by the U.S. Department of Agriculture Forest Products Laboratory supports these findings establishing that moisture content affects wood strength. Wood is a hygroscopic material that will seek equilibrium with local humidity conditions, also known as the equilibrium moisture content (EMC). EMC describes the variation in moisture content of wood under particular conditions of temperature and relative humidity.
Another study by the USDA found that wood strength is directly related to the moisture content in the wood fiber cell wall. Wood exhibits the greatest strength when the moisture content is between oven-dried (less than eight percent WME) and about 12 percent WME. However, when the moisture content is above the fiber saturation point (30 to 35 percent WME), the strength of polymers in the cell wall is reduced.
OSB exhibited measureable dimensional swelling (more than 30 percent) when either immersed in water or exposed to water vapors.
Among the materials tested, for each exposure scenario, pine planks exhibited the strongest SPT results, while OSB exhibited the weakest.
Wood samples immersed in salt water and fresh water resulted in no appreciable change in screw pull or nail pull strength when the samples were returned to their original moisture contents (less than 12 percent WME).
The duration of time wood materials were immersed in salt water or fresh water (up to six weeks) made no difference in screw pull test strength as long as the moisture content was uniformly low when tested.
Samples immersed in salt water reported high apparent moisture content as they dried because of residual salt content and resulting increase in conductivity.
Screw pull test strength varied depending on the material density and moisture content.
Screw pull test strength was highest when dry (less than eight percent WME) and lowest when saturated (more than 30 percent WME).
Pine planks exhibited the strongest SPT results while OSB exhibited the weakest.
Solid wood structural elements (e.g., interior wood framing, exterior posts and support members, floor and roof joists) that are flooded with salt water and fresh water will return to their original dimension and strength and could be retained if the fasteners remain competent.
Composite wood products (e.g., OSB) exhibit irreversible swelling when saturated or exposed to sustained conditions of high humidity. Distortion and cracking of overlying cementitious coatings, stucco, and cladding may occur when in close proximity to wet OSB.
Pressure-treated lumber, solid lumber, and exterior grade plywood should be considered as preferred materials when building or reconstructing structures with exposure to elevated humidity or potential flooding.
Nonstructural wood elements, such as cladding, handrails, trellises, roof fascia, and window trim, are more prone to damage by high wind forces and consequential movement when they are saturated with water.
The authors would like to thank Nichols Albergo, P.E., DEE, Robert Braun, P.E., Bruce Bosserman, P.E., John Marquardt, P.E., Chin Yang, Ph.D., and John Barkey, P.G. for their editorial comments and suggestions.