Lag screw withdrawal strength are a measurement of how much force is required to pull a lag screw out of a piece of wood. Pullout failure occur when a lag screw lose it’s grip on the wood fibers, and pullout failure causes a deck ledger to sag or detach from a house. You must understand lag screw withdrawal strength because lag screw withdrawal strength determine if a connection will remain secure over many years.
The withdrawal strength of a lag screw depend on the diameter of the lag screw and the density of the wood. A thicker lag screw have more surface area for the threads to grab, and a thicker lag screw provides more resistance against being pulled out. Wood density also affect the strength of the connection because dense wood provides more resistance to the lag screw threads than soft wood provides.
What Makes a Lag Screw Hold in Wood
For example, oak is a dense wood that provides high resistance, but Douglas fir is a less dense wood that provide less resistance than oak. Therefore, you must select a lag screw that match the density of the wood you are using. The depth of the lag screw penetration is a critical factor in calculating the total strength of the connection.
If you increase the depth of the lag screw penetration, you increase the total load that the lag screw can hold. For example, if a lag screw provides a certain amount of resistance per inch of penetration, doubling the depth of the lag screw penetration will double the total resistance. However, the direction of the lag screw installation affect the strength of the connection.
Installing a lag screw into the side grain provides more strength than installing a lag screw into the end grain. If you install a lag screw into the end grain, the capacity of the lag screw will decrease because the wood fibers can slip more easy in the end grain. Moisture content in the wood also affects the strength of the lag screw connection.
Wood that has a moisture content below nineteen percent provide full strength, but wood that has a high moisture content provides less strength. When wood has a high moisture content, the wood fibers swell, and the swollen wood fibers loosens the grip on the lag screw threads. Because moisture affects the grip of the lag screw, you should use galvanized or stainless steel lag screws for exterior projects where moisture is present.
You must drill a pilot hole before you install a lag screw to ensure the connection is strong. If you dont drill a pilot hole, the lag screw may split the wood, and a split in the wood will reduce the strength of the connection to zero. If you drill a pilot hole that is too large, the lag screw threads will not compress the wood fibers, and an oversized pilot hole will reduce the strength of the connection by twenty to forty percent.
You should drill a pilot hole that is approximately seventy-five percent of the diameter of the lag screw shank so that you can achieve the full rated strength of the lag screw. Using multiple lag screws is a method to increase the total load capacity of a connection. If one lag screw is not strong enough for a load, you can use two or more lag screws to increase the total strength.
You should space the lag screws at least three times the diameter of the lag screw apart so that the lag screws does not split the wood. Additionally, you should stagger the lag screws so that the connection distributes the strength across the wood. Finally, you can calculate the withdrawal strength of a lag screw using a mathematical formula.
The formula for withdrawal strength per inch involve the specific gravity of the wood and the diameter of the lag screw. Because the formula is complex, you should verify your calculations with a structural engineer to ensure the connection is safe. If you follow these steps and use the correct lag screws, the lag screws will provide a secure connection for the deck ledgers.
